Draft
8    ENFORCEMENT WORKSHOP ON

     PLANT INSPECTION AND

1    EVALUATION PROCEDURES
0)
P

     VOLUME VI
8    CONTROL EQUIPMENT OPERATION
I    AND MAINTENANCE - FABRIC FILTERS
CO

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     U.S. ENVIRONMENTAL PROTECTION AGENCY
     OFFICE OF ENFORCEMENT
     OFFICE OF GENERAL ENFORCEMENT
     WASHINGTON, D.C. 20460

-------
REFERENCE MATERIAL FOR ENFORCEMENT WORKSHOP

ON PLANT INSPECTION AND EVALUATION PROCEDURES
                 Volume VI
        Operation and Maintenance of
               Fabric Filters
               Compiled by
         PEDCo Environmental, Inc.
            505 S. Duke Street
      Durham, North Carolina  27701
          Contract No. 68-01-4147
               PN 3470-1-CC
               Prepared for
   U.S. ENVIRONMENTAL PROTECTION AGENCY
           Office of Enforcement
 Division of Stationary Source Enforcement
          Washington, D.C.  20460
                 July, 1979

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                             FOREWORD









          The following document is a compilation of selected



technical information and publications on the evaluation of indus-



trial air pollution control equipment operation and maintenance



practices. The reference manual is intended to be an instructional



aid for persons attending workshops sponsored by the U.S. Environ-



mental Protection Agency Regional Offices.
                                n

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                          TABLE OF CONTENTS

                                                        Page No.

Volume VI:  Operation and Maintenance of Fabric Filters


VI-1.  Fabric Filtration Systems Design, Operation
       and Maintenance, S.A. Riegel                       1-1

VI-2.  Baghouses -  What to Know Before You Buy,
       S.A. Riegel, R.P. Bundy, C.D. Doyle, Pollution
       Engineering, May 1973, pp. 32-34.                  2-1

VI-3.  The User and Fabric Filtration Equipment,
       Donald H. Rullman, Journal of the Air Pollution
       Control Association, Vol. 26, no. 1, January 1976,
       pp.  15-31.3-1

VI-4.  Ins and Outs of Gas Filter Bags, J.C. Walling,
       Chemical Engineering, October 19, 1970.            4-1

VI-5.  Baghouses: Separating and Collecting Industrial
       Dusts, Milton N. Krauss, Chemical Engineering,
       April 9,1979, pp. 94-106                           5-1
                               111

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              VI-1.

    FABRIC FILTRATION SYSTEMS

DESIGN, OPERATION AND MAINTENANCE
               by
        STANLEY A. REIGEL

           10001 Briar


   Overland Park, Kansas 66207
               1-1

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                        INTRODUCTION

     This report will give you an overview of the design,
operation, and maintenance of air pollution control systems
using fabric filters  (i.e., baghouses) as the control device.
Both engineering and cost factors will be discussed.

                      TYPES OF SYSTEMS

     Baghouses are used in two fundamentally different types
of systems:

     a)  Dust control systems which may be described as a
system which is not really critical to a plant's continued
operation or to the immediate health or wellbeing of workers
in the plant or the general public.  Dust control is used to
collect dust particles from the air, say in a woodworking
shop or in material transportation.  Dust collectors usually
handle low volumes of air.  High temperatures, continuous on-
line cleaning, long life fabrics, corrosion and other factors
usually are not considerations in their design and operation.

     b)  Process gas systems, on the other hand, are critical
to the production process itself.  They may be called upon to
operate twenty-four hours a day, 365 days a year.  They often
handle extremely large gas volumes and encounter high tempera-
tures, highly abrasive materials, sub-micron particles, high
dust loadings and corrosive agents.  The real definition, how-
ever, is not necessarily that it is a heavy duty system—
rather that if it shuts down, the plant shuts down.  For ex-
ample, in an asphalt plant if the baghouse system is not opera-
ting properly, the plant may not be able to achieve adequate
production rates.  If the baghouse has to be shut down, pro-
duction also comes to a standstill.  Also, where toxic gases
are present, operation of the process baghouse is critical.
In a plant where lead oxide is generated, for example, there
would be a hazard to workers if the baghouse malfunctioned.
Because of these factors,  process baghouses are maintained in
a different manner from dust collectors.  They are inspected
more freguently--where a dust collector may be inspected once
a month, a process baghouse may be inspected once a week.  Al-
so, the inspections will be more thorough.

     Process baghouses also must normally be more closely
monitored, and monitoring equipment may be more sophisticated
than on a dust collector.   Where a visual check of the stack
of a dust collector usually is sufficient, a process gas bag-
house may have automatic stack monitors.  Pressure gauges
usually will be inside the plant, at a monitoring station,

                             1-3

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rather than outside, attached to a baghouse leg.

     Process gas baghouses also may represent a considerably
higher investment than dust collectors, which makes a good
maintenance important.  However, due to increasing knowledge
about the effects of inhaling various particulates in the air,
the differences in maintenance of the two systems may be be-
coming less significant.

                     TYPES OF BAGHOUSES

     No other air pollution control device is subject to as
wide a variety of modifications as is the baghouse.  Baghouses
are classified according to;  the type of fabric used;  where
they collect the dust—either on the inside or outside of the
bags;  the type of cleaning mechanism used—shaker, reverse
air, compressed air or sonic cleaning;  or whether they are
continuous automatic or intermittant in operation.  A baghouse
can be a combination of these classifications, such as inside
bag collectors, continuous automatic, shaker cleaning on a
process gas stream.

Inside Bag vs. Outside Bag Collectors

1.  Inside bag collectors

Bag are attached both at the top and at the bottom of inside
collectors.  See figures 1 and 2.  Dust enters the hopper,
and the division between the clean and dirty air plenum is
near the top of the hopper, with the majority of the housing
being clean air side.  Air flow is from the inside to the out-
side of the bags.  It is necessary to enter the housing to
change bags.

2.  Outside bag collectors

Typically, bags are suspended from the top of the housing.
Only the top portion of the housing is the clean air plenum,
with the major portion of the housing and hopper being on the
dirty side.  Most outside collectors are designed so bags can
be inspected and removed from the top without entering the
housing.  Since air flows from the outside to the inside of the
bags, they must be supported from collapse by cages on the in-
side of the bag.

Intermittant vs.  Continuous/Automatic Baghouses

1.  Intermittant baghouses

Intermittant baghouses are used only in applications in which
the production machinery is shut down at regular intervals, say

                            1-4

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Stainless
Steel
Clamp
   -Cell PJate
                             V7777A      \ZZZ7,
Cell
Plate
                                               1
        /
 Cuff with
• Spring Steel—•—
 Band
                          V/Z2
             THIMBLE CONNECTION
        SNAP BAND CONNECTION
                     Figure 1   Bag-cell plate attachments

-------
         Rocking Motion
                               Shaker Bar
                                    Tension Nuts
                                         Bag Cap
                                           Clamp
Figure  2..  Typical shaker arrangement
            1-6

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during lunch periods, breaks, or at the end of the working
shift.  The unit must be sized so it can operate without
cleaning for the time interval necessary.

2.  Continuous automatic baghouses

These are units which clean their bags automatically during
operation at pre-set intervals.  With the exception of pulse-
jet baghouses, continuous cleaning units must be compartmented
so a section of the baghouse can be dainpered off during the
cleaning cycle.  In pulse-jet units, the compressed air blast
interrupts the air flow through the bag for the instant the
bag is being cleaned, and no damper valves  or compartmentation
is necessary (although the effect is the same).

Types of Cleaning Mechanisms

1.  Shaking

The most common type of inside collector is the shaker bag-
house.  See figure 2.  Shakers can range in size from small,
intermittant cleaning dust collectors with a few bags that are
shaken manually through a hand crank arrangement, to huge
compartmented systems with bags up to 40 ft. long and over a
million square feet of filtering area.  The hopper of a shaker
baghouse is covered with a sheet of steel with holes in it
called a cell plate.  See figure 1.  Bags are fastened to the
cell plate and, at the top of the housing, to the shaker me-
chanism.   Each row of bags is joined to a rod and each rod to
the shaker framework.  Each shaker assembly consists of a
motor, eccentric, connecting arms and bearings.  As the rod
rotates,  a mechanical shaking is initiated which actually
shakes dust off the insides of the bags, where it then falls
into the hopper.  Shaker baghouses normally utilize woven
fabrics and operate at low air-to-cloth ratios.

2.  Reverse air

The reverse air baghouse is basically the same in design as
the shaker and operates similarly.  The sole difference is
in the cleaning mechanism.   Insteam of mechanical shaking of
the bags,  a fan is used to reverse the air flow from insie-
outward to outside-inward.   The result is that clean air is
backwashed into the dirty air plenum, through the bags from
the clean side to the dirty side, blowing collected particles
free.   Reverse air cleaning is more gentle than shaking and
is used primarily with delicate fabrics.  Bag construction is
similar to that of shaker systems;  however, metal rings must
be sewn into the bags to keep them from collapsing during
cleaning cycles.


                             1-7

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 3.  Combination reverse air and shaker

 Some baghouses may be designed for both shaker and reverse air
 cleaning.  During the cleaning cycle, the bags are first col-
 lapsed with the reverse air fan, then gently shaken.

 4.  Pulse-jet

 A pulse-jet baghouse uses short bursts of compressed air to
 clean its bags which are attached only at the top to a tube
 sheet.  See figure 3.  Over each row of bags is a compressed
 air manifold with an orifice directed down toward the top of
 each bag.  During the cleaning cycle, an extremely short burst
 of compressed air flows from the top to the bottom of each bag,
 creating a shock wave that pushes the fabric out away from the
 cage, knocking dust particles loose.  Although several rows of
 bags may be pulsed simultaneously, each bag receives its own
 individual burst of compressed air.

 The cleaning particles from the bag in a pulse-jet collector
 is not accomplished by the backflow of air through the bag.
 In fact, a typical pulse puts no more than 0.5 cubic feet of
 air back through each bag;  up to 2 cubic feet with induced
 air flow from a venturi.  Compared to the square feet of cloth
 area in a typical bag, the effect is negligible.  What acutally
 cleans the bag is the sudden expansion from a shock wave,
 which moves from the top to the bottom of each bag.  Most
 pulse-jet baghouses are manufactured with some type of venturi
 over each bag, which most manufacturers consider necessary.
 However, recent venturi design seems to have become more of a
 marketing gimmick than a proper engineering design, since tests
 have shown that all the different configurations manufacturers
 placed over their bags made little difference in real cleaning
action.  -Moving-horns, d±ffusers, exotic venturi shapes, para-
 bolic velocity stacks, hyperboloid diffusers and even nothing
 at all have been used over bags.

Although pulse-jet baghouses can be compartmented for certain
 applications,  compartmentation is not normally necessary.
 Pulse-jet systems use densely felted bag materials to achieve
 a high level of filtering efficiency.  They also operate at
 air-to-cloth ratios significantly higher than inside bag col-
 lectors, so the baghouses are usually much smaller in size for
equivalent gas volumes.

Most pulse-jet baghouses are designed with hinged top doors
 for inspection and maintenance of bags and electrical and
pneumatic systems, so there is no need to enter the baghouse
itself for routine maintenance.

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 5.  Plenum pulse

A variation of the pulse-jet  is  the plenum pulse baghouse.
Basically, it operates  in  the same manner and also uses com-
pressed air or reverse  air to clean the bags.  The difference
is that the clean air plenum  is  compartmented into sections.
Each of these sections, when  isolated by a damper, becomes a
semi-compartment and is pulsed all at once during cleaning.
ONe large burst of high pressure air, then, cleans several rows
of bags.

Plenum pulse baghouses may be equipped either with compressed
air or with high pressure  blowers.

6.  Blow ring

A reverse air variation in which a ring travels up and down the
length of the bag blowing  air back through the bag as it moves,
These are very seldom used anymore although at one time they
were popular because they  were the only type of baghouse that
permitted high air-to-cloth ratios that did not infringe on the
pulse-jet patent held by Midro.

               TYPES OF BAGS  AND BAG MATERIAL

Cloth Construction

1.  Woven

Most commonly used with inside bag collectors.  Fibers are woven
on conventional looms into specified fabrics.  The type of
weave will vary depending  on  the application, and also, thread
size and type can be altered  as  necessary.

2.  Felted

Most commonly used with outside  bag collectors.  Normally, a
light, loosely woven fabric,  called a scrim, is used as a base
and fibers are needled into this base to form a dense fabric.

Bag Construction

1.  Bag

Common name for all filters, but specifically, means a tube
shaped filter.   This is the most common type of construction
and is used with both inside and outside collectors.

2.  Envelope

Flat unit sometimes used with outside bag collectors.  These
units offer the advantage of the maximum amount of cloth in a

                            1-10

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minimum space, but internal velocities are higher.

3.  Cartridge

These are relatively new in design and resemble an automotive
air filter in that the cloth is pleated to increase filtering
area.  So far they have been limited in application to low
temperature dust collectors.

Cloth Material

At least for process gas systems, the single most important
design decision is the cloth material to use.  High tempera-
tures, corroosive chemicals, and small abrasive particles com-
bine to make the wrong decision very expensive.  Bags that
were expected to last for three years may last only three months
(or even three weeks) if the wrong decision is made.  Table 1
gives the properties of the most commonly used bag materials.

                  MECHANISMS OF FILTRATION

     When a baghouse is started up the first time with brand new
clean bags it is a very inefficient device.  In fact it leaks
like a seive.  Clean bags are not good filters.  The following
mechanisms are important because they make the bags get dirty;
these collection mechanisms are only important until the bag
builds up a "filter cake" of collected particles.  After the
bags have been in operation a.while and the filter cake has
developed the bags are no longer, strictly speaking, the fil-
ters.  Rather the filter cake of collected materials is itself
the actual filter and the bag merely supports it.

1.  IMPACTION—the particle carried by an airstream collides
with a fiber and has enough inertial force so it doesn't de-
flect along with the airstream.

2.  STRAINING--the particle is larger than the opening between
fibers and is trapped between them.

3.  DIFFUSION--random movement of fine particulate causes par-
ticles to avoid following gas stream lines around the filter.

4.  INTERCEPTION—a particle tends to follow airstream lines,
but as it passes the fiber, it comes close enough so that one
side impacts with the fiber.

5.  AGGLOMENTATION—cohesion of one particle to another, and
adhesion of particles to fibers so that,  in effect, the target
size is increased.

6.  ELECTROSTATIC ATTRACTION—fiber has an opposite charge from
particles.
                             1-11

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                                 TABLE 1.


                           ^ >   >TJ   x-v T3  2!   >TJ     H     OOZ2


 FIBER PHYSICAL PROP.      §§§§§!§     P     &   §   P

                           O f   >   O t*l  X   *Tl     O     C/5   O   *
                           •O M   S   T> C/5       PO     25         Z
                           OOMOH       O     *          *

 E = Excellent             ^     w   «< po       K
G = Good
A = Average
F = Fair
P = Poor
Max. Cont. Op. Temp. Of
Abrasion Resistance
Toughness
R = Recommended
S = Satisfactory
N = Not Recommended
0 = No Information


285
A
A



225
E
G

CHEMICAL




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275
G
E



375
G
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RESISTANCE




w
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170
E
A



450
P
F



500
P
P



180
F
A



200
A
E

TO REAGENTS










MINERAL ACIDS
Aqua Regia
Chromic
Hydrochloric
Hydrofluoric

Phosphoric
Sulfuric
S
R
R
R
R
R
S
N
N
N
N
N
S
N
S
S
S
S
S
S
S
N
N
S
N
S
R
N
R
S
R
R
S
R
R
R
R
R
R
R
R
R
R
R
R
N
R
R
R
N
N
N
N
N
S
N
N
N
S
S
S
S
S
ORGANIC ACIDS
Acetic
Benzoic
Carbolic
Formic
Lactic
Oxalic
Salicylic
R
R
R
R
R
R
R
S
N
N
S
S
R
S
R
R
S
R
R
R
R
S
S
N
S
S
N
S
R
R
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
N
S
S
S
S
S
S
N
BASES
Ammonium Hydroxide
Calcium Hydroxide
Potassium Hydroxide
Potassium Carbonate
Sodium Hydroxide
Sodium Carbonate
S
R
S
S
S
R
S
R
S
R
S
R
N
R
S
S
S
R
S
R
S
R
S
R
R
R
S
R
S
R
R
R
R
R
R
R
S
S
N
N
N
N
S
R
R
R
R
R
N
N
N
N
N
N
*USED INFREQUENTLY - Glass,  Polypropylene, -Nomex,  and  Polyester account  for

 the vast majority of baghouses.



                                1-12

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TABLE 1 (continued)
^nr/nj.»_.ftjLi i\to.i_o irtiiv-.c.
TO REAGENTS


R = Recommended
S = Satisfactory
N = Not Recommended
0 = No Information
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SALTS
Calcium Chloride
Ferric Chloride
Sodium Acetate
Sodium Benzoate
Sodium Bisolfite
Sodium Bromide
Sodium Chloride
Sdoium Cyanide
Sodium Nitrate
Zinc Chloride
R
R
R
R
N
R
R
R
R
S
N
S
R
S
S
R
R
S
S
N
R
R
R
R
R
R
R
R
R
R
S
S
S
S
S
R
R
S
S
S
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
S
N
S
S
R
N
N
S
S
N
R
N
R
R
R
R
R
R
R
S
S
N
S
R
S
S
R
S
S
N
OXIDIZING AGENTS
Bromine
Calcium Hypochlorite
Chlorine
Fluorine
Iodine
Ozone
Peracetic Acid
Potassium Chlorite
Postassium Permanganate
Sodium Hypochlorite
Sodium Clorate
Sulfur Trioxide
S
R
S
S
R
R
S
R
N
S
0
S
N
S
N
N
N
0
S
S
S
S
0
N
S
R
S
S
R
R
0
R
R
R
0
R
0
0
0
• o
0
0
S
S
0
0
0
N
R
R
R
R
R
S
R
R
N
R
S
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
N
R
R
R
R
R
R
R
R
S
S
S
N
R
0
0
S
S
S
R
N
N
N
N
N
N
N
N
N
N
N
N
N
     1-13

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                       BAGHOUSE SIZING

     Baghouses do not operate according to fractional efficiency
curves.  In other words, a baghouse cannot be designed to yield
a given collection efficiency no matter how much information is
available on the process gas flow, particle concentration, par-
ticle size distribution, etc.  A designer of a baghouse system
knows that the collection efficiency of the baghouse will be
very high (and the outlet concentration to atmosphere will be
very low) but he has no idea precisely what it will be.

     Baghouse systems are designed from experience on the same
or similar applications.Most often the choice of what parti-
cular type of baghouse to use (i.e., inside vs. outside;  shake
vs. reverse air vs. pulse;  etc.) is based on one of the fol-
lowing:

          a)  The manufacture only builds one kind.
          b)  Although the manufacturer builds several types,
              he has had good results with a particular type
              (conversely, he may have had terrible results
               with a particular type and will never design
               it into a system again on this application).
          c)  The customer specifies a particular type be-
              cause either he personally has had good results
              with it or because his industry has, more or
              less, standardized on it.

     The fundamental design parameter around which the baghouse
is sized is the air-to-cloth ratio.  Size is related to air-to-
cloth ratio by the following simple equation.
     NBS =
                                                        (1)
            AC

where :
     NBS = Net baghouse size - square feet of filter area.
       Q = Gas flow rate through the baghouse - actual cubic
           feet of gas per minute.
      AC = Air-to-cloth-ratio - feet per minute.

     The actual baghouse is usually larger than the calculated
net for compartmentized baghouses because an extra compartment
must be added for cleaning and often an extra is also added for
on-line maintenance.  The gas flow rate is usually calculated or
measured and the air-to-cloth ratio is selected by the designer
and depends on the type of baghouse and the application.  Air-
to-cloth ratio is never calculated, it is selected.  The air-
to-cloth ratio range commonly used for the various types of
baghouses is given below:


                              1-14

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     1)  Outside bag collectors using felted bags -  (6 to 12)
     2)  Outside bag collectors using woven bags -  (approx. 4)
     3)  Shaker -  (2 to 3.5)
     4)  Reverse air -  (1.5 to 2.5)
     5)  Combinations of above vary widely.

                          OPERATION

     The two most  important operating parameters for any bag-
house are the emission level and the pressure drop.

1.  EMISSIONS.  A major operating parameter of any baghouse is
that of the emission level.  Either the system meets emission
standards or it doesn't.  Generally, a properly operating bag-
house, under most conditions, should have no visible emissions
(with the exception of steam).  Excessive emissions will be
caused anytime the integrity of the interface between the clean
and dirty air plenum is broken.  In other words, damaged or
worn out bags, or broken welds or other structural damage to
the tubesheet, cell plate or housing.

2.  PRESSURE DROP.  This is the resistance the gas stream en-
counters as it flows through the baghouse.  The greater the
resistance, the more power is required to force the air through
the system.  If resistance is too great, there will not be
enough power to move the air and ventilation at the source will
not be adequate.  Each baghouse is designed to operate at a
certain pressure drop or range of pressure drops.  Pressure
drop is usually measured in inches of water.  The correct pres-
sure drop is whatever it was designed to be;  in some cases this
may be 2" and in others 10" - 12".  During normal operation, if
the baghouse is functioning properly, the pressure drop should
remain fairly steady.  If it begins to rise or drop suddenly,
or gradually creeps up during the day, that is an indication of
a problem.

Competent operation of a baghouse will,  coupled with good main-
tenance, insure low emissions at an acceptable pressure drop
(assuming an adequate system design).  Competent operation means
proper initial start-up, proper routine start-up, and shut-down,
and proper monitoring.

Initial Start-up

When a baghouse system is first installed, it is essential to
conduct a pre-start-up inspection, not only of the baghouse it-
self but of the entire system, including such items as motor
rotation, electrical functions, calibration of all indicating
equipment,  etc.  This procedure should be followed even if the
unit was factory assembled.


