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      3.1.5.8  Reverse Pulse Collector.- Installations such as depicted
 In Figure 3.3J tire typical of the equipment now increasing most rapidly
 Jn application.   Hie pulse of compressed air is released at the top of
 the filter element and travels downward removing most of the deposited
 dust.   The pulse is usually quite sharp initially,  but attenuates as it
 travels,  losing  its effectiveness.   Thus,  the filter elements are
 limited to several feet.   The equipment shown is designed to entrain
 additional air with the  pulse, giving  not  only a shock front, but also
 a  brief reverse  flow.  The two effects apparently combine for added
 effectiveness.  The advantages of this kind  of equipment include  high
 air/cloth ratios and thus small plant  floor  occupancy,  very high  dust
 loading capability and high efficiency by  virtue of the felts used.
 The compressed air requirements may  cost as  much as the power to  achieve
 filtration,  although initial costs are generally fairly low.   This  kind
 of equipment comes in  practically all  sizes.

       3.1.5.9 Ultrafiltration Equipment.- Ultrafiltration equipment,  such
 as  shown  in Figure 3.3j,  achieves efficient  removal of low concentrations
 of submicron particles by an extension of  standard  fabric filtration tech-
 nology.   In this  process  a "filter aid" is used to  provide the initial  dust
 cake and  subsequently  to  provide good  cake release.   By adding a  means  of
 introducing the  filter aid as  a powdered or  fibrous  material  ahead  of the
 filter, several  manufacturers  have modified  their standard equipment lines
 to include  this  capability.  Ultrafiltration  is  further  discussed in Sec-
 tion 3.5.4.
 3.2  FILTER CONFIGURATIONS
     Filter  elements are  available in  numerous  shapes, styles  and sizes.
This range  of  choices  is  due to  the need for equipment to  serve totally
different applications and  the different relative evaluations of space,
power,  initial cost and maintenance time.  Filter cloth  is now arranged
in panels or envelopes, in  tubes and bags and in variations on these.
Looking ahead, it  is not  likely  that any radical new geometry will be
developed, although somewhat more cloth per cubic foot of baghouse      ;

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           Manufacturer:  Flex-Kleen (Research
                          Cottrell)
           Model Not  (several)
           Brochure No: T-7

           Filter Element Type: Bag
           Diameter/spacing: 5 3/4"
           Length: 18 to 84"
           L/D ratio: 3 to 15
           Flow direction: Up
           Dust caught on the outside.
           Retalnment: hung and clamped;
                       supported
           Cleaning method: reverse pulse
           Control method:
           Power required:
                 automatic
                 approx. 10 SCFM,   ?
                 100 psig per 500 ft
                 of fabric
Baghouse shape: Rect. or cycllndrical
Kilter elements per compartment:
            8 and up
Baghouse size: variable
Operation: continuous
Accessibility:  Intermittent
Fabric Types: various felts
Normal Air/cloth ratio: 6to 15
Normal pressure across cloth:
Applications:  High loadings continuous
               service
Temperatures: moderate
Loadings:  high, moderate
Dust types:
Efficiency:  good
O
                                                       FLEX-KLE6N filter bags are constricted
                                                       of quality materials to precise dimen-
                                                       sions with double stitched  french-fell
                                                       seams.  Felted materials  are used  for
                                                       their high filtering efficiency at high air-
                                                       cloth ratios.
                                                       EASY Tfc ASSEMBLE—Filter bags are
                                                       slipped over the bag cages with the top
                                                       2 inches turned in over the  top of the
                                                       cage to form a seal on the bag cup. The
                                                       bag-cage assembly is slipped over the
                                                       bag  cup  and clamped with a quick-act-
                                                       ing clamp.  A split ring on the cage en-
                                                       gages with a groove on Jfte bag cup to
                                                       form a positive lock.
                                Figure 3.3a.  Reverse Pulse Collector

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                                                                                                     /"N
HOW THE TOTALAIRE  FILTRATION
SYSTEM OPERATES
The basic operating unit of the Totalaire system is
a standard  Pangborn cloth tube type dust collector
which is adapted for ultra high efficiency air fil-
tration by coating the tubes with a special, inex-
pensive filter aid at the rate of .075 Ib/sq ft.  Even
the finest particles of atmospheric dust and tarry
materials are collected and entrapped in the result-
ing filter mat.  As these particles are fine and light,
they cause no holding problem and they actually
increase filtration efficiency. In a National Bureau
of Standards Spot Dust  Test, Totalaire  filtration
proved 94 7' efficient. By weight, the efficiency of
the unit will exceed 99%.  It  will clean atmos-
pheric air to below .04 mg per  100 cu ft with in-
take concentration of 9 to 12 mg per 1000 cu  ft.
  Filter aid material is supplied  in 30 and  50 Ib
bags. Under normal conditions one application will
last from 2 to 3 years.  On units operating under
suction, air moving  equipment  of the ventilating
system  is  used to feed the filter aid  through  the
hopper  sections. On units operating  under pres-
sure, it  is fed into the system  ahead of the fan.
Charging time  ranges from a few minutes on small
collectors up to an hour or two on larger units.
  Filter aid material adheres to the inside surfaces
of the tubes when fed into the  collector.  After a
few hours of operation, it can only be removed by
operating the  shaker mechanism.  Under normal
conditions, removal of the filter aid  material and
accumulated dust  is required only after 2 to 3
years  of   24-hour-a-day  operation.   Obviously
maintenance costs are extremely low.
 Manufacturer:  Pangborn
 Model:  Totalaire
 Brochure No:   931

 Fabric types: standard
 Normal air/cloth ratio;
 Normal pressure across cloth:
 Applications:
 Temperatures:  Ambient
 Loadings:       Very  low
 Cleaning frequency:   very seldom (years)
 Dust  types: Small particle
 Efficiency:
      48
to o
m m
ra mv
                                                      S«
   I
   i"
                 12   18   24   30   36  42

                        MONTHS
                                            46 —
  This graph nlmwn Ihn lypir/n/ <,/mrolin« limn rmfnrn ahnkndiiwn '
for a Totalnim unit undnr wirioi/s dust l,,,,rix. whim tha unit i»
started 
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              volume may be  obtained  by closer  folding,  pleating,  or packing.   Possibly
              new designs  that  simplify cloth replacement  will be  introduced and higher
"•"            entrance  and exit velocities will be  developed for those  baghouse systems
  i;            not limited  by particle abrasion.  At the  present time, however,  we
 J            must continue  to  examine filter configurations on the  basis  of:  (1)
              panels vs tubes or bags; (2) upward vs  downward flow;  (3)  inside  vs
  !,            outside collection; and (4)  length, diameter,  and length/diameter ratio.
 £'
 >                 3.2.1   Pane1 vs Tube FiIters
  !
  '                        About 20 percent of the collector  models available use flat
              panels, the  rest  using  either  tubes or  bags.   The advantages of one type
              over the  other seem to  depend  on  the  application, since both are  widely
              used.  One major  manufacturer  of panel  type  filters cites  these advan-
 ''            tages:
                                                                               3
                   (1)  The  flat filter  gets 20  to  40 percent more cloth per ft  of
 |                     collector volume.
 "                  (2)  The air is moving more slowly when it enters the panel, than
                       air at  cylindrical bag entrances, hence abrasion is  less.
                   (3)  A  set of panels  can be inspected more easily since both sides
                       of the  cloth  on  every panel can be seen.
                   (4)  A  single panel can be changed more easily than a single tube,
                       although it may  be easier to  completely rebag the  tube  col-
                       lector  once it  is cleaned out.
                   (5)  Dust getting  through any leak in the  cloth piles up  under the
                       panels where  it  is more easily removed than dust piled  around
                       the  lower ends of tubes.
                   (6)  There are more choices of inlet and  outlet locations on a
                       panel baghouse than on a bag  type,where inlet must be at  the
                       bottom and outlet must be at  the top  (or vice versa).
                  (7)  The panel may be ghaken or pulsed more uniformly than a bag
 »                     which is shaken  or pulsed from one end only.
                  (8)  In the event  fibrous materials are encountered,   the panels
                       can be brushed down on the dirty side, whereas the dirty
                       side of the bag may be inaccessible.   If the dust bridges
                       between the panels, every other one can be removed.
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 A manufacturer of bag or tube filter equipment might counter with the
 following opinions:
      (1)   Since bags can be very long,  more cloth can be installed or
           otherwise  serviced per ft^ of floor space.
      (2)   In most installations, abrasion is not a problem and when it
           is it can  often be alleviated by suitable fabric design.
      (3)   It is easier to repair a leaky bag in place without removing
           it than a  panel.
      (4)   A single tube can be changed  more easily than a single  panel.
      (5)   Dust piled around the base of the tube is often the quickest
           way to locate a leak.
      (6)   Unless the baghouse inlet  and outlet are located logically
           from the standpoint of air distribution and dust settling,
           expensive  baffles or plenums  are necessary.   Standard inlet
           and outlet locations reduce costs.
      (7)   Shaking a  baghouse takes  less power and is quieter  than
           shaking a  panel collector.
      (8)   The spaces between panels,  being smaller,  are more  apt  to clog
           with coarse fibrous matter or in the event of a high surge  of
           dust than  the relatively open bag or tube.
The views  of both manufacturers obviously  have merit in certain in-
stances,  and have to be considered against  the collector application  of
interest.
     3.2.2   Upward vs Downward  Flow
             Traditionally,  the  collector inlet was  located at  the  bottom
of the baghouse where the heaviest dust settled  immediately to  the
hopper, thereby minimizing  fabric abrasion  and extending  the period
between fabric cleanings.  Since the  compartment  or  entire baghouse was
shut down  for  cloth  cleaning, the dust never had  to  fall against an up-
ward gas  flow.  Furthermore, downflow required the use  of an extra  tube
sheet and perhaps even more expensive, the use of an extra bottom plenum
(see Figures 3.3b and 3.3f).  Thus, the upward flow design was  logical,
s imple and  leas t  expensive.
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                                                  •   x/
             The introduction of reverse jet cleaning, and with it con-,
 tinuous on-line operation, brought in the extra tube sheet and intro-
 duced downward flow to encourage the dust to fall during filtration.
 Otherwise,  the dust mi^ht collect again on the nearby cloth.   In fact,
 this  may happen even with downward flow,  especially near the  bottom of
 the tube where the downward air velocity  approaches zero.  The four
 reverse jet models available today all use downward flow.
             It would seem that the same principle of downward flow for
 continuous  filtering would apply to pulse cleaning as well, but in fact
 less  than 10 percent of the approximately 30 pulse type models use down-
 ward  flow.   The contradiction is partly explained by the theory that
 pulse cleaning removes the cake without breaking it up as much as the
 reverse jet does,  so the dust falls as larger pieces.  Also,  when the
 pulsed air  is directed downward, as it usually is,  it tends to carry
 the dust downward  although only briefly.
             Because the settling velocity and primary flow velocity are
 additive in downward flow,  all sizes  of particles  tend to travel farther
 in  downward flow than in upward flow before being  caught by the fabric.
 Particles in downward flow have some  probability of falling completely
 through the filter tube to the hopper without being caught.   This is
 true  of all particle sizes, whereas  in upward flow  only particles
 larger than a size determined by the  tube entrance  velocity can escape
 the fabric  (Section 3.2.4.2;  for most  filter  applications  this  size  is
 of  the order of  3|jm.)  Consequently,  it would appear  that  the choice of
 upward or downward flow might reduce  the  average cake weight, depending
 on  the particle  size distribution.  Dusts  predominately larger  than  the
 limiting size might  be  more suited  to upward  flow filtration.    Smaller
 dusts,if filtered  downward,might result In slightly  lighter cake weights.
            A consideration that probably  overrides the minimization of
 cake weight  is the distribution  of particle sizes along the filter
surface and  the effect  this has  on dust deposit permeability and subse-
quent  local  filtering velocities.  Both large and small particles would
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 be distributed more evenly over the length of the tube in the case of
 downward flow.  This should result in a more uniform use of the entire
 filtering surface than in upward flow.
             Another important consideration is that the upward flow col-
 lector is generally less  expensive, having one less tube sheet or plenum.
 It is  harder to adjust the tube tension with two tube sheets.   In addi-
 tion,  with downward flow  there is  dead  air in the hopper which increases
 danger of condensation.
             Some panel types  are distinctly upward or downward flows,
 in which case  the above considerations  apply,  even though the  panels
 may be small in dimension.  However,  most  panel-type  filters  tend to
 use horizontal flow,  that is,  the  inlet and outlet are approximately at
 the same height on the baghouse.
             These are  the considerations in selecting the direction of
 flow.   To summarize, although  the  upward flow  collector  is  slightly less
 expensive initially, the  downward  system should  give  slightly  the better
 filtering performance  and lower  power requirement.  However, the  data
 available are  insufficient  to  quantitatively confirm  this  conclusion.
     3.2.3   Inside  vs. Outside Filtering
            While  this is not  a  consideration  in most  panel filters, bag-
 type equipment  offers  the choice of filtering  on either  the inside  or  the
 outside  of the  cylinder as was shown  earlier in  Figure 3.1.  Of course,
reverse  jet equipment  provides inside filtering, as does all but one
model  of  downward  flow equipment.  About 60 percent of all upward flow
equipment is the  inside filtering  type.  Clearly, the choice has nothing
to do with the  fabric which can be sewn with either side out.  An impor-
tant advantage  to inside  filtering is being able to enter the filter
compartment for inspection and maintenance during operation.  Also, in-
side filtering does not usually require the use of supporting mesh which
can increase maintenance difficulties.  However, inside filtering tends
to involve more fabric flexure during cleaning.
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      3.2.4  Length, Diameter and Length/Diameter Ratio   /
             Most filter bags are 5 to 12 inches in diameter and 5 to 40
 times the diameter in length.  The choice of diameter and length does
 affect filter performance, but the dimensions are more traditionally
 determined by their effect on the initial cost of the bags and that of
 the bagliouse.
             3.2.4.1  Tube Diameter.- Filter cloth is usually woven in
 standard widths,and in general,  bag diameters are constrained by the
 available widths  of fabrics as woven.  One common size is approximately
 38  or 39 inches wide,  from which two 5  or 6 inch diameter bags can be
                   '     ;                                             i
 obtained, allowing for the necessary overlap at seams.  For certain
 applications, an  11*5  or  12 inch diameter glass fabric bag is the most
 economical size  from the  available 38 inch wide glass cloth.   A few bag-
 houses are designed for use with 7  or 8 inch diameter bags.   This size
 is  probably based  upon a  54 inch wide cloth from which two bags can be
 obtained from a single width.  Wool felts,  used in reverse jet baghouses,
 are generally either 9, 10 or  18 inches  in diameter.
             The diameter  of the  filter bags also influences  the size of
 the baghouse.  For  example,  about 1,750  square  feet of filtering area
 can be provided in  about  80 square feet  of floor area by using 6 inch
 diameter by 10 foot long  bags.   If 12  inch  diameter bags  were used in-
 stead, they would need  to be about 14  feet  long  to  provide  the same  fil-
 tering area in the  same floor  space,  though 12  inch diameter  bags  can
 easily be  obtained  20  feet  long  or more when  there  is  adequate  head  room.
 This  (12  in.  x 20 ft.) would result  in a baghouse having  about  2,500
                                                            2
 square feet  of filtering  area  in the same  floor  space  (80  ft  ).
            While using a smaller  diameter  increases  the  filter  capacity
 per unit  occupied floor area,  there are disadvantages associated with
 small bags.   Small  bags may bridge across,  particularly in cases of un-
usually coarse collected dust  or extreme surges  in dust loading.   If the
hopper plugs, any bag can become filled at  the bottom with collected
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 dust and the smaller bag is less easily emptied afterward.  Smaller bags
 require  a larger number of clampings,  tension adjustments and inspections
 for the  same cloth area, but they are  easier to handle during maintenance
 and repair.
             3.2.4.2  Length-Diameter Radio and Tube Entrance Velocity. -
 The air  (and dust)  velocity at the entrance of a cylindrical filter tube
 is  given by:
                         Ve - Vf x 4 (
For  example:     Filtration velocity =  3.0  FPM,  Vf
                 Length/Diameter ratio  =  25,  L/D
                 Entrance  velocity » 300  FPM,  Ve
This velocity  is typical of many collector  installations.
             In upward  flow,  the greater the entrance  velocity  the more
large particulate will be  carried into  the  tube,  (The largest  particle
that can be  lifted by  an air current of given velocity and viscosity  is
discussed  in Chapter 2).    Thus,  the greater  the entrance velocity, the
faster the dust deposit  increases.   The permeability  of  the deposit may
be expected to be diminished by the  larger  particles,  however, as dis-
cussed in Section 2.4.8.
            The upward velocity will decrease  along the  tubular filter
to zero.  Thus, the size of  particles that  can be lifted (in upward flow)
decreases along the tube.  This results in  a partial  distribution of par-
ticle  sizes and a consequent  variation  in deposit permeability and weight
along  the tube.  These variables  are affected  by the  entrance velocity
and  the size distribution  of  particles  entering  the collector.
            The cost of power required  as a consequence of entrance head
loss  is negligible, being  less  than  one dollar per KCFM-year at 300 FPM.
However, in the case of abrasive dust,   the bag may be scoured and excess-
ively worn near the entrance  tube sheet, (Attempts to define the poten-
tially abrasive entrance velocity have not been successful;  see Section
4.5.1).
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             Length/diameter  ratio  affects  the  swaying  stability  of the
 vertical  tube.   Bags  should  not  rub  together either  during  filtering  or
 during  the  cleaning cycle.   Thus,  the higher the  ratio the  greater the
 advisable bag separation, partly offsetting the floor-saving advantages
 of  long tubes.   Also,  the lengthwise seam  of the  bag will stretch  dif-
 ferently  than the rest  of the cloth, making the high length-diameter
 ratio bag bend as the  tension or dust load changes.
             The  short  stocky tube may shake more  easily than the thin
 longer bag,  but  the stresses at  the shaking end may  still be higher.
There is  no  accepted way to  generalize this at present, as  amplitude,
 frequency, dust  weight  and several cloth parameters  are involved.  Also,
the relation between  fabric  stressing and  fabric  life  is not clear.
             Length/diameter  ratios are usually between 5 and 40, but more
typically between 10 and 25.  In purchasing, the  choice of  length/diameter
ratio seems  to be mainly determined by whatever equipment is available,
and this  is  more dependent on other considerations.
             3.2.4.3  Tube Length.- In addition to its  relation to dia-
meter, tube  length is also limited in other ways:
             (1)  The tension along the length of the  bag increases as a
                  function of height, starting with whatever tension is
                  being applied  at the bottom.   It may be so large at the
                  top as to  pull holes in the cloth at the cuff seams or
                  otherwise  damage the fabric,  in cases of heavy dust de-
                  posits .
             (2)  The longer the bag, the more expensive it is per square
                  foot, because  of the difficulty of handling and sewing
                  a long bag.  The choice of long bags requires a more
                  elongated  dust collector for the same fabric area and
                  since elongated housings require more siding and stronger
                  structural members, the collector cost will generally in-
                  crease.
             (3)  Some cleaning mechanisms are length  limited,  notably air
                  pulse and  shaking, wherein sufficient, energy for the en-
                  tire  length of the tube must be applied at one end to the
                  detriment  of that end.   The energy requirement increases
                  faster than length, in general.
             (4)  Continuously cleaned dust collectors require the removed
                  dust to fall to the hopper during filtration.  Instead
-------
                  of falling, many particles must be caught again on the
                  fabric.  This results in relatively heavy deposits near
                  the bottom of the bags.  The longer the bags, the heav-
                  ier this deposit will be.  In a similar sense, intermit-
                  tently cleaned collectors require a pause after cleaning
                  to allow the dust to settle to the hopper.  The shorter
                  the bags, the sooner the collector compartment can be
                  returned to service.

             (5)  Different thermal expansions between cloth and baghouse
                  require latitude in the bag fastenings or shorter bags,
                  or both.  For example:

                                                    Resulting
                                    Diff. Length   Stress Change    Tension

                     Steel-Glass:   .00042 ft/ft       4.2 psi      2.5 Ibs
                                     of bag

             (6)  Very long bags may require two or more people to install
                  and inspection and other maintenance may also be difficult.

             (7)  The baghouse may be ceiling limited if it is to be in-
                  stalled indoors.

             (8)  One cl&irn is made that bag surface scouring increases,
                  due to sliding of the loosened deposit downward along
                  the bag surface.  Scouring may assist the cleaning pro-
                  cess, but it may also damage the fabric surface fibers.
                  It is not clear which in general, is the more important.

             A review of bag lengths in a number of specific filter in-

stallations has indicated the use of slightly longer bags where smaller
particles are involved.  There is no clear physical reason for this.

Recent air pollution control applications are chiefly concerned with small

particles.  There is also apparently a slight trend in the fabric filter

industry toward longer bags and tubes, which may be coincidental.

             As will be seen from a review of typical costs of fabric
filtration (Chapter 7), plant overhead and especially the cost of plant
  *Assuming the fabric expands as a solid; per 100°F.
                                 4
 **Assuming tensile modulus of 10  psi and no bag fastening latitude;
   per 100°F.
***Assuming a 6-inch diameter bag and .03-inch thickness; per 100 F.
-------
 floor space are very important factors.   It is entirely possible that
 two collectors may be intended for almost identical service,  the only
 difference being that one will be used where floor space is plentiful and
 free, and  the other where floor space will contribute as much as 10 per-
 cent to the annual cost of operation. One would prefer to select equip-
 ment with longer bags in the second case.
             3.2.4.4  Panel Geometry.- The average velocity entering a
 space (W) between two filter panels of distance L from entrance to closed
 end is  given by V  = V,. x 2 (L/W) .   For  example,  if the filtering velo-
 city is 2 FPM and L and W are 3 ft  and 1.5 inches respectively,  the
 average entrance velocity is 96 FPM.   It is generally less than for bag
 and tube  filters and consequently,  many  of the above comments  relating  to
 entrance  velocity do not apply.  Furthermore,  as  the flow is  often hori-
 zontal,  there may be more settling.   There should be less abrasion of
 the fabric  surface because of the  lower  velocities.
             In the normal interest  of saving floor and  plant  space,
 panels will  be designed as close  together as possible.   This  is  subject
 to  the  need  to inspect  and replace  the cloth;  clogging  of the  space
 between the  panels;  touching during  cleaning,  such that the dust cannot
 fall  or such  that  one panel abrades against  the next; entrance abrasion;
 .and structural requirements.  Most panel spacings  are between  one-half
 and three inches  and  equipment  using  oval bags  or  envelopes have  inside
 dimensions  on about  the same scale.
             Panel  length and height vary  from a few inches to a few feet.
 In  addition  to the above  limitations,  the manufacturer's  choices are
determined by  such things  as  the width of  the original bolt of cloth;
 the need to maintain tension over the  panel; the ease of  installation
and replacement of panels; and  thoroughness  of cleaning, which depends
on cleaning methods.
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      3.2.5   Other Configurations
             Panel type filters  vary from single flat fabric sheets  to
 oval envelopes  with only one end open,  really more like a bag than  a
 panel.   A standard item with Pangborn In the multi-tube Filter <• lament,
 two panels  sewn together at Intervals so that when Inflated six cylin-
 drical  filter elements result,  fastened together side by side.  This
 approach packs  more fabric in a given collector volume, but may increase
 maintenance costs, since a failure  of one tube requires maintenance or
 replacement involving  all six.   Also, tension adjustment of such  a  large
 filter  element  may be  slightly  more difficult.
             Reported in some of the early literature,  and perhaps still
 in common use in Europe,  is  the conical filter element which tapers from
 a  circular  bottom of ordinary diameter  to an apex at the top of the bag-
 'jouse.   This design should give nearly  constant upward velocity the
 entire  length of the element and ,therefore,  a more uniform particle size
 deposition.   The elements  can also  be packed more closely together  with-
 out  danger  of inter-abrasion, as long as  they are not  made too  long,
 except  that  space  must  be  provided  for  their maintenance just as with
 any  filter  element.  Tapered elements are  relatively expensive  to con-
 struct.  A  thin cone may bridge  across  the inside and  be difficult  to
 clean.   The  shake  dynamics of a  cone  are no  doubt substantially different
 than for a  cylinder.
            A few  other novel fabric  filter  configurations are discussed
 in Section 3.5
3.3  CLEANING MECHANISMS
     Except  for  studies of specific air pollution  control problems,  most
development toward better fabric filter equipment appears to go into
 improved methods of  removing the accumulated dust deposit from the
 fabric.   As a result, we have the variety of cleaning mechanisms already
mentioned in  this chapter.  Just as the dust particles cohere to one
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 another,  the  deposit  cake  adheres  to  the  surface  fibers  of  the  cloth,  or
 in the  case of deep or  napped  surfaces, among  the surface fibers.   The
 problem is to remove  the desired amount of  the deposit from the fabric
 quickly and uniformly without  either:
      •  removing too much of  the residual  deposit  which greatly  improves
        the collection efficiency at start-up on a subsequent filtering
        cycle,  for woven fabrics
      .  damaging the cloth, or  using too much power, either  of which can
        be a substantial part of operating cost
      •  excessively dispersing  the  removed dust particles, as these  would
        probably then  have  to be re-filtered.
     The standard cleaning methods are listed  in  Table 3.1  along with  a
 number  of superficial characteristics frequently,  although  not  always,
 associated with those methods.  More quantitative  discussion of some of
 these characteristics (efficiency, cost,  filtration velocity, etc.) can be
 found elsewhere in this handbook.  The cleaning principles  are  discussed
 here.
     The time required  for completion of  the cleaning process is an im-
 portant consideration in comparing cleaning methods.  This  time should
 be much shorter than  the time between cleaning periods, which as shown
 in Section 2.4  is determined by filtering velocity, dust loading, pres-
 sure drop tolerances  and the "K" value of the dust.  For a high dust
 loading, the cloth might have  to be cleaned very often - perhaps every
 30 seconds, for example.  For this, a manual cleaning process would be
 out of the question.  Likewise, a shaking process which might require
 1 minute or more just to accomplish the cleaning,  would mean that at
 least two thirds of the fabric in the system would be out of use at all
 times.  On the  other hand the reverse jet method accomplishes the
 cleaning quickly, but is limited in repetition by the slowness of the
 carriage.  Pulse cleaning is  very fast,  but  also has comparative dis-
advantages.  It seems  that  every cleaning  method in use has  at least
 one advantage  over each of  the others, and conversely.
-------
                                                     TABLE 3.1

                                           COMPARISONS OF CLEANING METHODS1
10
-p-
Cleaning Uniformity
Method of
Cleaning
Shake
Rev. Flow,
no flex
Rev. Flow,
Collapse
Pulse-com-
partment
Pulse-bags

Reverse jet

Vibration,
rapping
Sonic assist
Manual flex-
ing
Ave.
Good

Ave.

Good

Ave.

V . Good

Good

Ave.
Good

Bag
Attrition
Ave.
Low

High

Low

Ave.

Ave . -Hi

Ave.

Low
High

Equipment Type
Ruggedness Fabric
Ave.
Good

Good

Good

Good

Low

Low

Low
--

Woven
Woven

Woven

Felt,
Woven
Felt,
Woven
Felt,
Woven
Woven

Woven
Felt,
Woven
Filter Apparatus Power
Velocity Cost Cost
Ave .
Ave.

Ave.

High

High

V.High

Ave.

Ave.
Ave.

Ave.
Ave.

Ave.

High

High

High

Ave.

Ave.
Low

Low
M.Low

M.Low

Med.

High

-High

M.Low

Med.
--

Dust Max. Sub-
Loading Temp.** Micron
Efficiency
Ave.
Ave.

Ave.

High

V.
High
High

Ave.

--
Low

High
High

High

Med.

Med.

Med.

Med.

High
Med.

Good
Good

Good

High

High

V. High

Good

Good
Good

        These value judgments do not permit comparison of performance aspects,  only of methods.

       **
-------
      3.3.1   Shake
             Bags are  most  commonly  shaken from the  upper  fastening.
 Several  combinations  of horizontal  and  vertical motion are  used.   The
 bags  may all be  fastened to a  common  framework which moves  horizontally.
 The. frame may have  slight  additional  upward  or downward swing,  depending
 on the linkage holding  the framework.   The framework may  instead  be
 oscillated  vertically.   Alternatively,  the bags may be attached in rows
 to a  rocking shaft.   In this case the location of the  point of  attach-
 ment  with respect to  the center  of  rotation  determines whether  the motion
 is predominantly horizontal or vertical.
             Rarely  are  the bottoms  of the  bags  shaken  instead of  the
 tops.  Panel filters  on the other hand, are  usually shaken  top  and
 bottom,  that is, the  entire filter  bank moves  from  a single drive  point.
 The shake amplitude is  usually designed into the equipment  and  may be
 anywhere  from a fraction of an inch to'a  few inches.   The frequency of
 shake is usually several cycles  per second and  can  often be adjusted to
 obtain the most suitable fabric  motions.  Most  shaking is essentially
 simple harmonic or sinusoidal  motion.  When  it  is,  the peak acceleration
may be easily computed;  it  is  usually from 1 to 10  g s.
             During the  shake,  the filtering  should  be  stopped.  Other-
wise, the dust will work through the cloth, reducing the efficiency and
 possibly damaging the cloth by internal abrasion.   In  some  equipment,
 the flow  is  slightly reversed  during shaking, both  to  prevent penetration
and to aid in cake removel.  Still more elaborate, but possibly more
effective, cleaning procedures involve a series of alternate flows and
shakes which  take time, but are  claimed to give a gentler net treatment
 to the cloth, plus more uniform and thorough cleaning.
             In a typical cycle, the inlet flow to the compartment is
first dampered off by an automatic timer and  valve mechanism.  If neces-
sary,  the outlet vent is also closed (Figure  3.4).  In the absence of an
air lock  between adjacent hoppers it may be necessary to close a damper
-------
0




*

1
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Filtering —^
                                                                  Outlet
                                                                  Shut
                                               Cleaning
                           Remaining Compartments
                                                            Reverse  Flow
                                                              Option
                                    Time
              Figure 3.4.   Shake Cleaning Process and Associated
                           Pressure Cycles.
-------
 there to prevent the intrusion of dirty air from hoppers still operating.
 There should be zero forward pressure across the fabric during shaking,
 since otherwise dust will work through the fabric.   The timer starts the
 shaker motor and the bags are shaken horizontally at 1 to 5 cps with an
 amplitude of up to 2 inches; that is, the shaker imparts accelerations
                        i
 on the order of a few g  s.   Shaking continues for 10 to 100 cycles.
 Then the timer may start a small flow of clean reverse air using an
 auxiliary blower or a secondary duct-and-damper system for 10 to 20
 seconds.   The shaking may be repeated,  this time during the small re-
 verse flow.   Finally,  the cleaning  is stopped and after pausing to
 allow the dust to settle,  the inlet and outlet dampers are opened and
 the  compartment begins its  filtering again.   The entire cleaning cycle
 may  take  from 30 seconds  to a few minutes.   Some installations do not
 return the compartment on line  until the next one is ready to be cleaned,
 thereby achieving a fairly  steady overall flow through the baghouse
 system at  the expense  of  some over-capacity.
            The pressure  across  the compartment during the cleaning
 cycle  is  sketched in Figure  3.4.  If there  are only  a few compartments
 in the baghouse system, shutting  one down will increase the flow and
 pressure  drop across each of the  others,  the  amount  depending  on the
 fan  curve.  The pressure across  the baghouse  while the above  cleaning
 is performed  is also indicated  in Figure  3.4.   This  increase  in  filtering
 velocity  and  pressure across  the  fabric  in  the  other  compartments  can  be
 expected  to have  some effect, adverse but probably slight,  on  the  filter
 cakes  forming  in  those compartments.
            The mechanics of  cleaning by  shake  have apparently not been
 studied in any  detail.  The  cloth is  flexed to  some extent  and the cake
 is thereby cracked  or loosened from  the cloth  fibers.  Shaking produces
 inertial stresses at the cake-cloth  interface, both shear and tensile,
and if these exceed  the adhesive strength, then some of the dust cake
 falls  off.  It  is reported that a fine balance of bag tension, shake
 frequency and shake amplitude is necessary in order to obtain the "s"
-------
 shaped wave  in the moving  bag  giving  best results.   On the other hand,
 simple manual  shaking  equipment  seems to work with  a minimum of science,
 although  careless manual cleaning  is  hard on the  fabric.   The amount  of
 shake energy transmitted along the bag decreases  from a maximum at  the
 shaken end (usually  the top) to  a  smaller value as  the energy is absorbed
 along the bag.  A heavy residual dust cake,not removed by  the cleaning
 process (usually in  the case of  sticky fine  dusts),  changes  both the
 mass and  the flexibility of the  bag and must affect  the rate of energy
 absorption locally.  Further,  the  added residual  dust weight accumulates
 upward and increases the tension toward the  top of  the bag,  affecting
 the shape of the shake "wave"  for  better or  for worse.
             Shake equipment has  few limitations in application.  As
 noted above, the dust  loading  time cannot be too  short.  The dust should
 be fairly easily removable from  the cloth as it is easy to damage the
 cloth by  over-shaking.  This is  especially true of glass bags.   Although
 it was at first believed that  glass could not be  shaken, the  introduc-
 tion of certain glass finishes has made this possible on some dusts which
 separate  fairly easily from glass  fibers.  Shake  baghouses are  popular
 for both  very small systems (manual shake) and very  large  continuous
 sys 'cems.
             Two problems in particular  are often  reported with  shake
 equipment.   First, in an effort  to keep manufacturing  costs  down, a
 number of shaker mechanisms are underdesigned.   They wear and as
 they do,  they shake less effectively.   The bags load up or the operator
 intensifies  the shaking, and the mechanism then destroys itself unless
 given an  inordinate amount of maintenance.   Second,  as  the free end of
 the bag is shaken, the cloth must  flex  extensively at  the fastened end,
 resulting in fiber-fiber abrasion.   This may explain  the common occur-
 rence of bag failures within the first  couple of  feet of bag.  These
 entrance end bag failures  may also be related to dust deposit weight,
which is  often heavier at  the lower end of the bag and may result in a
higher rate of cleaning energy absorption in this  region.   Another
possible  factor is particle entrance abrasion.
-------
     3.3.2  Reverse Flow - No Flexing
            If the dust releases fairly easily from the fabric, a low-
presoure reversal of the flow may be enough to loosen the cake without
flexing or mechanical agitation.  To minimize flexural attrition of the
fabric, it is supported by a metal grid, mesh or rln^s, and is usually
kept under some tension (Figure 3.r>).  The support is usually on the
clean side of the tube or bag, although dirty-side support can help to
keep the sides of the bag or the panels sufficiently apart to allow the
cake to fall to the hopper.  Some filter equipment relies solely on low
pressure flow reversal, while other models use it in conjunction with
another method, shaking:for example, or use the much higher reverse
pressures of pulse or jet cleaning.
            There are several ways of accomplishing flow reversal.  In
addition to the standard dampers on each compartment,  each one can have
its own reversing fan.   A few models have a traveling apparatus that goes
from bag to bag or from panel to panel, blocking off the primary flow
                    Figure 3.5.  Reverse Flow Cleaning.
-------
and  introducing  some  air  in the  reverse  direction.with a secondary
blower  (Figures  3.3e  and  3.3g).   Perhaps a  simpler method is  to take
advantage  of  a suction  on the  dirty  side, or  a  relative pressure on the
clean side, without using another blower as sketched in Figure  3.6. If in
using this cleaning system it  becomes  necessary to increase the suction
or pressure being  used  for cleaning  by partially closing off  the inlet
or outlet  primary  duct, then to  avoid  the added power  consumption,  it
may  be  better to use  a  small secondary fan  or fans.
            Any  flow  volume reversed through  the filter must  be refil-
tered.  This  means that in addition  to taking cloth out of the  system
for  cleaning, this cleaning method increases  the total air flow in the
remainder  of  the system.   The  net increase  in air/cloth ratio is  nor-
mally 10 percent or less,  but  this may cause  a  large (perhaps 40  percent)
increase in power  consumption.   This is  partly  through a decrease  in
cake permeability.
           ' The  pressure  across  the  compartment  and baghouse will be
similar to that  sketched  in Figure 3.4.  However, some equipment users
believe repeated reverse  flows  introduced suddenly   to  "pop"
the cloth are beneficial,  in which case  the pressure-time  cycle will be
different.
            Reverse flow  equipment with a minimum amount  of cloth  flexing
finds the same wide range  of general applications as shaking equipment,
the one main requirement being that the dust release easily from the
fabric.  Thus, felts are not cleaned by  low pressure reverse flow.
            Disadvantages  of reverse flow equipment include the in-
creased filtration velocity  (or alternatively a proportionally larger  in-
stallation),  the costs of any additional fans, ducts  or dampers, and
when necessary the cost of a support grid.  This grid  tends to be a
nuisance when it comes to changing the bags  or panels and considerable
effort has gone into its design.   The grid or rings may chafe  against
the cloth unless the cloth is so tight it can't  move,  which introduces
-------
                                          Optional,  to  avoid
                                          temperature changes
                                                 (Pressure)
      F:  Compartments filtering

      R:  Compartment being cleaned by dampered
         control  from suction side of system.
Figure 3.6.   Schematic  for  Reverse  Flow Cleaning
             During Continuous  Filter Operation.
-------
tension problems,  or unless  the  cloth  is  fastened  to the  support,  as  for
example, rings  sewn to  the tube.   Even such hard  fibers as  glass  can
apparently  fail by chaffing.   Chaffing is  such  a problem  that  the  sup-
ports are occasionally  done  away with  at  the risk  of excessively  flexing
the cloth.
     3.3.3  Reverse Flow With  Collapse
            Even though cloth  flexure  is detrimental,  especially when
the fabric  is penetrated by  grit,  flexure  is frequently the preferred
means of cloth  cleaning.  It is sometimes  used  in  conjunction  with  other
methods.  The cleaning  method  excludes panel type  filters and  felts,
but cloth bags  having the dust cake  on the inside  are  often cleaned this
way.  The cylinder is often  reported as collapsing  for some reason  into
a cloverleaf pattern (see Figure 3.5).  The  collapse  is not 100 percent,
however, or the cake could not fall  to the hopper.  There should be just
enough reverse  flow to  crack and shear the cake until  it falls off.   It
may then be carried to  the hopper by the reverse air current,  depending
on collector design.
            With this kind of  cleaning  there  is an  optimum  tightness  of
the cloth, said to be somewhat between 25  and 100  Ibs  for a standard
11.5 inch diameter glass bag.  More tension  than this  is hard  on the
fastenings, or prevents the bag from collapsing enough.  Less  tension
results in too sharp a  cloth bend at the tube thimble where the cloth
is fastened (see Figure 3.5).  The tension of a suspended bag can be
automatically changed during the cleaning  cycle if necessary,  by adjust-
ing the position of the hanger frame.
            The collapse method of cleaning uses essentially the same
damper,  fan and ducting equipment as the method of reverse  flow without
collapse.   Just as  repeated shaking is standard practice,  the  bag or
tube may be collapsed more than once.  It may also be "fluttered" by
lightly and rapidly pulsing the reverse air to give a shaking  effect,
-------
at  some risk  of damage  to  the  cloth.  Such  flow changes may also  trans-
mit pressure  pulses  to  adjacent compartments and damage the cake  struc-
ture  there.
            Chief disadvantages of collapse cleaning are  flexural and
chaffing attrition of the  cloth, and some lack of control of cleaning
intensity.  For instance,  if the cake fails to come off sufficiently
and begins to stiffen the  cloth, there will be still less cleaning and
consequently, extra  flexing in adjacent areas and faster wear.  There is
little one can do to increase  cleaning intensity, although one can re-
peat  the process or change the rate of flexure.
      3.3.4  Pulse Cleaning
            This method attempts to overcome several of the difficulties
associated with other methods  of cleaning.  In this kind of equipment a
sharp pulse of compressed air  is released in the vicinity of the fabric
giving rise to some combination of shock, fabric deformation and  flow
reversal.  Depending on the design, and there are over 25 models  of
pulse cleaned equipment. The result is the removal of the dust deposit
without more  than a brief  interruption of the filtering flow.  The fabric
receives a minimum of flexural wear and the filter installation is
smaller because the fabric is  in use practically all the time.  Also,
for reasons on page 3-46,  most pulse equipment utilizes felt rather
than cloth.  With felt  the filtration velocity can be typically 3 to 4 times
that used in shake or reverse  flow equipment,  so the pulse filter in-
stallation is smaller.
            The main distinction of pulsed equipment is the brief clean-
ing time,  typically around one-tenth of a second.   The very low ratio of
cleaning time to filtering time makes pulsed equipment uniquely useful
at very high dust loadings, up to several hundred  grains per cubic foot
on  large particles.   Thus, this equipment is widely used for pneumatic
conveying  systems,  and it  also has  application at  moderate and low dust
concentrations.
-------
            The earliest designs of this kind, as shown in Figure 3.7,
use open top bags with a pulse nozzle in or just above the top of each
bag.  A timer controlled solenoid valve releases 60 to 120 psi air
through the nozzle for a small fraction of a second.  As the air pulse
moves down the bag, it may draw in other air and the combination tends
to bulge the bag.  In one variation the filtering is on the inside of
the bag and the dust is partly removed by reverse collapse, but then a
pulse similar to the one shown above snaps the bag open and completes
the dust removal.  Note that this forward pulse cleans without appa-
rently plugging the fabric.  A second variation in pulse cleaning design
is the compartment pulse, in which several bags or envelopes are served
by the same nozzle located in a plenum at one end of the group; or,  the
entire compartment is served by one or more nozzles.  For example, at
least one model injects a reverse air blast into the compartment inlet,
which temporarily stops the flow and reverses it,causing partial col-
lapse of the filter elements.
     Filtering
Pulse
Cleaning Wave
                    Figure 3.7.  Reverse Pulse Cleaning.
-------
           These variations in clean vs. dirty side pulsing and  inside
vs. outaidv'-the-bag pulsing, raise the question of the mechanism or
imichnniHtroi uf dust removal.  Sovrrnl imThaniBtiw nppt-ar poHHlhlt-  «lppiMi«l-
Ing on dt'
           1.   The  bag or  tube  stretches  au the air balloons through it,
               cracking and blowing off the cake,  (inside pulses only).
           2.   The  air pressure change, even after expansion and dilu-
               tion,  is still  long enough and intense enough to cause
               a flow reversal  which blows off the cake,  (clean side
               pulses only).
           3.   The  inertia of  the air mass pulls a vacuum behind it which
               pulls  off the cake, (dirty side pulses only).
           4.   The   shock  of the pulse accelerates the fabric away from
               the  dust cake,  even though the distance traveled by the
               fabric is very  small, (probably dirty side pulses only).
           5.   The  fabric  and  dust accelerate outward together, but only
               the  fabric  has  the elasticity to snap back into shape.
               Outside dust is  left behind; inside dust is thrown off
               at the end  of the snap-back, (inside pulses only).
 Among  the approximately 25 pulse-type collectors the pressure-time func-
 tion varies  considerably from a true shock to little more than a quiet
 reversal of  flow,  so it is entirely possible that several of these
 mechanisms apply.
              Efficiency of use of compressed air is important because the
 electric power needed to run the compressor can equal that for the pri-
 mary fan. Some models claim improved efficiency by using Venturis sur-
 rounding the pulse nozzles as was shown earlier in Figure 3.3.i.   These
 entrain a certain  amount of additional air which expands the bag,  causing
 the  collected dust to drop off the outer surface.   The entrainment may
 also set up  a shock wave which accelerates or flexes the filter cloth.
-------
             It  is expeciaily interesting that only one of the approxi-
 mately 25  available  pulse-cleaned models uses downward flow.   Apparently,
 the  dust is  able  to  fall through the  upward flow to the hopper despite
 upward velocities typically  3 to 4 times those in shaker and  reverse
 flow equipment.   Evidently the pulse-removed deposit falls  in large
 pieces, but  even  so,  a  large quantity of dust must be redeposited  on
 nhe  fabric.   Possibly,  these large pieces are beneficial in acting as
 filter aid,  but on the  other hand they increase the total deposit  on
 the  filter.
             The components of a pulse cleaning filter include  an air
 compressor,  a storage or surge tank,  piping,  solenoids  and  nozzles,
 and  some models use  Venturis and fabric support gridwork as well.  Few,
 if any, ducting dampers  are  needed along with their associated controls.
As there are no moving parts required,  the pulse method has an advantage
 in terms of  maintenance.
             Just  as  in shake cleaning,  in which all the cleaning energy
must be applied at one end of the  bag,  so in pulse cleaning the effect-
iveness of cleaning decreases  with length of bag.   This  is  evident in
the short bags typically used  in pulse  equipment,  rarely if ever over
ten  feet in  length.   Since there is little  fabric  motion, the  bags or
 tubes  can be packed slightly closer without  interchaffing.  Even so,
 large  installations still occupy more  floor space  than  types with  30- to
40-foot bags.
             Both  the  intensity and frequency  of pulse cleaning are
easily adjusted, but  there is  little control  of top to^bottom uniformity
of cleaning.  Woven cloths tend  to be over-cleaned by pulsing, resulting
in excessive  leakage  following cleaning, whereas felts are more easily
maintained at a satisfactory  level of residual dust deposit over the
entire bag length with excellent cleaning efficiency.
-------
            There  are  several  other  reasons  for using  felts  rather  than
woven  cloths,  although in  a  few rare cases tightly woven  fabric  is  used.
The  dimensional  properties of  felts  may make them more amenable  to  pulse
cleaning  involving "ballooning".   Bag inlet  velocities are high  in  pulse
equipment and  felts having softer  surfaces may withstand  more abrasion
than cloths.   Felts form a more porous dust  cake  having lower specific
resistance, so they can be cleaned less often than cloths for the same
pressure  build-up.  Equipment  using  felt  can be smaller because  higher
filtering velocities are possible.  Finally,  felts can be used even
though they are  initially  more expensive, because there is little mech-
anical wear of the fabric  in pulsed  equipment.
            Like other types of equipment pulsed  cleaning has its dis-
advantages.  It  is limited in  temperature to around 450 F at present,
because felt materials  are not  available  for higher temperature. High pulse
pressure  can damage the  fabric  by  over-stretching it.  Felts tend to
plug in depth  rather than  blind.   They may have to be  cleaned rather
than discarded because of  their greater value.  Felt,  compressed air,
power  and  the  compressor are all relatively  expensive  so  that economic
balances  are different  for pulse equipment.   As  a result, equipment
sizes  and  shapes are different.  Pulsed equipment may  be  best for some
applications and simply not economical for others.
     3.3.5  Reverse Jet Cleaning
            Introduced  in  the early  1950's,  this method of cleaning
became known for its high  filtering efficiency and high filtration  velocities,
It was the first method to give felted materials a good,  uniform cleaning.
It did this by using a small-volume jet of moderately pressurized air
spaced at moderate time intervals.  Figure 3.3f shows how this can be
done.  A carriage carrying the jet rings moves up and down the filter
tubes,  driven  originally by chains and sprockets,  or by cables con-
trolled by limit switches.   On the carriage a slotted ring close to the
fabric surrounds each filter tube.  Most of the jet air goes  through the
felt toward the dirty side, blowing the dust  out of the deep  felt surface.
-------
                                                                                J
             Since  the  four reverse jet models on the market are based on
 the  original Hersey patents,  the models are much the same.   One type
 uses adjustable  segmented rings  around the tubes,  another uses  a flat          —*
 plenum through which are  cut  holes about the same  diameter  as the tubes.
 The  traveling carriage is easily designed for either cylindrical or
 rectangular  baghouses. Tube  lengths  are in the  range of 6  to 30 feet.
                                                                                  i
 In all models the  flow is downward and outward.  For this reason,  the            j
 collector  can be open  sided,  but as reverse Jet  equipment is often used
 on toxic materials  because of its high efficiency,  health codes usually           j
 require that the filtered gases  be contained until  exhausted outdoors.
             There are  times when a very low dust concentration  must be
 filtered,  e.g.,  valuable  or toxic dusts.   At such  times  the reverse jet
 collector  is an  early  consideration,  not  only because of high efficiency,         i
 but  because  of high air capacity,  typically 10 to 30 FPM depending             -*
 mainly on  particle  size.   The time required for  cake buildup can be as
 long as a  month, depending on concentration.   To prevent useless running        ~->
 of the carriage  the limit switches may be over-ridden either manually
 or by a pressure switch.   Once the carriage starts  to move,  it  travels            '
 at its normal rate  of  a few feet per  minute.   It pauses  between trips
 for  cake buildup, thus lengthening the mechanical life of the equipment.          I
                                                                                _j
             Conversely with moderate  dust loadings,  the  carriage usually
 runs  continually.   On  higher  loadings  it  can be  speeded  up  to a limited
 extent if  necessary.   Note that  in continuous  operation  the  pressure
 drop  across  the compartment is not quite  steady, since compared with  the          j
midpoint of  the tubes  the  end of the  tube is  first  cleaned  too  soon and
                               i                  '
 then cleaned  late on the  next pass.   How  much  this  causes the primary            ,
 flow through  the system to fluctuate  depends  on  rate  of  pressure buildup,       —'
 rate  of carriage travel,  the  fan curva, etc.
            Reverse jet equipment  offers  control of both  intensity and          —i
 frequency  of  cleaning,  and even  offers  the possibility of adjusting the           ,
                                                                                  i
 intensity according to carriage  position  if necessary, although there           ^
-------
 I
 L
               is  no report of this  being  done.   The cleaning is practically uniform
 >              from top to bottom, the  more so since the deposit tends to be uniform
 '—             due to downward flow. For  these  reasons  the felt should last longer
   0            without plugging than with  any other method  of cleaning.  Unfortunately,
 <_             these advantages arc  partly offset by wear problems  in both the  felt
               and the moving  parts.
 *
 L.;                        Reverse jet  equipment is well suited  to  fine particles,
               expecially  if these cohere  well.   Otherwise,  there is  considerable re-
 [__             entrainment when the  deposit is removed and  the collector operates at a
               higher pressure drop. This equipment is  also suited to coarse particles,
 !              especially  non-abrasive  ones.
                           Regarding wear,  it is good design policy and almost  necessary
 !              to  have  as  much of the moving  mechanisms  as  possible outside  the com-
               partment.   Even the clean side of the compartment sooner or later gets
 ]              dusty,  if not from seeping  dust then from perforated fabric.   When this
 ****
              happens  the mechanism tends  to wear  and,  eventually, to stick and jump.
              The limit switches may become  fouled and  unless fail-safe the drive
               chains will break, cocking  the carriage and probably ruining  the fabric
               tubes.  Once  fouled it is very hard  to get the mechanics  back into the
 L.             original clean  and unworn condition.   Consequently,  the  frequency of
              maintenance increases.
 i
 L.                        In  a number  of  installations  a hard deposit  of  dust  and
               fiber  bits has  built  up  on  the jet rings  over a. period of time so close
 ^_             to  the cloth as  to cause abrasion.  This  deposit  can wear through the
               fabric unless removed.
 ^                        The air required to clean  the fabric  is usually supplied by
              a high-speed blower at pressures of  5  to  20 inches.  The volume required
              is roughly 5 percent  of the primary  filtering flow which increases the
*_
               filtration velocity only slightly.
-------
      3.3.6   Vibration and  Rapping Cleaning
             Although  several methods  of cleaning  involve motion and even
 fluttering  of  the  fabric,  higher  frequency  agitation with little cloth
 travel  can  also be effective.   In vibration and rapping, either trans-
 verse or  longitudinal elastic waves  travel  over the  fabric,  accelerating
 the  fabric  surface through displacements that  are usually small. The
 inertia of  the dust cake causes  stresses at  the cake-fabric interface
 which detach the dust cake from the  fabric.  Since it is easier to
 vibrate or  rap a compact group  of taut   filter elements, envelope filters
 are  the most commonly rapped type.
             This method of cleaning  is  especially successful with deposits
 of medium to large particles adhering relatively  loosely.   Since larger
 particles are  typically from low  or moderate temperature sources, high
 temperature  fabrics are not  usually used.  Woven  fabrics rather than
 felts are used because they  clean less  expensively and even  paper filter
material can sometimes be  used.
             The vibrating  mechanism should be  located outside  the housing
 to minimize  dusty  abrasion.  At least one end  of  the  frame supporting  the
 filter  envelopes is floating and  attached to an eccentric  arm  or vibra-
 tor.  If the cleaning  is to  be done manually,  a common rod may  protrude
 or the  housing may be  opened to strike  each filter frame.
             The compartment  is shut off during the vibrating or  rapping.
If continuous  filtering is necessary, several  compartments can be used
with sequential automatic  cleaning.  The length of the vibrating time
varies.  However,  as with  shaking, about 100 vibration cycles should be
sufficient.   Longer vibration is probably not as detrimental to  the
cloth as in shaking.  The power requirement is low and the cloth life
can be several years barring heat, chemical, or other problems.
            For larger particles,  for which this equipment is especially
suited,  the  filtration velocity can be higher than with other types  of  in-
termittent equipment.   Therefore,  vibration/rapping equipment is compact.
-------
                  With the  larger  particles,  higher  pressure  drops  across  the  cloth  can be
                  tolerated without  as much danger of blinding  the  cloth or  blowing  the
     __           cake through  the cloth.  For  this  reason  small,  light, unit-type inter-
                  mittent equipment  may  operate  safely without  attention for relatively
     i>             long periods, even without  instrumentation.   Eight hours is  a  typical
                  operating period between cleanings.  Even though  the  cost  of filtering
                  power at higher  pressure drops across  the small collector  is high,  it is
     fr"           frequently  only  a  small part of the plant process cost.
                              The  main disadvantage  of this kind of equipment  is probably
     *""           the  relatively few dusts for which it  can be  used.  If the equipment  is
                  located in  a confined  space it can be  noisy.
i
!                       3.3.7   Sonic Cleaning
                              Agitation frequencies still higher than those used  in vibration
I                  and  rapping have been  attempted with ultrasonic and sonic  cleaning methods.
'                  Although these frequencies are known to slightly  improve the preagglomera-
i     ^^
i                  tion  of a few fine dusts, they have not,  on the whole, been  very effective
!                  in fabric cleaning.  Lower sound frequencies  are used, however, with  success
]     *-            in a,  few installations.  The Fuller Company (Dracco) has installed sonicly
i
I                  cleaned equipment  in cement plants for over ten years.  They also used a
     ^_            successful  combination of reverse  flow-collapse plus sonic cleaning in
                                                                               2
                  Bethlehem Steel's  open hearth furnace baghouse about 1964-65.   Usually,
'                  sonic cleaning must be supplemented by some other method.
i                            The frequency band used( can be  estimated from the  sound,  des-
i                  cribed as low and mixed like a fog horn or railroad whistle.   The cloth
i     '
J                  motion is not apparent, but the vibration can be felt by touch.  At the
!                  Bethlehem installation the sound was generated by compressed air, with
i     —                                           "                               2
I     „             less  than 300 SCFM needed for 50 horns serving about 80,000  ft   of cloth.
                  The pressure required was not determined.   Following reverse flow and
i
     ~~            collapse the horns  gave three five-second blasts, after which the collapse
                  and horn blasts were repeated a second time.
-------
               In  all  reported  installations  of  sonic  cleaning  (cement  and
 open hearth  fumes), glass  cloth has  been used,  at  filtration velocities
 of  2 FPM  or  less.  However,  it was claimed  in 1959 that  use of sonic
 cleaning  would  reduce the  normal  cloth  pressure drop  of  4  inches  by 25
 to  50  percent,  raising the  possibility  of a  higher filtration  velocity.
              Sonic cleaning costs have  been reported as lying between
 those  of  reverse  nir  and shaking  systems.  The  compressed  air  equipment
 is  apparently  less than for  pulse cleaning equipment.  However, sonic
 cleaning  has never achieved  the popularity of most of the  other cleaning
 methods,  for reasons  which have not  been reported.
       3.3.8   Manual  Cleaning
              Any of  the automatic cleaning  cycles can be  initiated man-
 ually  at  convenient times, as between shifts.   An  even simpler method
 of  cleaning  is  to approach each bag  or  panel and remove the cake  by
 hand as in beating a  rug.  The method has no hardware to wear out and is
 as  reliable as  maintenance can be.   However, the  cloth itself generally
 wears  rapidly and endures  fewer cleaning  cycles than  other methods.
              This method  is only practiced on  smaller dust collectors
 having, at most, only a few filter elements.  They  can be brushed in
 place, or thumped to  knock the cake  loose.  Alternatively,  they can be
 removed and turned inside out for more thorough cleaning.
              When the more expensive fabrics blind or plug,  well before
 their  other qualities  are depleted,  they are reclaimed by various
 methods such as vacuum cleaning,  dry cleaning,  or  laundering.   An advan-
 tage here is that weak spots may ;be detected and repaired before a hole
 in  the fabric actually occurs.   A certain amount of skill goes  into re-
moving dust without damaging the  fabric.  Manual cleaning of  any large
 amount of ordinary cloth is not economical.
-------
 3.4  CONSTRUCTION AND MATERIALS
      Fabric filter manufacturers are rarely able  to guarantee  the per-
 formance of their equipment,  because performance  depends  so much  on how
 the equipment is  installed and used.  Any manufacturer  can, however,
 guarantee the quality of his  dust collector as  sold.  As  a  check  on
 quality, the manufacturer has standard specifications for the  production
 of the  equipment.  These specifications include a large amount of detail
 for construction  and  materials,  an example of which is  presented  in
 Appendix 3.2.
      3.4.1  Housing
             External  framework and configuration  design is  available  in
 three types,  the  choice  depending mainly on moisture condensation con-
 siderations.   The open-sided  pressure  design (inside filtering) is
 cheapest and used mainly below 135 F.   The closed pressure  type is  next,
 and followed by the slightly  more expensive closed  suction  installation.
 Above 160°F,  the  equipment is usually  closed and  insulated.  The  cost
 increases with the area  and thickness  of metal  used, which  depends  on
 the pressure the  walls must withstand.   Cost will also  reflect any neces-
 sary weather protection,  insulation,  etc.,  as well  as gas corrosiveness
 and temperature.
            Most  small unit collectors  are  assembled at the factory,
 usually  welded, while  larger  units may  be  either  assembled at  the
 factory  or  on location.  The  largest unit  that can presently be shipped
 by  railcar  is  approximately 10' x 40' x  12' high;  consequently many
 larger designs are assembled  from standard preassembled modules by
 either welding or bolting  on  location.  It  is difficult in field assembly
 to make  good  pressure or vacuum-tight seals between panels,  modules and
 flanges  and seal quality is a chief complaint among fabric filter users.
            To create a strong structure and provide good seals against
weather  and condensibles, under the large deformations  due to changes
-------
in  temperature and pressure, some variations  in construction are neces-
sary.  Several styles  of housing joints and sealants are  indicated  in
Figure 3.8.  For steel, one manufacturer uses galvanized  14 gauge sheet,
others use  10 gauge welded hot-rolled steel.  Much heavier material can
be  used, up  to one-quarter inch or more.  The open hearth FF installation
at  Bethlehem Steel used 3/16 inch for walls, roof and partitions, 1/4
inch for floor plates  and 5/16 inch for hoppers and inlet plenum (Fuller-
              2
Dracco design) .  In passing, let it be noted that competition forces
many manufacturers to  underdesign unless otherwise instructed (see
Section 8.3).
            Materials  other than steel may be used for housings; for
example, cement plant  collectors are often made of concrete and one
fabric filter manufacturer has a design for precast reinforced concrete
panels.  An aluminum company stated it feels that aluminum is definitely
the best all-round material to use.  Corrugated asbestos cement paneling
is  often used for exterior roofing and siding, with the interior walls
and partitions made of steel.  Various other composite panels can be
used.  Insulation can be sandwiched in the paneling or added to the
outside at factory or  later as needed.  The main precautions in selec-
tion of material are corrosion and changes in temperature that cause
thermal stresses in the seals and even in the bags.
-------
            fSealant)   !
        Vertical     ) !
         Siding  Joint)*
         Horizontal
          Siding  Joints
                                   mi
                    (Insulated)
                                                        U
[frfWTi
. y r|r
» B
Roof Joint

u



1
1
!
i *

Siding


J


tj
          Panel
           Stiffeners
i^^_

I
                        1
                      Main
                      Frame
i
Figure 3.8.  Examples of Some Styles of Fabric Filter Compartment  Joints,

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     3.4.2  Hopper and Disposal Equipment
            Hopper material  In usually similar to  that used  for  the hous-
ing with similar joints and  seals.  The  inside may be treated  to minimize
adhesion of the dust.  Hoppers are commonly available with either 45  or
60  slope depending on the need to conserve height or to provide adequate
sliding, as affected by the  plugging properties of the dust.
            Most hoppers are under a few inches of pressure  or vacuum.
Consequently they require some kind of valve at the bottom to  let the col-
lected dust out without admitting air to reentrain dust and  redeposit it.
Automatic filter equipment usually uses a rotary valve consisting of a
sealed paddle wheel as depicted in Figure 3.9.  These valves rotate at
about 5 RPM, and are typically 6 to 12 inches in diameter.  Thus they
handle around 2 CFM of collected material.  If the solids flow is higher,
the valve can be run faster or more or larger valves can be used.
            For low solids flow,and for many non-automatic installations
requiring periodic attention,sliding gates may be used.  Unless the hopper
is under practically zero pressure these gates are used only when the com-
partment is shut off.  When the hopper is under no pressure a gate may not
be necessary.      A stocking perhaps 6 feet long reaches from the hopper
bottom to the dust bin.  Likewise, the rotary valve or gate seldom dis-
charges with an open cascade of dust.  A stocking is used or more commonly
the solid flow enters a conveyor, usually screw-type.
            If for reasons of compartment cleaning or maintenance the com-
partments must be completely isolated then the rotary air lock valves are
located between the conveyor and the hoppers.  Otherwise it may be less
expensive to install the conveyor directly on the hoppers and use one valve
at the end of the conveyor.   The air slide, bucket, and belt methods  of
conveying powdered materials are normally used only for high solids  flow.
There are .however, times when the screw conveyor doesn't work as with
gummy dusts and those that lock up under pressure.
-------
\ /
J L
, i r
i i i




r, p




j
1






Chain
Drlva






4
^



c



);.


<

—





D;

r

— —
hj


i
                                                                      Full
                                                                      H«lf-skeleton
                                                                      Skeleton
    Rotary Valve
                                                    Manual  Sliding C«ten
   Trip Gatf

(Automatic or Mcnual)
     Dlgcharge Duck

(Knotted, Clamped, etc.)
            Valve #1
                  I
      Drive
                  L.
                              Valve #2, etc.
                        Screw Conveyors
                                                        Discharge
                                                                Rotary
                                                                Valve
  Figure 3.9 - Types of  Hopper  Discharge Equipment
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3.5  EXTENSIONS OF FABRIC FILTRATION  EQUIPMENT
     In addition  to  the  standard  FF designs  already  discussed,  from time
to time novel  filter  equipment arouses  interest  among  those  acquainted
with the  limitations  of  standard  equipment as indicated  earlier  in  Chap-
ter 1.  The  patent literature records a  fascinating  history  of design
variations,  of which  only a  small  percentage are now in  practice.   Be-
sides  these  efforts  to control dust,  fabric  filtration has demonstrated
other  capabilities including control of gases and perhaps odors, control of
mists, and ultrafiltration for very finu  particulates  in atmospheric dusts.
     3.5.1   Variations in Standard Fabric Filter Designs
     3.5.1.1   Bourdale Rotary Filter.- The filter media  surrounds a central
shaft  and rotates "until such time that  the  cake of  dust has such a  mass
that its centrifugal  force exceeds the  fan suction,  when it  flies off the
surface and  settles down into the hopper."   This is a compact unit in use
on a wide variety of  dusts up to  210  F.   It  is made  by Wheelabrator Alle-
vard of Paris.
     3.5.1.2   Tower Collector.- Two tiers of glass cloth bags are used, one
above  the other,  in a 60 foot or higher cylindrical  chamber.  The flow is
downward; cleaning is by sonics.  Although the brochure  of the company
that produces  this configuration  is not specific, the  bags are probably
butted end to  end on  an  intermediate  tube sheet to relieve the tension that
would  otherwise be extreme on a single 60 foot bag.
     3.5.1.3   Self-Emptying  Bag.- In one such design  up to 8 bags,  each
about  6 ft by  1.5 ft  dia., are held at the top by a guy cord during filter-
ing.   For cleaning the cord  is released and under ;the weight of the  dust,
the bag sinks  through the lower tube sheet, turning  itself inside out and
cracking off the cake.   In another design used in a copper mine in Zambia
the bags are held up  by  inflation pressure, and their top is guided  up
and down by a  taut wire through a grommet in the top center of the bag.
Dust dumped as the bag inverts itself is later washed away by a stream
                         Q
through the  inlet plenum.
-------
      3.5.1.4  Granular Bed Plus Screen.- This equipment uses pulses to
 clean a combination sand bed plus filtering screen,  as sketched in Fig-
 ure 3.10.  Large particles form a cake on the screen(s), and fine particles
           SCREEN
SAND
                     Filtering
                 Cleaning
           Figure 3.10.  Ducon Sand and Screen Filter Cleaned by
                         Back Flow.
are caught in the sand(s).  "The channels in the filter element are not
completely filled with sand.  Thus, during the cleaning cycle, short blasts
of high-pressure air expand the beds.  The air dislodges filter cake from
the outer screen and restructures  the granular material into a filtering
                                          9
bed essentially the same as the original."   The Ducon Co. says the device
can operate at filtering velocities of 30 to 40 fpm  at temperatures
well above 1000 F and on sticky dusts.
     3.5.1.5  Foreign Equipment.- Filtration and Separation, a British
publication,  regularly lists developments in dust control equipment in
Great Britain and Europe.  Much of this equipment is slightly different
-------
  from U. S. Designs  although  several U.S. manufacturers have  subsidiaries
 in  Europe, and  this  handbook  survey of available  equipment  styles  could
 be  extended to  include European  equipment.

     3.5.2  Control  of Gases  and Odors
            Several  experimental studies have shown that finely divided
 absorptive or adsorptive  granular materials aerosolized into  the gas stream
 entering the fabric  filter, can  effect the removal of certain gaseous com-
 ponents.  The combination of  good powder control with high  specific surface,
 and long, intimate gas-powder contact especially on the fabric, are advan-
 tages offered by fabric filtration equipment over most other  equipment.
 Most recently the removal of  up  to 98.4% of SO- by sodium bicarbonate has
 been demonstrated at a coal-fired power plant by Air Preheater Co. under
 NAPCA contract.  Fly ash  and  N02 are also removed at the same time.
 The optimum equipment design  for gaseous control would probably not be much
 different from  present filter designs, except for the system  for introducing
 and possibly recirculating the powdered material.  This addition is esti-
mated to be a small part  of the  fabric filter system cost.
            The addition  of particle aggregates to a filter cake should
 reduce power costs by reduction  of the cake porosity.   The partial recir-
culation of agglomerates  from the collector hopper seems attractive but
 there is no report of this practice.   It would appear that agglomerate
recirculation in conjunction with absorptive powder recirculation would be
attractive in numerous possible  gas and odor control applications.
            Similarly, although  as far as is known absorbing additive in
fabric filter equipment has not been used to control odors,  it is a likely
prospect.  Granular charcoal, catalysts,  or fritted impregnated material
could be introduced to and removed by the filter, regenerated in special
closely confined equipment, and then recirculated.
     3.5.3  Control of Mists
            It  is widely observed that the fabric in an ordinary fabric
filter collector must be kept dry or  it  will rapidly plug  with dampened  dust.
-------
  This may  be  reversible, however, as  shown by pilot plant experience at a
 Southwest  Portland Cement Company plant  some years ago using silicpnized glass
 fabric,  collapse without shake cleaning, and cement kiln effluent normally
 about 500°F:  "...During subsequent months of testing, the bags at times were
 saturated  with water  from condensation during normal variations of atmos-
 pheric and kiln conditions  (wet-process) and were even deliberately wetted.
 In  all cases, rising  temperature dried the bags and, at the next collapsing
 cycle, they discharged readily and returned to normal pressure drop.  In-
 spection of the fabric showed that no blinding had occurred, and the lack
 of  deterioration suggested  an entirely satisfactory life of at least one
     ,,11
 year."
            Other filter beds, more open than normal filter fabrics, are
 used for large particle demisting in a number of processes.  These are
 usually  self  draining, but  if they are of intermediate porosity and there
 is  danger  of  clogging they  can be kept open with water sprays.  One such
 study obtained effective removal of submicron TiO. particles plus around
                                                                  12
 90% removal of sulphuric acid mist, by irrigating synthetic felts.
 Filtration was at 70 to 165 FFM, the spray rate was around 10 gal. per KCFM,
 and felts  fairly similar to felted fabric filtration materials were used.
 The pressure  drops encountered were 8 to 10 inches of water and were nearly
 independent of filter velocity, possibly because the air permeability of
 the wet  felt  depended on the amount of water remaining in the felt.   Pro-
 jected initial costs of a complete 56,000 CFM system were cited as $2.40
 per CFM  plus minor installation costs.
            These examples of effective particulate control with fabric at
 conditions below dewpoint may point the way to solution of condensation
 problems in fabric filtration, enabling cooler and more economical operation
without  fabric blinding aid plugging.   There are many possibilities  for
 research and development in this area including the opportunity for  coin-
 cidental wet-process odor control.   Although the field is not a new  one by
 any means,  new synthetics and changing economics make it a viable  and
 inviting challenge.
-------
     3.5.4  Ultrafiltration

            The high efficiency made possible by filtering with a dust

cake is used to obtain very low effluent concentrations of toxic and radio-

active dusts, bacteriologicals, and atmospheric dust.  Very low concentra-

tions of liquid or tarry particles may also be controlled.  Aside from a system

for introducing the filter aid, the equipment is much the same as that for

any fabric filter system.  Since the fabric is cleaned only very infre-

quently, however, perhaps once a year or longer, the cleaning mechanism and
dust disposal equipment can be minimal.  A high fabric packing density can

be used without concern of chaffing.  Maintenance is low and the equipment is

available for operation with a number of filter aids.