                            1-15

-------
      a)   Inspect the baghouse interior  for debris.  Pieces of
metal, nuts, bolts, welding rod pieces, etc., can destroy bags,
screw conveyors and even  fan blades.
      b)   Inspect baghouse interior  for  leaks.  A baghouse should
be  air tight.  Even pinpoint holes  can  cause future problems.
An  easy method for checking leaks,  before initial start-up,
is  to go  inside the baghouse, close the doors, extinguish the
lights and  look for "stars".   (NOTE:  Always follow recommended
safety procedures when entering a baghouse).
      c)   Check bolted together sections to be sure all bolts
are in place and tightened down.
      d)   Check bag connections for  tight clamps.
      e)   Check bag tensioning on inside collector systems.
      f)   Check gaskets on all doors.
      g)   Adjust cleaning  mechanism  as recommended by manufac-
turer.

Routine Start-up and Shut-down

On  any process other than the most  simple dust collection opera-
tions, a  specific start-up procedure should be provided by the
manufacturer.  If hot, moist gases  are  to be filtered, pre-
heating will raise the interior temperature above the dewpoint
and reduce  chances for destructive  condensation.  A baghouse
can be pre-heated either  by heaters in  each compartment or by
recirculating gas through the system and heating it, or by
starting  a  process on some fuel that will not cause dust or
acid dewpoint  (such as gas or oil firing a boiler before switch-
ing over  to coal firing).  Pre-heating will vary with the appli-
cation as to both time and temperature  necessary to avoid con-
densation. «'

After routine shutdown, keep the fan running long enough after
the process has been shut down to purge corrosive gases from
the system.  Also, it may be important  to run the cleaning cycle
long enough to clean most of the dust from the fabric so that
you start with clean bags  the next  shift.  It may also be im-
portant not to do this so you start with an adequate filter cake.

Routine Monitoring

As  mentioned before, the  two major  indicators of a baghouse's
performance are its collection efficiency and its pressure drop.
As  long as pressure drop  is satisfactory, the proper amount of
air  is moving through the  baghouse;  and as long as the air is
clean, the  system is doing what it  is designed to do.  Simply
speaking, someone needs to visually check the baghouse's pres-
sure gauge and stack at regular intervals.Continuous stack
monitors and pressure drop recorders can be used where necessary.
Careful monitoring and reporting on a dust collector is not con-
sidered as essential as it is on a process baghouse.  But the
operator should monitor and keep records on any system as if he

                              1-16

-------
were sure that one day he would have a major problem and will
have to call the baghouse manufacturer for assistance.  If the
manufacturer has no idea of the history of the system, it is
not uncommon to have to rebag the entire unit and then watch
it fail again simply to monitor what happens as it fails so the
problem can be corrected.  Routine inspection, therefore, should
always involve routine recording and accurate record keeping.
Special monitoring may be necessary in certain high tempera-
ture applications.  There are heat sensitive materials, for
example, which can be placed on bags and which will change
color if the bag is exposed to excessive temperatures.

One good monitoring technique is to draw up a gridwork of the
baghouse and plot bag failures on this grid.  It should also
be noted where, on the bags, failure usually occurs, such as
top, bottom, or middle, and the nature of the failure, whether
it is, say, a burn, a hole, or a rip.  Typical monitoring and
indicating devices include:

     --Pilot lights to show that the baghouse is operating
       properly.   Pilot lights show what motors are opera-
       ting, which compartments are off or on-line, which
       rows of bags are being pulsed, frequency of pulsing,
       etc.

     --Opacity meters which can show even a slight drop
       in filtering efficiency which would not be detect-
       able by the human eye.

     --Pressure drop indicators such as magnehelic gauges
       or manometers to show any change in pressure drop
       during operation.  Recorders may also be used for
       a permanent record of pressure drop.

     --Temperature indicators and/or records to show
       when meximum operating temperatures are reached.

     --Gas flow meters indicate the amount of air
       moving through the system.

     --Corrosion chips can be placed at strategic
       points in the dirty air stream.   These should
       be made of the same metal as the baghouse and
       should be inspected and measured regularly to
       determine if corrosion is a serious problem.

                         MAINTENANCE

     Here we are concerned with maintenance of the baghouse and
several important system components.   Bag maintenance, per se,
is discussed later.

-------
 Routine  Maintenance

 The key  to  a  good  maintenance  program  is  record keeping.  The
 hallmark of a good routine maintenance program is no breakdowns.

      1)   Establish proper inspection intervals for all moving
 mechanical  assemblies  (and bags) based on manufacturer's
 recommendations.   A  typical  scheme  is  listed  in Table 2.

      2)   Keep a record of all  inspections and maintenance work
 performed.  A typical speciman is shown in Figure 4.

 Troubleshooting

 The key  to  good troubleshooting is  smart  maintenance men and
 adequate spare parts on hand.   The  hallmark of good trouble-
 shooting is getting  back in  production fast after a breakdown.
 A  list of suggested  spare parts is  given  in Table 3 and a
 troubleshooting guide is shown in Table 4.

                       BAG MAINTENANCE

      Bag maintenance is divided into two  distinct categories:
 1)  location and repair of individual bag  failures, and 2) com-
 plete bag changeout  of an entire unit.

 Location and  Repair  of Individual Failures

 On  a  process  baghouse, individual repair  requires the most
 amount of time.  In  an ideal baghouse, all bags would last their
 required time and  then all would fail  on  the  same day.  This,
 of  course,  never happens (unless there is a fire in the bag-
 house) .   There are variances in fabric quality, in bag manu-
 facturing techniques, tolerances, gas  flow distribution inside
 the baghouse,   areas  of greater  and  lesser condensation pro-
 blems, variations  in the bag cleaning  mechanism, etc.  Any one
 or  any combination of these and other  factors can cause bags to
 fail  prematurely.

 It  is typical  to find any baghouse  experiencing some small num-
 ber of failures during the first month or two of operation.
 These failures generally are due to manufacturing or installa-
 tion  defects.   Later on there will be  a period of few bag fail-
 ures  until you begin to reach the end  of  the operating life
 of a  set of bags.  Then occasional  failures will show up, fol-
 lowed by an increasing rate of  failure to the point where it
becomes expeditious to change out all of  the bags.

The first indication that bag failures have occurred can come
through routine inspection or by noticing a visible emission
                               1-18

-------
                           TABLE  2
                    PERIODIC MAINTENANCE
Daily

1.  Check pressure drop.

2.  Observe stack  (visual or with opacity meter).

3.  Walk through system listening for proper operation  (audible
    leaks, proper fan and motor  functions, bag cleaning  systetn,
    etc.).

4.  Note any unusual occurrence  in the process being ventilated.

5.  Observe all indicators on control panel.

6.  Check compressed air pressure.

7.  Assure that dust is being removed from system.

Weekly

1.  Inspect §iscrew conveyor bearings for lubrication  (do  not
    lubricate).

2.  Check packing glands.

3.  Operate all damper valves (isolation,^ by-pass, etc.).

4.  Check compressed air lines,  including line oilers and filters

5.  Check bag cleaning sequence  to see that all valves are
    opening and closing porperly.

6.  (Inside bag collectors)  Spot check bag tension.

7.  (High temperature applications)  Verify accuracy of  tempera-
     ture indicating equipment.

8.  Check pressure drop indicating equipment for plugged lines.

Monthly

1.  (Shaker)   Check all shaker mechanism moving parts.

2.  Inspect fans(s) for corrosion and material buidl-up.

3.  Check all drive belts and chains for wear and tension.

6.  Check all hoses and clamps.

                              1-19

-------
                     TABLE 2  (continued)



7.  Check accuracy of all indicating equipment.



8.  Inspect housing for corrosion.



Quarterly



1.  Inspect baffle plate for wear.



2.  Thoroughly inspect bags.



3.  Check duct for dust build-up.



4.  Observe damper valves for proper seating.



5.  Check gaskets on all doors.



6.  Inspect paint.



7.  Check screw conveyor flighting.



Annually



1.  Check all bolts.



2.  Check welds.



3.  Inspect hopper for wear.
                             1-20

-------
                                                    Figure
H'
I
to
       Company
             Check
                            Needs
                      Good  Attn.
Serial No.
                            (
Structural-Bolts
Ladder Assembly
Airlock
Drive Assembly
A. Gear Reducer
   Drive Shaft Align (
   Coupler Shaft    (
   Bearings        (
   Belts            (
   Sheaves         (
   Serial No. (Motor)
   H.P.
B.
C.
D.
E.
F.
G.
II.
I.
3.
Transfer Screw^ssy.(
Fan
A. Serial No.	
B. Model No.
             R.P.M.	
             Sheave Size
    D. Make _
    E. Sheaves         (  )
    F. Sheave Diameter (  )
    G. Shaft Diameter  (  )
    H. Series of Belts
          1.  Shaft Diameter	
          J.  Make	
   .  (  ) Water Trap        (   )
   .  (  ) Air Regulator      (   )
   .  (  ) Bin Indicator       (   )
   .  (  ) Magnehelic        (   )
   .  (  ) Magnehelic Tubing  (   )
   .  (  ) Baffle Wear        (   )
                                  (  )
                                  (  )
                                  (  )
Item

 15. (  )

 16. (  )
 17. (  )

 18. (  )
 19. (  )

 20. (  )

 21. (  )

 22. (  )
 23. (  )
 2*. (  )
 25. (  )
                                     26.
                                     27.
                                     28.
                                     29.
                                     30.
                                              Baghouse Inspection Report
                                                   Application	
                                                        Date
                                                       Name
        Check

 Top Door Hold Down
   Straps
 Top Door Leaks
 Manifold Pipes
   Anchored
 Manifold Pipes Holes
 Manifold Pipes Center
   Over Venturi
 Venturi Properly
   Seated
 Cages Properly
  Installed
 Bag Clamps „
 Bags-Visolite
 Bags-General Appear.
 Service Module
 A.  Wire Connections
       Terminal Box
 B.  Wire Connections

 C.  Diaphram Valves-
       Leaks
 D.  Solenoid Valves
       Operating
 E.  Hoses 
-------
                           TABLE 3
                    SUGGESTED SPARE PARTS
Following is a list of spare parts that should be kept on hand.
Quantities of parts will vary as to manufacturer's suggestion
and the type of process.

          - Bags

          - Bag support cages (reverse pulse and plenum pulse)

          - Bag clamps

          - Seals and caulking material

          - Solenoids

          - Diaphragms

          - Timer components

          - Baffle plates or wear plate sections for baffle

          - Bag connecting rods (shaker and reverse flow)

          - Tensioning springs (reverse flow)

          - Belts for shaker mechanism (shaker)

          - Motor for shaker mechanism (shaker)

          - Fan belts

          - Spare bearings and gasketing for all mechanical
            components
                             1-22

-------
                                TABLE 4
                         TROUBLESHOOTING GUIDE
The following chart lists the most common problems which may be found in a
baghouse air pollution control system and offers general solutions to the
problems.  There are a number of instances in which the solution is to
consult the manufacturer.  This may not be necessary in plants that have
sufficient engineering know-how available.

Where the information applies to a specific type of baghouse, the following
code is used:

                    RP	Reverse Pulse
                    PP	Plenum Pulse
                     S	Shaker
                    RF	Reverse Flow
Symptom
     Cause
     Remedy
High Baghouse
Pressure Drop
Baghouse undersized
                         Bag cleaning mechanism
                          not adjusted properly
                         Compressed air pressure
                          too low (RP.PP)
                         Repressuring pressure
                          too low (RF)
                         Shaking not strong
                          enough (S)

                         Isolation damper valves
                          not closing S,RF,PP)
                                    1-23
Consult manufacturer
Install double bags
Add more compartments
 or modules

Increase cleaning
 frequency
Clean for longer
 duration
Clean more virorously

Increase pressure
Decrease duration and/
 or frequency
Check dryer and clean
 if necessary
Check for obstruction
 in piping

Speed up repressuring
 fan
Check for leaks
Check damper valve seals

Speed up shaker speed
                              Check linkage
                              Check seals
                              Check air supply on
                               pneumatic operators

-------
                          TABLE 4 (continued)
Symptom
     Cause
     Remedy
Low Fan Motor
Amperage/Low
Air Volume
                         Bag tension too loose (S)

                         Pulsing valves failed (RP)


                         Cleaning timer failure
                         Not capable of removing
                          dust from bags
                         Excessive re-entrainment
                          of dust
                         Incorrect pressure reading
High baghouse pressure
 drop
                         Fan and motor sheaves
                          reverse

                         Ducts plugged with dust
                         Fan damper closed
Tighten bags

Check diaphragm
Check pilot valves

Check to see if timer
 is indexing to all
 contacts
Check output on all
 terminals

Condensation on bags
 (see below)
Send sample of du3t
 to manufacturer
Send bag to lab for
 analysis for blinding
Dry clean or replace
 bags
Reduce air flow

Continuously empty
 hopper
Clean rows of bags
 randomly, instead of
 sequentially (PP,RP)

Clean out pressure taps
Check hoses for leaks
Check for proper fluid
 in manometer
Check diaphragm in gauge

See above
                              Check drawings and re-
                               verse sheaves

                              Clean out ducts and
                               check duct velocities

                              Open damper and lock
                               in position
                                   1-24

-------
                          TABLE A (continued)
Symptom
      Cause
     Remedy
                      System static pressure
                       too high
Dust Escaping At
Source
                      Fan not operating per
                       design
Belts slipping

Low air volume


Ducts leaking


Improper hood design
Dirty Discharge
At Stack
Bags leaking
                      Bags clamps not sealing
                      Failure of seals in
                       joints at clean/dirty air
                       connection

                      Insufficient filter cake
Measure static on both
 sides of fan and re-
 view with design
Duct velocity too high
Duct design not proper

Check fan inlet configu-
 ration and be sure even
 air flow exists

Check tension and adjust

See above
Patch leaks so air does
 not by-pass source

Close open areas around
 dust source
Check for cross drafts
 that overcome suction
Check for dust being
 thrown away from hood
 by belt, etc.

Replace bags
Tie off bags and replace
 at later date
Isolate leaking compart-
 ment if allowable with-
 out upsetting system

Check and tighten clamps
Smooth out cloth under
 clamp and re-clamp

Caulk or weld seams
                              Allow more dust to build
                               up on bags by cleaning
                               less frequently
                              Use a pre-coating of dust
                               on bags (S,RF)
                                   1-25

-------
                          TABLE A (continued)
Symptom
      Cause
    Remedy
                      Bags too porous
                              Send bag in for permea-
                               bility test and review
                               with manufacturer
Excessive Fan
Wear
Excessive Fan
High Compressed
Air Consumption
(RP.PP)
Fan handling too much
 dust

Improper fan
Fan speed too high

Build-up of dust on
 blades
                      Wrong fan wheel for
                       application

                      Sheaves not balanced
Bearings worn

Cleaning cycle too
 frequent
                      Pulse too long
                      Pressure too high
                      Damper valves not
                       sealing (PP)

                      Diaphragm valve
                       failure

                                   1-26
See above
Check with fan manufac-
 turer to see if fan is
 correct for application

Check with manufacturer

Clean off and check to
 see if fan is handling
 too much dust (see above)
Do not allow any water in
 fan (check cap, look for
 condensation, etc.)

Check with manufacturer
Have sheaves dynamically
 balanced

Replace bearings

Reduce cleaning cycle
 if possible
                              Reduce duration (after
                               initial shock all other
                               compressed air is wasted)

                              Reduce supply pressure
                               if possible

                              Check linkage
                              Check seals

                              Check diaphragms and
                               springs
                              Check pilot valve

-------
                          TABLE k (continued)
Symptom
      Cause
   Remedy
Reduced Compressed    Compressed air consump-
Air Pressure (RP,PP)   tion too high

                      Restrictions in piping

                      Dryer plugged


                      Supply line too small

                      Compressor worn
Premature Bag
Failure -
Decomposition
Moisture in
Baghouse
Bag material improper
 for chemical composition
 of gas or dust
Operating below acid
 dew-point

Insufficient pre-heating
                      System not purged after
                       shut-down
                      Wall temperature below
                       dew-point
                      Cold spots through
                       insulation

                      Compressed air intro-
                       ducing water (RP.PP)
                      Repressuring air
                       causing condensation
                       (RF.PP)
                              See above
Check piping

Replace dessicant or
 by-pass dryer if allowed

Consult design

Replace rings

Analyze gas and dust and
 check with manufacturer
Treat with neutralizer
 before baghouse

Increase gas temperature
By-pass and start-up

Run system with hot air
 only before starting
 process gas flow

Keep fan running for
 5-10 minutes after
 process is shut down

Raise gas temperature
Insulate unit
Lower dew-point by keeping
 moisture out of system

Eliminate direct metal
 lines through insulation

Check automatic drains
Install aftercooler
Install dryer

Pre-heat repressuring air
Use process gas as source
 of repressuring air
                                   1-27

-------
                          TABLE 4 (continued)
Symptom
      Cause
   Remedy
High Screw
Conveyor Wear
High Airlock Wear
Material Bridging
in Hopper
Frequent Screw
Conveyor/Airlock
Failure
High Pneumatic
Conveyor Wear
Screw conveyor under-
 sized
Conveyor speed too high

Airlock undersized



Thermal expansion



Speed too high

Moisture in baghouse


Dust being stored in
 hopper

Hopper slope insufficient

Conveyor opening too small

Equipment undersized



Screw conveyor misaligned

Overloading components
Pneumatic blower to fast
                      Piping undersized
Measure hourly collection
 of dust and consult
 manufacturer

Slow down speek

Measure hourly collection
 of dust and consult
 manufacturer

Consult manufacturer to
 see if design allowed
 for thermal expansion

Slow down

See above
Remove dust continuously


Re-work or replace hoppers

Use a wide flared trough

Consult manufacturer
Align conveyor

Check sizing to see that
 each component is capable
 of handling a 100% de-
 livery from the previous
 item

Slow down blower
                              Review design and slow
                               blower or increase pipe
                               size
                                   1-28

-------
                          TABLE 4 (continued)
Symptom
      Cause
   Remedy
Pneumatic
Conveyor Pipes
Plugging
Fan Motor
Overloading
Air Volume Too
High
Reduced Compressed
Air Consumption
(RP.PP)
High Bag Failure-
Wearing Out
                      Elbows too short radius
Overloading pneumatic
 conveyor
Slug loading of dust

Moisture in dust

Air volume too high
                      Motor not sized for
                       cold start
Ducts leaking


Insufficient static
 pressure

Pulsing valves not
 working


Timer failed

Baffle plate worn out


Too much dust

Cleaning cycle too
 frequent

Inlet air not properly
 baffled from bags

Shaking too violent (S)


              1-29
Replace with long radius
 elbows

Review design
Meter dust in gradually

See above

See below
Dampen fan at start-up
Reduce fan speed
Provide heat faster
Replace motor

Patch leaks
Close damper valve
Slow down fan

Check diaphragms
Check springs
Check pilot valves

Check terminal outputs

Replace baffle plate


Install primary collector

Slow down cleaning


Consult manufacturer
                                                    Slow down shaking
                                                     mechanism

-------
                          TABLE A (continued)
Symptom
      Cause
   Remedy
High Bag Failure-
Burning
Repressuring pressure
 too high (RF)

Pulsing pressure too
 high (RP.PP)

Cages have barbs
 (RP.PP)

Stratification of hot
 and cold gases

Sparks entering baghouse

Thermocouple failed


Failure of cooling device
                                                    Reduce pressure
                                                    Reduce pressure
Remove and smooth out
 barbs

Force turbulence in duct
 with baffles

Install spark arrestor

Replace and determine
 cause of failure

Review design and work
 with manufacturer
                                   1-30

-------
or an increased reading on a stack monitoring device.  At this
time, it is necessary to inspect for bag failure.

In a compartmented baghouse, it is possible to monitor the
stack while isolating one compartment at a time.  Through this
technique, any compartment which has experienced significant
bag failures can be discovered easily because stack emissions
will be reduced when the compartment in question is turned off.

In non-compartmented baghouses, or if no single compartment is
responsible for the majority of emissions, it may be necessary
to check the entire unit for failed bags.  There are no dif-
ferent approaches used to locate failed bags - the major dif-
ference being in whether it is necessary to enter the unit to
look for failed bags.

There are three basic ways to search for leaking bags:  1) hunt
for the hole itself, 2) hunt for an accumulation of dust which
can be related to a nearby hole, and 3) use some detecting
device.

Inside collectors experience their most frequent bag failure
near the bottoms of the bags.  If this is the case, an accumu-
lation of dust on the cell plate or near the failure sometimes
will be visible and relatively easy to spot, provided the fail-
ure is near the walkway.  If the failure is higher up on the
bag, it will be more difficult to find because dust will be dis-
tributed over a wide area of the cell plate.  In this case, it
is necessary to inspect the entire circumference and length of
every bag.

With top bag access on outside collectors, it is normally very
difficult to see the exact failure;  however, in many applica-
tions a dust accumulation on the tubesheet or on the blow pipe
running over a failed bag will be readily noticeable.

The difficulty in finding a failed bag will vary with the ap-
plication and type of material being filtered.

One of the newest and most effective techniques that has been
found to locate failed bags is to inject a quantity of flore-
scent or phosphorescent 'dust into the baghouse and then inspect
the clean air plenum with a black light.   The dust from even
very small leaks is easily visible as it glows under the black
light.

On inside collectors it is necessary to scan the entire length
of the bag with the black light to pinpoint the failure.  On
outside collection units the florescent powder will be drawn
through the hole in the bag and will be easily visible either on
the venturi or around the blow pipe over the venturi.


                             1-31

-------
Use of this florescent powder technique is also effective in
spotting broken welds or other leaks in the baghouse tubesheet,
cell plate or housing.

In the past it was considered good maintenance procedure to
immediately replace a failed bag with a new one.  Recently,
however/ it has been found that a new bag in the vicinity of
old ones will be forced to take a higher percentage of the
dust loading because the resistance is lower than that of the
"seasoned" bags.  It was not uncommon to replace a bag one
month and then the next month to find the very same bag once
again needing replacement.

Also, individual one at a time bag replacement is quite costly
in man-hours on a per bag basis.  More recently, it has become
an accepted technique to simply seal off the failed bag until
a significant number of bags have been sealed that the pressure
drop increase, at which time all the sealed off bags are re-
placed simultaneously.  In most cases, 2% to 3% of the bags
(sometimes even up to 10% or more) can be plugged before no-
ticing a change in pressure drop.

On inside collectors if the failure has not occurred too close
to the cell plate, the bad bag can be cut, tied in a knot and
stuffed inside the cell plate.  If the failure is too close to
the cell plate, it is advisable to plug the hole.  In top ac-
cess outside collection baghouses, a plug is simply placed
over the failed bag.

There are commercially available plugs, although a plate of
steel with gasketing material will also work.  The gasketing
material should be made out of the same type of material as the
bag.  When using homemade plugs, it should be noted that they
must be heavy enough to resist the pressure differential,
otherwise the seal will be knocked out of place.

It is important on inside collectors to replace a plug failed
bags as soon as possible because of the "domino effect"in which
a hole in one bag will cause one to appear in an adjacent bag
and then that bag will cause a hole to appear in another bag
and so forth.                                                   >

It is important to keep track of the rate of failure of indivi-
dual bags.   As the rate of failure begins to rise, it is possible
to predict the end of bag life and schedule complete changeout
of all bags at a convenient time.  This is less costly than
changing individual bags, say month by month.

Complete Bag Changeout

1.  Baglife


                             lr-32

-------
Depending upon a variety of factors, sooner or later, probably
within two to five years, the original bags will wear out.  At
that time a complete changeout is required.

If inadequate bag life is obtained through no fault of the opera-
tor, it may be possible to charge the manufacturer due to
either an express or implied warranty.

     a)  Express warranty.  Several years ago baghouse manu-
     facturers uniformly refused to give express bag life
     warranties.  Now the opposite is true and virtually all
     manufacturers will give at least a pro-rated warranty.

     b)  The UCC, which is law in every state (with the pos-
     sible exception of Louisiana) will give the operator
     the benefit of the implied warranties of "merchantibility"
     and "fitness for a particular purpose" unless they are
     specifically disclaimed by the manufacturer in writing.
     "Merchantibility" means that the baghouse is of good
     quality and "fitness for a particular purpose" means
     that if the operator makes his intended use of the
     baghouse known to the manufacturer and relies on his
     expertise,  the manufacturer must supply a baghouse
     "fit" for that "purpose".

2.  Changeout

When bag replacement is necessary, first de-bag the collector
and save all the useable parts such as cages, Venturis, and
clamps.  It may be desirable to vacuum out the baghouse to
provide a more comfortable and safer working environment.  Care
should be taken when installing new bags to avoid tears, punc-
tures and rips in the bags.  Also, all clamps, seals, gaskets,
etc., should be checked after rebagging.  When putting bags
on cages, the bags should not be twisted, as damage could occur.

If bolt-on clamps are used to hold bags in place, it should be
noted that pneumatic wrenches are available in the $100 to $200
price range.   One pneumatic wrench will pay for itself not only
in time saved during the first bag changeout, but in assuring
that all clamps are properly tightened down.

There are new mechanized bag changeout procedures today whereby
an entire compartment of bags can be withdrawn all at once and
a new pre-bagged cartridge installed through the use of a crane,
usually mounted on top of the system.   The old cartridge is re-
moved to a remote location where it can later be re-bagged at
a convenient scheduled time and held in readiness for use on
another compartment of the system as the need arises.

                            COST

     What is the total cost of owning,  operating, and maintaining

                             1-33

-------
 a baghouse system?  Figure  5 attempts to give a breakdown of an
 answer to this question that would satisfy both an engineer and
 an accountant  (if such a thing  is possible).

     Some equations are presented below that predict  (probably
 within plus or minus 25%) the annual cost of operating and main-
 taining a typical large baghouse system using a reverse flow
 baghouse.  The annual operating cost a reverse flow baghouse
 is composed of:

     1.  Main fan power cost.
     2.  Reverse flow fan power cost.
     3.  Dust handling system power.
     4.  Direct operating labor.

 ID Fan Power Cost

 Te ID fan power cost for a  baghouse is given by Equation 2.

     FPC  = l 17 . 10"4 . Q . AP . AH . PC              (2)
                              Ef

 where:

     FPCB = Fan power cost  attributable to baghouse,  1978 dol-
            lars/year,
        Q = Gas, flow rate, acfm,
       AP = Operating pressure drop, kn. (probably about 6"
            W.G.),
       AH = Annual hours of operation, hr/year
       PC = Local cost of electrical power, dollars/kWh
            (see Table 3),  and
       Ef = Efficiency of ID fan, fraction,
          = 0.6 approximately.

 Reverse Flow Fan Power Cost

 The annual cost incurred to clean bags via the reverse flow
 fan is given by Equation 3.

            ,  , . 10~4 .  GCA . AP . AH . PC             (3)
                              (NCN + 2)

where:

     BCRR = Annual bag cleaning cost for reverse flow baghouse,
            1978 dollars/year,
      GCA = Gross cloth area,  f t.2,
       AP = Operating pressure drop, in WG (probably about
            6" W.G.),
       AH = Annual operating hours, hr/year,
       PC = Local power cost,  1978 dollars/kWh, and

                            1-34

-------
 I
(-0
                              Operation
         Material
Utility
Consumption
                                                    Total Use and Ownership Cost
                                     Maintenance
Material
Consumption
                                                             Capital
                                                             Charges
                                            Depreciation
                                  Financial Charges
                                                   Investment
                                                   Tax Credit
                                                                             Miscellaneous
                                                                             Charges
                                       Taxes,
                                       Insurance,
                                       Contingency
Overhead
Absorption
                                                   Figure  J>-  Cost Breakdown

-------
      NCN = Net number °f baghouse compartments  (total  number
            of compartments less one for cleaning and one  for
            maintenance) .

Dust Handling System Power

The annual power  cost to run the dust handling system is given
by Equation 4.

     ASHp = 0.435 ' PC  * DB • AH                        (4)

where :

     ASHp = Annual power cost to run system,  1978 dollars/year,
       PC = Local power  cost, 1978 dollars/kWh,
       DB = Dust  burden, ton/hr, and
       AH = Annual hours of operation, hrs/year.

Annual Operating  Labor

The annual labor  and maintenance cost, other  than bag replace-
ment cost, is given by Equation 5.

     OLRB = AWF  (0.249 GCA° * 966+0 . 089 GCA+5.850)        (5)

where:

     OLRjj = Annual direct operating labor, 1978 dollars/year,
      AWF = Area  wage factor (see Table 3), unitless, and
      GCA = Gross cloth  area, ft2.

Maintenance Costs

The cost of maintaining  a baghouse is divided into:

     1.  Direct labor (other than replacing bags) .
     2.  Parts and materials (other than replacement bags).
     3.  Direct labor to replace bags.
     4.  Bag cost.

Direct Labor (other than replacing bags)

     MLB = 0.04 • GCA •  AWF                             (6)

where:
         = Annual direct maintenance labor  (other than bag re-
           placement) , 1978 dollars/year,
     GCA = Gross cloth area, f t2 f ancj
     AWF = Area wage factor, unitless.
                               1-36

-------
Parts and Materials  (other than replacement bags)

     MMB = 0.04 • GCA                                   (7)

where:               ,

     MMB = Baghouse annual maintenance material cost, 1978
           dollars/year  (other than bags) , and
     GCA = Gross cloth area, ft2.

Bag Replacement

Bags represent a substantial portion of the total baghouse
system investment and they must be replaced periodically as
they wear out.  This section deals with the cost of bag re-
placement and focuses on:

     1.  Material cost, i.e., the cost of the replacement bags
    themsleves.

     2.  Labor cost, i.e., the direct labor necessary to install
         the replacement bags.

     3.  Frequency, i.e., the number of times the bags must be
         replaced over 30 years or, in other words, in-service
         bag life.

Material Cost

The cost of a set of replacement bags is given by Equation 8.

     RBCR = 0.78 • GCA                                  (8)

where:
          = The cost of a set of replacement bags, 1978 dol-
            lars/year, and
      GCA = Gross cloth area, ft2.

Equation 8 is based on one foot diameter by 35 ft. long fiber-
glass bags.  The equation can be used for other bag materials
by biasing with information contained in Table 5.

Labor
                                      t
The labor required to change a set of bags in a reverse flow
baghouse is given by Equation 9.

     BRLR = 0.021 • GCA • AWF                          (9)

where:

                              1^37

-------
BRLR = Bag replacement labor, 1978 dollars,
 GCA = Gross cloth area, ft2, and
 AWF = Area wage factor, unitless.
                     1-38

-------
                          TABLE 5
            APPROXIMATE COST OF REPLACEMENT BAGS
                                              1977 Cost
          Material                            (dollars)


Nylon (5.3 oz/yd2)                            0.64/ft2

Sewn in ring                                  2.00 each

Polyester (7 oz/yd2)                          0.31/ft2

Sewn in ring                                  1.30 each

Fiberglass (silicon/graphite finish;          0.26/ft2
 9 oz/yd2)

Sewn in ring                                  1>25 each

Fiberglass (10% PTFE finish;  9 oz/yd2)       0.42/ft2


Sewn in ring                                  1.50 each

Top caps  (mild steel, 12 in. dia.)            2.80 each

Stainless steel clamps                        1.75 each

-------
           TABLE 6
COSTS THAT VARY WITH LOCATION

State
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada .
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
$
kWh
0.0322
0.0354
0.0451
0.0315
0.0371
0.0346
0.0450
0.0529
0.0433
0.0457
0.0489
0.0232
0.0460
0.0320
0.0435
0.0382
0.0281
0.0356
0.0335
0.0470
0.0479
0.0475
0.0349
0.0445
0.0381
0.0194
0.0297
0.0330
0.0387
0.0564
0.0366
0.0783
0.0326
0.0371
0.0435
0.0314
0.0209
0.0461
0.0458
0.0332
0.0326
0.0270
Area
Wage
Factor
0.89
1.79
1.00
0.78
1.06
1.03
1.01
1.11
0.84
0.81
1.00
1.03
1.12
1.19
1.15
0.96
1.01
1.06
0.81
1.08
0.93
1.33
1..07
0.76
1.02
1.27
0.97
1.05
0.82
1.04
0.80
1.02
. 0.74
0.92
1.20
0.95
1.13
1.05
0.80
0.77
0.87
0.85
          1-40

-------
                     TABLE 6  (continued)


State

$
kWh
Area
Wage
Factor
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyoming
0.0364
0.0277
0.0362
0.0432
0.0143
0.0342
0.0358
0.0221
 0.99
 0.93
 0.84
 0.85
N/AVAIL
 1.10
 1.11
 1.03
                           1-41

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                     VI-2.

                  BAGHOUSES
              WHAT TO KNOW BEFORE
                   YOU BUY
                  S. A. Reigel
                  R. P. Bundy
                  C. D. Coyle

                  Standard Havens, Inc.
Copyright (c) 1973 by Pollution Engineering.
Reprinted with permission from the May, 1973
issue.
                      2-1

-------
By S. A. REIGEL, R. P. BUNDY and C. D. DOYLE, Standard Havens Inc., Glasgow, Mo.
With (he advent of stricter emission standards designed!
to produce the national quality standard  oil 75pg/m'
of paniculate, only the most efficient removal devices
will be suitable. The baghouse traditionally yfotds high
removal afficieraces (99.9 +  percent).
   A baghouse is  a large metal box  divided into two
functional areas. The first of these, the dirty air plenum,
may be  a part of  the baghouse proper or  it may take
the form of a distribution manifold. The function of the
dirty air plenum is to distribute the fouled gas evenly
to the filtering elements or bags. The baghouse hopper,
which  is the receptacle for collected material, is part
of the dirty air plenum. The clean air  plenum is that
part of the baghouse  where recombination of the  air
circuits from each of the individual bags takes place.
The interface separating the clean air and dirty  air
plenums is the  filtering media or bags.
   There are three important  mechanisms  involved  In
removing particles  from  a  flowing gas stream using
filter  media:  impaction,  interception,  and  diffusion.
These  three mechanisms which are responsible  for
collision of a particle on a target, are of paramount im-
portance when  the  baghouse is first brought  on  line
and the bags are clean. After a short time, however, the
bags become caked with a  layer of dust and this dust
cake actually becomes the filtering medium. No matter
how vigorously the bags are shaken,  collapsed, or
pulsed, a residual dust cake is retained after each on-
line cleaning. The bags act primarily as a  matrix to
support  the dust cake.
Baghous© Types
   All  baghouses operate in basically the  same way:
dirty gas is ducted  to the unit where it is  filtered by
cloth tubes or bags. This filtering action is extremely
efficient  and  results in virtually 100 percent  of the
entrained particulate remaining in the  unit on the bags.
The bags must be periodically purged of this collected
material. The  method  used for this  cleaning charac-
terizes one type of baghouse  from another.
   All  baghouses are either intermittent or continuous
automatic. Intermittent  baghouses are designed to be
cleaned  after the unit  has ceased filtering, say at the
end of  the  work day.  Intermittent  baghouses  cannot
be cleaned while on line, and thus are limited to low
dust loadings  or infrequent operation.  They have the
distinct  advantage  of  being  low priced.  Continuous
automatic  baghouses,  on  the other  hand, are more
expensive  but  are  able  to operate  24 hours a  day
without rest and can handle  high dust  loadings.
   Baghouses are characterized and identified accord-
ing to  the method  used to  remove collected material
from the bags.  This Is accomplished in a variety of
ways, including shaking the bags, reversing the direc-
tion  of air flow through the bags, blowing a jet of air
on the bags from a  reciprocating manifold, or rapidly
expanding the bags by a pulse of compressed air.
  The bags in shakoir-ltype baghouses are supported
by a structural  framework. The  structural  framework
is free  to oscillate  when  driven  by  a small electric
motor, Fig. 1. Periodically,  on  a timed basis, a damper
isolates a compartment of the  shaker baghouse so  that
no air flows. The bags in  that compartment are then
shaken  for approximately a minute during which time
the collected dust cake is dislodged from the bags.  The
dust falls into the hopper for subsequent removal.
  Rovers® flow baghouses  are equipped with an auxili-
ary fan that forces air through the bags in the direction
opposite to filtration. This  backwash  action collapses
the bag  and  fractures the  dustcake. When  the bag is
reinflated by being brought back  on-line, the fractured
dustcake is  dislodged  into  the   hopper.  If the  unit
operates under suction (the main  fan is located on the
"clean" side of the baghouse) reducing pressure in the
baghouse may elimate the need  for an auxiliary fan.
            ®8  baghousss  incorporate a  jet case or
manifold that surrounds each bag. The manifold travels
the length  of the bag in  a constantly repeating cycle.
As it passes over the surface of the bags, a jet of high
pressure air issues from orifices in  the  manifold  and
blows the dust cake off  the  bags.
   In recent years  ravers©  pulse  baghousas,  Fig. 2,
have  enjoyed a  rapidly  increasing  use.  This  design
utilizes  a short (usually less than  100  milliseconds)
pulse of compressed air  through a venturi, or diffuser,
directed from the top to  the bottom of the bag,  Fig. 3.
This primary pulse of air  aspirates  secondary  air as
it  passes through the  venturi. The resulting  air mass
violently expands the bag  and casts off the  collected
dust cake. A modification of this technique is the pres-
surized  plenum type of cleaning. In  this  case  an  iso-
lated  compartment above several rows of bags is sup-
plied  with   pressurized air. The  change  in  pressure
differential  across the bags when the damper is oper-
ated causes the bags to flex and cast off the dust cake.
   The fundamental criterion used in applying any bag-
house to any  application  is  the  "air-to-cloth  ratio,"
defined  as  the  ratio of actual volumetric  air  flow rate
to net cloth area.
whera: AC = Air to cloth ratio, fpm
         Q = Volumetric air flow, acfm
                  cloth area, sq. ft
                                                     2-2

-------
 " AC Is equal to  the superficial face velocity of the
air as it passes through the cloth. Shaker and reverse
air baghouses normally  operate at an alr-to-cloth ratio
of from 1  to 3, while reverse pulse baghouses operate
at about 3 to 6 times this range. Units which are clean-
ed with a compartment  off-line must be outfitted with
an extra compartment in order to keep a minimum net
cloth area on  line at all times. In extreme cases, this
can  double  the size of  the baghouse.
  A  second important operating  characteristic is the
"filter drag" given by:
        S  =
             A  P      •
              V
where:  S =  Filter drag, in.  H2O/fpm
       AP =  Pressure  drop across filter, in. H2O
        V —  Superficial face velocity, fpm
  As this equation indicates,  baghouses  do  not  obey
the fan law, which slates that the pressure drop varies
as the square of the  volumetric air flow. Virtually all
other devices  obey this  law; Figure 4 "shows the varia-
tion  of filter drag during a cycle varying from  "just
cleaned"  to "just before cleaning."
Filtering  Bags
  The most important components of any baghouses
are  the filtering elements, or bags. Perhaps  less is
known about  predicting their in-service  performance
than  about any other  component  In  general,  bags
are either woven or felted.  Woven bags  are used in
shaker and reverse air baghouses; felted bags are
used in  reverse  pulse, plenum pulse,  and  jet  case
baghouses.
  Woven  bags are usually fgrnished with a weight of
5 to  10 oz per sq yd and a permeability (acfm passing
through one sq ft of cloth with pressure differential of
0:5 in. H2O) of approximately 10  to 30 icfm. Felted
bags are heavier and  much fuzzier, weighing 10 to 20
oz per sq  yd. They also have a permeability of approxi-
mately 10 to 30 acfm.
  A  common problem with many  man-made  fibers is
their tendency to elongate under  load  and shrink at
higher temperature. This lack of dimensional stability
may cause a reduction  in cleaning efficiency or pre-
mature mechanical failure of  the  bags. To  avoid or
minimize  these problems, many fabrics are  heat-set.
Spun fabrics may also be singed or sheared for reduc-
ing the surface  hairs in order to  present a smooth
 surface to the dust cake, thus making it easier for the
 dust cake to dislodge during the cleaning cycle. Many
 other surface finishes and  mechanical operations may
 be performed on the cloth to accomplish specific aims.
 Resin  finishes  provide  a smoother surface; graphite
 finishes  may be of value  in eliminating  build-ups of
 static  charge;   napping  a  fabric  may help  improve
 collection efficiency. Resins or graphite  are  used to
 improve  abrasion resistance.

 Limitations
   Although baghouses operate at the highest collection
 efficiency level  (99.9 +  percent) all is not perfect. For
 a given application baghouses will probably be one of
 the more expensive solutions and will probably require
 the most space for installation  (see  Table). The cost
 of maintaining such facilities as settling ponds or pro-
 viding chemical  additives in wet systems may, however,
 offset  this price disadvantage. Baghouses will gener-
 ally require much less power to achieve high efficiency
 operation than water scrubbers and, of course,  have
 no water requirements.
   The highest maintenance component of a baghouse
 is the bags. The bags represent 20 to  40 percent of the
 equipment cost  and probably have an average life of
 18 to 36 months. This means that the unit will be re-
 bagged from three  to seven times assuming a 10 year
 amortization  of the unit.
                    i
.   There  are  several causes for bag  failure—blinding,
 caking, burning, abrasion, chemical attack, and aging.
 The circumstances  that can cause the above to occur
 are varied—some due to normal operation, and some
 due to misapplication or improper operation. Blinding
 occurs when dust is captured within  the bag  material
 and the  cleaning mechanism is unable to  dislodge it.
 Either the cleaning .mechanism is not  powerful  enough,
 or the nature of the  dust is such that it can readily enter
 the fabric. Obviously blinding  is the result of either
 misoperation (the cleaning mechanism is not powerful
 enough)  or misapplication (the  nature of the dust is at
 fault).
   Caking is the  formation of a solid mat of self-adher-
 ing dust  on the  dirty air side of the bag which cannot
 be removed by the  normal cleaning mechanism of the
 dust collector. The most common cause  of caking is
 the presence of  water droplets in the gas stream which
 causes mud  to form on the bags. This later dries into
 a hard cake. The water droplets can be caused by the
Fig. 1  Installation of shaker mechanism in  shaker-type    ' Fig. 2  Venturi and manifold in reverse pulse-type baghouse.
       baghouse.