3.6  REFERENCES FOR CHAPTER 3

     1.  Wheelabrator Corporation, Pustube Collectors.  Bulletin No.  565-E
         page 10 (1967).

     2.  Herrick, R.A., Olsen, J.W., and Ray, F. A.,  Oxygen Lanced Open
         Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
         Jnl. A.P.C.A. 16:  1, 7 (January 1966).

     3.  Sonics Clean Dust Collector Bags Soundly, Chemical Week.
         (14 November 1959).

     4.  Adams, R.  L., High Temperature Cloth Collectors, Chem.  Engg.
         Progress.  62;4, 66 (April 1966).

     5.  Bourdale Rotary Filter Dust Collector,  Filtration and Separation.
         170 (March 1969).

     6.  Fuller Company, Collecting Hot Dust and Noxious  Fumes with  Dracco
         Glass-Cloth Dust Collectors. Bulletin No.  DCB-1B (1967).

     7.  The Ducon  Company, Dueon Dust Control. Equipment. Bulletin No.
         F-9069, (1969).

     8.  Flannel Bags Key to  New Dust Filter,  Engg. and Mining Jnl.  166:12,
         98 (December 1965).

     9.  Chem.  and  Engg. News.  57 (15 December 1969).

    10.  Liu, H., and Chaffee,  R., Evaluation of Fabric Filter as  Chemical
         Contractor  for  Control  of S02  from Flue Gas, Final  Report to NAPCA,
         Contract PH22-68-51,  by Air "Preheater Co., Wellsville,  N. Y.
         (31 December 1969).
-------
11.   Hot Kiln Gases in Glass Cloth Bags, Pit and Quarry. 80,
     (October 1958).

12.   Morash, N.,  Krouse, M., and Vosscller, W. P., Removing Solid and
     Mist Particles from Exhaust Gases, Chem. Engg. Progress. 63.:3, 70
     (March 1967).
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                               CHAPTER 4
                            FABRIC SELECTION

4.1  INTRODUCTION                                                  4-3
4.2  MATERIALS                                                     4-3
     4.2.1  Natural Fibers                                         4-4
     4.2.2  Man-Made Fibers                                        4-7
            4.2.2.1  Acrylic Fibers                                4-7
            4.2.2.2  Modacrylic Fibers                             4-9
            4.2.2.3  Nylon, Polyamide                              4-10
            4.2.2.4  NomexR Nylon                                  4-12
            4.2.2.5  Olefin                                        4-12
            4.2.2.6  Polyester                                     4-15
            4.2.2.7  Teflon Fluorocarbon                           4-18
            4.2.2.8  Vinyon                                        4-20
            4.2.2.9  Glass                                         4-20
            4.2.2.10 Other Man-Made Fibers                         4-22
     4.2.3  Physical and Chemical Properties                       4-22
4.3  YARN PRODUCTION                                               4-28
4.4  YARN PROPERTIES                                               4-29
     4.4.1  Threads and Yarns                                      4-31
     4.4.2  Yarn Twist                                             4-31
     4.4.3  Yarn Number                                            4-32
     4.4.4  Simple and Plied Yarns                                 4-34
4.5  FABRIC PRODUCTION                                             4-35
     4.5.1  Weaving                                                4-35
     4.5.2  Felting and Needle Punching                            4-41
     4.5.3  Fiber Additives, Yarn Treatments,  and Fabric            4-42
            Finishes
4.6  FABRIC PHYSICAL CHARACTERISTICS                               4-46
     4.6.1  Abrasion Wear                                          4-46
            4.6.1.1  Sandblast or Surface Scour                    4-48
-------
            4.6.1.2  Internal Chafing                             4-50
            4.6.1.3  Internal Abrasion                            4-52
     4.6.2  Flexibility                                           4.52
     4.6.3  Strength                                              4-55
     4.6.4  Permeability                                          4-58
     4.6.5  Dust Deposit Release                                  4-59
4.7  AVAILABLE FABRICS                                            4-59
4.8  REFERENCES                                                   4-60
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                              CHAPTER 4
                           FABRIC SELECTION
4.1  INTRODUCTION
     Satisfactory performance of a fabric filter on a specific applica-
tion requires selection of a fiber material compatible with the gas-
particle environment, and a fabric design appropriate to dust collector
geometry and collector cleaning requirements.  Fiber, yarn, and fabric
parameters influence the ability of the fabric to collect dust at a, reas-
onable pressure drop.  !A brief summary of developments in fiber, fabric,
treatments, and applications has been presented in Chapter 1.  Man-made
fibers have been developed over the past 30 years with superior resistance
to physical and chemical environmental requirements.  While traditional
fabrics of cotton and wool are still employed for many fiber applications,
the impact of man-made fiber fabrics has extended the range of application
of fabric filters to a much broader gas cleaning market.  Continuing de-
velopments in fiber and fabric technology, as in dust collector design,
can be anticipated.
     The purpose of this chapter is to present information on fiber and
fabric properties important to their application for industrial gas
filtration.  In addition, pressure drop during fluid flow through fabrics
and dust deposits has been considered in Chapter 2; effects of fabric
structure on filter performance has been discussed in Chapter 6; and fabric
costs in dust collection have been considered in Chapter 7.   In what fol-
lows, a number of terms unique to the textile industry are used, and these
are defined in Appendix 4.1.
4.2  MATERIALS
     Production of textile fabrics begins with the basic structural  unic,
a single fiber.   Fibers are obtained from traditional natural vegetable
or animal sources, such as cotton (cellulose)  or wool (protein).   Fibers
are also produced from man-made or modified natural organic  polymeric
-------
materials through modern chemistry and chemical  engineering.   Industrial
filtration fibers are indicated in Table 4.1 by  generic group  manufacturer,
                2                                                      3
and Trade names.   A more complete index has been compiled by  Dembeck.
The internal molecular orientation and crystallite  structure of  the  fiber
determines its  basic physical  and chemical  properties.  End use  fiber
characteristics are substantially controlled by  additives and  treatments.
Within a generic group, polymers, co-polymers, and  homo-polymers may be
produced.  For  all these reasons, there are differences in physical  prop-
erties and chemical resistance for each manufacturer's product in each
      2
group.
     Fibers are produced as staple (relatively short  lengths (1  < length <
10 inches long), as monofilament (single long continuous filament L  » 10  in.)
and as multifilament (many continuous parallel filaments).  Natural  fibers
are used for industrial filtration staple.  Man-made  filtration  fiber is
used in both staple and filament lengths.  Cross sections of typical nat-
ural and man-made fibers are shown in Figure 4.1.   Fiber microscopy such
as this is a useful technique  in textile identification and filtration re-
search, as discussed in various references. '
     4.2.1  Natural Fibers
            Cotton and wool fibers are commonly used  in fabric filter dust
collectors.   Cotton fibers are elongated single hollow cells, often
flattened (Figure 4.1), with characteristic irregular lengthwise twists.
Cotton fibers from different varieties of plants vary in length from 1/2
to 2 inches, and in width from 12 to 25um.
                                t
            Sheep wool also varies with specie and with geographical loca-
tion.  The surface of a wool fiber is made up of flat scales  (cuticle)
which overlap like shingles.  These scales are spaced approximately 20 ^m
apart along the fiber.   They are very important in determining the mechanical
properties of wool fiber assemblages, and probably influence  dust collec-
tion and release properties as well.   Wool fibers are 10 to 70 urn in
diameter,  and 1 1/2 to 15 inches in length.  They are almost  uniformly
cylindrical  except for the ends which are tapered.
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                                        TABLE 4-1

MANUFACTURERS AND  TRADE NAMES  OF  INDUSTRIAL  FILTER FIBERS**
       ACETATE AND TRIACETATE
       Calanesa.-"Acetate".  "Arn.l"*
       DuPont--"Acele"*
       EaetsMn Chemlcal--"Estron"*
       r.M.C Corp., American Vlscoss Dlvlslon--"Avlsco"*
       ACRYLIC
       American Cyanamld--"Creslan"*
       Chsmstrand--"AcrHen"*
       Dow Cheulcal--"Zafran"*
       DuPont--"Orlon"*
       HOOACRYUC
       KastlMn Chenical--"Verel"*
       Union Carbide--"Dynel"*
       POLYAHIDBS
       Allied Cheolcal--"CaproUn 6"*
       American Enka--Enka nylon 6
       Cheast rind--Chens t rand 6.6
       DuPont--nylon 6.6 "Nomex"* nylon
       Nylon Induetrles--Celanese Nylon
       Firestone Synthetic Flb*r<--Flr«tton« nylon 6
       U.S. Rubb*r--nylon 6.6
       V«etr«--nylon 6.6 nylon 6
       POLTOTMS
       AMrlcaii Knka Corporation- -EnV«  Polyeiter
       B««untt  Plb«r«--"Vycron 2.5"*
       DuPont--"D»cron"*
       EMMn Chalc*l--"Kodel 2.4"*
       Flb«r bidu«crl« (Cel«nen)--"Portrel"*
       Ktotlon--"Cry*t*l Milt"*
       Vectr«--Poly««ter
       POLYBTHYLBNE
      Alun Folywr  Corpor«tlon--"R«evon"»
      American Th«m>pl«itlc PToduct«""A««rfll"*
       Dawbarn--"DLP"*
       Flraiton* Synthetic Ptb«n--Plre«ton« Polyethylene
       G.P. Chealcal>--"Gerfll"*
       Induatrlal  Pla»tlc--"Tuff-Llta-L"*
      Vectra--"Wynene"*
      Vogt Manufacturlng--"Vopl«ii"*
      POLYPROPYLENE
      Alamo Polymer Corporation--"Reevon"*
      American  TharoopUitlc Producte--"ABerfll"*
      Dawbarn--"DLP"*
      Flraatone Synthetic  Flbare--Plreatone Polypropylene
      Herculee  Powdero'o"Herculon11*
      Industrial  Ple»tlc--"TuMLlca-P"*
      Vectra --Polypropylene
      Voft Menutecturlng--"Voplei"*
      RAYOU VISCOSE                     ,
      Aaarlcan Enka--"3up.r«nka"*, Fiber 700"
      Beaualt Fibers--Beavmlt Rayon
      Celaaa«e--Celaneee Rayon, "Portlian"*
      Courtauldj North Anerlcen--"Plbro"*
      F.M.C. Corporation, Anarlcan Viscose Dlv.--"Avlsco"*,
         "R«ytl«x"*, "Super  Raytlex"*,  Fiber 40"*, "XL"*
      Industrial Rayon--"Lekroeet"*, "Tyron"*,  "VlUwyte"*
      SARAM
      Dov CheMlcal--"Rovana"*
      Firestone Synthetic Fibers--"Valon"*
      Vectre--Saren
      TBTLOW
      DuPont--"Teflon"*
      vniYOH
      F.N.C. Corporation, American Viscose Division--
         "Avlsco Vlnyon HH"*
      Rhodl* Aceta (France)—"Rhovyl"*
      Vogt Manufacturln(--Voplax Vlnyon
      CLASS
      FaeBO--Fa»co Fiber
      Johns ManvlUe--nber  Glass
      Owene-Cornlng-Flbercles Corp.--"Fiberflas"*
      Plttabursb Plata Claes--Flber Class
       CerssElc, Aabeatoa, Metal     i
      COTKM AMD WOOL
      Available as natural fibers
  *    Registered Trade  Mark
  **  from  J.P.   Stevens,  Inc.,  Ref.   2
-------


(a) Cotton  (X280)
 (A) Acrylic
      (AcrilanR) (X410)

(b) Wool  (X410)
(e)  Modacrylic
     (DynelR)  (X410)
(c) Polyacrylonitrile
     (Orion*)  (X380)

 (f) Polyamide  (Nylon)
      (X380)
(g) Polyester            (hj Vinyl (Vinyon)         (1)  Giant   (X150)
     (DacronR) (X380)         (X380)

    Figure 4.1.   Cross Sections of Filtration Fibers,  (From Harris, Ref.4 ,)

                                  L.f,

-------
      4.2.2  Man-Made  I-'ihers
            "Certain  (man-made)  fibers  offer  outstanding  resistance  to
 acids,  others  to  alkalies.  Most are  impervious  to  mildew.   Some....
 have  high temperature  resistance so that....operations  such  as  cooling
 of  exhaust  gases  can be  reduced  or eliminated.   Because of their  inherent
 toughness and  resistance to abrasion  and  other environmental conditions,
 many....offer  substantial overall savings  due to increased service life
 and reduced equipment  down-time.  The initial cost  of fabrics made from
 (man-made)  fibers may  be greater than fabrics made  from natural fibers.
 However, the increased service will make  them the most economical in  the
 long  run	A careful choice of filter or dust  collection fiber  for  each
                                                          2
 application can pay dividends in efficiency and  economy."
            While the  chemical processes used to manufacture the various
 polymeric materials used as filtration  fibers may differ,  '  '   the fibers
                                                  348
 themselves  are  made by somewhat  similar processes.  '  '   The liquid material
 is  spun (wet,  dry, or  extrusion)  to mono-  or  multifilament,  stretched to
 orient the  constituent molecules, then processed into yarn strands; and
 by  spinning, texturizing or through-tow and staple, to yarn.
                                                                    9 10
            4.2.2.1  Acrylic Fibers.- An acrylic fiber is designated  '
 as  "A manufactured fiber  in which the fiber-forming substance is any
 long chain	polymer composed of at least  857<> acrylonitrile  units	",
      CH
 (-CHo- V -)  ,  the remainder frequently being a  copolymer such as vinyli-
    L CN    n
 dene chloride  to provide  dye sites.   Process  steps in the production of
 acrylic fibers  are discussed in References 4  and 7.  Among other acrylics
 commonly used  in fabric  filtration are Orion  and Acrilanv   Both are some-
what dumbell-shaped in cross section (Figure  4.1) and their  surfaces are
 irregularly striated to  various degrees.  Acrylic fiber diameters are
 about 15 to 35 (am.

            Characteristics, properties,  and  available producer's forma
 are given in Table 4.2.   The acrylics offer a good  combination  of
            "....abrasion resistance and  resistance to heat  degrada-
            tion  under both wet  and dry conditions.  An outstanding
-------
                                 TABLE 4-2

         CHARACTERISTICS, PROPERTIES, AND FORMS OF ACRYLIC FIBER FOR

                           INDUSTRIAL FILTRATION*
  HEAT  RESISTANCE:

  Dry Heat;   Inferior  to  "Teflon"  and
  Nomex" nylon, which  are  outstanding
  in this  respect.   Inferior  to  poly-
  esters,  but  superior to  nylon  and
  natural  fibers.
  Moist Heat;  Rated below "Teflon"
  and "Nomex"  nylon, but considerably
  superior to  polyesters,  nylon, rayon
  and natural  fibers.

  CHEMICAL RESISTANCE:

  Acids:   Satisfactory resistance  to
  most mineral or organic  acids.
  Superior to  polyamides,  polyesters
  and cellulosic fibers but less resistant
  than other synthetics.
 Alkalies;  Inferior  to most fibers
  in this respect, except natural
  protein fibers, silk and wool.
 Oxidizing agents;  Fair  to good
 resistance to most oxidizing
 agents.   Superior to polyamides,
 polyethylene and natural protein
 fibers.
 Organic  Solvents;  Excellent
 resistance to most common organic
 solvents.  Superior to modacryl-
 ics polyethylene, polypropylene
 vinyon and protein fibers. .

 PHYSICAL PROPERTIES:
 Tenacity and Elongation:

 Filament;  Dry and wet tenacity
 ranges from 3.8 to to 4.8 g.p.d.
 Dry and  wet elongation ranges
 from  13  to 23.

 Staple;   Dry  and wet  tenacity
 ranges from  1.8 to  3.5 g.p.d.
 Dry and wet elongation ranges
 from 20 to 557,.

*from J.  P. Stevens & Co., Inc.  (Ref 2)
  Specific  Gravity;   1.12  to i.18
  Abrasion  Resistance;   Good,  but
  less  resistant  than polyamides,
  polyesters,  and polypropylene.
  Superior  to  "Teflon."

  FORMS AVAILABLE:
  DuPONT  "ORLON"

  Staple; 1.5  to  16 denier/filament.
  Cut   lengths 3/4" to 5%".   (Can be
  converted to spun yarns  of many
  sizes).
  Tow; 2  to 10 denier/filament.
  All of  the above are available  in
  bright, semi-dull and black.
  CHEMSTRAND "ACRILAN"
  Staple; 1 to 15 denier/filament.
  (Can be converted to spun  yarns
  of many sizes).
  Tow;  2 to 8 denier.  Available in
  high shrink form.
 AMERICAN CYANAMID "CRESLAN"
 Multifilament;  Deniers of 75,150
  and 200.  Semi-dull only.  Special
  industrial 166 denier—107. shrinkage
  in boiling water.
  Staple;   1.5 to 15 denier/filament.
 Bright and Semi-dull. Regular and
 high shrink.  (Can be converted to
 spun yarns of many sizes).        :
 FARBENFABRIKEN BAYER AG.  "DRALONG T"
 Multifilament;  200-800 denier, bright.
 Staple;  2-10  denier, white, dull and
 doped-dyed light green for identifica-
 tion purposes.   (Can be converted  to
 spun yarns of many sizes).
 RHODIACETA "CRYLOR"
 Multifilament;  Denier of  100 to  225.
 Staple;  Net yet .available  in U.S. Market
DOW CHEMICAL "ZEFRAN"
 Staple;  3  and 6 denier per filament
 cut 2 inches  (Can be converted to  spun
 yarns of many sizes).
-------
            characteristic is the ability of acrylics  to withstand
            a hot acid atmosphere, making this fiber a good choice
            in the filtration of exhaust gases.  Acrylic fabrics are
            used for dust collection in the manufacture of ferrous
            and non-ferrous metals, carbon black, cement, lime and
            fertilizers.

            "Other dry filtration or dust collection applications
            include the drying of raw  flour, sand and  coal, mining
            and ore dressing.  Wet filtration applications include
            the manufacture of dyestuffs, paint, varnish, solvents,
            storage batteries and mineral oil; also galvanizing
            and copper ipining.  Acrylic filters function well in
            the presence of steam, as  in heat exchangers.

            ". . . . .Acryl.lc fibers made  especially for industrial
            use, such as "Dralon T" and "Crylor," are  available
            as homo-polymers, composed of 100% acrylonitrile
            units.  The homo-polymers  offer a good hydrolytic
            resistance, and are recommended for temperatures of
            up to 284°F, while copolymers are recommended for
            temperatures of up to 248°F.  Dow Chemical Company's
            new acrylic fiber is a homo-polymer and will operate
            in the higher temperature  range recommended for homo-
            polymer acrylic fibers.  Wet performance of Dow's new
            acrylic is maintained to be superior to other acrylic
            fibers."2

                                                                       9
            4.2.2.2  Modacrylic Fibers.- Modacrylic fiber is designated

as "...a manufactured fiber in which the fiber-forming substance is any long

chain. .. polymer composed of less than  85%, but at least 357» by weight of
                                           P
acrylonytrile units..."  For example, Dynel  is a 40-60 acrylonitrile-

vinyl chloride mixture.  The cross sections of Dynel fibers resemble dumb-
                                        4
bells, and their surfaces appear uneven.

            Although few gas filtration applications use modacrylics, the

one most used for filtration and dust collection is manufactured by

Union Carbide Chemicals under the trade name 'Dynel'.   'Verel' (Eastman

Chemical Products)  is also an important available modacrylic fiber.

(Other producers are indicated in Ref.  3.)  "Modacrylics have good chemi-

cal and abrasion resistance generally,  offer excellent dimensional

stability and are unaffected by many acids and alkalies,  even at high

concentrations.   Water has no adverse effect on the fiber which retains
-------
 oyer 95% of its  strength when wet.   Dry and moist heat degrade modacrylic

 mere than the other fibers discussed in the brochure,  with the exception
            2
 of vinyon."   Characteristics and properties of modacrylic fiber are given
 in Table 4.3.
                                                                  9
             4.2.2.3  Nylon,  Polyamide.- Nylon fiber is designated  as....

 "a manufactured  fiber in which the  fiber-forming substance is  any long

 chain...polymer  having  recurring  amide  groups ...(-(J-NH-)...as  an integral

 part  of  the  polymer  chai.n."   Table  4.4  indicates  the basic  polymer in  the

 several  types of nylon  currently  available.   Nylon fibers  are  cylindrical,

 as  shown earlier in  Figure 4.1, with smooth  surfaces devoid  of markings.

 The  fibers  are uniform  in diameter  (10  ^m  and upward)  and  appear round  in

 cross  section.   Continuous filaments and staple  fibers  (1  to 5 in.  long)

 are  produced.

             "Three  types  of  nylon are available  for filtration:
             Nylon 6,6  (DuPont, Celanese and  Chemstrand), Nylon 6
             (Allied  Chemical,  Enka  and  Firestone)  and  'Nomex1
             nylon (DuPont).

             " 'Nomex' nylon  differs  considerably  from  the  other
             types, and  therefore  is  treated  separately.

             "Nylon  6,6  and nylon  6 have similar characteristics,
             except that  the  latter has  a lower melting  point and
             somewhat poorer  heat  resistance  at extreme  tempera-
             tures.   Possibly for  this reason, and  because  nylon
             6,6  was  introduced in this  country before nylon  6,
             the  former  is used more  widely in filtration.  Where
             elevated temperature  is  not a  factor, however, it
            would seem  that  the two  types would give comparable
             service.

             "Because of nylon's high abrasion resistance,  it is
            used in  filtration of abrasive dusts or wet abrasive
             solids at low temperatures.  Nylon's good elasticity
            makes it ideal for conditions where continuous flex-
             ing  takes place.  This fiber is  a good choice  in wet
             filtration  applications, at low  temperatures and in
            an alkaline atmosphere,  for example in dyestuffs.
            Fabrics of  nylon provide good cake discharge."2

The general characteristics  and properties of nylon fibers in use for

 filtration are indicated  in  Table 4.5.
-------
                                TABLE 4.3

          CHARACTERISTICS,  PROPERTIES AND FORM OF MODACRYLIC

                     FIBER  FOR INDUSTRIAL FILTRATION*
 HEAT  RESISTANCE:

 Dry Heat;   Shrinkage  starts  at  250 F
 but can be  heat stabilized at higher
 temperatures below the  stiffening
 state.  For prolonged use, 180°F
 should be considered maximum.
 Surpasses acetate, polyethylene,
 saran and vinyon in heat resistance,
 but inferior to other fibers.   This
 fiber will  not support  combustion.
 Moist Heat;  As with most fibers,
 moist heat  has more effect on the
 fiber than  dry heat.

 CHEMICAL RESISTANCE:

 Acids;  Little effect even at high
 concentrations for most mineral and
 organic acids, including aqua regia,
 chromic acid, nitric acid, phosphoric
 acid and sulfonic acid.  Can be used
 in the presence of moderate concen-
 trations of nitric and  sulfuric acids.
 Adversely affected by phenol and high
 concentrations of acetic acid.
Alkalies;  The fiber has good alkaline
 resistance under most conditions.
 Oxidizing Agents;   Excellent resistance
 to nearly all oxidizing agents.
Organic  Solvents;   Softens or
dissolves  in warm acetone and
some other ketones, otherwise
would be considered good for
organic  solvents.   Not  affected
by dry cleaning  solvents or gas-
oline.

PHYSICAL PROPERTIES:

Tenacity and Elongation;  Depending
on type  2.4 to 3.0  g.p.d. dry
and wet  tenacity.   Dry  and wet
elongation ranges from  32 to 39%.
Specific Gravity;   1.30 to 1.37.
Abrasion Resistance;  Good, but
inferior to polyamides, including
"Nomex"  nylon, polyesters and
polypropylene.   Superior to
"Teflon."

FORMS AVAILABLE:

Staple Only;

EASTMAN  ("VEREL") 3-24  denier.
(Can be  converted to spun yarns of
many sizes).
UNION CARBIDE ("DYNEL") 2-24 denier.
(Can be  converted to spun yarns of
many sizes).
 from J. P. Stevens & Co., Inc., Ref. 2
-------
                               TABLE  4.4
                    CHEMICAL COMPOSITION  OF  POLYAMIDES
          Nylon-4             Pyrrolidone
                5             Valerolacturn
                6             Caprolactum
                6T            Hexamethylene  terephthalamide  (aromatic)
                7             Heptanoamide
                8             Caprylamide
                9             Amino-nonanlc  acid
               11             Amino-undecanoic acid
               12             Lauryl  lactum  from butadiene
               66             Hexamethylene  adipamide
               68             Hexamethylene-diamine and suberic acid
              610             Hexamethylene  diamine and sebacic acid
             MXD-6            Metaxylylene adipamide (aromatic)
                          j^
            4.2.2.4  Nomex  Nylon.- Notnex Nylon is a proprietary aromatic -
polyamide linked structure developed  by DuPont for applications requiring
good dimensional stability and heat resistance.  "Nomex can be used at
temperatures at which other fibers melt.   Unlike glass, it is resistant
to fluorides and has good abrasion and flex  resistance.  It has a wide
range of filtration applications, including  the cement industry, carbon
black, non-ferrous metals and steel."^  Characteristics, properties, and
producer forms are indicated in Table 4.6.
                                                               it
            4.2.2.5  Olefin.- An olefin fiber is designated  as	 a
manufactured fiber in which the -fiber-forming substance is any long chain.
polymer composed of at least 85% by weight of ethylene, propylene, or
                      9
other olefin units..."  Polypropylene is  one olefin used for industrial
gas cleaning applications.
            "The production of polypropylene fiber varies among
            the manufacturers.... to achieve certain properties,
            such as dyeability,  light  stability,  heat sensitivity,
            and shape of the filament  cross section.   The pro-
-------
                                 TABLE 4.5

   CHARACTERISTICS, PROPERTIES, AND FORMS OF NYLON FIBER FOR FILTRATION*
 HEAT RESISTANCE:

 Dry Heat: Up to 250°F. nylon  has
 reasonably good dry heat resistance.
 It is superior to acetate, modacrylic
 and saran.  However, the other
 synthetics are superior to it.
 Moist Heat;  Nylon performs adequately
 in moist heat at temperatures ranging
 up to 225°F.  However, its high initial
 tensile strength and abrasion resistance
 will make it a preferred choice in a
 number applications.

 CHEMICAL RESISTANCE:

 Acids;  Most mineral acids cause
 degradation and partial decomposition.
 Soluble in formic acid.
 Alkalies;   Good resistance to alkalies
 under most conditions.   In this respect
 nylon is better than acrylics but
 not as good as the olefins.
 Oxidizing Agents;   High concentrations
 and temperatures  may cause complete
 degradation.
 Organic  Solvents:  Withstands common
 organic  solvents  very well.   Some
 phenolic compounds cause  solubility.

 PHYSICAL PROPERTIES:

Tenacity and Elongation:  Both nylon
 6 and nylon  6.6 are  produced  in  a wide
range  of strengths.
Filament;  Dry and wet  tenacity  ranges
from 4.0 to 9.2 g.p.d. with dry  and
wet elongation running  from 16  to 42%.
Staple;  Dry and wet  tenacity ranges
from 3.5 to 7.2 g.p.d. with dry  and
wet elongation running  from 16  to 50%.
 Specific  Gravity;   1.14.
 Abrasion  Resistance;  One  of nylon's
 outstanding  characteristics is
 abrasion  resistance and in this
 respect nylon  is superior to all
 other  fibers.

 FORMS  AVAILABLE:
 NYLON  6.6:
 Mpnpfilament;  7 to  30 denier
 Multlfilament; deniers of 20,30,40,
 50,60,20,80,90,100,140,200,260,400,
 420,520,630,780,800,840,1050,1260,
 1680,
 (NOTE: The underlined deniers are
 those  that are used most  often).
 Staple;   1.5 to 18  denier/filament.
 (Can be converted to spun yarns
 of many sizes).
 Tow;   3 to 18 denier/filament
Above  forms available in  bright,
 semi-dull, dull and black.
 NYLON  6:
Monofilament; 15 to 20 denier
Multifilament;  deniers of: 30/40/
 50/70/100/140/200/840/1050/1260/
 2100/2400/2500/3150/3360/4200/
 5000/7500/10,000/15,000
 Staple;   2.0 to 15  denier/filament.
 (Can be converted to spun yarns of
many sizes).
 from J. P. Stevens & Co., Inc. Ref. 2
-------
                                TABLE 4.6
CHARACTERISTICS, PROPERTIES, AND FORMS OF NOMEX
                              FILTRATION*
       NYLON FIBER FOR  INDUSTRIAL
 HEAT RESISTANCE:

 Dry Heat;  "Nomex" nylon does not melt,
 but at temperatures above 700°F degrad-
 ation sets in rapidly.  In dry heat,
 up to and including 450°F., this fiber
 may be used satisfactorily, as long as
 there is no acid dew point problem.
 Moist Heat;  Small amounts of water
 vapor at elevated temperatures, and
 in intimate contact with water or
 saturated steam, "Nomex" nylon exhibits
 a progressive loss in strength.  How-
 ever, it withstands these conditions
much better than Nylon 6.6 or many
 other fibers.

 CHEMICAL RESISTANCE:

Acids;  Withstands both mineral and
 organic acids much better than nylon
 6.6 or nylon 6 but not as well as
 polyesters and acrylics.
Alkalies;  Excellent resistance to
 alkalies at room temperature (better
 than polyesters and acrylics)
 but degraded by strong alkalies at
 elevated temperatures.
 Oxidizing Agents;  Like nylon 6.6
or nylon 6, "Nomex" is degraded by
oxidizing agents.
Organic Solvents;  Highly resistant
to most hydrocarbons and many
other organic solvents.

PHYSICAL PROPERTIES;

Tenacity and Elongation;  Dry
tenacity: 5.5 g.p.d.  Wet tenacity:
4.1 g.p.d. Dry elongation: 18%
Wet elongation: 15%.
Specific Gravity; 1.38
Abrasion Resistance;  Superior to
acrylic fibers and about equal
to polyesters and nylon 6.6.

FORMS AVAILABLE:

Multifilament;  Deniers of 100,200
and 1200.
Staple; 2 denier/filament (can
be converted to spun yarns of
many sizes).
Both filament yarn and staple
are available in natural (off-
while), international orange
and olive green.
* J. P. Stevens & Co., Inc., Ref. 2
-------
    '         duction of  polypropylene  fiber results  in a
             relatively  economical  and inexpensive  fiber be-
             cause  the basic  substance from which  it is  made
             is  propylene  gas--a  by-product of  petroleum dis-
             tillation.  The  most costly  aspect is  the manu-
             facturer's  initial  investment  in research and
             plant  establishment."8

             "Polypropylene combines the  virtues of  lightness,
             high strength and excellent  resistance  to most
             acids  and alkalies with the  important  added at-
             traction of low  cost.  Having  the  lowest  density
             of  any synthetic fiber in filtration and  dust
             collection, polypropylene offers the greatest
             cloth,yield per  pound  of  yarn  and  is one  of the
             most economical  synthetics.

             Polypropylene fibers are  manufactured under such
             trade  names as "herculon" (Hercules), "Reevon"
             (Alamo Polymer)  and  "Vectra" (National  Plastic
             Products).

             The sleekness of the fiber allows  for a fabric
             providing good cake  discharge  and  resistance to
             blinding.  Since its moisture  absorption  is  vir-
             tually nil, polypropylene is a good choice  for an
             application such as  dye production, where pigment
             changes  can be made  with  only  a  light wash  be-
             tween  batches.

             "Polypropylene has been used successfully at
             165°F  in the  greige, and  in  the heat set  stage
             at up  to 250°F."2

Characteristics, properties, and producer  forms are given in Table 4.7.

             4.2.2.6  Polyester.- A polyester fiber  is designated as
                                       t
             "...a  manufactured fiber  in which  the fiber-forming
             substance is  any long  chain...polymer composed of
             at least 85?0  by weight of an ester of dihydric
             alcohol  and terephthalic  acid  (p - HOOC - C,H,  -
             COOH)	"9                                  *

The general  characteristics of this material, which is a commonly used

dust collection fabric particularly in the cement industry, are presented
                          D
in Table 4.8.  The Dacron  polyester  fibers are round (see Figure 4.1),

but variation in shape of the spinnaret holes will affect the appearance
-------
                               TABLE 4.7

      CHARACTERISTICS, PROPERTIES, AND FORMS OF POLYPROPYLENE FIBER
                       FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:

Dry Heat;  Polypropylene has the
lowest heat resistance of all the
synthetics except the modacrylics,
and loses tenacity in direct propor-
tion to increases in temperature.
It should be remembered, however,
that the fiber's very high initial
tenacity will leave a generous margin
for many applications.
Moist Heat;  Since polypropylene is
non-hygroscopic, its heat degradation
characteristics are essentially the
same under moist conditions as they
are under dry conditions.

CHEMICAL RESISTANCE:

Acids;  Very good resistance to both
mineral and organic acids.  Attacked
at high temperatures by nitric acid
and chlorasulfonic acid.
Alkalies;  Generally good, except
poor resistance to sodium and
potassium hydroxide at high tem-
peratures (above 200op) at high
concentrations.
Reducing Agents;  Good resistance to
most reducing agents.
Organic Solvents;  Good resistance to
most organic solvents.  Exceptions
are ketones, esters, aromatic and
aliphatic hydrocarbons at high
temperatures.   Soluble at  160UF
in chlorinated  hydrocarbons.

PHYSICAL PROPERTIES:

Tenacity and Elongation;
Filament;  Dry  and wet tenacity
4.8 to 8.5 g.p.d. with dry and
wet elongation  15 to 30%
Staple;  Dry and wet tenacity
ranges from 4.5 to 6.0 g.p.d.
with dry and wet elongation 15
35%.
Specific Gravity;  0.90 to 0.91
are of the lowest specific gravities
of any fiber.
Abras ion:  Re s istance;  Excellent
abrasion resistance wet and dry.

FORMS AVAILABLE:

Monofilament;   6 to 12 mils  round
and flat.
Multifilament;  165 to 4000 denier.
Staple;  1.5 to 15 denier. (Can
be converted to spun yarns of
many sizes)
Available in natural and solution
dyed colors.  ,
 from J. P.  Stevens & Co.  Inc., Ref. 2
-------
      TABLE 4.8.   CHARACTERISTICS,  PROPERTIES AND FORMS OF POLYESTER
                           FIBERS FOR INDUSTRIAL FILTRATION*
 HEAT RESISTENCE:

 Dry  Heat:  Polyesters  are  not  compar-
 able to "Teflon"  and  "Nomex"  nylon,
 which have usually high heat  degrad-
 ation resistance,  but they  are
 superior  to most  other synthetics
 in this respect,  though subject  to
 hydrolytic degradation under
 certain circumstances.  However,
 polyesters are used for filtra-
 tion and  dust collection  under
 conditions of wet  heat, particu-
 larly where the initial yarn
 strength  exceeds minimum  re-
 requirements sufficiently to
 allow for  hydrolysis.   Below
 350  F there is little  shrinkage
 of polyester fiber.

 CHEMICAL RESISTANCE:

 Acids;  Good resistance to most
 mineral and organic acids except
 high concentrations of nitric,
 sulfuric and carbolic  acids.
 Alkalies;  Good resistance to
 weak alkalies and moderate resis-
 tance  to strong alkalies  at low
 temperatures.  Strong  alkalies
 at high concentrations and temper-
 atures dissolve polyesters.
 Oxidizing  Agents; Good resistance
 to most oxidizing agents.
 Organic Solvents:  Excellent  to
most organic solvents, but
 unsuited for some phenolic
 compounds  and affected by
 cyclohexanone at 313°F.
PHYSICAL PROPERTIES:

Tenacity and Elongation:
Filament;  Dry and wet tenacity
ranges from 4.4 to 7.8 g.p.d.
Dry elongation from 10 to 2570.
Water has little effect on either
the strength or elongation of
polyester fibers.
Staple; Dry and wet tenacity ranges
from 2.2 to 5,5 g.p.d.  Dry and
wet elongation from 18-50%.
Specific Gravity; 1.35 - 1.38.
Abrasion Resistance;  Polyester
fibers have excellent abrasion
resistance ranking next to nylon
in this respect.

FORMS AVAILABLE:

Multifilatnent;  Deniers of 30, 70,
100, 140, 150, 220, 250, 420,
440, 840, 880, 1000, 1100, 1680,
and multiples of 1100 up to 16,500.
Staple;  1.5, 2.25, 3.0, 4.5, 6.0
8.0, 15.  (Can be converted to
spun yarns of many sizes).
Direct Spun Yarn;  From 4s/I to
30s/l.
*From J. P.  Stevens & Co.,  Inc., Ref. 2.
-------
and mechanical properties of the fiber, as will the extent to which the

fiber is drawn.  The usual diameters are 10 to 30 urn.  The following

quotation provides some additional information on this material.

            "Polyester fibers are manufactured under such trade
            names as "Dacron" (DuPont), "Fortrel" (Fiber Indus-
            tries /Celanese) , "Vycron"  (Beaunit), "Kodel" (East-
            man Chemical Products) and Enka Polyester (American
            Enka Corporation).  They can be woven into filter
            and dust collection fabrics affording very good
            resistance to chemicals, abrasion and dry heat
            degradation, plus excellent dimensional stability."^
                           P
            4.2.2.7  Teflon  Fluorocarbon.-  The characteristics, prop-

erties and forms of these fibers are summarized in Table 4.9 and described

in the following quotations.

            "Teflon - Dupont's trade name for their fluorocarbon
            fibers - is available in two forms.  The multi-fila-
            ment yarns are made from the homo-polymer polytetra-
            fluoroethylene (TFE).  The monofilament yarns are
            produced from the co-polymer of tetrafluoroethylene
            and hexafluoropropylene (FEP).   Both polymers are
            composed of long-chain molecules in which all of the
            available bonds are completely  saturated by
            fluorine.  These carbon-to-fluorine bonds are ex-
            tremely strong, resulting in fibers which are excep-
            tionally stable to both heat and chemicals.   Teflon
            fiber has no known solvents except certain perfluor-
            inated organic liquids at temperatures above 570°F.
            The fiber is inert to concentrated mineral acid,
            organic acid,  alkalies,  oxidizing agents and
            organic solvents at elevated temperatures.

            "The fiber remains flexible and non-brittle  from
            minus 100°F to plus 550°F.   Teflon withstands
            prolonged exposure at 450°-500°F without degra-
            dation.   Above this temperature some  decomposi-
            tion results but even at  500°-550°F decomposi-
            tion is slow.   The gaseous  decomposition products
            evolved at high temperatures are highly toxic
            and must be removed from the work areas  through
            adequate ventilation.

            "Teflon fibers have a very  low  coefficient of
            friction resulting in excellent cake  discharge.
            This fact,  coupled with  its chemical  inertness
            and resistance to dry and moist heat  degrada-
-------
      TABLE 4.9.   CHARACTERISTICS,  PROPERTIES AND FORMS  OF TEFLONR
                    FIBER FOR INDUSTRIAL FILTRATION*
 HEAT RESISTANCE:
            "Teflon" TFE can be used
                               o.
Dry Heat;
in continuous service up to 500"F.
"Teflon" FEP can be used in continuous
service up to 450 F.
Moist Heat;  Due to zero moisture
absorption, these fibers withstand
moist heat temperatures about as
well as dry temperature.

CHEMICAL RESISTANCE:
Acids; Inert
Alkalies: Inert
Oxidizing Agents; Inert
Organic Solvents;  Inert
The only substances known to react
with these fibers are alkali metals
fluorine gas at high pressure and
temperature, and chlorine trifluoride.
This is the most chemically resistant
fiber produced.

PHYSICAL PROPERTIES:

Tenacity and Elongation:
Filament and Staple; Dry and wet
tenacity for type TFE is 1.6 g.p.d
and for type EEP is 0.5 g.p.d.
Dry and wet elongation is 15%.
 Specific Gravity;  2.1
 Abrasion Resistance;  Teflon
 fibers are  inferior  in abrasion
 resistance  to the  acrylic, nylons
 and polyester fibers, but are
 superior to glass  fibers.

 FORMS AVAILABLE:
 "TEFLON" TFE
 Filament Natural Brown
 100/200/400/1200 denier.
 2400 up to  26,400  denier plied
 yarns
 Filament Bleached White
 225/450/1350 denier.
 2700 up to  29,700  denier plied yarns
 Staple Natural Brown
 6.67 denier/filament
 Staple Bleached White
 7.50 denier/filament
 Staple available in cut lengths
 of 0.5" up  to 5.0" in 0.5" incre-
ments (Can be converted to spun
yarns of many sizes).
 "TEFLON" FEP
Monofilaments (clear, colorless)
 3/5/8/11/16/20/32/50/60 mils.
 from J. P. Stevens & Co., Inc., Ref. 2
-------
             tion, makes  Teflon suitable  for  filtration and
             dust  collection  under  severe conditions.   Its
             major weakness  is  abrasion resistance  in  which
             Teflon  is  inferior to  all  other  synthetic
             fibers  except glass.   The  high price of Teflon
             fiber limits its use  in  the  filtration field.
             However, for uses  under  extreme  temperature
             conditions,  Teflon may prove to  be most
             economical in the  long run.   It  would  further
             be  expected  that as the  demand for Teflon in-
             creases and  production expands,  the price of
             Teflon  will  decline."2
                                                        9
             4.2.2.8 Vinyon.-  Vinyon fiber is designated   as  ..."a  manu-

factured  fiber  in which  the  fiber-forming substance is any long  chain...

polymer composed of at least 8570 by  weight of vinyl chloride  units  (-CH-  -

CHC.0-)1.1   The characteristics,  properties and producer forms of the  fiber

are presented in Table 4.10.   The  following  quotations summarize these

characteristics.

             "Characteristic  of vinyon  fibers is the dumbbell
             shape of their cross sections....Fibers with an
             occasional twist are observed.   Both continuous
             filaments  and stable fibers  (1-5 in.)  occur in
             trade,  the width of all  types being 16 to 18 u."^

             "Vinyons are made  of 100%  polyvinyl chloride by
             the French Societe Rhovyl, and as a copolymer
             of  polyvinyl chloride  and  polyvinyl acetate by
             the FMC Corp., American  Viscose  Division.
            Vinyons offer very good  resistance to  most
             chemicals, even  in highly  concentrated form.
            They can be  made into  fabrics providing extremely
            smooth  cake  discharge, and are used for filtra-
            tion of air  and  (numerous  liquids)."2
                                                       :
            4.2.2.9  Glass.- Glass fiber  is  a product  of fusion, a non-<
crystalline silicate analogous  to  other  fiber polymeric materials,  (-SiO,-)
Selected silica sands, limestone,   soda ash,   and borax  or other ingredients

are melted at about 2500 F and  the mixture is extruded through spinnarets.

The resulting filaments may be  drawn while still molten and later twisted

and plied into filament  yarn.   Or, the extruded glass may  be drawn and

broken by jets of compressed air into staple of lengths 8  to 15  inches.
-------
          TABLE 4.10.   CHARACTERISTICS,  PROPERTIES, AND FORMS  OF
                VINYON FIBER FOR INDUSTRIAL FILTRATION*
 HEAT  RESISTANCE:

 Dry Heat;  The maximum working
 temperature  for Rhovyl's  type
 "Clevyl T" is 350°F., provided
 the fabric is clamped to  prevent
 shrinkage.
 Moist Heat;  Due  to vinyon's low
 water absorption  the effects of
 moist heat are similar to those of
 dry heat.

 CHEMICAL RESISTANCE:

 Ac ids;  Excellent resistance at
 room temperature  to mineral acids,
 including hydrochloric, nitric,
 and sulphuric acids, aqua regia
 and organic  acids with the exception
 of carbolic  acid.
 Alkalies; Very good resistance to
 alkalies such as  potassium hydroxide,
 sodium hydoxide and ammonium
 hydroxide.
 Organic Solvents;  Dissolved by
 ketones, and partially dissolved
 or softened by esters and  ethers.
 Certain other organic solvents
 cause swelling at certain  temper-
 atures.  In general, this  fiber should
 not be considered for use with
most organic solvents other than
mineral oil, aliphatic hydro-
 carbons, alcohols and glycols.

 PHYSICAL PROPERTIES:

Tenacity;  Filament and Staple:
 "Clevyl T" (Dimensionally  stable
 to 212°F) Staple 1.7 to 2.0 g/d
 "Type 55" (557, shrinkage at 212°F)
Filament 2.7 to 3.0 g/d.
Abras ion Res is tance;  Not as resis-
tnat as polyesters or nylons, but
comparable to the acrylics.
Specific Gravity;  1.34-1.38

FORMS AVAILABLE:

RHOVYL STAPLE:
"Clevyl T" (Dimensionally Stable
to 212°F) 3.5/5/8/15 d/fil.
"Type 30" (30% shrinkage at 212°F)
3d/fil.
"Type 55" (55% shrinkage at 212°F)
 1.8/3.0/6.4 d/fil.
RHOVYL FILAMENT:
"Type 55" only: 75/100/200/400
800/1600 denier.
FMC VINYON H.H. STAPLE ONLY
Deniers of 1.5 to 5.5 staple
lengths of 1/2 inch to 2 inches
but not in all deniers. (Can
be converted to spun yarns of
many sizes)
 From J. P. Stevens & Co., Inc.,Ref. 2
-------
The  fibers  are  then  treated with a lubricant  which  is  of great  importance
 in the  durability  of the  eventual fabric.   Following drying,  the  fibers
are  processed much like the more conventional fibers.
             Glass  fiber photomicrographs are  shown  in  Figure  4.1.   The
fibers  are  perfectly round  with  very  smooth and  structureless surfaces.
Diameters range from 5 to 16 urn  for most textile fibers.  Characteristics
and  forms of glass fibers are given in Table  4.11.
             4.2.2.10 Other Man-Made  Fibers.- Other generic organic  fiber
materials that  could be used for industrial gas  filtration include  acetate
and  triacetate  (cellulose acetate), rayon  (regenerated cellulose),  manu-
factured rubber, saran (vinylidene chloride),  and vinal  (vinyl  alcohol).
They appear  to  offer no significant advantages for  industrial gas filtra-
tion in most instances, although saran has  some  application in  spray
tower mist elimination.   (Other  unclassified  fibers are  summarized  in
Ref.  3, pp.  218-221).
             Several  fiber materials and finishes are currently  available
in pilot plant  quantities having potential  for high temperature applica-
tions or control of  electrostatic effects  in  fabrics.  Some metals and
                                                                          3
ceramics having  potential filtration uses are summarized  in Appendix 4.2.
         R              R
Fiberfrax ,  and Brunsmet  ,  have  been laboratory  tested and found satis-
factory for  filtration at high temperatures ( >  1000 F), but require further
development  in  filtration systems.
     4.2.3  Physical  and  Chemical Properties
            Relative  physical and chemical properties of filter fiber
materials are summarized  in Tables 4.12,  4.13 and 4.14;.  Table 4.13
shows general quantitative  physical properties and a qualitative estimate
                                           2
of chemical, heat,  and abrasion  resistance.   Where both high and low
tenacity fibers are  produced, the range of strength and breaking elonga-
tion is given.   Since many  fibers are produced in both filament and staple
fibers, the general  physical properties are shown separately for each
form.  Fiber resistance to specific chemical compounds  is indicated in
-------
                               TABLE 4.11

            CHARACTERISTICS, PROPERTIES, AND FORMS OF GLASS FIBERS
                         FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE
CHEMICAL RESISTANCE
PHYSICAL PROPERTIES
FORMS AVAILABLE
Fabrics made from glass yarns actually gain in
strength as the temperature rises from room
temperature to 400°F.  From that point, strength
and flexibility decrease.

The recommended operating temperature is 500 F
with surge limits up to 600°F

Glass is resistant to acids of ndrmal strength
and under ordinary conditions.  It is attacked
by hydrofluoric, concentrated sulphuric, and hot
phosphoric acids.  Overall resistance to acids
is slightly above average.

Hot solutions of weak alkalies will also attack
glass.  Overall resistance to alkalies is poor.

Operating a glass baghouse at or below the dew
point can be particularly damaging if acid
anhydrides or metallic oxides are entrained in
the gas stream.  Fluorides and the oxides of sul-
fur are particularly damaging to glass.

Glass is considered to be incombustible because
it is completely inorganic.  In addition, it
has a low coefficient of linear expansion and
hence is dimensionally stable.  Although glass
has an extremely high tensile strength it has
poor flex-abrasion resistance.

Various chemical treatments to the glass fabric
improve the flex-abrasion characteristics of
glass bags.

Filament
Filament and Textured
Filament and Spun
      From Albany Felt Co., Ref. 11.
-------
          TABLE 4.12.   RELATIVE PROPERTIES OF MAN-MADE FIBERS*
 Resistance  to
 Acids"
 "Teflon"
 Polypropylene
 Vinyon
 Modacrylics
 Acrylics
 Polyesters
 "Nomex" nylon
 Nylon 6.6 &  6
 Rayon

 Resistance to
 Alkalies
 "Teflon"
 Polypropylene
 Vinyon
 Modacrylics
 Nylon 6.6 &  6
 "Nomex" nylon
 Polyesters
 Rayon
 Acrylics

 Resistance
 to Oxidizing and
 Reducing Agents
 "Teflon"
 Polypropylene
 "Nomex" nylon
 Modacrylics
 Polyesters
 Acrylics
 Nylon 6.6 &  6
 Vinyon
 Rayon
tensile
Strength
Nylon  6.6 &  6
Polyesters
Polypropylene
"Nomex" nylon
Rayon
Acrylics & Modacrylics
Vinyon
"Teflon"
Resistance to
Abrasion__(Wet_& DryJ
Nylon  6.6 &  6
Polypropylene/Polyesters
"Nomex" nylon
Acrylics & Modacrylics
"Teflon"
Rayon
Vinyon
           J-9.
Dry Heat
"Teflon"
"Nomex" nylon
Polyesters
Acrylics /Rayon
Nylon 6,6
Nylon 6
Polypropylene
Modacrylics
Vinyon

Resistance^ to
Moist Heat
"Teflon""
"Nomex" nylon
Acrylics
Nylon  6.6 &  6
Rayon
Polyesters
Polypropylene
Modacrylics
Vinyon
Max. Recommended
For Continuous
"Teflon"
"Nomex" nylon
Polyesters
Acrylics
(homopolymers)
Acrylics
Rayon
Nylon 6; 6
Nylon 6
Polypropylene
Modacrylics
Price Relationshi
Service
      ,o
   5001-
   450°*
   300°*
   284°

   275°
   275°
   250°*
   250°*
   225°
   180°
(Highest to Lowest)
STAPLE           FILAMENT
"Teflon"
"Nomex" nylon
Glass
Nylon 6.6 & 6
Polyesters
Acrylics
Modacrylics
Vinyon (Rhovyl)
Polypropylene
Viscose Rayon
  "Teflon"
  "Nomex" nylon
  Acrylics
  Modacrylics
  Polyesters
  Nylon 6.6 & 6
  Glass
  Vinyon (Rhovyl)
  Polypropylene
  Viscose Rayon
*  These fibers are subject to hydrolysis when exposed in hot, wet atmospheres
   in varying degrees.  Polyesters degrade to the greatest extent.  "Nomex"
   nylon next, then nylon 6, and nylon 6.6 least.
   The following is quoted from DuPont's NP-33, "Properties of 'Nomex1.11
   "At elevated temperatures, "Nomex" fiber in intimate contact with water
   or saturated steam exhibits a progressive loss in strength with water
   vapor.  (Nylon 6,6 completely deteriorates in less than 100 hours under
   the same conditions)"

** Based on prices per pound at time of publication (1965 est) - See Chapter 7.
   From J. P., Stevens and Co., Inc., Ref. 2.
-------
                                                                                                I  o
                                                     TABLE 4.13

                       SUMMARY OF PHYSICAL AND CHEMICAL PROPERTIES OF  INDUSTRIAL FILTER FIBERS
-O
I
///***// //
* / / / / / / / / /
FILAMENT i
Breaking Tenacity gpd (Dry, \\ '« ' " " « ^ "^i
B,eak,ng Tenacity gpd, We,, °] 3| 38 40 J° 79 * ' ^ 7 J
Breakmg E.onoa.ion « s , Dry, » « » 1< |3 « 18 ; J° ;
Breaking Elongalic. • , , We,, » « 23 '« Jf » '5 £ \
STAPLE
Breaking Tenacity gpd, Dry, {* *° ». 3° » » 38 ":»j
Breaking Tenacity gpd, We,, ?8 J« J« » J* *J ^ 4'° ! *| !
Breaking E,onga,,o,.^Dry, » » « » ; » | » « " ' «
Breaking Elongation -., We,, S S S » M » « '"'.£'

Specific Gravity 1 33 1 18 1.18 1 12 1 15 i 1.14 . 1.30 1 14 ; 1 38 • 1.38 '
Mai Recommended Operating f ; 175 275 275 275 284 .. 275 : 180 250 ! 450 300 i
i i i '• ' • i
Resistance to Abrasion G : G G G G G G E E ( E ;
Resistance to Dry Heat FjG.G G'GjGJFG :E!G!
:••?•• ' ' " : -H -..».-.. 	 , 	 , 	 ,-
Resistance to Moist Heal F ; G G G E-GiF G E'Fl
Resistance to Mineral Acids • p '• G G G'G^GG'P F G
Resistance to Organic Acids • P ' a G G G : G G F E G!
Resistance to Alkalies P|F F F.FiFiGG GiGi
Resistance to Omdizing Agents FG;GGG:GG F |Q Q|
Resistance to Solvents !F-E EE'EiE'GE E;Ei
f * J?
///////
S / / f f f 3
: i
44 4.6 ; 1 0 4.8 ! 1.5 ' '
78 ! 54 70 . 8.5 , 57 23
44 46 1.0 4.8 : 07 : •
7.8 • 5.4 70 8.5 36 23
10 19 . 10 15 9 15
25 25 80-30-30 30
19 19 20 15 l» "5
25 - 25 80 30 40 25

22 ' 25 53 45 15 i 0
40;55'S4 6.0 46 15
2.2-25 53 4507 10
40 5 5 5.4 6.0 3.6 , 1 5
18 24 . 32 15 9 15
50 : 45 40 35 , 30 25
18 : 24 32 15 14 15
50 45 40 ' 35 40 25

1.38 i 138 : 1-38 ' .92 . 90 ; 1.52 170
• • r- | :
300 I 350 j 300 ; 150 225 | 275 ; 1.60

	 • 	 1 ' ' !
E'EJE,G;E.,GG
GiGJG'FlG.GF
F 1 F 1 F F F • G i F
GjGJG-G'E'.PiG
	 1 	 j 	 r- -t ! !
GJG:G,G EiGjG
G G G:GjE|FjG
	 1 	 ! 	 " 1 :
G G | G . P ; G ; F F
E:EiE;GJ'GJGjG
/ f f f
/ / oj /
; i i
• 6 C? 60
30 73 j
• 6 07 39 j
30 47 1 i
•5'23 ;
•c.
•i '2 25 j
•C J
t
16 07' i 1
30 '
• 6 37 1
30 i ;
15 40 I
60 ^
15 40
60 i 1

210 1 34 254 . 1
500 350 ; 550 , 200
I | j—

F , G ' P | F .
'ill
E : P i E | F L
E P ' E : F ,
E E \ E j P
E . a: E| e|
E G P ! G |
« F E ! F j
E| F E j E i
// /
i i
i
: 3B i
i : ;
i -
i i
i 1T
,
i- -i- '
30 '• ' 10 !
19 { M7 i
J3 ! OB j
i.| ,6,
3 ! ! 25 1
7 i j 35 ;
3 i i 25 ;
7 J 50 i

50 ; 1 25 i 1 32 !
J25 ; 175 j 200 !
	 ,, 	 » 	 *
! i
. i J
G ; G j G
G F i F !
!
G F 1 F j
P ! F ' F
-4— j 	
GIF F
G ! P P
F i P P
j 	
EG F
-------
Table 4.14.   (Additional detailed  engineering  test  data on  fiber  proper-
                                   2  12
ties is presented in Appendix 4.3.  ').

            "It is possible  that a fiber  indicated  as being
            fair within a general  chemical class  (in Table
            4.13) may be rated as  good for a particular chem-
            ical within the  general  grouping.  Each fiber has
            its own strong and weak  points and must be evalu-
            ated on its own  merits."2

     The evaluation of the resistance of fibers to chemical reagents pre-

sented in Table 4.14 is based on liquid filtration requirements.  However,

the data presented in the table have direct applicability to dust filtra-

tion as the time,  temperature, chemical concentration,  type of polymer,

treatment of fabric, etc., all have their effect on final selection of a

filter or dust collection medium.  For these reasons,  tests should be made

under operating conditions,  as a composite table cannot cover all possible

conditions to which a filter or dust collection fabric  may be subjected.

     The maximum temperature, degrees Fahrenheit ( F), as enumerated in

Table 4.14 is the maximum temperature that should be employed normally

for dust collection or liquid filtration of dilute solutions.  A fiber

marked R (recommended) can be expected to usually withstand most conditions

reasonably well.  S (satisfactory)  fibers should be considered only for

low concentrations and moderate temperatures.   N (not  recommended) fibers

can be expected to give poor results under any conditions.

            "Chemical composition of synthetic fibers  greatly
            affect their chemical resistance.   This is  true
            within a generic family as well as from one type
            of fiber to another.   Chemical resistance  of a
            co-polymer fiber is usually lower  than that of a
            pure homo-polymer base  fiber.   The co-mono-mers
            used to form "links" are particularly sensitive
            to external effects....

            Using  acrylic  fibers as an example, in acidic  or
            oxidizing solutions,  in dry or humid  air be-
            tween  275°-350°F, the difference  in behavior  be-
            tween  homo- and co-polymers  becomes very important.
            The homo-polymer is superior.   The influence  of
            composition of the basic  acrylic  polymer has
            little effort  when considering organic  snlvpnts,
            weak acids, most mineral salt solutions and
            cleaning or bleaching agents.   In alkaline  media
            up to intermediate concentrations  and tempera-


-------
                                       TABLE 4.14

                           RESISTANCE OF FIBERS TO  CHEMICAL REAGENTS

BtaB. Twv. QapMi FaWriwn * n 3
laanjuu. *ooa
Aqua R«0i« N

Pho»pho«C JS j
Sulfunc IN i
Tmilf|rrT. agp, |
Acetic S
6«ruO»C N
Carbolic N
Formic N
Lactic S
Diane R
Salicylic N
Aoimoniitfn HydrovidB S
Catciwn HydronMM N
Potautum Hytfroiida N
Pobmnm Carbonate N
Sodnm Hydrouda N
MLTC
CateiuaiCMDrida 8
FavricCMorUa N
SodHMAcaMa N
SodMi BanzoaH R
SodwmBroaMa 8
SediMmCMonda R
SodwmCyankla S
SottwnlMraia S
SoawMSrffaii S
£ncCMorid» S
ajKMUjna 8

CMortoa S
FluoMW N
IT • i <
T8;JT5 7TJ
i *
R -R-nr
s s s
s s is




s s s










m?
sl


si




s










ni
j


5
11



S










•oaooiaaoitfo-xoaoD jiupw:r%it«o.uDii?ojsu!2aoi2is IISDOD
4-4—1- :.,..,::;']•; j

,isU|s:R . s s ..R:R!R;RI..«:S!«;

Ring's B'R'R'R-^ ~K~ R • 5 ; R | a ! R 1 R ' s '

,!•'. s ..M-.:,-. sVR'sls^J..:

n is i « ;P S:SiR,R s R R;R.N:R R N N
	 4 	 f 	 . 	 . 	 j 	 . 	 . 	 . 	 \ 	 , 	 . 	 »— , 	 ; 	 •
1 i i j I i i i ; 1 1 :


-M 	 !— I— I 	 tlii.il
.|.|.!.L.!.vJ.4i^r^

S!N!R S'NJR S!R:S SIN'N

! i i i i 1 j 1 i ; i 1 i I i
                                                                 * /-*/   '//
                                                           */// * // *•/ [I ff// /////./
                                                           //if IIffft / e r i///_//1 if

-------
             Cures,  both  types  of  acrylic  polymers  behave  in
             a  simiLnr  manner.   As alkaline  conditions  become
             more  severe  the  homo-polymer  again shows  super-
             iority.

             Under weak or  intermediate  conditions  of  chemi-
             cal concentration  and/or  temperature,  generic
             family  only  will need to  be considered.  As
             severity of  conditions  increase,  then  special
             fibers  within  a  generic type  become  important
             considerations.  "Tailor-made"  fibers  are  being
             manufactured within a generic family.   "Nomex"
             nylon will withstand  450°F  whereas nylon  6 or
             6,6,  the common  types of  nylon, are  useful up
             to 250°F."
             "In the acrylics,  "Crylor"  and  "Dralon T"  can
             be used at higher  temperatures  than  most other
             acrylics because of their higher  acrylonitrile
             content.   All  manufacturers of  synthetic  fibers
             are striving to  produce better  fibers  for  fil-
             tration and  dust collection."

4.3  YARN PRODUCTION

     The standard definition of a yarn  is as  follows:

             "Yarn: a generic term for a continuous  strand
             of textile fibers,  filaments or material in
             a  form  suitable  for knitting, weaving,  or
             otherwise  intertwining to form  a  textile
             fabric.  Yarn  occurs  in the following  forms:

             a.  A number of  fibers twisted  together
             b.  A number of  filaments laid  together without
                twist
             c.  A number of  filaments laid  together with
                more or  less twist
             d.  A single filament,...monofilament
             e.  A narrow strip  of material  such  as paper
                cellophane, or  metal  foil...

             Varieties  include single  yarn,  plied yarn,
             cabled yarn, cord,   thread,  fancy yarn."

     Yarns formed of short staple fibers are called spun yarns.  Yarns

may also be  formed of a  twisted bundle  of individual fine monofilaments
(typically several hundred, each monofilament of order 10 ^m diameter), in

which case the yarn is called mult ifilament  or filament.   Single or mono-

filament yarns (e.g. as  in monofilament fishing  line,  5 to 10 Ib. test)
are not as widely used in  fabrics  for industrial gas filtration as multi-

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      Multifilament  yarns  are  made  directly from individual  monofilaments,
 frequently  as  they  are  extruded,  in which  case  the  desired  number of spin-
 narets  is used.   Following varying amounts of thermal  and mechanical
 molecular crystallite orientation,  a finish lubricant  may be  added and  a
 slight  twist is  given the yarn  to  maintain multifilament order.   This  low-
 twist basic yarn is then  coverted  to the specific yarn form required for
 the  desired fabric  design either by the fiber producer, by  an intermediate
 processor, or  by the fabric manufacturer.   Man-made  fiber staple  may be cut
 frommultifilament yarn.   Staple yarns  are  made  by the  fiber yarn  or fabric
 manufacturer by  parallelizing the  short fibers  and  twisting them  together
 for  the desired  strength.
      The method  or  "system" of yarn production  is determined  by the type
 of fabric desired for dust collection.  The major fabric types  are pre-
 sented  in Table  4.15 based on information  provided by  Albany  Felt Company.
      A variation of the woolen system  is the  worsted system,  in which the
 fibers are given additional parallelizing  treatment, or combed, before
 being spun.  As  a result  the  worsted yarn  is  smoother  and of  finer quality
 (Figure 4.2).  The  type of spinning machine as well as the  amount  of
 combining influences the  fiber orientation, and  thus has an influence on
 the  amount of  free  fiber  available  for dust filtration.
4.4  YARN PROPERTIES
     Yarns of  staple fibers can be  made with  various fiber  lengths and
various fiber  diameters.  The diameter of  the yarn itself can range from
relatively fine  to  comparatively heavy and  thick.  The amount of  twist
 is also variable.  All yarns  composed of staple  fibers must possess
sufficient twist to hold  the  fibers  in place.  The amount of  twist  in
staple yarns also depend upon staple length,  shorter staple requiring
higher twist for adequate weaving strength.
     Filament yarns are smooth and  even unless they have been deliberately
formed in an uneven manner for novelty effects.   They may be  thick  in
diameter and heavy, gossamer  sheer  and light, or of any intermediate
weight and diameter.
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                                    TABLE 4.IS

                                 MAJOR FABRIC SYSTEMS *
Woolen System Spun Fabrics

     Thli  terminology relates  to « manufacturing system and  not  Just wool flb«ra.
Synthetic  can be  woven on the  'woolen system1.

     Woolen  System Spun Fabrics  are made  with soft,  lofty, open, heterogeneous,
low  twlat  yarns,  which result  In a fabric with many  small pores  for high capacity
and  efficiency.   This type  fabric Is Ideal  for vary  flna dust up to and Including
coarse grades ....

     This  type construction was  designed  primarily for  the Shaker type baghousa.

Cotton System-Spun Fabrics

     This  terminology relates  to a manufacturing system and  not  Just cotton fib-
ers.   Synthetic  fibers can  be  woven on the  cotton system.

     In this type fabric, the  yarns are smaller,  tighter and of  high twlat.  Be-
cause  of this,  the yarn count  is Increased  to prevent duat leakage and the result-
ing  fabric is generally lighter  in weight and offers less capacity than a woolen
system spun  fabric.
     This  type fabric could be used for a Shaker type baghouse.

Filament Fabrics
     Filament filtration fabrics are characterized by slick  yarns without any pro-
truding fiber ends along the yarn and therefore  the  surface  of the fabric Is very
smooth .

     Since this type  fabric Is completely without a  surface  "cover" or "nap", the
yarns  are  packed  quite closely together,  even tighter than a cotton system fabric
of equal permeability.
     This  results  In  a fabric  which has excellent release characteristics, but
limited  capacity.
     This  type construction was  designed  for the  Reverse-Air type baghouse.

"Combo"  Fabrics (Spun- filament)
     This  type fabric Incorporates  some of  the advantages of both the woolen system
fabric and the Filament  fabric.   In this  unique  design,  the  filament yarna ere
manufactured  in the lengthwise or warp direction  of the  bag whereas the woolen
system spun  yarns  are manufactured  in the crosswise or filling direction.

     The "combo"  fabrics  are a so-called  one sided construction whereby the fila-
ment yarna are predominant  on  one side, giving the Inherent advantage of cake re-
lease.  The  spun yarns are  used  to  add strength and to cover the Inters tlcea of
the yarn and  thereby  reduce pore  sice.                                      /
     This  fabric was  designed  primarily for Shaker baghouses, however,  it  also has
limited use  in Reverse Air  Collectors.
     The "combo" fabric offers a  good  balance of capacity and cleanabllity where
plugging problems are encountered.

Needled Fabrics
     This type fabric la manufactured by mechanically interlocking  layere  of fibers
by inserting a multitude of reciprocating barbed  needles. Needled  fabrics should
uae a woven base fabric (sometimes referred  to aa a scrim) for strength and  stability.

     This type fabric offers excellent capacity and wear with the reverae-Jet and
pulse jet type collectors.       '
  From Albany Felt Company, Ref .  11
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Figure 4.2.  Diagram of Fiber Lay- a. Woolen Yarn  (note, Fibers Usually  Lay
             in Much More Random Manner); b. Worsted Yarn  (Ideal  Fiber Lay).
             (From Joseph, Ref. 14).
      4.4.1   Threads  and  Yarns
             Thread  and yarn are  basically similar.   Yarn  is  the  term usu-
 ally applied when the assemblage of  fibers is  used  in the  construction
 of  a fabric.   Thread is  the product  used  to join pieces of fabric  together
 in  the  construction  of textile products.   Thread is  frequently of  plied
 construction.   It is fine,  even,  and strong.   Several types  of thread are
 available on the  market:   there  are  simple ply threads, cord threads,
 elastic  threads for  special use,  monofilament  threads of  the man-made
 fibers,  and  mult ifilament  threads.
      4.4.2   Yarn  Twist
             As  the staple  fibers  or  filament fibers  are formed into  yarns,
 a certain amount  of  twist  is added to hold the fibers together.  The
 amount  of twist is measured by the number of turns per inch.  The more
turns per inch the stronger the yarn  becomes,  up to a  point.  Beyond  the
optimum,which varies depending upon  fiber  content, staple  length, size
of the yarn and appearance  desired,  the yarn will become somewhat brittle
and tend to break easily.  According  to Gurley   yarns with  low twist
have  less than 5  turns per  inch; medium twist  yarns from 5 to 20 turns
per inch; and high twist yarns from  20 to  30 turns per inch.  It has  been
suggested that optimum ranges are 3 to 6  turns per inch for  filament  fiber
yarns and 10 to 20 turns per inch for staple fiber yarns.
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            The direction of twist is also  important.  Yarns may be
twisted either with a right-hand twist or a left-hand twist.  The right-
hand twist is called a Z twist, while the left-hand twist  is an S twist,
as in Figure 4.3.  Various effects can be obtained by combinations of
yarns ot: different twist direction, and the  durability of  yarns may be
increased by efficient plying of S and Z twist single yarns.
     4.4.3  Yarn Number
            Yarn number is a measure of linear density.   Direct yarn num-
ber is the mass per unit length of yarn; indirect yarn number is the length
per unit mass of yarn.   Yarn number is frequently called yarn count in the
indirect system.  To some extent the yarn number is an indication of the
diameter when yarns of the same fiber content are compared.
          Figure 4.3.   Diagram of S and Z Twist in Yarn.  (From
                       Joseph, Ref. 14).
            Over the years various methods of determining yarn number
have  been developed.  Cotton yarns have been numbered by determining the
weight  in pounds of 840 yard hanks, or, aiore frequently, the number of
840 yard hanks required to weigh one pound.  For example, if 840 yards
weigh one pound, the yarn number is Is; if it requires 30 such hanks to
weigh one pound, the yarn is a 30s.  A heavy yarn would be Is; a medium
yarn  is considered a 30s, while a very fine yarn might be a 160s.
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            Woolen system yarn is measured by  the number of  300-yard hanks
per pound while worsted system yarn  is measured by the number of 560-yard
hanks per pound.  Man-made fiber yarns are usually measured  using the denier
                                                                           *
system.  The denier is equal to the  weight in  grams of 9000  meters of yarn.
            Recently the textile industry has  considered among other systems
the Tex numbering system for all fibers.  A kilometer of yarn is weighed in
grams and the weight becomes the yarn number.  In this system the larger the
number the heavier the yarn and, conversely, the smaller the number the
finer the yarn.  Note that:

                         Tex = 	Wt in grams	
                               1000  meters of  yarn               ^   '

and
                             9 Tex = 1 denier                    (4.2)

The denier or Tex may be used to determine monofilament fiber size and
approximate yarn diameter from the appropriate fiber material density.
                                        j^
            Consider for example a Nomex  nylon yarn stable  fiber (see
Table 4.6, for forms available) of 2 denier/filament (2/9 tex).  The
diameter of this fiber is obtained as follows.  Since
                               f\
                           T D   L p. = Wt in  grams,             (4.3a)


           J Df   (9 x 10   x 10  cm)(1.38 gm/cm3) = 2 grams       (4.3b)

from which
                               Df =  14.3um

The fiber density, Pf, was obtained  from Table 4.13 (also see Appendix
4.3).
     *
      This system dates back to early Roman history where a coin, the
"Denier" (~ 0.5 gm) was used as a medium for buying and selling silk.
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                                            p
            A  100 denier multifilament Nomex  nylon  yarn  (Table 4.6) would
be composed of approximately 50-2 denier  filament ends.   The yarn dia-
meter would be of the order

                            (1-0 I Df2 N- f Dy2                 (4.4)

where 1-e = a. = the  packed  fraction, depending upon  twist and yarn tight-
            ness
      Df  = the individual  filament diameter
      N   = the number of filaments in the yarn
      D   = the yarn diameter
       y
from which (assuming 1-e = 0.6), D  & 14.3 v30 % SO^m, or approximately
the thickness of a human hair.
            Breaking tenacity  (c.f. Table 4.13 or Appendix 4.3) in grams/
denier can be converted to stress from the fiber size and breaking weight
                                P
applied.  For the 2 denier Nomex  nylon staple fiber above, breaking ten-
acity (dry) is 5.3 grams/denier, or 10.6 grams.  Stress at rupture is
thus determined as follows:

                           Stress % Force/area
Stress = 10.6 x 980/(14.3 x 10* )  n/4 = 6.5 x 10  dynes/cm  or about
9.5 x 104 psi.
     4.4.4  Simple and Plied Yarns
            Yarns  that are even in size,  have an equal number of turns
per inch throughout and are relatively smooth, are called simple yarns.
A simple, single yarn is the simplest assemblage of  fibers suitable for
operations such as weaving and knitting.   These yarns may be made from
any of the fibers  and by any of the basic systems.
            A simple-ply yarn  is composed of two or more simple-single
yarns plied or twisted together.   In naming a ply yarn the number of
singles  used precedes the word "ply."  For example,  if two singles  are
used,  it would be  called 2 ply.  Typical  plied and cord yarn are shown in
Figure 4.4.
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  a-

                 4.5  FABRIC PRODUCTION
                      4.5.1  Weaving
                             The production of a flexible textile  fabric  for use  in  filtra-
                 tion systems involves  weaving;  rarely are completely non-woven materials
                 such as paper used. Most felts used in filtration are first woven, then
                 given further treatment.   Figure 4.5 shows the major weaves used in non-
                 felted filter fabrics.  Woven fabrics are formed  by (interlacing  yarns at
                 right angles on a loom, after which the raw or "greige"  fabric may  be
                 further treated.   While there are many patterns of interlacing,  the fabrics
                 in most common useage  in  gaseous filtration are classed  generally as twill
                 and sateen (satin). Plain weave fabric is also sometimes used.   The engi-
                 neering technology of  these and other weaves is discussed in detail else-
                      16                                          '         8
                 where   and the following quotation from Potter and Corbman  summarizes
                 the fabric production  process:
                   (a) Photograph of Cord,  Composed
                       of Four Plies, Each  Ply Com-
                       posed of 7 Single Yarns.
(b)  Four Ply Yarn,  Slightly Mag-
     nified,  Showing Fibers Sep-
     arated in One of the  Single
     Yarns Forming the Ply.
.
                          Figure 4.4.  Plied Yarns.  (From Joseph, Ref. 14),
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               (a)   Plain Weave,  Showing Loose or Open Construction
                    and Close Construction, Such as Muslin.
                                       (b)  Twill Weave.  "This drawing shows
                                            a three-shaft twill-two warp yarns
                                            in each repeat.  This is a right-hand,
                                            filling-faced twill because the
                                            diagonal moves from the upper right
                                            down to the lower left, and more
                                            filling than warp appears on the
                                            face of the fabric.  It is also re-
                                            ferred to as  a one up and two down
                                            twill (1) because the warp goes over
                                            one and under two filling yarns.""
(i)  Warp Floats are
    Seen Interlacing
    Every Eighth Filling.
(ii)  Warp Floats  are
     Seen Interlacing
     Every Fifth  Filling.
(iii)  Filling Floats
      are Seen Interlacing
      Every Fifth Warp.
    (c)  Satin Weave,  Showing (i & ii )  Warp-Face and (iii)  Filling-Face
        Construction.
      Figure 4.5.   Weaving Styles for Filtration Fabrics.  (From Ref.  8).
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 PREPARATION FOR WEAVING
 "In the  weaving operation,  the  lengthwise  yarns  that
 form the basic  structure  o£  the fabric  are called  the
 warp.  The  crosswise  yarns  are  the  filling,  also re-
 ferred to as the weft or  the woof.   The filling  yarns
 undergo  little  strain in  the weaving process.  In  pre-
 paring them for weaving,  it  is  necessary only  to spin
 them to  the desired size  and give   them the  amount  of
 twist required  for  the type  of  fabric  for  which  they
 will be  used.

 Yarns intended  for  the warp  must  pass  through  such
 operations  as spooling, warping,  and slashing  to pre-
 pare them to withstand the  strain of the weaving
 process.  These operations  do not improve  the  quality
 of  the yarn.  In spooling,  the  yarn is  wound on  larger
 spools,  or  cones, which are  placed  on  a rack called a
 creel.   From this rack, the  yarns are wound  on a warp
 beam, which is  similar to a  huge  spool.  An  uninter-
 rupted length of hundreds of warp yarns  results, all
 lying parallel  to one another.  These  yarns  are  un-
 wound to be put through a starch bath  called slashing,
 or  sizing.   The slasher machine covers  every yarn with
 a starch coating to prevent  chafing or  breaking  during
 the  weaving process.   The sized yarns  are  passed over
 large steam-heated copper cylinders  that remove  the
 moisture  and set the  size.   They are then  wound  on  a
 final warp  beam and are ready for the  loom.

 TWILL WEAVE
 A distinct  design in  the  form of diagonals  is  charac-
 teristic  of the second basic weave,  called the twill.
 Changes  in  the  direction of  the diagonal lines pro-
 duce variations,  such as the herringbone,  corkscrew,
 entwining,  and  fancy  twills	The  values of  the
 twill weave include its strength and drapability.
 The  diagonally  arranged interlacings of  the  warp
 and  filling provide greater  pliability  and resili-
 ence than the plain weave.   Also, twill  fabrics  are
 frequently  more  tightly woven and will  not get dirty
 as  quickly  as the plain weave,  though twills are
more difficult  to clean when they do get soiled.
The yarns" are usually  closely battened, making an
especially  durable fabric.   Twill weaves are there-
 fore commonly used	where strong  construction
 is essential.

 In the twill weave,  the filling yarn interlaces
more than one warp yarn but never more than  four,
as strength would be  sacrificed by so doing.  On
each successive  line,   or pick,   the filling yarn
moves the design one  step to the right or to the
left, thus  forming the diagonal.  Whichever  the
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 direction of the diagonal on the face of the
 fabric,  the design runs in the opposite direction
 on the reverse side.

 When the direction of the diagonal starts from the .
 uppcT left-hand side  of the fabric and moves down
 toward the lower right, it is called a left-hand
 twill.  When the direction of the diagonal starts
 from the upper right-hand side of the fabric and
 moves down toward the lower left, it is called a
 right-hand twill.  Although there is no advantage
 of one over the other,  the direction of the diagonal
 can aid  in the recognition of the face of the fabric.

 The steepness  of the  diagonal can indicate strength
 and durability in the fabric.  In order to obtain
 a  steep  twill, more warp yarns must be used than
 filling  yarns.  And since warp yarns have a higher
 twist and are  stronger  than filling yarns, the
 steeper  the twist the stronger the fabric is
 likely to be.

 Twill weaves are named  according to the number of
 harnesses required to make the design.   A three-
 of four-harness twill is frequently used.   The
 word "shaft" may be substituted for "harness,"
 as in three-shaft or  four-shaft twill.

 Twill weaves are also classified as even or
 uneven according to the number of warp  and fil-
 ling yarns  that are visible  on the  face of the
 fabric.   The even twill,  for example,  shows  an
 equal number of warp  and filling yarns  in the re-
 curring  design,  such  as two  over and two under.
 This  pattern makes  what is called a four-shaft
 twill, and  it  requires  four  harnesses.

Most  twill  weaves  are uneven.   An uneven twill
may  show more  warp  than filling yarns  in the re-
 curring  design;  this  is  called  a warp-face  twill.
 If more  filling  yarns than warp  yarns show on
the  face, the  weave is  called  a  filling-face  twill.
Warp-face twills are generally  stronger  than fil-
 ling  face twills because the  stronger warp yarns
on the face  of  the  fabric can  take more  abrasion
and wear.  Warp-face twills  generally have much
more  warp than filling  yarns;  consequently,  such
 fabrics  hold their  shape better  and  drape better
due to the warp's greater twist  and  resilience.
Twills are  described  in  terms  of  the interlacing
of the warp  yarns over  and under  the filling  yarns.
An uneven four-shaft  twill,  for  example,  that  has
three warp  yarns riding  over  one  filling yarn  is
referred to  as  a three up and one down,  or   ly.
On the other hand,  a  three-shaft  twill  that  has
one warp yarn  riding over two  filling yarns  is
referred to  as  a one up  and  two  down, or  "z-

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SATIN WEAVE
In the basic construction, the satin weave  is
similar  to the  twill weave but generally uses
from five to as many as twelve harnesses, pro-
ducing a five-  to twelve-shaft construction.  It
differs  in appearance  from the twill weave  be-
cause the diagonal of  the satin weave is not
visible; it is  purposely interrupted in order to
contribute to the flat, smooth, lustrous surface
desired.  There is no  visible design on the face
of the fabric because  the yarns that are to be
thrown to the surface  are greater in number and
finer in count  than the yarns that form the reverse
of the fabric.  The satin weave may have a warp-
or filling-face construction.
Warp-Face Satin Weave.  Warp-face satin is woven
so that the warp may be seen on the surface of
the fabric.  For example, in a five-shaft con-
struction, the warp may pass over four filling
yarns and under one; in a twelve-shaft construc-
tion, the warp may pass over eleven filling yarns
and under one.  Since  the warp lies on the sur-
face and interlaces only one filling at a time,
the lengths of warp between the filling are called
floats.  These  floats  lie compactly on the surface
with very little interruption from the yarns going
at right angles to them.  Reflection of light on
the floats gives satin fabric its primary charac-
teristic of luster,  which appears in the direction
of the warp.

The long floats found  in the satin weave might be
considered a disadvantage because they represent
a minimum of interlacings, and therefore a poten-
tial weakness in the fabric.   Furthermore, to
increase the smoothness and luster of the fabric,
the yarns are given a minimum of twist and are
therefore relatively weak.  The longer the float,
the greater the chance that the surface of the
fabric will snag, roughen, and show signs of
wear	
Satin-weave fabrics  drape well because the weave
is heavier than the  twill weave,  which, in turn,
is heavier than the  plain weave.   More harnesses
are used for satin weave, thus compressing a
greater amount of fine yarn into a given space
of cloth.  This compactness gives the fabric more
body as well as less porosity	
Filling-Face Satin Weave.  The filling-face
satin weave is also  called the sateen weave;
however,  this sometimes causes confusion because
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some  cotton  and  rayon  fabrics  are  also  identified
a.s .sali'c'ii.   In thi.s  construction,  the-  Tilling
yarn  lies on the  surface of  the  fabric  as  it
pauses  regularly  over  and under  the warp yarns.
For instance, a  filling yarn may pass over four
warp  yarns and under one.  The floats are  con-
sequently made up of the filling yarns, and the
luster  appears in the  filling  direction.

"On the conventional loom, the warp yarns
that  are to  run  lengthwise in  the  fabric are
wound on a cylinder  called the warp beam,  which
is at the back of the  loom.  The warp also ex-
tends to a cylinder  called the cloth beam, which
is at the front of the loom  and  on which the
fabric  is rolled  as  it is constructed.  Sup-
ported  on the loom frame between these  two
cylinders, the warp  yarns are ready to  be  inter-
laced by the  filling yarns that  run in  the width
of the  cloth, thus producing the woven  fabric.

In any  type  of weaving, four operations are
fundamental.  They are performed in sequence
and are constantly repeated	The essential
parts of the  loom are:  warp beam, cloth beam,
harness or heddle frame, shuttle, and reed.  These
parts perform the following operations.

        Shedding—raising warp yarns by
        means of the harness or heddle
        frame
        Picking—inserting filling
        yarns by means of the shuttle
        Battening—pushing filling
        yarns firmly in place by
        means of  the reed

        Taking up and letting off-
        winding the  finished fabric
        on the cloth beam and re-
        leasing more of the warp
        from  the warp beam.

CLASSIFICATION OF WEAVES
The manner in which groups of warp yarns are
raised by the harnesses to permit the inser-
tion  of the  filling yarn determines the pattern
of the weave, and in large measure the  kind of
fabric  produced.  Weave patterns can create var-
ying  degrees of durability in fabrics,  adding
to their usefulness and also to their appear-
ance.    In a simple weave construction,  consisting
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            of the filling going under one warp and over the
            next, two harnesses are needed:  one to lift the
            odd-numbered warp yarns, and a second to lift the
            even-numbered warp yarns.  More than two harnesses
            are required for advanced weaves, and as many as
            forty for figured weaves	

            PLAIN WEAVE
            The plain weave is sometimes referred to as the
            tabby, home-spun, or taffeta weave.  It is the
            simplest type of construction and is consequently
            inexpensive to produce.  On the loom, the plain
            weave requires only two harnesses.  Each filling
            yarn goes alternately under and over the warp yarns
            across the width of the fabric.  On its return, the
            yarn alternates the pattern of interlacing.  If
            the yarns are close together, the plain weave has
            a high thread count, and the fabric is therefore
            firm and will wear well."8

     4.5.2  Felting and Needle Punching

            The felts used in fabric filtration in their early stages of

production are also woven, but subsequent steps completely change the

character of the material from that of a woven fabric.   While felts can

be made simply by matting fibers together and by other non-woven

methods  (see below), the use of a woven base fabric called a scrim greatly

increases the strength and stability of the fabric.

            The production of a felt depends on whether its fibers are
naturally binding.  Because woolen fibers are scaley, and also shrink
when exposed to heat and moisture, a woolen scrim shrinks when mechani-

cally worked in warm water in the presence of certain lubricants and

chemicals.  The identity of the separate yarns tends to be replaced by

a more homogeneous character.  The material becomes felt-like in density,

stiffer and thicker.  Minor amounts of synthetic fibers added to the

woolen yarns can modify the properties of the felt thus produced.

            To further increase the homogeneous character of the felt
surface, it may be napped.  This has always required the use of teasels,
woody, thistle-like parts for a weed plant, the barbs of which pluck
fibers from the surface of the felt.  When enough nap has been raised in
this way, it may be singed or otherwise trimmed to the desired thickness.
-------
             Need It1  punching is  a method of combining two or more layers          —'
 of liber  into a  1'elt-likc fabric.   Usually one layer is a scrim for
 strength,  while  the  other(s)  may consist of fibers of almost any descrip-        _
 tion or combination.   Thus considerable control over separate properties
 of the  finished  material is possible.   For example,  the scrim may con-
 tribute the  desired  dimensional stability while the  top layer'contributes
 the  ideal  properties  for dust control.   The surface  layer might be 100%
 dacron, the  scrim 1007o nylon, for example.
             The  technique used  in needle punching is to prepare a scrim            '
 and  a batting separately.  The batting may be formed by carding, by air-
 lay  of  fibers, or by  other random web-forming equipment.   The batting
 is generally of  the  same order  of  weight or lighter  than the scrim.               •—'
 It is unrolled or otherwise spread over the scrim.   Needles having for-
 ward barbs are punched from the batting side into or through the scrim,          _j
 and  the batting  fibers thus laced  into  the  scrim remain behind  when the
 needles are  withdrawn.   Production is at the rate of about 10 FPM.
 Variations in the needling process include  needle angle,  number of
 repetitions,  two-sided  needling, etc.   When a  shrinkable  scrim  is used,
 the  needled  material may later  be  felted  in various ways  to  produce  a
 still more dense  and uniform  material.                  :
                                                                                  _j
             Non-woven  production methods  include  resin  bonding, wet  bond-
 ing  (paper-like materials), spun bonding  (while  the  fibers  are  still
 tacky from their  extrusion  stage), heat  bonding,  chemical  bonding, spray          "^
                                                                n  1 f
 bonding, and  stitch bonding (a  sort of  knitting within  a matt).  '
 Nearly all fibers used  in fabric filtration can be used  in non-woven              —'
 fabrics.  Because non-woven fabrics can  often  be  produced more  rapdily
 than by weaving,   it appears that filtration fabric or even  filter ele-            _,
ments might eventually be produced by non-woven methods.
     4.5.3  Fiber Additives, Yarn Treatment, and  Fabric Finishes                  _J
            Natural fibers are  produced with an outer molecular  film; cot-
 ton  cellulose fibers are  covered with a wax-like  adhesive; and wool  pro-          __
 tein fibers contain oils, fats, and waxes from glandular secretions dur-
 ing  growth.  Both may contain other agricultural  chemicals added to  pro-           '
 tect or preserve  the fibersto aid in the harvesting process, e.g., in-

-------
secticides, defoliants, etc.  Wool will have much of  the grease removed
in a washing, scouring or solvent extraction process.  Additional oil
(animal, vegetable, or mineral) may be provided as a  lubricant for  spin-
ning.  The production of fabrics from fibers and yarns Involves many
chemical, physical and mechanical processes.  ProporLlca of  the fiber,
yarn, and fabric are continually modified  through treatment  and additives
to meet the requirements of production machinery.
            Each additive or  treatment may result in  some residual  mater-
ial attached to individual fibers that ultimately affects its strength
in service or its ability to  act as a substrate for filtration.  The
potential combination of additives, treatment or finishes is nearly
limitless.  Prediction of fabric performance in service  (pressure drop,
efficiency, life) on a given  application is presently empirical through
service testing, and thus the science of fabric treatments for improved
performance is relatively undeveloped.  Furthermore the addition of
materials to improve filtration performance, particularly life in service
is often proprietary.  Many techniques are relatively new, and not  subject
to competitive or comparative testing procedures.   The entire field of
the role of finishes and additives in gas  filtration requires further
analysis before analytical generalizations can be produced for optimiza-
tion studies.
            Each of the generic man-made fiber materials  discussed above
is a complex chemical structure with varying physical properties and
surface characteristics.

            These polymeric materials frequently contain  one or more of
the following additives:
            •   Plasticizers reduce  flow viscosity or temperature
               in melt spinning the  resin,  such as  DOP in PVC;
               and are also employed to improve low temperature
               flexibility.
               Solvents  are used in  wet  spinning as  with  acrylic
               plastics  or rayons;  and in coating,  adhesives, etc.
            .   Organic peroxides are used as  polymerization  initia-
               tors  or for cross-linking reactions  in thermoplastic
               materials  to transform them into thermosets.
-------
               Antioxidants are added to reduce oxldative deter-
               ioration during manufacture, processing or storage,
               and to provide heat protection especially among
               vinyls.

These agents may bt; required in varying amounts.

               Flow-control agents may be required to control melt
               viscosity and transformation of the polymer to a
               stable form for end-use.
               Colorants, pigments, and delustrants include powered,
               colloidally dispersed or dissolved materials added to
               provide color or reduce brightness.

            ./Flame retardants are employed to provide fire protection
               for flammable polymers.

               Stabilizers are used to import thermal stability or
               mechanical protection during processing and to protect
               the mix against changes induced by other additives,
               e.g. by neutralization of contaminants, residues,
               or impurities.

               Ultraviolet absorbers are added to reduce UV absorption
               by the polymer or to quench molecular reactions, in
               order to limit physical degradation of the plastic ex-
               posed to sunlight.
            .  Antistatic agents may be applied as a coating to ex-
               ternal fiber surfaces or added internally; the agent
               acts as a hygroscopic material to assist the charge to
               leak away (make the fiber electrically conductive) and
               may modify the charging process, reverse the sign of
               the charge, or promote dissociation of ionic material
               present on the fiber.
               Other additives, filters, and processing aids may be
               employed as viscosity depressants, parting agents,
               emulsifiers, coupling agents, internal or external
               lubricants, or adherents.

            Many compounds ,are used in varying amounts to assist in manu-
facturing and processing operations and to provide stability,  compatibility,

and other desirable end-use properties.   Since filter fiber is a small

proportion of the total annual market for man-made fiber textiles, mater-
ials used in production and processing are developed,  produced, and used

primarily for the larger market applications,  rather than for fabric fil-

tration objectives.   Each producer's fiber,  while generically the same
-------
(i.e., same basic polymer), will be produced differently and will have
different additives which may affect dust particle-fiber attachment, de-
posit formation, and stability.  The reduction of these phenomena to
engineering design parameters in filtration will be difficult and complex.
            The yarn may be treated as well as the fiber.  Yarn treatments
now include surface addition of lubricants, antistatic agents, and var-
ious mechanical operations such as attenuation or stretching, heat set-
ting, and bulking or texturizing.  Warp yarns are subjected to greater
strain during weaving and may be sized or coated to prevent chafing and
breakage.
            Fabric finishing includes those processes to improve appear-
ance or serviceability of the fabric after leaving the textile machine
(greige  goods).  Cotton and wool fabrics are usually cleaned  (washed or
scoured) and bleached, and may be chemically treated to provide water-
proofing, mothproofing, mildewproofing, or fireproofing.  Mechanical •
finishes applied to cotton and wool include singeing, napping, shearing,
felting, or shrinking.  Synthetic fabrics are usually heat set to re-
lax internal stresses in the yarns set up during weaving.  This causes
shrinkage, although it enhances dimensional stability for subsequent
exposures to temperatures below the setting temperature.  Additional mois-
ture protection (water repellents) and antistatic agents may be applied.
            Glass fiber fabrics are subjected to high temperatures
(~10  F) to relieve yarn twisting and weaving stresses and to heat clean
the weaving size.  Glass fabrics are usually lubricated with silicones,
graphite and other proprietary finish agents to reduce fiber-fiber abra-
sion resistance during filter cleaning.  Graphite finishes of the order
of 5% by weight have been indicated to provide extended service life of
glass fiber fabrics at temperatures < 500 F, as illustrated in Figure 4.6.
Largely as a result of improved finishes, the use of glass fiber fabrics
for applications at temperature > 275 F has increased greatly.  Competi-
tive fabrics now include NomexR (< 425°F) TeflonR (< 450°F),  and newer
                         R           R
man-made fibers (Brunsmet , Fiberfrax , and Polyimides).  Improved lubri-
cants and finishes for glass fiber fabrics to provide longer life at
higher temperatures (> 600 F) are under continuing development.
-------








M
*
VI
Ol
u
(0
3
T3
C
W




800

700


600


500



400



300


200
100
1 1 i i i
O Legend:
\ & - Commercial ulieone fim*
~ 1 O - Commercial silicon* plut
\ tjrjphite finish
\
\
\
.\
\
\
\
\
\
\

\
0\
\ 0
*v^ 1 — "1^2* ~
i i i i i
3.5

3.0


2.5


2.0



1.5



1.0


0.5
0.0
                                                               CO
                                                               01
                                                               X
                                                               01
                                                               CO
                                                               C
                                                               O
                   200
300  400  500  600   700  800
            Figure 4.6.   Effect of Graphite on Glass Fabric Tempera
                         ture Endurance.  (From Ref.  18).
4.6  FABRIC PHYSICAL CHARACTERISTICS
     4.b.l  Abrasion Wear
            The mechanical abrasion of fabric is only one of several kinds
of wear, other kinds including chemical and thermal degradation, handling,
etc.  As Table 4.16 indicates, a fabric could conceivably experience many
kinds of wear at one time or another during its lifetime of manufacturing
and use. Most of these types of wear are terms from popular usage and con-
sequently are ill defined.  For present purposes the term "abrasion" will
be defined and used as an eroding away of fabric fibers or fiber surface
material, through moving contact between (a) the fiber and dusjt particles
or (b) adjacent fibers.
-------
                                TABLE 4.16
                 SOME  TYPES OF MECHANICAL WEAR IN FABRICS
                 Fatigue                    Migration
                 Sliding                    Slippage
                 Rubbing                    Breaking
                 Galling                    Cutting
                 Scoring                    Grinding
                 Snagging                   Plowing
                 Picking ;                   Penetrating
                 Flexing :                   Rupturing
                Welding                    Shearing
                Compression                Catching
                Plucking                   Bending
                Fretting                   Polishing
                Fracture                   Chaffing

            Material science describes the forms of abrasion as either two-
body or three-body abrasion.  Fiber-to-fiber rubbing or fiber-to-particle
collision (sand blasting) are two-body systems.  Interstitial dust particles,
caught and chaffing under pressure between adjacent fibers or yarns, repre-
sents three-body abrasion.  The mechanics of the two processes are somewhat
related, however, and involve the relative sizes of the bodies, the hard-
nesses or relative hardnesses, the pressure between the bodies, the coef-
ficient of friction and the nature of the motion between them.  The laws
of material abrasion are nevertheless only partially determined because of
other effects which are more difficult to evaluate such as humidity, rate
of oxidation of freshly exposed surfaces, smoothnesses of particles, any
accumulation of freshly abraded particles, etc.
            The laws of material abrasion are undeveloped, and fabrics,
being of complex geometry and having extremely varying stresses and strains,
are especially difficult to analyze.   Neither can fabrics be analyzed in
detail for abrasion during their fabric filtration performance, as abrasion
-------
 hi  usually reported  simply as "holes" in the filter element (Section 8.3).
 Kv«-n  tliouKh  i-t  appears  that abrasion represents up to 25% of fabric failures
 llu-rc- JM insufficient data relating to particle hardness, coefficient of
 I rl.(-tlon, ft cetera, to establish the rules of filtration fabric abrasion.
             Manufacturers of filter fabrics have little technology for
 the abrasion resistance of their fabrics, although they do measure the
 resistance of fabrics to abrasion by standard testing procedures.   Typi-
 cally a swatch of the fabric is rubbed back and forth over a rough surface,
 sometimes in the presence of grit,until the fabric becomes perforated.  The
 fabric abrasion resistance is rated according to the time required for per-
          19
 foration.    Other methods include sandblasting, and flexure or tension
                20
 during rubbing.     Thus it appears that fabric manufacturers as well as
 users rely on an evaluation of the destruction of the fabric without attemp-
 ting  to determine the basic cause or causes of failure.  Considerably more
                                                21 22              23
 analytical approaches have been taken by Backer  '   and Hamburger  , as
                    10 24           16
 reviewed by Kaswell   '    and Hearle
       4.6.1.1   Sandblast or Surface Scour.- Particles entering a fabric col-
  lector  filter  element  are apt  to strike the fabric surface with sufficient
 velocity to abrade  and remove  portions of fibers,  eventually perforating the
  fabric.   This  is  analogous to  the process called sandblasting, and although
  sandblasting  involves  perpendicular impact while the fabric is struck at a
                                                          25
  shallow angle,  the  mechanics should be related.   Raleigh   studied perpen-
 dicular sandblasting.   He assumed that the rate of wear is related to the
 maximum strain  produced in a surface by an impact.   This strain is propor-
  tional  to
                                 -. V N/'jJ
                                      Y
  where V is  the  impact  velocity,  p the density and  Y the tensile modules
  of the  impacted material.   To  the extent  that this  strain  exceeds  the
  tensile strain  limit of the material,  bits of the  material  will chip  out.
*
  Another test that is frequently used is to abrade the fabric against it-
  self with a rolling crease.  Cycles to failure are an indication of re-
  sistance to flexure and two-body abrasion.
-------
 KvidenLly  this applies better  to hard, brittle materials than to plastic
 ones, where,  the mechanism  is more probably plowing than chipping.
     Zcnz  and Othmer   reported a general agreement that 90  sandblast
 crrosion is  proportional to
                                 -LV3

 where L is sand loading or concentration, although the exponent was  reported
 to vary from study  to study between  3 and 6.  The exponent of 3 can  be  shown
 to be consistent with the  assumptions that wear  rate  (depth/time)  is propor-
 tional to  both the  particle impact rate  (number/time) and the average parti-
 cle kinetic  energy.  In the above case it is assumed  that the mechanism is
 plastic plowing, rather than brittle chipping.
     Fabric  abrasion using sandblast equipment has been studied experimen-
              27
 tally.  Parker   found with approximately 60 mesh granules that harder  abra-
 sive (circa  silicon carbide hardnesses) and higher blast pressures shortened
 fabric life; for example:

                  Pressure (psi):    30    40    50    60
                  Life (seconds):   100    54    35    23
Unfortunately Parker did not state the rate of sand flow which may or
may not have varied with blast pressure.
     Generally Parker found the rate of removal of fiber was constant over
the life of the fabric, although it slightly accelerated during the life of
                   2                   24
heavier (8-12 oz/yd ) fabrics.  Kaswell   also indicated an approximately
linear relationship between fabric strength loss and the time of operation
of the abrasion equipment.
     Types of surface abrasion test machines other than sandblast are more
widely accepted, and their data may be related to filter element scouring.
       24
Kaswell   reviewed these methods and a large number of studies of fabric
surface abrasion made chiefly by the garment industry.  He credits Backer
             22
and Tanenhaus   with an examination of fourteen fabric properties that re-
late to abrasion resistance:
-------
     Geometric Aspects
        1. Area o£ contact between  fabric and abradant
        2. Local pressures developing on specific yarn points
        3. Threads per inch
        A. Knuckle height
        5. Yarn size
        6. Fabric thickness
        7. Yarn crimp
        8. Float (knuckle) length
        9. Yarn conesiveness
       10. Congressional resilience
       11. Fabric tightness
       12. Cover factor
     Abrasion Aspects
        1. Direction of abrasion
        2. Magnitude and direction of tensions developed during abrasion
     In conclusion it appears that fabric surface scouring is related to
dust loading and impact velocity, and to particle sharpness and hardness.
Fabric designed with a dense surface (level; few knuckles) should retain
its strength longer. Nap, pile or sacrificial surface fibers will extend
the life of the fabric   A soft fiber material, flexible fibers, and re-
silient yarn and fabric will also reduce the rate of surface fiber degradation.

     4.6.1.2  Internal Chaffing.- The rubbing together of two fibers as the
fabric is flexed is also two-body abrasion.   Glass fibers are notoriously
weak in this respect.  Originally glass fabric had almost no durability;
crossed glass fibers apparently became nicked due to chaffing or due to
dissolution at the point of contact and then snapped easily.   However when
silicone and other lubricants were added, their life extended almost in-
definitely in comparison.
     When a yarn is bent its elementary fibers slip past one another, un-
less they are bonded together or under a large lateral pressure from twist-
ing or weaving.   In the latter cases the yarn is beam-like in rigidity and
liable to rupture in tensile failure if bent.   More normally,  fibers slip
to some extent as the yarn is bent; and also adjacent yarns tend to slip
as the fabric is flexed.   Thus the shaking or collapse cleaning of a filter
fabric is associated with  myriad tensile stresses and fiber slippages with
chaffing.
-------
     To a first approximation, the strain associated with a bending fabric
is  (ft = •« R) where t is the fabric thickness and R the radius of bend.
Also approximately, the amount of slippage (distance) between fibers of
diameter D  in a yarn undergoing bending strain 6 Is 1 ~ £D. , or
                                J ~ ^f                            (4.1)
                                     R
A precept of two-body abrasion is that the volume of material removed during
                          28
a rubbing of distance 1 is
                                        ;
where k is a wear, coefficient, L is the: loading between bodies, and H the
hardness of the body being abraded,  (v and L may alternatively be expressed
per unit area).  It appears that k depends on several aspects of the materials
and their geometry, but is frequently of order unity.
               29
     Rabinowicz   discusses four common types of two-body wear, any of which
may apply during the fabric deformations active in dust collectors:

             •  Adhesive wear.   One  surface  adheres  so strongly  to
               a  second because of  atomic bonds at  their contact
               that  on sliding, a portion of  one  is  torn loose.
             •  Gouging wear. A proturberance  on one  surface
               plows a groove  in the second.   It  appears that k
               (wear coefficient) may be of the order of the
                                           28
               protuberance size in microns.
             .  Corrosive wear.  Corrosive films that  normally tend
               to resist further corrosion  may wear  away, exposing
               fresh surface to corrosive attack.
             •  Fatigue wear. Tensile stresses  trailing the sliding
               body  may crack  the supporting  surface,  leading to
               chipping of fragments from the  supporting surface.
     It appears from these brief reviews that chaffing inside  the fabric
may be reduced by minimizing  the:
-------
             .  number  of fabric deformations (cleaning cycles)
             .  degree  (radius)  of deformation (cleaning intensity)
             •  tightness of the fabric  weave
             •  softness  of the  fiber  material
             •  fabric  thickness
             .  fiber diameter
             •  fiber roughness
             •  interfiber friction coefficient.
      Table  4.17  lists frictional properties for  several  fibers.  These  affect
 both  the  rate  of wear between  fibers and  the stiffness of  the yarns,  to be
 discussed below.
      4.6.1.3   Internal  Abrasion.- Many dust particles are  abrasive  in con-
 tact  with the  fibers  ordinarily used in filtration  fabrics.  Since  most
 industrial  dusts and  fumes include some particle of sizes  substantially
 smaller than the fiber  diameters (10-30um)  there is great  likelihood  of
 penetration between adjacent yarns and adjacent fibers.  Here the fine
 particles may  be trapped  by interfiber pressures, and as the fabric flexes,
 they  cut  and gouge the  fibers.   This is three-body  abrasive wear.   This
 wear  also follows Eqn. .(2) as  far as can be  determined, except that k may
                                                       28
 be an order of magnitude  smaller  than  for two-body wear
      The  evidence for wear of  a  fibrous structure due to imbedded grit  is
 scant, despite the claims  of carpet  cleaning services and the manufacturers
 of laundry detergents.  Soiling  of carpets and garments has received much
                               24
 attention according to Kaswell    but primarily from the standpoint of color
 changes rather than mechanical degradation.  Apparently dust filtration
 fabrics have not been tested for  degradation due to  imbedded particles.  While
 reports from filter operation  indicate a general weakening of the fabric
with  time, thermal degradation,  surface scouring and other mechanisms are
more  readily shown to be the causes than imbedded particulates.
     4.6.2  Flexibility
            The flexibility of a filtration fabric is important  for  at
least two reasons:  Removal of the dust deposit may be improved by flexing
-------
I
Ln
U)
                                                              TABLE 4.17
                                                   FIBER FRICTIONAL PROPERTIES*
THE COEFFICIENTS OF FRICTION OF VARIOUS AIR-DRY TEXTILE
FIBERS AND FILAMENTS IN COMMERCIAL CONDITION
Rubbing Surfaces

Nylon/nylon
Very fine
6 denier
27 denier
Bristle
Viscose rayon/viscose
rayon
Acetate rayon/acetate
rayon
Cotton/cotton
--from sewing thread
--from cotton wool
Wool/wool
from tops

commercially scoured

coarse, "clean"

Coefficient of
a
Friction


0.14
0.15
0.23
0.6 (0.5-0.8)

0.19

0.29

0.29
0.57

u2 = 0.38
Hi = 0.24
u2 = 0.49
Ul = 0.20
u2 = 0.42
ul =0.25
Fiber Daimeter(u)


18
28
62
500 approx.

30 (variable)

41 (variable)


18 (variable)

20

18

26

STATIC COEFFICIENTS OF FRICTION OF FIBERS
(Average Values)
Wool
With-scale
Anti-scale
Wool in water
With-scale
Anti-scale
Cotton
Jute
Viscose rayon .
Acetate
Nylon
Saran
Terylene

Steel









	 0.11
	 0.14

	 0.15
	 0.32
	 0.22
	 0.46
	 0.43
	 0.56
	 0.47
	 0.55
	 C.58

. 0.29









           In  the  case  of wool  (il  is  the  coefficient  of  friction in the direction of the scales (root-to-tip)
           and n2  is the coefficient  of friction in the direction against the scales (tip-to-root).


-------
of the fabric substrate and conversely, such flexure may cause  i'abric  de-
gradation as noted above.  Thus, fabric flexibility may be both necessary
and harmful.  Since the. role of flexibility in fabric filtration  is not
well established, only the principles of fabric flexibility are stated here.
A single fiber flexes beam-like according to the well-established

                                                       (4.3)
        24                          •>         /
relation                          ,3         4
                                        _     _
                             y ~ 48YI '    ~ 64
where t,  is the span length and y is the center span deflection under
       s
center load W.  Y is the elastic modulus and  I the moment of inertia of
the fiber which as expressed above is specifically for a circular fiber.
            A parallel group of N fibers with zero friction between them
would deflect 1/N as much, under the same  load conditions as the single
fiber.  On tho other hand with high friction the bundle would deflect as
                                                            2
a  single beam, and the deflection would be approximately 1/N  as much as
ior the single fiber.  A yarn of parallel  fibers, having intermediate
                                           2
friction, would deflect between 1/N and 1/N   as much as given by Eqn. 4
for only one fiber.
            A twisted yarn deflects somewhat more easily, but the exact
expression for deflection is highly involved.  The amount of inter-fiber
friction in a yarn is proportional to the  coefficient of friction and to
the pressure between fibers.  This pressure results from' twisting, and it:
usually changes with bending.  One simple measure of the amount of friction
in a yarn is the amount of permanent set it acquires when bent; a perfectly
frictionless yarn would normally recover elastical-ly.
            When woven, each yarn contributes its own stiffness to the
fabric.   The yarn stiffness will generally be increased however by in-
creased inter-fiber pressures from bending the yarns and packing them
together.   Thus a section of fabric N' yarns wide will generally be more
-------
 than N'  time as  stiff as N' single yarns.  Extenuating factors in calcu-
 lating fabric stiffness are slippage vs. friction between the yarns, the
 thickness of the fabric, and the position and the straightness of yarn
 through  the fabric.
            Fabric flexibility as a property of the fabric can be designed
 via any  of the above factors.  Other design variations include texturized
 (teased) yarns,  lubricants or bonding agents, fibers of several diameters
 or lengths, and a variety of finishing treatments including napping and
 calendering.  Many fabrics are designed to be more flexible in one direc-
 tion than in the other.   This opens the possibility in filtration fabrics
 of using relatively delicate fibers or yarns in one direction for collecting
 the dust deposit, and using more flexibly durable yarn in the other direc-
 tion for removing the deposit.
            Flexing of filtration fabric is of necessity associated with
 interfiber tensions and frictional chaffing.   Any grit present may cut
 the fibers during chaffing (Section b).  In addition all fibers undergo
 some molecular fatigue wj Li. repeated flexing.
     4.6.3  Strength
            Although fabric breaking strength is not usually an important
 parameter in filtration, per se, it is frequently specified because it  is
one check on the quality of yarns and fibers in the fabric.  Strength is
also one indication of the aging a fabric has undergone.  Filtration fabric
 rarely tears in use unless the filter element has been sewn in such a way as
 to concentrate the tension, i.e. at a cuff seam.  Tensile modulus is more
closely related to the performance of the fabric since it determines the
distribution of tension over the filter element and thus the distribution
of cleaning energy in mechanical cleaning.

            Fabric strength is related to fiber strength or denier, defined
 as the weight in grams of 9000 meters.   Appendix 4.3 lists the denier
 strengths (i.e.  weights) and other fiber mechanical properties of a variety
of fibers.  Strengths and moduli are also indicated in Figure 4.7.
-------
                                Tvpicul Mlri'MH Mlriiin curve  for n  textile liber. (Dillmi,
                              .1  II.. Inil.  I.UK  Clinn  44: 21 IT),  lil.V.M
                                                                              I  FiMrflM
                                                                              t  Collo* SO/1
                                                                              1  Ctlwctl
                                                                              4  H T
                                                                              5  Canon it/I
                                                                                Sow Mon»n N
                                                                             20  V««wi NOMU IWIiflMlMM
                                                                             ti
                                                                             (2  own.
                                                                             25  C«M Report No. M, Office of the QoarUrmMUr
General.
  The immediate elastic recovery is determined by extrapolating to sero load the initial straight-line portion of the recovery
                                                                        lieally defined M: 100 -  (ii
  Delayed recovery is the recovery which take* place slowly and is ma
ivcovery (%) - permanent aet (%)).
  Permanent a*t ia mtaeursH after retnoving the •train and allowing the sample to recover for
                       Figure  4.7.  Stress  Strain Curves  for  Fibers*

 * From Harris,   Kef.  4
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             'flu- strength of a yarn can obviously not be greater than the
 summed strength of its fibers and actually the yarn strength is practi-
 cally always less.  This is for two reasons: first, the central-most fibers
 in the yarn are nearly straight and tighten first; hence they break before
 the outer spiraling fibers reacli full load.  Second, part of the tension
 in the outer fibers goes into pressure directed into the yarn rather than
 into longitudinal tension; hence a tightly twisted yarn may be a very weak
 one in tension.  In the-case of staple fibers which must be twisted to ob-
 tain yarn strength, there is a twist giving maximum yarn strength; this
 maximum depends on fiber properties number of fibers, etc.
             If a typical fiber in the yarn is examined, it will lie at a
 twist angle  to the yarn axis as depicted in the following sketch :

 The stresses  (cr) in yarn  (y) and fiber  (f) are related by
                                        2
                             ,)  = n   cos  •!>                           (4. A)
Thus the strength of the fiber contributes less strength to the yarn.
Fiber and yarn strains are also related by a similar but more complex
relationship, since the yarn has a widely ranging fiber packing depending
                                  *
on fiber geometry within the yarn.   Consequently even though the fiber
tensile modulus is determined (See Figure 4.7 and Appendix 4.3), the ex-
pression for the yarn's tensile modulus, that is, the ratio of yarn stress
to yarn strain, is not algebraically simple.
            The process of weaving further modifies the contributions of
yarn strength and tensile modulus to the fabric.  The yarn is generally
weakened by being bent around orthogonal yarns, depending on its twist and
the sharpness of the bend.   The tensile modulus of the fabric is generally
increased by the weaving process over that of the unwoven yarns, depending
* Loose twisted yarns may elongate considerably before the fibers become
  tightened, in analogy to the members of a folding gate. Small elastic
  strains in homogeneous compressible media can be described by Poisson's
  ratio, but this approach is misleading in fibrous structure applications
  where the geometric deformations are often large.

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 on  the  tightness  of  the weave.  Consequently  the  fabric  is  less  resilient  in
 tension than  a  parallel assemblage  of  the  same number of fibers, that is,  it
 absorbs  less  strain  energy before rupture.  Kaswell   cites the  importance of a
 fabric's elongation-recovery properties  in numerous  industrial  applications.
            Strength and modulus arc further complicated by dependencies on
 humidity, mechanical fatigue, and of course the manufacturer's processing
 and finishing variables.  Backer and other at MIT and elsewhere  '   have
 done much to relate  fabric and fiber physical properties, and much of this
 developing science may  be useful in obtaining improved filtration fabrics.
     4.6.4  Permeability
            The resistance to flow of gases through porous materials has
 been discussed  in Chapter 2.  The permeability of filtration fabrics is so
 decreased by the residual dust deposit that the permeability of the clean
 fabric appears  to have little to do with its use.
            The objective in fabric design is to maintain a highly permeable
 residual dust and fabric combination, while yet passing a minimum amount of
 dust.  Toward this end the pores through a fabric  must be closely controlled.
 They must not exceed a certain bridging diameter.   (Section 2.3.2).  If the
 pores are too small they will cither plug or pass  too little gas, which dis-
 tributes itself according to the square of the pore diameter.   In an ideal
 filtration fabric probably all the fabric pores should be the same size,
 the size depending on dust properties, etc.
            Pore size and thus fabric permeability are dependent on cloth
                          24
design structure.  Kaswell   indicates reductions  in permeability with in-
creasing pick count and also with pick diameter as would be expected with
 the decreasing pore sizes.  Filling twist was found to have a greater effect
on fabric porosity and permeability for a given fabric than any other con-
structional variable; the tighter the filling yarn the more permeable the
 fabric.   This would be expected for any fabric passing more air between than
through the yarns.
            Pore size must not be visualized as  simply the  distance between
crossed cylinders, however.   For all but  filament  yarns  there  will  be nume-
-------
 rous  Cibor  ends  protruding  into the pores, and  in the extreme ease  (napped
 or  felted materials)  the pores will be primarily between fibers rather than
 between yarns.   In  such cases the uniformity of spacing is equally  important,
 but the means of achieving  it are less dependent on weave.  Yarn texturizing
 and post-weave surfacing treatments contribute much to the permeability of
 the clean fabric, and perhaps also to the residual dust deposit permeability.
     4.6.5  Dust Deposit Release
            The ability of  the fabric to release the deposited dust will
 dopond on the mode  and intensity of cleaning and also on the adhesive.
 character of the fabric.  Cleaning and fabric adhesion are discussed in
 Chapters 2  and 6.   The way  in which fabric construction relates to  deposit
 release has not  been  determined, but is presumed to depend partly on the
 electrical  resistances of selected fibers.  Resistance is seen to depend
 on  humidity, which  is independently known to have a marked effect on fil-
 tration fabric performance  (Section 2.2.2)
 4.7  AVAILABLE FABRICS
     There  are at least 50  U.S. manufacturers of dust filtration fabrics
 or  filter elements  for dust collectors.  The list of manufacturers  in Appen-
 dix 4.4 resulted from a 1969 survey of nearly 200 companies believed to have
 interests in the kind of filtration fabrics used in dust and fume collectors.
     The list is representative of U.S. fabric manufacturers and suppliers,
 but undoubtedly does not include all such firms, and may not accurately
 represent the interests of  every firm listed.  For example a firm although
 not specifically mentioning a fabric product in its brochures is often able
 to supply the product on short notice.   Other firms prefer to specialize in
 certain fibers, filter element types, etc.
     Chapter 7 discsses the purchase costs of filter elements, filter fabric,
 and typical fibers  in some  detail.   As indicated in Chapter 1, the market
 for dry filtration  fabrics  is estimated at $15 to 30 million annually,  about
half the fabric going into new collection equipment and half replacing fabric
which has worn out.
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      The  distribution  of  fabric manufacturers,  while  not  analyzed  in detail,

 may  be  assumed  similar to the  distribution  of  filter  equipment  manufacturers.

 That is,  the  typical fabric manufacturer  has one  half to  one  million dollars

 in sales,  and the  largest manufacturers have sales  of several million

 annually.

 4.8  REFERENCES FOR CHAPTER 4

 1.   J.  J.  Press, Man-made Textile Encyclopedia, Textile Book  Publishers,
      Inc.,  Div., Interscience Press  Inc.,  New York (1959).

 2.   J.  P.  Stevens  & Co.,  Inc., Selecting  Fabrics  for  Filtration and  Dust
      Collection. Bulletin,  New  York  (1961  est).

 3.   A.  A.  Dembeck, Guide  to Man-Made Textile Fibers and Textured Yarns
      of  the World,  3rd  Ed., The United Piece Dye Works, New York, (1969).

 4.   M.  Harris, Ed., Handbook of Textile Fibers, Textile Book  Publ. Inc.
      (Interscience) New York (1954).

 5.   A.  N.  J. Heyn, Fiber  Microscopy; A Textbook and Laboratory  Manual,
      Interscience Publishing, New York, 1954.

 6.   J.  M.  Preston, Modern  Textile Microscopy, Emmott  & Co., Ltd., London,
      1933.

 7.   H.  Bunn, Ind.  Eng. Chem.. 44: 2128 (1952).

 8.   M.  D.  Potter and B. P. Corbman, Textiles; Fiber to Fabric,  4th Ed.,
      McGraw-Hill Book Co.,  New York  (1967).

 9.   P.  L.  85-897,  85th Congress-Second Session, Textlie Fiber Products
      Identification Act. 15 U.  S. Code 70, 72 Statutes 1717 (1960).

10.   E.  R. Kaswell, Wellington Sears Handbook of Industrial Textiles,
     Wellington Sears Co.,  Inc., N. Y. (1963).

11.   Albany Felt Company, Dry Filtration Manual, Technical Bulletin,
      Industrial Fabrics Div., Albany, New York., (1968).

12.  W.   E. Morton and J. N. S.  Hearle, Physical Properties of  Textile
      Fibers, The Textile Institute, Butterworths, London,  (1962).

13.  American Society for Testing and Materials, Standards on  Textile
     Materials, Part 24, p. 39  (1965).

14,  M.  L. Joseph,  Introductory Textile Science, Holt,  Reinhart & Winston,
     Inc.,  (1966).
-------
15.  M.H. Curley, Man-Made  Textile  Encyclopedia,  New  York:  Textile B<«ok
     Publishers,  p.  229,  (1959).

16.  J.W.S. Ik-arlr,  P.  Grosberg, and  S.  Hacker, Structural  Mechanics  of
     Kiln-fa, Yarns,  and  Fabrics, Vol.  1, Wiley-Intcrscience,  N.Y., (1969).

1.7.  Modern i'lastics  Encyclopedia,  45:  14A,  (1968).

18.  T.W. Spalte, J.K.  Hagan, and W.F.  Todd  "A Protective  Finish  for  Glass-
     Kibcr Fabrics",  Chcm.  Eng'g. Prog.  59:4, 54  (April, 1963).

I1).  J.A. Sal vatorr.  A  Study of. the Influence of  Selected Paramo tors  on
     tin- Work to Abrade  a Fabric Using  a Modified Touting Device,  M.S.
     Thesis, Lowell  Technological Inst., Lowell,  Mass.  (1968).

20.  E.H. Allard, Wet and Dry Abrasion  Resistance of  Substrates as
     Measured by the  Stoll  Abrader  with  the.  Flex  Element, K.  S. Thesis,
     Lowell Technological Inst., Lowell, Mass. (1969).

21.  S. Backer, "The  Relationship between the Structural Geometry  of  a
     Textile Fabric  and  its Physical Properties;  II.  The Mechanism of
     Fabric Abrasion, Textile Research  Jnl.. Q, 453  (1951).

22.  S. Backer and S.J.  Tenenhaus, "The. Relationship  between  the Structural
     Geometry of a Textile  Fabric and its Physical Properties; III. Textile
     Geometry and Abrasion  Resistance", Textile Research Jnl., 21, 635  (1951)

23.  W.J. Hamburger,  "Mechanics of Abrasion  of Textile Materials", Textile
     Research Jnl. .  15_,  169 (1945).

24.  E.R. Kaswell, Textile  Fibers, Yarns, and Fabrics, Reinhold Publ. Co.,
     N.Y., (1953).

25.  Lord Rayleigh, "The Sand Blast", Nature. 9_3, 188 (1914).

26.  F.A. Zenz and D.F.  Othmer, Fluidization and Fluid-Particle Systems,
     Reinhold Chem. Engg. Series, Reinhold Publ.  Co., N.Y., (1960).
                            ;                      :      i
27.  E.P. Parker, Abrasion Resistance of Substrates as Measured by the
     Sand Blast Method, MS. Thesis, Lowell Technological Inst., Lowell,
     Mass. (1967).

28.  E. Rabinowicz, Department of Mechanical Engineering, Massachusetts
     Inst. of Technology, Cambridge, Mass., Unpublished Notes, (1964).

29.  E. Rabinowicz, "Wear", Scientific American.  206:2.  127 (1962).

30.  W.D. Freeston, M.M. Platt, and M.M. Schoppe,  "Mechanics of Elastic
     Performance of Textile Materials; Part XVIT:  Stress-Strain Response
     of Fabrics under 2-Dimensionai Loading", Textile Research Jnl...
     37.: 11,  948 (1967).
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                               CHAPTER 5
               ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS

TABLE OF CONTENTS
5.1  DESCRIPTION OF PROCESS EFFLUENT TO BE FILTERED                 5-4
     5.1.1  Gas Flow                                                5-5
     5.1.2  Gas Properties                                          5-6
     5.1.3  Dust Flow                                               5-6
     5.1.4  Dust Properties                                         5-6
     5.1.5  Variability in Aerosol Composition                      5-7
     5.1.6  Emission Requirements                                   5-7
5.2  DUST COLLECTOR DESIGN                  •                        5-8
     5.2.1  Pressure Drop                                           5-8
     5.2.2  Air/Cloth Ratio                                         5-9
     5.2.3  Cleaning Mechanism and Fabric                           5-11
     5.2.4  Cloth Area                                              5-12
     5.2.5  Cloth Life                                              5-13
     5.2.6  Housing Configuration                                   5-13
            5.2.6.1  Number of Compartments                         5-13
            5.2.6.2  Fabric Arrangement                             5-14
            5.2.6.3  Compartment Structure                          5-14
     5.2.7  Capital Cost Estimates                                  5-15
5.3  FAN AND DUCTING DESIGN                                         5-16
     5.3.1  Ducting Layout                                          5-16
     5.3.2  Ducting Costs                                           5-17
     5.3.3  System Pressure Drop                                    5-18
     5.3.4  Fan Selection                                           5-19
     5.3.5  Minimizing Fan and Ducting Costs                        5-22
5.4  PERIPHERAL EQUIPMENT, INSTRUMENTS, AND CONTROLS                 5-22
     5.4.1  Particulate Pre-Conditioning Equipment                   5-22
     5.4.2  Gas Pre-Conditioning Equipment                          5-23
     5.4.3  Instrumentation                        '                 5-25
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TABLE OF CONTENTS (Continued)

     5.4.4  Control Equipment                                       5-25
     5.4.5  Dust Disposal Equipment                                 5-26
5.5  FINAL SYSTEM DESIGN                                            5-26
5.6  PROCUREMENT AND RESPONSIBILITY                                 5-28
5.7  REFERENCES FOR CHAPTER 5                                       5-30
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                               CHAPTER 5
              ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS

      The design of a  fabric  filter system is  similar  to most  engineering
 design assignments in that there is no single rigorous approach  to the
 solution of the problem at hand.  Many parameters must be  identified  and
 their inter-relationships understood.   Placing an important variable  in
 proper perspective whether it  be,  for  example,  the  fabric  cost or  gas
 stream dew  point,  represents a key step in the design process.   The pri-
 mary  purpose  of this  Chapter is to highlight  the many factors that must
 be considered in designing the overall system.   Since many of these inter-
 relationships are  discussed in greater detail in other sections, a very
 general procedure  for selecting and/or designing the  system components
 is presented  here,  with the emphasis placed upon the  pitfalls and  expec-
 ted problem areas.
      The  overall performance of the  filter system will only be as  good
 as that  of  the  poorest  functioning  component.   Therefore,  only those
 well  experienced with fabric filter equipment  should  assume the  respon-
 sibility  for  system design, especially  of new equipment and new  appli-
 cations.  The replacement of existing  equipment  is somewhat easier  since
 one does  have some' practical guidelines.  In  general, the  first  decision
 that  one must make  is whether  to do the job in-house, or to contract  for
 a "turn-key"  job in which the  entire system is  provided by an outside
 firm.   In either case,  the best  available consultation should be sought
 and the purchaser  should retain  the responsibility for complete descrip-
 tion  of the process effluent to be  filtered.  Once the above groundwork
 is established, the experienced engineer is presented with a tractable
 design problem which  he can undertake  in the conventional iterative
manner and which involves the  following main design steps:
      1.  Define the effluent -- mass flow rate, dust properties,  gas
         properties,  and process variations with  time.
     2.  Approximate  the collector design
     3.  Approximate  the fan and ducting needs.
     4.  Based on  (2)  and (3)  select peripheral equipment,  instruments,
         and controls.

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     5.  Repeat  (2) and  (3)  to minimize  the  estimated  total  system cost.          ,-
     6.  Review  for alternative  effluent control methods;  budget;  and
         procure.                                                                 '
The  first  three  steps  can  be carried  out approximately without  the cost
data for individual system components by using  the  guidelines set  forth
in this handbook.  In  later  stages, specific apparatus must  be  evalu-
ated,  in conjunction with  equipment supplier support,  to arrive at real-
istic  estimates  of initial costs.                                                 '•'
     Operating,  maintenance  and  overhead costs  are  also essential  con-
siderations in preparing design  specifications. Even the accounting
practices  of the purchasing  company may  decide  the  relative  attractive-
ness of one system over another, e.g., the depreciation of a low first           ~'
cost,  higher maintenance system  versus that  of  a higher capital  cost in-
stallation.  Total annual  cost is probably the  best criterion to use (see        _,
Chapter 7).  Other criteria  of performance that are difficult to assign
a cost to, but which affect  the  overall  quality of  the system,  include
                                                                                 ..'•y
the  emission level of the equipment, its  reliability, simplicity  of
operation, etc.  Although  the latter  factors are difficult to quantity
the  design engineer must take them into  account.
5.1  DESCRIPTION OF PROCESS  EFFLUENT TO  BE FILTERED
     The definition of the problem, which is common to any engineering
design effort, constitutes the basis  for its effective solution.   In the
case of fabric filtration, the crux of the problem is  to define  as  com-
pletely as possible the process  effluent properties.  The minimum  infor-
mation required  is listed  in Table 5.1.  So  important are these  data             ~
that,  in almost  all cases, preliminary stack samplingishould be  perfor-
med and, in some instances,  tho  entire operation should be simulated on  -        _^
a bench or pilot scale with  experimental dust generation and filtration
facilities.
     If a process is characterized by variations in gas flow and/or gas
particulate composition, the equipment must operate at  peak loads with-         	,
out media plugging,  as well as at reduced flows  where condensation may
occur.   Any potential  future increase in effluent loading should be con-
-------
sidered in estimating design capacity, since an initial overdesign is

often less costly than subsequent reconstruction.

     It appears highly advisable to prepare a brief summation as out-

lined below in which the key properties of the effluent and the control

requirements are Listed.  Most fabric filter manufacturers submit a

similar questionnaire to potential customers prior to quoting on new

equipment.


                                TABLE 5.1

                  EFFLUENT AND FILTERING REQUIREMENTS
  1.  Process Effluent

      (a) Gas flow:
           Average:
           Maximum:
          Temperature:
          Water Content:
          Other Constituents:

      (b) Dust flow:
           Average:
           Maximum:
          Six.e distribution:
          Size: <1   <5  <20 -50 <80 <95
          %:   __   _  	  __  _  _
          Particle density:    »
          Bulk density:
          Est. range of K2:
          Other properties:

                         3.  Exhaus t
  2.   System

       (a) Preferred  location  -
          (in)  (out)side

       (b) Space  limitations,  if  any

       (c) Ambient weather
           Range:
           Temperature:
          Snow, water, wind loads

       (d) Weight requirements

<9Q  um (e) Cost considerations
(a)
(b)
(c)
(d)
Particulate level:
Gaseous req'ts:
Visibility req'ts
Preferred exhaust location:
(in) (out) side
Distance from collector:
       5.1.1  Gas Flow

              Determine the volume of gas emitted by the dust generating

  process prior to any corrective adjustments of temperature or dew point.
-------
Should  there  be  temporal  variations,  the  maximum,  minimum,  and  average  flows
should  be  estimated.   The cost  of the fabric  filter  system  will be  approx-
imately proportional  to the  volume of gas emitted  by the  process.   There-
fore, it is imperative that  one establish what  gas volumes  will obtain
when practicable  process  changes and  closure  or  all  unnecessary vents
of  leak points are  considered.   One should anticipate, and be  prepared  to
defend, process changes that  constitute minor  expenses  relative  to the
savings achieved  through  reduced gas  handling capacity.
     5.1.2  Gas Properties
            Determine  the temperature and pressure of  the carrier gas
stream  and estimate its approximate water content.   Identify  any abnormal
gaseous constituents such as acid vapors,  toxic  and/or corrosive fumes,
combustible or explosive  materials, condensibles,  etc.  Determine whether
composition and/or concentration vary significantly  with  time,  particu-
larly during  process start-up or shut-down operations.
     5.1.3  Dust Flow
            Determine  the weight  (mass) rate  of  dust or fume  generation
by  the  process, again making certain  that  the quantity has  been minimized
as much  as possible by process  adjustment.  Variable load conditions,
particularly  peak values,  must  be  considered  in  determining filter capa-
city if  overloading or plugging  is  to be  avoided.  Standard procedures
for measurement of effluent properties are described in several  test
manuals  (for  example,  see  reference 1).
     5.1.4  Dust Properties
            The better the system designer understands the properties of
the dust particles (see Chapter 2), the easier his task of designing the
filtration system becomes.  Minimal information for developing a functional
system, however,  must include a characterization of the particles in
terms of mean or median diameter and,if possible, the distribution of
sizes.   A knowledge of effective densities for discrete and bulk par-
tides  as well as an estimated permeability (K») for the dust  is also
useful   in establishing filtering conditions.
-------
             The  value  i>£  (K?), which  is  thought  to  reflc-ct  tlie  integrated
 effoctfl  of  particle dimensions;  e.g.,  length  to  diameter  ratio,  cohesive-
 ness between particles, dust  cake rigidity, and  spatial arrangement  of
 particles in the dust  cake, is a valuable design tool.  On  the  other
 hand,  individual measurement  of  the factors which presumably determine
 the K- are  not as yet  sufficiently understood to make their quantifica-
 tion possible.   Additional dust  properties for which no strict  quantita-
 tive definition  is currently made, but which  constitute important  in-
 puts for system  design are: the  softness or stickiness of the dust as a
 function of temperature or humidity; abrasiveness;  agglomerating charac-
 teristics;  "seeping" tendency; adhesion of the dust cake  to the  fabric.
     5.1.5   Variability in Aerosol Composition
             Allowance  should be made for the  fact that.even without  inten-
 tional modification of the gas temperature or the particle  size  proper-
 ties, there  may  be radiation cooling, moisture leakage into the gas
 stream, agglomeration  of  the smaller particles and/or sedimentation  of
 the larger  particles,  or  other changes during transit through the system.
 Therefore,  one must attempt to define the aerosol as it enters  the filter
 unit.
     5.1.6   Emission Requirements
             The  degree of particulate control which must be attained
with the overall  filter system should be determined early in the design
 process.  This will usually be stated as a maximum tolerable weight
emission rate rather than as a system efficiency.  The requirements may
also specify other factors that must be considered,  e.g.,  toxic gases,
odors,  or visibility of particulate or steam plumes.

             It must also  be decided whether the  filtered effluent can
 be discharged directly to the outside environment (with the attendant
 problems of heat loss, make-up ventilation and visible exhaust)  or re-
 leased within the building.  In  the latter case,  the problems of heat,
 materials toxicity, nuisance and/or hazard in the event of  filter rup-
 ture, and noise  take on added importance because  of confinement.
-------
 5.2  DUST COLLECTOR DESIGN
     The design  criteria applying  to  the  collector  itself will,  in  turn,
 affect  those  for other system components.  During the early design  phase,
 however, it may  not be possible  to predict the  interrelationships be-
 tween design  criteria and costs.   Therefore, until  realistic trade-offs
 can be  established among collector size,  fan requirements, and hoods and
 ducting, cost estimation within  approximately + 50  percent is acceptable.
 As a subsequent  aid to cost optimization, it may be helpful to construct
 a table by which one can estimate  how the collector cost will increase
 with respect  to  an increase, say 10 percent, in any significant  variable:

^
•rl
8
r~*
t>

M
0)
4J
r-l
•rl
A 10% increase in this variable: V


0)
M
s
cd

4)
(X
0)
H
T
0)
M
3
M
0)
£

U
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,a o
P


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M

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•rl M
O 01
Qu O«
si
DT
H
01
4J
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Q
Ol
r-l
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a
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P
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a
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H
Q


$
•rl
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Q
.J

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I
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tance
w
•rl
M
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U
•rl
M-l
•rl
U
8.
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K2
Will add this much to the
     INITIAL COLLECTOR COST: ($)	
     ANNUAL SYSTEM COSTS: ($)   	

 The effects of  selected  variables on  the annual operating and maintenance
 costs may also  be estimated,as  Indicated, since the collector design has
 a  strong affect on  the annual costs (Chapter 7).
      5.2.1  Pressure Drop
             Estimate the average pressure differential across the filter
 media and deposited dust layer  during normal operating conditions.
 Although the value  selected may be somewhat arbitrary, several practical
 considerations,  e.g., collector strength under pressure or vacuum, fan
 power requirements, and dust cake mechanics, point to a few inches of
 water as the optimum pressure drop.  Lacking any better design guides,
-------
3  inches  of water  is acceptable as a  typical value.  On  the  other  hand,
the use of high velocity  filtration,  felted fabrics, or  the  presence  of
a  sticky  or low porosity  dust cake often require  that  optimum pressure
drop be of the order of 8 to  10 inches  of water.  Pressure loss  through
the collector alone, exclusive of the media drop, is usually small com-
pared to  that of the loaded fabric.  The pressure loss associated  with
the duct, hood and stack  system will probably be  in the  same range as
that for  the collector with fabric.
            The pressure  drop through the combined fabric and dust layer
can be treated as an independent variable in the sense that  the  design
engineer  can exercise considerable control over the cleaning mechanism.
By increasing the intensity and/or frequency of cleaning it  is possible
in some cases to reduce pressure drop to levels approaching  those  of  the
clean fabrics.  If this concept is carried too far, however, the collec-
tion efiiciency may be lowered, the fabric itself damaged, and the power
costs for driving the cleaning mechanism increased to prohibitive  levels.
Thus, the selection of the optimum operating pressure loss becomes a
matter of trade-offs based upon engineering judgement and field  trials.
Since the final operating pressure loss may not necessarily  conform to
the original design point, it is not practical to over-refine the  pre-
liminary estimates of average and peak pressure drops.