                                                 2-3

-------
        Rulo-of-Thumb Costs of Typical Collectors ol
             Standard Mild Steel Construction
   Type of collector
  Dollars per cubic foot per minute
                       Yearly
                     maintenance
Equipment   Erection     and
  cost       cost     repair cost
 Mechanical collector
 0.07-0.25   0.03-0.12   0.005-0.02
 Electrostatic
   precipitator
 0.25-1.00    0.12-0.50   0.01-0.025
 Fabric filter
 0.35-1.25    0.25-0.50   0.02-0.08
Wet scrubber
 0.10-0.40   0.04-0.16   0.02-0.05
 malfunction of water spray cooling equipment, or more
 commonly, from condensation.
   Water,  in  the  form  of condensate,  is  a  common
 cause  of  malfunction.  If the temperature of the  hot
 humid  gas falls below the dew point temperature in
 the baghouse, water  will condense,  combine with  the
 collected  dust and  blind  the bags with  mud.  The
 gross gas temperature  does  not  have  to  reach  the
 dew point for this phenomenon to occur: only the metal
 skin of the baghouse need  be below this temperature.
 For this reason  many baghouses are insulated to re-
 tard heat transfer and maintain skin temperature above
 the dew point.
   Two types of burning can destroy bags. Baghouses
 have a rather low temperature limitation ranging from
 180 to  550  F. Consequently, on those applications
 generating high temperature  off-gas, there  is a prac-
 tical limitation of using those  fabrics able to  withstand
 at  least 400 F. Even then, a very reliable means  of gas
 cooling, such as evaporative cooling, is required, (POL-
 LUTION ENGINEERING, Nov./Dec. 1970). Even when
 the gas temperature is closely controlled, there is  the
 possibility of a hot spark reaching the bags and burn-
 ing a hole.
   Abrasion  is a  natural phenomenon  resulting from
 handling dust-laden gases. Some wear on bags occurs
 from direct  impaction,  but  wear  is greatly increased
1
1
-J
c

TOTAL CYCLE REPEATED
( TO ATTAIN EQUILIBRIUM ^

INTERVAL
OFCAKE
REPAIR

+RFSCUA
*t>RAG \

*> D W
DEPOSITION OF HOMO-
GENOUS DUST MASS
to
d
«-

START Of
NEXT CYCLE
FILTERED DUST MASS. W (GRAINS/FT2)



                How to Buy • Baghous*
    If you have an air pollution  or dust control  problem In
your plant and are considering purchasing a baghouse, here
are some do's and  don'ts:
   DO:  Write specifications  that Include  volume,  tempera-
        ture, dust  and water content of  the gas stream.
        Specifications should clearly  state  the  application.
DON'T:  Specify  crucial  parameters  such  as  air-to-cloth
        ratio.  No one knows the manufacturer's equipment
        like the  manufacturer. Trust  him  to make the  cor-
        rect  decisions based on experience and require a
        performance guarantee.
   DO:  State  the  applicable code  requirements and  per-
        formance expected.
DON'T:  Request  a  bag-life guarantee; most manufacturers
        won't give one.
   DO:  Spell  out  peripheral equipment you  want quoted
        with  baghouse;  i.e., fan,  motor, screw conveyor,
        etc. If you want a turnkey job,  require that the equip-
        ment price be broken out separately from the erec-
        tion price.
DON'T:  Require  detailed proposal if  "budget" figures will
        do.
   DO:  Expect the manufacturer to explain his features and
        evaluate them as  they pertain to  your  application
        and plant operations,
DON'T:  Arbitrarily  request  changes  to  standard  design.
        "Specials" cost much  more  and  require  fantastic
        lead time.
   DO:  Check  with  the   manufacturer's  customers—they
        know his ability to perform.
DON'T:  Expect to  visit an  installation identical  to the  one
        you're considering.  No  two  operations are alike.
        Also, improvements are  being made daily  that  may
        make what you buy better than what is in  the field.
   DO:  Require factory assembly and prebagging if at all
        possible. This  will be  less  expensive  and cause
        fewer headaches than doing  It at  your  plant.
                                     when  dust  strikes a  bag  tangentially. Elimination of
                                     abrasion  is impossible, but  proper design  of  a bag-
                                     house will retard this cause of bag failure by channel-
                                     ing the  air in the  least destructive way,  and/or by
                                     holding gas velocity to a minimum.
                                       The destruction of  a bag  by chemical  attack is
                                     generally due to misapplication or misoperation. Pro-
                                     cesses which generate gases at elevated temperature
                                     frequently produce  water  vapor  and  acid  radicals
                                     which, at a given temperature (the acid dew point), will
                                     react and form an acid vapor. The acid  dew point
                                     is  dependent  upon  the  concentrations  of  the  con-
                                     stituent in the gas, and is therefore very  difficult to
                                     predict.
                                       Bags also tend to wear  out from aging. After hun-
                                     dreds  of thousands  of  cleaning  cycles,  the  fabric
                                     weakens and will ultimately fail. The occurrence is as
                                     unavoidable as human deterioration, but,  like humans,
                                     the better the bag is cared for,  and the better the ori-
                                     ginal equipment design, the longer it will last     5S
Fig. 3   Fitter drag cycle.

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                        VI-3. THE USER

                              AND

                FABRIC  FILTRATION EQUIPMENT
           Copyright  @  1976  by Air  Pollution
           Control Association.   Reprinted with
           permission  from the Journal of the
           Air Pollution Control Association/
           Vol. 26, No.  1.
That fabric filters offer unmatched opportunities for attaining op-
timum control of particulate emissions is generally conceded. It
is also accepted, however, that difficulties arise In predicting
and achieving  consistency of performance when seemingly in-
significant changes occur in processing conditions, in baghouse
design, in maintenance, or in operation.
   The following package of information draws on field experi-
ence  to address this unpredictability and to examine specific
causes of success and failure. Several papers, and portions of
papers, were  selected from those  presented at the second
APCA specialty conference on The  User and Fabric Filtration
Equipment, held in Buffalo, New York in October 1975. The pa-
pers which follow  are authored by baghouse "users" in various
categories of industry; the one exception is a condensed version
of the keynote  paper by a "manufacturer" which was designed
to bridge the gap between the first and second conferences.
The entire second conference will be reported in a Proceedings
whose availability will be announced in JAPCA.
                           3-1

-------
                Baghouse Technology: A Perspective
                Donald H. Rullman
                American Air Fitter of Canada, Ltd.. Montreal. Quebec
 As presented at the Second Conference on The User and Fabric
 Filtration Equipment, this keynote address served to bridge the
 two year period between the first and second conferences. In this
 condensation, the author discusses the current state of the Indus-
 try, and points out some of the user's responsibilities to the goal of
 achieving regulation compliance In a cost-effective manner.
 The dialog of the First Conference on The User and Fabric
 Filtration Equipment, held in Buffalo in 1973, and this
 Second Conference two years later comes down to two basic
 elements:
   —proper design of system and equipment
   —living with the results.

   The Clean Air Act Amendments of 1970 provided  three
 methods for mandating air pollution control for stationary
 sources.
   1) Setting  of  primary  and secondary air quality  stan-
     dards,
   2) Setting standards for hazardous air pollutants, and
   3) Setting new source performance standards.

   Ambient air quality standards have been established for
 six pollutants.
   1) Sulfur dioxide
   2) Paniculate matter
   3) CO
   4) Photochemical oxidants
   5) Hydrocarbons
   6) Nitrogen dioxide

   Of these, primarily particulate matter is controllable by
 bag  filtration. However,  in  1973, of the 23  industries for
 which new source performance standards had been set or
 were under consideration, 17 had a particulate matter stan-
 dard.
   Updating, as of June  1975, the Environmental Protection
 Agency has promulgated  22 new source performance stan-
 dards. An additional 16 have been proposed and 55 are cur-
 rently being developed. A spokesman from EPA informed
 me recently that their objective is to issue 22 standards per
 year.
   What the EPA tells us, essentially, is that they have de-
 termined, fairly accurately, the scope of the problem by in-
dustry and by process, and are continuing to apply research
 emphasis  in  measurement and filtration  of fine  particu-
 lates. They have expressed an  opinion that there is sub-
stantial evidence that fine particulate can be controlled in
a cost effective manner. That of course is why we are here.
                                                      3-2
How do we design the process, the system and the bag-
house to be compliant in a cost effective manner?
  Throughout those presentations it will  be evident that
there is still—and perhaps always will be—a mixture of art
and science in the  application and performance of fabric
collectors. Let's review briefly where we stand today. What
is happening in this "arty science" of fabric filtration?
  On the large collector scale there is considerable activity
in yet another major production area of the integrated steel
mill. That is in "hot metal operations," or those operations
involved in moving, tapping and handling molten iron from
the blast furnace to the basic oxygen furnace shop. Specific
applications include:
  —cast house
  —torpedo car reladling
  —desulfurization, in the ladle or in a torpedo car by sev-
    eral processes
  —BOF secondary  ventilation  which involves fugitive
    emissions control at the BOF during charging and tap-
    ping.

  Japanese steel producers have the bulk of experience in
these areas—particularly cast, house  ventilation.  Their
major installations, including BOF secondary emission con-
trol, are virtually all structural, reverse  air collectors with
polyester fabric and 12 in. diameter bags. Two such collec-
tors will also be installed soon in England. Several  BOF
secondary ventilation collectors have been installed in the
United States. These are shaker collectors. A similar instal-
lation is also being installed in Mexico  and the first such
unit will be installed in Canada within a year.
  We are well aware that the  metric system  is being  in-
creasingly  used  in  North America. If you  travel  into
Canada now, the reported temperature is given in degrees
Celsius. In fact, snow depths in Canada this year will be re-
ported in centimeters. Having  lived in Montreal for a year
it would seem to me that the kilometer  might be a better
unit of measure for snow fall.
  In the metric system the 5 in. diameter bag becomes 125
mm, the 8 in. bag becomes 200 mm and the 12 in. hag be-
comes  300 mm.  For the most  part these  bags are inter-
changeable with their North American counterparts.
  While this is a North American conference, there is no
doubt that the European and Japanese producers can teach
us a thing or two with respect to certain applications,  for
instance, ferroalloy applications. In North America two
basic collectors have been applied successfully on ferroalloy
and electrometallurgical processes. These are the glass or
Nomex  fabric reverse air unit  and the  Nomex equipped
shaker collector. In Europe the predominant baghouse in
use on these applications is a glass reverse air collector, and
I understand they are going a step further.  Other than for
ferrosilicon furnaces, many ferroalloy furnaces can be vir-
tually completely enclosed extracting a relatively small gas

             Journal of th« Air Pollution Control Association

-------
                                  TROUBLE SHOOTING GUIDE*
            Trouble
     Possible  cause
          Remedy
       Loss of Air Flow
       Insufficient air How,
       lack of dryer capacity
       or puffing at burner
       Excessive air flow,
       poor fuel consumption
       or low temp.

       Dusty Stack
 Fan stopped
 1) Cleaning cycle too long
 2) Low compressor pressure

 3) Low compressor pressure

 4) Excessive dust load

 5) Excessive Tiring rate

 6) Mud plugging

 7) Oil or carbon coating

 8) Damper closed
 9) Duct leaks
10) Fines system leaks

11) Plugged precleaner or duct
12) Plenum bypassed (reverse
   air or plenum pulse types).

 1) Cleaning cycle too short
 2) Damper open
 3) Inadequate firing rate


 1) Holes in bags
 2) Poor bag seating
 3) Housing leaks

 4) Bleed through
Check motor interlock circuit or
for tripped overload.
Check for broken or slipping belts.

Decrease cycle time and cycle timer.
Check comp., if ok, check blast
time.
Check for sticking blast valves or
broken lines.
Reduce  feed rate or use cleaner
aggregated.
Check moisture removed vs cap.
chart and reduce feed rate.
Clean bags, maintain internal col-
lector temperature  above dew pt.
Clean buys, clean burner and check
for proper air/fuel ratio.
Check damper position.
Weld and repair.
Check airlock seals and housing
discharge.
Clean out build-up.
Check plenum valves and bypass
switches.

Increase cycle timer.
Check damper position.
Check burner, if ok, reduce feed
rate.

Replace or repair.
Repair bag mounting device.
Repair leaks between bag section
and clean air plenum.
Replace or repair worn bags, in-
crease cleaning cycle. Also reduce
air flow  to minimum to reduce
operating air/cloth  ratio.
    * "Special Problems with Fabric Filters on Hot Mix Asphalt Plant Dryers,
    pany, Charlotte, NC.
                                1 W. Con Proctor, Rea Construction Corn-
 volume off the process. Whereas this application had pre-
 dominantly used scrubbers, the idea now is to use a com-
 bustion chamber followed by a heat exchanger—with or
 without heat recovery—and a relatively small reverse air
 glass collector.
   There are also municipal and commercial incinerator in-
 stallations in Switzerland which have proved that the bag-
 house is a viable option to the precipitator in this applica-
 tion. Virtually every form of bag cleaning is in use in this
 application with apparently no significant data indicating
 one achieves better results than another. Since in most in-
 stances incinerators actually discharge relatively low sulfur
 levels Nomex can be used. Chlorides are the big problem;
 therefore these units are well insulated. Also, considerable
 attention  has been  given to material handling  systems on
 such collectors.
   In the cement and rock products industry, the fabric col-
 lector continues to  be an alternative to the  precipitator in-
 certain applications previously considered  as  exclusively
 belonging to ESP. The increased use of direct reduced pel-
 lets in steel producing arc furnaces should augment lime
 consumption, and there is an increasing number of success-
 ful lime kiln fabric collector installations.
   We should also say a word  or two about collector con-
 struction. In our 1973 conference, several  manufacturers
                      discussed the merits of modular versus structural collector
                      construction. I do not intend to  repeat those discussions
                      but I do wish to stress that  both methods are widely ac-
                      cepted today. One has not eclipsed the other and will not.
                      Both techniques can give equivalent years of suitable en-
                      closure for the filtering system.  Two years ago it  would
                      have appeared that the modular unit would soon displace
                      the structural design for all small and medium range collec-
                      tor sizes. But then the cost of fabricated steel started mov-
                      ing higher and higher. Already having a weight disadvan-
                      tage the fabrication cost of modular units soon began to
                      outweigh the remaining erection cost advantage.
                        Under certain  conditions the  modular collector  has  a
                      higher equipment cost (than a structural  unit) but a more
                      than compensating lower field erection cost. However, the
                      cost of fabricated plate can in certain cases destroy that ad-
                      vantage. Thus, responsible manufacturers today do, and
                      must, investigate more  than just  the technical factors we
                      have reviewed before making a final decision on whether to
                      recommend modular or structural.
                        Recent  advances in  fabric filtering technology involve
                      making the selected fabric collector more dependable, con-
                      ceivably more efficient, and easier to maintain as this de-
                      vice is applied to an increasing  number  of applications.
                      However, as the  baghouse is in each case  simply a compo-
January 1076    Volume 26, No. 1
                   3-3

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THE  USER AND  FABRIC FILTRATION EQUIPMENT


nent of a system—albeit the major component—advance in
system design technique is truly important to the success
of a baghouse installation.

The User's Responsibility

   Before  I close  I would like to discuss briefly a serious
matter—the  user's  or  purchaser's responsibility toward
proper analysis of process, proper specification of system
and collector, and ultimate selection of manufacturer.
   To the  user's accounting department, purchase of an air
pollution  control system undoubtedly represents a major,
non-return, capital expenditure except where there is sig-
nificant reclaim value in the collected material.
   To the  user's management, this situation also represents
a high capital outlay with increased operating costs. But it
also represents far more—risk. Risk that the equipment
will not perform, and perform dependably,  and risk that
the letter of the law will not be met.
   To the  engineering and operating department of a com-
pany contemplating purchase of pollution control equip-
ment, the assignment  to select  the proper system  and
equipment still occasionally represents  a very real dilem-
ma. Unless a portion of the engineering department is spe-
cifically assigned continued responsibility for air and water
pollution  control, too often involvement in these problems
presents new  technologies with new jargons, and there is
inevitably the pressure of a tight time schedule.
   This is  normally the right situation to choose a qualified
consultant However, with or without this consultant, the
baghouse  manufacturer has a serious responsibility to de-
termine what he can about the process and to inform the
user. The user has the responsibility to listen, to study, to
inspect installations, and to confer with other users of such
equipment.
  With some fear of being misunderstood we can say that
the primary purpose of the steel envelope, shell, or building
around the filtering system (by which we mean  bags, bag
suspension members,  bag cleaning devices, instruments,
dampers, and material handling) is simply to provide an
optimal working environment to allow this filtering .system
to function properly.  Design of collector housings  entail
normal building design disciplines considering wind, snow
loads, seismic conditions, and the like. But occasionally it is
difficult  for a manufacturer to get a potential user to men-
tally open the door of such a structure and fully and equal-
ly analyze the filtering system—that portion of the  struc-
ture that has entailed  far more design research and years of
practical engineering.  In such cases, final analysis can too
easily dwindle to cost per ton of fabricated steel.
   Fortunately such situations  are becoming increasingly
rare.  Often the deliberate,  technical approach  taken by
users and consultants in our work is impressive and  highly
respected. The continuation of conferences such as this,
play a large part in allowing each of us, from our own per-
spective, to establish that necessary technical dialog.
     Mr. Rullman is manager, Engineered Systems Depart-
   ment, American Air Filter of Canada, Ltd., 400 Stinson
   Blvd., Montreal, Quebec, Canada H4N 2G1.
                Innovations in Ferroalloy Baghouse System  Design
                R. Neil Paylon
                Airco Alloys, Niagara Falls. New York
System design used by a typical ferroalloy producer I* reviewed,
with special emphasis on shaker cleaning of Nomex fabric In large
structural baghouses. The use of air curtains at the furnace hoods,
duct size venturl for flow measurement, and other techniques «re
described to permit evaluation of Individual bags as well as com-
plete compartment* of bags.  A unique hook-up of magnehellc
gauges provides an Instant assessment of the performance of
each compartment to assist operators In achieving optimum oper-
ation. A critique on waste heat recovery, gas coolers, and Induced
draft fans on dirty gas service is Included.
Airco Alloys, a Division of Airco, Inc., is the second largest
producer of ferroalloys in the United  States. This paper
deals with the application of large filter fabric collectors to
collect fume from submerged arc electric smelting, open-
hooded furnaces. At the present time, Airco has  three bag-
house systems controlling five furnaces, with an  additional
three systems under construction for  six other furnaces.
There are also two submerged arc furnaces  equipped with
electrostatic precipitators, and one covered  furnace with a
wet scrubber. This paper will describe the  philosophy of,
and experience with, the components of the air pollution
systems which are in use  or under construction. Following
                                                       3-4
                                                                        Journal of the) Air Pollution Control Association

-------
this, we will go into the evolution of some of the unique fea-
tures of our systems which are at the heart of filter opera-
tion, and the reason for the title of this paper.
  Ferroalloy furnaces producing fume considered most dif-
ficult to collect are from silicon metal, 50% to 75% ferrosili-
con, and ferrochrome silicon. These  fumes  are largely sili-
con dioxide, in submicron sizes, reported  to average less
than  0.3 microns.  It is the collection of these fumes that
will be discussed.
  Thus far, we have used only two shaker type cleaning
methods;  namely,  fiberglass with deflation air  and gentle
shake, and combination Nomes with deflation air and vig-
orous shake. Another popular cleaning method  which is
used  in our industry, both here and abroad, is reverse air
through ringed glass bags. Although glass filter media have
advantages of  a 500° F  plus  operating temperature and
lower capital cost, the air-to-cloth ratio is limited to 2 to 1
or less. Thus far, Airco Alloys has elected to use the combi-
nation Nomes filter cloth and shaker mechanisms that per-
mit a 2.75 to'l ratio at equivalent bag pressures. Bag pres-
sures up to 16 to 18 in. WG on this type of application are
not uncommon. All bags are a standard llVj in. diameter by
30 ft-6 in. high. In order to cool gas from 500°F or mora
down to  375°-400°F, we  use  forced convection, tubular,
air-to-gas cooling. We prefer to put an investment in cool-
ers instead of more bags.  Our experience  with glass and
Nomex has shown good bag life but fewer bag failures with
Nomex. Perhaps the most significant difference is  the in-
creasing filter drag of glass using gentle shake, compared
with the stable filter drag of Nomex.
  Our experience with conventional dust handling by screw
conveyors, bucket  elevator, and live-bottom storage  hopper
has been  satisfactory. We did have an initial bad applica-
tion of a vertical screw conveyor which taught us the value
of a dependable dust removal system. The collected fume is
being slurried in concrete misers and hauled away as lend
fill. One mixer truck has been  able to keep up  with about
20 tons/day of fume, hauling  a distance of one mile, and
washing out the bowl after each load and scouring out with
crushed stone or slag at least once a day.