      5.2.2  Air/Cloth Ratio
                                                   2
             This ratio (CFM of air filtered per ft  of cloth filter area)
 is very important in determining collector performance.   The ratio (or
 its equivalent,  filtering velocity)  is discussed throughout this handbook
 in connection with dust  deposit  characteristics,  collector configurations,
 collector efficiency and  pressure drop and maintenance requirements,  to
 mention some  principle effects  related to air/cloth ratio.   Ratios in
 current use  range from less  than 1:1 to more  than 20:1.   The choice de-
 pends  on cleaning method  and  fabric,  and on characteristics  of the par-
 ticles .
            There is  no precisely determinable ratio  for a given applica-
 tion as  the choice also depends  on  estimates  and  trade-offs, such as

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 initial collector cost and recurring power costs.  Consequently, there is
 no precise analytical method for determining the best air/cloth ratio,
 although Chapters 2 and 6 indicate several approaches to the construction
 of analytical models.  Instead, it is customary to select ratios based
 on similar previous experience, that is, ratios that have been.proven on
 similar combinations of cleaning method, fabric, and dust.
            Each dust collector manufacturer has guidelines for the selec-
 tion of air/cloth ratio, based on his experience with a variety of appli-
 cations.  These guidelines vary from manufacturer to manufacturer,  largely
 as a result of differences in equipment.  Four such guidelines are sum-
marized in Appendix 5.1 for a shaking bag collector,  a glass cloth col-
 lector employing principally reverse flow plus flexural cleaning,  a
reverse jet collector, and a reverse pulse collector.  These are typical
 of the guidelines that manufacturers have made publicly   available.
Normally, these guidelines should enable estimates to within at least
25 percent of the optimum design ratio.   In unusual cases,and for  a more
exact estimate, consultation with an experienced manufacturer is advis-
able.  Frequently, in new applications  a pilot study  has been used  to
determine the best air/cloth ratio.   Such studies  can be misleading,
 however, unless they accurately model the proposed equipment and use a suit-
 able aerosol.
            There is an optimum air/cloth ratio for each set of filter
 system design parameters: i.e.  dust to be filtered, configuration of  filter
system, cleaning mechanism to be employed, fiber material to be utilized and
configuration of the fiber media.  However, for a given set of the above
parameters, the total system cost versus air/cloth ratio relationship is
rather flat near the optimum ratio.   Thus there is a tendency to minimize
initial costs by selecting an air/cloth  ratio toward  the high end  of  the
range.   On the other hand it is frequently reported that, with lengthy
operation at two different filtration velocities,  the lower of the  two
filtration velocities results in lower  operational costs.  Thus one must
carefully weigh the traditionally cited  advantages of lowered air/cloth
ratio (i.e. lower power costs,  decreased maintenance  and higher collection
efficiency) against the larger  initial  capital costs  associated with

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 increased  collector size and  the penalty for  its  space  occupancy.   Further,
 as  pointed out  earlier in Chapters  1  and 2, it  is anticipated  that  with
 advancing  fiber,  fabric media and cleaning  technology the  optimum
 range  of air/cloth ratios,  for any  set  of design  and operating parameters,
 will tend  to  increase.
     5.2.3 Cleaning Mechanism and  Fabric
            The selection of  the cleaning mechanism arid the  filter  fabric
 are best made together, since both  items ate  closely related (Section
 3.3).   For example,  felted  fabrics  are  almost exclusively  cleaned by
 pulse  or reverse-jet air, whereas most  woven  fabrics are cleaned by
 other  means.  Of  the relatively few choices of  fiber media (approximately
 eight,  see Section 4.2),  most will  be eliminated  for reasons such as
 poor temperature  and/or corrosion resistance  or excessive  cost.  Of the
 several cleaning  mechanisms used in filtration  systems,  only two or
 three  will meet the specific  requirements for a given installation: i.e.,
 high,  low  or moderate dust  loadings;  continuous or intermittent operation;
 ease of removal of dust from  the fabric;  small  floor area; minimal  pressure
 drop;  high efficiency;  etc.
            By  a  process  of elimination,  therefore, a review of past
 successful filtering performance will usually show that  only a few
 cleaning mechanism - fabric combinations  are  compatible  and  sufficiently
 attractive to warrant economic  evaluation.    The  time required for
 cleaning also determines  the  choice of  cleaning mechanism.   This time
 should  be  a small  fraction of  the time  required for dust deposition,
 since  otherwise too  large a fraction  of  the fabric will  be out of ser-
 vice for cleaning  at any given  time.  It  is common with  shake cleaning
 equipment,  for  example, to have  a cleaning-to-deposition time ratio of
 the order  of 0.1 or  less.  Applying this  criterion, having a ten compart-
     *It is not intended here to exclude or impugn the merits of a
hitherto untried combination.  The design approaches set forth in this
chapter, however, are based upon successful precedent or at least qual-
itative findings which lend some substance to cost estimates.  If it
is desired, or is absolutely necessary to consider some novel combination,
one should view the problem as a research and development effort and not
as a routine fabrication process.  The path that should be followed will  .
be more apparent once preliminary cost estimates have been prepared,
based upon conventional design approaches.
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 ment  baghouse  would  mean that  one  compartment  is  out  of service at all
 times.   Therefore,  the  choice  of cleaning  mechanism affects  system size
 as well  as  fabric  life,  maintenance,  etc.
      5.2.4   Cloth Area
             The  amount  of  fabric in actual use  at  a given  time  is  found
 by dividing  the  estimated  total  flow  entering  the  collector  by  the sel-
 ected air/cloth  ratio.   The  volume flow rate of the effluent entering
 the collector  will  not  necessarily be the  same  as  that  discharging from
 the generating process,  owing  to temperature changes, the  added volume
 of vaporized cooling water,  and  dilution air which may  be  added deli-
 berately for cooling purposes  or accidentally by air  leakage.   The latter
 factor may contribute to a significant  flow increase  in systems operating
 under large  negative pressures.  In filter applications  involving  a vary-
 ing flow, some judgement is  required  to decide  whether  to  size  the
 equipment for  the  peak  flow, the average flow,  or  for some intermediate
 point.   It  is  again necessary  to seek a compromise between the  increased
 cost  of  larger equipment and the increased cleaning cost,  and the  possible
 risk  of  fabric damage associated with short term,  high  pressure drops.
             Except  for  certain systems  that are operated intermittently,
 e.g., a  few  hours on line  followed by cleaning  only during down time,
 most  filtration  units will require reserve fabric  capacity to allow for
 off-line cleaning,  inspection  and maintenance.  Since it is  common prac-
 tice  to  isolate  temporarily defective  filter units  until it  is  convenient
 to replace them, additional reserve capacity may be required.   The  total
 fabric area  to be installed can  be estimated by multiplying  the  area to
 be in actual use at  any  time by  the term

                             (1 + TR+F)                        (5.1)

where (T^) is  the time ratio (cleaning  to  deposition) discussed  in  the
        K
 preceding section and (F) the fraction  of  the fabric area expected  to be
 out of service at any time due  to replacement,   inspection,  or mainten-
ance operations.   Judicious timing  of the above procedures  in relation
 to peak flow periods may, however,  reduce  the multiplying factor.
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             Costs for Hie installed fabric media as  a function of unit
 size may be estimated from data given in Section 7.3.
      5.2.5  Cloth Life
             Fabric deterioration often results  from  the  combined  assault
 of several factors (Chapter 8),  rather than from any single  effect such
 as thermal erosion,  mechanical  stress through repeated  flexure, chemical
 attack,  abrasion,  etc.  All possible  modes  of failure should be con-
 sidered  during  the preliminary  design phase.  Again,  previous experience,
 especially that relating  to similar fabric-cleaning-dust applications,
 may  be the best and  only  guide.   Extrapolating  from  experience, one might
 estimate  that the  reduction in  fiber  life  through  thermal erosion might
 double for a 20 F  rise  in temperature,  or  that  the mechanical attri-
 tion rate might double  when the  frequency  of cleaning is doubled.
             Generally,  it should  be possible to estimate fabric life
 within a  factor of two  in situations  where  no direct  experience can be
 cited.   If performance  data are  available,  estimated  reliabilities may be
 upgraded  to perhaps  + 20  percent, which  is  the  order  of dependability
 of the best pilot  plant data.  Having established  a  reasonable estimate
 of fabric life,  one  can then reach an annual cost  figure for fabric media.
      5.2.6  Housing  Configuration
             5.2.6.1  Number  of Compartments. -  Selecting the  number of
 separate  compartments for a fabric filter  installation is a  relatively
 easy  decision.   The  basic information required  is  the allowable variation
 in gas flow with respect  to  process or plant ventilation, the  availabi-
 lity  of sizes of commercial  units (compartments  or filter house modules)
 and the expected frequency  of maintenance.  In  small  collectors,  indivi-
 dual  compartments may contain as  little as  100  square feet of  fabric sur-
 face, although  collectors as large as approximately 50,000 CFM capacity
may also have only one compartment.  Multiple compartments of  almost any
 size may be chosen,  subject  to availability.  With the exception of re-
verse jet  and pulse  jet units, at lease one compartment will be out of
service during  the cleaning  cycle.  It may also be necessary to provide
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                                                                                            .1
 additional compartments  for  emergency,  extended maintenance,  or unexpec-
 ted  increases  in process  effluent.
             5.2.6.2   Fabric  Arrangement.  -  Fabric  filter  media  in panel,
 tube,  and  bag  form represent the  most  commonly used  industrial  configura-
 tions.  Although the  selection of filter  geometry  may  occassionally  be
 a matter of preference,  the  type  of fabric  and the cleaning method
 usually dictate  the configuration.   Panel filters, for example,  cannot
 be cleaned by  flexure, and reverse  jet  cleaning requires  open ended
 tubes.  Other  important  considerations  are  discussed in Section 3.3.
             Panels,  tubes,  or bags  are commercially  available over a
 broad range of dimensions as discussed in Section  3.1. Once  the require-
 ment for  fabric surface  per  compartment is  determined, it remains to
 decide what combination  of filter length, diameter,  and spacing will be
 the  least  expensive.   This  is too often interpreted  as a  maximum filter
 packing and a  rather  compact filter housing,  i.e., a container  with  no
 dimension  much greater than  twice the  smallest dimension.  Closely packed
 filter elements  tend  to wear against one  another,  and  make inspection and
 maintenance difficult (Section 8.3).  Compact  housings incur  a  greater
 cost  for plant floor  space than taller  units.   Recently,  one  manufacturer
 has  introduced super-long bags  (—60 ft), partly to  conserve  floor
      (3)
 space   .   Compact housings  may,  however, be more  suitable in outside
 locations  where  land  use  costs  less  than  the erection  of  tall structures.
             Other considerations  in  selecting  the  fabric configuration  are
 downward vs. upward flow  and  inside  vs. outside  filtering.  The ease and
 the  frequency  of fabric replacement, and  the uniformity of flow distri-
 bution and  associated danger ;of condensation in  stagnant air  pockets are
 also considerations.
             5.2.6.3  Compartment Structure.  - Although many kinds  of
 fabric collectors can, in principle, be operated either under negative
 or positive  pressure,  the larger units  are often custom designed for one
condition or the other to minimize costs.   Least expensive is  the instal-
 lation needing no housing at all,  in which the particulate is  collected
on the interior surfaces  of a positive  pressure bag system. The danger
of cooling  below the  dew  point is  an important consideration,  particu-

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 larly in a suction housing where infiltration leakage  often occurs  unless
 a more expensive gas-tight design is  adopted.   Thermal excursions of  the
 collector during startup and shutdown can effect  the sealing character-
 istics of critical gasketed connections,  depending  on  the  structural
 materials.   Interconnection of  compartments  via ducting or hoppers  and
 their isolation, especially during the cleaning cycle,  are important
 considerations (Section 3.4).   The generalized  rules for designing  the
 compartment structure  are not well defined.   Construction  materials are
 likewise not generally prescribed. The most  common housing material  is
 steel, as discussed in Chapter  3.   The choice of  material  depends on  the
 nature of the dust and gas mixture and their  flammability,  corrosiveness
 etc.   Lacking a more specific guideline,  one would  do well to follow  pre-
 cedent in selecting materials of construction,  metal gages and dimensions.
      5.2.7   Capital Cost Estimates
             A detailed discussion of  initial and  annual cost  factors  is
 presented in Chapter 7.   The cost  of  the  baghouse,  including  its amorti-
 zation schedule  and plant overhead costs,should  now be  estimated based
 upon  the tentative equipment selections.   The designer  should now devote
 primary attention to initial installation costs with emphasis upon how
 these initial costs will contribute to the total  annual  cost  of the fabric
 filter system operation,  in accordance with specific company costing  pro-
 cedures.  At this  time,  sufficient  information  has  been acquired to  com-
 plete the working  estimate guide at the beginning of Section 5.2.
             Some of  the  design decisions  cited  above may have been based
upon  very limited data.  Consequently, one should review the compilation of
preliminary  estimates,  particularly those reflecting choice of fabric,
cleaning mechanism, and average pressure drop through the  filter media.
The objective should be to describe a  collector that will be within ~10
percent costwise of the optimum design.   It is not advisable, however, to
finalize the design until other components of the  overall system have
been  selected and evaluated with respect to their  possible influence on
collector design.
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 5.3   FAN AND DUCTING DESIGN
      The design and selection of filter system components, other than the
 collector discussed previously,  also require some iterative
 approaches during the planning phase.  Fortunately,  the alternatives are
 fewer, and in most cases,  the design features are simpler.   The main
 trade-off area is between the pressure drop through  the ducting and stack           i
 systems  (an inverse function of  duct diameter)  and the fan size and cost.          —'
 In many  cases, the pressure drop through the fabric  filter unit alone
 will  largely determine fan size.                                                   _j
      5.3.1  Ducting Layout                                                          ,
             The positioning and  amount of ducting are  determined mainly
 by the expected locations  of effluent source,  collector,  fan,  and venting
 point.   Ordinarily,  minimum duct lengths and diameters will be used un-            —'
 less  the ducting is  also  intended to provide radiant cooling of the gases
 prior  to filtration,  or to function as settlement chambers to remove the           _J
 larger particles.   The first step in design is  to establish the locations
 and lengths  of main,  branch and  riser ducts showing types  of junctions               '
 and bends.
             The distance  from the dust collector  to both the dust  source            !
 and the  point  of discharge  of the filtered  effluent, must  be considered
 with regard  to duct  costs  and space  availability.  The  cost of plant floor           i
 space may be a reason  for  locating  the collector  outside the plant,  other
 possible reasons being ceiling limitations  and  safety.  On the other hand,           i
 the outside  collector must  usually be  protected against weather and insu-          ~
 lated against  temperature changes.  Also, not to  be overlooked in  locat-
 ing the collector are space  clearances  for,  and accessibility to; the nee-           _J
essary ducting;  crane or hoist requirements, especially for fan repair;
 the weight of  the collector;  the  feasibility of removal and disposal of
 the collected  dust; and plant  insurance  costs.
            In  laying out the duct plan, the locations of  cleanout  doors,            '
                                                                                    •••*«••
dampers,  dilution valves and any  auxiliary  cleaning equipment,  i.e., dry
inertial collectors, scrubbers, or cooling  towers should be  taken into               '
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 consideration.   Whenever possible, all ducting should be located so as
 to be easily installed and maintained.
             It  is important to use as little horizontal ducting as  pos-
 sible and to select diameters which allow for complete transport of the
 dust load to the collector.  Sharp bends and abrupt changes in dimensions
 should also be  avoided to prevent dust accumulation.  Since the above
 approach suggests high velocity systems, it should be recognized that
 power costs also increase with higher duct velocities and that abrasive
 dusts can cause rapid duct erosion, particularly in bends.   Useful  velo-
 city guidelines have been set forth in Table 2.42(b) and in various engineer-
 ing manuals to  aid in this aspect of system design.
             Duct sizes through the system are estimated by  the recommen-
 ded transport velocities and the calculated volume flow rate through each
 section.   The latter value is computed from the  source flow,  corrected
 for any added volume attributable to dilution air,  and adjusted to  the
 temperature and pressure conditions at the location of interest in  the
 duct system.  In cases of fluctuating flow,  it is  recommended that  the
 ducting be sized for some intermediate flow between average and peak
 loads.
            The ducting  sections  located downstream of the  collector may
 be  sized  somewhat larger,  owing  to absence of settlable  particulate  in
 the  gas  stream.   The final dimensions  should  represent a balance between
 the  increased cost  for larger duct size  and  the  lower pressure  drop
 (power  cost).
     5.3.2  Ducting  Costs
            Standard  steel  ductwork  is normally used, unless  there is
danger  of  corrosion,  abrasion, adhesion, high  temperature,  thermal dis-
 tortion, need of  insulation,  or unusual  surges in pressure  or in tempera-
ture.  Aluminum, glass, plywood, etc., can sometimes be used as substitutes
for steel.  While the  duct material may be suggested by  the collector man-
ufacturer, the ducting is normally made up and installed  locally.
            Cost estimates  or quotations are readily obtained as soon as
diameter,  length, and guage specifications are available  (Section 7.1.3).

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 One  must  consider the coats of dampers,  ports,  Tee sections,  elbows,
 flanges  or other special  components,  keeping in mind the fact that fit-
 ting or  adapting the ducting to the above parts may involve more cost
 than that for assembling  straight vertical or horizontal runs.
       5.3.3  System Pressure Drop
              The total system pressure drop is  that of the combined  losses
 in the duct and  the fabric  filter unit.   Ducting losses vary  approximately
 as the square of the gas  velocity and can be readily calculated  by standard
 formulas  or from tabular  or graphical data (see Section 2.5).  Pre-
 sure drops for non-linear shapes  (elbows,  Tee's,  reducers,  etc.)  are
 usually expressed in equivalent length of straight duct of the same  dia-
 meter.  In conventional practice,  one traces the  largest branch  (usually
 from the  most remote source of dust generation),  noting the temperature
 and  gas flow through each succeeding  section up to the collector inlet.
 A similar procedure is followed from  the  collector exit to the point
 where  the filtered  effluent is discharged  to the  inside or outside atmos-
 phere.  Ordinarily,  the total  pressure drop associated with the  ducting
 alone  will be in the range  of  3 to  6  inches of  water,  although other
 values may apply in some  circumstances.
              One then  adds  to  the estimated pressure drop  through  the
 ductwork,  the collector pressure drop  and  that  of  any  other component of the
 system, e.g.,  a  centrifugal  collector.  The  result will  be  the net pre-
 sure to be  supplied  by the  fan  when ambient  pressures  are the same at
 system inlet  and  outlet.  Should the ambient pressure  at the system  in-
 let  exceed  that  at  the outlet,  the net pressure requirement for the  fan  ,
 is decreased  by  this difference.*  The opposite applies  if  the pressure
gradient  is reversed.
*The kinetic energy or velocity pressure retained by the air leaving
the fan may be deducted from the fan static pressure requirement, except
when the air is charged to a large stagnant space, e.g., exhausting the
fan outdoors.  The kinetic deduction is frequently overlooked, however,
on the justification that by omitting it, a safety factor is added.  See
the excellent discussion on fan selection in "Fan Engineering" by the
Buffalo Forge Company(6).
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     5.3.4  Fan Selection

            Of the types of fans commonly used in industrial ventilation,

centrifugal fans are most often chosen for the primary flow in fabric

filtration systems.  These are described below and in Figure 5.1:


           1. "Forward-curved blade types;  A multi-bladed, "Squirrel Cage"
               wheel in which the leading edges of the fan blades curve toward
               the direction of rotation.  These fans have low space require-
               ments, low tip speeds and are quiet in operation.  They are
               usually used against low to moderate static pressures such as
               encountered in heating and air conditioning work.  Not recom-
               mended for dusts or fumes that would adhere to the short curved
               blades, causing unbalance and making cleaning difficult.

           2.  Straight or Radial-blade (paddle wheel,  long shaving wheel):
               The "workhorse" for most exhaust system applications, they are
               used for systems handling materials likely to clog the fan
               wheel as the name indicates.  Such fans  usually have a medium
               tip speed and a medium noise factor and are used for buffing
               exhaust,  woodworking exhaust or for applications where a heavy
               dust load passes through the fan.

           3.  Backward blade type;   The type in which  the fan blades are in-
               clined in a direction opposite to the fan rotation.  This type
               usually has a high tip speed,  provides high fan efficiency and
               has non-overloading characteristics.   Except in case of direct
               driven arrangements,  non-over-loading feature is over emphasized
               in exhaust ventilation work as the exhaust system acts as a
               load limiting orifice to make  overloading  of any exhauster motor
               from variations  in system conditions  improbable.  Blade shape is
               conducive to buildup  of material and  fans  in this  group should
               be used only on  clean air containing  no  condensible fumes or
               vapors."(7)

         The  first consideration  in  fan selection is which of  the numer-

ous fan types  is best suited to the estimated air flow and pressure  re-

quirements.  Reference  to manufacturers' data depicting  the performance
of various types over a range of speeds and delivery volumes will estab-

lish this.  Another consideration in selecting the fan type is whether

it is to be used on the clean side of the filter or on the dirty side

where maintenance will probably be higher.  (See also the discussion of
types of collector construction, Section 5.2.6.)

         Although several fan sizes of a specific type  can meet the de-
sired pressure-volume requirement by changing the speed,  only one size
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  \	
BACKWARD CURVED BLADES
i
I
             VOLUME - CFM
STRAIGHT OR RADIAL BLADES
             VOLUME-
FORWARD CURVED  BLADES
             VOLUME-CFM
         Figure  5.1.  Some Fabric Filter System Centrifugal

                     Fans.  (From Ref. 7).
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will  perform at hits  peak  efficiency.  Therefore, there may be no parti-
cular economy in  selecting a smaller size unit of lower cost, since opera-
tion  at  a higher  speed will lend  to proportiately greater power consumption.
On  the other hand,  a  slight oversizing of the fan will result in lower
fan speeds  and probably  less maintenance without a serious drop in ef-
fiency.   Extra fan capacity is a safety factor against future process
expansion or unexpected  peak flow requirements which the fan must be capable
of  handling.   Another consideration in fan selection is the reliability re-
quirement:   Can a fan outage be tolerated, or should more than one fan be
installed?
          The process  of fan selection  from  the  several trial  sizes  in-
 volves the  following  steps.  First, choose  a maximum rated  capacity,
 roughly  20  percent  more than  the  required capacity.  Determine  from  the
 table or fan curve  the  power and  speed requirements  at the  required  pres-
               *
 sure  and flow, and from these data, estimate the installed fan and  motor
 costs and maintenance costs.   Using an acceptable amortization  rate, per-
haps  ten years and  the  local cost of power,  compute  the total annual cost
of  operating this particular fan  size.  Note that the  cost  of the drive,
mountings,  and motor  may easily double the cost of the fan alone.
         Note that  one  is  limited to available motor speeds if a direct
drive is to be used.  Direct drives consume  less power, need  less main-
tenance, and present  no  belt slippage problems.  On  the other hand, many
small and moderate  sized fans  are belt driven, both  for reasons of initial
cost  and to permit  future  speed adjustments.
         This initial cost estimate for fan  and associated equipment is
probably adequate for the  early phase of the system design process.  To
minimize fan costs, however, the  above computation should be repeated for
slightly larger or  smaller fans until a minimum cost is reached.  It may
become apparent before undertaking this step that a change in fan style
*Fan test data is usually based on standard temperature and pressure,
i.e., standard air density.  If the fan is to handle non-standard
density gas, different curves will be used.
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 or size should be considered, due to revisions in collector design pres-
 sure drop and in ducting requirements.  Having determined the fan arrange-
 ment of minimum cost, again consider whether there is a sufficient mar-
 gin of capacity and of reliability.
      5.3.5  Minimizing Fan and Ducting Costs
             Preliminary estimates of ducting requirements and costs should
 be reviewed.  It may be determined, for example,  that a small increase in
 duct diameter,  say 5 percent, will allow use of a smaller fan with a net
 lowering of overall cost; that is, the lower operating and capital costs
 for the fan assembly overrides the increased initial costs for the duct-
 ing.   In this example,  the decrease in pressure drop through the duct
 system would be of the  order  of 20 percent  based  upon the inverse fourth-
 power relationship between pressure loss and duct diameter.   Having es-
 tablished  a revised value for duct pressure drop,  new criteria are avail-
 able  for resizing the fan and motor drive.   If  the combined  cost of the
 duct  and fan system decreases appreciably as a  result of  this  reassess-
 ment,  the  process should  be repeated until  a minimum point is  identified.
             It  also is  recommended that  the above  cost  analysis  be exam-
 ined  in terms of the  key  variables affecting the  initial  and annual  costs
 for the combined fan-ducting  system.  Again,  a  tabular  array of  the  system
 cost  increase,as a  function of change  in the major design variables,con-
 stitutes a  useful working  tool.
 5.4   PERIPHERAL  EQUIPMENT, INSTRUMENTS,  AND CONTROLS
      In earlier  sections  of this chapter, a  preliminary design approach
has been outlined for the main components of  the fabric filter system.
As  the  few  remaining system parts  are  less  dependent upon  the primary
system  dimensions,  they should  now be selected so that  the overall fabric
filter  system design can be finalized (Section 5.5).
      5.4.1  Particulate Pre-Conditioning Equipment
             The pre-filter particulate treatment process, if any, may be
 designed for:  (a) the enhancement of particulate  agglomeration,  thereby
 precipitating out of the  gas  flow some material prior to  deposition on
 the fabric and/or improving the resistance  properties of  the dust cake,

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 (b)  the prc-separation of coarse particles by gravity sedimentation or
 inertial techniques,  or (c)  the injection of additives to produce a dust
 cake sufficiently dense to provide high efficiency collection of low con-
 centrations of fine material,  or alternatively additives to produce a
 more porous dust cake with lower resistance.
             Use of any of the  above techniques may require reappraisal
 of preliminary cost estimates  for the fabric filter system for the fol-
 lowing  reasons (see Section  2.4.8 for details):
             (a)  Change of deposit permeability and hence cycle time,
                  due  to alteration of particle size distribution.
             (b)  Change of inlet loading.
             (c)  Change of cake removability or residual permeability.
             (d)  Change in properties of the collected material such as
                  bulk density  and total volume.
             (e)  Changes  in  gas stream properties.
 One  must balance the  economic  advantages of changes in dust  properties
 against  the  costs  of  installing and operating  the  pre-conditioning equip-
 ment.  Of course,  if  the  treatment process  is  used  to elevate  collection
 efficiency  to  levels  satisfying pollution  control  regulations,  the equip-
 ment costs  become  a necessary  investment.
      5.4.2   Gas Pre-Conditioning  Equipment
             Hot process effluents  are usually  partly  cooled  before  fil-
 tering to reduce the  flow volume  which,  in  turn, decreases the  filter
 fabric area  requirement.   Lowering  the  gas  temperature  toward  the  dew
 point often  extends fabric life and may  permit the  use  of  less  expensive
 or more durable  materials.   Prior  to  entering  the collector, the process
effluent may also be  altered through  combustion, absorption, chemical
reactions or humidification, any  of which may  influence  the  design  of
other fabric filter system components.  Temperature conditioning, a com-
mon  treatment,  is usually accomplished by the methods described in
Table 5.2.   In  other  situations,  it may prove more  economical to filter
the gas essentially at process effluent temperature,or even  at  increased
 temperature, provided that the  temperatures do not  exceed the upper  limit
 for  the fabric.

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                               Table 5.2
                  METHODS OF TEMPERATURE CONDITIONING
 Radiatton-Conyoction Coo1ing  (long, uninsulated inlet ducts)

         Advantages:   Lowest  flow volume of the three methods
                       Smoothing or damping of flow, temperature, pressure
                          or other surges or peaks in the process effluent
                          stream
                       Saving  of heat (building space heating).
      Disadvantages:
Cost  of  extensive  ducting
Space requirements of  ducting
Possibility  of  duct plugging by sedimentation.
 Evaporation (by water injection well ahead of the filter)

         Advantages:
      Disadvantages:
Low  installation cost, even with  automatic controls
Capability  of  close and rapid control of  temperature
Capability  of  partial dust removal and/or gas control
   via  scrubbing.

Danger  of incomplete evaporation  and consequent
   wetting  of  the filter or chemical attack of the
   fabric or filter
Increased danger of exceeding the dewpoint and
   increased possiblity of chemical attack
Increased steam plume visibility, a hazard near highways
Possible increase in volume filtered.
 Dilution (by adding ambient air to the process effluent stream)
         Advantages:
      Disadvantages:
Lowest installation cost, especially at very high
   initial temperatures.

Substantial increase in total filtering volume
Automatic control of both temperature and filtering
   velocity is not possible
Uncontrollable intake of ambient moisture, dust, etc
   without prior conditioning of the dilution air.
* See also Ref. 8.
-------
             Somewhat related to the three cooling methods outlined in
 Table 5.2  are the special systems for emergency protection of the equip-
 ment.  These include:C02 devices that automatically release high-pressure
 gas to cool the effluent, should its temperature exceed a critical pre-
 set limit; fail-safe dampers held in position by set-point melting links;
 burst-out  panels to prevent duct or collector rupture  due to pressure or
 vacuum excursions;  and various types of alarm systems.  Such equipment
 can be reliable and relatively inexpensive compared to the cost  of re-
 placing a  collector system.  Rapid-cool equipment can  also serve in shor-
 tening the time required to take equipment out of service.  Heating equip-
 ment may facilitate start-ups by preventing condensation.
      5.4.3  Instrumentation
             Instrumentation which provides a continuous record and/or
 direct display  of the  important  factors describing the overall filtration
 process is an essential  part of  a good  fabric filter system.  The cost is
 relatively low  (~ a few  percent  of the  total system investment)  and the
 judicious  selection of instrument type  and function may permit the use of
 less expensive  fabrics or materials  of  construction, e.g.,  ordinary sheet
 steel rather  than a special alloy.   A discussion of recommended  instru-
 mentation  is  presented in Section 7.1.5.   The  measurement  of  those system
 characteristics  that affect its  overall function,  e.g.,  temperatures,
 pressures  (absolute and  differential),  primary gas  and  dilution  air  flow
 rates,  water  rates,  dew  point  levels, and  possibly  continuous monitoring
 of stack oftluont concentrations, all enhance  the probability of succes-
 ful  system  operation.  Any  or all these devices  can be  designed  to alert
 the  operator  through appropriate warning systems  (horn, alarm bell,  flash-
 ing  lights) when abnormal conditions arise.
     5.4.4  Control Equipment
            Solenoid valves, damper actuators, timers,  etc., associated
xtfith the filter need not be considered here, since they are usually  in-
cluded  in  the basic  filter  price.  Auxiliary blowers such as those for
reverse cleaning may or may not be included and air compressors for pulse
-------
 cleaning arc seldom included as part of the filtration unit.   Special
 ducting  dampers  that must open and close rapidly or provide high leak
 tight  integrity  (^  0.1 percent leakage  under adverse temperature,  pres-
 sure and corrosion  conditions) should be specified in the  system design.
     5.4.5   Dust Disposal Equipment
             The  physical  and/or chemical properties of the dust  collected
 in  the hopper require appraisal at this  point.   Although the  disposal tech-
 nique  is usually independent of the primary system operation,  some  pre-
 treatment of the process  effluent  may contribute significantly to  the
 ease of  handling of the collected  dust.   For example,  the  injection of
 an  inert mineral dust might  reduce the  tackiness of some resinous mater-
 ials to  the  point where bridging problems  in hoppers  or plugging in screw
 conveyors could  be  reduced,  and the overall system economics  improved.
             The  final selection of hopper  design,  hopper outlet valving,
 vibration equipment  (if needed), screw conveyors,  chutes,  pneumatic  con-
 veyors,  and  other ancillary  equipment will  depend  upon several factors.
 These  may include the angle  of repose of  the  dust,  its bulk density  and
 the volume and/or weight  to  be  handled.  Another  factor is the tendency
 of  the dust  to flow  freely or  to bridge, agglomerate or become sticky
 under mechanical stresses  (stirring, rapid motion,  vibration) or under
 changes  in temperature or humidity.  One must consider whether the hopper
 is  to  operate under  positive or negative pressure, and also what labor
 requirements are necessary for dust handling, i.e., whether the dust is
 to be reprocessed, sold or dumped as waste.  Several types  of disposal
equipment are discussed in Section 3.4.2.
5.5  FINAL SYSTEM DESIGN
     Having established preliminary design features for the individual
                                      V
parts of  the system,  it is now necessary to integrate these into a com-
patible system.  This process calls for  more engineering judgement  and
experience than that required for the preliminary design phase.  In addi-
tion,  the final planning should be geared to management's  philosophy on
the relative importance of initial costs and total annual  costs.
-------
      Summary guideline  tables  outlined  in previous  sections  should  be  re-
 examined  to determine whether  overall costs  can be  lowered by  adjustments
 in any  of the  principal variables.  Should any  appreciable differences  in
 costing be  observed, design  optimization  procedures should be  repeated  at
 this  time.   Since  the basic  types of equipment  required are  now apparent,
 appropriate  manufacturers' equipment lines should be reviewed  for prepar-
 ation of  specifications.  It is highly  advisable to consider standard
 items when  possible.  Quality, expected service life, ease of  repair and
 delivery  times, as well as initial cost will be evaluated.   Where possible,
 preliminary  cost estimates or  final quotations  may now be obtained.
      Before  fixing the design  specifications, a final search should be made for
unique  or extenuating factors  that could  possibly influence  performance
 or total costs, such as safety features,  noise, general plant  ventilation,
 insurance, relocation of system components, tax rebates, etc.  One should
also re-examine future needs with respect to plant expansion or process
changes.  If there remain some questionable design areas,or  if certain
design aspects are controversial, it may be advantageous to  seek the pro-
 fessional opinion of some competent outside person(s) or agency.  Other-
wise, final design and cost estimates can be submitted for approval and
procurement.

      No further design or cost changes  of significance are expected.
 However,  it  may be possible  to trim the overall costs a few  percent by
                               A;.
 the  following  analytical procedure.  Let  the total annual cost (C) be
 expressed as the sum of: power  costs (P);  all costs related to  fabric life
 including materials and labor  (F); annual distributed initial  costs in-
 cluding interest, taxes and any other item that is directly  related to
 initial installed cost (I); and fixed costs not considered elsewhere (X)

                          C=P+F+I+X                     (5.2)

Select a key design parameter, such as  filter velocity (V),   and introduce
it into each of the terms of this equation where applicable.    In each
term, assign to the parameter its appropriate exponent (Table 5.4)."
-------
 Power,  for example, is approximately proportional to V .  This means that
 if a slightly different velocity (V) were chosen,  the revised estimate
                                                          2
 of power cost (P1) would be approximated by P' = P (V'/V) .   Substitution
 of the  design parameter into the other terms of the equation yields the
 following expression for a revised total cost (C1)

              C-  = P(V'/V)2 + F(V'/V)~°-5 + ICV/V)"1 + X         (5.3)
 That is,  the result of changing from the design velocity previously
 selected  to a new value can be estimated by the expanded equation.   Cost
 factors P,  F, I,  and X have already been determined by careful design pro-
 cedures.   To estimate the effect of a 5 percent increase in filter  velo-
 city,  the ratio (V'/V = 1.05)  is used,  and  (C1) is  readily evaluated.
      By few repetitions of this process,  the velocity  that will give the
 minimum total cost may be estimated.   If this new estimate is  appreciably
 different from the original design value, a re-examination of  the system
 design is called  for.
      By this process, the effect of minor changes in any design parameter
 may  be estimated.   For parameters  having both positive  and  negative
 exponents  in Table 5.3 (filter  velocity, Ave.  Cloth Ap,  etc.)  an optimum
 design value may  be  estimated.   Note, however,  that  the  parameter exponents
 listed in Table 5.3  should  be  accepted  as estimates  only,  owing to  the
 limited field  and  laboratory data  available  from which  they derive.  With
 respect to  trends,  i.e.,  direct  or  inverse  relationships,  and weak or strong
 dependency  of  a given cost  category,  the exponents  cited  in Table 5.3 may
 be interpreted as  tentative typical values.  These  exponents will not
 apply  uniformly to all  systems.  One  is also not constrained to  use  the
 precise grouping of  cost  factors cited here.
 5.6  PROCUREMENT AND RESPONSIBILITY
     The  final responsibility for the design,  fabrication, procurement
 of materials, approval  of specifications, performance testing,  mainten-
 ance, and the overall performance of the fabric filter system should fall
within one department and preferably with one individual of decision

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                                Table 5.3
                  APPROXIMATE OPTIMIZING EXPONENTS OF COSTS'
PARAMETER
Filter Velocity
Temperature
Particle Size
Ave. CLoth Ap
Cleaning Intensity
Flow Volume
Loading
Humidity
POWER
+2
+1
--
+1
-1
+1
+ .5
+--
CLOTH-RELATED
-.5
-1-2.75
-2
--
+1
+1
+1
+--
INITIAL
-1
+1.75
-1
-1
-1
+1
+.5
- —
 Note:   Fixed costs (X)  are invariant by definition.
 *
  May vary from above,  values  according to circumstances.
 insufficient information.
Blanks indicate
 making capability.   Although  several  departments  or  divisions  within a
 company may  contribute  their  services,  any  joint  sharing  of  responsibi-
 lity  should  be  avoided  to minimize  oversights,  schedule conflicts,  or
 other committee type problems.
      Following  receipt  of quotations  for system components and installa-
 tion  costs,  one should  verify compliance with specifications,  including
 the warranty aspects  of all components.  This is  equally  important when
 a  turn-key package has  been purchased.  Although  there will  be occasions
 when  completely defensible design changes may be  proposed by equipment
 suppliers, one  should not expect any extreme deviations in dollars or
 design with  a carefully executed program.
     Fixing  the  responsibility for on-line  fabric filter system perfor-
mance cannot be  overstressed, especially with smaller installations.
Frequently, a manufacturer is blamed for faulty equipment operation,
whereas the problems have actually arisen because of user abuse and neg-
 lect.   This has  led to dissatisfaction and experimentation in some plants,
-------
sometimes  leading  to  the  purchase  of  several  fabric  ftlt.er  designs  to

handle one  type  of process effluent.  This makes  maintenance  routines

unnecessarily complicated.  When dissatisfaction  has  resulted in purchase

from several different manufacturers, one cannot  expect  to  receive  inter-

ested customer service from any of  them.

     As a  final  point, few if any manufacturers will  guarantee  the  per-

formance of fabric filter equipment,  because  its  function is  highly sen-

sitive to  process effluent changes  as well as to  the  quality  of the in-

stallation job and subsequent maintenance.  All of these are  difficult
to document.  Perhaps one of the best procurement policies is to request

the manufacturer to provide and guarantee installation and startup as

part of a package.  He should also provide a set of guidelines  for

operational routine and maintenance.at no cost,  and he may provide

training for the men who will use the new system.

5.7  REFERENCES FOR CHAPTER 5

     1.   American Society of Mechanical Engineers, Determining  the Properties
         of Fine Particulate Matter: Power Test  Codes. PTC-28, New York (1965),

     2.   The Buffalo Forge Company, Guide to Shaking Bag Aeroturn Selection.
         Buffalo, New York,  September 1968.

     3.   The Fuller Company,  Collecting Hot  Dust and Noxious Fumes with
         DraccoR Glass-Cloth Dust Collectors,  Bulletin DCS-IB (July 1967).

     4.   R. Frey and  T.  Reinauer,  New Filter Rate  Guide,  Air Engineering.
         (30 April 1964).

     5.   F. A.  Zenz and  D.  F.  Othmer,  Fluidization and Fluid-Particle
         Systems. Reinhold Pub.  Co.  (I960).

     6.   The Buffalo  Forge Company,  Fan  Engineering.  6th  Edn., Buffalo,  NY,
         (1961).

     7.   American Conference  of  Governmental  Industrial Hygienists,  Indus-
         t;rial Ventilation,  10th Edn., Lansing,  Michigan  (1968).

     8.   P.  W. Spaite, D.  G.  Stephan and  A.  H. Rose,  High Temperature Fabric.
         Filtration of Industrial  Gases,  J.A.P.C.A.  11,  (5)  243, (May 1901).
-------
                              CHAPTER 6
                        FABRIC FILTER PERFORMANCE

6.1  INTRODUCTION                                                  6'3
6.2  LABORATORY PERFORMANCE OF CLEANED EQUIPMENT                   f-9
     6.2.1  Bench Scale Performance                                6-9
            d.2.1.1  Pressure Drop - Time Relationship             6-9
            6.2.1.2  Dust Collection Efficiency                    6-10
     6.2.2  Single Bag Performance, Pilot Scale Tests              6-13
            6.2.2.1  Pressure Drop - Time Relationship:             6-13
                     Basic Concepts                                6-13
                     Liquid Filtration Analogy                     6-17
            6.2.2.2  Dust Profiles                                 6-20
            6.2.2.3  Analysis of Mechanical Shaking                6-30
                     Residual Drag                                 6-33
                     Filter Capacity                               6-38
            6.2.2.4  Effects of Fabric Structure                   6-49
            6.2.2.5  Effects of Humidity                           6-62
            6.2.2.6  Effects of Velocity                           6-6*
                     Hopper Fallout                                6-6V
                     Particle Size Stratification                  6-70
                     Deposit Consolidation                         6-72
                     Particle Penetration                          6-74
     6.2.3  Single Compartment Performance                         6-75
            6.2.3.1  Shake-Type Collector                          6-76
                     Light Dust Loading                            6-76
                     Filter Aid                                    6-78
                     Periodic Shaking                              6-80
                     Normal Dust Loading                           6-81
            6.2.3.2  Other Single Compartment  Studies               6-83
                     Hopper Fallout                                6-83
                     Particle Size Stratification                  6-84
                     Deposit Consolidation                         6-84
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6.3  LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT             6-85
     6.3.1  Basic Pressure Drop Equations                             6-85
     6.3.2  Performance of a Multicompartment Collector               6-90
            6.3.2.1  Studies with Light Dust Loadings                 6-91
                     Basic Media Performance                          6-91
                     Further Studies of Filter Aids                   6-92
            6.3.2.2  Studies with Heavy Dust Loadings                 6-94
     '               Frequency and Number of Raps                     6-94
                     Reverse Flow Air                                 6-96
                     Inlet Dust Loading                        '       6-96
                     Comparison of 5 Fabrics                          6-9'J
6.4  LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLEC-     6-99
     TORS
     6.4.1  Reverse-Jet Filter (Mersey Type)                          6-99
     6.4.2  Pulse-Jet Collector                                       6-101
6.5  FIELD PERFORMANCE                                                6-101
6.6  REFERENCES                                                       6-105
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                               CHAPTER  6
                       FABRIC  FILTER  PERFORMANCE
 6.1   INTRODUCTION
      The  determination  of  fabric  filter performance includes the specifi-
 cation  of pressure  drop, collection efficiency, component life, and costs
 during  operation, in  terms of dust, gas, and fabric parameters.  Pressure
 drop  with a given dust, gas, and  fabric is determined by collector filter-
 ing velocity, by the  method or mechanisms of cleaning, and by the amount
 of cleaning.  These same factors  simultaneously influence efficiency, life,
 and operating cost.   Every fabric filter system in service can, in principle,
 he described by one or  more analytical statements relating pressure drop to
 dust  and  gas flow rates, dust-fabric resistance, and cleaning mechanism
 operating parameters.   This Chapter considers pressure drop performance
 and its variation.  Data are also presented on fabric filter efficiency
 as effected by operating parameters.  Efficiency, cost, and life have not
 been  reduced to analytical relationships as has pressure drop and, as a
 consequence, overall  optimization of performance across the several vari-
 ables has not been  reported.  Supplementary cost data are contained in
 Chapter 7, and maintenance, service life, and failure modes of components,
 principally fabric, are discussed in Chapter 8.
      There, are approximately 100 fabric filter models commercially avail-
 able  in the U.S.  (Chapter 3), each available in a range of sizes.   These
 differ principally  in the type and arrangement of fabric used and in the
method of cleaning  the fabric to control operating pressure drop.   The unit
 operation of all these models can be represented, as indicated in Figure
 6.1, by a system into which the particles and gas flow, and to which power
 is applied to produce external observable changes in gas and particle flow
rates.  (Additional related system inputs include costs and additional out-
puts  include component life considerations.)  With proper quantitative
statements about the input parameters shown on the left, the output vari-
ables may be estimated with reasonable assurance.   Not all the factors
indicated are equally important, and in specific instances many may be
relatively unimportant to the total performance of the system.
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                                         POWER
                         I
FLUID FLOW               -
  Temperature            I
  Density                I
  Viscosity              .
  Moisture               I
  Chemical Species       I
    Inert
    Acid
    Alkaline
    Corrosive
    Condensible
  Flow Rate, Average
  Rate of Change of Flow
  Flow History
PARTICLE FLOW
  Concentration
  Particle Size
  Size Distribution
  Shape
  Density & Packing Char.
    Bulk Density on Fabric
    Density Changes with Pres
  Surface Characteristics
    Moisture Adsorption
    Adhes ion
    Electrostatic Properties
    Oxidation
    Sintering or Bonding
  Chemical. Species
    Inert
    Ac id
    Alkaline
    Corrosive
                                      FABRIC  FILTER
                                         S YSTEM
I
V	
                                               FLUID FLOW
                                             PARTICLE FLOW
          PRESSURE DROP
         COLLECTION EFFICIENCY
          POWER EFFICIENCY
          COSTS
             Capital
             Operating
             Maintenance
             Repair
          COMPONENT LIFE AND  FAILURE ELEMENTS
             Operating History
             Fabric
             Seams
             Supports
             Mechanisms
          ANCILLARIES
             Dust  Handling System
             Control
             Other Systems
                 Figure 6.1.  Parameters Controlling and/or Describing The
                              Performance of Fabric Filter Systems.
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     TheHe external system parameters are determined by the functioning
of the  internal components of the  fabric filter, as illustrated  in
Figure  6.2.  Each of the components listed can exist over a range of
sizes,  shapes, operating variation, and arrangements within the  collector.
Table 6.1 presents a summary of the effects on performance of  fabric  fil-
ter  internal component configuration and operational functioning.
     This chapter discusses the relationships of the external  performance
of fabric filters to the form and  functions of the internal components,
insofar as this information is available.  Performance relationships  are
highly  dependent on properties of  the particulate material described  in
Chapter 2.  Chapter 4 contains fabric characteristics  which,  in conjunc-
tion with the deposited dust, also influence performance.  Details of the
internal configurations of alternate collector systems are discussed  in
Chapters 3 and 5.
     Laboratory testing of fabric  filters under controlled conditions has
provided much basic knowledge and handbook data for collector  design  and
interpretation of performance.  Laboratory or pilot plant research and
development studies require a simulant dust for the process to be modeled;
dust feeding and redispersion apparatus; flow, pressure, temperature,
and humidity measuring instrumentation; appropriate duct work; stack
sampling equipment; and other special equipment for measurement  and
analysis of performance parameters.  Most major fabric filter manufacturers
have development laboratories available for trials on specific dusts,
including particle size measurement instrumentation, one or more commercial
filters set up for test, fabric evaluation test rigs at bench scale, dust
generation apparatus, etc.  Data developed from these sovirces  is usually
proprietary.   However, summary charts, tables or nomographs are often pre-
sented to aid in selection of appropriate fabric area for specific filter
applications  (Section 5.2).
     Discussed below are the data available on filter performance in
terms of internal component configuration and function and input para-
meters.   The  basic types of collectors for which comprehensive labora-
tory data are available include:
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                  INTERNAL COMPONENT CONFIGURATION AND
                             CHARACTERISTICS
1.  Fabric
    A.  Size of Fabric Element, L.D.
    B.  Fabric Design, Weave, Arr.
    C.  Fabric Structure, Nap
    D.  Strand Structure, Arr.
    E.  Yarn, Fiber, Fibril Size, Arr,
    F.  Fabric Materials, Service-
        ability
    G.  Manufacture, Finishes,
        Lubricants
    H.  Permeability; clean;
        in-service
    I.  Fabric Weight
    J.  Fabrication, Sewing

2.  Collector
    A.  Compartments
    B.  Headers,  Dust Inlets
    C.  Fabric Attachments
    D.  Construction Materials
    E.  Hoppers,  Dust Outlets
Cleaning Mechanisms
A.  Mechanical
B.  Gasdynamic
C.  Combined, Sequencing
D.  Coupling Characteristics

Performance Re lat ionsh ip.s
A.  Local Pressure Drop
B.  Local, Efficiency
C.  Residual Drag, Local.
D.  Deposit Drag, Local
E.  Local Velocity
F.  Local Particle Concen-
    tration
G.  Changes in above as re-
    lated to internal con-
    figurations, external
    system inputs, and
    microscopic  processes.
              Figure 6.2.   Fabric  Filter Internal  Component  Parameters.
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                               TABLK  (). I

            EFFECTS  ON PERFORMANCE  OF FABRIC  FILTER  INTERNAL
                         COMPONENT  CONFIGURATION
 Component
   Typical
 Configurations
                                            Effects,  Remarks
 Housing
Dusty
Gas
Inlet
Class-
ifier
Dust
Hopper
 Rectangular,
 cubical,
 cylindrical
Top,  bottom,
tangential
Rectangular
chamber, one
module without
cell plate or
other internal
components
Inverted
pyramidal,
conical, see
Appendix 6.1
 On outside  filtering  fabric  configura-
 tions,  the  gas  approach  velocity  is  de-
 termined  by housing size,  relative to
 space occupied  by  filter medium;  housing
 acts as settling chamber in  this  case, to
 reduce  dust concentration  reaching fabric,
 especially  for  the larger  size  particles;
 and may amount  to > 50%  reduction in dust
 load to fabric  at C   > 5 gr/ft3.

 Inlet at  bottom, hopper, or  on  side  usually
 followed  by baffle to prevent direct jet
 of dusty  gas on fabric;  inertial  effect of
 baffle  reduces  the dust  concentration
 reaching  the fabric,  particularly for larger
 particles (> 50 urn);  tangential inlet en-
 hances  this effect; top  inlet designs with
 inside  filtering fabric  configuration tend
 to have less stratification  of  particle
 size on fabric; bottom inlet designs have
 larger  particles depositing  nearer bag in-
 let, produces wear, scour, and  abrasion
 problem on  the  fabric with some dusts; in
 large multicompartmented designs, inlet
 header must be  properly  proportioned for
 flow-pressure balance.

 Classifier  reduces dust  load to fabric by
 settling and inertial turn effect, espec-
ially for the larger more abrasive particles;
 usual large empty chamber adds  little or no
 increased pressure drop; same function can
 be  accomplished with  inertial skimmer or
 cyclone in  smaller space but with greater
 pressure drop requirement.
 In  conjunction with bottom inlet  location
 and inlet baffle,  hopper acts as  inertial
 skimmer or gravity settling chamber;  sized
hopper relative to gas flow determines
 particles  inertially  removed; dust build-up
 in hopper can reduce  effective flow  to
bottom entry bags,  hopper dust flow
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                          TAHLK  6.I  (Continued)
Contponont
    Typical
 Configurations
                                             Effects,  Remarks
Cleaned
03 S
Outlet
 Fahri c
 Arrange-
 ment
Fabric
Arrange-
ment and
Suspens ion
Tube
Sheets,
Thimbles
Cleaning
Mechanisms
Side, top
  Bag, tented
  top, open
  bottom
  Sleeve, open
  both ends
  Sleeve, open
  top, over
  wire frame
  Multibag
  Envelope


  S leeves-clamp,
  spring insert
  Closed ends-
  hook,  buckle,
  cap
  Envelope-
  button, hook
  (Numerous)
 problems, bridging, etc., associated with
 high humidity  (>  70%) and greater  adhesion
 (see Ch. 2); recent studies on hopper  de-
 sign and dust  flow  indicated  in Appendix
 6.1; bin vent  and silo  filter applications
 may discharge  directly  to storage  without
 hopper; hopper must be  air-tight when  under
 suction to  prevent  redispersion of collected
 dust to fabric.

 No effect of configuration on performance
 except insofar as outlet gas  flow  related
 to back flow air utilized for cleaning;
 large multicompartment  units  require careful
 design of outlet header to assure  even  in-
 let flow distribution.
 Fabric cotifiguration and utilization de-
 termined local dusty gas velocity  which in
 turn controls  Local dust-fabric resistance;
 fabric arrangement  relative to cleaning
 method and mechanism determines both resid-
 ual drag and effective  drag; too much  fabric-
 crowded into housing becomes difficult to
 service, thereby limiting maintenance and
 producing poor performance,  i.e. dust  leaks
 and accelerated deterioration of fabric and
 other components.
 Tension and tension changes  affect cleaning
 distribution, and indirectly, fabric por-
 osity.   Installation and ease of maintenance.
Wear often results at stress points.
Spacing of elements affects wear, aero-
dynamics and sedimentation, and maintenance
ease.  Thimbles can cause wear.
See Sections 3.3, 8.8.
-------
      . Single compartment intermittently cleaned designs
      . Multicompartment periodically cleaned designs
      . Continuously cleaned designs
 (Gradations between available commercial designs tend to make these cate-
 gories a matter of degree.)
     Most laboratory tests are directed primarily at pressure drop,
 and at ways to control, pressure drop within reasonable  limits by
 changes in operation, usually changes in cleaning.  Less data on the
 field performance of fabric filters in specific applications are avail-
 able, due to the usually limited range of variation of  parameters.  Per-
 formance from such pilot plant or full scale tests is discussed in a
 final section of this chapter.
 6.2  LABORATORY PERFORMANCE OF INTERMITTENTLY CLEANED COLLECTORS
      Intermittently cleaned collectors are designed and operated on a
 relatively light duty cycle.  Sufficient fabric area is furnished, in
 conjunction with low dust concentration, so that the pressure drop in-
 creases slowly over a time period in the order of hours.  The collector
 fan (exhauster) is shut off and some form of shaking or vibration is used
 to clean the fabric.  Typically, the filter may operate on its dust
 source over a full shift, and be shut down for cleaning when the produc-
 tion machinery is idle.  Most intermittently cleaned collectors operate
 with a filtering velocity in the vicinity of 2 to 3 fpm with dust
 loadings less than about a grain per cubic foot.  They  are usually em-
 ployed for relatively coarse dusts on dust control, venting, or nuisance
 applications.
     6.2.1  Bench Scale Performance
            6.2.1.1  Pressure Drop-Time Relationship.- Specific dust-fabric
 filter resistance coefficients (K«') discussed in Chapter 2 have been ob-
                                 ^                                  2
 tained in many cases with bench scale apparatus on the scale of 1 ft
          1 2
 of fabric.  '   The pressure drop increase for a stated operating time
may be estimated (see Chapter 2)  by the following equation:

                        & Ap(t) = KZ' cVt/7000                  (6-O

-------
where C. -  inlet dust concentration  (grains pur  ft'), V =  filtering  vel-
ocity (ft/min) and t - time  (min) .   Experimental values of K«/ determined
l>y Williams et al.,  Tor a constant  filtering velocity and a  single
(cotton) fabric-, were presented earlier  in Table 2.37.  They  were shown
to be a function of particle material and approximate particle size,  in
accordance with the expected influence of these  parameters on deposit  por-
osity.
                                              2
             Later studies by Snyder and Pring   demonstrated  that K  '  is
a function of dust properties and type of yarn or fabric (amount of  free
fiber surface available to the dust  deposit), as shown in Tables 6.2a  and
b.  Their data indicated that K,,' is a resistance coefficient specific for
the dust and fabric.  The quantitative separation of thfe effects of  fabric
from the dust properties in  the specific resistance coefficient was  not
attempted.  Effects of dust  particle size and shape, and fabric yarn or
nap on  the resulting pressure drop increment are shown in Figures 6.3a, b
and c.  Snyder and Pring's pressure  drop - deposit curves are seen to  be
non-linear with a marked upward curvature evident in several  instances.
Typical K»' values ranged from 234 for freshly-formed MgO fume on napped
     D
Orion   to 7 for petroleum coke fines on spun staple Orion napped both  sides
Napping resulted in about a  10% reduction in K«', but usually resulted in
a higher residual dust deposit after shaking.
            6.2.1.2  Dust Collection Efficiency.- Collection  efficiency
was found to range from 84°X,'  for MgO  fume to greater than 99.47 lor petro-
leum coke.  However, the Snyder and  Pring^ data are of limited value  as
they do not represent typical industrial usage.  Fabrics were not utilized
for long periods to achieve  equilibrium  priming  or aging, and cleaning
parameters were not quantified.  The principal value of their study  was
to direct attention to the effects of free available fiber on K,.', and
to the  non-linear variation  of K2' with deposit weight.   They also pro-
vided experimental validation of the predicted effect of «as viscosity
on pressure drop.
-------
                              TABLE 6.2a
               PROPERTIES OF VARIOUS ORLON FILTER FABRICS'

                       Dust:  fine petroleum coke
                    Results:  average of 5 cycles
Dust Residual
Cloth Weight Perme- Loading Dust, Collect-
Oz./Sq. ability Grams/ Grams/ ing
Yd. New Used Sq.Ft. Sq.Ft. Efficiency
Napped filament 3.9 35 10 29.00 4.18 99.91
Orion, 3/1 twill
Knit Orion, 7.6 85 58 15.40 4.58 99.70
napped
Orion spun 7.5 60 33 18.21 1.48 95.19
staple
Orion spun 9.0 100 50 19.95 2.15 99.64
Filtra-
tion
Constant ,
K '
K2
17.1
12.5
15.0
7.4
staple, napped
both sides
Orion spun fiber-
stock, 3/2 twill
4.9
110   62
17.96
0.99
93.42
13.0
      From Snyder and Pring, Ref. 2.
                              TABLE 6.2b

             FILTRATION CHARACTERISTICS OF NAPPED AND UNNAPPED
                            SIDES OF ORLON*

              Orion:  1 oz. napped filament, 76 x 72 count, 3/1 twill
            Results:  average of 4 runs
              Fume:   freshly generated magnesium oxide




Napped side
Unnapped side

Used
Perme-
ability3
4.2
8.1
Dust
Loading
Grams/
Sq.Ft.b
12.0
13.8

Residual
Dust ,
Grams0
15
4.5

Collection
Efficiency
%
89.4
84.0

K '
K2

234
251
      Permeability after shaking,  cu.  ft/min./sq.  ft.  at 0.5 inch w.g.
      Corresponds  to weight of dust removed by shaking,  grams/sq.ft.
     c
      Dust remaining on cloth after shaking.
t
 From Snyder  and Pring,  Ref.  2)
-------
o
 CN
re •
.5*
                                          Figure  6.3a.  Values of Ap vs.
                                          Deposit Weight (LTV) in filter-
                                          ing  fine  petroleum coke dust.
                                          Figure 6.3b.  Values  of Ap vs.
                                          LTV covering dust  generated in
                                          abrasive biasing of steel  paint
                                          drums on high twist,  unnapped
                                          Orion with an extremely  low fiber
                                          surface area per square  foot and
                                          fiberstock Orion.
     eo-~ 400—soa~ *6o—
     COO  2000  MOO  4000  SOOO
TOO	600  BOO
LTV GRAINS /fT*
Figure  6.3c.   Effect of particle
size and  shape on filter resistance
of cotton sateen cloth. Silica gel
dust, 43% less  than  10 microns;
Scale dust, 27, less  than 10 microns;
Limestone  dust,  32%  less than 48
microns.

(From Snyder and Pring,  Ref.  2).
-------
      6.2.2  Single Bag  Performance, Pilot  Scale .Testa
             6.2.2.1  Pressure Drop-Time  Relationship
             Basic Concepts.  - When textile fabric is used  as  a filter for
dusty gas in the form of  a bag, tube, or envelope, and then cleaned repe-
titively, an equilibrium  pressure drop-time behavior is observed.  After an
                                                                            2
initial  priming or aging  period (which.may range from < 10 hours to > 10
hours for certain dusts and  fabrics), the  pressure time response appears as
illustrated in Figure 6.4.   The pressure-time trace will then be repeated
cyclically, provided that the gas and dust flow rates are  maintained con-
stant and if the cleaning energy patterns,  durations, etc., are applied
uniformly.   The pressure-time curve depends upon the gas viscosity, particle
properties, (size, shape,  surface phenomena),  fabric properties, (fiber,
yarn,  weave, finish, nap)  and operating  characteristics of the cleaning
mechanisms.  If the flow  of  dust to the  fabric  filter is stopped at any
point, and  a pressure-flow curve is determined  using cle^an air,  it will be
found  that
                               Ap - S
   (I}"
                                                                     (6.2)
                    n
                    10
                  »
                  *  9
                  I
                  w
                  e  a
                  °~
                 §  4
Du*t:
tutlcli 111* Mi'-
Du«t Coocutratlo*
a - gralna/ft*
Fabric Warn  cc,  vt, AT?,
    |: Typa a«?Utudt (raq. duratioa
                                        Incrtuad pr«iur« loi> ciuM
                                        1>)F lncrt.it u K,' .t hljhtr
                                        vtloclty     2
                                2.1 x 5 - A.9 In.HjO. calculattd prtilurt drop
                                            •t iqual dtulcy, at
                                            hlghtr flov ratt
                        - Kfftctlvt prtnurt drop
                        -Rttldual pr«iure drop
           v • 3.0 fpm
                              "' '
                                      ?0
                                                 32  36
                                                        40  44
                                     TIMl, minutet a V, lbi/ft
    Figure 6.4.   Pressure Response  in Constant Flow  Rate  Gas Filtration
                  (Constant Gas and  Particle Flux).  (From  Hnt^wardl and
                  Durham, Ref. 3).
-------
where n -- 1 as long as viscous flow obtains  (for a granule Reynolds
number < 10).  The property S or "drag" is a variable function of
fabric, flow, and dust parameters as described  in Chapter 2;  it  is
analogous to the resistance in a purely resistive electrical  cir-
cuit described by Ohm's Law, i.e., Voltage = Resistance x Current.
                                            4 5
            The drag, S, may thus be defined '  as
                                                                     (6.3)
for small Reynolds numbers, which is the case of most fabric filtration.
S is a property of the  filter and applies to any filter area.  If S varies
from area to area, due  to variations in permeability for example, then
the overall effective drag S  is given by the following, again analogous
to the electrical case  of resistances in parallel:

                                                       j:            <6'4>
where the a. are the incremental component filter areas and the S. are
the local drags.
            The use of the drag concept allows direct comparison of filter
media from one filtering situation to another, regardless of filtering
                      6"
velocity or unit size.
            The Darcy permeability, K  , which defines the intrinsic per-
meability of the filter medium in terms of its structural form, is analogous
to electrical conductivity.  It is related to the specific resistance, K9 ,
of the filter medium by the relation
                        K      (l-e)pp   S ' (l-c)pp                   -

The ratio, W/S, provides another measure of cake permeability in terms of
the density of filter deposit per unit area, W and the resistance to air-
flow per unit velocity or drag S.  The latter description of permeability
is merely the reciprical of the previously cited specific resistance,  K«.
It should be noted that the permeability is an intensive property
-------
 (independent of mass) whereas  the  drag,  S,  is  an extensive  property (mass
 dependent) that relates filter resistance  to  flow rate.  While  the drag
 will always increase as additional material  is  deposited on the  filter,
 permeability may  increase,  decrease,  or remain  constant over the same
 period, depending upon  flow,  fabric,  and  dust interactions.
            Iltil izing concepts of filter drag and areal density  of deposit,
 the typical pressure-time response during equilibrium operation  of a
 fabric filter element shown in Figure 6.4 can be represented as  a drag-
density relationship as presented in Figure 6.5.  Several typical  features
                           FILTERED OUST MOSS, VMgrtlm/ft*)
              Figure 6.5. Schematic Representation of Basic
                          Performance Parameters for Fabric
                          Filters.

 of performance  are  illustrated.   The  total  drag of  the  filter medium in
 service  is  the  sum  of  the  drag  produced  by  the  fabric with its irreduc-
 ible residual deposit  plus the  drag of the  dust deposit added during
 the filtration  cycle.
             All  practical  cleaning methods  so far developed result  in
 some nonuniformity  in  the  effectiveness  of  cleaning obtained in different
 areas of a  filter cloth.   Because of  this,  when filtration is resumed on
 a recently  cleaned  cloth area,  the rate  of  change of drag  of the medium
 will vary in the early part of  the filtering cycle.  The curve in Fig-
-------
urc (>.r>  indicates that cleaning  leaves a nonhomogeneous and discontinuous
surface-  that is subject to rapid increases  in resistance when a new de-
posit is first being formed.  Curve segment A represents the increase
during this part of the cycle.  After the discontinuities have been
largely  eliminated by preferential flow of  the dust-laden gas through  low-
resistance areas, a relatively uniform deposit has been formed.  Sub-
sequent  deposition of dust generally results in a linear increase  in re-
sistance to fiow as dust accumulates, curve segment B.  Efficiency of  the
filter is lowest during the early part of the cycle while the discontin-
uities are being repaired.
            This relationship will hold for any dust-fabric combination
but the  shape of the curve may be significantly altered by differences
in the cleaning or the nature of the fabric.  Highly effective cleaning
will lower the residual drag value, S , and poor cleaning will increase
it.  For that part of the cycle represented by curve segment B where the
homogeneous deposit is formed, the specific properties of the dust tend
to control  the rate of increase of resistance.   The permeability, K, of
the dust-fabric combination will be reflected by the slope of the curve.
These general relationships relating to pressure loss through the medium
in a filtration process apply whether the area  being considered is a
single bag or many bags in a single compartment, so long as all  of the
area is  put in service at one time.
            Because of the dominant importance  of pressure drop in
equipment of this type, the drag of hhe filter  medium must be given
primary consideration.   Filter drag,  however,  is but  a single element  of
the total pressure drop that determines fan and power requirements.  In
practice, the total pressure drop to  be overcome may,  because of  duct
losses,  be  twice that attributable  to the filter medium alone.  The total
system will contain sections with gas in turbulent  as  well  as viscous  flow,
so that  total pressure  drop will vary with  flow exponentially by  some
power between 1.0 and 2.0 instead of  directly with  flow as  it does  for
the Ap across the medium alone.
            The residual drag,  S ,  is determined by  the properties  of
the dust and the fabric,in conjunction with the operation  of  the  cleaning

-------
mechanism.  Cleaning energy  input, as distributed through accolor.ition,
frequency, and duration, do fines the amount of dusL removed down to  some
irreducible minimum value.   The  effective drag, S ,  is defined as that
value of  the draj; obtained  from  extrapolation of the  linear portion of
the S-W curve to zero W, at  the  ordinate.  For practical purposes,  the
filtration cycle appears as:

                             S(W) = |£ = Se + W/K                      (6.6)

where S   is some function of S  , fabric, and dust parameters.  The actual
shape of  the S-W curve  in region A has not been presented; instead  it
is usually approximated by this  equation.
            In terms of the  specific dust-fabric filter resistance co-
efficient ,
                                 K2' = 7000/K                          (6.7)
                                              2
providing W is calculated as grains of dust/ft  of fabric.  Then  the
pressure-time relation  is generally of the form

                           Ap(t) = SgV + K2' C^V2 t/7000              (6.8)

when the  S-W curve is linear from the extrapolated value of S  .   The  form
of the S-W relation is not always linear, and seems  to depend on  the amount
of fiber  available at the fabric surface for dust holding, as will be dis-
cussed below.
            Liquid Filtration Analogy.- Because investigations of liquid
filter performance have been treated in much greater detail in the tech-
nical and engineering literature, it is of some value to examine  their
applicability to fabric filtration.  A typical liquid filter utilizing a
fabric, as applied in the Chemical Process Industry, operates at constant
pressure drop and the flow rate  (or total flow volume) is allowed to de-
crease until a fixed volume has been treated.   The filter is then stopped,
drained, and the resulting cake  (deposit) is removed by manual or semi-
automatic means,  which may involve the physical removal of the fabric
system from the main flow housing.   In rotary pressure or vacuum filters,
the deposit and removal steps are continuous and automatic.   In the manu-

-------
        nl paper Liu' rake formed from a slurry of wood  fibers  is contin-
                                                                        •7
nous I y withdrawn as product.  The use of. rapid sand  filters  (2-'J gpni/l'L")

for water purification (in Sanitary Engineering) involves operation of  a

fixed hod of granules as a filter for fine participates until  the deposit

storage capacity of the bed is exceeded, whereupon the  filter  is then

backwashed.

            In each of these areas there are extensive  investigations and

analyses of the filtration process under a condition of variable deposit

and variable flow or pressure drop.  However, the following reasons

preclude the direct utilization of these data to the processes of fabric

filtration of gases, even when the same filtration substrates are employed;

            1.   The adhesion forces between small particles at a
                fibrous or granular substrate are several orders of
                magnitude smaller in liquids.  Adhesion forces for
                particles in air or gases are typically of the order
                of 1 dyne (Chapter 2,  Table 2.10) for sizes in the
                range of 1 to 100 urn,  and are dependent upon humidity,
                capillary condensation, and surface effects such as
                contamination, roughness, electrostatic charge,
                etc.  Table 6.3 indicates that adhesion forces in
                liquids are. typically  less than 10"^ dynes.  They
                depend upon van der Waals forces operai ive at
                molecular dimensions (~- 10~8 Cm).

            2.   Pressure drop in liquid filtration is generally
                much greater than in fabric filtration of air
                and gases.   Typical terminal pressure drop in
                fabric filtration is less than 15 inches of
                water, i.e.,  less than 1/2 psig.   Pressures
                used in liquid filtration in the chemical pro-
                cess industries are generally greater than
                10 psi (rotary vacuum filter) and may exceed
                100 psi (plate or frame type).   Rapid sand fil-
                ters typically backwash at 3 to 5 psi.

            3.   The combination of lower adhesion forces and
                greater pressure drop  are believed to have sub-
                stantial effects on the characteristic of the
                deposit in liquid filters as contrasted to de-
                posits likely to occur in fabric filtration of
                dusty gas.   In liquid  filtration through fab-
                rics,  one typically observes a deep cake of
                order of several millimeters thick,  much
                thicker than the fabric media.   Calculated
                values of deposit thickness in fabric filters
-------
                   (Table 2.36) are generally less than 1 milli-
                   meter.  Measurements of deposit thickness in I'abric
                   filtration have not been reported.

                   The irreducible residual deposit pressure drop
                   in fabric filters, measured by Sg or Sr as shown
                   in Figure 6.5 is a major portion of the total oper-
                   ating pressure drop in every cycle.  In the case
                   of liquid filtration, Ruth (see Appendix 6.2)
                   and other investigators have shown that the
                   pressure drop of the filter medium at the be-
                   ginning of the filter cycle is of little sig-
                   nificance in the resistance of tho medium nflor
                                     T
                   a cake has formed.
                               TABLE 6.3

               ADHESION OF VARIOUS PARTICLES TO SUBSTRATES OF
                        VARIOUS MATERIALS IN WATER

                         (for F    = 7 x 10"5 dyn)*
Substrate Material
Glass

Steel

Bronze
Glass
Paraffin
Particle
Material
Quartz
Graphite
Quartz
Graphite
Graphite
Quartz
Quartz
V"
11.0
7.5
11. 0
7.0
7.0
5.0
5.0
•v 7 **
V /0
0
20
33
40
40
76***
0***
      *
       (From Zimon, Ref. 8).

        7  = fraction of particle numbers remaining attached after
        application of  indicated force.

         For F,   = 1.6 x 10   dyn.
              det

             The structure of the deposit formed at the surface of a

fabric dust filter is markedly influenced by relative humidity and ad-

hesion, and electrical characteristics of the dust and fabric.  The aggre-

gates formed are probably very filamentous and chainlike for fine part-

icles, although more compact, similar to the liquid filtration case, for

larger dust particles.  For these reasons, deposit formation in dust


-------
filtration probably differs from that  in liquid  filtration.   Liquid  fil-
tration concepts such as complete pore blockage, standard  blocking,  cake
filtration, and the like, are very  likely to be  inappropriate models for
gas filtration phenomena.  While analogies between  liquid  and gas  filtra-
tion are attractive to consider, especially the  concept of cake  formation
in the latter case, they require further investigation before their  utility
can be evaluated.  A summary of pertinent studies in  the field of  liquid
filtration applied to fabric filter technology is given in Ret".  3.
            6.2.2.2  Dust Profiles.-  Pressure-time  data shown in Figure
6.4 and S-W curves given in Figure  6.5 represent a  macroscopic average
over the total fabric surface for a single cylindrical filter tube (or
                                             9
for many bags in parallel).  Stephan,  et al.,  have investigated local
interactions of dust, gas, fabric,  and cleaning  parameters at various
locations on a single bag.  Local mass deposited (areal dust  density)
was measured with a I'-gaging mass probe and local flow rate with a
velocity probe.  These two instruments are shown in Figure 6.6a  and  b,
respectively.  The improved mass probe containing a fi-source  directed
toward an end window G-M tube is held adjacent to the filter  bag.  The
resulting signal is interpreted through calibration as mass per  unit
area.  The filter velocity probe measures air flowing through a  section
                                                       2
of the bag.  Both devices measure an area of 1 to 2 in. .   Clean  unused
                                         22               2
fabric has an areal density of ~500 gr/ft , 10 oz/yd , or 0.07 Ib/ft  ),
so that the dust deposits encountered in practice are 1 to 10 times  the
                                                                      2
weight of the fabric.  Residual dust weights are typically <  0.1 Ib/ft .
            Typical results obtained with the mass  and velocity  probes
on a 9-inch diameter by 60 inch long bag are shown  in Figure  6.7.  De-
posited mass is greatest at or near the center of the bag height (~ 800
     2
gr/ft ).  Filtering velocity is lowest in this same region ( < 1 fpm).
Figure 6.8 illustrates the variation of filtering velocity with dust de-
posit density over the bag.   Values of the local relative  specific dust-
fabric filter resistance coefficient (K') have been calculated  (assuming
a uniform 1 in.  H_0 pressure drop over the total bag) as  shown in Figure
6.9.   Dust resistance is lowest at the bottom (probably as  a consequence-
-------

                                           End window
                                           G-M tube
                                           detector
                                   Personnel
                                   dosimeter
                                    Figure 6.6a. Improved Mass Probe,
   -

                                1.83 in.
                                Air Flow
                                Aperture

                                                                       Haystings-Raydi
                                                                       thermal anemone
                                                                       flowmeter
                                                         I   Cellular  foam rubber
                                                            baffle
c
                                   Figure 6.6b.  Filter Gas Velocity Probe.

-------
                   600   700   800



                 WALL DENSITY \q,t. tl ? )
                                      HI.TCR VtLDCiry
        Figure 6.7.  Corresponding Mass and Filter Velocity

                     Profiles. (From Stephan, et al., Ref.  9)
U
o
                200       400         600        800


                         LOCAL WALL DENSITY, groins/ ft2
    Figure  6.8.  Variation of Filtering Velocity with Dust

                 Deposit Density, from Figure 6.7.
-------
                    00
                    01
                    00
                    n)
                    01
                   TD
                    3
Operating cow) it tow:
9 x 60 In. b«|, cotton i«t**n,
fly «ih c««t duic
                                                  7000 x 1 In. HjO
                                                     W~x~V
                         
-------
 100

  90

  eo

  TO

  «>
t:  30

<  20

    IO

     0
                             o.a
                                                      _L
'o,n
                                                                         I
          I
           30  4O    60  80 100
                                          ZOO   300 400   600 800 K>00     2000  3OOO
                                  Log Duit H»i (Cr«ln>/

                          Log Dust Drag (In.  HjO/fpn)
 2.0    3.0 4.0
             Flgur. 6.10b.   Developmont of Duel: Drag Prof 111 Through • nitration Plxlod
             rfrom R«f.  A)


Hot«« to Fltur*  6.10
Pllt.r tub.. v.r.  6 In.  In dl«a«t«r by 63  In.  long fabricated  of  atandanl cotton aatun  (National
Flltar Madia Styla Ho. 74, 96 narp x 60 fin, 9.7 ot/yd?) having a ratad Frailar Poro.lty of
about 15 ft3/mtn.   In tarn of raalitanc.  and  paraaablllty, tha fabric would ba ratal  at Sg «
0.033 In H20/«pm and K - 14,300 (gralna/ft2)/(ln H20/£pm).   Shaking action vaa vartlcal  at
tha cop. Kith a  2-1/4 In. anplltud. at a fraquancy of 6.79  cpa for a duration of ona alouta.
Taata vara conduct.d at avaraga flltar valocltlaa ranging fron 0.75 to 7.5 fp», and  duat coocan-
tratlona In tha  rang. 1
-------
develops which  Is maintained  for  the remainder of  the  filtration cyclo.
Hoth S and W profiles  tend  to become nearly  flat at high mass deposit
values.  Effects of particle elutriation and of velocity on dust-fabric
resistance (K~' ~ V) are no doubt factors in the departure of the pro-
Tiles  from flatness.  The variations in profile slopes near top and
bottom of the  filters  appeared  in all experimental runs, indicating that
even when starting, with a perfectly  flat profile,  dissimilarities develop
along  the bag  length near the top and bottom of the bags.  This investi-
gation indicated that  filtration does not occur uniformly over the length
of a filter bag.  Different quantities of dust are collected and local
filter velocities vary by factors greater than 2 in the initial part of
the filtering  cycle at different  locations on the  tube.  The correspond-
                                                        4
ing values of  local permeability vary in space and time.   These varia-
tions  reflect  the differences in cake removal by the pattern of cleaning
energy applied, the relative adhesion of different fractions of dust as
a function of  cleaning energy applied, and the differences in dust struc-
ture over the  length of the tube.
            The approach to profile  linearity is expected since areas of
low resistance will handle higher than average air volumes, and vice
versa.  Consequently more dust  is deposited on the low resistance areas,
less on the areas of high resistance, and a self-balancing system exists.
                                                                  2
It is  likely that the  period 0,1 •- 0,2 (i.e., addition of 50 gr/ft )
corresponds approximately to the period of deposit repair and non-linear
K«'.  Values for the effective  average resistance  coefficient for the
total bag structure, (K' = 7000/K = 7000 S/W) are shown in Figure 6.11
for each of the increments, 0,1 -+ 0,n.  The resistance coefficient in-
creases rapidly during addition of the first increment of dust, then falls
to a relatively constant value  through much of the remainder of the cycle.
The terminal value is somewhat higher, possibly because of dust deposit
compaction.   Dust permeability  profiles for another test series are
shown in Figure 6.12.   Average K«/ increased from 2 to 12.7 during the
test.   Effects of particle stratification are apparent at the top and
bottom of the bag,  at the various intervals during filtration.
            General conclusions drawn from these studies are:

-------
BltSI
•* «J
*J O
t-1  (VI
•^ X
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U>  -
a y
•o c
  01
O -H
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£8
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         1.
        0
l-'igure 6.11
                   09
                             10

                        At - Ap
                        in. H20
     1.5        20         2.!

bAp, Dust  Deposit Pressure Drop,
               Variation of Average  Specific  Dust-Fabric Resistance Co
               efficient During  a Filtration  Cycle.
            1.  The local specific dust-fabric  filter resistance
                coefficient is a variable with  respect to both
                location on a filter tube,  and  time  '
                dust deposited.

            2.  The dust mass and drag profiles reflect  interac-
                tions of particle size, flow, and structure
                during deposit formation.
            3.  The average specific dust-fabric filter  re-
                sistance coefficient tends  to rise during the
                filtration cycle.
            Usiag the same techniques of mass  and  velocity measurement,
 the deposit characteristics may be  studied during the  process of cake
 removal.  Effects of shaking on dust mass and drag profiles  are illus-
 trated in Figures 6.13 a and b.  Figure 6.13a shows a  mass profile just
 before cleaning, and after one and  20 minutes of  shaking.  In this par-
-------
                                                 Ave.  Av«. AP.   W, ,
                                                  K   JC9, "H90 gr/ft*
                                              0,1 "5577   2.4 To?  1060
                                              0,2  991   7.1  1.0
                                              0,3  671  10.4  2.0
                                              0,1)  553  12.7  3.0  2290
           200  900 400  600 800 1000    2000 3000 40OO 6000   10/300
                 DUST MASS PERMEABILITY (GRAINS / FT2)/(IN. H20 / f pm)
       Figure 6.12.  Decrease in Dust Mass Permeability  through  a  nitration
                     Period  (from Stephan ,et. al., Ref.  4)
 ticular  case,  89.5 percent of the dust deposit was removed by one minute
 cf  shaking and 97.2 percent by a 20-tninute shaking period.  The reductions
 in  drag  are illustrated in Figure 6.13b.   The first minute of shaking re-
 duced  the  pressure drop by 89.7 percent and the next 19 minutes reduced it
 by  only  an additional  2.8  percent.   However,  dust removal itself is of
 secondary  importance to resistance  decrease,  which determines the pressure
 drop after shaking.  Before shaking,  the  cake had an average resistance of
                    2
 7.4 inch H20/(lb/ft -fpm)  and this  value  was  slightly decreased to 7.2 by
 one minute of  shaking.   After 20 minutes  of shaking,  specific resistance
was increased  to 20.  Continued shaking may reduce dust mass proportionally
more than  resistance.   Reduction in effective resistance is  more meaning-
 ful than quantity of dust  removed in  determining  optimum lengths of clean-
 ing periods.
-------
                                          MIN CLI ANIN'.
                                            ITRIOOI
                                       I.I   J  .  1  1_J
                                  0   4(1  bO m loo  I'll'1  4illt 600
                                  OUSI MASS -graint / ft2 t
           Figure 6.13a.  Fiffect  of Cleaning on  Residual Dust Mass
                          Profiles (From Stephan,  et al., Ref. 4).

  100

  so
  80


  60 -
              a
              § 30h
              \~
              _l
              < ZO

                10

                 0
                          r
                       B  (20 MIN. CLEANING
                       °-'       PERIOD)
                    EFFECTIVE
                              * 0.973 in
u ^0 _    (I MINI S^-O.IOO
        (20 MIN.) sn,« 0.073
    I	.1	L	
            .Ol    .02    04 .C6 .06 .1
                                           S (I MIN. CLEANING
                                                   PERIOD)
M0.r -
                          .    .        .  .
                          DUST MASS Drag
                                                   A  .6 .a i.o
                                                              2.0
                              (Inches H.O/fpm)
          Figure  6.13h.   Kflucl of Cleaning on Residual  Dra^ I'ro-
                          f Mi's (From  Stephan, et al . ,  Re L'.  4).
             Additional  data on development  of residual dust  mas.; pro-
files during cleaning are  shown in Figure  6.14.   An essentially uniform
terminal  dust mass of 1650 grains per square  foot was reached  under the
filtration conditions given in the Figure.  The  filters were then shaken
for 5 seconds (= "1st shake," consisting of 34 strokes) and  the residual
profile was measured.   Approximately 8870 of the  terminal dust  was removed
during this first period.   Successive incremental shaking periods were
then conducted,  and profiles  were measured.   Only 3"/, of the  terminal
mass was  removed in the  second period.  The profiles were progressively
flattened  with further  shaking.
-------
                                          isi THROUGH 7tn SHAKE, EACH 5 SEC
                                          8m THROUGH nth SHAKE, EACH 2 WIN
                                          i;ih SHAKE, 5 MIN
                                          Ulh SHAKE, IOMIN
                                             TERMINAL Lf  30>r> H?0
                                             AVG F,,.TER VELOCITY oiii/m."-
                                             IV.ET DUST CONC , 75grom»';. (I
                               150    200     250
                              DUST MASS, groms/tq ft
                                                   JOO
                                                         J50
                                                                ••00
      Figure 6.14.  Residual Dust Mass  Variation with Cleaning Dura-
                    tion.  WT =  1650  gr/ft  .   Shake,  6.75 cps. (From
                    Stophan and Walsh,  Ref.  10.)

             Stephan and Walsh also discuss  aging,  equilibrium residual

profiles,  filtering velocity, and particle  penetration into the fabric.

They  emphasize  that a non-uniform residual  profile exists at the start

of  every  filtration cycle.  Filtration never  occurs uniformly over the

total  filter surface.  Different filter velocities,  amounts of dust col-

lected, and deposit structure occur at various  locations.   They conclude

that data  from  laboratory bench-scale determinations  of  dust-fabric

resistance  coefficients cannot  be equated for design  purposes  to the

effective or  average  resistance coefficients of the  same  dust  on full-
;ized filter  tubes.

            One practical implication of the Stephan-Walsh  study:

            "....concerns the differences  in local cleaning
            intensities which exist  between very short clean-
            ing periods and appreciably longer ones.... Filter
            cleaning  follows  the  law of diminishing returns  -
            i.e., each  shaking  stroke contributes  less and
            less, with  the degree of cleaning approaching
            asymptotically some  limiting value for a given
            set of cleaning conditions."
-------
             "This.... suggests...the hypothesis that  apprec-                       ^
             iably  shorter  cleaning periods  than are  now common;v
             in  use  may  be  employed without  proportionately
             reducing  the length  of the  subsequent  filtration
             periods.  Such a  situation  would  have  two distinct
             advantages:
             1.   "Rate of filter  wear, which will always                            —•
                 be  related in some way  to the total  number
                 of  cleaning strokes experienced by the
                 filter, could potentially be  reduced.                              _
             2.   With  the use  of  shorter cleaning periods
                 at  more frequent  intervals, increased
                 filter  ratios -  i.e., smaller equipment                            ""*
                 size  -  might  well  be possible." ^
This hypothesis  is  supported  by  the recent  successful development of pulse-        —
jet filters, which  are  continuously cleaned on-line  by a short (< 1 sec)
burst of high pressure  compressed  air (100  psi) every 30 seconds or so.            	
Higher filtration velocities  are used (~10  fpm), and  fabric  life appears
to be at least as long as  in  many  shaken designs.
                                                                                   •^
            6.2.2.3  Analysis of Mechanical Shaking  - Mechanical shaking of
a single bag or  compartment is discussed below.  Analysis  of multi-compart-
                                                                                   —s
ment shaking with and without simultaneous reverse air flow, and reverse-
jet and pulse-jet cleaning are considered in  following sections.
             The  fabric  is  cleaned  periodically to remove deposited material
by  any of  several  possible means (Section 3.3), of which mechanical shaking
                                                                                   _/
is  one of  the more  common.  A minimum pressure differential  at the start
of  the filtration  cycle can be achieved by  vigorous  cleaning to  keep aver-
age residual accumulation  and drag small  (Curve A, Figure  6.15).  However,         ~"
too much energy  applied to the fabric to remove accumulated  dust shortens
fabric life and  reduces collection efficiency (Curve  B).   There  is, thus,           _j
an economic optimum to  be  determined (Curve C), which depends  on many pro-
perties of dust, fabric and cleaning.
            Fabric  cleaning can be  generalized to considerations of force
applied to the deposited powder  layer as a consequence of:   (a) motion
induced in the fabric,  or  (b)  interaction of  the motion of cleaning gas
with the deposit and fabric.  Removal forces  produced on the deposit are
                                                                                   _/
resisted by adhesion  (particle-to-fabric) and  cohesion (particle-to-

-------
                              InCrnalCy and Duration of Clewing

          Figure 6.15.  Cost Analysis in  Fabric Filter  Cleaning


particle) forces within the deposit  matrix.   In principal,  the force app-
lied to the deposit can be determined from consideration of the  energy
applied during cleaning.  In practice, cleaning forces  are  produced by a
configuration having  fixed frequency and  amplitude,  or  other characteristic
patterns of application, and deposit mass variations.   The  point of opti-
mum cleaning is produced by field adjustments of  one  or more of  these
variables.
            A comprehensive analysis of mechanical shaking  in fabric fil-
tration has been reported by Walsh and Spaite  .  They  studied the  effects
ol shaker amplitude,  (c ) shaker frequency (u>) and duration (number of
                       9
strokes, u>t) on residual drag (S ) and filter capacity  (weight of dust col-
lected during the filtering cycle, W without exceeding  some terminal fil-
ter drag, S ).   Tube  geometry, fabrics, shaking direction,  etc., were held
-------
constant..  The  pilot scale test unit  is  shown in Figure 6.16.   bags were
installed with  zero tension and zero  slack.   The range of variables inves-
tigated  included  V = 3 fpm, 0.05 <  e  <  2  in. ,  5.2 < cu < 56 cps,  and 5
                                  """  S """*             ~~
< o)t < 700 strokes.   Maximum acceleration  produced by the simple  harmonic
motion is:
                                =  (2 in>  c                         (6.9)
where -    -  amplitude of the motion,  1/2  the  peak-to-peak stroke  length.
The muxJmum occurs at the limits of  the  shako motion.  The range oJ'
accelerations  studied was 0.72 <. ir v.  15.5 g's (10  < y physical  limits as illustrated in Figure 6.17.  Shaking
            r
I  ISLE
3  JNH
3
                                   . Bi.C*£«S
                       &)  5 OJST FEECE"
                          £ MiC
» SHAKING MCOivSv
0 :uST CATCH |;NS
  •N KACC*
1 1 OJST :ATCH iiNS C.J"-
                                               i2  HOPPER OX"
                                               .s  :U*T CATCH
                                             Fly /isli  test  dust;
                                             
-------
          10
       i
       i
       i
       ut
       O
       a!
       2°'
                                 V»- LIMIT OF PR* i ICAL
                                     AMPLITUDES («MO)
«'•
                        09 g'«
        0 01
          0 I
        * u«i,h », S|i«lt» t) produce an initial  reduction in residual  drag,
S .  A point is soon reached when increased duration produces little
further  reduction in residual drag (S   =  S  .  ).  The initial cleaning
                                      r    mm
-------
                JUT ciiwmoHi;:
                Ml.TUS i;nTWN 4ATKKN MhKIC
                KI.Y ASII rrsr imsi, t..1 m rtii).  • I.H. cm
                MI.TKK Vr.U*:\T> -  I-" 11'»>   K
                              I INCH
                              I// INCH
I *x
\ x^
1
,\
1 ^~0
I |
11
^-.?S
	 A--
1
                                        W'iLV!.1! IJ
                                                  /.inlllh |n I, In/
.M>»IU in./.In.;
I.I»H>'' In./nln.'
                                                  jo ,,,.t. ,,h,ik.-
                                                       'T~
               20    4O    6O   aO    tOO    I2O   I4O    ISO
                                     I IWIMBtH OF STROKES),
                                                     180   200   210
       Fi.j';uro b. L8.   Effect of Cleaning Duration on Residual
                      filter Drag for Several  Shaking Conditions,
                      (From Walsh and Spaite,  Ref.  1.1.)
strokes  accomplish most of  the  cleaning.  A practical  limit exists beyond
which  increased shaking duration did not substantially decrease residual
drag.     This limit, indicated  at N  in the figure,  ...  "was approximately
                                     s
reached  when oscillatory motion was developed over  the entire filter tube.
Physically,  N  was the number of strokes required to produce significant
              s
discontinuities in the dust mass structure so that  ...  drag was at a prac-
tical  minimum because of low resistance areas."     Maximum capacity was
achieved with a greater number  of strokes.   In either  case,  15 seconds of
shaking  was  sufficient to achieve an effective minimum drag and maximum
capacity.  Commercial fabric filters typically shake for  30 seconds or
longer.

             The effect of shaker acceleration on minimum  residu; I  dv.ig
(S     at N ) is shown in Figures 6.19a and b.   These data indicate that
v min     s
the minimum  drag produced (the  nearly horizontal portion  of the curves
in Figure 6.18) is a function of the acceleration, as
                       mm
                             (7
                                     (in.  H20/fpm)
              (6.10)
for 0.7 v.,v< 6 g's.   For i» 6 g's,  S  .   -^constant, independent  of
        ~~~  •*-*                •        m i. n
-------
         6
         o
         c
         i
        to
        o
        <
        s
           o a
         u. o 6
           0 4

                        111 : i u •. - COT i UN i*Ti t N r«b».c
                        ,,i'M  I.: uTHUH 0 'I > «S»i
                        I .LTI N vllOCi'T - 1 0 torn
                        rfRMINAl. FiLTf'NORAC-     -^ ?.0m
      Figure   19a.
o
 6, S   • constant, (0.19 for
                                          fly alls  on sateen weave cotton)
                                          0.7 _. O < 6 g's
                                                      I
                 1.0         235          10

                       SHAKER ACCELERATION  (da's)
                                                             20
  Figure 6.19b.  Minimum Residual  Drag as  a Function of  Shaker  Acceleration.
-------
             The  form  of  the  S  -cut:  function appears  to he relatively in-
dependent of  the  type of dust  (Figure  6.20),  within the  limits  of  the
data available.   Properties  of the dust  (particle size distribution,
shape, adhesion  forces,  density) and characteristics  of  the  fabric  (free
available fiber,  weave,  yarn,  fiber material)  as well as filtration
parameters will affect the S -tot values.   Hard-to-clean  dusts,  or  napped
fabrics, for  example, will require greater shaking  duration,  but the
shape of the  drag-duration curve would be.  expected  to be similar to those
shown in Figures  6.18 and 6.20.
            The path  followed  by residual  drag (S   <*  S .  ), as a function
of cleaning duration,was analyzed  using  the data of Figure 6.18 and found
to be represented approximately by:
                   s*. ' s      •}              i
                    t	r     3      .     x   1
                   S  - S  .  =4  los  (au)t)  ' 4                (6.11)
                    t    mm                                   v     '

where S  is the terminal drag prior to shaking, S  is the residual  drag
produced by the number of cleaning strokes (< N ), and S  .  is the  mini-
mum drag obtainable with a given  input acceleration (Equation  6.10).
Data points taken from Figure 6.18 (for cot < N ) are shown in  Figure 6.21,
                                           ~~  S
            The number of strokes  (tut) required to reach minimum resi-
            >  . )  is a functioi
            mm
Figure 6.22.  For these data,
dual drag (S .  ) is a function of shaker amplitude (e ) as indicated in             J
                          Ns '  (a*>s =                        (6.12)
where (  is the shaker amplitude (1/2 the peak-to-peak total motion of
       S
the shaker) .
            In summary, residual drag appeared to be determined by
(number of strokes x peak acceleration), Equation 6.11.  However, resi-
dual drags smaller than a certain limit were not obtainable, the limit.
depending on acceleration, Equation 6.10.  At that acceleration, N
strokes (Equation 6.12) were required to reach the limiting drag.

-------
Figure 6.20.
             CLEANING DURATION, UK, STROKES

        Effect of Dust on Residual-Drag-Cleaning  Dura-
        tion (S -ot) Relationship.  (From Walsh and Spaite , Ref. 11)
                          -H I tin A I Draft ••[!»
Figure 6.21.  Effect of Acceleration and Shaking Dura-
              tion on Residual Drag.
-------
          W
        01   3.0
          0
         4-1 CO
           01
         ^ jC
         tfl U I 0
         01 C
         CM M
         
-------
I.IOO
"U 1.000
U-
v>
z
< BOO
(C
S3
£ 600
o
2
3 *°°
-------
6.13) for a  given  input  acceleration.   Data points taken from Figure fa.23
(for o>t < N  )  are  shown  in Figure 6.25.
        —  w
                            02    04    0.6    0.8    10
                             Q— FILTER CAWCITY RATIO
             Figure  6.25.  Effect  of  Acceleration and Shaking Dura-
                           tion  on Residual  Deposit.
            The required  number  of  strokes  to achieve N  is given
approximately by

                            «-  ^w* I7s                    (6.15)

for 2e  > 1 inch.  Smaller amplitudes  (2e  < 1 inch)  appear to arrive at
      S                                   S
N  sooner than estimated  by Equation  6.15.
 S
            In summary, Equation 6.13  indicates the maximum amount of
dust  (W   .  that can be removed  from the  fabric by a  given acceleration
input.  Equation 6.15 indicates  (approximately) the number of  shaker
strokes (N ) required to  reach the  maximum dust removal.   If the re-
          w
quired number of strokes  to reach maximum dust removal  is  not  achieved
during the shaking cycle  (cot < N )  an  (
                             —   w
removed (W)  is given by Equation 6.14.
during the shaking cycle (cot < N ) an estimate of  the  amount  of  dust
                             —  w
-------
            Thf.sr results arc indicated in Figure 6.26.  Data i'rom
i:tirvc C of Figures 6.18 and 6.23 have been plotted to indicate uhr
dust mass relationship through a single filtering and cleaning cycle.
These represent equilibrium conditions at constant filtering velocity
and dust concentration during the filtering cycle.  The relationship
i'or drag produced by added dust mass has been discussed above and in
Chapter 2.  For many dusts and fabric combinations, this is
               SOO = l^li - !iZ + s  - K2 ci vt + se           (6.16)
                        V     7000         7000
                    2                              3
 where  W  is  in gr/ft  of  fabric  and C.  is  in gr/ft  of  gas.   The  interval
 .shown  at A  in Figure  6.26  represents  the  process  of deposit  repair  when
 the  rate of change  of drag (resistance) is  a decreasing function of dust
 deposit.  The interval S^-S   is approximately 0.2 in.H.O/fpm.  Vnlues
 pf S -S  are discussed below, for  various dust, fabric,  and  cleaning
 combinations.   Tho  corresponding value of W at the  point where the
 linear part  of  the  S-W filtering curve actually starts is approximately
 100  grains/ft  .  The  filtering  cycle  (path  A & B) can be approximately
 represented  by

                         K ' C,  Vt
            Upon reaching the terminal drag  (S ), tho dusty gas flow is
stopped and shaking begins.  Drag is reduced  (path C) according to the
shaking mechanism inputs, as given in equations  6.10, 6.11 and 6.12.
(summarized in Table 6.4).

            Equations  6.9   through  6.17 define the loop shown in Figure
6.26, approximately.   For an assumed inlet loading of 5 gr/ft  and a
                                     2
filtering velocity of  3 fpm (15 gr/ft -min),  the pressure drop-time
history of the filtering and cleaning cycles  appear approximately as
shown in Figure 6.27  (from  Figure 6.26).  Equations and data presented
above provide an estimate of the transient conditions in filtering and
-------
     1.0-
   0-75
B
a.
O
 CM
33
CO

M

o

o>
    0.5-
         Irreducible
         Residual
         Dust Mass
         Held in
         Fabric In-
         terstices
         as a Func-
         tion of
         Fabric
         Dust Prop-
         erties
         Amount ob-
         tained tron
         shake -
         cleaned
         filter tube
         weight
  0.25-
                                Number
                                of
                                Clean-
                              ing Strotes
                       15 sec shake
                                                 Shaking Conditions
                                                 21=1 inch
                                                 a = 1.44g
                                                 a, = 5.25 cps
                                                 Curve C, Figs.  W
                                                  6.21, 6.26   -max
         New fabric
                   0
       Equilibrium
       Res idual
       Interstitial
       Dust Mass
 200        400

Filtered Dust Mass,  W,  gr/ft
600
  2
                                                                 800
                   Figure  6.26.  Fabric Filter Performance
                                With Intermittent Mechanical
                                Shaking (15 sec shake).
-------
                                TABLE 6.4
           SUMMARY OF CLEANING EQUATIONS FOR FABRIC FILTER DRAG
            AND DUST DEPOSIT DURING SHAKING WITHOUT AIR FLOW
6.9
6.10

6.11
              a .
                - (2
              S .   =      ;  0.72 < « < 6, For a > 6, S .  -» Constant (0.19)
                     a 1/2        ~   ~                rain
            S  - S
                   .
                  mm
                                            S  - S
                                                   .
                                             t    mm
6.12
                   (cot)  =
6.13

6.14
              W      810 (l-e"a)
               max
              W                       W
              ^  -f logfaeut)- f ;   jj£-  < 1
               max                     max
6.15
                 = (a*)w«     ;   2 es > 1 inch
                            s
cleaning cycles  for various values of  filter operating and cleaning
parameters.  As  an illustration of their utility, Figure  6.28 has been
constructed for  a cleaning time of 5 seconds, keeping all other para-
meters constant  (taken from the data shown in Figures 6.18 and 6.23).
Much less dust is filtered prior to reaching terminal drag.  The corres-
ponding pressure drop history is shown in Figure 6.29.  The number of
cleaning strokes used per hour for the 5 second cycle is 75, (Figure 6.28),
as compared to 108 per hour for the 15 second shake cycle, (Figure 6.26).
The amount of dust collected is 900 gr/ft -hr with either cycle.

            In contrast, a typical pulse-jet collector operated under
                                                           -I
similar conditions of inlet concentration (fly ash, 5 gr/ft ) at a fil-
tering velocity  of 8 fpm would clean each bag approximately 88 times per
hour, and each cleaning cycle  (0.16 sec duration, 100 psi jet air) would
                             2
remove approximately 27 gr/ft  of dust.  Equilibrium pressure drop for
-------
                                                1} tec
                                                •hake   •
                                                cycle (80
                                                •trokei)
                                   Number of Cleaning Strokei/Hr
                                                108
                             20        50
                            OmtATIM TME,
       Figure 6.27.  Filter Preieure Drop Hletory (Coniteat Velocity ead Duet Concentr«tloo).
this collector  would bi1 effectively constant at 4.0  in. H_0.   Amount of
                                 2
dust removed  would be 2400 gr/ft -hr.  Short-cycle on-linp cleaning has

been developed  rapidly by filter manufacturers during  the past few years.

The number of cleaning operations per hour in these  designs  is approxi-

mately equal  to or less than the number of cycles required in shaking

designs, so fabric life is probably at least as long.   Most  use felted

fabric having inherently high collection efficiency, so that short-cycle

cleaning does not  adversely affect average collection.   Dust handling

capacity is much higher per unit fabric area with on-line cleaning.

            Walsh  and Spaite   discussed the effects of the  duration of
cleaning cycle  on  filter capacityin terms of practical  applications of

the research  results presented above.  They indicate:

            "...that the influence of factors affecting the  transmission
            of  motion may be directly related to N , and it  has been
            shown  that longer shaking amplitudes might  be desirable
            since  they favor the transmission of motion.   A  definitive
            conclusion cannot be reached, however, until  the influence
            of  amplitude, on filter wear is known."
-------
      1.0
   •O.75
rt
(X
O
 CN
PC
    0.50
t>n
   0.25
                                  m
             New Fabric.
                                           5  sec.  shake
                                           Shaking conditions
                                           2e=l  inch
                                           c*=1.44g
                                           W=5.25cps
                                           Curve C.Figs.  6.21 &6.26
                                            I
                                                200
                                      i
                                     400
             Equilibrium
             Residual  DnsL  Mass
                     Filtered  Dust Mass,  W,  gr/Et
                                                 2
         Figure  6.28.
Fabric Filter Performance with  Intermittent
Mechanical Shaking (5 sec).
            "Another application of the data is related to reducing
            filter wear for a given shake by minimizing the total number
            of strokes applied over a period of time.  The cleaning dura-
            tion which will achieve this effect can be found by drawing
            a line from the origin and tangent to the curve of a "capa-
            city vs duration" relationship, as shown in Figure 6.30,,
            The duration at the point of tangency will allow the most
            dust to be filtered per shaking stroke, such that the total
-------
  o
  C
  •H
  Q.
  O
  t-l
  •a
  D
  v.
  -U
  J-,
  PL,
          3.0
          2.0-
                Data  1'rom
                Figure b.31
               Assume C.=5gr/ft
                     V = 3 fpm
Stop
main
dusty
gas
t: 1 ow >
                               3
                    Number of cleaning  strokes/hr
                           25 x 3 =  75
                            I
                            10

                      Operating  time,  rain.
       20
         0'
Kigurt- h.29.  filter Pressure Drop History (Constant Velocity
              and Dust Concentration.
       number of strokes utilized  over  a  period  of  time  (say  one
       week), will be significantly less than  the  tptuJ number usod
       with a duration of Nw strokes.   The  total  amount  of  dust
       collected over this  period  and maximum  filter  drag  for each
       cy.-le would be the same  in  both  cases.  For  the illustration
       shown, I he fotnl number  of  strokes required  for operation  at
       this optimum condition For a 40-hour interval would  ho  approxi-
       mately one-third the number required if N  strokes wore used
       for each cleaning period".„.(also see a similar comparison in
       Figures 6.26 and 6.28 using 2/3 the number of strokes  in going
       to N  vs N , respectively).
       "In practice,  operation unde?  this condition probably  would
       produce a residual dust  ...(deposit)... less uniform than  that
       after Nw strokes,  so that significant aging  (gradual increase
       in residual drag with time) may complicate operation in this
-------
 region...The extent of such nonuniformities can be seen by
 considering local velocities of filtration as shown in Figure
 6.31.  These data were taken while determining the curve of
 Figure 6.30 and indicate the degree of nonuniformity that
 existed after several cleaning durations.   For instance, the
 profile after cleaning with 28 strokes shows velocities on
 the order of five to six fpm through the upper portions of
 the filter, while the profile after cleaning with 688 strokes
 shows a uniform velocity of three fpm over the entire filter
 surface.  The former situation indicates that the shaking
 motion was effective in reducing local filter drag only at the
 top of the bag; hence, velocities there are higher than aver-
 age.   Repeated operation with this shake would result in a
 gradual build-up of drag at the bottom of  the filter and a
 gradual increase in the effective; drag of  unit.  This type of
 drag  Increase would not occur when operating with a duration
 of 688 strokes because the  entire filter area would be affec-
 ted by the shaking  motion".„.(also see Section 6.2.2.2 above
 for residual profile uniformity with duration of shaking).

 "Figure 6.19a,  which contains  several curves representing
 residual drag as a  function of acceleration and terminal drag,
 has certain practical implications.   These data indicate that
 in the region below the optimum acceleration,  residual drag
 was dependent on terminal conditions (see  equation 6.11),
 while in the region above the  optimum acceleration it was
 practically constant (see equation 6.12).   In other words,
 changes  in conditions  such as  increased  cycle  times or in-
 creased  dust concentrations, which would increase the terminal
 drag  of  a  unit,  will increase  residual drag  if the  accelera-
 tion  of  the  shake is  below optimum for the  system.   Also,  if
 a  unit  is  operated  on a  time-cycle and the acceleration is
 below optimum,  both  residual and  terminal  drags will increase
 from  cycle  to  cycle1'until the  drag reduction during cleaning
 equals  the  drag  increase  during  filtration.  This effect  will
 be  in addition  to any  other normal increases associated with
 an approach  to  equilibrium.  Assuming  that cleaning  duration
 is  in  excess  of  N ,  a  reduction in residual  drag  can be
 accomplished  only by  increasing acceleration.   This  type  of
 phenomena may explain  the results  reported by  Lemke,  et  al.,
 (12)  (ZnO   from hot  dip  galvanizing kettles),  wherein the
 residual drag of two  pilot-plant  filter units,  operating  on
 a  timed cycle,  increased  gradually over a period  of  time.
 The increase was of  such  an extent that vigorous hand  shaking
was periodically required to maintain  reasonably  low filter
 pressure differentials.  The vigorous  shaking by hand,  then,
was comparable  to increasing the acceleration  of the shaker.
 It would seem possible,  therefore, that a dependence  of res-
 idual drag on terminal conditions may be a general  indicator
 of  insufficient acceleration, and that the  region of opera-
 tion  (i.e., above or below the optimum acceleration) may be
determined by varying  filtration times an^.  monitoring  the
effect of such changes on residual drag."•'•I


-------
   000
   eoo
z
4
a
4  400
a
             A-MAKlMUM
             /  AVtHACC
/
                SLOPE
                            I     I     I     I
                            TtuwWiLWWO • i OiN M,0/t»«
        IDURATION FOR MAXIMUM CAPACITY PER SHAKING STROKE
     *    I     I     I     I     I     I     I     I     II
         100
              ZOO  300   400   500  6OO   700   800   900
                 CLEANING DURATION (NUMBER or STROKES)
                                                      1000 MOO
          DETERMINATION OF DURATION FOR MAXIMUM CAPACITY
                        PER SHAKING STROKE
    Figure (>..}0.
       Determination of  Duration for Maximum
       Capacity Per Shaking Stroke.  (From Walsh
       ;


' / '
/
/
*

PROFILE A
DURATION
OF
28
STROKES


1 1 1
1
-

—




/
f\
} '
(
\

1
'PROFILE B
DURATION
OF
344
STROKES


1 1 1
1 /I 1 1




PROFILE C
DURATION
OF
; 688
STROKES


I/ 1 1 1
                         t      :   »  «  e
                          FILTER VELOCITY 
-------
  Walsh and Spaite presented the following conclusions:
             "1.  A well-defined limit will exist beyond which increased
                 shaking with a given shaking stroke will not significantly
                 contribute to further cleaning (see equations 6.11  and
                 6.14).

             2.  Filter capacity and residual filter drag are a  function
                 of shaker amplitude and the square of shaker frequency
                 (maximum acceleration of bag cap), but  again a  well-
                 defined limit exists beyond which increases  in  these
                 factors will not result in further cleaning  (see equa-
                 tions  6.10 and 6.13).

             3.  Shaker amplitude has an influence on the number of clean-
                 ing strokes required for a given degree of cleaning.
                 This influence is independent of its effect  on  the maxi-
                 mum acceleration of the shaking  stroke  (see  equations
                 6.12 and 6.15).
             4.  A relative measure of the influence of  factors  affecting
                 the transmission of shaking motion can  be obtained by
                 considering cleaning durations required to produce mini-
                 mum filter drag.

             5.  For any given shaking arrangement there will exist a
                 cleaning duration which will produce the least  bag wear over
                 a period of time." 11

             There is a definite requirement for  study of the effects of

these same shaker variables  on fabric wear  and bag life for  optimization

of cleaning  costs.

             6.2.2.4 Effects  of Fabric Structure. - There are more than a

dozen generic  types of man-made fiber materials  that can be  used in fabric

for gas filtration,  in addition to cotton and  wool.   Fiber,  strand,  twist,

yarn, yarn count,  weave,  and  finish parameters are discussed in Chapter 4.
                                        2
The initial  studies of Snyder  and  Pring   indicated an effect of fiber

material, fabric  weave,  and mechanical  finish  on  filtration  performance

(See Figures 6.3a,  b,  and  c, and Tables  6.2a and  b).  More recent  inves-

tigations of the  effects  of  fabric  structure in pilot scale  systems  have
                                   13                11
been presented by  Spaite and Walsh    (two bag unit,   Figure  6.19),
      14                       15
Durham   and Borgwardt, et al.    Other data have  been  presented in

Chapter 2.

                            13                                           R
            Spaite  and Walsh    studied the performance  of three Fiberglas
              £
and two Dacron  fabrics, woven  from continuous filament yarns (specifica-

tions and photographs of test  fabrics are given in Figure 2.26a).


-------
            "Basic performance curves  for  the Fiberglas   fabrics  when
            operated at a nominal velocity  of 2  fpm are  shown  in  Figure
            6.32a.  Each set of test bags were exposed over  the entire
            range of nominal velocities  investigated  (2  fpm  to 10 fpm),
            and two runs were made at  low velocity to determine the
            extent of changes in the residual medium during  the tests.
            The lower curve for each fabric was  obtained  after new bags
            were operated at 2 fpm for 48 hours.  The upper  curve for
            each pair shows results of a repeat  run made  at  2  fpm under
            conditions that were identical, except that  the  cloth had
            been exposed for one week  to the higher filtration velocities,
            The higher resistance after exposure is a reflection  of  the
            effect of dust penetration into the  fabric,  or "aging".^

            Figure 6.32a shows an increased effect of aging  on the more
              n
open Fiberglas  fabric No. 1, as compared to No. 3:

            "...the tighter weave, No. 3, exhibited a higher resistance
            to flow when it accumulated a semi-permanent  residual dust
            mass.   Much of the advantage in low residual  drag  shown
            initially by the open fabric, No. 1,  was cancelled out by the
            effect of aging; ...dust cake permeability was affected,   but
            little by .aging.

             The importance of changes in filter drag and filter  resis-
            tance  coefficient is most evident when their  combined effect
            is shown as total resistance to flow.  In Figure 6.32b, the
            increase in filter pressure differential that occurs during
            filtration is  shown as a function of the mass of dust fil-
            tered.   These  curves represent the pressure increase during
            operation at velocities  of 2 fpm, 4  fpm,  and 6 fpm on the
            tightest (No.  3)  and loosest (No. 1)  FiberglasR fabrics.
            The filtration capacity  of the fabrics,  in terms of the mass
            of dust than can be  collected before  a given differential
            pressure is exceeded,  depends on the  fabric structure  most
            strongly at high filter  velocities.   At higher velocities
            this dependency becomes  greater as the mass deposited  becomes
            greater;  at lower velocities  it becomes  less  significant  as
            the total mass  increases.
            The effect of  fabric  structure and nominal velocity  on the
            specific dust-fabric  filter resistance  coefficient  is  shown
            in (the  upper  pnrLion  of)  Figure 6.33.   In every  instance
            the dust  deposit  creates  an increase  in  1'he  specific resis-
            tance coefficient  as  filtering  velocity  is  initially in-
            creased.

            These  changes  in the  resistance coefficient  are  substantial
            over the range  of velocities  that would  exist in multicompart-
            mented  units  operating at  nominal velocities  of 2 to 3 feet
            per minute.  Thus,  they  are an important  consideration in any
            analysis  of flow and resistance patterns  in such  an installa-

-------
                                        WET GROUND MICA TFST OUST
                                        NOMINAL VELOCITY  (3) 2 Ipm
                                        INLET CONCENTRATION (5) 5 GRAINS/FT
                       Dust Deposit Density, W  (gr/ft^)
 Figure  6.32a.  Effect of Fiberglass Fabric Fill  Count  Variation
                                          WET GROUND MICA TEST OUST

                                           —	FIBCRCI ASS N«3
                                                 flBfHGLASS N« I
                      Dust Deposit Density, W (gr/ft2)
 Figure  6.32b.  Effect of Fiberglass Fabric Construction
                                         WET OAOUNO MICA TEST OUST
                                         NOMMAL VELOCITY (g) t «»m
                                         I^LET COWCENTWTK5N ® S ORA(HS/FTS
                 0     100    200    100   400    SOO    tOO    TflO    *X
                       Dust Deposit Density, '.-.'  (gr/ft^>

Figure 6.32c.  Effect of DacronR Fabric Fill  Count Variation.
                (From  Spaite and Walsh,  Ref. 13)
-------
                     •o
                     4O
                     JO
                    « 20
                   1  -
                   I
                   U

                   i
                               (bottOB f««<). mlct
                               duit) J u»,
                               D R O.SM*
                                P
             FIBERCUS
             (top feed,
             fly uh.
             I) tin
                                           COTTON SATEEN
                                           (Cop (<«d, fly 
-------
                                         n
  The basic performance curves for Dacron , shown in Figure
 6.32c are similar to those obtained with FiberglasR.  The.
 filter drag at the end of the period of deposit repair was
 again found to be higher for the tighter weave fabric,
 Dacron  B.  The accelerated increase in initial drag for both
 Fiberglas  and DacronR indicates that such an effect is char-
 acteristic of the continuous-filament yarns.

  The effect of velocity on the specific resistance coefficient
 for the mica dust deposited on Dacron  followed the same gen-
 eral pattern shown for Fiberglas^ in Figure 6.33 (upper
 portion).

  The increases in pressure differential across the filtering
 medium during the filtering cycle are essentially the same
 for both Dacron^ and Fiberglas^.:  The previously discussed
 similarity in the relationships for the individual components
 of  the pressure loss (i.e., loss across the initial dust
 fabric medium and loss through the deposit...) are reflected
 here in curves that  are so nearly identical to those shown
 for Fiberglas^ in Figure 6.32b that it is unnecessary to show
 both sets.

  A  second measure of performance that must be considered when
 fabric structure  and velocity are varied is the amount of dust
 discharged through the fabric.  Most of this discharge occurs
 in  the early stages  of filtration.

  Figure 6.34 shows the manner in which dust discharge varied
 as  a function of  nominal velocity and Fiberglas^ structure.
 Each filtering cycle was terminated at a drag level of 1.5
 in. H20/fpm.   Because of this,  some cycles were ended before
 they could progress,  substantially beyond the region of for-
 mation or repair  of  the uniform filtering medium.   Such tests
 are  designated by circles  on  the curves.

  A most  interesting  aspect  of  Figure  6.34 is  the  curve  which
 shows  dust discharged per  cycle  as  a  function of  velocity  for
 FiberglasR No.  1.  The  data points  numbered  1 through 7 repre-
 sent  the  results  of  7  consecutive  tests  of  this  fabric,  each
 made  after 24  hours  of  operation at  test  velocity.

  The  efficiency was  poor  in early  tests,  but  showed  a marked
 improvement, associated with the aging  of this  loosely  woven
 fabric.   The interim runs at low velocitiep  (points  5 and  7)
 indicate  that  the  improvement  in efficiency  tended to be per-
manent.   During Run  7 the dust discharge  changed  from 11.8
grains/sq.ft.  for  the second cycle  to  10.9 grains/sq. ft.  for
 the fifty-second cycle.  Subjective estimates  of  filter effi-
ciency support these data: before aging,  a number of  pinholes
and high velocity  air jets were  in evidence  over the  surface
of the fabric; after aging, these  largely disappeared.  During
the tests, the efficiency of the fabric changed from  85.5
percent for Run J  to 96.1 percent for Run 7'.*3
-------
             The curves i.n Figure 6.34 show that dust discharge was approx-

 imately  proportional to velocity and inversely proportional to Tilling

 yarn count.
             "Conditions that developed in the filter enclosure were simi-
             lar to those observed in industrial fabric filters.  The bags
             developed a slight dust layer on the exterior surface, and
             some dust collected on the floor of the clean air side.  The
             weight of this material was not included in the dust discharge
             data.  The dust discharge data are probably higher than would
             be encountered in actual use because of the intensive cleaning
             employed for these tests.
                            •p
              For the Dacron  fabrics, the effect of nominal velocity on
             dust discharge per cycle is shown in Figure 6.34b.  A general
             linear relation is again shown, but the relationship is dif-
             ferent in that the Fiberglas* fabrics allowed greater dust
             discharges at low velocities and Dacron^ generally showed a
             lower rate of increase in dust discharge with velocity.
             These differences might be attributable to a more even dis-
             tribution of air flow through the DacronR fabric.  An
             indication of the distribution of areas available fui.  high
             air flow is  obtained by considering light transmission
             through the  fabrics (Figure 2.26a).

              The most striking point in the comparison of the two  cloth
             types is  that the two materials with nearly equal clean-cloth
             permeability  values (Fiberglas^ No.  1 - permeability  15.84
             and Dacron^ B -  permeability  14.62)  have different  performance
             characteristics.   The difference  is  reflected in dust  mass  vs.
             filter drag curves (Figure  6.32),  and  the  efficiency curves,
             (Figure 6.34).  The comparisons  demonstrate the  impossibility
             of predicting  performance  of  an untried fabric  from permeabi-
             lity data and  known performance data for a  second  fabric,  even
             when the  same  dust is  involved.  Thus,  cloth  permeability
             per se seems  to be of  less  importance  to these  filtration
             parameters than the manner  in which  the fabric structure is
             changed to alter  permeability1^^

             The effect of  filtering velocity on  the specific dust-fabric
filter resistance  coefficient  was also investigated by  Borgwardt, et al

Their flyash data  are shown in Figure 6.33  (lower portion).  They found
that:

                              K2' ~ V372                        (6.18)
for 1 < V < 4 fpm.  The earlier data of Spaite and Walsh (Figure 6.33,
upper portion, mica dust 5-10  urn D  x 0.5|im thick) are less well-
-------
                               N*l
                             WIT MOUNO MIC* TC8T OUST
                             TCNMNM. OHM $ 1.9 IN. M,0 / fpm
                             ( ) INDICATE EXKMMENTM. WOW
                             0 MOttATC K.EEOMO THMUOHOUT CYCLE
                                    FIKMLAM N»2
             r      4      *      *     10     it     i4     it
             Filtration Velocity, V  (fpm)

Figure 6.34a.  Effect  of Nominal Velocity  on Dust
                Discharge for Fiberglass Fabrics.
Irt
I
       40

     ~ li
     UJ
     d >o
             »«T GROUND MCA TEST OUST
             TERMINAL DRAG $ I S IN H.O / Ipm
           • INDICATE BLEEDING THROUGHOUT CTCLE
                                      OACRON A
        o     r     «     t      <     >o     ie     <4     K
             Filtration Velocity, V  (fpm)

Figure 6.34b.  Effect  of Nominal Velocity  on  Dust
                 Discharge for  Dacron^  Fabric.
                 (From Spaite and Walsh, Ref.  13)
-------
 behaved, but indicate that K- ~ Vn for n > 0 and 2 < V < 8  fpm.   In each
 of these studies fabric, dust, and velocity all affect K~.   (See  also
 Section 2.4.8.4).
             Figures 6.32(a) and (c) can be used to determine the  unknown
 interval (S  - S ).  The nonlinear portion of the S-W curves occurs during
            e    r                                      2
 the period of dust deposition less than about 100 gr/ft .  Approximate
 values of the interval (S  - S J are ~ 0.2 in.H 0/fpm.
                                  14
             Durham and colleagues   have extended the above investigations
 to nine additional woven fabrics indicated in Table 6.5.  Six different
                                    R        R                      R
 fiber materials were tested:  Nomex , Teflon ,  Polypropylene, Orion -
                   R         R
 Acrylic (Microtain ),  Dacron  and Cotton.  Yarn configurations included
 continuous  multifilament warp and  fill,  filament  warp  and  spun fill,  spun
                                 T>
 warp  and  fill,  and  napped  (Dacron  )  woolen system yarns-warp and  fill.
 Weaves  and  thread  counts were different  for  each  fabric, as  was  fabric
                                                         3
 weight  and  clean air-flow  permeability.   Three grains/ft   of flyash test
 dust  (  ~  10 (am  D )  was  filtered  at  4  fpm (constant mass  flux 1.7 x 10
       2         P
 Ibs/ft  min)  in all tests  (top entry,  single 5.5  in. diam. x 66.6  in.
 long  bag, under suction).   Cleaning was  accomplished with a  mechanical
 reciprocating shaker attached to a  flexible  connection at  the bottom  of
 the bag (2 e = 1.75  in.; co  = 5.6  cps;  duration, 0.5,  1, 3, and 5 min;  cot
 =  170,  340, 103, 1.7 x  103  strokes; ,1 =  2.85 g's).   (See equations 6.10
 through 6.15  for effects of shaking parameters on minimum residual drag
 and maximum dust capacity  per  filtering  cycle).  Filter drag-deposit
 density (S-W)   curves are  shown  in Figures 6.35a, b, and c,  and 6.36.
 Fabrics have been grouped qualitatively by yarn free available fiber
 area, according  to  the classification  hypothesis indicated in Figures
 2.69 a-d  (Chapter 2).  Four classes of fabric were identified according
 to a qualitative estimate  of  free available fiber:
            (a) Continuous  multifilament warp and fill, Figure 6.35a
                         j^
                 1.  Nomex
                2.  TeflonR
                3.  Polypropylene
* W is dust deposited on the fabric, as calculated from dust fed and
hopper fallout (see discussion below).

-------
                                                       TABLE 6.5
           Fabric Filter Media Specifications  and Performance  with Constant Particle  Flux
                                               (1.7  x 10"3lbs/ft2-min)
Fabric
••bar
1

2

3

4


5


6

7

1

9

	 T5» —
Conpoelttoo
•0.1'

Teflon*

Poly-
propylene
*-.»


D«ro.»


Cotton

Orion*

ee
acrylic

Dae too*

Tarn
Fl leant
»arp & fill
FUnent
•arp & fill
filament
varp 6 fill
fllanent
varp;apun
fill
Pliant
uerp:»pun
fill
Spun varp
6 fill
Spun varp
4 fill
Spun varp
4 fill
Woolen
ayet.
Heave Count ,
threads /In.
3*1 96*78
tvlll
3,1 76r.il
tvlll
3*1 74*73
tvlll
3*1 95x58
tvlll

3*1 77*81
' tvlll

Sateen 97*63
SH-U
)*2 85x77
tvlll
2*2 39x35
tvlll
2*2 41*39
tvlll
Clean
Air Flov
FeraeeMllty
15-20

20-40

10-20

20-25


20-30


15-20

15-25

to

55-65

Clean
Drag
0.028

0.017

0.033

0.022


0.020


0.028

0.025

0.008

0.008

Fiber
Denalty,
../c.5
1.38

2.1

0.9

1.38


1.3*


1.50

1.14

1.14

1.38

fabric
Veltbt
o»/ys*
4.5

8.6

4.3

5.4


5.8


9.5

5.7

9.8

12.5

Teredeal
drag.S,
la.ayvfpaj
1.5

1.0

1.5

1.0


1.0


1.0

1.0

0.7

1.0

Drag.S ,
In.HjO'fi
0.40

0.15

0.21

0.22


0.05


0.41

0.20

0.05

0.09

tlve
" J"»- 1.
In.BjO/fpn
0.99

0.54

0.92

0.55


0.41


0.56

0.32

0.25

0.20—

t-s
0.59

0.39

0.71

0.33


0.36


0.15

0.12

0.20

0.11

Terminal
Bepotlt,
' It. /ft2
0.045

0.055

0.095

0.060


0.075


0.077

0.12

0.10

0.205

Storage
Capacity.
"t««
0.030

0.055

0.06)

0.060


0.075


0.077

0.12

0.14

0.205

Specific KiUtance
Coefficient,
(IWf?2)-'
11.5

8.7

6.2

7.8


7.7


5.8

5.8

4.8

2.4~*

»|lpiMl.a»e
0/1000 cfv. '
0.85

0.51

0.68

0.47


0.39


0.39

0.33

0.23

0.17

 • - cfWft  at O.S la. EjO. eatuved linear Of - V.
 e* - Ubaay felt Hlcroula*
*** - latiaated from linear portion of S-W curve up to U < 0.1 lbe/ft ,  then t^' Increaalag to 8.5 at V - 0.2 lee/ft
 •ote: n« eah teat duet. 10 u« dlam., C, - 3 graln»/ftT, » - 4 fp., cleaning tin. 180 aec. 5.5 1«. «la». « 66.6 In. lea* bag*.
-------
             (b)  Filament warp and spun fill, Figure 6.35b                         ^

                 4.   Nomex
                 _   _      R
                 5.   Dacron
             (c)   Spun warp and  fill,  Figure 6.35c

                  6.  Cotton Sateen

                  7.  OrlonR                                                       -'
                                        T>
                  8.  Acrylic (Microtain )

             (d)   Napped,  Figure  6.36                                               _,
                            T>
                  9.  Dacron
                                                                                     i
             Effects  of  increasing  amounts  of free available fiber should          —<

include reduction in K/ and increased  amount of  dust  storage capacity

(W /S ).  More free  fiber  (A -* D)  should also reduce  the  amount  of dust           ^

deposit released  for a  given shaking cycle,   An  increase  in o)t on class A

fabrics should produce  little additional dust removal  (AW  small as   0) .                                /

            o  Dust  storage capacity  (Wt/St)  increases  with  amount of free
               available fiber (except for  polypropylene,  which is higher
               than  expected).  Capacity varies  from 0.03  to 0.2  Ibs dust/
               ft  of fabric per  unit  drag.                                        ""*

                                                                                   i
-------
   1.5
e
•a
o

c
00
n)
£
0)
c5
   0.5
                   1. NomexR Filament
                      warp and fill
     \3. Polypropy-
        lene Fila-
        ment , warp
        and  fill
    Te.flonR  Filament
    warp and fill
V .t^   .

  t  30 sec
0 ° O L80 sec
  0  300 sec
    cleaning time
                                           1.5 '•
     0
                                                40 NomexR
                                                  Filament warp,
                                                                       DacronK
                                                                       Filament
                                                                       warP.  Spun
                                                                       till
                                                                4-   )0 HIM'

                                                              OO- 180 sec
                                                                0 300 sec
                                                                 cleaning  tinu1
                                                    _L
          .02   .04   .06   .08   .10   .12
                                                    .02   .04   .06   .08   .10
                                                                                 T
    (a) Dust Deposit Density, T7,  lbs/ft2      (b) Dust  Deposit  Density, T7,  Ibs/ff
                       1.5
                    a
                   •H
                    s
                                             7.  Spun OrlonR

                              6. Cotton  Sateen
                                                8, Spun
                                                   Acrylic
                                                 30 sec
                                                180 sec
                                            Q° 300 sec
                                               cleaning time
                                              i	t    i   i
                         0    .02   .04   .06   .08   .10   .12   .14

                         (c) Dust Deposit Density, ft,  lbs/ft2

                 Figure 6.35.  Effect of Fabric on Filter Performance
                               (From Durham, Ref. 14).
-------
s   1.0
W

c
•H

c/1

u  °5
(0
                                       9.  Napped Dacron^
                                                        +  60 sec
                                                        o 180 sec
                                                        o 300 sec
                                                         cleaning time
                                                            '
                                                                  1
           Q02  Q04  0.06  0.08   O.I2  0.14   0.16  0.18   0.20  0.22  0.24
                                                 _        2
                            Dust Deposit Density W, Ibs/ft

                                                          P
        Figure 6.36.  Effect of Shaking Duration on Dacron  Fabric
                      Filter Performance (From Durham, Ref. 14).
             o  Amount of dust removed by an increase in shaking duration
                (cot)  is relatively lower with filament fabric (No. 1).
                Duration increase from 30 to 300 sec (170 to 1.7 x 103
                strokes) produced a 0.005 lb/ft^ increase in dust holding
                capacity.  Spun yarns (Nos.  5,  6, 3) tend to release dust
                more  slowly,  an increase in duration from 30 to 300 sec.
                producing a 0.015 lb/ft^ increase in capacity.  Napped
                fabric (No. 9)  shows the largest effect of_increased
                shaking duration, 60-300 sec. producing   W of 0.03 Ib/ft
                At_an inlet flux of 1.7  x 10"3  Ibs.  dust/ft2-min, these
                AW values correspond to an increase in operating time
                between cleaning cycles  of (Col. 4):
                Fabric
                Filament
                Fil./Spun
                Spun
                Spun
                Napped
No.
 1
 5
 6
 8
 9
W
                                    30-300
Time of
W
                          30-300
Time to
0.005
0.015
0.016
0.014
0.03
(60-300
2.9
8.8
9. it
8.2
17.6
sec)
min





                                                                         '180
                              26.5 min
                              44.0
                              45.0
                              59.0
                             121.0
-------
                                            V
             o  Increasing cleaning Duration from 30 sec. to 5 min. on fil-
                ament fabric (No. 1) produces about 3 min. longer filter-
                ing time in a 26 min. cycle, or no net improvement.  The
                filtering cycle is lengthened less than the amount required
                for the additional shaking.
             o  Increasing cleaning duration from 30 sec. to 5 min. on
                spun yarn fabrics (Nos. 5, 6, 8) produced about a 9 min.
                longer filtering cycle in a 45 to 60 min. cycle, or about
                15 to 20 percent longer filtering cycle.
             o  Increasing cleaning duration from 1 min. to 5 min. on the
                napped fabric (No. 9) produced an 18 min. longer filtering
                cycle (15 percent increase in filter operating time be-
                tween cleanings).
             Dust characteristics will modify the quantitative observations
 above,  but general trends shown are expected to be the same.  For most
 dusts,  increased cleaning duration beyond 30 sec. or 1 min. will probably
 not produce a net increase in filtering time greater than about 10 to 15
 percent.   The effect of increased cleaning time on reduction of fabric
 life  is assumed  to be  linear.   A five  to  ten times  increase  in cleaning
 duration  (30  or  60 sec  to 5 min) will  probably  reduce  fabric life  by  a
 similar factor.   Assume  fabric  life  is  of order 5 x  10   shaking  cycles.
 Cleaning  for  30  sec uses  170 cycles  (5.6  cps).   Let  the  filtering  cycle
 be  45 min long followed by a 30 sec  shake and 30 sec dwell  for settling.
 Then every  46 min will require  170 shake  cycles.  Fabric  life  is esti-
 mated as  (5 x  10   x 46/170 x 60) 2.8 years.  If the cleaning duration is
 extended  to 3 min (6 x, or 10   shaking  cycles per cleaning cycle)  the
 filtering cycle may be extended about  10  percent  or 49.5 min.  The com-
 plete cycle will  consist  of 49.5 min of filtering followed by  3 min shake
 and 30 sec  dwell  for settling,  or 53 min.  Fabric life may be  estimated
          f         O
 as  (5 x 10  x 53/10 x 60) 0.55 years,  or  about  l/6th of the life  obtained
with shorter cleaning duration.  Pulse jet collectors cleaned  on-line
 every 30 sec  (120  cycles/hr) would be expected  to have an average  fabric
 life of order 5.2  years, under 'the above assumptions.  None of these esti-
mates include effects of factors tending to reduce fabric life during fil-
 tration, such as scour and abrasion, temperature, condensation, acid  or
alkaline deterioration and other gas or particle factors.
                    14                         —
             Durham   observed anomalies in S-W curves for different
shaking times for  tests at different parts of the year.  For example, in

-------
Figure 6.35a, Fabric No. 1, it was found that data produced for 3 min.
shaking did not agree with data for 30 sec and 5 min shaking obtained
several months later.  Similar anomalies are evident in data for fabric
                                R                       ~
Nos. 6 (cotton) and 8 (Microtain ).  The shift in the S-W performance-
curves arc positive (Nos. 1 and 8, to the right, higher W  per units)
and negative (No. 6, smaller W /S ).

              6.2.2.5  Effects of Humidity - Durham and Harrington
have shown that the apparent anomalies in performance just described
probably result from changes in ambient moisture content (Relative
Humidity, R. H.).    Relative humidity was controlled between 20 «nd 60
percent R.n. In the same experimental configuration described above. ^
              Resuspended fly ash was used as a test dust, and the  11 fab-
rics enumerated in Table 6.6 were tested.  Increasing relative humidity
generally reduced overall resistance of the filter.  Three responses were
calculated to identify the system resistance: effective drag, S , specific
                         /                                     e
resistance coefficient, K2, and terminal drag, S .  (Figures 6.5 and 6.35
show typical values of these three parameters.)
              Table 6.7 presents results of the effects of relative humi-
dity on K™, S  and S .  The specific dust-fabric filter resistance coeffi-
cient was significantly reduced by increasing the relative humidity.  Effec-
tive drag exhibited no particular trend.  In most instances, terminal drag
was reduced with increasing relative humidity.  The specific resistance
Coefficient determines the rate of increase in resistance during the linear
portion of the filtering cycle to achieve any given value of terminal drag.
Thus, reduction in K« offers potential for increased bag life and reduced
maintenance (i.e., less frequent shaking required).  Figure 6.37 illustrates
the effect of R.H. (20 < R.H. < 60 percent) on K_ for three acrylic fabrics
and for polypropylene.  An increase in R.H. from 20 percent to 60 percent
reduced K' by a factor of order 2.
-------
ON
U)
                                                           TABLE  6.6

                                                  FABRIC  CHARACTERISTICS

                                       (From Durham and Harrington, Reference  16)
Fiber
Composition*
Nylon
Creslan
R
Dae r on
Polypropylene
Crylon
R
Dralon
OrlonR
Cotton sateen
Glass filament
Glass Combination
Glass texturized
Type
Warp
CF
CF
CF
CF
CF
CF
CF
S
CF
CF
CF
Yarn**
Fill
CF
CF
CF
CF
CF
CF
CF
S
CF
S
T
Thread Count ,
Yarn Denier threads/in;
Warp Fill Warp Fill
210 210 74
200 200 80
250 250 76
210 210 81
200 200 77
200 200 76
200 200 76
95
54
48
46
68
76
66
69
63
71
62
58
56
22
24
Fabric
Weave Thickness,
Pattern Mils
2 x
3 x
3 x
3 x
3 x
3 x
3 x
2
1
1
1
1
1
1
Twill
Twill
Twill
Twill
Twill
Twill
Twill
Satin
3 x
2 x
3 x
1
2
1
Twill
Twill
Twill
9
10
9
12
10
9
8
24
9
24
16
.4
.8
.1
.4
.2
.8
.7
.1
.6
.6
.5
Fabric
Weight
oz/yd
4
4
3
4
5
4
4
10
9
16
14
.1
.0
.9
.6
.1
.4
.3


.5

                 R                                R                           R
        * Creslan  acrylic, Amer. Cyanamid; Dacron  polyester, Du Pont; Crylon  acrylic, Crylon S.A.(Fr);

          DralonR, Farberfabriken Bayer (W.Ger.); Orion  acrylic, DuPont.