Furn® Mood Ventilation

  The type of hood venting used on our ferroalloy furnaces
is an open hood approximately the  same diameter as the
furnace, with a 5  ft vertical opening between the furnace
and  hood to allow charging and stoking equipment to go
right up to the furnace. Thus, the hoods are upward of 30 ft
in diameter and about 10 ft high, with one or more offtake
ducts leading to stacks, and then to air pollution control
systems. (See Figure 1.) Sufficient ventilation must be pro-
vided to prevent an outflow of gases from over the furnace
into the furnace building. Starting in 1971, Airco Alloys has
used air curtains to  help seal in the gases and  to comple-
ment the  use of vertical sliding doors  in some cases. Al-
though the primary object at that time was to eliminate the
use of doors to reduce the required ventilation flow into the
furnace, the full evaluation of this technique has been diffi-
cult without accurate and continuous flow  measurements.
What has  been accomplished,  unquestionably, is  an eco-
nomical   way  to   dispose of furnace  taphole  fumes
amounting to approximately 50,000 cfm, depending on the
size of the furnace. Building drafts make open hood venti-
lation difficult, requiring a face velocity through the venti-
lation space of at least 250 ft/min. In principle, the air cur-
tain jets blow downward and inward to the furnace, elimi-
nating inflow for 2 to 3 ft below the bottom of the hood
skirt. If the jet velocities are carefully  controlled, the in-
duced secondary flow of room air is minimal, and any
culation outward from  the air curtain is immediately re-
entrained into the horizontal inflow entering the furnace.
   As a measure of our  confidence in the air curtain tech-
nique, Airco has built these devices around the hoods of
three furnaces and is building three  more in both new and
existing furnaces. Another  ferroalloy  producer has built
one unit with the likelihood of adding three more on exist-
ing furnaces. Low capital cost is possible,  since the device is
an annular duct or ducts built either inside or outside the
hood skirt and requires  only an additional \ in. static pres-
oura, over and above the hood and duct Icasae. The flow can
ba provided by in-duct vane asial fans.
1. Furnace
2 Hood
3. Tap
4. Offtaho
5. Fallout
 6. Flow venturi
 7. Coolers
 8. Cooling air
 9. Fallout
10. Suction monifold
U. 1.0. fans
12. Dampers
13. Baghouse
14. Dust elevator
15. Dust silo
16. Emergency dampor
Plgrro H.  Fabric ffto oyotcm.
       107®    Volumo 23, Wo. 1
                                                       3-5

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      1 om 2    4

Flguro 2.  Variation In flunl otach tompcraturoo.
                     6    8     10   Wcon   2
                         Timo of doy
  In our recently designed offtake ductwork from the hood,
we are using at least two offtakes with fall-out chambers in
which a change of direction and a reduction in velocity are
incorporated. Since the offtake gas temperatures will be of
the order of 600° to 700°F, we are using automatic control
dampers to limit large excursions of temperature  in the
ductwork. An indication of temperature fluctuations in two
equal ducts under a single fan suction is shown in Figure 2.
In the  latest design, we use a single damper, controlled by
ratio control, in an effort to keep both offtake temperatures
equal in value. This avoids the complication  involved in
having separate temperature control dampers  in each off-
take, since upon furnace shut down, the dampers will close
and transfer fan pressure from the bags to the suction oids
of the baghouoe, inviting & disastrous duct collapse.
                               (0°)
                              Full open
                          \  dampor
                                                               Aforaoton
                                                             On a dual single inlet fan installation, severe fan wear oc-
                                                           curred on  blade wear plates  and at the junction  of the
                                                           blades with the back and side plates. The fan wheels did
                                                           not originally have weldments or wear plates on the back
                                                           and  side plates. On the  scroll, which was originally % in.
                                                           thick, up to % in. wear had occurred. It was  necessary to
                                                           make costly wheel replacements on both fans. For future
                                                           installations on dirty gas service, these wear points should
                                                           be adequately protected and backed up by regular inspec-
                                                           tions for wear.
Para Characteristics

  I.D. fans in this industry are usually controlled by inlet
box dampers. In selecting fans, we have found that serious
losses of static efficiency occur when the fan system pres-
sures are overestimated. We have also found that fan man-
ufacturers sometimes use overly optimistic factors for esti-
mating fan brake horsepower, based on percent of full load
volume. With radial tip fan wheels, we find that the prom-
ised horsepower savings are not achieved.
  Figure 3 illustrates  that the static efficiency at the rated
capacity can drop from 69% to 55% if the system pressure
changes from 30 in. to 24 in. In this example, 4 fans rated at
30 in. SP would cost $150,000 more per year in power costs
than  if properly rated in 24  in.  SP. From our experience
with several manufacturers, we would recommend that pro-
spective purchasers of industrial fans should ask for perfor-
mance curves at various inlet damper positions, and not ac-
cept the customary horsepower reduction factors given by
the fan manufacturer.
                 20     30      40
                      Flow, cfm H 10,000
         Ptgao 8.  EWc« o4 btot teatgy on ten offtetency.
                                                             Waste heat recovery from ferroalloy furnaces is not prac-
                                                           ticed to date in the U.S. To recover waste heat, the furnace
                                                           must  be closely  hooded to restrict excess room  air from
                                                           tempering the combustion gas temperatures. We have con-
                                                           tinued to use open-hooded furnaces for several reasons, in-
                                                           cluding easier stoking, safer operation, and the ability to
                                                           change products  to meet market demands. To be economi-
                                                           cally feasible,  waste heat recovery requires that a user of
                                                           steam, hot water, or warm air is close by and can use  the
                                                           heat throughout the year.
                                                             Studies made by Airco Alloys in 1972 indicated that fur-
                                                           nace offgas temperatures of at least 1000°F (538°C), would
                                                           be required to generate up to  150,000 Ib/hr of steam at 400
                                                           psig and 650°F from two of our largest furnaces located at
                                                           Calvert  City, KY. Steam turbines would  drive two large
                                                           I.D. fans and exhaust 100 psig steam for use in a nearby
                                                           chemical plant. In subsequent  tests on one of these fur-
                                                           naces, offgas temperatures up to 800°F were obtained, but
                                                           ventilation was marginal and  the existing air-cooled hood
                                                           would not withstand these temperatures. The furnace con-
                                                           struction also made such high operating temperatures too
                                                           risky,  and so a maximum offtake temperature of 650°F
                                                           (343°C), was  arbitrarily selected. It was decided  to build
                                                           forced convection coolers, of a type first  used  in  1971, ns
                                                           this design fitted  into  the available site, was self-cleaning,
                                                           required little maintenance, and had only 2 in. of pressure
                                                           drop. The cooling air fans were selected for 100°F ambient
                                                           temperature, and have inlet louver dampers controlled by
                                                           cooler outlet temperatures. Two fans in parallel are used
                                                           for each pair of coolers.
                                                             Elsewhere in the industry, radiation-convection trom-
                                                           bone typs coolero are being used and are reported to be of
                                                        3-6

-------
less expensive construction. We believe that this design has
more pressure drop, requires greater site space, and lacks
outlet temperature control to the extent that it could over-
cool in winter and undercool in summer.
  Similarly, evaporative cooling was rejected because of
possible acid dewpoint corrosion in the  ductwork and bag-
house, requiring adequate insulation of these parts. Finally,
the possibility of sulfuric acid dewpoint in the Nomex bags
would make glass filter media a better selection, but  this
requires lower air-to-cloth  ratio and, thus, more filter bags.
As previously stated, we prefer an investment in coolers to
reduce baghouse size, and the number of operating parts.
Baghouse Maintenance

  Contrary to our original expectations, bag replacement
cost is not the major maintenance cost in our structural
baghouses. Rather, it is the dust handling from compart-
ment hoppers through to the trucking of the mixed paste.
Plugups in the conveyor to elevator transfer point are com-
mon, and there are periodic replacement of conveyor bear-
ings and seals around dampers and conveyor drives. We
prefer to use  poppet type dampers for  their tight closing
characteristics, and prefer not to use them in horizontal po-
sitions due to failure of supporting rollers. Next, we have
sought  better access to the dampers, and this  required a
new structural design of the baghouse substructure. Final-
ly,  we have gone to extra large compartments in order to
limit the number of compartments to 12 on new systems.
We have found that the larger compartments not only re-
duced the number of operating valves, conveyors, and rota-
ry valves to be maintained, but there also was a 10% reduc-
tion in cost. There should also be less danger of re-entrain-
ment of dust from  the hopper when inlet and deflation
dampers open.

Gas Flow Measurements and Bag Evaluation

  The primary variable in the dirty air side of baghouses is
flow.  There must  be sufficient ventilation to capture the
paniculate. This aspect of air flow measurements has been
lacking, and in the past, there has been a substitution of
temperature and pressure values,  which are of secondary
use in indicating baghouse performance. We will be mea-
suring flows in the following manner in our latest baghouse
project.

                    Total System Flow

  Main gas flow will be measured by a duct size venturi
with a design  AP of 1.5 in. WG and a pressure loss of 0.25
in.  WG. The flow differential will  be transmitted to  a 30-
day strip chart.

                  Bag Flow Measurement

  Individual bags in a single compartment  can  be  com-
pared by a simple test apparatus, with results as shown in
Figure  4. These are multiple venturi  tubes,  which are
mounted within the dust compartment and aligned with
the test bags  and  directly below the tube sheet. We have
been  using a 4 bag cluster, of which at least one is a  stan-
dard  service bag identical to those used in the other com-
partments. The calculation of flow through test bags, as de-
termined from the velocity head measurements,  can  be
compared with the calculated flow through the standard
bag or bags, which in turn can be compared with the total
flow to  the baghouse passing through X compartments of
bags identical  to the standard test bags.
              Compartment Flow Measurement

  By measuring the pressure drop across the inlets of each
compartment  in a baghouse, and knowing the total flow
from a flowmeter or velocity traverses, the  changing and
relative flows  into each compartment at any time can be
calculated. This permits a ready tool for evaluating the ef-
fect of changes in bags from one compartment to another,
different shaker cycles per  minute,  or malfunction in any
compartment which may be seriously affecting the flow ca-
pability of that compartment. Bag  filter drag coefficients
from a furnace operation on a certain product can be used
to estimate system pressures in a future installation.  Bag
pressures are the major pressure components in the calcu-
lation of system losses for I.D. fan selection.
        S 4
          2
                        Shaking frequency
                            155 cpm
                         32 min. cycle
                *       Bagsll.W"x30'-6"
Combination Nomex
    P.M. 1607
 S*
                       Fiberglass
                     W.W.C. 425-04
                         -Collecting-
            1&4  7   8 245  9  10  3&6  11  12 1&4
                     Compartment number
               Figure 4. Test bag performance.
January 1976    VoJuma 26, No.
                                                      3-7

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 THE USER  AND FABRIC FILTRATION  EQUIPMENT
                 Compartment Monitoring

   By connecting the manometer or differential pressure
gauges, as shown in Figure 5, the effectiveness of each com-
partment can instantly be monitored. This is quite impor-
tant when you consider that an operator rarely can afford
to spend an hour or so watching all compartments isolate,
clean, and return to service. The causes of the assumed loss
in flows may be: inoperative inlet, reverse air or reinflation
dampers, leaky dampers or loss of shaking and/or reverse
air flow.  Defective compartments have  been known to stay
on-line for  weeks at a time  because  the deficiency was
never detected during casual daily inspections.

                   Automatic Air Purging

   In an  effort to maintain the operation of the compart-
ment flow monitors  by  preventing  dust plugging  of the
measuring lines, we are installing automatic purging valves
between  the manometers and the measuring points. These
small valves are cam driven, and isolate each gauge for a
few seconds while they apply a 50 psig air supply pressure
to  the measuring  line. The purge timer starts the purge
cycle after a preset interval, and the cycle is stopped by a
limit switch on the camshaft.
   To low pressure -7
    side of gage  /
                                        To high prev.ure
                                         side of gage
                               Inlet plenum —
   Instrument air
     SOpsig
              5"-0-5"
          magnehelic gage
    020"
magnehelic
    gage
 Atm.
                       -0*3-
        T
Instrument air
   50 psig
                          r
                        3 way
                      solenoid valve
  I       \—Cam-operated 4-way air purge valves-^-/ 	  	|

Figure S.  Typical compartment monitoring system.
     Mr. Payton is with Airco Alloys, a Division of Airco, Inc.,
   Box 368, Niagara Falls, NY 14302.
                Review of Baghouse  Systems for Boiler  Plants
                M. J. Hobson
                Acres American Incorporated, Buffalo, New York
The principle of fabric filtration has only recently been accepted
for fossil fuel fired boiler partlculate emissions control. Information
on design criteria Is, therefore, limited. The paper provides data on
the few Installations where baghouses have been Installed and
successfully operated. The general conclusion Is that a strong re-
lationship exists between air to cloth ratio and bag life.
Fabric filtration is accepted as a standard method for par-
ticulate removal from the majority of industrial process
emissions. One application that is just beginning to achieve
commercial acceptance is the control of fossil-fueled boiler
emissions.
  Until now, the reasons for choosing other emission con-
trol systems for boilers have been:
• High cost compared to less efficient, but acceptable, con-
    trol methods;
• High flue gas pressure  loss compared to other control
    methods;
• Acid attack problems;
• Unacceptably short bag life;
• High totalized costs compared to switching to a cleaner
    fuel;
• Possibility of high temperature excursions and  flamma-
    ble gas ignition.
  In recent years, particulate emission regulations for both
industrial and  utility size boilers have tightened to  the
point that fabric filtration is now a serious competitor to
                                                        3-8
             Journal of tha Air Pdutton Control Association

-------
 electrostatic precipitation and wet scrubbing on moot in-
 stallationR. The alternative of switching to a cleaner fuel is,
 in most cases, no longer viable due to high price and alloca-
 tion priorities.
   It is important, therefore, to reexamine the application
 of fabric filtration to boilers and determine whether those
 remaining potential problems have been solved by recent
 developments.
  This paper was prepared to document the experience of
 those  few experimental  and commercial installations of
 boiler plant baghouses.

 Boiler Flue Gas Characteristics

  Flue gas characteristics, such as gas  composition  and
 temperature, depend on the capacity of the boiler unit, the
 type  and extent of  heat  recovery equipment, the  type of
 fuel firing equipment and the fuel analysis. The flue gas
 from industrial installations  fired by spreader stokers is
 usually in the temperature range 300°-400°F with high ex-
 cess air and large participate size.
   Large industrial  and utility installations are pulverized
 coal  fired with flue gas  temperatures in the range 250°-
 350°F and particle  size distribution ranging from 50 to 0.5
 ^m, with the bulk of the fly ash between 2 and 16 »/m.
   Plant operation can vary considerably. Many small in-
 dustrial boilers shut down in the summer or operate at a
 low load. Larger industrial and utility units usually operate
 continuously at a fairly high capacity factor.
   As  many designers and operators are aware, acid attack
 on the gas side of flue gas systems is often proportional to
 the frequency of unit shutdowns and the extent of low load
 operation.

 Pilot Studies

   A considerable number of fabric filter pilot tests  have
 been performed on  various boiler  flue gas systems. These
 results have been documented in previous technical papers
 and I do not propose to recapitulate the results in detail.
   The pilot tests showed that  bulk woven fiberglass and
 Teflon felt fabrics can provide the performance required on
 most boiler fly ash  removal applications. Both fabrics are
L
 resistant  to acid attack and abrasion and are sufficiently
 impervious to give satisfactory dust removal efficiency.
   Woven fiberglass was found to operate well at air to cloth
 ratios between 2 and 4. When using Teflon felt air to cloth
 ratios can be higher.

 Commercial Installations

   Data were collected from 10 installations of baghouses on
 boilers. Four of these are power utility plants, the remain-
 der are industrial boiler plants supplying steam for process
 and heating. One  of the power  utility plants burns heavy
 fuel oil, all the remainder burn solid fuel.
   Each baghouse  installation will  be reviewed for design
 data and  operating experience. This information is given in
 brief tabular form  in Tables I, II, and III.

          Southern California Edlton, AlamHo* Unit 3

   Number 3 boiler  at Alamitos Plant is an oil-  and gas-
 fired unit of 2,305,000 Ib/hr capacity. For 6 months of the
 year, residual oil is fired which, until  1965, caused a visible
 stack plume. A program of pilot tests showed that fabric
 filtration  could  be utilized for heavy oil-fired boiler  stack
Table I. Boiler data.
Case
no. Plant and location
1 Alamitos, CA

2 Sunbury
Shamokin, PA


3 Carborundum
Niagara Falls, NY
4 Amal. Sugar
NYSSA, OR
5 Amal. Sugar
Nampa, 10
6 Colorado UTE

7 Holt wood, PA

8 Winston-Salcm, NC
9 Winston-Salem, NC
10 du Pont
1 1 du Pont
Fuel
No. 6 Oil
1.5-1. 7% S
0.8% S anthracite
and up to 6% S
petroleum coke
(75/25 mixture)
Bituminous coal
7% ash 2.3% S
Coal

Coal

0.6-1. 9% S coal
12% ash
Anthracite, fines
and petroleum coke
0.8% S Coal
0.8% S Coal
Coal
Coal
Fuel firing equipment
Tangential tilting
return flow burners




Spreader

Spreader



Spreader

Pulverized coal

Spreader
Spreader
Spreader
Spreader
Plant capacity
on baghouses
2,305,000 Ib/hr

175Mw



75,000 Ib/hr

200,000 Ib/hr

150,000 Ib/hr

3 units each
132,000 Ib/hr
50% of 80 Mw unit

75,000 Ib/hr
40,000 Ib/hr


January 1976     Volume 26. No. 1
                                                         3-9

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THE USER AND  FABRIC FILTRATION EQUIPMENT
Table II. baxhousc desixn data.
Case no. Flue gas temp. ° F
1 258

2 325

3 350
4 310

5

6 310

7

8 ' 300-350
9 300-350
10
11

Gas now acfm
820,000

220,000


191,800

145,000*

86.000

200,000

38,000
23,000



gr/icf entering
baghoute


2.0






1.53



0.6
0.3



Air/cloth
ratio cfm/ft*
6.5

2.0

3.5
3.56

2.36

3.35



8.5
8.3
4.48
2.32

Clolh
cleaning; method
Deflation
& reverse air
Deflation
£ reverse air
Reverse air
Deflation
& shake
Deflation
&.• reverse air
Deflation
& shake
Deflation
& shake
Reverse air
Reverse air
Pulse-jet '
Reverse air

Suction or
pressurized
P

S

S
S

S

a-

s

s
s



Fabric type
Fil>«TKlass60IST

Filter Kluss
Tcllon coated
Fiberglass
Fiberglass

Fiber Klass

Fiberglass

Fiberglass

26-ciz fiberglass
26-0/ fiberglass
Teflon felt
Fiberglass
Teflon coated
a in 8 modules.
 gases provided that an alkali is injected ahead of the bag-
 house to neutralize SO.i. In 1965, a large horseshoe shaped
 baghouse was installed at the base of the stack of No. 3
 Unit at the Alamitos Station.
   The air to cloth ratio is 6.5:1 when one compartment is
 out  for cleaning. The outside diameter of the baghouse is
 90 ft, the height 80 ft and the depth 23 ft.
   The alkali additive  is injected at the  outlet of the  air
 heaters. Flue gas temperature is 258°F and the bags are
 cleaned about once per hour by deflation and reverse air.
   The baghouse equipment has passed through five phases
 of modification. Operation of the baghouse has steadily im-
 proved due to the many equipment  modifications made.
 Bag failures are now  reduced  to less than 1% over a 5
 month period. The stack plume is clear.

              Sunbury, Pennsylvania (P.P.AL.)

   Each of the two boiler units at the Sunbury Station have
 two  baghouses installed. The two boiler  units generate a
 total of 175 Mw and are fired with a mixture of anthracite
 and  petroleum coke. The anthracite has a sulfur content of
 0.8% and the coke sulfur content can be as high as 6%.
   The gas flow to each baghouse is 220,000 acfm and the
 dust burden is 2 gr/fts at a temperature of 325°F. The bag-
 houses were  supplied by Western Precipitation and  have
 an air to cloth ratio of 2. Bag cleaning is by deflation and
 reverse air and the I.D. fan is located on the dirty side. The
 original mechanical collectors were retained ahead of the
 baghouses. The  mechanicals were modified to reduce the
 pressure drop.
  Of the 5040 bags, only 50 were replaced after over 2 yr of
 operation  and many of these failures were caused  by im-
 proper installation. The pressure drop for the baghouse
 and  ductwork is less than 5 in. water gauge. It is planned to
 replace the bags every 2 yr.
  Discharge loadings from the baghouse were measured to
 be 0.017 gr/acf.
       Carborundum Company, Niagara Fate, New York

  The boiler plant is fired with coal and oil. One of the coal
fired units was fitted with a baghouse 6 yr ago and it has
operated for 9 mo/yr ever since. Numerous modifications
have been made to the baghouse  and the boiler firing
equipment has also received attention. The last 18 months
of operation have been very successful ,from the viewpoint
of hag life.
  The 75,000 Ih/hr boiler is fired with 7% ash,  bituminous
coal with a sulfur content of 2.3%. The firing equipment is
a spreader stoker. No  mechanical collectors are installed
ahead of the baghouse. The gas temperature entering the
baghouse is 350°F. The bags  are  fiberglass with an air to
cloth ratio of 3.5. The  bag cleaning method is reverse air
and the baghouse is operated  at negative pressure. The
cloth resistance is 6 in. maximum and the bag  failures ex-
pressed as a percent per year of operation amount to 0.3%.

                   Amalgamated Sugar  .

  At one plant of Amalgamated Sugar in Oregon, a 200,000
Ib/hr coal  fired  spreader stoker has operated  since 1973
with a fiberglass baghouse. The gas flow to the  baghouse is
191,800  acfm at  310°F. The air to cloth ratio is 3.56. The
bag cleaning method is deflation and shake. The baghouse
is operated at a suction. No significant operating data were
available on operating experience from this installation.

                   Amalgamated Sugar

  At another plant of Amalgamated Sugar, an 8-module
baghouse of Western Precipitation design is installed on a
150,000  Ib/hr boiler. The gas  flow is 145,000 acfm and the
air  to cloth ratio is 2.36. The bags  are fiberglass cleaned by
deflation and reverse air. The baghouse is operated under
suction.

                     Colorado UTE

  The Colorado UTE Electric Association has three 13 Mw
boilers,  each  with  mnximum evaporative capacity  of
132,000  Ib/hr. The boilers are fired by spreader  stokers
consuming low sulfur coal with 12% ash.
  Wheelabrator-Frye baghouses were installed  on all boil-
ers. The first unit commenced operation in early 1974. The
flue gas temperature is 310° F and dust burden at the bag-
house inlet is 1.53 gr/scf.  Fiberglass fabric bags  are in-
                                                       3-10
                                                                        Journal of the Air Pollution Control Association

-------
 stnllod with nn air to cloth ratio of 3..'lf> when five of the nix
 compartments are in  operation (i.e.,  when  one compart-
 ment is cleaning or down for maintenance).
   Pressure drop across the collector (headers, house, and
 fahric) is maintained at 4.2 in. water gauge by initiation of
 the cleaning cycle. Cleaning normally occurs every 3-4 hr.
   To  inhibit  SO.-i corrosion, a heating system is installed
 which circulates hot gas when the haghouse is down, during
 startup and during low load conditions.
   The dust collection  efficiency is high; discharge concen-
 trations are less than 0.003 gr/acf on  all three units.
   After some initial problems relating to bulk density of
 the fly ash, frequency of emptying  the  hoppers, and flue
 gas distribution, bag life is exceeding 1 yr of operation and
 2 yr plus are expected.
                 Holtwood Station (P.P.&L.)

   The boiler plant at the Holtwood Station is a 75 Mw unit
 fired with anthracite fines and petroleum coke. The fuel is
 pulverized and 50% of the flue gases discharge to a Wheela-
 brator-Frye baghouse. The other 50% pass through a wet
 scrubber.
   The gas flow to the baghouse is 200,000 acfm at full load.
 The  dust  burden entering the baghouse is  not  available,
 but it is estimated to be approximately 4 gr/acf. The bags
 are  fiberglass  with  deflation  and  mechanical  shake
 cleaning.
   Little operational data are available but  the  discharge
 concentration was measured at 0.007 gr/acf.

                  Hanes, Wlnston-Salem

   The  boiler plant  consists of two spreader stoker-fired
 boilers; 1—75,000 Ib/hr, 1—40,000 Ib/hr.  The fuel fired is
 low sulfur coal and the dust burden entering the baghouses'
 is 0.6 gr/acf for the 75,000 unit and 0.3 for the 40,000 unit.
 These concentrations are low because the original mechani-
 cal collectors were retained in service.
   Two  Dustex baghouses were installed after a program of
 pilot testing. The baghouses were provided with I.D. fan8
 and stacks and a draft control system to allow the existing
 equipment to function without modification.
   High air to cloth ratios of 8.5 and 8.3 were selected for 26
 oz woven fiberglass fabric bags. The gas temperature enter-
ing the baghouses is maintained between 300° and 350°F
by bypass and control dampers.
  Bag life on the 40,000 unit was poor due to abrasion at
the cage wire contact points. It was better for the 75,000
unit, possibly due to the coarser particle size distribution,
which resulted in a lower cloth resistance.
  Gas cleaning performance was high. A discharge concen-
tration of 0.006 gr/scf was obtained in tests. Woven Teflon
and Gore Tex (microporous PTFE membrane on a woven
teflon grid) bags are now under test in this installation.

                      du Pont Plant

  Four baghouses were  installed at a 6 unit boiler plant of
E. I. du Pont de Nemours and Company, Incorporated to
arrive at source emission compliance for the overall plant.
Both pulse-jet and reverse air cleaning systems were select-
ed to compare performance. The pulse-jet design was fitted
with Teflon felt bags; the reverse air design was fitted with
Teflon coated fiberglass bags. The original mechanical col-
lectors were retained ahead of the baghouses. Air to cloth
ratios are 4.48 for the pulse jet and 2.32 for the reverse air.
  During 12 mo of operation, only one bag failed in the
pulse  jet units and that is reported to be due to contact
with the wall. The baghouse efficiency is 98.4%. In series
with the  mechanical collector, the overall efficiency is
99.8%. The fiberglass bags have a life of 1-1/2 to 2 yr.
Table III. Baghouse operation experience.
Case no.
1

2
3
4
5
6
7
8
9
10
11
Months of
operation
23

30
54
14

20
4
9
9
12

%Bag
failures/year
2.2"

1.25
0.3




150
800
0.03

Clotha resis-
tance in. W.G.
5.7

5C
6.0
5.0

4.2

8.0
9.0


gr/scf leaving
baghouse
Clear
Stack
0.017
N.A.
N.A.

0.003d
0.007d

0.006


Comments
System extensively modified in five phases over 3 yr.
Alkali injection ahead of baghouse
Mechanicals ahead of baghouse
No mechanicals in series
No mechanicals ahead of baghouse

Heating and hot gas circulation during shutdown


Bags fail at cage wire- contact
Mechanicals ahead of baghouse
Mechanicals ahead of baghouse
a Maximum at full load.
b In last two phases.
c 1m hiding ductwork.
     i f.
January 1076     Volume 26, No, 1
                                                          3-11

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THE USER  AND FABRIC FILTRATION EQUIPMENT
  10

  8
.9
e 6
I
o 4
          .01       .1        1       10      100
                      % bag fiilures/yr
Figure 1.  Trw relationship between air to doth ratio and bag falurw*.
                                                   1000
                                                            Conclusions

                                                              Although the data available on boiler plant baghouses
                                                            are scant, the information reviewed above  indicates that
                                                            satisfactory results can be obtained with careful design and
                                                            intelligent boiler operation.
                                                              The relationship  between air to cloth ratio, cloth resis-
                                                            tance and bag replacement rate is also evident even though
                                                            these factors are strongly influenced by the large variation
                                                            in boiler equipment, flue gas systems, fuel characteristics
                                                            and operating conditions.
                                                                 Mr. Hobson is with Acres American Incorporated, Buffa-
                                                               lo. NY.
                Glass Bag Filters  for  Lime  Kiln  Exhaust
                James R. Gage
                Bethlehem Steal Corporation, Bethlehem, Pennsylvania
The Information presented In this paper l» directed toward those
concerned with specifying and operating larger size fiberglass bag
                                  *
filter houses. Information on emissions, bag (He, Instrumentation,
and maintenance Is presented from an operating Installation. The
solutions to particular problems of short bag IHe, one due to erratic
start-up and the  other due to coarse dust are presented and dis-
cussed. The bag filter collectors are satisfactory for these applica-
tions and a bag IHe of 22 to 34 months Is being realized. Proper
hopper Inlet arrangement, hydraulic dampers, and reverse air draft
control are believed to be partially responsible for the good opera-
tion and bag life.
Many press releases announcing the installation of an in-
dustrial fabric filter dust collector will describe its function
as similar to an ordinary household vacuum cleaner. Well,
some bag filters do serve such cosmetic needs but others
are not that simple. The purpose of this paper is to explore
                                                            the inner workings of a large baghouse as it operates to con-
                                                            trol the paniculate emissions of a lime kiln exhaust. Useful
                                                            information is presented on cleaning cycles, extending bag
                                                            life and avoiding operating problems.

                                                            Background

                                                              In 1969 it became necessary to abandon efforts to collect
                                                            our lime  kiln exhaust particulates with a low energy wet
                                                            scrubber. An estimated 125 tpd of dust was then .being
                                                            emitted from one kiln of which 40 tpd was  high quality
                                                            burned lime dust. This dust was highly reactive with water
                                                            and produced a lime mortarlike deposit on all equipment
                                                            interiors. In addition, the high alkalinity of the scrubber
                                                            water would have required the added installation of a large
                                                            acid  gas  neutralizing system. Electrostatic precipitators
                                                            were seriously considered as a solution but the hij;h calci-
                                                            um dust was too resistive to collect without excessive col-
                                                            lector size and electrical power  costs. Therefore, a fiber-
                                                            glass type baghouse was  selected for both installations, one
                                                            for a single large 650 tpd lime  kiln, the other a common
                                                            baghouse for four smaller lime kilns of 700 tpd total  pro-
                                                        3-12
                                                                          Journal of the Air Pollution Control Association

-------
duction. The characteristics of the gas stream for these two
applications are shown in Table I. The large kiln emits 180
tpd of baghouse dust, the four kilns emit a total of 150 tpd.
The presence of sulfur trioxide in  the four kiln baghouse
exhaust is due to different kiln combustion controls and in-
ternal heat exchangers,  which create a leas reactive dust
than at the large kiln. All the kilns  use the same coal. Also
the stone  feed  is  the same  chemical  analysis with some
kilns burning a finer size.  The single kiln  baghouse oper-
ates continuously  except for  minor kiln repairs every 6 to
12 months. The four kiln baghouse runs continuously ex-
cept for repairs to the main I.D. fan  every 12 to 15 months.

Baghouse Construction Features

  These baghouses were viewed as a part of the kiln pro-
cess equipment, therefore specifications were developed to
emphasize good operation  and reliability. Plant visits and
literature  searches  were  made  to develop  background
knowledge in writing the purchase  specifications. Figure 1
shows the cross section  of the haghouses purchased. The
large kiln bnghouse has 14 modules, the four kiln unit h-is
 16. The lower exterior steel siding  prevents chilling of the
screw conveyors and hoppers which are not insulated. Both
systems include I.D. fans with inlet  box dampers to provide
automatic draft control for the kilns. Both systems were
provided with generously sized fans and baghouses to elim-
inate the process restrictions due to inadequate draft.

System Controls

  As special features to maintain  proper and continuous
operation, the specifications  included an inlet pressure re-
corder, a  hydraulic control system  for the dampers and a
reverse air fan draft control system.
                dusty  air from  the compartment hopper,  collapsing the
                bags. This further volume is delivered to the filtering areas
                through the inlet duct causing an additional pressure rise.
                A typical  cycle is illustrated in Figure'4. By watching this
                pressure trace, the general area of the trouble can usually
                be discovered for the compartment being observed. The ac-
                tion of the tipping valves is also a good indicator of the ac-
                tual dust  removal  during the cleaning cycle. A rising pres-
                sure trace with regular lines would indicate a change  in the
                process volume as  shown in Figure 5. It is necessary to ser-
                vice the pressure tap weekly  by disconnecting the hose to
                the pressure transmitter and  blowing compressed air back
                through the piping to the duct. Otherwise the tap will clog,
                giving  an  erratic trace as shown in Figure 6. The informa-
                tion from the pressure rise can be  used to estimate the
                flows in the compartments. Table II shows the typical re-
                sults of the 14  compartment  baghouse filtering  160,000
                acfm with bags 8 months old.
Inlet Pressure Recorder

   This provides a visible record of the internal functioning
of the baghouse. A pressure tap located on the I.D. fan out-
let duct senses baghouse inlet pressure which is transmit-
ted to a continuous strip  chart recorder at the baghouse
control panel. With all compartments working  well, the
pressure trace is  a regular series of evenly sized vertical
lines  showing the normal rise  and  fall  caused by  the
cleaning cycle as shown in Figure 2. The pressure trace will
thus show if the baghouse is operating well without a tedi-
ous inspection. However, if there are  missing lines or no-
ticeably shorter vertical lines as in Figure 3, the  corre-
sponding compartment is not doing its share of the filter-
ing. A direct check on that compartment can be made by
observing the pressure trace on its next cleaning cycle. The
inlet valve, while closing, should cause a pressure rise as the
flow increases to the remaining compartments. Next the re-
verse air (or collapse air) valve should open and remove hot
Table I. Gas stream characteristics.
                          4 Kilns
Large kiln
Inlet temperature
Inlet (lust loading
Dew point
SO, in outlet
SO, in outlet
Inlet volume
Outlet temperature
460° F
8 Kr/acf
13U"F
15 ppm
225 ppm
158, 000 acfm
340° F
490° F
lOgr/acf
145QF
None
25 ppm
160.000 acfm
380°F
                 suspension -v 1
                  grating   ^
                                                             {, — Insulating
                                                             \ __ Fiberglass
                                                                bags (60)
                                                               11 >/." dia
                                                                31' long
                                                             Gather up
                                                             conveyor
                                                                                        i
                                                                 V Fiberglass baghouse cross section.
January 1876     Volume 26, No. 1
                                                          3-13

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THE  USER AND  FABRIC FILTRATION  EQUIPMENT
           Figure 2. Normal pressure trace with aH com-
           partments functioning.
  The volume control valve provides a 10 sec stroking time
for all the cylinders without resorting to any individual cyl-
inder speed control valves. An additional volume control
valve would he helpful, one for the reverse air dampers and
the other for the inlet air dampers.  These central control!)
allow a  fine degree of cleaning cycle tuning which would
not be practical using individual damper controls.
  The slow damper stroking minimizes the rate of volume
change  in the process during  the cleaning cycle and  pre-
vents the bags from reinflating too  rapidly after their col-
lapse. This helps the I.D. fan to maintain  control of the fir-
ing hood draft which  is important in providing the proper
kiln combustion atmosphere.  This hydraulic feature for
control is thus some compensation for the baghouse being a
variable draft loss device in contrast to the steady state op-
eration of either precipitators or scrubbers.
  There are serious consequences for ignoring a problem
compartment. Eventually a module of hags which are on
line but are not being cleaned will fill up with a dust load
until the hanger springs fail dropping the bags. Their filter-
ing load is also transferred to the remaining compartments,
shortening the overall  bag life. If the total cleaning mode
fails, the baghouse will build up pressure drop until it in-
terferes with the process. At this point the bags are well
loaded with dust. If either the main I.D. fan is shut down or
one compartment is cleaned normally, following the resto-
ration  of the cleaning cycle, the whole baghouse dust load
will drop into the screw conveyors. This should be antici-
pated in the dust removal system design.  If not, the han-
dling equipment may stall or flood causing further prob-
lems.
Hydraulic Control System

  An hydraulic damper control system was chosen to pro-
vide repeatable damper motions at controlled speeds. The
hydraulic power-pack was located next door to the  bag-
house control panel. Copper instrument tubing bundles
each with eight % in. tubes were run thru an 8 in. under-
ground conduit pipe to  the baghouse interior. The system
includes a variable volume 5 gpm 500 psig pump, a volume
control valve and V4 in.  direct solenoid operated hydraulic
valves. The cylinders (2 in. dia X 8 in. stroke) are located
along an interior walkway and drive  through a pipe exten-
sion of the rod to the damper arms overhead. All cylinder
rods are retracted when  the compartment is on line.
           Figure 3. 14 compartment baghouee wRh one
           compartment malfunctioning.
        4.0
     o
     $
        3.5
     •&  3.0
EXPANDED
PRESSURE
 TRACE
                0  10  20  30 40  SO  60
                 Cleaning cycle time, seconds
     Figure 4. Variation of baghouse Inlet prewure during cleaning
     cycle.
Reverse Air Draft Control System

  This  system adjusts the reverse air manifold  draft by
damping the fan performance to suit the variable pressures
to be expected at the baghouse inlet. Draft is maintained at
2.5 in. of water using a control damper in the duct operated
by a 25  sec/100" rotary actuator. The control instrument is
on the baghouse panel where the draft fluctuation can be
observed during the cleaning cycle. A fan ammeter is also
helpful  in discovering any reverse air damper malfunctions.
For convenience, the dedusting line from the dust elevator
and  storage  bin is  connected upstream of this control
damper. This provides some flow to prevent controller
wind-up should the  reverse air dampers be unusually gas
tight.
  One aim of the iystem is to prevent the collapsed  bags
from enduring the  full  reverse air fan suction if damper
timing or operations are maladjusted. It also provides  for a
constant cleaning effort even though the baghouse  inlet
pressure may be rising or falling requiring more or less fan
static pressure to effect the cleaning.
  The draft  pickup tap on the manifold  rarely plugs in
contrast to the baghouse inlet pressure tap.
                                                        3-14
                                                                         Journal of trw Air Pototion Control Association

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            Figure 5. Pressure trac« with changing pro-
            cess volume.
Bag Collapse Adjustment

   The baghouse cleaning cycle has been adjusted to pro-
vide a very gentle bag collapse. Primarily the reverse air ac-
tion is interrupted at a time when the bag is nearly col-
lapsed but well before  the bag is drawn into  the full star
shape and drawn tightly down towards the tube sheet col-
lar. This type of cleaning cycle is successful both due to the
precision of the hydraulically-operated dampers and to the
controlled supply  of  reverse air volume. It  is noteworthy
that this type of cycle has been in use for the past 9 months
and spanned the end of the life of one set of bags and the
start of the new set of replacements.
   This operation was instituted to try to correct two of the
worst problems in fiberglass bag  life  under  reverse air
cleaning.
1. The severe stresses that occur at the bottom of a fully
   collapsed  bag  causing premature failure at the maxi-
   mum flexure points.
2. Overcleaning of the  bottom few feet of the bag during
   the maximum collapse of the cleaning cycle.
   Operating the baghouse under this type of cleaning ac-
tion should provide benefits for any type or style of bags,
and allow the maximum possible bag life based on failure
due to flexure.

Baghouse Performance

   Figure  7 shows  emission concentrations versus bag ser-
vice life based on emission tests conducted on the exhaust
stacks of both  baghouses. As expected, it indicates an in-
crease in emissions over the life of the bags.
    BagLIf*

      Figure 8 shows that there  is some  correlation between
    lower filter rate and longer bag life. The net air quantity as
    used here includes an estimated 10,000 acfm of reverse air
    volume. Bag life is the point in the life cycle where bag fail-
    ures become so numerous that it is more economical to
    rebag the whole baghonse. No attempt was made to keep
    track of individual  bags and  their average life. The bag-
    house with  the longer life enjoyed a normal 2 to ,T in. water
    pressure  drop whereas the other was  normally 4 to 5 in.
    water. However, the first set of bags for the large kiln were
    not included in the data as they were  replaced after  6
    months of operation. This is an unusually short life and a
    result of a combination of start up problems. The following
    corrective measures were taken and a resultant improve-
    ment to a 22 mo life was realized.
            .015
            -010
          O  005
                          \
                 Pin holes found ^
                  m a tew bags
                             o
                                         DUST
                                       EMISSIONS
                         Single point sample
                           I
                                       I
                0          6           12
                            Months in service
            Figure 7. Increase In dust emissions with service lite.
18
          Figure 6. Pressure trace with plugged sensing
          tap.
    1. The cooling tower outlet temp was reduced from 550° to
       510°F and the shutdown interlocks reset from 600° to
       550° F.
    2. The cooling  tower water spray atomization system was
       improved  to reduce and later eliminate the bleed in air
       cooling which had been inflating the total volume.
    3  Doghouse  hoods were installed over the reverse air con-
       nections at the hoppers in order to reduce clogging of
       the reverse air ducts. Figure 1.
    4. The cleaning program system was revised to give damp-
       er operations in sequence. This insured that the cleaning
       cycle could function properly even if there were small
       amounts of dust in the ducts slowing down the damper
       strokes, which were also adjusted to 10 sec, to give maxi-
       mum power  and minimum  bag popping during reinfla-
       tion.
    5. The  slipping  damper operator arms,  which caused
       damper malfunctioning, were corrected.
    6. The system was adjusted to provide a gentle collapse of
       the bags.
January 1076     Volume 26, No. 1
3-15

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THE  USER AND  FABRIC  FILTRATION  EQUIPMENT
Table II. Cleaning cycle description.
 Start of cycle
 Ihlct valve closet
 Rev. valve opens
 Kcv. valve closes
 Inlet valve opens
 Initial drag
 Final drag
                        Ba^house
  3.3 in. W.C.
  3.5 in. W.C.
  3.8 in. W.C.
  3.5 in. W.C.
  3.:.' in. W.C.
2.1 in. W.C./fpm
1.3 in. W.C./fpm
                                           Volume
    9,000 acfm
    None
(-) 14,000 acfm
    None
   13.700 acfm
                                                                    25
2.0
                         | 1.5
                         i
                                                                   • Large kiln
                                                                                                  • 4 kilns
                                                                                1
               LIME KILN
               BAG LIFE
                                                                                        J_
                                                                                                   I
                                                                      0        12       24        36
                                                                                   Bag life, months
                                                                  Figure 8. Effect of net fitter rate on bag life.
                                                                                     48
Moisture and Corrosion

   After 3 years of operation, the extended walls have pre-
vented dust or mud buildup in the screw conveyors or bag-
house hoppers, even though these portions are not insulat-
ed, and there are no vibrators on the hoppers. The side wall
insulation has prevented any stiffness or acid salting in the
bags near the exterior walls. The baghouse modules interior
steel  is scale free and in good condition. The 316 stainless
steel  outlet stacks and  H.R.S. manifolds  are also in good
condition.  The  ground level access doors have  been re-
moved and a louvre installed above the inlet duct to pro-
vide some air circulation for cooling.

Cyclone Precleaners

   The coarseness of the dust has not caused wear on the fi-
berglass bags even though precleaners were not incorporat-
ed, although a dropout effect is obtained in the down flow
cooling tower. However, precleaning is a problem at one of
the two other Bethlehem Lime Plants and at a preheater
type lime kiln plant in Nevada.1 At  the  latter, two kilns
each with 10 ft dia cyclone precleaners provided very good
bag life after 18 months of operation. A third kiln, with a
multitube precleaner converted  to a dropout box, experi-
enced unusually bad bag life during the same period. Plans
were to reinstall the multituhes at the earliest opportunity.
   Bethlehem's problem occurred in collecting dust  from
the lime cooler  portion of a rotary  hearth (or doughnut
shaped) kiln at one of two different locations. The first in-
stallation is a quite  successful reverse pulse type cleaner
operated at 4 fpm, 5 to 6 in. water, 240° F while the second,
                                      a modular shaker type unit was spectacularly unsuccessful
                                      when operating at a net 2.5 fpm, at 2 to 3 in. water, 200°F.
                                      Table III shows the dust size for all three type units. The
                                      dust from the fiberglass baghouse is as coarse as the hopper
                                      sample from the shaker. The dust from the successful pulse
                                      collector is coarser than the shaker sample collected at the
                                      tube  sheet  after passing through  the  broken bags. The
                                      coarse material found on the tube sheet was  blamed for the
                                      bag failures and was removed  by installing a retrofit cy-
                                      clone precleaner. The shaker baghouse had, by then, oper-
                                      ated  12  months  with daily bag  replacements  totaling
                                      $10,000/mo in direct bag and labor costs. The bag life has
                                      been good since the cyclone installation.
                                        However, it should also be noted that both the successful
                                      fiberglass baghouses and the successful reverse pulse bag-
                                      house have an inlet arrangement conducive to good dust
                                      settling in the hopper. They have  an overhead duct with
                                      round 90° elbows from the duct into the upper portions of
                                      the hopper. The down comer elbows promote dust migra-
                                      tion to the outside or bottom of the elbow where it can drop
                                      into the  hopper without being  swept upward with the air
                                      flow into the bags. The unsuccessful modular shaker and
                                      the fiberglass baghouse in  Nevada do not  have this same
                                      feature and instead must rely on precleaners. The shaker
                                      has an opposite  mounted inlet duct with side outlets com-
                                      ing directly into the hoppers  at a  point too low in the hop-
                                      per as it was chosen to clear the cross bracing. The manu-
                                      facturer's literature for the  Nevada fiberglass  baghouse
                                      shows a  square  tee connection feeding opposite compart-
                                      ments which would also be  a very turbulent and non-segre-
                    Table III. Dust size comparison.

Unit
Source
Topsi/c
-100 Mesh
-200 Mesh
-325 Mesh
—5 micron
Baglifc, mo
Fiber
Klass
Hopper
20 Mesh
99%
88%
10%
N.A.
22-34
Reverse
pulse
Hopper
1 ti Mesh
28%
6%
N.A.
N.A.
18+


Shaker
Tube sheet
1 0 Mesh
27%
17%
11%
N.A.