-------
                                             TABLE 6.7
   EFFECT OF RELATIVE HUMIDITY ON SPECIFIC RESISTANCE COEFFICIENT, EFFECTIVE DRAG, AND  TERMINAL  DRAG
                                  (From Durham and Harrington, Reference  16
                Specific dust-fabric filter
                resistance in.  H.O/ft-min
                      lb/ft2    L
                                        Effective  drag,
                                        in. H 0/ft-min
                                          Terminal drag,
                                          in. H 0/ft-min
Description*
    Relative humidity,  %
 20   30   40   50   60
           Relative humidity,7=
  20    30    40    50    60
  20
                                                                                    Relative humidity,  7=
                                                                                      30    40    50    60
       R
Nylon

Creslan

Dacron

Polypropylene
      Ti
Crylon

T,  -,  R
Dralon
     t>
Orion

Cotton

Filament glass

Combination
   glass

Texturized
   glass
11.2  7.6  6.4  3.9  2.6

 9.9  9.6  7.8  5.6  4.0

 8.4  7.7  7.2  4.2  3.0

 9.6  7.8  7.8  5.0  3.6

 6.4  3.8  4.2  4.2  2.3

 9.3  5.2  4.2  4.1  2.3

 6.8  6.5  4.3  3.8  3.4

 8.0  7.5  7.3  5.8  5.4

 5.7  5.6_ 6.5  6.2  5.3


 5.1  4.3  4.4  3.7  3.2
0.24   0.23   0.32  0.32  0.30

0.12   0.14   0.23  0.30  0.29

0.69   0.61   0.63  0.60  0.62

1.05   0.98   0.99  0.94  0.95

0.12   0.12   0.18  0.21  0.32

0.45   0.40   0.38  0.37  0.32

0.78   0.82   0.74  0.59  0.61

0.46   0.48   0.45  0.45  0.49

0.53   0.55   0.48  0.63  0.68


0.25   0.24   0.20  0.18  0.20
                  7.7  7.0  6.4  5.5  4.3  |  0.49   0.45   0.40  0.41  0.42
                                           J_
0.60   0.48  0.55  0.45  0.38

0.45   0.48  0.50  0.48  0.40

0.98   0.88  0.88  0.73  0.75

1.37   1.25  1.25  1.10  1.08

0.34   0.33  0.36  0.36  0.30

0.76   0.68  0.56  0.50  0.41

1.00   1.03  0.90  0.76  0.73

0.74   0.73  0.66  0.53  0.74

0.73   0.75  0.73  0.85  0.88


0.43   0.38  0.35  0.32  0.30


0.75   0.70  0.58  0.58  0.58
* See Table  6.6  for generic fiber description and manufacturer.

** Mass  flux 1.7 x 10"3  Ibs/ft2-min,  for  20 min. filtering  cycle, W   = 0.034  lbs/ft'
                     L
-------
a
01
u
•l-l
<4-l
41
U
c
id
•H C-J
VM  4J
•H  VM

-------
                                               TABLE 6.8
                EFFECT OF RELATIVE HUMIDITY ON OUTLET DUST CONCENTRATION AND  EFFICIENCY
                              (From Durham and Harrington, Reference  16)
Outlet dust concentration, grains/1000 ff3
Bag
Description*
Nylon
-, R
Creslan
R
Dacron
Polypropylene
CrylorR
Dralon
Orion
Cotton
Filament glass
Combination glass
Texturized glass
20
130
168
34
36
148
26
12
0.04
148.1
10.4
63.8
Relative humidity, °L
30 40 50 60
148
177
32
32
89
24
7.5
0.2
135.9
10.3
40.2
61
100
13.1
35
56
17
6.9
0
106.4
2.2
19.9
4.4
37
1.9
7.0
13
0.8
3.9
0
25.4
0.1
6.0
0.02
3.1
0.7
2.7
1.3
0.6
0.8
0
9.1
0.1
1.1
95
94
98
98
95
99
99
99
95
99
97
20
.62
.47
.86
.80
.12
.11
.59
.99+
.00
.65
.84
Efficiency, weight 7=
Relative humidity, 7=
30 40 50
95.02
94.02
98.95
98.96
97.29
99.23
99.75
99.99+
95.31
99.66
98.66
98.02
96.35
99.12
98.85
98.14
99.43
99.78
99.99+
96.55
99.92
99.31
99.86
98.78
99.94
99.78
99.56
99.98
99.87
99.99+
99.18
99.99+
99.80
60
99.99+
99.90
99.98
99.91
99.96
99.99+
99.97
99.99+
99.71
99.99+
99.96
Note: Inlet dust concentration C. =3.0 gr/ftj, fly ash, 4.0 urn median- diameter.
      * See Table 6.6 for generic fiber description and manufacturer.
      (From Durham and Harrington, Ref. 16).
        I ._    L ..
-------
              probably tlve major factor in producing uniformly high effi-
              ciency.  Figure 6.38 illustrates the effects of relative hum-
              idity on the outlet dust concentrations for Crylor^, CreslanR,
              Orion**, and  polypropylene fabrics.

              Relative  humidity also significantly affects the outlet par-
              ticle concentration at various times during the filter cycle.
              Figure 6.39 indicates an initial outlet particle concentra-
              tion of about 1.5 x 1C)6 particles per cubic foot at the begin-
              ing of a 20-minute filter cycle.  As the filter cycle proceeds,
              the particle concentration drops off much faster as the rela-
              tive humidity increases.  Curves similar to those in Figure
              6.39 were obtained with all fabrics except cotton sateen.
              Relative humidity affects the rate at which the interstitial
              openings of the filter medium are bridged, the structure of
              the deposit, and the filtration characteristics of the system.
              The net effect of increasing the relative humidity of the
              carrier gas was to improve the efficiency of the system."16
o  I8°
 <4-
 g  160

 I
 2  140
 Q
 2  120
   100
 o
 o
    80|-
 UJ
 O
40|-


20

 0
           O   Creslan  Acrylic
           •   CrylorR Acrylic
               Polypropylene
                      Acrylic
                 10
                       20          30        40

                           RELATIVE HUMIDITY, %
50
60
         Figure 6.38.   Effect  of  Relative Humidity  on Outlet Dust Con-
                       centration.
                       (From Durham and Harrington, Rof.  16).
-------
10'
                                                 RELAXIVE  HIIMIDITY, 7»
                                                       D  20
                                                       O  30
                                                       A  40
                                                       •  50
                                                       •  60
   0
Figure 6.39.
              8
   10     12     14
FILTER TIME, min.
16
20   22
Relationship of Particle Concentration and Filter Time at
Various Relative Humidities. (From Ref. 16)
-------
              These results are consistent with observations of increased
particle  adhesion  force at higher R.H.  (c.f. Figures 2.17 and 2.19, and
Equation  2.15).  The  increased force  should tend to produce a more open,
porous, and  filamentous deposit having  lower K' and better retention  (less
particle  migration and bleed).  Humidity may also affect the size of  the
particles collected on the filter.
              It should be noted that the above effects were obtained only
with  fly  ash, while humidity had no apparent effect on resistance or col-
lection efficiency when using cement dust, pulverized limestone, or amor-
phous silica under identical test conditions.  Clearly, further research
is needed to determine what aspects of  fabric and particles affect, through
humidity, the resistance and collection efficiency of a filter system.
              6.2.2.6  Effects of Velocity - Several aspects of filter per-
formance  have been observed to depend upon total gas flow rate or average
filtering velocity.  These include:
              o  Increase in specific resistance with velocity;
              o  Amount of dust depositing directly in the storage
                 hopper before reaching the filter (hopper fallout);
              o  Particle size stratification of dust passing to the
                 fabric because of fallout, and within the bag, as
                 velocity (flow) decreases throughout the length of
                 the bag;
              o  Deposit reorieptation or consolidation if velocity
                 increases during filtering cycle (deposit collapse);
              o  Particle penetration through thin or open areas on
                 the fabric surface (deposit puncture).
              Hopper Fallout - Hopper fallout has been observed during
single bag and single compartment studies.  A fallout factor,  7, has been
defined  as:
                _ amount of dust deposited in hopper,  W - W _
                  amount of dust entering collector inlet,  W  '  or
                      w                                               (6.19)
              7 = 1 - ~
                      W
-------
 That is,  / i-s  the fraction of the total dust entering the inlet of a
 fabric filter deposited directly in the hoppers, without being filtered
                                                                         14
 by the fabric.   Hopper fallout is reported in the fabric study of Durham
 above, (but  without 7).  Values of 7 depend upon dust particle size and
 shape, as  related  to  hopper,  inlet;, baffle, flow, and collector configura-
 i. ion parameters.  Typical values Tor y  from fabric I'll Lei: cumpai LiueuL
 studies (discussed below, see Figure 6.50) are 0.1 - 0.25 over a flow
                                                 3
 range of  four for  either top  or bottom  bag inlet .
             Particle  Size Stratification.  - Velocity also affects dust
 resistance by altering the particle size distribution in the  deposit.
 The  bulk  flow of gas  and dust is parallel  to the filter  surface and de-
 creases in velocity from a maximum at the  entrance of the bag to zero vel-
 ocity at  the other end.  Filter arrangement also produces changes in par-
 ticle size distribution by inertial separation and settling of larger par-
 ticles as  fallout.  During bottom feed  operation,  a greater portion of
 larger particles enter the bag as the gas  throughput is  increased (i.e.,
 dust fallout decreases).   Thus,  the average particle size of  the fallout
 varies with  filtration rate,  as  does  the average particle size of the
 filter deposit.  Since the resistance coefficient  is sensitive to the por-
 osity of the cake  and  the surface-to-volume ratio  of the  particles,  as
 given by the Kozeny equation,  size  stratification  will contribute to
 changes in the resistance of  the dust cake.
             The  effect  of velocity  on particle  size  statification along
 the  length of a  single  bag  is  shown in Figures  6.40  and 6.41.   In these
 tests, made  on a single  filter bag, samples  of  the  dust cake were  taken
 from several different  vertical  positions  along  the  bag.  Figure  6.40  in-
dicates the  effect of velocity on median particle  diameter deposited  at
 four  different elevations  (of a  60  inch  bag  height).  Just above  the  bot-
tom  inlet  (2  in.),  median particle  size  varies  from  8 to  15 ^m, as aver-
age  filtration velocity is varied from 2.5  to 8.5  fpm (approximate upward
velocity at  bag  inlet,  109 to 327 fpm).   Just below  the top, most  of  the
dusty gas  has been filtered, upward velocities are  low, and median par-
ticle size varies from 5  to 13  um as average filtering velocity varied
from 2.5 to 8.5 fpm.   The deposit at  the top is composed of finer particles
(higher K~).   Figure 6.41 shows  the effect of filtering velocity on par-
ticle size distribution at the center of the bag.  Higher gas  flow supports

-------
                           POSITION ON BAG. Inch*! from bottom
                              2   •
                              16   .
                              27   *
                              58   «•
                                                        10  n
Figure 6.40.
               Effect of Filtration  Rate on Particle  Size in Deposit  at
               four  bag altitudes  (Single bag filter  unit, bottom feed)
               (From Bongwardt and Durham, Ref. 3).
                                                       7.4 fp«
                                                       6.1 fpn
                                                       3.7 fpi"  A
                                                       2.5 fp»  «
                          10
                                  IS       20       25
                                  FAHXCU SIZE, «ietOM
                                                          30
                                                                  35
                                                                           40
Figure  6.41.
              Effect of Filtration  Rate on Particle  Size Distribution in
              deposit at Center of  Bag (Single Bag Test Unit, Bottom Feed)
              (From Borgwardt and Durham, Ref. 3).
-------
 larger particles,  the median particle size increasing from about 6um at
 2.5 fpm to about 14 urn at 7.4 fpm.   These figures show that elutriation
 of the particles has  occurred inside the bags and that the degree of elu-
 triation depends upon rate of gas flow.   Changes in particle size distri-
 bution between different positions  on the filter surface result in the
 formation of non-uniform filter cakes.   These phenomena indicate that the
 specific resistance coefficient will depend upon filtration rate, through
 effects  on particle size.
             Deposit Consolidation.  - Deposit consolidation (collapse)
 occurs when forces  produced on and  in the granular matrix exceed frictional
 and adhesive forces supporting the  particle aggregate structures. Forces
 tending  to produce  consolidation arise from static pressure differentials
 across the deposit  (1 in. H_0 = 0.036 psi = 5.2  psf),  viscous  drag of the
 fluid  on the particles,  kinetic energy imparted  by depositing  particles,
 flow pulsations, or physical contact with the  filter.   If these  forces
 are greater than interfacial particle-particle-fiber  forces, the  structure
 can shift  to a more compact,  stable  orientation.   Permeability  is reduced,
 and pressure drop rises  .
             Deposit consolidation and collapse has  been  observed  experi-
                                                                       4
mentally on both bench-scale  apparatus and  in a  pilot-scale  2-bag unit.
 In  experiments with fly  ash  on  bench-scale  equipment, cake  collapse was
produced by  gradually  increasing air  flow through a dust  cake supported
on  a flat  circular  filter  1.5  inches  in diameter.  Deformation of the
matrix occurs in steps over appreciable intervals of filter velocity.
Deposit  thicknesses were measured with a microscope having a graduated
micrometer  fine adjustment.  Irreversible compressions up to 50 percent
                                                4
of  the original deposit  thickness were observed.    (Actual deposit thick-
ness was not reported.)
             In an experiment with a pilot-scale 2-bag unit, filters were
                                               2
loaded to an average dust mass of 950 grains/ft  at an average filter
velocity of 3.4 fpm as shown in Figure 6.42.  The resultant pressure  dif-
                                                                2
ferential was 3.4 in.  H~0 (effective permeability 950 (grains/ft )/
-------
I
-si
UJ
i rtuuu
E
o.
t:
x^eoo
z
V}
Z cnn
'ERMEABILITYJJGRA!
A C
o <
o <
«- 200
en
(O
2
Q <
1 1 J II 1
** '* ** **** ^2^1st COLLAPSE, A P=b-9 IN. H20
\ 2. nd COLLAPSED pr 8.7 IN. H20
~~ ^>- J^^(t<^3rd COLLAPSE, A P=I04 IN. H20
•^B" O CT O w*^ H2U
_ \.,xAP=l5.8 1N.H20 _
NOTE:
DUST MASS CONSTANT AT 950 GRAINS/ft2,
CAKE DEPOSITED AT 3.4 fpm
KEY:
m m INCREASING Ap
o 	 o DECREASING A P
1 II 1 1 1
324 6 8 10 12 U
                                     AVERAGE  FILTER VELOCITY (fpm)
                          Figure 6,42.  Successive Deposit Collapse Observed On Pilot-
-------
 (in. II  0/fpm)  ).   Filter  velocity was  lowered  to  about  0.75  fpm and  then         __,
 raised  to  5.9  fpm during  which  time  permeability  remained  constant at  950
 (K' =7.2).  As  filter  velocity was  gradually  increased above  5.9 fpm,
 however, permeability decreased rapidly  to  785  (K_  =  9.9)  at V = 6.5 fpm.
 From 6.5 to 7.2  fpm, the  permeability  of 785 was  maintained.   Between  7.2          >
 and 7.6 fpm a  second collapse occurred,  further reducing permeability  to         ~'
 745 (K' =  10.1),  a value  which  was maintained up  to 8.1 fpm.   At this  point,
 filter velocity  was reduced  to  5  fpm to  substantiate  that  a new matrix            —';
 structure  of lower permeability had  been created.   Two  more distinct col-
 lapses were observed as filter  velocity  was ultimately  raised  to 9.8 fpm,         _,
 and after  each of these,  filter velocity was lowered  and then  raised, to
 demonstrate that  a permanent consolidated structure had been produced in
 each instance.  After the final collapse, permeability  had been  reduced  to
 585 (K' =  11.9),  or to  about 60 percent  of  the permeability of  the uncol-           \
 lapsed deposit.   Filter drag was  increased from 1.0 in.   HoO/fpm  for  the uncol-
 lapsed cake to 1.62 for the final collapsed matrix, or by more than  60 percent.     ,
The same effect has been  observed in single compartment  and multicompartment      ^
 filters, as discussed below.  These effects also depend  upon R.H. in the gas,
and fabric parameters.                                                             ^J
             Particle Penetration - Woven fabrics arc susceptible to pin-
hole flow  leakage, a common phenomena with many types of porous media.
Deposit puncture,  which occurs  when small pinholes  are opened through the
matrix, should be  more prevalent on filament yarns  or worn staple fabrics.
Local resistance  is lowered and high flow occurs through the opening.
Local disintegration of the filter deposit is a more severe manifestation
of the application of external  forces that cause deposit collapse.   The           •~f
puncture may be self-repairing,  since increased flow through the area
brings more dust to the vicinity of the puncture.   Thus, inertial deposi-         _j
tion mechanisms are enhanced by the higher velocity.  If the holes  are
                                                                                    ;
too large,  however, penetration will continue.                                       !
             Deposit puncture has been confirmed experimentally as  shown
in Figure 6.43.  On a bench-scale flat filter paper it could not be  in-            _j
duced even at very high pressure differentials.  However, when  a cotton
sateen filter fabric was used as the support,  puncture occurred as  shown
in Figure 6.43.  It is  seen that the unpunctured cake  had a drag of  1.0
-------
    •$.
       2.0
       1.5
       1.0
      OS
NOTE: ORIGINAL DUST MASS APPROXIMATELY
      1,000 groins/ft.2, CAKE DEPOSITED AT
      5.3 fpm
            BEFORE
                           •—• INCREASING AP
                           o—oOECREASING AP
                     AFTER PUNCTURE
                                  , FABRIC RESISTANCE
                  10
            20       30       40
             FILTER VELOCITY, fpm
50
61
          Figure 6.43.  Deposit Puncture Observed on Bench-Scale
                       Filter.  (From Ref. 4).
in. H-O/fpm.  As filter  velocity was raised above 10 fpm,  drag began to
decrease sharply and continued to decrease, approaching a limit  erf
approximately 0.47.   Reduction of filter velocity to 10 fpm verified the
constant resistance  of the punctured deposit.   The pinhole punctures crea-
ted were easily visible  by microscopic examination of the  fabric against
a bright light.
              An example of pinhole healing is described by Stephan, et.
   4
al.   The air velocity through the perforated  area dropped from approxi-
mately 100 fpm to about  2.3 fpm during a few minutes of normal filtering.

     6.2.3  Single Compartment Performance
            The laboratory performance of single compartment intermit-
tently cleaned fabric filters is effectively represented by single bag
performance tests discussed above.  Differences are associated with scal-
ing and in distribution  of flow to each of the several  bags.
-------
             6.2.3.1  Shake-Type Collector - Billings, et. al.,   reported         -"
                                                             p
 laboratory performance data for a small Wheelabrator Dustube  (2 #35A)
                             2                      3
 cloth tube collector (500 ft  of fabric, nominal 10  cfm capacity).  This         _j
 equipment is representative of several commercial collectors.
                                                                                     i
             The test unit consisted of two identical steel chambers, each         —I
 containing 32-5" x 70" cloth tubes.  In normal operation, one chamber fil-
 tered the dusty air while the second was cleaned by mechanical shaking of         _^
 the cloth tubes.  Operational time between cleaning cycles varied with dust
 loading and desired pressure drop characteristics.   Tubes were usually
 shaken when they attained a pressure drop of 2 to 4 in.  H^O.
             As the number of compartments increases, different sections             !
                                                                                   —^
 may be shaken sequentially, but the proportion of bags not in operation
 decreases so that large multi-chambered units operate at nearly constant            j
           ,                                                                        —i
 pressure drop.

             The objectives of the test program were as follows:                    _J
             1.  To evaluate the resistance-efficiency characteristics
                 with a variety of aerosols at different  dust load-
                 ings and filtration rates.                                        ~J
             2.  To compare laboratory results with  those obtained
                 in the field, since only field results indicate
                 service life and the effect of maintenance pro-
                 cedures on filter performance.
             3.  To investigate the feasibility of this type  of
                 unit for removal of low concentrations (e.g. toxic
                 materials such as Be, radioactive dusts, etc.)
                 from air and gas streams by means of augmentation                   j
                 with filter aids.                                                 —'

            Efficiency and pressure drop tests were  conducted in three               j
operating modes:  (a) performance of woven cotton medium with light dust
loadings and no bag shaking (i.e., basic cloth performance),  (b)  per-                '•
                                                                                   _j
formance of asbestos-flocked bags with light loadings and no shaking, and
(c) performance with higher dust loadings and frequent, periodic  shaking.
            Light Dust Loadings.- Tests with dust loadings in the range
0.04 to 0.1 gr/1000 cu. ft. (atmospheric dust) illustrated initial fil-
                                 6-76
-------
ter performance and provided basic  information on cloth characteristics.
Figure 6.44  illustrates the efficiency and pressure drop  increase  as  new
cotton bags slowly acquired a deposit of atmospheric dust.  These results
were obtained during a period of 484 hours of operation on atmospheric
dust, but are presumed to illustrate what takes place during a  few  min-
utes when higher dust loadings are  being filtered.  Both efficiency and
   100
                                              FILTRATION RATE 3CFM/SPFT
                                              SATEEN WEAVE COTTONBAM
                                              KM X 68 THREADS PER INCH
                             i.s       a
                   •MAINS OF OUST PCM SOU Aft E FOOT Of CLOTH
Figure 6.44.  Efficiency and Pressure Drop; New Cotton  Bags With Atmo-
              spheric Dust,  (prom Billings, et. al, Ref. 17).

pressure drop rise as  a deposit forms but the rate of increase fails off
after  only  a  few grains of dust have been deposited.   (From observation
of  industrial operations  it is  known that this increase would continue
for1  several weeks  with  the very low dust  loadings  found in ambient air.)
Over the period  of this experiment  penetration decreased 3 fold  (from
27  percent  to 9  percent) while  pressure drop  doubled  (from 0.09  to 0.18
in.H20).
                                                                          o
                                                                            CM
                                                                           (X
                                                                           o
                                                                           £
                                                                           0)
CO
CO
-------
            Filler Aid. - Figure 6.44  indicates that  i.L  takes many hours
of operation (at dust  loadings less than 0.1 gr/1000 cu.ft.) to  increase          ^J
filter efficiency to 90 percent.  For recovery of atmospheric dust,and
                                                                                   I
chemically toxic  or radioactive dusts, a high initial  efficiency is re-           j
quired.   This can be  achieved with deep beds (up to 5  feet) of  fibers or
granular solids, or with glass fiber  paper filters which are discarded             ;
when their resistance  becomes excessive.  However, operations with  a
fabric filter on low dust concentrations can be simplified and mechanized          ;
                                                                                   j
by using a filter aid.                                                            —'
            Asbestos floats (fibers too small for other use) were dis-             j
persed and filtered onto clean cloth of the same type as above.  Table
6.9 indicates the effectiveness of this treatment in increasing the
collection efficiency  of oil smoke (Diol 55, mass median diameter 1.2 urn).        -J
Efficiency was measured optically.
                                                                                  —•
                                TABLE 6.9
        EFFECTIVENESS OF FILTER AIDS FOR LOW PARTICULATE LOADINGS                  '•
Quantity of
Asbestos on
Filter Cloth
gr.V sq.ft.
0
32
50
82
154
182
194
Pressure Drop

(at 3 cfm/sq.ft.)
in H20
0.098
0.135
0.160
0.170
0.187
0.202
0.212
Smoke
Efficiency

per cent
20.3
44.9
52.7
84.7
96.4
99.4
99.9
            This shows a very substantial gain in efficiency) 20.3 to              i
99.9 percent) for a modest pressure drop increase (0.098 to 0.212 in.
H~0) .   Since bag filters are usually designed for resistances of 3 to              J
4 in.  H»0 before shaking, flocked bags can be operated for many months
on low loadings before shaking of the bags and renewal of the flock.
                                                                                   i
-------
             Table 6.10 indicates how efficiency decreased as the filter
 aid was removed.  Some asbestos fibers were held permanently by the
 fabric; hence, after shaking, only a small smount of additional flock
 restored the high efficiency characteristics of the filter.

                               TABLE 6.10
      REDUCTION IN EFFICIENCY OF ASBESTOS FLOCKED BAGS DURING SHAKING
Time of Bag
Shaking
:Minutes
ob
0.5
1.0
2.0
3.0
5.0
7.0
10 0
15.0
20.0
30.0
40.0
.Pressure Drop
(at 3 cfm/sq. ft)
in. H20
0.212
0.208
0.204
0.196
0.182
0.180
0.176
0.171
0.167
0.166
0.166
Smoke
Efficiency3
percent
99.9
97.7
97.0
96.0
94.0
93.3
90.0
89 .0
88.4
85.3
83.0
83.0
 a.   1.2  micron droplets  and  optical  penetrometer
 b.   Flocked with  194  grains  of  asbestos per  square  foot of cloth
 (rrom Billings, et.al, Ref.  17).
Table 6.11 lists the efficiency of plain and asbestos flocked cotton bags
for a variety of other aerosols and demonstrates the wide application
of the technique.
            The quantity of asbestos required to produce high filtration
efficiency is dependent,to a large extent, on how uniformly the fibers are
dispersed on the cloth.  With the flocking apparatus used a  resistance
rise of only 0.1 to 0.2 in. H^O was adequate.

            Commercial systems lor high efficiency performance with filter
aids have been discussed  in Chapter 3 (see Figure 3.31).  Wheelabrator
                              *
now uses a preliminary precoat ,  prior to addition of asbestos or other
       *U.S. Patent No. 3,041, 808.
-------
                               TABLE  6.11
             EFFICIENCY OF BAG COLLECTOR FOR VARIOUS AEROSOLS*
"— •• • ' " 	 ' "• '
Aerosol
Atmospheric dust
Uranium trioxide
microsphcres
Copper sulfate
microspheres
Aluminum chloride
fume
Ammonium bi-
fluoride fume
Ammonium bi-
fluoride fume
Talc

Loading
gr./lOOO
ft3.
0.025
0.0079

0.86
166

15

15

0.1 -
6.0C

Median
Size by
Count
microns
0.5
0.8

0.9
0.6

0.5

0.5

1.4

Pressure
Drop
(at 3 cfm/sq.
in. H20
0.212
0.212

0.236
0.244

0.244

0.11

0.157 -
3.54

Bag
Treatment
ft2.)
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
None

None

Weight
Efficiency
per cent
99. Ob
99.9

99.1
99.2

99.7

74.0

99.9+
a. Sateen weave cotton bags, filtration velocity 3 cfm/sq.ft.
b. 85 percent efficient by discoloration test, 68.2 percent efficient
   by particle count
c. Grains per cubic foot
(From Billings, et. al., Ref.  17).
fibrous filter aid, to promote release of the filter aid and any tarry
atmospheric constituents.  The specific resistance coefficient for fil-
                                                                    2
tration of atmospheric dust on filter aid is 16 (in. H20/fpm)/(lb/ft ).
            Periodic Shaking.- Figure 6.45 indicates variations in pres-
sure drop during normal filtering and shaking operation.  The cycle
includes a 15 minute filtration period, 2.5 minutes for shaking, and a
2.5 minute off-period for dust settling.  There are some variations in
maximum and minimum pressures attained.  For these thoroughly aged cotton
_J
-------
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Q/C? • OUST LOADINt IN Oft*
	 • FILTCftlNa PERIOD
	 • SMAKIN9 PCNIOO
- acrM/so.Ft or COTTON •*• ,
TALC DUST (NYTAL 500) /




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-------
     10  20
             30   40  50  60  70  SO  90  KX> HO  120  ISO 140  ISO ISO  170
                            FR.TMATION TIME.- MINUTES
                                                                     2OO
Figure 6.46.  Effect of Dust Loading on Rate of Filter Pressure  Drop
              Increase.  (From Billings, et. al., Ref. 17).
greater than 99.9  percent  in  all  cases.   Pressure  drop,  when filtering
         3                     2
O.L gr/ft  of talc at  3  cfm/ft  ,  increased  from 0.3  to 0.7 in.  HLO in
                         2
30 minutes.  At 1.0 gr/ft  , the pressure  drop  increased  to 1.5  in H-O at
the end of 30 minutes.
            The outlet dust loading  increased  somewhat with inlet load-
ing, but collection efficiency remained above  99.9 percent.  These data
indicate that while there are some differences  in  specific cases, the
general outlet dust concentrations from this type  of collector  using
                                                              3
cotton sateen fabric will probably be  less  than 0.1  gr/1000 ft   (200 vg/
Data presented earlier in Table 6.8  for cotton  sateen and fly ash tend to
confirm this general observation.    Field  performance data (Table 6.23)
have indicated similar outlet loadings.   However,  outlet loadings up to
              3          3
180 gr/1000 ft  (0.4 gm/m ) were  observed with  man-made  filament  yarns.
Relative humidity also affects filter  efficiency as  discussed above.
                                                                         3
                                  6-82
                                                                                   J
-------
                               TABLE 6.12
      EFFICIENCY AND PRESSURE DROP OF "DUSTUBE" FILTER AT VARIOUS
                     LOADINGS OF "MICRONIZED" TALCa
Inlet
Loading
gr./cu.ft.
0.1
1.0
2.0
Pressure Drop

in. H20
0.3 to 0.7
0.3 to 1.5
0.5 to 3.0
Outlet
Loading ~
gr./lOOO ft .
0.012
0.034
0.070
Weight
Penetration
percent
0.012
0.0034
0.0035
Weight
Efficiency
percent
99.988
99.9966
99.9965
a.  Mass median diameter 2.5 microns, geometric standard deviation 1.6,
b.  Pressure drop increased from lower to higher value in 30 minutes.
c.  Air to cloth ratio 3 cfm/ft.^; sateen weave cotton bags.
    (From Billings, et. al., Ref. 17).
            6.2.3.2  Other Single Compartment Studies.- The data discussed
below were obtained on multi-compartmented collectors operated in parallel
mode, (no compartment off-line for cleaning) such that effects studied
relate primarily to single compartment performance.  The 3-compartment
                                                          3 18
pilot fabric filter system used for most of these studies, '   is shown
in Figure 6.47.  Each compartment contained 8 cotton sateen bags 60 in. long
x 5 in. diameter.
            Hopper Fallout.- Hopper fallout, 7, was defined in Equation
6.21 as the fraction of dust entering the inlet of a filter collector
that does not reach the fabric.  Figure 6.48 shows values of 7 when fil-
tering upward and downward, at several gas volume through-puts.  Average
                                                2
air to cloth ratio ranged from 1.3 to 4.5 cfm/ft  .  in bottom entry, up-
flow operation:
            . more material was carried into the  filter bags as
              velocity increased, and
            . The fallout varied from 27% at 1.3  fpm to 8% at 4.5 fpm.
In top-entry, down-flow operation:
            . the fraction falling into the hopper increased as
              velocity increased, and
            . the fallout varied from 8% at 1.3 fpm to 30% at 4.5 fpm.

-------



         Figure  6.47.
Three Compartment Baghouse (From Robinson,
et. al.,  Ref.  18).
Lacking internal configuration dimensions, an analysis of flow velocities,
particle sizes, settling velocities and stratification has not been made.
            Particle Size Stratification.- A particle size distribution
of fly-ash from hopper and filter deposits is shown in Figure 6.49 ob-
tained from a 30,000 cfm prototype 4-compartment field unit operated on a
                             18
pulverized coal-fired boiler.    Hopper fallout varied from 8 to 16%.
Fallout sizes were considerably larger than those reaching the fabric.
            Deposit Consolidation.- The effect of filtering velocity on
the specific resistance coefficient has been reviewed above (see Figure
6.42).  Further experimental data on the three compartment collector
operated in parallel mode is shown in Figure 6.50.   Dust was deposited
at low velocity (point a), the dust feed stopped, and the air flow in-
creased without further dust deposition.  At points b and c the gas flow
                                   6-84
                                                                                     o
                                                                                  J
                                                                                  u
                                                                                  -
                                                             i
                                                           — •
                                                              9
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  §
  u,
  o
  •-C
  u.
Ab
10
35
30
i . -1
2C

15

K)
',
0
^ ( i I I I ' i '
Top Feed = top inlet, downflow, dust deposition inside
_ sleeve
Bottom feed = bottom inlet, upflow, dust deposit on in-
side of sleeve
TOP FEED DOWN
BOTTOM FEED UP FLOW . FLOW
. 9
+v* /
2^
/• ^^^ ^
9/. 4 ^*^
^^^ >,
»x^» •»•
of
•
-
-
H
\
-i

-
3-comp. collect
(8)5 Y. 60in. cotton
~ sateen bags/comp. *"
(collector fabric area,
. J 1 1 l l i ' 1 .
) .'00 100 600 800

100
  Figure 6.48.
               TOTAL  GAS  FLOW_,  cfm
Fly Ash Fallout vs. Gas Throughput For Top and Bottom
Feed (Parallel Flow, Constant Rate Filtration) (From
Borgwardt and Durham, Ref. 3).
was decreased  toward  zero  and  raised  again.  The  test  shows  that  as  flow
is decreased the filter permeability  remains constant, but when flow  is
increased the  permeability decreases.  The apparent compacting effect
may be due to  a displacement of particles from the deposit at higher  gas
flow and redeposition deeper in the filter matrix.
6.3  LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT
     6.3.1  Basic Pressure Drop Equations
            In discussing the performance of single compartment filters,
the parallel functioning of the separate filter elements was compared to
-------
                                                               1000
                            MRTCLC SIZE, MtorMw


Figure  6.49.  Particle Size  Distribution of Fly  Ash (from
               Robinson,  et al,  Ref.  18)
  10
                  HOW INCREASING

                  FLOW DECREASING „
                            •>..—.—. •
                              (a)
                                                  _L
                            2           3           4

                        AVERAGE GAS FILTER VELOCITY (»). fpn
 Figure 6.50.   Dust Cake Compression (Pilot  Baghouse,  Parallel
                Flow Operation)  (From Borgwardt and Durham, Ref.  3)
-------
 that of electrical resistances in parallel.  The same  analogy applies  to
 two or more compartments operating side by aide with  the  same differen-
 tial pressure driving force.   The equations  are essentially unchanged if
 the compartments are all shut down for cleaning at  the  same time.   If
 however,  the compartments are taken off line one at a time  for cleaning,
 they operate at different cycle phasings,  and the equations must  account
 for this.

             When a just-cleaned compartment  is  returned to  servi.ce, its
 drag will  be less  than that of the  other compartments  (Figure  6.51).
 Consequently the flow through the  compartment will  be greater  (Figure  6.52),
 This means  that  the  rate of deposition of  dust  in this compartment will
 be  greater  than in the other  compartments, and  the  compartment drag will
 increase more  rapidly.   The compartment will  rapidly approach  a level  of
 operation  similar  to the average  of  the other compartments.
             The  flow through  the  overall system,  which also  increases
 when the cleaned compartment  is  first  returned  to service,  gradually de-
 creases as  the  drag  in the separate  compartments  increases.  When the
 next  compartment is  removed from  the system  for cleaning, the overall
 flow will decrease again.  Thus  the  overall  flow  follows a cyclic pattern.
             The  variation  in  overall flow  depends on the characteristic
 curve of the fan driving the  system  and other system flow resistance
 characteristics.  As  a  consequence of  overall flow variation, the dif-
 ferential pressure across  the  separate compartments will also undergo at
 least some variation.   This is  indicated schematically and described in
 Figure 6.53.
             Velocity variations may occur not only from compartment to
 compartment, but also  from filter to filter and along  the length of a
 filter, as discussed above in single bag performance analysis.  Therefore
 the use of nominal velocity to describe overall filtration is arbitrary,
since for a given nominal velocity many combinations of internal flow
distribution may exist.
-------
raw
•0
i
fc •«
3 4O
£
= «
A
II 1 1 1
1
•
">•*-•
- 1 5
J±t|-
	 tn fc 	
M
1 1 1 1

0.0         IX)        1.5
   FILTER DRAG (INCHES
                                                       8.0
Figure 6.51.   Instantaneous Filter Drag  Profile  for Six Compart'
                ment Baghouse.  (From Walsh and Spaite,  Ref.  5).
            10
          £ •
                   "i	T—r
                        —r
                         AVERAGE VELOCITY : 3jO 
-------
      c\j
    CO
    UJ
    X
    z
    UJ
    tt:
    UJ
    u.
    u.
    o
    UJ
    o:
    :D
    co
    CO
    UJ
    a:
    a_
                           TIME
Figure 6.53.  Schematic Pressure Differential Curve for
              Multicompartment Baghouse)(From Ref. 5).
Explanation:   Time t.
               Time
               Time t
A compartment has just returned on-line
with a lowered drag; the system flow is
higher than usual, and the collector
pressure drop is lower than usual.
The next compartment has jus t been removed
for cleaning, and total cloth area is
less than usual. Thus filtering velocity
is slightly increased, and pressure diff.
has also increased. System flow has decreased.
The compartment has been cleaned, and is
returned on line.  The system returns to the
same state as at Time tft, and recycles.
-------
            Residual  conditions  are  those which  exist  in a  particular             ^
compartment as  it  is  put on  stream after cleaning,  and  terminal  conditions
are  those which exist  in a compartment  just  before  it  is taken out  of             _j
service  for cleaning.  The residual  drag (set  by cleaning method and dust-
fabric parameters) and terminal  drag( set by fan and dust-fabric resistance
characteristics) establish overall limits for a particular installation;
therefore they  are critical  operating parameters.   Variations in filter            :
drag,  in establishing differences  in filter  velocities,  also influence
dust penetration  into the  filter,  filter blinding,  and  the  structure of
the  dust cake.  Drag  itself  is affected by these changes.  Some  of  the
equations describing  multicompartment system dynamics  are developed in
Appendix 6.3, and  are of a form  perhaps best studied by computer tech-            ->
niques.
     6.3.2  Performance of a Multicompartmented  Collector                        ^
            Billings,  et al.   have  reported a laboratory performance of           j
a  commercial multicompartment collector cleaned  automatically on a  time           ~~*
       *
cycle.   The test  unit consisted of  four compartments,  each  containing             (
                                                    2
eight  bags 8 in.  in diameter and 6 ft.  long  (100 ft  of  cloth/compartment),       .^"
as shown in Figure 6.54.  Dusty  air  entered  the  bottom  of es<-h section,
passing  upward  and through the fabric tubes.   Once  every 5.2 minutes each
compartment was shut  off from the  fan by means of a damper,  vented  to
atmosphere, and the bags cleaned.  Dust removal  was accomplished by               |
lifting  the whole assembly about 1-1/2" and  dropping suddenly.
                                                                                   t
            In  addition, outside air was drawn into the  compartment by the
negative pressure of  the hopper  and  inlet plenum, and passed through the
bags in  a reverse direction  to help remove dislodged dust.   Reverse air
passed down the inside of the bags and  through the inlet header  to  the
other compartments.  This cycle was then repeated on another section.
Sateen weave cotton bags were normally used  in this equipment at  a  fil-           —'
                         2
tration  rate of 10 CFM/ft .
            Performance characteristics investigated on  this collector            V
were the effects on pressure drop and efficiency of changes in reverse             ,
flow air, amount and frequency of rapping,  dust  loading, air velocity,             __,
and aerosol particle material.   Performance of five different bag'mater-
ials was also compared.
-------
 c
   -



                                                              OUTLET,
                                                              TO FAN
                                                                 A.  BACK FLOW
                                                                     AIR OPENING
                                                                 B.  SHAKING
                                                                     MECHANISM
                                                                 C.  AIR VALVE IN BACK-
                                                                     FLOW POSITION
                                                                 D.  8-6' BAGS PER
                                                                     SECTION

                  Figure 6.54.  Cloth Tube  Filter  Cleaned by Mechanical Rapping and
                               Back Flow Air.   (From BilUa**,  «t.  «!.,  fcef.  17).
                   An adaption of the British Simon Suction Filter manufactured  in  the
               U.S. by Entolater Div.,  Safety Car Heating and Lighting Co.,  New  Haven,
               Connecticut.
   .-
c

                          6.3.2.1  Studies With Light Dust Loadings
            Basic Media Performance.- The combination of rapping bags,  as  an
integral unit, to minimize distortion and the use of reverse air flow per-
mits the choice of several bag materials other than sateen weave cotton
cloth.  Such fabrics are:  (1) wool felt with its inherently higher effi-
ciency but less porous structure which requires reverse air for adequate
cleaning and (2) woven glass cloth capable of withstanding high temperature
but having poorer flexing characteristics.  Several bag materials were  tested
including heavy and light wool felt, woven glass cloth lubricated with  a sil-

-------
                          R                           17
 iconc1,  woven  napped  Orion ,  and  sateen weave  cotton.    The  basic  pressure
 clrop-efIficiency  characteristics  of  these  fabrics were evaluated on  light
                                  3    3
 dust  loadings  (0.01  to  0.1 gr./lO  ft ).  No  bag cleaning was  employed
 during  these  tests.
            The  pressure  drop and efficiency  of various  fabrics is  shown
 in Table  6.13  for atmospheric dust  and copper sulfate microspheres.  On
 atmospheric aerosol  at  10 cfm/sq. ft., the  fabrics may be rated in  order
                                  p
 of increasing  efficiency  as: Orion  .cotton,  light wool, woven glass, and
 heavy wool.
            The  last, two  columns  of Table 6.13 show  the  inlet  loading and
 weight  efficiency determined with copper  sulfate microspheres  for each
 of the  fabrics shown in the  first column.   The clean cloth  pressure drop
 for each  fabric  is related to copper  sulfate  efficiency.  The  relationship
 between weight collection efficiency  and  resistance for  these  five  fabrics
 is *
                          Wt. eff.  =  98.5 (Ap)°-33,                 (6.20)
 for Ap  inches of water, at a filtration velocity of 10 fpm with a
                                                                          19
 copper  sulfate particle size of  2.7 microns(MMD).  Billington  and Saunders
 report  the exponent  of pressure  drop  as 0.43  for 13 air conditioning filters,
 using a stain density measure of  efficiency with particles  in  the same size
 range.  They also mention an exponent of 0.1  found by Bigg  for weight effi-
 ciency  of fabric filters  using particles in the range 25 to 100 microns.
            Further  Studies of Filter Aids.  - To extend the knowledge of
performance of filter aids to other fabrics and higher filtration velocities
                                              R
 than those described in Section 6.2.2.1, Orion , cotton, heavy wool felt,
and woven glass were flocked with asbestos floats and tested with copper
 sulfate aerosols at  a filtration velocity of  10 fpm.   Bags were not shaken
during  the flocking  process,  nor during the testing with copper sulfate.
The results of these tests are given  in Table 6.14.  It is possible to in-
crease  the efficiency of  these materials to over 98 percent by the appli-
cation of asbestos floats as  a filter aid.
                                 2
            The use  of 200 gr/ft.  of asbestos floats had been suggested
as an optimum amount in the study described in a previous section.  To

-------
                              TABLE 6.13

                      FABRIC COMPARISONS WITH LIGHT LOADINGS3
I'abr it-




Light Wool
Felt
Orion,
Napped
Cotton,
Sateen
Weave
Heavy Wool
Felt
Woven Glass
Clean
Fabric
Pressure
Drop in H?0

0.07

0.10

. 0.29


0.34

0.56
Atmospheric
Inlet Loading
gr./lOOO
cu.ft.

0.184

0.130
K
0.045


0.220

0.058
Dust
Weight
Efficiency
percent

74.8

60.4
/>
35. 5°


85.4

81.9
Copper
Inlet
Loading
gr./lOOO
cu. ft.
0.89

0.81

1.02


1.01

0.90
Sul fate
Weight
Efficiency
percent

41.1

45.6

63.7


71.3
.
81.1
'Filtering velocity 10 cfm/sq. ft.

 Believed to be experimental error.
£
 Probably nearer 60 percent.

(From Billings, at. al.,  Ref. 17).
                               TABLE 6.14


        FABRIC COMPARISONS USING ASBESTOS FLOATS AS A FILTER AID3
Fabric

Orion
Cotton
Dense Wool
Woven Glass
Pressure Drop"
in H20
Initial
0.16
0.36
0.36
0.81
Final
0.95
1.43
0.90
2.20
Copper Sulfate Weight
Efficiency
per
Initial
45.6
63.7
71.3
81.1

Final
98.56
98.95
99.28
99.60
3200 gr./ft2
  Filtering velocity - 10 fpm

 (From Billings, et. al., Ref. 17),
-------
 confirm this  on a different aerosol and at a different filtration rate,
 glass  bags  were flocked in small increments, and the efficiency was evalu-
 ated after  each step with copper sulfate microspheres (Table 6.15).  Each
 increment of  filter  aid caused  an increase in pressure drop and a decrease
 in  amount penetrating.   Effluent loadings at a velocity equal to 3 fpm were
 not appreciably different from  those at 10 fpm.

            Test  7 shows  the increase in penetration when  the unit was
 shaken five times, i.e.  five complete cycles or  20  strokes per compartment.
 About  60 percent  of  the original material,  by weight,  was  recovered from
 tVie hopper  at  this point.   The  remaining asbestos on the bags then amounted
                   2
 to  about 80 gr./ft .  Penetration and pressure drop  at this  time were dif-
 ferent than during loading due  to the uneven removal of the  flock and chan-
 neling of the  flow through less  flocked areas.   Use  of asbestos  floats  as
 a filter aid permits  control of  penetration and  pressure drop.   The primary
 aerosol particle  concentration  cannot be too high or frequent  replacement
 of  the filter  aid will  be  necessary (see Figure  3.31).  The  use  of glass
 bags with asbestos flock  can substantially  increase  collection efficiency
 for high temperature  work  in such applications as the  final  gas  cleaning
 stage  in incineration processes.   Field trials will  be required  to estimate
 service life of the  combination.   Present applications  of  filter  aids (1970)
 also include use  of  granular reactants  for  recovery  of  gaseous contaminants.

            6.3.2.2   Studies With Heavy Dust Loadings
            Frequency and Number of Raps.- The standard cleaning cycle
consisted of four raps per section,per 5.2 minutes, in conjunction with
entering reverse flow air.  Table 6.16 shows the variation of pressure
crop and penetration  as  the number of raps i's decreased.  Efficiency in-
creased as  pressure drop increased witn decreasing number of cleaning strokes,

            An investigation of  the effects  of the frequency of  rapping was
 made.   This was done  by reducing the  cycle  time  and  the number of raps,  in
 direct proportion, to keep  the number of raps  per section  constant  over any
-------
                                                                        f
                                         TABLE 6.15

        EFFECTIVENESS  OF FILTER AIDS  FOR LIGHT LOADINGS  OF COPPER SULFATE MICROSPHERES'
Test
No.
1
2
3
4
4a
5
5a
6
6a
7
Filtration
Velocity
cfm/ft.2
10
10
10
10
3
10
3
10
3
10
Cumulative
Filter Aid
on Fabric
gr./ft.2
0
25
50
100
100
150
150
200
200
b
Pressure Drop Copper Sulfate
in H«0 Loading
gr./lOOO ft.

0.86
1.56
1.76
1.80
0.63
2.00
0.66
2.20
0.72
1.46
Inlet
0.92
0.79
0.87
0.88
1.41
0.88
1.68
0.80
1.62
0.80
Outlet
0.130
0.024
0.019
0.012
0.007
0.008
0.008
0.003
0.003
0.026
Weight
Penetration
percent
18.9
3.1
2.2
1.4
0.5
0.9
0.5
0.3
0.2
3.4
Weight
Ef f icienc
percent
81.1
96.9
97.8
98.6
99.5
99.1
99.5
99.7
99.8
96.6
a.  Woven glass bags lubricated with silicone, no fabric cleaning during tests, except prior
    to Test 7.

b.  Recovered 60% from bags by rapping, five cycles = 20 strokes.
-------
                              TABLE 6.16
        EFFECT OF DECREASING THE NUMBER OF RAPS IN THE STANDARD
                     CLEANING CYCLE ON I^RESSURE DROP
                              AND EFFICIENCY
No. Raps/
Section in
5.2 min.
cycle
4
'}
2
1
Average
Pressure Drop
in. H«0

3.2
3.6
4.5
5.7
Dust
Inlet
gr./ft3

0.85
0.60
0.60
0.91
Loading
Outlet
gr./lOOO ft3

1.68
0.94
0.63
0.49
Weight
Penetration
percent

0.20
0.16
0.10
0.054
Weight
Efficiency
percent

99.80
99.84
99.90
99.946
 a.   Sateen weave cotton bags;  back flow air 100 cfm;  total air volume
     3000 cfm;  fly ash test dust.
     (From Billings, et. al., RcL.  1.7).
 long  time  interval.   Data presented  in Table  6.17  indicate  that  several  raps
 followed by  a  long pause  produced lower pressure drop  than  short cycling.  The
 rapping operation  appeared to  be  more  effective in removing dust than the
 reverse  air,  the  velocity of  which  was about  1 fpm,  (see below).
             Reverse  Flow  Air.-  The amount  of  reverse flow air  that  enters  a
 compartment  to aid in cleaning is often controllable.   The  above equipment was
 modified to  study  the effect of reverse air volume.  Results in  Table. 6.18
 indicate a 6-fold  increase  in  filter penetration when air volume was  in-
 creased 5-fold.  At  the same time, pressure drop decreased  about 50%.

             Inlet Dust Loading - The average pressure drop increases
with inlet dust loading as  shown  in Figure 6.55.  With fly ash, the relative
rate of pressure drop rise with dust loading decreases at higher loadings,
regardless of filtration velocity.  Talc produces  relatively higher pressure
drop at 2000 cfm and  the rate of rise with dust load is higher than with
fly ash.   These curves are  similar to performance  data for continuously
on-line cleaned reverse-jet and pulse-jet collectors.
-------
                                 TABLE 6.17

       KFFKCT OF VARIATION IN CLEANING CYCLE FREQUENCY ON PRESSURE DROP
                         OF A MULTICOMPARTMENT COLLECTOR*
Cycle No. of
Raps/Section
1
2
3
4
No. of
Minutes
1
2
3
4
Average Pressure Drop
in H20
Comp . I
5.8
5.4
5.0
4.8
Comp. IV
6.0
5.8
5.5
5.4
       *Sateen weave  cotton  bags,  back  flow air  100  cfm,  total  air
       volume 3000 cfm or 10  cfm/ft2,  inlet loading 0.65 gr./ft3,
       fly  ash  test  dust.   (From  Billings,  et.  al.,  Ref.  17).
                               TABLE  6.18

 EFFECT OF CHANGES  IN  REVERSE AIR VOLUME ON  PRESSURE  DROP AND  PENETRATION*
Amount of
Reverse Air
cfm
51
f.Q
85
120
180
250

Average
in H90
3.1
9 Q
2.6
2.6
2.3
2.2
Dust
Inlet
gr/cu.ft
1.10
i in
1.10
0.94
0.88
0.94
Loading
Outlet
gr/1000 cu.ft.
1.3
4.3
4.1
4.6
7.2
Weight
Penetration
Percent
0.12
0.39
0.44
0.52
0.77
Weight
Efficiency
percent
99.88
99.61
99.56
99.48
99.23
 *Sateen weave cotton bags; air volume 2000 cfm; 4/5 cleaning cycle; fly
  ash aerosol; 1 fr/ft-1.    (From Billings, et. al. , Ref. 17).
             Table 6.19 shows the penetration of fly ash as the inlet
                                                  3
dust loading is increased from 0.40 to 13.0 gr./ft .  Individual effluent

concentrations of each compartment are given in Table 6.20.  Compartment I

contributed almost twice as much to the effluent as the others, probably

due to uneven distribution of flow in the unit.
-------
o
 CM
CX
2
Q
CO
CO
01
0)
60
Cfl
H
01
                                    ENTOLETER UNIT

                              CURVE AEROSOL OUANTITV  RF AM
                               A  FLY ASH  3000 CFM  120 CFM
                               8   •  •   200O •   63 •

                               C   -  "   tOOO •   20 •
                               D    TALC   200O •   230 •
            Figure  6.55.
   INLET  DUST LQADING-GR./FT3


Variation of  Filter with Inlet  Dust Concentration

(From  Billings, et. al., Ref.  17).
                                 TABLE 6.19

       EFFECTS OF INLET  DUST CONCENTRATION  ON FLY ASH PENETRATION3
Dust
Inlet
gr . /cu. ft .
0.40
2.90
5.10
13.00
Loading
Outlet .,
gr./10J ft .
0.52
1.50
2.40
3.80
Weight
Penetration
percent
0.130
0.052
0.048
0.029
Weight
Efficiency
percent
99.870
99.948
99.952
99.971
 a. sateen weave, cotton bags;  back flow air  100  cfm;  total air volume
    1000  cfm or 3.3 cfm/sq.ft  ;  4/5 cleaning cycle.

    (From Billings, et. al., Ref.  17).
-------
            Compar J son of five fabrics.- The basic performance of  five  fabrics
 iltiM-ing light loadings of atmospheric dust and copper sulfate has been dis-
cussed above.  These fabrics were also used to filter heavy dust loadings to
obtain comparative performance data.  The pressure drop and penetration for
each fabric, used with the standard cleaning cycle, are given in Table  6.21
for fly ash and talc dusts.  Fly ash required a lower pressure drop on  all
fabrics.  Fly ash penetration was also greater.  Fabric order is similar in
                             n
each series, except for Orion  fabric, which had the highest penetration on
fly ash and near the lowest on talc.  This shift in order of penetration for
     D
Orion  may have been caused by humidity or electrostatic charge effects.
6.4  LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLECTORS
     6 . A . 1  Reverse Jet Filter (Horsey Type)
            Billings et al   reported on laboratory performance tests of a
reverse- jet continuously on-line cleaned fabric filter (see Figure 3.3F).
Pressure drop and efficiency were found to vary with inlet dust loading,
filtering, velocity and cleaning ring operation parameters.  Empirical
pressure drop performance equations were derived for test dust simulants.
            For resuspended vaporized silica (particle size, order of 1 (Jim),
the equilibrium operating pressure drop (in. H^0)was found to be:


        Ap = fo.31V(M + 1.5)°'19 + 800Q - Vi  x 0.9s] ( RJoV°<18     (6.21)
             L                        1000          J \100/
where    V = filtration velocity, 7 to 20 fpm
                                                2
         M = dust flux to fabric, 1 to 87 gr./ft  - min, (=^7)
       V .  = velocity of reverse- jet air, 2000 to 8000 fpm
       RJO = percent of time reverse- jet operates, 6 to 100%
g
            The pressure drop equation for resuspended fly ash
       16, cr  >3) was found to be:
           S
        Ap =(b.047V(M + 14)°'35 + 4200 - Vrj x 0.70~| (W\"°>25       (6.22)
            L                                     J
                                  1000              UOO
for 10 < V < 30 fpm, 1 < M < 76 fr/ft2 -min, 2200 < V   < 4200 fpm and
7 < RJO < 100%.
-------
                               TABLE 6.20

    INDIVIDUAL COMPARTMENT EFFLUENT CONCENTRATIONS WHEN FILTERING AN
    INLET DUST CONCENTRATION OF 1.1 GRAINS PER CUBIC FOOT OF FLY ASH3
Compartment
(I farthest from
II
III
(IV nearest fan)
Overall
Dust Loading
Outlet ~
gr./lO ft .
fan) 2.2
1.3
1.2
1.2
1.2
Weight
Penetration
percent
0.19
0.12
0.11
0.11
0.11
Weight
Efficiency
percent
99.81
99.88
99.89
99.89
99.89
Pressure
drop
in H20
3.4
3.5
3.5
3.6

a. sateon weave cotton bags; back flow air 100 cfm; total air volume
   2000 cfm or 6.7 cfm/sq.ft.; 4/5 cleaning cycle.
                               TABLE 6.21
      COMPARISON OF FIVE FABRICS FILTERING HEAVY DUST LOADINGS
Fabric
Cotton
Light Wool
Dense Wool
Orion
Woven Glass
Orion
Cotton
Dense Wool
Light Wool
Woven Glass
Operating
Pressure Drop
in Ho^
5.5 A. Talc3
5.0
5.6
5.5
5.9
B. Fly Ash
2.4
3.0
2.7
2.6
4.7
Weight
Penetration
gr./10Jft .
0.990
0.740
0.340
0.330
b 0.063
5.60
1.40
0.30
0.26
0.12
Outlet
Loading-,
gr./10Jft .
99.9010
99.9260
99.9660
99.9670
99.9937
99.440
99.860
99.970
99.974
99.988
   Mass median diameter 2.5 microns;  geometric standard deviation 1.6;
   filtration velocity 5 cfm/sq.ft.;  inlet loading 1.0 gr./ft.;
   4/5 cleaning cycle, 4-Compartment  Unit.
b.  Mass median diameter 16 microns;  geometric standard deviation >3;
   filtration velocity 10 cfm/sq.  ft.;  inlet loading 1.0 gr./ft  .,
   4/5 cleaning cycle, 4-Compartment  Unit.

    (From Billings, et. al., Ref.  17).
-------
             Pressure drop for fine talc (M'  = 2.5 |am, cr  = 1.6) was
                                            o     x _      o
 Lound to be:
Ap -  fo.l6V(M + 1.3)°')0 + 2000 - Vl
      L
                                      1000
0.8~| /RJO\
   -1 \100/
                                                            -0.14
                                                                      (6.23)
 for 7 <. V < 20 fpm,  1 < M <, 70 er/ft  -min, 2000 < V   < 8000 fpm, and
 10 < RJO < 100%.
 Average inlet and outlet dust concentration for 8 test aerosols are given
 in Table 6.22.   Efficiency varied with amount and magnitude of cleaning
      6.4 2  Pulse Jet Collector
                                 20
             Dennis and Silverman   reported on laboratory test of a
               o
 Micro-Pulsaire  pulsejet continuously on-line cleaned collector (See
 Figure 3.3H).
      Equilibrium pressure drop for resuspended fly ash was given by:
           Ap = 0.14 +
                0.48 (Ci)
                                  0.23
                                 0.5
                                                                    (6.24)
                         te\  t1
                        l\100/
             For resuspended vaporized silica, pressure drop was found
 to be
              Ap = 0.14
                    1.75 (Ci)
                                     0.27
                            WO
                                   ).5
                   (6.25)
for 0.06 < Ci < 1 gr/ft , and other parameters same as above.  Weight col-
lection efficiencies were reported to be greater than 99.9% on all tests.
6.5  FIELD PERFORMANCE
     Pressure drop and efficiency for several fabric filters tested in the
                                   21
field were reported by Dennis et al  .   Data for intermittently cleaned
tube and screen type collectors is shown in Table 6.23.  Average operating
efficiency was found to be greater than 99% in all tests.  Field test re-
sults for reverse-jet collectors are shown in Table 6.24.  Efficiency for
-------
                                  TABLE  6.22

     AVERAGE  INLET AND  OUTLET  DUST  CONCENTRATIONS  FOR A VARIETY OF AEROSOLS

                        TESTED  ON  THE  REVERSE-JET COLLECTOR3
Aerosol
Atmospheric Dust
it M
ii ii
Copper Sulfatc Microsphcres
ii ii n
it n n
Uranium Trioxide Microsph.
ti n ti
n it n
Talc
n
n
n
Vaporized Silica
n n
M n
n n
y n
U ii
M n
Fly Ash
n n
n n
M n
n n
V "
Alundum
Calcium Carbonate
n n
n n
Filtration
Velocity
ft/min
25
17
8
25
17
8
25
17
8
8
11
8
8
20
28
17
22
10
10
10
10
29
20
10
10
10
17
25
25
25
Average Dust Loading
gr./cu.
Inlet
3.2x10";?
2.1x10"?
3.3x10°
ISOxlO'c
250xlO~
470x10
640x10";!
770x10°
1800x10
0.14
0.16
2.0
8.7
0.06
0.09
0.22
0.87
1.07
1.21
1.26
0.37
2.1
4.0
6.8
11.2
13.4
2.0
0.0055
0.033
0.25
ft.
Outlet
0.6x10";?
0.2x10°
0.2x10
4.9x10";?
3.7x10";?
31x10
27x10"^
68x10°
120x10°
1.2x10";?
1.2x10°
3.9x10°
6.0x10
2.6x10",
4.9x10°
4.4x10°
5.8x10°
1.0x10°
41x10°
1.8x10"
8.6x10";?
153x10°
63x10°
86x10°
44x10°
61x10
13. 8x1 O"5
e
11.1x10 ^
17.3x10°
48xlO-5
Reverse-Jet
Velocity
fptn
b
b
b
b
b
b
b
b
b
2000
1620
2000
2000
2000
2000
4800
2000
4000
4000C
4000
5000C
4000
4000
4000
2000
3000
3000
b
b
1450
a. Single 18"x60" bag; AmericanPelt Co., #51002, 19 oz./sq,
b. No reverse-jet operation
c. Three 6"x60" bags; same as above.
(From Billings et al., Ref. 17).
yd.
-------
                                                               TABU 6.23
                                     FIEU) TEST RESULTS FOR CLOTH SCREENS) AND TUBE  (T) COLLECTORS CLEANED BY
                                                                     MECHANICAL SHAKING*



o»
l~>
o





Test
and
Unit
13(T)b
16(T)
16a(T)d
17 (T)
18(S)°

19 (S)
20(S)
21 (S)
22(S)
23(S)
24 (S)

25(S)
Operation
Founding
Found ing
Founding
Founding
Granite
chipping
Founding
Truing &
shaping
grinding
wheels &
sticks
Rubber
Material
DUST DESCRIPTION
Inlet Outlet
Loading Median Size
Grains per Microns

Iron Scale
Si02
Bronze Scale
SiO,
Bronze Scale
SIO,
Iron Scale
sio2
Granite

Iron Scale
SiO,
Ai6
SiO,



Talc
Cu. Ft.
0.68
0.44
0.595
0.13
0.032

0.39
0.19
0.15
0.10
1.33
0.88

4.3
Mass
2.6
105
3.8

6.2
48
4.3
4.4
4.0
3.2

~
Count
0.63
0.82
0.71
0.46

0.47
0.56
0.51
0.57
0.45
0.62

1.3
Loading
Grains per
Cu. Ft.
0.000015
0.000069
0.000048
0.000013
0.000028

0.00063
0.000013
0.00011
0.000074
0.00003
0.0025

0.0064
Median Size
Microns
Mass Count
0.57
-
1.1 0.41

3.4 0.37
0.87 0.43
0.96 0.43
0.59 0.38
0.46


Air
Flow
Rate
rs.i.p.)
c£*a
7,000
560
1,850
2,000
7,020

18,200
13,900
32,000
32,000
4,520
32,000

7,000
CFM per
Sc. Ft.
Cloth
2.5
0.8
2.6
2.2
2.3

1.5
1.3
4.1
3.2
1.4
3.2


Pressure
Loss
Across
Collector
in. Water
1.5
5.0
0.3
2.2
3.0

1.9
1.2
2.9
4.1
1.7
3.3


Collection
Efficiency
by Weight
Per Cent
99.99
99.98
99.99
99.99
99.91

99.65
99.99
99.93
99.93
99.99
99.72

99.85
Manu-
fac-
turer
American
Wheela-
brator and
Equipment
Corporation

Sly
Corporation

Pangborn
Corporation




*STP » 70°F and 760 am Hg
 Cloth tubes
°Cloth envelopes over «etal screens
 TJnit 16 following replacement of worn-out bags
-------
 I
I—
o
                                                                            TABLE 6.24


                                                  FIELD TEST RESULTS FOR CLOTH BAG COLLECTORS CLEANED BY REVERSE-JET AIR
Test
and
Unit

26
27
28
29
30
31
32
33
34
35
35a
35b
36

Operation

Truing and
shaping
grinding
wheels and
sticks,
etc.

Metal
polishing
Crushing &
grinding
Found ing
Drying

Material

A/ -0
£. J




B,C
Polishing
rouge
BeO

BeO;Si02
Tapioca
Starch
DfST
Inlet
Loading
Grains per
Cu. Ft
0.130
2.260
0.650
3.10
3.65
2.44
0.075
0.0094
0.0048
0.14
0.029
0.0054
3.17

DESCRIPTION
Outlet
Median Size
Microns
Mass
18.0
7.2
8.5
10.0
3.3
-
2.3
9.8
1.2
-
-
-
7.7

Count
0.52
0.70
0.59
0.52
0.54
-
0.48
0.55
0.43
-
-
-
4.8

Loading
Grains per
Cu. Ft.
0.001
0.0009
0.0008
0.0009
0.0025
0.0011
0.0021
0.00022
0.00012
0.00052
0.00057
0.000022
0.00054

Median Size
Microns
Mass
0.59
1.1
0.75
0.72
2.0
0.74
1.8
1.2
0.56
-
-
-
1.1

Count
0.42
0.43
0.41
0.44
0.45
0.43
0.43
0.42
0.43
-
-
-
0.43

Air
Flow-
Rate
f C f p 1
' S • 1 . r . '
cfra
18,800
930
1,230
9,500
8,000
12,000
2,200
6,000
5,000
9,600
9,600
9,600
2,130

CFM
per
S,, Ct-
c . r L •
Cloth
17.0
5.5
7.2
32.0
26.0
53.0
46.0
32.0
27.0
12.0
12.0
12.0
11.3

Pressure
Loss Across
Collector
in. Water
4.0
3.5
3.5
3.2
2.9
3.1
3.6
6.4
3.6
2.4
2.4
2.4
4.4

Collection
Efficiency
by Weight
Per Cent

99.23
99.96
99.88
99.97
99.93
99.95
97.20
97.62
97.51
99.63
98.03
99.59
99.99

             STP - 70F and 760 mm Hg

           Note:  All units manufactured by Turner and Haws Company ("Aeroturn")•


-------
                               3                   /
inlet dust loadings ^ 0.1 gr/ft  was found to be 97) to 98%, with filtering
velocities in the range of 25 to 45 fpm.  These latter data have been attri-
buted to poor maintenance and possible bag wear, but indicate typical field
values under as-found maintenance conditions.
     The literature gives pressure drop and efficiency data for a number
 of fabric filter installations, along with at least partial descriptions
of the dust, gas, fabrics used, and operative detail.  These data are sum-
marized insofar as reported in Appendix 6.4.  Due to wide variation in con-
ditions of filtration, the data have not been analyzed.  The data is indi-
cative of fabric filter field performances, particularly those of new and
unusual filter installations as these are the ones most frequently reported
in the literature.
     The efficiency of fabric filters in collecting particles of various
sizes has not been reported.   Preliminary laboratory tests on particle size
efficiency using monodispersed aerosols have been discussed in Chapter 2.
 6.4  REFERENCES FOR CHAPTER 6.

 1.  C. E. Williams, T. Hatch, and L. Greenburg, Determination of Cloth
    Area  for Industrial Air Filters, Heating, Piping, and Air Condition-
    ing,  U, 259  (1940).
2.  C. A. Snyder and R. T. Pring, Design Considerations in Filtration of
    Hot Cases, Industrial and Engineering Chemistry 47, 960 (195'i).
3.  R. H. Borgwardt and J. T. Durham, Factors Affecting the Performance
    of Fabric Filters.NAPCA. PHS, USDHEW, AICHE, Annual Meeting Paper
    (Nov. 1967).
4.  D. G. Stephan, G. W. Walsh, and R. A. Herrick, Concepts in Fabric
    Air Filtration, Am. Industrial Hygiene Assoc. Jour., 21, 1 (1960).
5.  G. W. Walsh and P. W. Spaite, Characterization of Industrial Fabric
    Filters,ASME Winter Ann. Meeting (Dec. 1960).
6.  R. A. Herrick, Theory and Application of Filter Drag to Baphouse
    Evaluation, Air Engineering, p.  18 (May 1968).
7.  K. J. Caplan, ed., Air Pollution Manual, Part II...Control Equipment,
    American Industrial Hygiene Association, Detroit,  Mich., Section 5.3
    (1.968).
B.  A. D. Zimon, Adhesion of Dust and Powder, Plenum Press, N.Y., 116 (1969).

-------
                         6.4 REFERENCES  (Continued)
  <>.   I).  C..  Stophan,  P.  T.  Bohnsl.av, R. A. HerricU, G. W. Walsh, and
      A.  II.  RO.JO ,  Jr., A New Technique for Fabric Filter Evaluation,
      Am.  Industrial  Hygiene Assoc. Jour. 19.   276 (1958).

 10.   D.  G.  Stephan and  C',.W. Walsh, Residual Dust Profiles in Air Filtra-
      tion,  Ind. Eng.  Chem.  _52, 999 (1960).