Hopper
100 Mesh
98%
83% •
63%
15%
0-2
Cyclone
and shaker
Hopper
3L'5 Mesh
All
All
1%
72%
11 +
                                                         3-16
                                                                         Journal of th« Air Pollution Control Association

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     14 TIPS  FOR  FIRE  PROTECTION  OF  FABRIC  COLLECTORS*

  ^  ....... ..,._..       	   i i«nr •>• .mi mini i	mi i •MMOTtM a iiiiimMWlNI. W (MtJIMIIP V00.
     ti.  I :se of only noiicomlnistible materials in construction of the collector.
     7.  Fabric filters should he. of low hazard materials. Where  combustible media must  be  used, flame retardant  treat-
        ment  must be provided.
     H.  K.ibnc fillers should be in metal  enclosures with provision made for denning the filters.
     9.  Collectors should be constructed so as to eliminate ledges or other points of dust  accumulation. Hopper bottoms 1
        should be sloped and the discharge system designed to handle maximum flow.
   10. The collector should be designed to handle the maximum possible operating pressure to which it might be subject-
       ed.                                                                                                 '     I
   11  Clean-out doors or panels should be provided for interior cleaning of collectors.
  . ]'£. Kxplosion relief venting should be provided.
   ];?. Coili••!  tors should be located outside of buildings unless the interior room has an exterior wall that incorporate*, ox-
       pli'sinn  relief venting, with  other walls,  floor and ceiling designed to withstand a pressure of  1'.'•  psi. The room
       should be equipped with automatic sprinkler protection.
   14. To protect ngainst damage,  in addition to explosion relief \enting or design of a collector to withstand maximum
       peak explosion pressure, the following features should  be provided:                                             -f
       a.  Kxplosion suppression system.
                                                           OR
       h.  Collector should be designed to operate using an inert or oxygen deficient gas as  an operating medium.
                                                           ''/?
       c.  Ci 11 lectors containing combustible  parts should be equipped wit h nutomnl ic sprinkle is.
  ^s			  ...  ..   ....^J
   * "Fire Protection for Fabric Collectors," Richard A. Gross, Factory Insurance Association, Buffalo, NY.
   ** Recommendations to protect collectors can be derived from loss experience. Also, NFPA Pamphlet #66 contains valuable guid-
   ance regarding collectors used in air conveying system* which can be applied to collectors used in other applications.
gating arrangement. We are now more aware of this impor-
tant feature and intend to review carefully the manufactur-
ers' design of this function in the future. The buyer should
be especially aware of standardized modular units built for
a variety of applications for the lowest cost, for a preclean-
ing cyclone retrofit is an expensive correction.

Conclusions
  The fiberglass baghouse has proven to be a satisfactory
dust collector for the exhaust of these lime kilns. We expect
to continue or to improve upon the bag lives experienced to
date. Above all, the  units have provided good collection of
the dust satisfying state emission limits.  Interruptions to

January 1876     Volunw 26, No. 1
the kiln operations have been nominal and the extra draft
provided has improved production on all the kilns to the
limit of other equipment in the kiln system.

References
1. David J. Krohn, "U.S. Lime Division's dust abatement efforts
  whip pollution problem," Pit & Quarry (May 1974).
     Mr. Cage is in the Mining Division of Bethlehem Steel
   Corporation. Bethlehem, PA.
                                                         3-17

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                     VI-4


                INS AND OUTS
                      OF
               GAS FILTER BAGS
                      by

                John C. Walling
                Joy Manufacturing Co,
Copyright  (c) 1970 by Chemical Engineering.
McGraw-Hill.  Reprinted with permission from
the October 19, 1970 issue.
                      4-1

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 Operation  & Maintenance,
                                                Ins   and  Outs
                                                Of   Gas
                                                Filter   Bags
Here is a guide to selecting the synthetic-fiber bag cloth and the bag-cleaning technique
that could most efficiently and economically handle your gas filtering problems.
                              JOHN C. WALLING, Joy Manufacturing Co.
  Until recently, the so-called natural fabrics-cotton,
wool  and  silk—were  all that  were  available  for
gas filtration, despite  their  limitations  with respect
to resistance to temperature, chemical  attack, abra-
sion and wear. The development of glass-fiber cloth
capable  of  withstanding continuous  temperatures
up to 600 F. made cloth practical in  gas filtration
applications from which it was once precluded.

Synthetic Fibers for Dust and Fume Control

  Acrylics—Synthetic fibers for filter cloth are made
from materials  having a molecular structure  that is
naturally,  or which can be  made crystalline.  The
proper orientation of this crystalline structure during
the extrusion process  gives  the fiber  the necessary
tensile strength. The most suitable member  of  the
acrylic family of plastics with regard to crystalliniry
is acrylonitrile, which yields an oriented fiber having
a tensile strength in the neighborhood of 40,000 psi.
  An acrylic fiber  by  definition is one  containing a
minimum  of  85%  acryonitrile.  Acrylic  fibers  are
available in the U.S. under  such  names at Creslan
(American  Cyanamid Co.), Orion  (Du Pont Co.),
and Acrylan (Chemstrand, div. Monsanto Co.). The
maximum recommended  gas temperature is  about
275 F. Fibers of 100% acrylonitrile resist a  some-
what higher temperature. Acrylic fibers exhibit ex-
cellent resistance to hot, acid atmospheres.
  Poll/amides (nylon)—Polyamide resins  are naturally
crystalline materials that can be quenched and cold
worked to  yield  a tough, oriented fiber having a
tensile strength of about 80,000  psi.   Nylon  fiber
is available from American Enka Corp., Allied Chem-
ical Corp. and Firestone Tire & Rubber Co. (Nylon
6), Celanese  Corp.,  Du Pont  Co.  and  Chemstrand
(Nylon 6.6).  It is suitable  for gas temperatures to
about  250  F. This is lower than  that  for many of
the other synthetics. However, the excellent abra-
sion resistance of nylon makes it very  suitable for
filtering abrasive  dusts when temperature is  not a
factor. It  resistance  to alkalis is good  under  most
conditions, but its resistance to mineral acids is poor.
  Nomex (nylon)—A.  special nylon  capable  of con-
tinuous service in  gas temperatures as  high as 450
F., Nomex's (Du Pont) resistance to fluorides makes
                                            4-3

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OPERATION AND  MAINTENANCE . .  .
                             Properties of Synthetic Fibers for Gas Filtration
                  Material                     Acrylic

Maximum temperature (dry). °F.....	  275

Abrasion resistance	  Good

Resistance to acids	  Good

Resistance to alkalis	  Fair

Tensile strength, psi	  40.000
 Nylon    Nomex    Teflon   Polypropylene   Polyester

250       450     500       225           275

Excellent  Good    Poor      Good          Good

Fair       Good    Excellent  Good          Good

Good      Good    Excellent  Fair           Good

80,000    80,000  20,000    110,000       80,000
Cost/sq. ft. of surface	  Moderate  Moderate  High
                   High
Low
Moderate
                    :iai!iBiBinraui«»™iEi™i»im
it  superior to glass in many high temperature appli-
cations.  It  resists  acids  and  alkalis  better  than
conventional nylon. It is,  incidentally,  the material
used  in the Apollo space suits.
   Fluorocarbon.1 (Teflon)—Fluorocarhon  resins  have
unique properties because of the presence of fluorine
atoms in  the  molecular structure.  They  are prac-
tically  inert to  chemical  attack.   In  addition,  the
temperature  resistance of the fluorine-carbon bond  is
such  that fluorocarbons retain their  properties  at
temperatures  higher  than  any .other  carbon-chain
plastic. Fluorocarbons are  crystalline in  nature, and
suitable  for  filler  applications.  Teflon  is  produced
in multifilament  and monofilament  form. The  multi-
filament material can be used  continuously  in gas
stream  temperatures  up  to 500 F. Its main  disad-
vantages  are its high price  and poor  resistance  to
abrasion.
   Polypropylene—\ polymeric  resin of the polyolc-
fin family,  polypropylene's  qualities are similar  to
polyethylene except that its heat resistance is higher.
The  action  of  the  catalyst  induces  a crystalline
structure  that makes polypropylene very suitable  in
the fiber form.  Its tensile strength is in the neighbor-
hood  of 110,000 psi.
   Polypropylene fiber offers the combination  of low
price  with high  resistance  to acid  and  alkali  attack.
However, it  should not be exposed continuously  to
gas temperatures higher than 225 F.  Its  abrasion
resistance is good.
   Polyesters—Certain of the long-chain polyester resins
can be converted to fiber form by cold working after
extrusion  to  produce  an enforced  crystalline  struc-
ture. Their tensile strength averages  about 80,000 psi.
   Polyester fibers (Dacron, Fortrel,  Vycron,  etc.) are
a  popular  synthetic filter  material  having  good all-
around  resistance to chemical  and abrasive  attack,
combined  with  gas   temperature   tolerance  up  to
275 F.
   Manufacturers of synthetic cloth filters can suggest
finishing techniques—such  as heat  setting, scouring,
resin  treating, calendering  and napping—appropriate
to the application that will improve bag life, dimen-
sional stability, permeability and ease of cleaning.
  Evaluating Cleaning Methods

    A  comparison  of the gas-cleaning efficiencies  of
  bag filter  cloths is  not basically important, because
  almost all commercial filter materials remove 99+% of
  the dust and fume  from gas streams. All  else being
  equal,  the greatest  variety  in  design  is  found  in
  the technique by which the filter bags  are cleaned.
  This is  an  important factor because it not only affects
  the size of the baghouse,  but also  its cost, mainte-
  nance requirements  and  dependability.
    For a specific filter  material,  the  size and cost  of
  a  bag filter  depends  primarily on  the area of filter
  surface required and  the amount of space available
  for filtration  within  the filter  housing. The  basic
  parameter  of  haghouse si'/c vs. capacity  is the air-to-
  cloth ratio, i.e., the total cu. ft./min. of gas filtered
  divided by the total sq. ft. of filter cloth in the filter.
    Most  of the  factors  that  affect  the air-to-cloth
  ratio  are linked to  application,  such as  gas temper-
  ature,  dust  loading,  filter cloth material  and dust
  characteristics. However, a factor that bears directly
  on the  ratio  is the percentage of bags out of service
  for cleaning  at any  one time. Another is the amount
  of flexing  and creasing  imposed on the filter cloth
  by the  cleaning mechanism.
    In  recent  years,  new  filter-cleaning techniques
  have  been developed  that  allow  significant increases
  in  the  air-to-cloth  ratio  without affecting  the life
  of  the  filter  cloth,  filter dependability, or mainte-
  nance  requirements.  These advances allow  a cor-
  responding decrease in  filter  area  and filter space
  requirements  for the same application and capacity.
  In  the  case  of some of the  new  high-performance
  synthetic  fabrics  (which are  costly  on  a square-
  yard  basis)  high air-to-cloth  ratios  are essential  if
  first cost is to remain low enough to make their use
  economically practical,

  Popular Cleaning Methods

    Backwash   (Fig.  1)—The principle of   removing
  collected material from  a filter medium  by  back-
  washing is probably  as old as  filtration  itself.  In
                                                    4-4

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 OPERATION & MAINTENANCE .  . .
         Filtering cycle        Cleaning cycle
       Filtering cycle      Cleaning cycle
                                                             Shaker
 BACKWASHING removes dust by collapsing bags—Fig. 1       SHAKER cleaning requires isolation of bags—Fig. 2
         Filtering cycle    4   Cleaning cycle

      Compressed
      air headers
     Filtering cycle
Cleaning cycle
Ejector
retracted
         Ejector
         in position
                                                                                                    Pressure
                                                                                                    waves
                                                                                                    plus
                                                                                                    backwash
EJECTOR cleans bags with a short burst of air—Fig. 3        IMPROVED EJECTOR (patented) also backwashes—Fig. 4
                                                    4-5

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the case of ban-type industrial gas filters, filter cake
is discharged partially  or completely by collapsing
the hags during the backwash  cycle. The resulting
flexing  and  creasing  of  (he filter material  is  the
major  cause of bag deterioration in this  design. In
addition,  the section of the housing containing  the
bags being backwashcd is necessarily out of service
during  this cycle. These two factors  limit the air-to-
cloth  ratio  of  backwash-type filters  to from  1:1 to
3:1, depending on  the severity of the application.
   Shaker (Fig.  2)—In   some  applications,  the  filter
cake  can  be  discharged  by  simply  isolating  the
bag from  the  gas stream  and  shaking it,  either
manually or automatically. This  method reduces bag
flexing  and extends bag life, compared to the back-
washing technique.  In some cases, both backwashing
and shaking are used simultaneously. Air-to-cloth ratios
for shaker-type bag  filters range from as low as 1:1 to
as high as  5:1.
   Ejector (Fig.  3)—Both backwash- and  shaker-type
bag filters  suffer  from  the same  limitations:  (1)
frequent cleaning cycles that cause rapid deteriora-
tion of  the filter material,  and (2) a large proportion
of  the  filter must  be removed  from service  during
the cleaning cycle.  The ejector  principle is a recent
development that is suitable for  many  applications,
and is capable of operating at air-to-cloth ratios up to
15:1 with  reasonable material life.
   Contrary to  the  conventional  backwash or shaker
arrangement,  gas  flows  from  the  outside  to  the
inside,  and the bag shape  is maintained by means
of a wire frame  (Fig.  5). A compressed  air  no/./.le,
usually  augmented  with  an ejector tube, is positioned
above each bag outlet. A short burst of compressed air
creates  a pressure  wave that travels down the  bag
and pneumatically  inflates it as it passes.  This action
discharges  the filter cake from the bag.
   Gas flow through the bag effectively stops during
the period  this  pressure  wave  exists,  even  though
the bag is  "onstream"  as far as the system  is con-
cerned. There is  therefore no need to isolate sections
of  the  filter for cleaning.  Theoretically,  each  bag
can be blown individually.  As   a practical matter,
                                    Cleaning
FRAME retains bag shape in ejector cleaning—Fig. 5
however, each row of bags  is usually piped to  the
same compressed air header, and blown simultane-
ously.
   Improved Ejector (Fig. 4)—The conventional ejector
design depends  on a controlled  flexing of the filter
cake to  break and  dislodge  it from  the filter cloth.
In many cases, this  is inadequate for proper cleaning.
Unforeseen conditions-such as moisture  condensation
on  the filter surface,  electrostatic attraction  between
the  cloth  and the  filtered material—will cause  the
filter cake to "ride" with  the cloth  as it  flexes,  in
which  case the ejectors arc ineffectual and the filter
must be  taken out of service.
  The improved  design  mechanically seals  off each
bag as it is blown and causes  a positive  backwashing
action  that, in addition  to the flexing action of  the
pressure  wave,  thoroughly removes hard-to-dislodge
filter  cakes.  This  development greatly  extends  the
application range and reliability of the  ejector-type
filter, while retaining its other advantages.

Maintenance and Access

  The failure of  even  a  small  proportion of  the
filter bags in an industrial gas filter  seriously  com-
promises its efficiency. Present day concern for pollu-
tion makes it imperative that broken filter  bags be
replaced promptly. A well  designed filter must there-
fore allow  quick and easy  access for  the  removal
and replacement  of  individual bags.
  Conventional  filters  depend on walkways within
the filter housing for access to individual bags. These
walkways  take up valuable filter space. In addition,
manhandling  filter  bags into  place  within the  re-
stricted  confines  of the housing is   expensive  and
time consuming.
  This problem  can be resolved by an  arrangement
in which the bags  and bag  cages can be inspected
and removed from  outside  the  filter housing.  Dag
replacement becomes a simple matter of  (1) push-
ing aside the ejector, (2)  releasing three hold-down
bolts, (3)  lifting the old bag out, (4) dropping  the
new bag in,  (5) refitting  the the hold-down  bolts
and (6)  repositioning the ejector.
  A complete filter can be rebagged  by this method
in  a  fraction  of the  time formerly  required,  and
access  walkways  within  the housing  are also  elim-
inated. •
John C.  Walling  is a senior
applications   engineer  with
the Western Precipitation Div.
of Joy  Manufacturing  Co.
(1000 W. Ninth St., Los Ange-
les. Calif. 90054). A graduate
of University  of Washington,
where  he received a B.S. in
mechanical engineering  and
a B.A. in business administra-
tion, he  joiiied Western  Pre-
cipitation after a  stint with
American Standard Co.  His
background includes 10 years
of experience in gas handling
and cleaning, mass transfer,
and air  and water pollution
control systems.
                                                     4-6

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                     VI-5.

                 BAGHOUSES:
          SEPARATING AND COLLECTING
               INDUSTRIAL DUSTS
                      by

                 Milton N. Krauss
Copyright  c  1979 by Chemical Engineering,
a McGraw-Hill publication. Reprinted with permission
from the April 9, 1979 issue, pp. 94-106.

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                                     Baghouses:
                                  separating and
                                                 collecting
                                 industrial  dusts
                      The selection and operation of baghouse collectors is mainly
                        an art. Here is practical information on filter fabrics and
                          cleaning; baghouse design, operation and servicing; and
                        methods for protecting the system from Jire and explosion.
                                  Milton N. Kratts
Q The separation of dusts from industrial-gas streams
may often be done by using filters made of natural or
synthetic fibers. These filter elements are bag-shaped,
and placed in structural enclosures called baghouses.
  In addition to supporting the filter' elements,  the
baghouses contain baffles for directing airflow into or
out of these elements, equipment for cleaning the fabric,
and a hopper for collecting and discharging the dust.
  Baghouses are used to control dust nuisance when-
ever dust-laden air must be discharged to the atmos-
phere, or to, recover valuable dust 'from a process
venting system. Sizes may range from small bin-venting
filters to large multicompartment filters that receive
dust from an extensive system of exhaust ducts.
  Dust filtration and collection is an art. The sizing and
selection of dust filters are based on (a) past experience
and (b) actual tests that use specific fabrics at specific
dust-to-air loadings. Because of the many types of col-
lectors and filter fabrics, combined with numerous mar
terial properties that affect filtration, the selection of a
filter should be made by actual test at the design load,
using the least-costly fabric for the requirements.

Separation principle for filters
  Dust filtration in baghouses is accomplished by pass-
ing the dust-laden air through a filter fabric that is
formed into cylindrical tubes or oblong bags. As the air
passes through the fabric, the dust is retained on  the
surface and in the interstices of the yam strands, build-
ing up a filter cake that also acts as a filter medium. In
order to reduce the resistance to airflow (so as not to
affect the flowrate), the filter cake must be periodically
dislodged. The method" of cleaning the fabric is the
basic difference between various filters.
  The greater the area .of filter cloth provided for a
given dust loading on a specific baghouse, the longer it
takes for a filter cake to build up that will affect the
airflow. In comparing the area of filter fabric offered by
different vendors, we  should consider the  different
methods of cleaning the fabric. The criterion should be
a constant pressure drop across the fabric for a specified
airflow rate and a specified dust loading, rather  than
the term "air-to-cloth ratio."
  If checked dimensionally, this term is the face veloc-
ity of the air (ft/min) through the effective area of the
fabric for a given type of filter. The air-to-cloth  ratio
only has significance when comparing the performance
of a particular manufacturer's line of baghouses when
handling different materials. The cleaning method es-
tablishes whether a baghouse is suitable for continuous
or intermittent service.

Properties of filter  fabrics
  In order to function properly, a filter fabric  must
have the following properties:
  Permeability—The fabric must be sufficiently porous to
94
                                     5-1
                             CHEMICAL ENGINEERING- APRIL 9, 1979

-------
permit a satisfactory flow of air. The permeability of a
fabric is generally  stated  as  the  clean  airflow  in
(ft3/min)/ft2  of fabric,  at  a pressure differential  of
0.50 in.  water column, as  determined by the Frazier
test.*  However,  the  Frazier  permeability  alone  is no
guide  to fabric selection because materials having the
same permeability may not have the other characteris-
tics required  to  successfully  filter a specific material.
Also, the filter cake  formed on  the fabric surface will
reduce airflow.
  Mechanical strength—The fabric must resist the tensile
forces caused by  the operating pressure differentials, by
mechanical shaking  during cleaning,  and by pulsing
during reverse airflow.  It  must  withstand  abrasion
where it is  clamped to tubesheet ferrules, where it is
supported on metal  grids or retainers, and where it is
subjected to the  impact of the filtered materials.
  Solids  retention—Fabric construction must  be  open
enough to prevent the accumulation of fines in the
interstices of the yarn, yet tight enough to prevent fines
from  blowing through  the fabric. This may  seem
anomalous, but  the  filter cake  on the surface of the
fabric also  acts  as a filter medium and narrows the
interstices.
  Corrosion resistance—The fabric  must resist attack and
weakening due to chemical action between the fibers
  "ASTM Standard D-737, Standard Method of Teat for Air Permeability of
Teatile Fabrics, American Soc. for Testing and Materials, Philadelphia, Pa.
and the filtered materials, especially if moisture is pres-
ent due to condensation.
  Heat resistance—From some processes, the fabric must
resist high-temperature exhaust gases. Each fabric has a
definite temperature limit beyond which it will tend to
disintegrate.
  Cleanability—The  fabric must have a  surface texture.
that is  conducive to  rapid release  of  the filter cake
during cleaning. This, too, may seem anomalous, since
the surface must also  retain  the cake.  However,  the
forces involved during cleaning are usually the airflow
in the opposite direction to  normal  airflow,  and me-
chanical flexing of the fabric. The fabric should have a
high rate of electrostatic-charge dissipation so as to shed
charged dust  particles.
  Dimensional  stability—The fabric must resist stretching
or shrinking that would  affect its permeability. Many
untreated fabrics tend to increase in permeability after
laundering.
  The properties of commonly used filter fabrics  are
shown in Table L

Fabric construction
  A knowledge of the construction of filter fabrics will
indicate how well the preceding property requirements
can be met. However, final selection should be made by
test and by an economic study that weighs  first cost
against  probable maintenance expense.  All fabrics will
                                                     5-2
                                         CHEMICAL ENGINEERING APRIL 9, 1979
                                                  95