 11.   G.  W.  Walsh  and P.  W.  Spaite, An Analysis of Mechanical Shaking in
      Air Filtration,  J.  Air Pollution Control Assoc.  12. 57 (1962).

 12.   E.  E.  Lemke,  ot  al.,  Air Pollution Control Measure for Hot-Dip
      Galvanizing  Kettles,  J. Air Pollution  Control Assoc. 10,  70 (I960).

 13.   P.  W.  Spaite  and G-W.  Walsh,  Effect of Fabric Structure on Filter
      Performance,  Am. Indust. Hygiene Assoc.  J. 24,  357 (1963).

 14.   J.  Durham, Filtration  Characteristics  of Fabric  Filter Media,  NAPCA,
      PUS, USDHEW,  internal  Technical  Report (Feb.  1969).

 1'3.   R.  H.  Borgwardt, R.  E.  Harrington,  and P.  W.  Spaite, Filtration
      Characteristics  of  Fly Ash  from  a Pulverized  Coal-Firec' Powe,:
      Plant,  J.  Air Pollution Control  Assoc.  18, (1968).

 16.   J.  R.  Durham, and  R. E.  Harrington,  Influence of Relative  Humidity
      on  Filtration Resistance and  Efficiency.  NAPCA,  PHS, USDHEW, AICHE
      63rd Annual Meeting  paper,  Chicago," 111.  (November  1970).

 17.   C.  E.  Billings,  M. W.  First,  R.  Dennis,  and L. Silverman,  Laboratory
      Performance of Fabric  Dust  and Fume  Collectors,  USAEC  Report No.
      NYO-1590-R, Harvard Air  Cleaning  Laboratory,  Boston, Mass., Rev.  Ed.
      (Jan.  1961).

 18.   J. W. Robinson,  R. E. Harrington, and P. W. Spaite, A New Method  of
      Analysis for Multicompartmented Fabric Filtration, Atmos.  Envir.  1
      495 (1967).                                        	  ~

 19.   N.S. Billington  and D. W. Saunders, Air Filtration,  Jour.  Institution
      of Heating and Ventilating Engineers,  (London),  15,  46,  (1947)

20.  R. Dennis and L. Silverman,  Fabric Filter Cleaning by Intermittent
     Reverse Air Pulse,  ASHRAEJ,    43  (1962).

21.  R. Dennis, G. A. Johnson, M. W. First and  L.  Silverman, How Dust
     Collectors Perform, Chem. Engg.»  196,  (February  1952)
-------
                               CHAPTER 7
                               ECONOMICS

7.1  INTRODUCTION                                                    7-3
7.2  INITIAL COSTS                                                   7-8
     7.2.1  Initial Filter Cost                                      7-11
     7.2.2  Initial Fan, Compressor Cost                             7-17
     7.2.3  Duct Costs                                               7-20
     7.2.4  Dust Disposal Equipment                                  7-22
     7.2.5  Instrumentation                                          7-23
     7.2.6  Planning and Design Costs                                7-27
     7.2.7  Foundations and Installation                             7-27
     7.2.8  Start-Up Period                                          7-28

7.3  OPERATING AND MAINTENANCE COSTS                                 7-29
     7.3.1  Power Costs                                              7-31
     7.3.2  Labor Costs                                              7-32
     7.3.3  Plant Overhead Costs                                     7-36
     7.3.4  Collection System Returns                                7-38

7.4  CLOTH AND BAG COSTS                                             7-39
7.5  ACCOUNTING COMPARISONS OF COSTS                                 7-44
     7.5.1  Annual Distribution Method                              7-45
     7.5.2  Anticipated Cost Summation Method                       7-46

7.6  ECONOMY IN FABRIC FILTER OPERATION                             7-48
7.7  REFERENCES FOR CHAPTER 7                                       7-48
-------
                           \
                           \  CHAPTER 7
                              ECONOMICS
7.1  INTRODUCTION
     Whether  fabric  filtration represents the optimum gas cleaning method
for a given set of performance specifications depends upon a detailed
cost analysis.  The  available data citing cost experience, however, are
limited and,  in many cases, oriented towards specific operations.  There-
fore, unless  the design engineer is confronted with a very similar and
proven fabric filter application, he should prepare a written specifica-
tion and seek expert professional opinion.  All cost factors associated
with the design, fabrication and installation of the fabric filter system
should be determined as accurately as possible before selecting the dust
control system.  In  those cases where the gas treatment equipment will be
of modest size or where it represents but a very small fraction of a large
capital investment,  one can justify a less rigorous approach.
     The intent of this chapter is to provide the design engineer with
practical guidelines for estimating the cost of various fabric filtration
approaches.  A concept underlying this chapter is that several trade-
offs must be considered between size of components and operating para-
meters in order to optimize costs.  For example, if low cost performance
and high efficiency were the only criteria,  one could never have too
much cloth area for  treatment of a fixed gas volume (Figure 7.1).  There
will be some point, however, when the initial costs for the larger fil-
tration system will override the lower recurring costs of power, mainten-
ance, etc.
     The cost data assembled here have been compiled from several sources;
e.g., equipment users, manufacturers of filter systems and ancillary
parts, and open literature surveys by many investigating groups.  Pur-
chase costs cited for past years have been converted to their 1969 equiv-
alents in accordance with the Marshall and Stevens Index, Table 7.1.
     Several  partial cost analyses have appeared  in recent years,  includ-
                      f\ f\              i
ing those by  Stairmand  '  and Stephan,  that relate the  collection efficiency

-------
                             TABLE  7.1

      MARSHALL AND STEVENS  INDEX FOR UPDATING EQUIPMENT COSTS*

1945
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
Example :
M&S
Index
106
170
181
181
183
186
192
209
226
232
236
Equipment
$1460 in
1969
Factor
2.64
1.65
1.61
1.61
1.53
1.51
1.46
1.34
1.24
1.21
1.19
costing $1000
(early) 1969.

1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
in 1955
M&S
Index
239
238
238
239
242
245
253
263
273
285
(296)
is estimated
1969
Factor
1.17
1.18
1.18
1.17
1.15
1.14
1.11
1.06
1.03
1.00
(0.95)
to cost
See Ref.  1,  pg.  26-8; and Chemical  Engineering. recent  issues,  last page,
                         Sli« ol collector (It" of fabric)
      Figure 7.1.  Trade-off in Costs  Due to Collector Size,  for
                   Fixed Gas Flow.
-------
of various types of particulate collectors )to control costs.   In Fig-
ure 7.2, for example,the efficiency is compared to control costs as a
function of particle size.  Although the costs quoted, which reflect
British experience, are about 50 percent lower than those reported in
this chapter, the relative  impact of particle size upon cost is directly
applicable.  Stephan's data, summarized in Table 7.2, gives purchase costs
and power requirements for  a spectrum of participate col led ors i-tuvtivr
on various particle sizes.
     In detailed cost analyses presented in this section, several appli-
cation-related variables will be shown to have an important bearing on
cost.  Therefore, in referring to rather generalized summations of the
types shown in Figure 7.2 and Table 7.2, one must keep in mind that
individual component costs, frequency of replacing parts, amount of main-
tenance, and cost accounting techniques may vary widely from one instal-
lation to another.  A reasonable and useful cost estimate can only be
prepared by examining in turn each contributing cost factor.  Figure 7.3
indicates several important aspects typical of most fabric filter systems:
                                                 1.50
                                                      fr
                                                      M
                                                      CO
                                                      0)
                                                 1.00 T
- •  .50
                                                      u
                                                      g
                                                  0
                   I  5 10 M M 60  80 90 95  98 99 99 5    99 9
                     Collection Efficiency (%)
               Figure  7.2.  Gas Cleaning Costs  for Dusts
                            of various Particle Sizes ,  for
                            300,, 000 CFM and 8000 hrs/year.
                            (From Stairmand, Ref. 2.)  (1968)
-------
                       TABLE 7.2
    APPROXIMATE CHARACTERISTICS OF DUST AND  MIST
                  COLLECTION  EQUIPMENT1
iqulpmrat Type
A.




1.










C.







D.








B.








r.














Settling
Chamber*
1. Simple
2. Multiple
tray
IMF till Sep-
arators
1. Baffle
chamber

2. Orifice
Impact Ion
3. Louver type

4. Gas re-
versal
Cyc lone*
1. SlngU


2. Multiple

3. Machsnl-
cal
Filters
1. Tubular


2. Reverse
J«t
3. Envelope


Electric*!
Preclplta-
tor»
1. One-itege


2. Two-stage


Scrubber*
1. 8pr«y
tower
2. Jet


3. Venturi

4. Cyclonic

J. Inert 1*1
6. Packed

7. Mechani-
cal
Purcha** Smallest Pressure Power
Cost* Particle Drop Uaad*** „_ .
(»/c£«) Collected (in H20) KV . «~«r"
(micron*)** (1000 cfa'
0.1 40


0.2-0.7 10



0.1 20


0.1-0.4 2

0.1-0.4 10

0.1 40


0.1-0.2 15


0.3-0.7 5

0.2-0.7 5


0.3-2.3 
-------
                                     Fan &
                                     Ducts
                                      67
Electric
 Power
      Dust
    Disposal
        77.
              Equipment
              Installed
                157,,
 General
 MaInt.
                   Cost of Capital
                         117,
                              Plant
                             Overhead
    Fabric
    Replace-
      ment
   Replace
   ment
Fabric Pur
chases
   107,
  Typical  Installed Cost,  $2.38/cfm

  Typical  Annual Cost,  $1.05/cfm-year


  Figure  7.3.   Fabric  Kilter Annual Cost Distribution.
                (From GCA Survey,  1969).
The cost of the collector is only a few percent of the total
cost; therefore, the collector cost taken alone is a poor
criterion to use in selecting the system.

Fabric replacement (labor plus material) represents about 1/5
of the total annual cost; therefore more serviceable fabric
may lead to substantial savings.

Fabric replacement cost is about 4 times greater than collector
cost; therefore the collector should be very carefully designed
to promote fabric life.

Labor costs are nearly 1/3 of the total annual cost, and are
twice as much as the initial costs.  Therefore the equipment
should be designed to need a minimum of attention time.
-------
      . Direct operating costs are half the total annual cost; therefore
       even after the system is bought and installed, substantial sav-
       ings may result from careful use.
     An itemization of the principal costs associated with the installa-
tion and use of a fabric system and "typical" values for each cost are
given in Table 7.3.  These costs and their normal apportionment as de-
picted in the "pie" diagram, Figure 7.3, are discussed in detail in the
following sections.
     Two basic cost categories, initial and annual, must be considered.
The initial cost includes the planning effort, site preparation, in-
stallation and other activities required to place the system into rou-
tine, effective operation.  The annual costs include not only those for
power, labor  and replacement parts but also various plant overhead
charges and a certain amortized fraction of the initial cost.   Methods
of combining these separate costs are discussed in Section 7.5.
7.2  INITIAL COSTS
     The cost of purchasing,  installing,  and placing the fabric  filter
system in routine service will  usually include the following items:
                                                         Reference
          Cost Item                 Typical Cost Range    Section
                                         ($/CFM)                  "
    Basic  filter  assembly  with           0.3 - 10.            7.2.1
      filter  media
    Fan,  blower,  compressor              0.05-1.              7.2.2
    Ducting  ($/ft)                       2-50                7.2.3
    Dust disposal equipment                --                 7.2.4
    Instrumentation                      0.01-0.1            7.2.5
    Planning  and design                  -0.1-               7.2.6
    Foundations and  installation         0.1-5.              7.2.7
    Start-up,  training, and shake-       -0.1-              7.2.8
     down period
     A more  detailed  check list  of  equipment  and  services  considered  as
 capital  cost items  is given in Appendix  7.1.  One  should also  prepare  a
-------
                          TABLE 7.3

                TYPICAL, COSTS OF FABRIC FILTRATION
 I.  Installed Cost - $2.38 per CFM

        Planning and       $0.10
          design
        F.O.B. baghouse     0.80
        Freight             0.05
        Fan and motor       0.25
        Ducting             0.65
        Disposal equip-     0.10
          ment
        Instrumentation     0.05
        Foundation and      0.28
         installation
         labor
        Start-up            0.10

              Total:      $2.38
 II.   Annual Cost - 0.77 per CFM per year

        Electric power     $  0.12    (
        Labor                 0.30    \  ;     .,
        Plant overhead        0.25--  \ "   '        .     A, ,
        01   L.     t-           n in    ( Insurance, etc.:. 045
        Cloth purchases       0.10    ^

              Total:       $  0.77
III.   Total Cost of Operation - $1.05 per CFM per year

        Annual cost        $  0.77
        15 yr.  amortization   0.16
          of the installed
          cost
        Interest on the       0.12
          unamortized
          portion of in-
          stalled cost,
          at 10%
               Total:       $  1.05
   (see text for explanation and variations)
-------
      detailed flow (lliiv.ram so that no vital components will be overlooked
      during the costing process,  Most ot the items listed above represent ex-
      penditures of funds that would be charged directly to the filter system.
      Certain costs, however, may not be charged to the system, but incorporated
      In the general plant overhead.
           The total initial investment is amortized over a period of time,
      usually 10 to 1.5 years, that is consistent with the expected equipment
      life.*  Since n fixed amount of capital is tied to the filter installation,
      thus preventing investment elsewhere, one must charge this loss of use
      of the capital to the project annually as interest.  Both amortization
      and interest distributions are discussed in Section 7.5.  They are com-
      bined with the annual operating and maintenance costs to establish the
      effective yearly cost of owning and operating the filter system.
           As a rough rule of thumb, the total initial investment cost includ-
      ing material and labor will average about $2.50 per CFM of gas filtering
      capacity, with an expected range of $1 to $7 from one installation to
      another depending on severity of the problems.  This overall average cost
      is based mainly on the results of a survey of 40 fabric filter installa-
      tions for which the ranges in size and cost are shown in Figure 7.4.
10
100
1000
     10
Ifl5
                                                                    10
                                      SIZES, ft  of fabric
                            ^ • >
                              r
10
         100
                          TOTAL INSTALLED COST,  $/CFM
             Figure 7.4.   Filter Installation Cost Data.   (Arrows Indicate
                          Median Installations,  ~5000 ft2 and ~$2.40/CFM.)
      This period varies; power plants anticipate a 30 to 35 year life
for most plant equipment, while 5 years may be used in corrosive chemical plants,
-------
The  breakdown of these costs, which has been given previously  in Table  7.3
and  Figure  7.3, is discussed  in  the following sections.  The average
Ligure  is apportioned more or less evenly among F.O.B. basic filter cost,
fan  and ducting costs, and other costs associated with the purchase and
installation of the system.
      Initial fabric filter system costs for many diverse applications are
listed  in Appendix 7.1.   Although these data are not always specific
;ts to what  equipment and  labor costs were  included,  tho  data should
onablo  cost estimates  to within  50 percent of actual,  cost  by the
experienced designer.
     While  the scale factor from F.O.B. collector cost up to installed
system cost is typically about three (Table 7.3),this value is obviously
highly dependent on the individual system.  It will be shown below that
both  the basic filter cost and the costs of ducting and  installation can
be quite different depending on the application.  Examples of  initial
cost variations in the Iron and Steel industry provided by an  experienced
contractor  in the air pollution control equipment field are shown in
Table 7.4.
     7.2.1  Initial Filter Cost
            The filter assembly as shipped by the manufacturer  normally
includes hopper,  cleaning mechanism with associated fans or motors,  and
one set of filter elements.  The equipment may or may not be assembled,
and it may not necessarily include supporting structures, paint or protec-
tive coatings,  and insulation.  Thus  "the initial filter cost"  is not
entirely meaningful until all these factors are  defined clearly.   Since
users of filtration equipment often fail to specify in their publications
the above details  on their initial  costs,  the data presented in this
survey are subject to  unavoidable uncertainties  in some areas.
            Initial costs are indicated in Figure 7.5 for four  common
types of fabric filter  dust collectors;  reverse  jet,  pulse  jet, mechan-
ically cleaned  multi-compartment  bag  collectors,  and  single compartment,
-------
                             TABLE 7.4
                 ESTIMATES OF CAPITAL COST BREAKDOWN''
    1.  Matt-rial breakdown for fabric filter systems (percent):

                                      BOF
Sinter Plant
  Windbox
Electric  Open   Shop Electric
Furnace   Hearth    Furnace
4
4
71
10
3
5
1
2
37
32
6
5
7
7
2
:«5
34
7
7
9
2
2
25
42
4
9
8
7
2
17
27
10
10
7
1
        Foundat ions
        Ductwork and
           stack
        Collector
        Fan and motor
        Structural
        Electrical
        Water treatment
           and piping
        Controls             2         443         5


    2.  Labor/Material ratio for various components of particulate con-
        trol systems(at $5/hr):

        Collector            0.35
        Fan, motor and       0.15
          starter
        Stack                1.00
        Ductwork             1.00
        Steel                0.30
        Foundations          1.30
        Electrical           1.50
         From Barnes, Ref.  25.



intermittently cleaned envelope collectors.  These costs are as the col-
lector is normally shipped by the manufacturer, with freight paid to the

shipping point (free on board, or F.O.B.).  Costs are given both as dol-
lars per square foot of fabric and as dollars per CFM of filtering capacity,

assuming whenever necessary a typical air/cloth ratio (filtration vel-
ocity).  These costs are based upon a survey of filter users, manufac-
turers' data, and the technical literature.  Certain generalizations may

be made from the information presented in Figure 7.5:
-------
10*
                      10*
 10   KX>    O    KT   10"    K
         Reverse Jet Equip-
         ment (15 fpm assumed)
                                           10'
                                           10*
                                          I01
                                                  I	I	I	1	I
      10    K»   O*   »4    I0»    10* "*
K>    100    10"   10*   10*    10*   won*
      Pulse Cleaned Equipment
         (10  fpm assumed)
 Note:
    (a)  All equipment without primary exhausters, but with secondary
         fans, blowers, etc.  used for cleaning.
    (b)  Symbol  (x)  indicates data points estimated using  the assumed
         aiWcloth ratio.
         Symbol  (T)  indicates high temperature equipment.
         Symbol  (0)  indicates miscellaneous data points.
    (c)  Numerals indicate data source:
         (1)  Kane,  American Air Filter Co., Ref. 26.
         (2)  Wright,  Dracco-Fuller Co., Ref. 27.
         (3)  Chetn.  Engineering, (9 Feb. 1970).
         (4)  Dennis,  Ref. 28.  Compressors and cleaning air fans
              included.
         (5)  US PHS,  Ref. 29.
         (6)  A manufacturer's price list.
    (d)  All prices  are updated, via Table 7.1, to reflect 1969
         price levels.

          Figure 7.5.   Initial Fabric Filter Costs - 1969  Basis;
                        F.O.B.
-------
                                         10
                                         10
                                         10
                                        10
                                        10
    10   100
                   10'
10
                                                    WO
                        10*
                                                  \
   DO
    Compartmented Bag-Type
    Equipment (3 fpm assumed)
                              CFM
                                          10
  100    10*
                     •0*
Unit Envelope-Type Equipment
   (3 fpm assumed)
Note:
   (a)  All equipment  without primary exhausters, but with secondary
        fans, blowers,  etc.  used for cleaning.
   (b)  Symbol  (x)  indicates data points estimated using  the  assumed
        ai*r/cloth  ratio.
        Symbol  (T)  indicates high temperature equipment.
        Symbol  (0)  indicates miscellaneous data points.
   (c)  Numerals  indicate  data source:
        (1)  Kane,  American  Air Filter Co., Ref. 26.
        (2)  Wright, Dracco-Fuller Co., Ref. 27.
        (3)  Chem.  Engineering. (9 Feb. 1970).
        (4)  Dennis, Ref.  28.   Compressors and cleaning air fans
             included.
        (5)  US PHS, Ref.  29.
        (6)  A manufacturer's  price list.
   (d)  All prices  are  updated, via Table 7.1, to reflect  1969
        price levels.
                     Figure 7.5.  (Continued)
-------
             1.   Reverse  jet  and  pulse  jet  collectors  are  about  twice
                 as  expensive as  other  types  in terms  of  fabric  area,
                 but are  comparable  on  the  basis  of  gas filtering
                 capacity.   Intermittently  cleaned equipment  is  least
                 expensive  because  it is  designed for  easier  dust  con-
                 trol  applications  and  interruptable operations.

             2.   Costs per  unit capacity  increase sharply  below  about
                 3000  CFM,  and approach levels  as low  as $.50 to $1.00
                 per CFM  in large installations.  For  a preliminary
                 estimate,  a  cost of $.80 per CFM of filtering capacity
                 might be considered as typical,  and the figure  of $2.
                 per ft2 of fabric would  represent a large number  of
                 collectors to within 50  percent  of  the true  value.

             3.   Although cost data  from  a  single source falls on  one
                 curve, costs  from different sources appear to vary by
                 as  much as a  factor of 3 for various  reasons  cited
                 previously.   This variation in cost is especially true
                 for bag-type  equipment,  which  is designed for a wide
                 variety of temperature and particulate applications.

             Part of the data  in Figure 7.5 is  shown in Figure 7.6 for

better comparison.  This was  published about 1965 by  a prominent  manufac-
      27
turer.

            The  type  of construction of  the fabric  filter assembly is

particularly important for bag-type units, and  for large equipment the

unit cost depends more on construction features  than  on size.  This var-

iability factor  is  shown in Table 7.5  for several collector configurations

designed primarily  for high temperature  applications.   These cost esti-
     30
mates   include  the baghouse,complete with the bag cleaning accessories,

and materials handling equipment to accommodate dust  removal from collec-
tion hoppers.  They do not include  the primary air moving equipment nor
any costs for field installation, piping, ducting,  or electrical wiring

or insulation.

            Except  for preliminary cost estimating,  prospective  purchasers

of filter equipment are advised to consult  reputable manufacturers for

accurate costs.   Purchasers are advised to  specify  quality of material

and construction, and to examine critically any  bargain prices.    Because

the trade skills involved  in  producing filter  equipment are  fairly modest,

the field is crowded.  Manufacturers are often compelled to seek  economies
-------
MCCHANICAL
tasRi
3- OOMIKNT WTICIINCV
                                LEGEND
                                             CiNTNIfUML
                                  6- HMH (NIMY-VtMTUMI
                                  NMHMC
                                  7-MTWWTTCNT
                                                JCT -
                                               DUTY
                                   AOTO*UTIC MUBURE TYW
            IOOO
 O.H  0.60  a 78
        COST/CFM
                                                                                J
Figure  7.6.   Cost Per CFM of 12 Different  Dust Collector
              Designs Compared to the Total Volume Handled
              Per Minute.  (To Convert to  1969 Estimated  Price
              Levels, Multiply Scale by Marshall and Stevens
              Index 1.14.)  (From Wright,  Ref.  27).
-------
L
                                        TABLE 7.5
L
                   APPROXIMATE COST FOR BAG-HOUSES OF THE INDICATED CONSTRUCTION
                                        PRICE RANGE
                                     $/SQ.  FT.  OF CLOTH
                    WITHOUT INTERNAL PARTITIONS:
                    Open pressure unit       $1.20 to 1.40
                    Closed pressure unit     1.55 to 1.70
                    All  welded  unit          1.85 to 2.00
L                 WITH  INTERNAL PARTITIONS:
                   Open  pressure unit        1.35  to 1.50
!                   Closed  pressure  unit      1.80  to 1.85
L,                 All welded  unit           1.95  to 2.20
L                   *
                    Based  on  11  1/2-in.  diameter  x  30.-ft.  long  fiberglas  filter
                    bags and  a minimum of  40,000  sq.  ft.  of  cloth  area.   Cloth
                    cleaning  by  repressuring.   (From  Adams,  Ref.  30}  1969 cost  basis,
               that are sometimes not  in  the best  long-term  interests of  the  buyer.  A
               dollar saved  initially  may cost several dollars  in subsequent  operating
i               and maintenance costs.  Furthermore, equipment that  is well designed  for
               one application may be  a poor selection elsewhere.   Rather than invest in
               cheap equipment, it is  advisable to consider,if  necessary, the  redesign or
               modification of good quality standard stocked equipment.  Although the
               purchase cost may be doubled, the long-term costs may be greatly reduced.
               It should be noted that about 15 percent of the  operational problems en-
               countered by fabric filter users are the result  of equipment inadequacies.
                    7.2.2  Initial Fan, Compressor Costs
                           The purchase cost of fans or blowers suitable for fabric fil-
               ter equipment, which includes motors, drives, and starters, varies in-
               versely with system capacity as indicated in Figure 7.7.  Purchase cost
               also increases with the design pressure to be delivered by the fan although
               this is not readily apparent in the figure because oC the limited data
               from any one source.  Cost is also determined by the style of the fan and
-------
                                                         ' Si-SO/CFM




                                          '  Ol?! '"anufacturer-
                                                 ,  several  <  Pr°duct
                                                     verai sizes  ft"
                                                                * *^ •
                                                        10
Figure 7.7.
             Fan and Blower Costs, Including Motor, Starter, etc.

             1969 Basis.   Fan Design Pressure Indicated.
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 its  design speed.   Generally,  fans  delivering  10,000 CFM or more will  cost
 between $.10 and  .20 per  CFM,  fans  delivering  about 1000 CFM will  cost
 $.30 to .55 per CFM,  and  the cost of  smaller fans  increases sharply.
             On the  largest  applications,  the fan(s) may  be custom  fabri-
 cated.   The gas properties  and type of  service affect  the trade-off be-
 tween initial  cost  and  annual  costs?  e.g.,  a smaller fan running at higher
 speed will  be  slightly  leas efficient,  and  will  probably require more
 maintenance than a  larger fan  operating at  less  than rated speed.  This is
 especially  true if  the  fan  is  located on  the inlet side  of the  filter,
 where slower speeds  are desirable if  the  dust:  is abrasive or gummy.  Fan
 maintenance, which  can  be a big problem in  some  installations, may justify
 a larger  initial size (and  cost).   It may be desirable to oversize the fan
 merely  to allow for  unforeseeable expansions of  the collector or increased
 process  effluents.   In  addition, it may be  advisable to  oversize the fan
 on collectors where  the overall pressure  differential  fluctuates for any
 reason,  to  minimize  reduction  of gas  flow during periods  of high resistance.
             Fan and  motor costs are approximately equal.  A starter for
 a 10 HP motor  costs  about $75,  and  the  drive cost should  not exceed $50.
 Fan-motor sets are  around 60%  efficient,  such that one air HP, e.g.
 1000 CFM capacity at  6.35 in.  water represents about 1.6  electrical HP.
 Installation labor cost which  is normally about 157= of materials cost and
 foundation  costs is not included above.
            The cost of an air compressor for pulse-cleaned equipment,
 pneumatically operated  instrumentation, and dampers in some filter houses,
 is normally  included with the  initial fabric filter cost  (see Section 7.1.1
 for  pulse cleaned equipment).  Otherwise a separate compressor must be
 purchased at considerable expense,  since the existing supply of shop air
 is often inadequate with respect to volume or pressure, or is  not con-
sistently available.  For pulse cleaned collectors, the amount of com-
pressed air varies according to the cleaning mechanism design.   A
typical design value is 2 SCFM per  1000 CFM of  gas filtered,  with delivery
pressures ranging between 75 and 100  psig.  The following estimates have
been  provided by compressor manufacturers  specifically for continuous
operation with dust collectors:

-------
I'll IS I'll
Air,
SCFM
135
100
10
Supp ly
Pr 1-8 Hiiro ,
psig
100
100
100
Motor Power,
IIP
25
25
3
Total Cost,
$ F.O.B.
2700
2500
700
Approx. Initial
Cost, $ per CFM
filtered
.04
.05
.14
 The cost  is  estimated to increase  with the 0.28 power  of the  volume re-
 quired.     Field  installation is estimated at  0.60 times the  cost  of the
 compressor,  and the  ratio of installation labor to total materials cost
 is  estimated at 0.27.
      7.2.3   Duct  Costs
             The cost  of  ducting depends  on the  gauge and diameter  as well
 as  on the length  and  the material.   It also depends on the  complexity  of
 the system,  i.e.,  the  number of elbows,  Tees,  transition sections,  etc.,
 and the amount  of fitting needed to  adapt  the  ducting  to existing  flanges.
 The ducting  may require  clean-out  or inspection ports,  dampers, or explo-
 sion panels.  The  cost of installing the ducting will  also  depend  on
 location accessibility,  as well as the method  of joining and  sealing the
 duct components.
                                                                 2
             One manufacturer of small  filter units  (up  to 1200 ft  ) has
 reported his  experience  in installing  ducting  systems which for present
                                                   32
 purposes would  be  considered smaller  than  typical.    He  indicated  an
 average cost  for  installed ducting of  $1.75 to  1.85 per  pound of sheet
 metal, depending on gage  and diameter.   This included both  fittings and
 straight piping on his medium and large  jobs, but  the cost was more for
 his  smallest  jobs.  For  example, a connecting duct assembly drop for a
 single lilter unit of this size averaged about  $53. (1969 cost basis).
In the Chicago area one duct  branch including headers  cost about  $250.
Using the  figure $1.85 per pound,  Table 7.6 gives the  estimated installed
cost per foot for several gages and diameters:  typically it  is around
$10 per foot.  In support of  this  Table,  a Boston sheet metal  contractor
independently estimated a cost of  $5  per foot for a 100-foot system of
6" 18 gauge  ducting including soldered joints and about 3 gates,  installed.
            Table 7.6 also lists  the  costs of typical duct fittings be-
fore installation.  The cost  of a  fitting depends,  of course,  on  the
-------
                              TABLE 7.6

                        APPROXIMATE DUCT COSTS

(a) Installed Duct
Systems
($ per foot)
Metal Gage
(U.S. Std.
6
14
16
18
20
22
24
26
30

Thickness
) (inches)
.203
.078
.062
.050
.0375
.0312
.025
.0188
.0125
(b) Duct Fitting
Duct Diameter (inches)
6 11 24 48
108. 216.
20.75 41.50 83.
8.25 16.50 33.00 66.
6.60 13.20 26.40 53.
4.90 9.80 19.60 39.
4.10 8.20 16.40 31.
3.30 6.60 13.30
2.50 5.00 10.00
1.65 3.30
**
Costs
($ per item, not installed)
Description
Straight pipe,
Same, with self
seal
Reducer
Flange
Tee, 45°
Gauge
per foot 20
24
-locking 24
26
20
20
20
Diameter (inches)
6" 12" 24"
2.40 3.90 6.50
.56 .95 2.97
1.50 2.80
1.18 2.24 7.60
$18. (12x6) $36. (24x12)
$23. (12") $35(24")
$33(12x12x8) $50(24x24x12)
 1969  cost  basis  of $1.85 per pound for galvanized  sheet  steel,  riveted

 and soldered,  including a nominal number  of  fittings.

T«
  1969 Boston area prices; in lots of 10 fittings each.
-------
 amount of work required to make it; typically the fitting costs 2 to 4
 times as much as the same length of straight pipe.
                                            31
             Installation labor is estimated   to be 85 to 100% of material
 cost; that is, the cost of the installed ducting is about half material
 and half labor.  This rule of thumb varies with material and difficulty
 of installation,  of course.
             Materials other  than galvanized steel are sometimes used.
 Estimates for shop fabricated, field erected ducting of aluminum,  gal-
 vanized steel, and stainless steel are $5.42,  8.00,  and 15.12 per  foot,
 respectively,  (no sizes or gauges  given).     These costs  are further esti-
 mated to increase With the 0.55 power  of the size.   These estimates  pre-
 sumably apply  to  the same  ducting  requirement  and therefore  should repre-
                                                          jf
 sent  thicker aluminum than steel,  for  structural reasons.
             There is little  in the filtration  literature  on  actual duct
 costs,  but  a survey of  nine  large  filter installations  averaging 50,000
 CFM indicated  the costs  of the associated  ducting to be $.60 per CFM
 (range,  $.25 to $1.00).  This  included  applications  of  both  high and low
 temperature  filtration,  and  both short  and  long  reaches.   In Iron  and
 Steel  Industry applications  the system  ductwork  cost is estimated  at
                                               2S
 around  30% of  the total  initial material costs.
      7.2.4   Dust  Disposal  Equipment
             The purchase of  dust handling and disposal equipment must be
 compared  against  doing  the job manually, but even so the  equipment is
 justified by the  high cost of  labor  on  all  but the smallest dust flow
 rates.  The  disposal equipment typically includes a  rotary valve and
 drive,  one or  more  dust  conveyors with  drives, and a collection bin  of
 some kind.   This excludes the hopper,  which is  usually provided as  part
 of  the  basic collector.
            A  good  quality 8"  dia. rotary air lock valve metering  on the
 order  of  1 CFM costs $400  to $500  including motor and drive.  Motor  and
     *
      Aluminum sheet goes by Brown and Sharpe gauge numbers, while steel
goes by U.S. Standard gauge numbers.  For approximately the same thickness,
use two B&S numbers lower than the U.S. Standard number.

-------
drive1 onch cost about $75 depending on size and detail.  It may be pos-
sible to drive the valve from the conveyor shaft and eliminate the need
of a separate motor.
            The most common type of dust conveyor in use is the screw con-
veyor consisting of a 6" to 15" diameter covered trough, a screw of 10
to 16 gauge carbon steel, end plates, motor and motor mount, drive, and
drive guard.  The total cost is on the order of $40  per foot, depending
on design.  The trough and screw alone cost approximately $25 per foot
(9" diameter, 14 gauge screw, 12 gauge trough, 2" screw shaft diameter,
30 ft. long).  The cost of conveyor is about $450 including motor.
            Normal power requirements are l/15th HP per foot of conveyor,
but nof all dusts flow this freely.  A number of dusts become sticky as
the temperature drops, or pack together due to vibration or mechanical
churning; consequently many problems arise with screw conveyor plugging
and breakdage, and other types of dust conveying equipment are sometimes
used.  Table 7.7 gives cost estimates for several such conveying methods.
The costs do not agree with the above estimates, and the data in
Table 7.7 is suspect for other reasons, but it may be useful in making
comparisons.
     7.2.5  Instrumentation
            The effective utilization of any fabric filter system,regard-
less of size, cannot be obtained unless certain basic measuring devices
are incorporated.  Optimum collector performance depends upon the observa-
tion or control of several major variables such as gas temperature,
system pressure, fabric pressure drop, gas flow rate, humidity, particu-
late properties (mass concentration and size), dust levels in hoppers,
etc.  The above listing of variables does not necessarily represent their
relative order of importance.  In a system where accidental condensation
of moisture could produce rapid fabric blinding the sensing of dewpoint
might take priority.  High temperature filtration, on the other hand, re-
quires that the temperature not exceed levels beyond the tolerance of the
fabric.  In applications characterized by high and possibly variable dust
loadings, the maintenance of gas handling capacity depends upon the meas-
urement of fabric pressure loss.  The number of separate sensing devices
-------
                              TABLE  7.7

                   REPORTED  ESTIMATES OF CONVEYOR COSTS'
                          (per foot of length)
Type, Size Cost Size /ov

Belt 18 in. wide 460
24 550
36 640
42 720
48 770
Bucket (height)
30 tons/hr. ,,
t r\ • r- • \ 4- £j
(8 in.x 5 in.)
75 tons/hr. ,._
(14 in.x 7 in.)
120 tons/hr. _.«
(15 in.x 8 in.) ^
Roller, 12 in. wide 7
15 8
18 9
20 10
Screw, 6 in. dia 235
12 280
14 300
16 310
Vibrating, 12 80
in. wide
18 115
24 125
36 135
*Frotn Ref. 31.
Exponent
0.65
0.65
0.65
0.65
0.65

0.65

0.83
0.83
0.90
0.90
0.90
0.90
0.90
0.80
0.75
0.60
0.80

0.80
0.90
0.90

0.33
0.33
0,28
0.28
0.28

0.44

0.44
0.44
0.33
0.33
0.29
0.29
0.25
0.25
0.25
0.25
0.28

0.28
0.26
0.26

(1) Although reported as $/ft, some of these costs appear more reason-
able as total costs
(2) Cost increases with
(3) Installation costs
. 1969 basis.
equipment length

raised to this
include foundations, electrical,

powe r .
paint and fie]
     labor, and add about 65% to purchase costs.
(4)  Total installation labor is about 30% of total material cost.
-------
 justified  and  their  degree  of  sophistication are related to  the size
 and/or  the capital investment  associated with  the  fabric filter system.
 Regardless of  system size,  pressure measuring  devices  in some  form should
 bo used  and  are usually  supplied  by the manufacturer.  Simple  draft gages
 provide  a  measure oJ: fabric  resistance  (and magnitude  of dust  loading),
 and give the static  pressures  at  various points within the system.  These,
 in turn, can be related  to  gas  flow rates and  power consumption.
             In a small system,  observation of  key  pressures  permits manual
 adjustment of  gas flows  and  actuation of fabric cleaning equipment,
 either manual  or motor driven.  Similarly, reference to visual display of.
 temperature  indicates whether  dilution air dampers or  pre-cooling sprays
 should be  adjusted.   In  the  large  fabric filter system, however, re-
 liance upon  manual control  becomes highly impractical  and the  sensing
 systems  are  coupled  directly to control locations  for  automatic operation.
 A well  instrumented  system  not  only protects the investment  and decreases
 the chance of  malfunction,  but  also enables the user to diagnose and
 rectify  many of the  simpler  operating problems without resort  to outside
 professional consultation.   In  any case, outside assistance  can be employed
 more effectively when the operating characteristics of  the system are
 defined  and  understood by the user.
            Table 7.8 gives representative  catalog costs  of  instrumenta-
 tion types commonly used in fabric filter systems.   Relatively more
 expensive equipment  is recommended for long-term rugged use,  high  tem-
 peratures,  abrasive conditions, corrosive gases,  etc.   The  additional
cost of installing and adjusting the typical instrument is  estimated  to
be of the same order as the instrument price.
            The instrument indicators  can be grouped on a  central  control
panel,  the cost of the panel running $200 to $500 per  foot  of panel de-
pending on the density of the instruments.   This  cost  includes wiring  and
pneumatic piping but excludes instrument cost.   The cost  of installed
 instrumentation will vary from $.01 per  CFM for a large baghouse  to roughly
$.10 for a 1000 CFM unit.
            Standard instruments should  be  used whenever  possible  for  re-
liability and ease of replacement  parts.  Standardization will minimize
-------
                                    TABLE 7.8
                  TYPICAL INSTRUMENTATION CATALOG COSTS - 1969
Variable, type
Temperature
      Flue gas thermometer          x
      Dial-type thermometer         x
      Recording thermometer         x
      Sensing switches
      Sensor 4- Rustrak recorder     x
      Thermocouple bridge + amplifier

Pressure
      Draft gauge, inclined         x
      U-tube monomcter              x
      Sensing switches, min-max
      Sensing switches, differential
Flow
      Hand-held tube-type kit       x
      Low velocity mass flow,
              electronic            x
      Time record clock             x
      Motor-type valve or damper
              controller            x
          Purpose:
   A: For indicating
   B: For recording
   C: For controlling
                                        B
x
x
X
X
X
X
    X

    X
                       Catalog Cost
45 -
30 -
25 +
20
200
200
 30
 15
 30
 40
275

178
 80

125
                              70
                              100
                               70
                               60
Humidity
      In-line industrial hygrometer x
      Recording hygrometer (Temp,humid.)
                                    x   x
      Transducer, electronic            x
      Dew point sensor              x

Mechanical Sensing
      Lever-arm, mercury switch     x   x

Particle variables                   •«.
      Continuous size monitor
      Fabric hole detector          x   x
Mass Concentration meter            x   x
                         110

                         325
                         350
                         15
                         100
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maintenance costs which will be one of the factors traded off against the
initial costs.
     7.2.6  Planning and Design Costs
            A certain amount of time goes into planning the dust collect-
ing system, principally in the form of calculations and drafting but
also as meetings, procurement, travel, etc.  For the experienced user
of fabric filter equipment this may not amount to much, especially if he
uses so many small collectors that he keeps them in stock.  For inex-
perienced users and those designing collection systems for new applica-
tions, the time may range from 10 to 1000 man-hours or more.  Large
systems justify careful planning, as do those that must perform with high
reliability, and those that involve a large risk of fabric damage.
            It is customary to charge small amounts of staff time to an
overhead account, which means the cost will not be reckoned against the
filter system.  The cost of larger planning programs may or may not be
added to the other costs of obtaining and installing the system.  Never-
theless, the costs should be estimated and included in all comparisons of
alternative dust control systems, unless the planning is to be furnished
free by an equipment supplier.
            Procedures for designing a filter system are discussed in
Chapter 5.  Engineering of this kind will cost approximately $15 per
hour, including the normal overhead costs of maintaining an engineering
staff.  Thus a typical planning cost for a good sized system may be $1500,
or on the order of $.10 per CFM of system capacity.
     7.2.7  Foundation and Installation
            A fabric filter system may be installed outside the main plant
building when no space is available within the main structure or when a
cost analysis shows that the outside location is on the whole less expen-
sive.  Although the cost of outside site preparation is discussed at
                         1 31
length in the literature, '   the basis for cost breakdown is sometimes
cbscure, particularly between direct and indirect costs.
            Since fabric filter units weigh from 0.25 to 1 ton per 1000
CFM capacity not including fans,  motors and ducting, they require substan-

-------
Lial foundations and supporting members.  These may account for 4 percent         _^
ol" the installation for material costs and an additional 5 percent for
associated labor (Table 7.4).
            Many fabric filter installations, even though themselves mod-
orntoly well scaled against weather, still are best housed for easier
maintenance in bad weather.  A separate light weight building to house
                                                     2
the equipment is estimated to cost $1.50 to $5 per ft  of floor area
based on reported costs for single-story garage and maintenance shop                ^
shells.  This includes foundation, structural frame, outside siding, in-
side partitions, and paint.    The filter may alternatively be located            —     I
indoors, where it requires typically 1 sq. ft of floor space for each
50 CFM capacity.  For this space a substantial annual charge is normally          _
made (Section 7.2..3a).  Outside or inside, the cost of installation, at
about $5 per hour wages plus 507* overhead, amounts to 30 or 40% of the
initial equipment cost on  large equipment.  It may be, however, 2 or 3
times higher in special cases.  Itemized installation costs are discussed
in preceding sections and shown in Table 7.4.                                     ~"J     j
            A frequent consideration is whether it is best to purchase
the filter assembled or to assemble it on site.  The net cost to the
purchaser will probably be about the same; the cost of assembly at either
location is estimated at around 10% of the total on-line cost.   Factory           —
assembly is usually preferred from the standpoint of job quality unless                 !
unusual skills are available at the site.   Railroads, however,  have size          _
                                                  *                                     !
limits of approximately 9.5 x 12 (high) x 40 feet.    The cost of painting
including protective coatings is estimated at 57, of the assembled cost,                 j
and the place at which the painting is done makes little difference to                  j
the purchaser.                                                                           ;
     7.2.8  Start-Up Period                                                             j
                                     •   '                                                i
            Although not usually reckoned as part of the baghouse instal-            •   !
lation cost,  there is in fact nearly always a period following  installa-
                                                                                     r
      *
      Freight  rates  for  steel  vary with  the  amount  shipped.  Minimum car-
 load  rate  in the Boston  area requires  at  least 40,000  Ibs.
-------
 tion  when personnel  are  becoming  accustomed  to  the  equipment  and unfore-
 seen  operating  problems  are  being corrected.  This  period  ranges from  a
 lew hours to  over  a  year.  Start-up  cost  is  herein  treated  as an initial
 rather  than as  an  annual  cost  for convenience,  even though  it may  extend
 into  the  useful life of  the  equipment.
      Analysis of 78  problems reported by  the users  of 26 fabric filters
 of many types and  sizes  indicated that at least 20  percent  of the  problems
 were  related  to becoming accustomed  to the equipment or to  faults  in the
 installation.   Half  of these start-up problems were connected with the
 filter itself,  such  as poor seals  between panels, bags located too close
 together, or  failure of  the shaker mechanism due to weak design.   Other
 start-up  problems  are cited in Chapter 8.  Whether  from inadequate tlesign
 or untrained use,,  problems do often  arise and they  do cost  the purchaser
 money.  Furthermore  the installation of the equipment may require a tempo-
 rary  plant shutdown, which may be  considered a start-up cost.
      It is estimated that the start-up costs may on the average add 50% to
 the normal maintenance cost for the  first year, that is, amount to about
 $0.10 per CFM.  Start-up cost may be much higher, or on the other hand the
 purchase of equipment known to be of superior quality, and  the use of well
 trained or experienced personnel,  can much reduce the learning costs.
 7.3   OPERATING AND MAINTENANCE COSTS
      The costs  of  operating the fabric filter system from day to day and
 keeping it in good working condition include:
                 \
Cost  Item                              Typical Cost Range        Section
Powe»-
  Fan power for filtering              $  .10 - .25 per CFM-yr    7.2.1
  High pressure air for cleaning           0 - .25     "
Labor
  Fabric replacement                      .02 - .20     "         7.2.2
  General maintenance                     .02 - .20     "
  Dust Disposal                           .01 - .15     "
Plant Overhead
  Space, heat, lights, insurance etc.    .05 - .50     "         7.2.3
-------
 Operating and maintenance costs considered to be "typical" are indicated
 in Tablo 7.3 and Figure 7.3, where together they make up $0.67 per CFM-year,
 or about 2/3rds of the overall yearly filter cost.  In addition there are
 the costs of purchasing replacement fabric which are discussed separately
 in Section 7.3, and various incidental costs discussed in the following
 pages.  To these are added a yearly amortized portion of the initial costs
 of the equipment, installation, and capitol costs (Section 7.1) as dis-
 cussed in more detail in Section 7.4.
      Operational and maintenance cost  data were available for about 50
                                                                       *
 fabric filter installations via both the literature and the GCA Survey .
 These varied in size from 330 to 820,000 CFM,   The mean operating and
 maintenance cost as reported, including power,  labor and fabric replacement
                       &
 but excluding overhead , was $0.52 per CFM-year.  Only 6 installations re-
 ported O&M costs in excess of $1.  per  CFM-year  as indicated in Figure 7.8.
 Overhead is estimated at typically an  additional $0.025 per CFM-yr.,
 (Section 7.2.3).
                                                         10.
           Figure  7.8.   Total  Operating and  Maintenance  Cost.
                        (As  reported,  including  power,  labor,  and
                        fabric replacement,  but  excluding  overhead)
                        (From  GCA  Survey,  1969).
     The  following indicates the difference in apparent costs of two
hypothetical plants, one of which meticulously includes overhead while
the other uses only direct costs:
* It cannot be determined precisely what indirect costs are in the figures
  reported in the literature.   Engineers are inclined to report only direct
  expenditures, while others may include the costs of plant space, employee
  fringe benefits, tax rebates for expendible items etc.   GCA surveyed direct
  costs by asking for task labor requirements in hours, prices as billed,  etc.
                                   7-30
-------
      Power
      Labor
      Space
      Heat
      Other overhead
      Fabric (see  Section 7.4)
Annual O&M Costs ($/CFM-yr)
    Direct Cost Only
    $      .12
           .21
           .18
Including Overhead
$       .12
        .38
        .05
        .06
        .04
        .09 (Tax rebate)
                 Total:      $      .51                        $       .74
 Each  of  these  costs  varies  widely  from  installation  to  installation.
      7.3.1   Power Costs
             The  cost of power for  pumping  the gas  through the filter is
 perhaps  the  most easily determined of all  the system costs; it is  given
 simply by
                      Power Cost = CQp/E
                      where C = cost of electricity
                            Q = volume  flow rate
                            j5 = average fan pressure
                            E = fan-motor  efficiency
      The cost  of electric power is typically $.012 per kilowatt hour,
 depending on location and the type of industry.   The power conversion
 efficiency of  fan-motor units is usually taken as 60%.  The average pres-
 sure  at the  fan  is the only indeterminate  factor and even this is partly
 determined by  the filter operator's control of the fabric cleaning process.
 The average  fan  pressure is typically 5 to 10 inches of water (See Figure
7.7)  and the portion contributed by the filter alone, indicated in Figure 7.9,
is typically 3 to 6 inches.   Power cost for fan, blower, or compressor oper-
ation may be determined using Figure 7.10,  which is based on the above equation
using a 60% electrical to air power conversion efficiency.
-------
             (a) lie fore Filter Cleaning  •
                                        I  ttl  I J
                _
           oTi                     iTo      *            io7
                            (in. H20/FPM)   •
             (b) After Filter Cleaning
           0.1
  ltll.1.1  -1.     .
    i.oVio7
.  H90/FPM)
                            (in. nn'
            Figure 7.9.  Typical Filter Pressure Drops.
                         (Assuming an average filter velocity of
                          3 FPM, median pressure drops  are 7.5 and
                          4.5 inches of water, or 6 inches average.
                          (From GCA Survey,  1969.)
Figure 7.10 is based on full time operation; therefore, if for some reason
the filter system only operates 3 hours per day the actual power  cost  would
be one eighth as much.
      7.3.2  Labor Costs
             Labor skill requirements range from supervisory and  instrument
repair categories down to unskilled labor for such tasks as dust  disposal,
and these wages currently range approximately from $2.50 to around  $6.00  per
hour.  Considering the typical  labor overhead of employee  benefits, medical
coverage, administration etc.  the net cost of average  labor to the  company
can be estimated at almost twice the actual wage.
             Among about 30 fabric filter installations reporting labor data,
about .047 man-hours were required per'CFM-year,  equivalent to about $0.21
per CFM-year in wages.   Costs  for specific filter  installations are listed
in Appendix 7.2.1.   These costs were reported to  be  distributed as  in  Table 7.9
among several tasks common to most filter installations.   The task  labor dis-
tributions are further  shown in Figure 7.11.   Fabric replacement is indicated
-------
          50
          10'
Inches of
   Water,

 Fan  or
 Blower
 Diff.
 Pressure
            10                    100
                     Cost, $/ye«r per 1000 CFM
                   1000
        100 i
   Pounds
    per
   sq.ia 10  .',
    (psig)

    Fan or  •
   Compressor
    Pressure
3.0 c/kwh
  Power Cost
                                 10                 100
                     Cost, $/year per CFM at Pressure

          Figure 7.10.  Air Power Coats.  (Based on a 60% fan-motor
                         power efficiency and full-time operation.)
-------
                               TABLE  7.9
                 REPORTED LABOR  DISTRIBUTION  COSTS
Re
'X.
No. Reporting:
No. Practicing:
Mean Practice:
Median Practice:
Bag
>p lac ing
47
27
26
.084
$2.00/bag
General.
Maintenance
39
26
26
.083
$.045/CFM-yr
Dust Bag
Disposal &
13
27
13
.073
$2.00/ton
Cleaning
Repairing
1
27
4
Var.
—
Total
100
(30)
(30)
207**
	
(Units)
%
Count
Count
$/CFM-yr
	
* GCA Survey data. Reported wages, before overhead.

** The average of  combined task costs was $.207/CFM-yr; the combined task
   average cost is $.24/CFM-yr.
    A.  Replacement Labor, $/year          D.  Other Maintenance Labor,
    B.  Replacement Labor, $/bag      A        $/KCFM-year

    C.  Other Maintenance Labor, $/year    E-  Disposal Costs, $/year

                      F.  Disposal Costs, $/ton
Figure 7.11.  Reported Labor Costs, GCA Fabric Filter Systems Survey,
              1969, (Wages, before Overhead)
-------
 to  absorb  almost half  the  labor;  on  the average  it  takes about half an
 hour  to  replace a  filter element  including preparation  time,  filter clean-
 out time etc.  Fabric  lasts about one year on  the average  (Section 8.3).
      Fabric  filters  typically  require about 10 hours of attention per KCFM
 per year for such  things as inspection of the  fabric, instrument adjustment,
 fan servicing, etc., excluding the other labor categories  shown and excluding
 unscheduled  repairs.   (Unscheduled repairs are so variable that they were
 not made a part of the survey,  but may perhaps be estimated at 1 or 2 hours
 per KCFM-year).
      The purchase  of dust  handling and disposal  equipment  trades off
 against  doing the  job manually.   Among 13 fabric filter installations
 reporting dust disposal costs  the average direct cost was about $2. per
 ton with variations  from $12.  per ton down.  These companies had already
 installed a  reasonable amount  of  disposal equipment, and the $2./ton figure
 represents the manual balance  of  the task.  Most of these companies did
 the work themselves, but about  a  third of them contracted the work out to
 a local  trucking firm.
      Fabric  cleaning and repairing is apparently practiced by only a
 small percentage of filter users, usually those with expensive fabric
 that  tends to bind or plug frequently.  One installation periodically sent
 the bags out to be dry cleaned.   Another found that ordinary cotton bags
 could be patched in place quickly and inexpensively.  (See Chapter 8).
      These reported costs could be adjusted in two ways although as given
here  they have not been adjusted.  First, as mentioned above the cost of
 labor overhead could be included  as a factor of from 1.5 to 2, except for
labor contracted outside the company.  Second,  these costs could be ad-
justed for the fraction of the year the equipment was in actual use.   For
example,200 hours a year spent in replacing fabric on a filter used only
8 hours per day might be interpreted as equivalent to 600 hours a year  on
a full-time filter.  However,  there is no assurance that the fabric life
would remain the same, as if the  fabric were being damaged by condensation
due to frequent shut-downs  the fabric life might actually increase if  the
operation went to full-time.   Lifewise most other labor requirements  cannot

-------
be  .scaled with assurance.  A  scaling  factor may  if  desired  be  obtained  from      _j
Appendix 7.2, where approximately  two-thirds  of  the  installations  operated
                                                                                  i  .
full-time and the other  third operated an average of 227.  of the  time, giving
an  average operation of  757« of  the  time and a scale  factor  of  1.33.
     7.3.3  Plant Overhead Costs                                                  ^
            Just as there is an overhead cost associated  with  the  use of               i
manpower, there is a cost associated  with the use of the  plant.  This can         __,   :
include a variety oi" charges — Rentals, utilities, administration costs,                .
taxes, etc., and the overhead cost  factor depends to a  large extent on              ;   ]
                                                                                  jraj   |
the company's accounting practices.   Some of  these costs  may go  on the                 i
books against the. filter system whether or not they  involve actual cash
expenditures, while other costs may never be  compared with  the filter oper-       "~*   j
ation even though they are directly related.   The cost  of space  occupied by
the filter is apt to be  one of  the  latter.  The  following presents a brief        —•'   '
summary of typical plant overhead costs per CFM-year:

                                      Direct  Expense     Actual Expense               '
                   Space             $    0                $      .055
                                                                                  *-**
                   Heat                   .15                      .15
                   Insurance, taxes       .045                    .045             _
                   Other                Var.                    Var.
                           Total:     $  .195+              $   .25+
          Variances  in these costs are discussed briefly below.
            7.3.3.1  Space - The filter housing and its peripheral equip-
ment should be considered as renting the plant floor space occupied, as
they prevent the space being used for something more profitable.  In certain
circumstances the upper room space may also have value.  On the other hand
equipment located in unused buildings or outdoors may not incur a space
                                                                          2
cost.  Plant space varies in value, but a typical rental value is $2.75 ft
year (1969).  Rental cost may not include heat, light, water, sewer, taxes,
etc.
-------
                     The filter capacity packed into one square  foot  of
       K base area depends on the filter height,  the  filtration velocity and
 MM- duaiiing mechanism, and consequently the packing varies  widely from
 limtollntlon to installation.  Figure 7.12 shows  the capacity packing for
                     IM
                     140
                      110
                     100
                      •0
                      •o
                      40
                      to
Apparent Upper /o
Limit
Practiced
                                     i   o  .
                            10    100   IOOO  DOOO  K)OOOO  10
                                MOW UMM.ITY (CfM)
        Figure  7.12.  Plant  Floor Area Required per Filter Capacity.
                      (Based on Collector Dimensions plus 3 ft.
                      Perimeter Clearance.)   GCA Survey Data.
30 installations  surveyed.   It appears there is an upper packing limit
in practice;  for  example,  few installations of 10,000 CFM capacity would
                     2
exceed 80 CFM per ft  although many would occupy the same amount of plant
floor area and  filter  less  air.  As several maintenance problems appear when
the fabric is packed too  close together (Section 5.2.6) or when the air/
cloth ratio  is  made  too large (Section 4.2.2) there may be good reasons
to avoid the  apparent  packing limit indicated in the figure.
                     Picking from the figure a typical capacity of 50 CFM/ft2
of floor, and the  rental cost  of $2.75/ft2-year, one obtains a typical
space cost of $0.55  per CFM-year, which is  one  of the overhead costs.
-------
              7.3.3.2  Heat - Some baghouses exhaust unwanted heat to the
  outdoors,  but  others  waste heat, particularly in winter months.   In the
  northern half  of the  U.  S., fuel usage costs  approximately $.40  per CFM         ~
  per year when  the air is exhausted  outdoors.   This is  an annual  average
  based  on a 154 hour week and  an  8 to 9 month  heating season.   Over the          	
  U. S.  an average heat cost may be $0.15 per CFM-year.   Other cost consider-
  ations are the exhausting of  air previously air conditioned,  and the cost
                                                                                  -^*
  or value of providing alternative ventilation by means other than the filter
  system.
              7.3.3.3  Insurance and  Property Taxes - These combined are
  estimated  to cost about  1.870  of  the installed cost of  the baghouse, or
                                                                                  N»>
  about  $.045 per CFM per  year.
              7.3.3.4  Lights -  Lighting should be installed around the
                                                                                  ->
  baghouse,  but  the annual cost  is probably  negligible.
              7.3.3.5  Supply Inventory  - Normally at least one  set of bags
  is kept on hand at  all times,  and spare fan parts etc. may also  be stocked.
 Annual inventory costs amount  to approximately 10% of  the inventory invest-
                                                                                  _/
 ments.
      7.3.4  Collection  System Returns                                                  i
             	                                                 I
             Offsetting  the  filter system annual  costs in  some cases is  an
 actual profit  from  operating the  system.   For  example, particle -collection         •   j
                                                                                       i
 may be a basic  part of the  manufacturing process,  or the  dust  collected          ""'
                                                                                       l
 may have a  high sale value  as  in many  non-ferrous  metals  refining applications.       i
             The  return on investment  for a  fabric  filter  installed  to  control    *"   I
 a community air  pollution  factor  is frequently difficult  to asses.   In-               j
 stallations which eliminate dust damage claims and  expensive and  time-consuming  —'   j
 complaints  or litigation will  represent a direct  economic return  to the
user in terms of  reduction of management problems.                                ^ •  i
             Since many dust  induced diseases have an etiology requiring              _,- \
several years exposure, and  are non-specific, the utilization of fabric           .^   :
filtration for health protection purposes usually makes excellent  economical           ;
sense.   Other direct benefits from the reduction of visible but otherwise
-------
 innocuous dusts include improved plant  housekeeping  and  reduced maintenance
 on plant machinery,  improved visibility,  increased employee morale,  improved
 product  quality,  etc.
 7.4  CLOTH AND BAG COSTS
      Bags fitted  for a specific  model of  fabric  filter are available  from
 a  number of suppliers, usually including  the manufacturer of  the  filter.
 The prices depend very much on the  cloth  used; that  is,  on the kind  of  fiber,
 the weave and  weight of cloth,  and  on specific treatments given the  cloth,
 yarn,  or fiber during manufacture to protect the fabric  in use from  fire,
 rot,  mildew, abrasion and  so on.  Table 7.10 indicates costs  for  a number of
 fairly typical bags  as purchased a  set  at a time.  Also  given for comparison
 are some fabric and  fiber  costs.  A typical bag,  ready to be  installed  by
 the purchaser's maintenance crew, costs $10; however, bag prices  range  between
 $1 and $100.   A breakdown  of the cost of  a typical bag in Table 7.11  indicates
 that  the bag cost increase accrues  in making the  fabric, and  the  rest in
 making the bag from  the fabric.
      As  basic  as  the purchase price of  the bag is, it is only one factor in
 the fabric-related costs of operating the filter  system.  Even more important
 is the fabric  life which depends  on many  things  as discussed  in other sec-
 tions  of this  volume.   By  maximizing the  length  of time  the fabric remains
 useful,  the fabric costs and related replacement  labor costs  measured in
 dollars  per year  are minimized.   Figure 7.13 indicates the results of a
 survey of about 40 filter  installations.  As indicated, most  fabric lasted
 about  one. year, with a resultant median cost of about $0.10 per CFM-year
                                      ft
 and a  mean cost of $0.18 per CFM-year.    Cotton and  glass were the most
 widely used fabrics  reported in  the survey.
     Associated with the installation of  every bag is a labor cost, typi-
cally  between $1.50  and  $2 per bag;  this  is discussed in Section 7.2.2,
     Fourteen fibers used in  fabric filtration are listed in Table 7.12
 (See also Section 4.2.1).   The approximate price of  each fiber relative
to cotton is given along with its temperature limitations.   Figure 7.14
rolato.s temperature and cost for some  of the  common fibers  and shows  why
glass is  a popular fiber despite its mechanical  durability  limitations.

-------
                                  TABLE 7.10

                   TYPICAL FILTRATION FABRIC COSTS
Basic
Material
Cotton
Wool
OrlonR
Dacron
Nomcx
Nylon
Fiberglass
TeflonR
i>
Fiberfrax
n
Brunsmet
Fiber
Cost/lb.
.40
(Wide Var.)
.66-. 80
1.40
2.50-6.00
1.00
.60
--
20.00
-^40.00
Woven Fabric
Cost/Yd
.41
1.77
1.01
— 'V
1.04
--
.70
.98-1.68
8.00
(8 oz)
38.00
--
Felted Fabric
Cost/Yd
--
3.97
4.82
4.82
11.50
--
N.A.
36.70(23 oz)
29.20(19 oz)
~'
--
*
Selected Retail Bags
Cost Lengths x Diameter
13.60
1.50
50. F
35. F
5.
3.
30.
22.
13.70
2.80
25. F
50.
27.
—
27.
20.
16.50
10.
9.63
2.80
75.
._
--
21' x 9"
5' x 5"
14' x 7"
7' x 12"
9.5' x 6"
13.5' x 5"
30' x 10"
25' x 11.5"
22. 5T x 12"
9' x 6"
5' x 5"
25« x 11.5"
20' x 8"
—
20' x 8"
25' x 12"
25' x 11.5"
22.5' x 8"
12' x 5"
6' x 7"
25' x 11.5"
—
--
* Note: Bag costs and fabric costs are not related, as
        separate sources (References 9, 10,11, and GCA
(F) Felted fabrics
(R) Registered Trademark
the information is from
Survey, 1969).
-------
                          TABLE 7.11

                    COSTS OF TYPICAL  BAG

Bag purchase cost:
Sizo:
2
Cost per ft :
Fabric cost:
Fabric cost per bag:
Fiber cost:
«- Fiber cost per bag:
Fiber-to-fabric factor:
Fabric-to-bag factor:
Typical Bag
$ 10.
20 ft2 ,
$ .50
$ 2. /yard
$ 4.40
$ .75/lb
$ 1.10
~ 4x
~ 2x
Range Available
$ 1- 100
1 - 100 ft2
$ 0.15 - 4.00
$ 0.40 - 40. /yard
$ 0.25.- 25.
$ 0.40 and up/lb
(wide variation)
.. ..
~0.5x - 5. Ox
Types of fabric used:
            Cotton                      33
            Glass                       33
            Polyester  (e.g. DACRONR)    15
            Acrylic  (e.g. ORLONR)        5
            Wool                        5
            Other types                 9
               100
Fabric Life, months
                                             1000
                     i. . * t
        .01                    0.10
                 Fabric  Costs,  $/CFM-
                                      1.00
          Figure 7.13.
       Fabric Usage Reported and Costs.
       (Costs as reported, before tax credit.)
       (From GCA Survey, 1969, and miscellaneous
        literature data.)

-------
                                TABLE 7.12




            FIBER, TEMPERATURE RANGE, AND RELATIVE COST
Type (Typical Name)
Cotton —
Rayon, acetate —
Wool
p
Acrylic (OrlonR)
(Dynel )
Vinyls 	
Polyester (Dacron )
Polyethylene, ^
polyolefin (Polyfain )
Saran
Polyamide (Nylon^
- modified (Nomex )
Polypropylene 	
Poly-TFE (TeflonR)
Glass 	
Asbestos 	
Ceramic (Fibref rax' )
Recommended
Temp. Range:
Max. Contin.
225

250
275
240
—
325


250
500

500
600

2800
(160-190)
210
(180-235)
(200-275)
(150-180)
250
(250-280)
200
160
200
425

(225-450)
(450-550)
500
2300
Approximate
Cost
1
1.1
2.75
2.75
3.2
2,7
2.8
2.
2.5
2.5
8
1.75
30.
5.5
3.8
— 75
Metal
(Brunsmet  )
-100
-------
    0


    6


    4


    2


  20
I'8
u  16
   14


   12



   10


   8


   6


   4


   2


   0
                                     Poly TFE
                                     Glatt
             100     200      300     400


                        UPPER  TEMPERATURE
                           500
600   700
        Figure 7.14.
Approximate Temperature Capability/Cost Rela-

tionship for Filtration Fabric Materials.

(Costs Relative to Cotton.)
-------
 7.5  ACCOUNTING COMPARISONS  OF  COSTS

     As  discussed  in  earlier parts  of  this  Chapter,  fabric  filtration

 costs  fall  into two classes,  initial or  one-time  costs,  and periodic  costs.

 The  variations  in  each  from  installation to installation can be many-fold

 and  each installation must be separately analyzed.   Furthermore,  as noted

 earlier,  certain costs,  notably overhead items  but also  others, may or

 may  not  appear  as  costs  of the  filter  system even though they are related.

 In evaluating the  costs  of fabric filtration one  should  consider  how  the

 cost figures are going  to be  used in order  to decide which  of the hidden

 costs  to include,  if  any.

     First  costs (Section 7.1)  may  include:

          Planning and  preparation  for installation
          Equipment and  parts purchasing
          Installation
          Start-up and  training

     Periodic costs (Sections 7.2,  7.3)  may include;

          Replacement fabric
          Power
          Labor  and associated  labor overhead
          Plant  overhead, taxes
          Planned  replacement parts
          Unexpected  repairs
          Deductions  or  costs of such extenuations as
          Sale  value  of  dust  collected
          Effectiveness, reliability, convenience, etc.
          Capital  costs

     First  and  periodic  costs may be combined in  either  of  two ways, as

 an annualized distribution of first costs plus the annual charges, or as

 a summation of  future periodic  costs plus the first costs.  The first

 treatment is the simpler; it  is adequate  for most cost estimating, and

has been  Implied in earlier parts of this Chapter.  The  second is more
*
 In order to compare one particulate control system with another, one
should attempt to reduce all criteria of performance to the same denominator,
such as dollars per CFM-year, or dollars per ton, etc.  There is no
completely acceptable way of doing this.  One may use the data of Stair-
mand (e.g. Figure 7.2) to assign a dollar value to a given collection
efficiency, but this must be modified in terms of the plant emission
requirement.


-------
precise;  it  is  preferred by cost accountants because  it enables a more
accurate  comparison of alternative proposals.  Each is discussed below
with examples.
     7.5.1  Annual Distribution Method
            To  estimate the total average yearly cost of owning and using
the filter system, one can simply divide the total initial cost by the
expected  life of the system, and add the result to the expected annual costs
of operation.   This is called amortizing the investment using straight-line
depreciation; in this method the book value of the initial investment de-
creases steadily through the life of the system.   Every year a fraction of
the investment  is written off the book value, and accepted as a filter system
cost.
            Since money is tied up in the investment,  and since the same
money could have been used elsewhere to generate profit,  the investor suffers
a loss of interest income as another cost.   Using an interest rate of 10% and
$1000 invested, the first year title interest cost is $100.   The second year,since
the investment has depreciated, say l/15th in anticipation of a 15 year life,
the remaining investment is $933, and the interest cost is $93.33.   For the
average year the equipment will be worth $500 and the  interest cost will be
$50; thus over the 15 year period a total of $750 will be acknowledged on
the books as interest cost.  This total interest  cost  may be annually distri-
buted for convenience as simply $50 per year; it  is equivalent to simply
(Investment) x  (Interest rate) f- (2)  per year.   Note that even though the
interest may not be an actual cash outlay,unless  the money was borrowed,
it is attributable to the filter system.

      Example;
      Total  initial investment:   $1,000 including materials,  labor,  freight,etc.
      Cost of investment  capital:   8%
      Anticipated equipment life:   12 years
      Anticipated yearly  costs:   $325 per year  including power, maintenance,
                                 fabric,  plant  space etc.
-------
      Straight investment depreciation:  $  83.33 per year
      Capital (interest) cost:
                        $1000 x .08 * 2 -   40.00      "
      Yearly O&M costs:                    325.00      "
      Total estimated cost of owning and
         operating the fabric filter system:  $ 447.33 per year
                                                or $5,380 Total.
 Likewise the rate of depreciation need not be constant; other acceptable
 methods include faster-than-straight line depreciation and slower-than-straight
 line depreciation.  Plant accounting policies may prefer any one of several
 commonly used depreciation schedules, and income tax laws accept any of
 several schedules.  Unless the design engineer has a specific reason for
                                                                        36
 using another method, the straight-line depreciation method is simplest
      7.5.2  Anticipated Cost Summation Method
             Alternatively the anticipated future costs of using  the equip-
 ment may be back-computed to the present time and summarized, in terms  of
 present dollars.
      Example based on the system just discussed:
             Total initial investment:      $1,000.
             Capital (interest) cost:
                $40./yr x 12 yrs              480.
             O&M costs:
                $325./yr x 12 yrs            3.000.
             Total cost of owning and
                operating the system:      $ 5,380.  or $447.33  per year.
 This is of course equivalent to the annual distribution method,  and equally
 useful  for estimating;  but it is  not  as accurate  as  the following method.

     An important  principle  in  finance  is  that future expenditures are
 greater than their present value.   For  example, suppose a cost of $404 is
 anticipated at a time one year  from now.  By investing $372 at the present
 time at  a return of 8% the cost can be met when the time comes.  Thus the
 future  expenditure is said to have a "present worth" of $372.   All such
 anticipated future expenditures including interest costs can be summarized
 in present dollars.  The preceding example may now be treated in the following
manner:
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                      Approx.
   Year   Book Value    Interest    O&M   Total  Yearly Cost
             Present  Value
             of Costs at  SI
0 $
1
2
3
4
5
6
7
8
9
10
11
12
Total:
1000
917
833
750
667
583
500
417
333
250
167
83
0
- __ _ _ _
$ 77 $ 325 $
70 "
63 "
57
50 "
43 "
37 "
30 "
23 "
17 "
10 "
3 "
$ 480 $ 3900 $
Total initial investment:
Present value of future costs:
___
404
395
388
382
375
368
362
355
348
342
335
328
4380
$1,000.
2,794.
_ - _
$ 372
339
309
281
255
232
211
191
174
157
143
130
$ 2794

             Present value of system
              lifetime expenses:
$ 3,794.
According to this example, $3794 invested now would pay for the same
equipment and its use as would the $5380 estimated by the previous methods.
The smaller figure in this case represents more accurately the expected
costs to the company, and this accounting method (or still more refined
methods) should be used whenever their increased complexity is justified.
         This method should be used whenever two or more systems having
different initial/annual cost ratios or different lives are to be compared.
Consider for example two systems, the one described above and an alternative'.
system:
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       lixamplt
System No. 1
Total initial investment: $
Capita] (interest) cost:
87,, 12 yrs
O&M costs: (for 12 yrs)
IJasos for comparison:
I) Initial costs only:
2) Annual distribution:
3) Present value of future costs
1000
480
3900

1000
5380
3794
System No. 2
$ 600
288
4512

600
5400
3660
Hardly anyone comparing two different systems would look only at their
initial costs.  Using the method of annual distribution of initial costs          —'
as a criterion of comparison would, in the above example, lead to the wrong
decision, as the more detailed present value method shows the second system       ^
will cost the company less.
7.6  ECONOMY IN FABRIC FILTER OPERATION                                           _,
     Chapter 5 discussed ways to minimize costs in designing fabric filter
systems, and Chapter 8 will discuss guidelines for operating and maintaining      _/
the system.  Operating personnel should be aware that even though they may
never see the electric bill or the invoices for the replacement fabric,  they
can do much to affect day-to-day costs.   This applies to the personnel run-
ning the dust generating process as well as those close to the filter system.       i
Temperatures can be kept down, minimum cleaning can be exercised, minimum air
can be filtered, etc.  Many ways to keep costs down are discussed in other         ,
parts of this handbook.                                                            —'

7.7  REFERENCES FOR  CHAPTER 7                                                      :
                                                                                  —'
1.  R.H. Perry, C.H. Chitton, Kirkpatrick, Perry's Chem. Eng. Handbook.
    McGraw Hill, 4th Ed.
2.  C. J. Stairmand, Some Industrial Problems of Aerosol Pollution, Proc.
    Rov. Society. 307:209 (1968).
                                                                                   i
3.  C. J. Stairmand, Selection of Gas Cleaning Equipment. A. Study of Basis        ~*
    Concepts, Filtration Society Conference on Dust Control, Olympia,
    London, Sept. 22-25 (1 Sept. 1969).
                                                                                  ~j
4.  D.C. Stephan, Dust Collector Review. Am. Foundrymen 's Sot-. Annual
    Meeting, Philadelphia, Pa. (May 1960).
                                                                                   t'
-------
 5.   R. L.  Adams,  Application of Baghouses  to Elec.  Furnace Fume Control,
      JAPCA  14:8. 299  (August 1964).

 6.   F.A. Bagwell,  L.F.  Cox and  E.A.  Pirsh,  Design and Operating Experience
      with a Filterhouse  Installed on  an Oil  Fired Boiler,  JAPCA 19;3.  149
      (March 1969).

 7.   C. J.  Lewis and  B.B.  Crocker, The  Lime  Industry:  Problem of Airborne
      Dust,  JAPCA 19:1. 31  (January 1969).   .

 8.   Organization  for Economic Cooperation and Development, Air Pollution
      in the Iron and  Steel Industry,  O.E.C.D., Great Britian(1963)

 9.   W. B.  Harris  and M. G.  Mason, Operating Economics of  Air Cleaning
      Equipment Utilizing the Reverse  Jet Principle,  Ind. & Eng.  Chem.  47:12,
      2423 (December 1955).

10.   B. P.  Harrison,  Baghouse Cleans  500° -  Cement Kiln Gases, Air Engin-
      eering.  14 (March 1963).

11.   R. J.  Plass,  Comparison of  the Cottrell Electrostatic Precipitator
      and the  Silicone Glass  Bag  Filter,  Proc.  Seminar  on Electrostatic
      Precipitation, Penn State U., University Park,  Pa.  (December 1960).

12.   U. S.  Senate,  Com.  on P.W.,  Hearings Before  the Subcommittee on Air
      and Water Pollution (18 May 1967).

13.   J A. Fife, and R. H.  Boyer, What Price  Incineration Air Pollution
      Control?. Metcalf and  Eddy,  Inc.,  Boston,  Mass,  (circa 1967).

14.   J. H. Weber, The Impact of  Air Pollution Laws on  the  Small  Foundry,
      62nd Annual Meeting,  JAPCA.  (June  1969).

15.  W. Muhlrad, Baghouse  Dust Collection of Brown Smoke from an Oxygen
      Converter, Stahlund Eisen 82:22, 1579 (October  1962).

16.   J. L.  Smith and  H. A.   Snell, Selecting  Dust  Collectors,  Chem. Engg.
      Prog.  64:1 (January 1968).

17.   R. A. Gussman, C. E. Billings, and  L. Silverman,  Open  Hearth Stack
      Gas Cleaning Studies.  Semi-Annual Report  to  American  Iron and Steel
      Institute SA-17  (August  1962).

18.  A. Little and B.  W.  Sutton,  Industrial Air Cleaners -  A Study of  Cost
     and Efficiency,  Filtration and Separation. 109  (March  1967).

19.  J. H. D. Hargrave and A.  F.  Snowball, Recovery of Fume  and Dust from
     Metallurgical Gases at Trail, B.C., Can. Min. and Met.  Bull.. 366,
      (June 1959).

20.  T. Killman,  Dust  and Fume Control for Electric Furnaces,  Texas Steel
     Co.,  Fort Worth,  Texas  (May, 1969).
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21.  National Asphalt Pavement Association, Guide for Air Pollution Con-
     trol of Hot Mix Asphalt Plants, Nat'l. Asphalt Pavement Assoc.,
     Riverdale, Md., Information Series 17.

22.  R.L. Chamberlin, and G. Hoodie, What Price Industrial Gas Cleaning,
     Proc. Intern. Clean Air Cong,. , (London) 133, (1966)

23.  H. R. Crabaugh, A. H. Rose, and R. L. Chass, Dust and Fumes from Gray
     Iron Cupolas-Control In L.A. County, Air Repair 4:3, 125 (Nov. 1954).

24.  R.L. Chamberlin, and P.B. Crommelin, Economic Aspects of Air Pollution
     Control for the World's Heavy Industries, presented at the First
     World Congress on Air Pollution, Buenos Aires, Argentina (November 14-
     21, 1965).

25.  T.M. Barnes, A Cost Analysis of Air Pollution Control in the Inte-
     grated Iron and Steel Industry. Battelle Memorial Institute for
     NAPCA, Fed. Clearinghouse No. PB 184576 (15 May 1969).

26.  J. M. Kane, Operation,  Application and Effectiveness of Dust Collec-
     tion Equipment. American Air Filter Co., Bull No. 270 P2, Louisville,
     Ky. (1952).

27.  R. J. Wright, Select Carefully - Dust Collectors  Fit Different Needs,
     Plant Engineering, 8 (circa 1965).

28.  R. Dennis, E. Kristal,  G.  Peters, and L.  Silverman,  Laboratory
     Performance of the Mikto-Pulsaire Collector,  Air  Cleaning Lab.,
     Harvard University, Boston, Mass., Contract No. AT(30)-1:841 (June 1962).

29.  PHS, USDHEW, Control Techniques for Particulate Air Pollutants.
     NAPCA No. AP-51, Washington, B.C. (January 1969).

30.  R.L. Adams, High Temperature Cloth Collectors, Chem. Eng. Progress 62:4,
     66 (April 1966).

31.  Chemical Engineering. 76:6, 132 (24 March 1970).

32.  F.R. Chase, Application of Self-Contained Dust Collectors.  The Torit
     Corporation, St. Paul,  Minn. (1963).

33.  M.S. Peters, Plant Design and Economics for Chemical Engineers,
     McGraw-Hill Book Co., Inc., New York (1958).
                          \
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                               CHAPTER 8
                       OPERATION AND MAINTENANCE

TABLE OF CONTENTS
8.1  OPERATION OF BAGHOUSE SYSTEM                                    8-4
     8.1.1  Start-up                                                 8-4
     8.1.2  Routine Operation                                        8-5
            8.1.2.1  Use of Instruments                              8-5
            8.1.2.2  Flow Variation                                  8-6
            8.1.2.3  Cleaning Cycle                                  8-6
            8.1.2.4  Changes in Operation                            8-7
     8.1.3  Shut-downs                                               8-7
     8.1.4  Safety                                                   8-8
8.2  MAINTENANCE OF BAGHOUSE SYSTEM                                  8-9
     8.2.1  Hoods and Collection Points                              8-10
     8.2.2  Inlet Ducting                                            8-11
     8.2.3  Blast Gate and Flow Control                              8-11
     8.2.4  Fans                                                     8-12
     8.2.5  Entrance Baffles                                         8-13
     8.2.6  Hoppers                                                  8-13
     8.2.7  Bag Retainment                                           8-14
            8.2.7.1  Thimble Sheets                                  8-14
            8.2.7.2  Fastening                                       8-14
            8.2.7.3  Tension                                         8-15
     8.2.8  Filter Elements                                          8-16
            8.2.8.1  Spare Stock                                     8-16
            8.2.8.2  Installation                                    8-16
            8.2.8.3  Inspection                                      8-17
            8.2.8.4  Salvage of Filter Elements                      8-18
     8.2.9  Collector Housing                                        8-19
     8.2.10 Specific Cleaning Mechanisms                             8-19
            8.2.10.1  Shake                                          8-20
            8.2.10.2  Reverse Flow                                   8-20
            8.2.10.3  Reverse Flow Plus Collapse                     8-21
            8.2.10.4  Pulse                                          8-21
            8.2.10.5  Reverse Jet                                    8-22
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                               CHAPTER 8

                   OPERATION AND MAINTENANCE (Continued)

     8.2.11  Rotary Valves and Conveyors                              8-22
     8.2.12  Instrumentation                                          8-23
8.3  ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION PROBLEMS          8-24
     8.3.1  Types and Frequency of Problems Reported                  8-25
     8.3.2  Specific Applications Reporting Problem Types             8-28
     8.3.3  Literature Survey of Maintenance Problems                 8-29
8.4  REFERENCES FOR CHAPTER 8                                         8-33
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                              Chapter  8
                        OPERATION AND  MAINTENANCE

     A  good  fabric  filter  system can last  15  years  and  longer,  providing
 it has  the continuing  interest  of its  personnel.  Those dust  control
 systems  located  in  the  center of plant activity, and  those  systems  col-
 lecting  a valuable  dust, are the ones  that usually  receive  the  most atten-
 tion and need  the least supervision.   On the  other  hand the filter  house on
 the roof, out  back,  in  a cold place, etc.  and the filter that seems to
 contribute nothing  but  trouble  will  not be popular  and  will require extra
 effort.
     A  few principles for  successful operation apply  to any fabric  filter
 system:
     1.  Reduce  operating  and maintenance  costs by  selecting  the most
         suitable equipment in  the first place.  Study  the  operating,
         the instrumentation, and the  maintenance manuals before pur-
         chasing.   Get  equipment of  adequate  quality.
     2.  Follow  the manuals.  Know what is in them, and why.
     3.  Know what  is entering  the filter  system.
     4.  Treat the  fabric with  care  at all times.
     5.  Keep  the flow  into the  filter as  low as possible,  limited
         only by the danger of  reaching the dewpoint.   If there is
         one single way of minimizing  operating and maintenance costs
         this  is it, according  to the  reports  of many filter  users.
Complete guidelines for the operation  and maintenance of specific filter
models are available from most  fabric  filter  manufacturers.   This chapter
discusses upkeep problems and practices common to most  equipment.   Be-
cause both operating and maintenance personnel are  involved in system up-
keep, these practices are separated in two sections, each of which may be
used as a general guide.  The last section discusses fabric filter equip-
ment problems reported  in a survey of  about 50 different fabric filter
installations.  The problems are  analyzed for  common causes.
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 8.1  01'K RAT ION  OF  BAGHOUSE SYSTEM
     Like  any new  cqviipment,  the  filter  system has  a  start-up period which
 can  last a few  minutes  or many months, depending  in part  on how well the
 system was planned.  Afterward the  system functions with  much less  atten-
 tion.  It  is important  to remember, however,  that any change  in the input
 conditions will make at least some  difference in  the  filter's operation and
 require a  period of special  attention.
     8.1.1 Start-up
            When the new equipment  is started for the first time, the  fan
 should be  checked  for correct direction  of rotation and speed.  The duct-
 ing, collector  housing,  etc.  should be checked for  leaks.   Gas  flows and
 pressures  should be checked  against the  design specifications.  Instruments
 should then be  checked  for correct  reading and calibration  adjustments made
 as necessary.   Control  mechanisms, and especially all  fail-safe  devices,
 should be  checked  for operability.
            At  the first start-up of the  system, and  also whenever  new
 bags have  been  installed by  the maintenance crew, the bags  should be checked
 after  a few hours of operation for  correct tension, leaks,  and  expected
 pressure differential.   Initial temperature changes or the  cleaning cycle
 can  pull loose  or burst  a bag.  It  is wise to  record  at least  the basic
 instrument readings during this start-up  period on  new bags,  for ready
 reference  and comparison during later start-ups.
            It  is not always  specified, but it is generally a good  idea to
 start  a new set of bags  at a  filter velocity  lower  than that  to be  normally
 used.  The reason for this is that new bags have a  low flow resistance, and
 if the fan is run at normal  speed with no  other flow  resistance in  the sys-
 tern, the filter velocity will at  first be  quite high.  As a result  the effi-
 ciency will tend to be low and dust particles  may be  driven into and plug
 the cloth.   In contrast, a low initial velocity will  allow  the  first dust
particles  to stay closer to the surface of the cloth, bridging  over the pores
 in the cloth and leaving the rest of it much more permeable.  Then  the velo-
city can be gradually increased until, after a small  percentage of  the normal
dust cake has formed,  the normal  filtering velocity can be  utilized.

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            In addition, when removal of the dust cake must be unusually
complete during every cleaning cycle, it may be worthwhile to begin fil-
tration at a lowered velocity at every cycle.  This can be done manually
by gradually adjusting a blast gate, or if the procedure must be done
frequently  or if precise control is needed, it can be accomplished via
automatic control equipment.  Some filter equipment is specifically de-
signed for this.
            During any start-up, transients in the dust generating pro-
cess and surges to the filter house are probable and ought to be anticipated.
Unexpected temperature, pressure, or moisture has often badly damaged a
new installation. In particular, running almost any indoor air or combus-
tion gases into a cold filter can cause condensation on thji_j*alls and
cloth, leading to blinding and corrosion.  Condensation'in the filter-
house may void the manufacturer's guarantee.  It can be avoided by pre-
heating the filter or the gas.
     8.1.2  Routine Operation
            Day-to-day use of a filter system requires frequent observa-
tion and occasional adjustments in order to determine and adhere to the
best overall compromise between performance, bag life, power costs, etc.
            8.1.2.1  Use of Instruments - A single monometer used across the
filter cloth can provide a wealth ot information.  It indicates the
permeability of the cloth, tells how heavy the dust deposit is before
cleaning, how complete the cleaning is, and whether the cloth is starting
to plug or blind.  It tells what surges in velocity the dust deposit is
undergoing, and whether there is any flow through the cloth during the
cleaning cycle.
            It is a good idea to post a list or a recording of the normal
differential pressures through one filtration cycle, as a means to quick
detection of later trouble.  A high differential may mean:
                     •an increase of air flow
                     •the beginning of blinding of the cloth
                     •hoppers so full as to block off the bags
                     •condensation in the cloth
                     •cleaning mechanism inoperative

-------
while a low differential pressure may mean:
                      •the  fan  is slowing down, perhaps due to
                           belt slippage or fan motor problems
                      'broken or undamped bags
                      •plugged  inlet ducting or valves closed
                      •leakage  between sections of the filterhouse
The reading of the manometer should be easily visible from the plant
operating floor, and one man should have responsibility for checking it.
On an expensive system an  alarm should sound if certain pressure dif-
ferential tolerances are exceeded.
            Other instrumentation can be nearly as valuable.  If it has
been installed it is probably  there for a good reason and it should be
used.  Transients frequently occur in pressure, flow rate, temperature,
and humidity.  Operating personnel should know at once when an indication
is excessive and what the  causes may be.  A trouble-shooting manual, such
as shown in Appendix 8-1,  should be kept handy to the filterhouse.
            8.1.2.2  Flow.  Variations - With multiple or variable dust pick-up
points there will be variations in flow rate and filtering velocity.  All
the branches of the inlet  ducting may be open or some may be shut off,
depending on plant activity.  Also while one collector compartment is
down for its cleaning cycle, another may be down for bag changes or still
another for inspection.  These system changes affect the flow through
the filter.  Too much flow through too few bags amounts to an overload
or too high a filter velocity, leading to inefficient filtering, plugging
of the cloth, loosening or bursting of the bags, or unsatisfactory build-
ing ventilation.  Too low  a flow is a frequent cause of condensation.  When
flow variations are anticipated, it is wise to have at least manual means
of flow control, and automatic control equipment is often justified.

            8.1.2.3 Cleaning Cycle - As the cloth ages, adjustments in the
cleaning cycle may be advisable either in the amount or the intensity of
cleaning or in the length of the cleaning cycle.   One tries to use as
little cleaning as possible so as to prolong the life of the fabric; but
one has to use enough cleaning to keep the differential pressure at an
-------
 economic  level.   There is  In principle  a point  of  optimization,  although
 this  may  be difficult  to locate  in practice.  Operating  at  Che  point  ot
 minimum cleaning is  indicated by & gradual  build-up  of differential  pres-
 sure,  perhaps  over  a period  as long as  a few  days.   Then before  the  pres-
 sure  gets too  high,  a  slight increase  in cleaning  action (frequency  or
 intensity or duration,  as  appropriate)  should reverse the trend  in pres-
 sure.   After a few  cycles  or a few hours the  pressure should  reach a suf-
 ficiently low  level  to reduce the  cleaning  action.   If this gradual  fluc-
 tuation in differential  pressure is not  observed,  it may mean that the
 fabric  is being  overc leaned,  because the only way  to be  sure  the lilter
 operation is near the  point  of minimum  cleaning is to continually operate
 around  it.   Changes  in- process or  fabric condition will  undoubtedly  cause
 the cleaning requirements  to shift  from week  to week, requiring  a continual
 hunting for them.  Normally  the  process  will  be sufficiently  stable  through
 automatic control so that  day to day observation and adjustment  of the
 cleaning  cycle can be  made manually by  a skilled person  in a  few moments.
 This  procedure of hunting  for the minimum cleaning requirement is normally
 worthwhile  on  all except possibly  the smallest  filter systems.
             8.1.2.4  Changes in  Operation - The fiber is  carefully designed
 to operate  with  a certain  flow rate, particle size and type,  etc., and
 any changes  in these conditions, expected to exceed ten percent or so,
 should  be analyzed for their  effect  on  the overall filter operation.  This
 should  be an economic  analysis by an engineer skilled in  fabric  filter
 systems.   If large changes are indicated or a large  installation invest-
 ment  is involved, a  short  pilot  study using,if  necessary, borrowed  pilot
 filter  equipment  may be justified.

     8.1.3   Shut-downs
             The main precaution  in shutting down the filter system is pre-
 vention of moisture  in the filterhouse.  Condensation can appear  through
 the cooling  of gases containing moisture, particularly combustion gases,
 if they are  not completely purged from the filter system and replaced
with drier air before the filter cools  down.  This can also happen with
-------
 air  at  ambient  moisture  levels  if  the  filter  is  in  a  colder  location.   It
 can  happen  if weather  leaks  into  the collector  particularly  during  ex-
 tended  shut-down periods.  To  prevent  condensation  some  plants  purge  their
 systems  carefully on shut-down  and then seal  the system  off  completely.
 Others  continue a flow of  warm  air through  the  filter during the  shutdown,
 which also  helps prevent condensation  when  they  start up again.   A  par-
 tially  sealed-off filter system can be lightly  pressurized with reasonably
 dry  air  to  exclude seepage of damp air.
             In  addition  to general maintenance during shutdowns,  other
 reported practices ^include cleaning corners and  crevices  of  any dust
 accumulations which  might  solidify during a prolonged shutdown; removal
 of any material which  might  catch  fire on contact with air;and removal of
 the  bags for storage under more suitable conditions.
     8.1.4  Safety
            The preceding  portions of  this chapter -mention a number of
 precautions against  system damage.  There are of course  any  number of
 possible accidents.  Baghouse structures and ducting  are usually ade-
 quate to prevent metal fatigue  or  panel collapse during normal pressure
 excursions.  It may, however, be less  expensive  to  include explosion
 panels than to  design  the  entire system to withstand  a large  surge of
 pressure.  A surprisingly  large number of dusts  are flammable and some
 are  spontaneously combustible.  (A good test is  to  ignite a  small conical
 pile of the dust and observe the rate  and amount of burning.)  The danger
 of fire in a high velocity air  system  containing cloth is apparent.  Some
 plants find that their ventilating hoods collect a surprising quantity of
 combustible material like  lunch bags and candy wrappers.   Instrumentation
 responding quickly to  sudden temperature changes, such as an automatic
 cooling system,  may  be justified in some installations.
            Operating  personnel should be accustomed  to think "Hazard."'
whenever a fabric  filter is being  used on abrasive dust or toxic fumes
 or gases.   (Many dusts have some degree of abrasiveness which can make
 trouble in other  plant equipment unless controlled.)  Safety codes in
 such cases will  not  normally permit the recirculation of  the  filtered air

-------
to the building'but  if the air  is recirculated, any bag failure can be
serious.   If the exhaust  is to  the outdoors then consideration has to be
given to providing make-up air  for ventilation to the building, as no
sizable exhaust rate can  be made up by leaks through doors and windows.
At times the closing of windows or doors may cut down the air flow through
the building so much that the dust pick-up hoods fail to ventilate ade-
quately.   In certain weather the makeup air inlet may entrain some of the
outdoor exhausted gases unless  precautions are taken.
            The disposal  of collected dust can be a problem.  If the dust
is poured  or stirred in the open or if there are leaks in the disposal
equipment  some of the material will re-aerosolize; it does not take much
dust escape (~0.1 percent.') to offset the remarkably high efficiency
of the fabric filter.  The procedures needed for disposal of radioactive
dust can use up the permissible exposure quotas of many men unless care-
fully planned.
8.2  MAINTENANCE OF BAGHOUSE SYSTEM
     There is a wide tendency to regard the dust collector as a piece of
trivial equipment until something goes wrong with it, whereupon it becomes
despised as a troublemaker.  Planned preventive maintenance is a better
policy, the amount depending on how much the plant has to lose in case of
a system failure.  A few companies successfully contract out the dust col-
lector maintenance, some contracting the entire job and others only the
supervisory portion, leaving the routine maintenance to in-house men.

            Timing of maintenance is important.  A supervisor or mainten-
ance forman may spend two hours every day looking over a large system,
because to let things go any longer than this is,  in his experience,  in-
viting trouble.  For example, one leaky bag can rapidly destroy the ad-
jacent bags unless detected.   Other procedures may be best done once  a
month, or every six months, depending on what they are.
            Maintenance procedures will be based on the operating and
maintenance manuals furnished with most fabric filter equipment,  and  later
they will be supplemented by experience.   Appendix 8.1 lists examples of
-------
maintenance procedures and troubleshooting lists extracted from Heveral such
manufacturers' manuals.  Records of bag changes as well as non-routine
maintenance may prove invaluable later in pinpointing high-cost parts of
the collector system and for projecting costs of any proposed similar
equipment.
            In the following paragraphs maintenance practices are sugges-
ted for the various parts of the system.  While these are extracted from
reports of over 100 specific installations, some comments regarding equip-
ment apply generally.  The one factor most responsible for high maintenance
costs seems to be excessive filter velocities.   Another factor is loca-
tion; centrally located equipment receives more attention than isolated,
inaccessible equipment and as a result often gives better performance.
Similarly a single large system has fewer servicing points than a number
of small dust collectors and will cbst less to maintain, other things
being equal.

            The system components should all be readily accessible.  Lad-
ders, walkways, and cranes necessary for maintenance should be provided
at the time of installation.  Lighting outside and if possible inside
the filterhouse should be installed.  If the equipment is outdoors it
should be especially well protected, and the hoppers, disposal equipment
and fans should be enclosed to make servicing easier during bad weather.
Wherever there is possibility of dust accumulation or plugging there
should be access doors or cleaning ports.  Doors, valves, bag clamps and
tension adjustments should all be serviceable by hand with a minimum of
tools, to save time.

     8.2.1  Hoods and Collection Points
            Check periodically for ill advised changes such as holes cut
in the hoods, more hoods added, ducts blocked off,  or dust intakes moved
away from the dust source.   Be sure any temperature sensor located in
the gas stream is where it will pick up a temperature truly representative
of the mixed gas.
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      8.2.2   Inlet Ducting

             This is a moderately large problem area  Cor several reasons:

              (a)  Abrasion.  Flat, heavy 'plates installed at bends may
                  increase elbow life.  Alternatively soft, rubber-like
                  material may out-last metal.

              (b)  Corrosion.  Insulate to reduce condensation.  Ducting
                  made of plywood may be compatible with mildly corrosive
                  gases.  Long ducts, such as "hairpin" cooling systems,
                  are particularly subject to corrosion unless condensa-
                  tion is avoided.

              (c)  Sticking.  Some dusts may "paint, out" on the inside of
                  the ducts.  If this is due to dampness or due to thermal
                  precipitation from hot gas to cold duct, insulation
                  should help.  An increase in pressure drop from end to
                  end is an indication of plugging.  While vanes in duct
                  turns can minimize pressure losses at the turns, they
                  may collect material.

             (d)  Settling.  Install clean-out doors or air lances in
                  long horizontal runs if there is any danger of settling.
                  Frequent plugging may require a Vee-shaped duct with a
                  conveyor in the bottom of it, or a higher air velocity
                  (i.e., smaller duct) at the expense of an increased pres-
                  sure drop.  If possible keep the. bottom of the ducting
                  straight; when tapers are necessary, put them in the
                  sides or top.

             (e)  Temperature surges. Peaks in gas temperature will not
                  be reduced or  dampened appreciably by short lengths of
                  ducting.  However, the removal of duct insulation or
                  the addition of a heat absorber like brick work or a
                  steam boiler in the duct system will help to dampen
                  temperature excursions.   Otherwise a high-temperature
                  by-pass of the filterhouse or some other variety of
                  fail-safe equipment may be needed.

      8.2.3  Blast Gate and Flow Control

             Problems with flow control equipment are frequently reported.

Any valve which may have abraded, plugged, become moved, etc. should be

checked periodically.  Especially the blast gate should have a positive lock

against vibrational moving.  It  should be accessible, not in a duct high

overhead, as it may have to be adjusted frequently.
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             Filter compartment inlet dampers are a high maintenance item
and spare parts should be stocked.  An indication of a bad damper seal is
a differential pressure across the cloth when there is supposed to be no             ~
flow through the compartment.  This is given as a common reason for short
bag life in shake-type systems.  The valves on the clean side of the bag-            —
house, if any, may also malfunction if the bags have been allowed to leak
extensively and the valves are fouled.  Either air cylinders or motor
driven actuators for dampers and valves may be used, depending partly on
the required speeds of opening and closing.  Motor driven actuators, al-
though slower, may be slightly more reliable.
             Being able to isolate a compartment from the rest of the
system without shutting the system down can be invaluable to baghousc
maintenance.  However, even the best valves may not be acceptable in all              '
cases; for example if there is danger of carbon monoxide, internal main-
tenance may be prohibited except during plant shutdowns.                               :
      8.2.4  Fans                                                                    ~"
             Fans and blowers are reported to be a large  problem area,
particularly those located on the dirty side of the baghouse where material
can accumulate on the vanes and throw off the balance.  Corrosion and
abrasion can also cause trouble.  It is wise to anticipate and prepare for            ~~
some fan maintenance in order to avoid long, expensive shutdowns.   For
example, the fan should have access doors, electric disconnects,  and                 —/
crane facilities for both ease and safety.  Fan maintenance ia roduced
by installing rugged equipment, and a large slow fan will probably need
less attention than higher speed equipment.  If one type  of fan blade
does not seem to last very well, another type may be better (see Section 5.3).
Having two or more fans on the system may enable maintenance of one of  them
without shutting down the system completely.  Furthermore, seal maintenance
problems on the filterhouse may be reduced by having fans both before and            -'
after the baghouse, enabling operating the house at nearly zero gauge pres-
sure.  Condensation and corrosion in the fan may be alleviated with duct and         _w
fan insulation; most fans come provided with drains in anticipation that
water will sooner or later get into the fan housing.
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              The  fan  should  be checked  for direction of  rotation  periodically,
 as  even  a  fan running backwards moves some air.  Air Clow and  Can speed  should
 be  measured,  not  just estimated,  checking belt wear and  adjusting the belt
 tension  as necessary.  These checks can be combined with routine  lubrication
 procedures.
              Vibration noise probably means the rotor  is out of balance,
 and/or the bearings are  going bad.  Abrasive or sticky material may easily
 change the fan blade  weight,  and  the faster the fan speed, the more serious
 this  is.   Sometimes a relatively  small  adjustment in speed can temporarily
 alleviate  a vibration until  such  time as the fan can be  shut down for repair.
 One remedy for accumulation  of material on the blades  includes sand-blasting.
 This  is  a  matter  of hours, if the rotor has to be removed, but abrasive
 blasting systems  have been installed in the fan housing  to expedite the  job.
      8.2.5   Entrance  Baffles
              Good baffling can reduce maintenance in the filterhouse by
 helping  to distribute  the gas flow and the dust load more evenly  to each
 compartment and to each bag.  A baffle can protect the. fabric nearest the
 compartment inlet from direct impact of abrasive particles, or it can pro-
 tect  the first couple  of feet of each filter element from the same thing.
 Used  skillfully,  baffles can  either direct the largest dust particles
 downward to the hopper or upward to the filter surfaces, thus in effect
 controlling the flow  resistance of the dust deposit.  For any of these
 purposes it might conceivably be advantageous to use baffles that are manu-
 ally adjustable.  On  the other hand, baffles sometimes have to be removed
when they  contribute to maintenance problems by accumulating dust or abrad-
 ing too  rapidly.
      8.2.6  Hoppers
             Bridging and backup in hoppers should be anticipated whenever
 the dust is expected to have a high angle of repose (a steep angle of  slide
when piled), a. low aggregate density,  or a high particle length/diameter
 ratio.  Steep-sided and/or wide-bottomed hoppers will presumably have  already
been installed.  Further assistance may  be obtained by antifriction coating
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the hopper surfaces, or by air-pulsed rubber liners in the hoppers.  Vibra-
tors  are often used to loosen the flow and if used, heavy low-frequency vi-
bration may be preferable.  However, some dusts tend to lock up when vibrated,
especially when the dust is already deep in the hopper.  If surges in dust
load are causing back-up, then over-running of the rotary valves will help.
Moisture introduced through a leaking rotary valve in a suction system can
cause sticking on the hopper walls.  If condensation is occurring in the
hopper, insulation should help.
             Sledge hammering is reported to be one method of freeing a
clogged hopper.  A better method is the installation of poke holes in the
side of the hopper; these can be designed in the original equipment if
the need is anticipated.  Any hopper will get better attention if it is
housed against inclement weather.
      8.2.7  Bag Retainment
             8.2.7.1  Thimble Sheets - One fairly common complaint is that the
bags are packed together so closely as to abrade against each other or
against the baghouse.  Close bags can neither be inspected adequately
nor installed nor adjusted easily.  If the bags are more than about three-
deep beside the walkway, or two-deep for large bags, the man cannot reach
from the walkway to do his job.  Hence damage often results as he misplaces
his feet.  In the event of high maintenance resulting from overcrowded bags,
some baghouse users take out part of the bags and block off the thimbles.
Unfortunately this overloads the rest of the bags.   It would be better to
take out the old thimble sheet and seal in a better designed one.
             The thimbles themselves are ideally seamless and blunt-lipped
to minimize stress points and chafe on the cloth.
             8.2.7.2 Fastening - The method of fastening and terisioning bags is
one distinction between equipment by different manufacturers,  and there are
many variations.  Generally, a minimum of tools should be needed;  clamps
should be both quick and finger-adjustable.   By the same token, both hands
should be free for the work; that is, the man should not have  to hang onto
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 a  ladder or  loan far over  as  he  works  on  the  bag.   Fastenings  should  be
 secure,  because  as  a taut  bag is collapsed  it becomes  tighter  and  the  fasten-
 ing  tends to slip.   Also large changes in baghouse  temperature can lead to
 differential thermal expansion,  bag  tension changes, and possible  slippage.
 Sometimes -a  bag  pulls completely loose from the  thimble, as  indicated  by a
 dusty  exhaust  and an unusually low manometer  pressure.
              8.2.7.3  Tens ion -  At present  there is no general rule  for how much
 tension  a bag  should have  for best overall  performance.  A very  slack  bag
 can  fold over  at the lower cuff  and  bridge  across or wear rapidly.  Too
 much tightness will  damage the cloth and  work against  the fastenings.
 Because  of the seam,  as the tube is  tightened it may arc, banana-like,
 enough to touch  the  next tube.   Whatever  tension the bag has when  it is
 installed will be increased by the weight of  the. dust  cake, especially at
 the  top  of the bag.   This can  be an  increase  of several times  the bag weight
 with some dusts.  Circumferential tension as  from inflation may  further cause
 the  tube  to  tighten  lengthwise.   The fabric properties may change as the  bag
 ages, and this may also change the tension.    For all these reasons the tension
 of the cloth needs to be checked  from time to time, especially a few hours
 after installation of the cloth.
             Correct  tension is mainly a matter of filter dimensions and
 cleaning mechanism.   Shake cleaning in particular seems to require a
 unique combination of tension, shake frequency, and bag properties for best
 results.  The manufacturer's recommendation should be followed until there
 is more pertinent experience to  go on.   One rule for obtaining adequate
 tension is the two finger method; when two extended fingers are slipped
over the uninflated,  flattened tube,  it is claimed that the wrist should
 just be able to rotate 90 degrees.  This amounts to about one-half inch of
 slack, not a   precise  amount but a very ready test.   Bags that clean by
collapsing may be under the best  tension when they take on a cloverleaf
pattern as they collapse.   Correct tension is reported  as being from 25 to
 100 pounds for glass bags of one-foot diameter, enough  to keep them suffi-
 ciently open during cleaning to let the dust fall to the hopper.
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      8.2.8  Filter Elements
                   *
             In most filter systems the biggest part of the maintenance
program is related to fabric upkeep.  The cloth has to be inspected regularly,
and it is replaced either on a preventive basis or on short notice as trouble
occurs.  Some filter users are able to salvage some of the removed filter
elements, while others find salvage does not pay.  While other parts of the
filter system may be maintained by the standard skills found in most plants,
filter elements need special handling at all times.  Glass fabric,for example,
is especially fragile and ca,n be ruined by kneeling on it or dragging a tool
across it.
             8.2.8.1  Spare Stock - At least a few replacement and.usually a
complete set of filter elements should be kept on hand, the quantity de-
pending on the expected bag life, the risk, and the delivery time.  The
margin of safety and risk of shutdown are balanced against the costs of
storage and inventory, which are annually about 10 percent of the purchase
cost of the fabric stored.  The spare filter elements should be labeled to
indicate type and quality, clearly enough to avoid any possible confusion
with other sizes, cloths, or manufacturers, and kept well separated from
used filter elements.  Elements arc stored safely against mildew, larvae,
crushing, etc. usually on a first-in, first-out inventory system.
             Filter elements can be purchased either from the equipment
manufacturer or from a firm that specializes in sewing various types of
filter elements (Section 4.6).   On the one hand, it is a good idea to avoid
splitting the responsibility for filter system performance between the equip-
ment manufacturer and a separate bag supplier.   On the other hand, most filter
users feel vulnerable if they are committed to getting all their stock from
one source.   At least one filter user solves this problem by buying cloth and
sewing his own bags.   Generally, the experienced firm supplies the most re-
liable products.  Inexpensive filter elements often turn out to require a
lot of maintenance and be more expensive in the long run.
              .8.2.8.2 Installation - In many filter houses the elements  can be
installed by one manj but in the interest of a  short down-time a crew of
two or three men usually do the job.   (Not all  types of equipment have to
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bo shut down for bag replacement; sec Section 5*2.)  Practices differ be-
tween keeping old and new elements in separate compartments and mixing them
together, which is customary when accidents happen only to a small percen-
tage or when there is no problem about down-time.  Keeping them separate
is better preventive maintenance, although it is more expensive since some
cloth will be discarded before its life is up.  Keeping them separate is
preferred when filterhouse conditions are noxious, or when entrance to the
compartment takes a lot of preparation time.  In either method records should
be kept showing date of maintenance, a description of the cloth installed,
and the location of the change by thimble number or compartment number.
             8.2.8.3 Inspection - External maintenance inspection of the filter
house and system is usually performed daily, while the filter elements
themselves are typically inspected once a week to once a month.  The appearance
of the air exhausted from the filter is not always a reliable indicator
of element condition; the inside of the filterhouse should also be seen.
             Any dust on the floor of the clean side of the filterhouse
indicates faulty operation, and the location on the floor is often a quick
indication of which element has failed.  More often, however, locating the
hole in the fabric is a difficult, time consuming job, sometimes because
the hole is hidden by other elements.  Fortunately experience often shows
that most of the holes occur in certain repetitive areas of the bags.
Holes (or more important, thin places about to become holes) may sometimes
be located by running a fluorescent light tube or flashlight through the
filter element, perhaps while it is still in the filterhouse.   Sometimes
on squeezing the bag or tube a puff of dust will show the location of
the hole.
             It is important to repair holes as soon as possible after
they develop, of course, especially when abrasive dust is being filtered,
because a hole in one element can quickly cut a hole through the adjacent
fabric.   If there isn't time to repair or change the perforated bag it
should be tied off until such time.   The same applies to a seam failure;
use whatever quick remedy will permit continuing the operation  until a
downtime can be scheduled.
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              The inspection procedure at an 80,000 CFM 500°F installation
                                                             2
 has been described in the literature and may be of interest:  .
       "At present an inspection of the bags while the baghouse
       is operating requires a supervisor, the operator,  and
       three mechanics.   The operator locks out one compartment
       at the panel board, one mechanic closes the tipping gate
       valve at the bottom of the hopper, and the other two
       mechanics open the top and bottom inspection doors.   The
       supervisor enters the bottom of the compartment, inspects
       the bags ?..;<•• general appearance, and any defective bags
       are tied off or capped at the cell plate.   The procedure
       is reversed to put the compartment back in service,  and
       the next compartment is inspected.  All 10 compartments
       may be inspected and serviced in this manner in less than
       four hours.  Inspections are made about once a month.   Bags
       are replaced during furnace rebuilds, when the baghouse is cool."

                8.2.8.4   Salvage  of  Kilter Elements  -  liags  can be removed
 for cleaning and  repair  if most  of  the  cloth  is  still  sufficiently valuable.
 Patches can be  sewn  on,  or  sometimes  applied  with quick-setting  glue
 without removing  the bag from  the baghouse.   Thermosetting and pressure
 sensitive adhesives have been  used  to  repair  glass  bags.  Of course that
 portion of the  cloth surface patched  is  lost  for filtration, and the
 surrounding cloth takes  a slightly  higher  load.  Bags with holes all in
 one end can have  a new end sewn  on.
             There are a number of ways ot cleaning a removed bag,  if this
 is judged to be worthwhile.  Dry methods include turning it inside  out and
 beating or brushing it,  tumbling it in a  drum, vacuum cleaning  it,  and using
 jets of compressed air on it.  These techniques may damage the cloth,  es-
 pecially glass cloth.  For example, cloths plugged to the point  of  rigidity
may be actually broken by bending them.  Wet cleaning methods may be more
 practicable when  a cloth has become blinded or plugged well before  its me-
 chanical life is  up.  In rare cases wet'cleaning of the bags inside  the
 filterhouse may succeed.  Felts have been dry cleaned successfully.  Un-
 specified cloths were reported to withstand 6 to 8 washings before they had
 to be thrown away.  Consulting the fabric supplier or manufacturer might
well salvage a large cloth investment that has met with a partial accident.
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             It  is n°°d practice to  install  salvaged  fabric in separate
compartments from new fabric in order to get full service from the. new
compartments.
      8.2.9  Collector Housing
             The importance of accessibility has already been discussed:
a central location, inclement weather protection, ladders, walks, and
lights are all conducive to good maintenance and will reduce maintenance
expenses.  Also the interior of the collector should be readily accessible,
if possible during operation.  High temperatures or noxious dusts may make
collector entrance hazardous or unpleasant and thus expensive.  Respirators,
coveralls, gloves, etc. if needed for entrance, should be kept close, by for
emergency repairs.
             Faculty seals are a frequent complaint in collector housings,
especially when the equipment has been assembled by an unexperienced crew.
Also, seals tend to be weakened by weather, heat and age.  Seals should be
checked every six months to a year; one technique involves placing one man
inside the compartment at night with a light and another man outside.  While
large leaks may be indicated by a manometer that does not zero when it is
supposed to, minor leaks as along a. thermocouple wire which can admit water
to begin a corrosion problem are harder to find.  If necessary, the filter-
house panels and flange connections can be dismantled, cleaned up, and
reassembled, probably using both cemented gaskets and bolts.   Seals around
the doors to the filu>rhouse must not be overlooked.
             The doors should be openable by hand without need of special
tools; they should be large enough for easy entrance, fabric maintenance,
and periodic cleaning of the dust which inevitably penetrates the cloth to
the clean side of the compartment.  Glass panels in the doors may be worth-
while for observing the filter elements in operation.
      8.2.10  Specific Cleaning Mechanisms
              Each of the standard mechanisms (see Section 3.3 and below)
requires some maintenance procedures not shared by the others.  Generally
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the more elaborate the cleaning apparatus the more maintenance ot all types
needed.  One should use a minimum of cleaning energy, if the cloth fails
to clean adequately then a small temporary increase in the cleaning process
may bring the pressure drop back into line.  The process of blinding tends
to be an accelerating one, because as the free area of the cloth decreases
the fan pulls harder in attempting to maintain the flow.  This increased
pressure drop in an already marginal situation must hasten the blinding
process.  For the maintenance man, this means being alert and ready to adjust
the cleaning procedure as necessary.  At the same time, however, all changes
in cleaning procedure should be fully cleared before they are. put into prac-
tice, because the cleaning process has far-reaching effects and a large part
of the filter system expense is related to the cleaning action.                    ~

         8.2.10.1 Shake - Any wear in the shaking machinery results in a
                                                                                   "*••
lessening of the shaking action, and unless the trouble is recognized for
what it is, the shaking intensity may be stepped up until the mechanism            __
destroys itself.  Regular lubrication and avoidance of gritty dust in the
mechanism can best be achieved by having most of the shaker mechanism out-
                                                                                   «••*•
side the filterhouse.
              If the cloth is not cleaning, check the shaker rack to be sure
                                                                                   *w
it is moving. (For doing this while the rest of the filter system is down,
a jog button located near the door of the compartment is useful.)  A small
adjustment of shake amplitude or frequency may markedly change the propa-          *"
gation of the shake wave along the cloth tube and improve the cleaning.  If         <
the cloth still won't clean sufficiently with a safe amount of shaking, it         —•
may be necessary to reduce the filtration velocity or alter the particulate
characteristics for a few hours.  The velocity should, of course be abso-          ^
lutely zero or even negative duringrthe shake part of the cleaning cycle.
                                                                                   
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Llie dust  into  the hopper.   The  rate of  flow  (or  the back pressure) and the
timing need adjustment  from time  to time to  keep the residual drag at an
economical lovel.
               8.2.10.3  Reverse  Flow Plus Collapse  - Whenever  the  cloth  is  flexed
as part of the cleaning process,  and wherever  it flexes, there, is apt to bo
a high rate of wear  indicated by  a thinning  and  eventual perforation of the
cloth.  This is especially  common near  the thimbles where the flexing is
3-dimensional.  A decrease  of air flow  or an increase in cloth tension will
help to reduce flexing.  Frequently cloth flexure  is reduced by installing
rings inside the tube,  sometimes  sewing them into  the tube.  In the cases
of reverse pulse and panel  filters, wire grids are generally used to back
up the cloth.  While these  do reduce flexure wear, they may also introduce
frictional wear between the  cloth and the ring or grid.
               Note that any type  of mechanical wear of the fabric--abrasion,
flexure,  or tensile wear--will  be amplified  by adverse environmental con-
ditions.  The  molecular structure of both synthetic and natural fibers
may be damaged by high  temperatures, moisture, and/or chemical conditions,
thereby weakening the fibers and  making them more susceptible to mechanical
wear.  Thus in a particular  case, lowering the filtration temperature may
alleviate a flexural wear problem.  The converse can also apply.   For example,
fibers are available for filtering at very high temperatures (nearly 2000 F)
but at present there are no  practicable ways of cleaning them without ex-
cessive fiber  flexure.  In  other  words, reducing the mechanical demands on
a fabric may make it more tolerant of tough  environmental conditions.  Fa-
brics with different finishes, weaves,  etc.  may also be put on trial to cir-
cumvent a particularly  troublesome mechanical or environmental problem.
               8.2.10.4  Pulse -  As there are  almost no moving parts in the pulse
cleaned apparatus, hardware maintenance is  certainly reduced compared to
other cleaning methods.   However, the excessive use of cleaning air pressure
may balloon the bags so much as to weaken them by overstretching.   If the
fabric is being damaged and cleaning cannot be diminished,  one must try
another fabric or attempt to reduce the adhesiveness of the dust  (Section 2.2).
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               8.2.10.5  Reverse  Jet  - As  there  is a  fair amount of mechanism
within the clean side of the baghouse, any penetration of the bags by dust
results in a wearing of the mechanism, gradual or rapid, depending on dust
abrasivencss.  Beyond a certain point, unless the equipment is completely
overhauled the mechanism will not only destroy itself but the bags as well.
Thus this method of cleaning while popular in some applications is at other
times associated with fairly high maintenance.
              Sometimes in reverse jet equipment a hard residue of felt
fiber and dust builds up on the cleaning rings so near the bags as to
begin to rub.  This accretion can usually be sanded off, but may be prevented
by antifriction coatings or rings made of stainless steel.   The cleaning
rings must stay smooth and level and at the prescribed distance from the
cloth.   The cloth tubes must be kept taut.   Equipment operability can
usually be prolonged by changing from continuous tn intermittent cleaning,
in which the carriage pauses for a period of time between trips.   Here
again is the principle of using a minimum of cleaning.
      8.2.11  Rotary Valves and Conveyors
              These can be high maintenance items if the dust packs to-
gether or adheres to the paddle wheel or other surfaces.  The intake of
moisture due to faulty seals is a frequent cause of sticking.  Ordinarily
a good quality valve will give long service if it gets lubrication and a
check of its seals and clearances from time to time'.  Anti-adhesion coat-
ings may be used to some advantage on the wheel blades.   A glass window
in the valve (kept clean) is a popular method of telling whether the valve
is discharging normally.  Kraus  lists a number of engineering and main-
tenance practices for rotary valves used in pneumatic conveying.
                                       *-\
              Smeary dust material in a screw conveyor can directly cause
repeated plug-ups and breakdowns.   Converting to a pneumatic conveying
system is not always an improvement, depending on the dust.   Possibly dif-
ferent conditions of dust temperature or moisture will change the properties
of the dust; otherwise it may be necessary to use separate collection bins
instead of conveying to a central point.
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              Air  leaks  in  the  rotary valve and  sometimes air  leaking along
the conveyor  from  one  compartment  to another can re-aerosolize dust  and
overload  the  filter.   Indications  of this arc non-zeroing manometers and
imusuu I .flit rr  ilr.a(.',s .
      8. 2.. I2  Instrumentation.
              The  operability of fail-safe mechanisms and automatic  control
instrumentation is very  important  to the safety  of the filter  cloth, and
it is usually up to a  maintenance,  man to be sure these are in working order.
He may advise the  installation  of  more or better instruments,  since  most
manufacturers supply a minimum  of  instrumentation with their equipment in
the interest  of economy.  Good  instrumentation often pays for  itself,
however.
              One  thing  to  check after equipment installation is the loca-
tion ot all sensing instruments because a small  difference in location can
be serious.   For example, the wrong temperature  may be measured if the gases
are not well  mixed at  the sensor location; or, a high gas velocity may give
an error  in the static pressure sensed at one side of a differential mano-
meter.  All instruments  should be  calibrated after installation, and re-
checked monthly for sensor  location, leaks (manometers)., sticking, legibility,
etc.
              A central  panel for most or all instrument readout,as opposed
to scattered  instruments,has been estimated to reduce instrument mainte-
nance costs by  as much as 50 percent, as well as making operation of the
collector system more convenient. One should record instrument readings
over  one normal  operating cycle for use in checking and troubleshooting
later in the  life of the equipment.  The record should be posted beside
each  instrument.
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8.3  ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION             ;
     During a 1969 survey of about 50 fabric filter installations in a
variety of industries the people most familiar with the day-to-day
upkeep of these filter systems were personally interviewed.  These people
wore asked to assist with about 130 questions relating to the equipment
and its performance.  The questions included:
     - What are your principal causes of fabric failure?
     - Have you tried other fabrics, and why are you using
         the present one?
     - Do you receive any complaints regarding the quality of
         your filtered effluent?
     - Do you have problems associated with fabric blinding?
     - What, if any, are the major difficulties with with your filter
         system?
     - What aspects of performance or operation could be improved,
         based on your experience?
     - What suggestions would you make for improvements in design
         or manufacture?
     - What do you see as being the principle requirements for
         research or development?
From the answers to these questions, nearly 100 different suggestions
for possible research and development investigation were obtained, as
discussed in another volume of the contract documentation.  The surveyed
installations also reported a total of 112 operating problems which are
summarized in this section.
     As considerable care was taken to distribute the survey across a
spectrum of filter applications, the operating problems reported here
may with some confidence be considered typical of most filter users.
The list does not include all possible problems because of the limited
size of the survey.  There is furthermore a wide variation in problems
encountered from application to application, and from installation to
installation.
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      Appendix 8-2 lists 43 installations  which  reported operational

 problems,  together with the applications   data  suspected of  being related

 to  the  problems,   These are:

      -  dust  source
      -  particle  size
      -  air/cloth ratio
      -  maximum differential pressure
      -  temperature
      -  fabric material
      -  cleaning  method
      -  particle  abrasiveness

 There are  often  other circumstances that  also contribute to  filter oper-
 ating problems.

      In addition to the 1969  survey of  fabric filter users,  the fabric
 filtration literature reports numerous  examples of operational and main-
 tenance problems.

      8.3.1  Types  and Frequency of Problems Reported

             Table  8.1 lists  23 types of operational problems encountered

 by  the  installations surveyed.  They are  grouped into 5  causality  cate-
 gories  as  follows:

      1.  Fabric-dust interactions     9 types of    60 problems reported
                                       problems

      2.  Filter  element difficulties  2    "         6      "

      3.  Filter  element-hardware      5    "        12      "
           interactions

     4.   Collector  design problems    5    "        27      "

     5.   System design problems       2    "         7	"	
                                     23 types of     112 problems reported
                                     problems
The largest category of problems reported was the first:   60 different
fabric-dust related problems were reported.  Of these the most frequent
-------
                              TABLE 8.1

             TYPES AND  FREQUENCY OF PROBLEMS REPORTED
 1.  Fabric  - dust  deposit  interactions             Frequency

    a.   interstitial  deposit related                   8
         abrasion,  wear

    b.   flexure wear  failure                           10

    c.   seeping                                         4

    d.   blinding                                       14

    e.   burning, heat                                  6

    f.   holes, pinholes,  shot holes                     6

    g.   hydroscopicity                                  4

    h.   condensation                                    5

    i.   deposited  dust  hardens,  cake                    3
         tears, cracks bag                             	
                                         Subtotal       60

 2.  Fabrication failures  not. particularly related to dust interaction,
    mechanical

    a.   seams, sewing                                  2

    b.   tears at top                                    4

                                         Subtotal        6

 3.  Design  or maintenance failures related  to  tensioning, supports, rings,
    collars, or cleaning  device  interactions

    a.   chafe on housing  or other bags                 3

    b.   tensioning, bags  too loose                     1

    c.   cage, wire, ring  abrasion,                     5
         wear  (also dust related),
         support mechanism interact


(continued)


-------
                        TABLE 8.1 (Continued)
    d.  cleaning carriage bag wear                     1

    e.  seals around cloth-metal collars               2

                                       Subtotal       T2

4.  Collector design problems, internal, external mechanisms, incl,
    auxiliaries, (ex. pipes, hoods)

    a.  unable to enter collector to service or        4
        maintain during operation

    b.  hole detection problems or performance         3
        effluent monitor

    c.  hopper dust sticking, holdup, screw            6
        conveyors plug

    d.  internal mechanism wear                        4

    e.  external mechanism wear, timer, shaker,       10
        fan, bearings, doors, seals, wall failure

                                       Subtotal       27

5.  Dust collecting system design problems, external to collector
    (incl. pipes, hoods)

    a.  piping, elbows, abrade                         4

    b.  hood inlet control poor                        3

                                        Subtotal       7

                                        Total:        112
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type was blinding:   14 problems of blinding of the fabric by the dust
were reported, as detailed  in Table 8.1.  Over 25 percent of all problems
reported were from blinding, flexure wear, and interstitial abrasion.
            From this brief analysis one is able to anticipate that most
fabric filter maintenance will center on interactions between the fabric
and the dust.  The dust may not collect in a convenient way on the fabric;
the dust may not remove easily from the fabric; the dust may damage the
fabric in some way.  Being  thus forewarned, maintenance labor may possi-
bly be minimized, for example, by running pilot tests before selecting
the fabric.
            Appendix 8.3 compiles the data in Table 8*1 and Appendix 8.2
to indicate the kinds of application in which each problem type occurs.
For example ,in Appendix 8»3 one may observe that "fabric wear apparently
related to interstitial abrasion" was reported in:

     - 6 installations with particle sizes over 10 microns, out of
         7 reporting
     - 5 installations with temperatures under 200 , out of 8 reporting
     - 2 installations with very abrasive dust, out of 6 reporting
     - 4 installations with cotton fabric, out of 8 reporting, etc.

While this breakout  does not necessarily demonstrate cause-and-effect
relationships, the table may be useful in troubleshooting operational
problems, as it enables comparison with applications reporting similar
problems.
     8.3.2  Specific Applications Reporting Problem Types.
            Appendix 8.2 lists the problems reported by each of the  various
installations surveyed.  These installations are grouped by industrial
category, enabling for example, the observation that all of the (four)
combustion applications reported only problems related to fabric-dust
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interactions.  Iron and steel applications also reported fabric-dust
problems, notably abrasion from fabric support hardware.  Again, this
analysis does not necessarily demonstrate cause and effect relationships;
instead, the analysis is presented for comparison with the reader's ope-
rational experience.
    8.3.3  Literature Survey of Maintenance Problems
           A review of about 500 documents published during the last 10
to 15 years yielded a large number of comments related to problems asso-
ciated with operating and maintaining fabric filter systems.  These in-
cluded comments based on experience with a single filter system, as well
as comments from users of many systems and from manufacturers usually with
many years of experience.   Much of this information has already been woven
into the previous sections of this Chapter and is not repeated here.  Some
of the literature, however, is so valuable in amount of detail, or in de-
scription of a specific installation, or in insight and experience, that
it is indexed here in Table 8.2.  Most of these reports are from the Iron
and Steel Industry and the Nonferrous Metals Industry.
           Three cases in point are described in Appendix 8.4.  These are
fabric filter applications of sinter plant  discharge,  oil combustion,
and copper smelting.  The original accounts make interesting reading, as
they demonstrate the range of problems associated with  the development
of new filter applications.  These accounts are typical of the better
literature describing filter system operation and maintenance.
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oo

o
                                                    TABLE  8.2

                                 MAINTENANCE  PROBLEMS AND  PRACTICES  REPORTED IN THE
                                            FABRIC  FILTRATION  LITERATURE
Industrv* Reference
1 4,5
5 6
5 2
5 7
7 8
7 9
7 10
7 11
7 12
7 13
8 14
8 15
8 3
8 16
8 17

8 18
8 19
Bust/Source
Oil combustion
Carbon Black
Soap, etc.
Cleansers
Cement Plant
Cement
Cement
Glass Furnace
Various applies.
Coal
Cupola,
Foundry
Cupola
Open hearth
General (Steel)
Electric Steel
Furnace
Sinter strand
General (Steel)
Fabric
Glass
Glass
-
Cotton
Glass
Var
-
Var
-
Var
-
-
Glass
-
-

Glass
Glass
Problem**
lb,3b,3c
G,le
Misc.
Id
3b,li
G
G
lh,li,ld
lh,lb
le,lh
G
4e
3a,3c ,4c
Misc.
le

la,3b,ld
4c , 4e , Ih
Remarks
System and procedures development
for new application
General
Pneumatic conveying equipment
Also chemical attack reported
General
General discussion of fabrics etc.
General
Several applications discussed
Panel discussion
General
General manual
General
System and procedures development
Misc. applications at Bethlehem Steel
plants ; Nui^erous problems
Labor and material cost analysis for
one year
Discussion of system upkeep
Misc. applications at Jones & Laugh 1 in
Fluoride attack.
                                                                         L_
-------
                        TABLE  8.2  (Continued)
                                                                                                 f
Industry
9

9

9

9
9

9
9

9
oo 10
t
U)
G*
G :

G

G

G
G
G
G
Reference
20

21

22,23

24
25

26
27

28
29

30
31

32

33

34
35
36
1
Dust /Source
Uranium plants

Copper Smelter

Secondary Cu
Refining
Arsenic salts
Zinc Roaster

Smelter
Cu Smelting;
ZnO
Zinc galvanize
Air filtration

General
General

General

General

General
General
General
General
Fabric
Wool

OrlonR
Glass
Glass +

-
Glass

TeritalR
OrlonR

Cotton
Various

-
-

-

-

-
Felts
-
(Cotton)
Problem
Misc.

lb,3a

Misc.

lb,3d
li.la.ld,
2a
-
1-, 3-

Id
-

Id
G

G

Misc.

Misc.
3d
G
Misc.
Remarks
Numerous problems on 18 R.jet
collectors; costs
Detailed experience with several
installations
General discussion of system
experience
Russian plant experience
System and procedures development

Designed for low maintenance
Routine inspection described

Occasional manual cleaning required
Maintenance economics of low efficiency
filter equipment
Residual cake profiles
General discussion by system manu-
facturer
Inspection schedule, disposal
methods discussed
Abrasion, adhesion, bridging etc.
discussed
Filter sleeve maintenance and storage
Reverse jet equipment
Unit sized filters
General discussion by system manu-
37
General
facturer

Troubleshooting checklist, general
-------
                                            TABLE 8.2  (Continued)
Industry
G
G
Reference
38
39
Dust/Source Fabric
General
General
Problem
G
Remarks
General upkeep of all types of
collectors
World-wide survey of maintenance
                                                                      costs reported
 * Industry Key:

      1 - Combustion Processes
      2 - Food and Feed
      3 - Pulp and Paper
      4 - Inorganic Chemicals
      5 - Organic Chemicals
      6 - Petroleum Refining
      7 - Nonmetallic Minerals
      8 - Iron and Steel, Foundry
      9 - Non-ferrous Metals
     10 - Miscellaneous
      G - General discussion

** Problem lb, 3c, etc: For explanation, see Table 8.1.

G = General discussion of system upkeep experience.

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8.4  REFERENCES FOR CHAPTER 8

     1.  R. L. Clement, Selection, Application, and Maintenance of Cloth
         Dust Filters, Plant Engineering, pg. 24, circa 1965.

     2.  R. A. Herrick, J.W.  Olsen, and F.A. Ray, Oxygen-Lanced Open
         Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
         JAPCA 16:1, 7 Jan. 1966.

     3.  M. N. Kraus, Pneumatic Conveying, I + II, Chem. Engg., 167
         (Apr.. 12, 1965) and 149 (May 10, 1965).

     4.  D.N. Felgar and W.E.  Ballard, First Years Experience with Full-
         Scale Filterhouse at  Alamitos Generating Station, Electric World,
         Circa 1966.

     5.  F.A. Bagwell, L.F. Cox and E.A.  Pirsh, Design and Operating
         Experience with a Filterhouse Installed on an Oil Fired Boiler,
         JAPCA 19:3, 149 (March 1969).

     6.  I. Drogin, Carbon Black, JAPCA 18:4, 216 (April 1968).

     7.  Dust Collection at Oakite, Anonymous,  Soap and Chemical Specialties,
         157 (June 1965).

     8.  R.E. Doherty, Current Status and Future Prospects-Cement Mill Air
         Pollution Control, Third Nat'l.  Conf.  on Air Pollution,Proc., PHS,
         USDHEW (December 1966).

     9.  R.F. O'Mara,  and C.R.  Flodin, Filters  and Filter Media for the
         Cement Industry,  JAPCA 9:2. 96 (August 1959).

    10.  G. Funke, Operation and Maintenance of Filter  Collectors, Staub
         (English Translation)  £6:4, 25 (April  1966).

    11.  A.B. Netzley and J.L.  McGinnity,  Glass Manufacture,  Air Pollution
         Engineering Manual, USPHS 999-AP-40, Cincinnati (1967).

    1-2.  Anonymous, Panel Probes  Dust Collection Problems,  Rock  Products,
         76 (January 1965).

    13.  H.E. Soderberg,  Considerations in the  Selection of Dust Collectors
         for Coal, Mining Congress  J.,  62  (October 1964).

    14.  American Foundrymen's  Society, Foundry Air Pollution Control
         Manual,  Des Plaines,  111.,  2nd Ed.  (1967).

    15.  R.W. Mcllvaine, Air Pollution Equipment  for  Foundry  Cupolas,
         JAPCA 17:8.  540  (August  1967).
-------
 16.   H. M. Chapman,  Experience with  Selected Air  Pollution  Control  In-
      stallation at Bethlehem Steel,  JAPCA  13:12,  604  (December  1963).

 17.   T. Killman, Dust and Fume Control  for Electric Furnaces. Texas
      Steel Co., Fort Worth" Texas  (May 1969)""     	"

 18.   T. A. Young, Gary Steel Works Experience with Dust Control  at No.  3
      Sinter Plant, Blast Furnace  and Steel plant, 1057  (December 1968).

 19.   S. Vadja, Blue  Ribbon Steel with Blue Skies, I&S Engr., 71  (August 1968).   ~

 20.   W. B. Harris and M. G. Mason, Operating Economics of Air Cleaning             :
      Equipment Utilizing the Reverse Jet Principle, Ind. &  Eng.  Chem.            —''
      42:12, 2423 (December 1955).

 21.   D. J. Robertson, Filtration of Copper Smelter Gases at Hudson Bay           _
      Mining and Smelting Co. Ltd., Can.  Min. and Met. Bull.. 326 (May 1960).

 22.   R, H. Graves, Discussion of Baghouse Design and Operation,  Nat'i.             i
      Assoc. of Secondary Materials Industries, Inc., Pittsburgh, Pa.             "^
      (June 1967).

 23.   Anonymous, Bag Life Extended at Copper Refinery, Air Enge (18 Jan. 1969).   J

 24.   V.G.  Matsak,   Smoke Purification Equipment, in USSR Literature, on
      Air Pollution Related Disease. USPHS TT60-21475 141 (1960).                   i
      	'	' —- 	-H J-L-LJi-- -   --!'_- - /  _—1- -LT.J—                                               ^J

 25.   L.P.  Landucci, and R.E.  Eyre,  Pilot Plant Filtration of Zinc
      Suspension Roaster Gases, Can. Min. and Met.  Bull.. 703 (October 1962).       i
                                                                                  —j
26.   Anonymous, Bag Filter for Smelting Plant is Fully Automatic.Filtration
      and Separation.  326 (July 1966).
                                                                                  —i
27.   H.E.  Schwartz, L.E.  Kalian,   and A. Stein,   Controlling Atmospheric
      Contamination in Smelting and Refining of Copper-base Alloys,
      Air Repair £: , 5 (May 1955) .                                                  •

28.   G. Thomas, Zinc-Galvanizing Equipment, Air Pollution Engineering
      Manual.  USPHS No.  999-AP-40, Cincinnati (1967).                               j
                                                                                  —/
29.   D.G.  Hill,  An Evaluation of the Costs of Air Filters,  Filtration
      and Separation,  297 (July 1967).

30.   D.G.  Stephan  and G.W.Walsh, Residual Dust Profiles in Air Filtration,       *""'
      Ind.  and Eng.  Chem. 52:  12,  999 (December 1960).
                          •"•                                                         I

31.   R.L. Adams,  How to Maintain Cloth-Type Collectors, Air Engineering,         —'
      20 (May 1963).
-------
32.   W.O. Vcdcler,  Filters Aid At-Source  Control  of Dusts,  Chem.  Eng..
      140 (May  1951).

33.   J.M. Kane,  Guideposts Tell How  to Select Dust Collecting Equipment,
      Plant Eng,, , (November 1954).

34.   R. Ashman,  Filtration Fabric Selection, Power and Works  Eng..
      53 (February 1959).

35.   K.J. Caplan,  Current Applications of the Reverse Jet  Filter Principle,
      A.M.A. Archives. Ind. Hyg. Assoc.. 21; 200 (March 1960).

36.   F.R. Chase,  Application of_S_elf-Contained Dust Collectors,  The
      Torit Corp., St. Paul, Minn." (1963)7	" """	""

37.   S. Levine, What You Should Know About Dust Collectors, Rock  Products,
      53 (April 1965).

38.   P. Staubung,  Operation Maintenance of Dust Removal Equipment,
      Wasser Luft Betr. (German) 12, 215 ( April 1968).

39.    R.L.  Chamberlin and P,B. Crommelin,    Economic Aspects of Air
      Pollution Control for ^he.World's Heavy Industries^ presented at
      the First World Congress on Air Pollution, Buenos Aires, Argentina,
      (November 14-21, 1965).
-------