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                                                   BACHOUSES
eventually blind when handling fine materials, or ma-
terials that crystallize or polymerize when embedded in
the fabric. It is better to replace all cloth in a filter at
regular intervals (as determined by experience or by
periodic permeability tests) than to operate the unit
until airflow is drastically reduced and other problems
are created.
   Filter fabrics may be felted or woven. Felted fabrics
are composed of fibers that are compressed  under high
pressure and are relatively thick. The pressure enmeshes
the fibers, and in so doing forms a strongly bonded
sheet of material having complex labyrinthine inter-
stices through which air can pass  in three directions.
Synthetic monofilament fibers  cannot form a strong
bond between themselves and are usually pressed over a
woven sheet of scrim fabric for strength. Wool fibers are
naturally barbed and have a stronger bond than syn-
thetics. Most felts can absorb moisture due to capillary
 Properties or cornmorvfilter fabrics
 •"~-	   •' -
action  between  the fibers—even though the  fibers
themselves are moisture resistant.      .
  A simple test for relative absorption of moisture is to
stand equal lengths of fabric on edge in ajar containing
a shallow  layer  of water. After  several  minutes, the
samples should be withdrawn and the height  to which
the water has risen in each sample noted.
  The  thickness  of felt provides for maximum dust
impingement. The many changes in  direction due to
lateral airflow traps the finest particles. Inspection (a)
by  ultraviolet light of the cross-section of felt fabrics
that have handled dust containing fluorescent materi-
als, and (b) by microscopic examination of the  cross-
section  of felts handling other types of dust, shows that
the particles rarely penetrate through' more than one
third of the felt thickness. While there is a reduction in
air permeability,  this is not as much as would occur in a
woven fabric having the same quantity of trapped dust
per square inch of filter area.
  Felted fabrics are used for'maximum product recov-
ery where filtered air is recirculated or discharged  into a
work space. Felts  are primarily used in collectors having
a high  face velocity for airflow (i.e., high air-to-cloth
ratio).
  Woven fabrics are composed of twisted yarns that are
woven into geometric patterns having various spacings
between the  yarns, and with a specific surface  finish
that is designed to retain or shed a filter cake, depend-
ing upon the application. The permeability of these
fabrics  depends on the type of fiber, the tightness of
twist and the size of the yarn, the type of weave (geo-
metric pattern), the tightness of weave (thread count),
and the type of surface finish. Correct coordination of
all these factors for a successful filter application cannot
be done by abstract analysis. The  fabric must  be  tested
in a dust collector considered suitable for a particular
application on the basis of operational and economic
factors.  Woven fabrics have a low ratio of weave  open-
ings to  yarn  area and  require a surface  filter cake to
retain the dust. This limits the face velocity for airflow
96
                                         CHEMICAL ENGINEERING APRIL 9. 1979

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                        Shaker
                        motor "\
Eccentric    Bap suspension  ^Partition
  rod       / bar      /
             Reverse-air
             damper in
              normal
              position ^

           To exhaust fan
            Compartment
              in service
           Level indicator	_•
             for dust
                                                  Reverse-air
                                                  damper in
                                                  cleaning
                                                  ^position

                                                To exhaust fan
                                               "" Fresh-air damper
             Top of silo-'"
                                                                                 	---Compartment
                                                                            -*--'"°~       ht»inn /•Ipanorl
to low values [1.5 to 3 (ft3/rnin)/ft2] in order to prevent
dust from blowing through the openings and destroying
the filter cake.
   The permeability of woven fabric is very dependent
on its construction. A review of textile terminology will
prove useful in understanding the effects of yarn design
and fabric weaving on permeability. The yarn may be
made from:
   1. Staple—the term for short fibers of cotton or syn-
thetic  material—drawn  into  parallel  strands  and
twisted  into yarn by spinning.
   2. Continuous filaments of synthetic  material, made
by extruding a solution  through a  finely perforated
nozzle and then spinning the filaments into yarn.
  -The degree of twist of the yarn affects permeability. A
tight twist resists penetration by  dust particles, and a
loose twist permits retention of dust, or "blinding"  of
the yarn. Filament yarns are stronger than staple ones.
Spun yams have more flexibility than  filament yarns
and more resistance to repeated flexing.
   Weaving is the interlacing, at right angles, of a series
of yarns to form a textured fabric. The basic weaves are
plain, twill and satin (Fig. 1). In the plain weave, each
crosswise or  filling yarn  passes alternately  over' and
under one warp or lengthwise  yarn. Each warp yam
passes alternately over and under each filling yarn. This
type of construction makes the tightest weave; by vary-
ing the thread count, the weave may be made as open
and  porous as required.
   In the twill weave, the filling yarn passes under one
warp yarn, then over two or more warp yarns. In each
succeeding row, the filling thread moves  one warp ei-
ther left or right, forming  the diagonal lines that iden-
                                                    5-4
                    tify a  twill weave. Twills have fewer interlacings than
                    plain  weave  and, depending on the  thread count,
                    greater permeability.
                      In the satin weave, the filling yarn passes over one
                    and under four or more warp yarns in the next row.
                    This weave  has few  interlacings, spaced  widely but
                    regularly. It has a smooth surface and greater permea-
                    bility than the other weaves. Cotton fabric in this weave
                    is known as cotton sateen. Regardless of weave, the size
                    of yarn and the number of warp  and filling yarns per
                    inch (thread count) greatly affect permeability. A low
                    thread count will permit higher  airflow rates at low
                    pressure drop, whereas a high count will trap more fine
                    particles and  produce a higher pressure drop.
                      The woven fabric may have various finishes. Cotton
                    fabrics may be preshrunk to maintain dimensional sta-
                    bility;  They may also be napped in order to  hold a filter
                    cake. (Napping is the process of scratching the  cloth
                    surface to raise the  fibers and form  a  soft pile.) Syn-
                    thetic  fabrics may be heat-set to  obtain dimensional
                    stability and a smoother surface with uniform  permea-
                   'bility.  All  fabrics may  be silicone-sprayed to obtain
                    abrasion resistance, improved  cake release, or low mois-
                    ture absorbency.
                      Many other fabrics are available for special  applica-
                    tions.  Selection of these should be  made only  after
                    determining that the common fabrics are not suitable
                    for the proposed service, and after discussing the appli-
                    cation with specialists in filter-media firms.

                    Example of performance
                      The following example illustrates the unpredictabil-
                    ity of fabric performance. Specifications for  the original
                                        CHEMICAL ENGINEERING APR|L 9_ ,979
                                                                     97

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                                                        BACHOUSES
  -2
  -1
c
co
                            Cleaning cycle
     ;;• '.'Atmospheric pressure .'.j,-;.';: -i/^-:?-^',*:. *'z.-.~~'"."- ",', !; :'V"-i  .
           100
                  200
300    400
  Time, s
                                       500
                                              600
                                                     700
      bag material as furnished by a filter manufacturer are
      compared to a replacement material supplied six years
      later by the same manufacturer for the same filter and
      the same service.
         Fabric manufacturer
         Material
         Thread dia., in.
         Thread count, yarns/in.
         Weave
         Permeability, (ft3/min)/ft2 20 to 25
           (Frazier)
        Original
        ABC Co.  .
        Orion
          filament
        0.0085
        70x68
        Twill
Replacement
XYZ Co.
Orion
  staple
0.024
90x60
Satin
43
        Despite the apparent  dissimilarity of these fabrics,
      they gave the same performance. In this case, filtration
      and pressure drop were evidently a function of the type
      of filter cake built up on the fabric during service rather
      than a function of the fabric.
        The fabrics are formed into filter bags having config-
      urations to suit the collector. The bags may be cylindri-
      cal tubes open at both ends for attachment to top and
      bottom tubesheets using band clamps or snap rings, and
      fitted with anticollapse spreader rings to permit the
      released dust to fall out. Or, the bags may be cylindrical
      tubes closed at  the top  to permit suspension  from a
      system of shaker bars, and  open at  the  bottom for
      fastening to a tubesheet. Or, they may be cylindrical
      tubes closed at  the bottom  and open at  the top for
      installation over a wire retaining-cage suspended from a
      tubesheet.
        The bags may be oblong—in one version, closed at
      one end, open  at the other  for slipping over a retaining '
      cage (like a pillowcase);'in another  version,  closed at
      one end  for top suspension, with several cylindrical
      openings at the lower end for clamping to a tubesheet.
        In any bag  design, the  following  details should be
      considered:
        Thread used to  sew the  bag should be of the same
      material as the bag, or compatible with it. The open
      ends of the bag, which are attached to tubesheet ferrules
                                                         5-5
      98
by clamps or snap rings, should have a reinforced cuff.
All hems and seams should have two rows of stitches,
with  edges  folded  or  treated  to  prevent  fraying.
Spreader rings should be enclosed  in  a pocket that is
doubly reinforced inside and outside the bag.
   The suspension end of hung bags should be doubly
reinforced and folded over to form a loop  for hanging
on a wide suspension hook, or fitted with a metal grom-
met if hung from an  eye  bolt or hook bolt.  Cuffs  of
oblong bags that are hooked over support  pins should
be fitted with metal  grommets at  the hook locations.
Where bags must be grounded,  a copper wire should be
sewn the full length of the bag, with sufficient lead  to
clamp to a tube ferrule.

Filler  cleaning
   The shape of the filter element, its method of support
and the normal direction of airflow through the filter
will determine the method of cleaning. Normal airflow
will hold the dust cake against the fabric. Stopping the
flow of air will tend to release the dust cake. Simultane-
ously flexing the element, or reversing the airflow, will
ensure release of the cake so that  it can drop into a
hopper.
   Flexing can be induced (1) mechanically, by motor-
driven crank mechanisms that oscillate closed-end tube
elements, or (2) pneumatically, by reverse airflow for
flow-through or closed-end tube elements.
   On flow-through elements that collect dust on the
inside surface, spreader rings may be used to form nodes
for more-effective reverse-air flexing  and  to prevent
collapse of the elements. (A collapsed element would
prevent  the  dust  from falling  into the hopper.) On
closed-end tubular elements that  collect dust  on the
inside surface,  a top-suspension tensioning device  is
depended  upon  to prevent collapse of the elements
during cleaning and to hold their surfaces taut to form
the dust cake during normal airflow.
   Envelope,  oblong or tubular elements  that collect
dust on their external surfaces are generally placed over
metal retaining cages or grids that keep the fabric taut
so as to retain the dust cake and maintain  the internal
open  area required for clean-air flow. During cleaning,
reverse airflow flexes the fabric away from  the retainer
grids;  restoring  normal  airflow  whips  the  fabric
against it.
   On collectors where normal airflow through the fab-
ric is not  stopped,  an  extra-high reverse airflow  is
needed to dislodge the dust cake. This may be done by
using high-pressure air jets or separate  air-reversal
blowers.

Cleaning methods for baghouses
   Commercial baghouses use one or more of the clean-
ing methods  previously discussed. These methods may
be further  described as follows:
   Shaking only is used on unit filters  having  woven bags
where service can be interrupted long enough for clean-
ing—usually 3 to 4 min. The bags may be (1) envelope
type, mounted over a metal spacer,  with the closed end
connected to a motor-driven shaker grid and tensioning
bar, and the open end fastened to  a clean-air plenum
wall; or (2) tubular type, with the closed end suspended
                                              CHEMICAL ENGINEERING APRIL 9. 1979

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from motor-driven shaker bars, and the open bottom
end fastened to a tubesheet for the entry of dust-laden
air. The cleaning cycle may be initiated by (a) a timer
set for a specific cleaning period, (b) a pressure-differen-
tial sensor, or (c) a manual stop-button connected to a
time-delay relay. In this method, the exhaust fan stops,
allowing  the  internal pressure  to  reach atmospheric,
and the shaker motor operates for a preset interval.
After cleaning, operation of the dust collector may be
restored automatically or manually, depending upon
the service.
   Shaking with reverse  airflow is used with either type of
bag described for unit filters. However, for continuous
service, the filter elements must be  distributed among
two or more compartments,  and the exhaust fan must-
remain in operation  during the cleaning cycle. Each
compartment is fitted with its own  shaker  mechanism
and a set of reverse-air dampers. Each set comprises an
outlet damper, ducted to the exhaust fan, and a smaller
damper opening directly  to atmosphere. A  program
timer  permits cleaning the bags in each compartment
successively, with the time interval  between cleanings
adjusted to suit the service.
   During cleaning, the timer activates the reverse-air
damper drive to close the clean-air outlet damper to the
fan, and simultaneously opens the atmospheric-air inlet
damper. Suction from the active compartments draws
air through  the bags in the reverse direction. Limit
switches make sure  the dampers are correctly positioned
and start the shaker mechanism. After the shake period,
the reverse-air damper drive opens the clean-air outlet
damper and closes the atmospheric-air  inlet damper,
reactivating  the compartment. During  the  cleaning
cycle,  a certain amount of dust is  picked up by the
reverse airflow and deposited on the bags in the active
compartments.
   Fig. 2 shows a two-compartment filter used for vent- •
ing a silo receiving  material from a pneumatic convey-
ing system. Fig. 3 shows the variation in bin pressure
during the cleaning cycle for this collector.
   Reverse-airflow cleaning is used for all  types of filter
elements (tubular or envelope, woven or felt materials,
dust inside or outside of the bag, elements  distributed
between several compartments or all in  one housing).
There are many arrangements for creating reverse air-
flow—all intended to enable continuous service. Several
common arrangements  are:
   1. A compressed-air nozzle at the clean-air discharge
of each bag injects high-pressure air into the tube in the
reverse direction from normal airflow in pulses of short
duration (1/10 to 1/25 s) for systems having closed-end
tubular bags  supported on metal  retainer cages sus-
pended from the clean-air plenum tubesheet. The pulses
are controlled by external timers  that operate special
air-supply solenoid valves, each of which serves a group
of bags. The time between pulses is adjustable, but is
factory-set for the intended service. See Fig. 4.
   2. A variation of this method is to divide groups of
bags (including the clean-air plenum) into zones by
means of internal partitions. The clean-air plenums are
manifolded into an outlet duct via poppet valves. Each
zone is equipped with a single pulse valve that supplies
compressed air to the group of bags. During the clean-
              Upper   Nozzle or   Venturi
             plenum    orifice    nozzle
                '
Solenoid  Compressed
 valve     air supply
  /      atlOOpsig
               \
     Exhaust
      outlet.
                                                   Collector
                                                    housing
                Discharge
ing cycle, the poppet valve in the zone to be cleaned
closes, stopping airflow  through the zone. The pulse
valve opens for 0.1 s, admitting a burst of air into the
plenum to clean the bags. The poppet valve then auto-
matically reopens, putting the cleaned zone  back on-
stream. This operation is repeated in  each successive
zone until all bags in the collector have been cleaned.
See Fig. 5.
  3. A motor-driven plenum chamber  receiving air
from a separate blower delivers reverse air to closed-end
oblong bags that  are supported as in the previous ar-
rangement  but not zoned. The plenum rotates and
blows  reverse air  into the clean-air discharge end of
each bag as its nozzles travel  over  the bags. Timing is
                                                   5-G
                                         CHEMICAL ENGINEERING APRIL 9, 1979
                                                                                                           99

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Solid
state
timer
                                                                          Door
                                                                         / seal
                                                                                 Internal
                                                                                hinge latch
                                                                                               Plenum
                                                                                             _ chamber
                                                                                            7  ,doors
                                                                                                  Door
                                                                                                 .retainer
             Housing'
                                                                             -'Internal partitions
   determined by  the  speed of rotation  of the  plenum
   chamber. See Fig. 6.
      4. A counterweighted frame carries a series of blow-
   rings that encircle each filter tube. The  frame is driven
   slowly up and down by chains supported from overhead
   shafts and sprockets. Air is continuously supplied to the
   blow-rings by an external blower or fan connected by a
   hose, or a specially designed air-supply connection to a
   manifold mounted on the blow-ring carriage. See Fig. 1.
      5. A traveling air-plenum, moving horizontally, is
   positioned over the clean-air discharge of each vertical
   line of envelope-type filter bags.  Diaphragms  on the
   traveling plenum seal off adjacent rows of bags. Reverse
   air may be supplied in two ways: (a) atmospheric air
   may be drawn  through the filter bag in the  reverse
   direction from normal airflow by the operating suction
of the exhaust fan, or (b) clean air may be blown back
into the bags from a separate blower mounted on the
traveling plenum. See Fig. 8.

Design requirements for filters
  Manufacturers offer standard designs for baghouses.
However, such  designs may have to be radically modi-
fied to suit the user's requirements. These may be fixed
by  the characteristics  of the material entering  the
baghouse,  and by the  operational needs  of the duct
system to which the baghouse is connected. Before pre-
paring a  specification  for a  baghouse, consideration
must be given  to  design details such as: housing con-
struction,   physical   location,  servicing   provisions,
method of operation,  and protection  from fire  and
explosion.
    100
                                            CHEMICAL ENGIK
                                                           AND APRIL 9, 1979

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                                                       Blank covers are provision for future filter elements
 Housing construction                     .
    Baghouses may operate  under positive  or  negative
 pressures. Baghouses that receive air from  a fluidized-
 bed process or from  a positive-pressure pneumatic con-
 'veying system generally operate under positive pressure.
" The pressure is limited by the pressure drop across  the
 bags because discharge is directly to atmosphere.
 .   Most  baghouses  operate under  negative pressure,
 which imposes stringent design requirements  for  the
 bag enclosure walls.
    Although the pressure drop across a baghouse may be
 3 to 6 in. water column, the negative pressure imposed
 by the exhaust fan  is determined  by the duct-system
 requirements that may be five to ten times those of  the
 pressure  drop.  If the air  inlets  to the baghouse  are
 blocked by powder or closed inlet dampers, the suction
 may increase to values well above the normal operating
 level. The user must determine 'the maximum  suction
 that may be imposed on the baghouse, and seek a unit
 that is rated for this condition.
    If the  dust-air mixture can become explosive,  the
 baghouse walls must also resist a positive pressure that
 is  determined from dust-explosion tests. These fix  the
 vent area in the walls, so as. to limit pressure buildup.
 The user must specify the allowable positive pressure for
 baghouse design.
    Cylindrical baghouses are simpler to design for high
 suction or high pressure than are flat-sided enclosures.
 Dust-laden
  air inlet
Reverse-jet supply blower.
Blow-ring
 carriage
  drive
  Blow-ring
  carriage—
ounterweights
 Blow-ring--
 carriage
                                        Clean-air
                                        discharge
                                                       5-
                                          CHEMICAL ENGINEERING APRIL 9. 1979
                                                101

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                                                  BACHOUSES
Standard suction  ratings, for cylindrical units range
from 15 to 160 in. water column, depending on diame-
ter. Rectangular or square baghouses have ratings that
range from 15 to 30  in. water column. These  units
require  external battens and cross-bracing  to stiffen
them. Multicompartmented  units have interior  walls
that require special bracing  to resist differential  pres-
sures  and  stress reversal during cleaning operations.
Tubesheets are usually made of heavier plate than are
the walls, and are fitted  or formed with stiffeners be-
cause the tubesheets not only serve to support the bags
and retainers  but  also act  as  walkways  during  tube
replacement.  Distortion of tubesheets can cause top-
suspended bag retainers to touch each other and retain
dust. Some baghouses are only  available as cylindrical
units; others may be square, rectangular or cylindrical,
depending on size  and application.

Fabrication of baghouses
  The enclosures for housing and supporting the bags
may be made in several ways:
  /. Modular  design—A  complete,   factory-assembled
                                                     5-9
unit consisting of bolted or welded components may
include the dust hopper, supporting legs, cleaning  ac-
cessories, timer, internal baffles, bags, etc. Installation
only requires rigging into location atop a base to hold
down the legs  before connecting to the inlet and  ex-
haust  systems, and  utilities.  A  group of  parallel-
connected units may be used to get additional capacity.
The parallel connections may be internal, using com-
mon clean-air or dusty-air plenums; or external, using
ductwork manifolds attached to inlet and outlet con-
nections on each  module. Module sizes  are limited to
those that can be transported over the road. The largest
(12 to 14 ft square in  cross-section) would require a
low-bed trailer.
  2. Factory-weldedsubassemblies—The bag housing, inlet
and outlet plenum chambers,  dust  hoppers,  etc.,  are
separately fabricated to form the largest subassemblies
that can  be shipped over  the road.  Connections to
adjacent components may be prepared  for bolting or
welding at thejobsite. The method of assembly depends
on  the materials  of construction and location  of  the
baghouse. Field welding is  precluded where bags  are
102
                                        CHEMICAL ENGINEERING APRIL 9, 1979

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 already  installed in the subassembly. The bag enclo-
 sures may contain all the bags attached to their retain-
 ers, and may have cleaning accessories factory-piped
 and factory-wired.
   3.  Knocked down—All structural components are fab-
 ricated as separate panels, with formed flanges for as-.
 sembly in the field by bolting or by welding. This type
 of construction is limited to very large baghouses fabri-
 cated of special materials or located  at jobsites where
 access by crane is not possible. The flanged joints also
 serve as stiffeners on the flat surfaces of hoppers, plenum
 chambers, tubesheets and bag enclosures.

 Materials of  construction and  unit size
   The materials of construction and the sheer size of
 some units may preclude  complete factory assembly,
 which is preferred in order to eliminate leakage and fix
 responsibility.
   Baghouses requiring  hot-dipped galvanized surfaces
 or internal linings of rubber,  polyurethane or other
 plastics for resisting corrosion or abrasion will  need
 bolted and gasketed joints because welding will destroy
 the coating. Hot-dipped galvanized sections are limited
 to sizes large enough  to fit  into available dip tanks.
   Welding of galvanized surfaces is possible, but special
joint preparation and field coating the weld with com-
 patible materials are  required in order  to  maintain
 corrosion resistance. Larger galvanized sections may be
 welded by using lower-cost electrogalvanized sheet steel
 instead of hot-dipped, in order to avoid the size limita-
 tion imposed  by dip tanks. However, special welding
 preparation and coating of the weld  will be  required.
   The weight of the zinc coating on electrogalvanized
 sheets should equal that normally specified for  hot-
 dipped galvanizing (a coating of not less than 1 oi/ft2
 on' each surface). Complicated surfaces such as tube-
 sheets on baghouses requiring interior coatings may
 have to be stainless steel or Hastelloy because coating
 these surfaces may not be possible. All bolted joints
 will require gasketing.or the application of  a  sealing
 compound  to  the  mating  surfaces.  These materials
 should be compatible with the interior coating and the
 dust entering  the collector.
   The large size of a unit may require fabrication and
 shipment knocked-down in order to erect the baghouse
 in areas inaccessible to a crane. Such areas could be (a)
 building interiors, (b) yards surrounded by high build-
 ings, or (c) roofs loaded with other equipment: In many
 cases, a crane could lift the components to a peripheral
 location, from where they could be trolleyed on a cable
 to the final position.

Access  for servicing
  The location of access doors is principally determined
by the method  of  grouping the bags and attaching
them to their supports. The closed-end tubular bag and
its metal retainer suspended from the clean-air plenum
tubesheet is the most common design because it permits
the closest grouping of bags in a given space. Access is
through hinged doors located entirely across the clean-
air plenum, atop the collector. The bag and its retainer
are simply dropped through the tubesheet and fastened
to it by clamps, snap rings or twist locks, using the bag
material as a gasket. Internal air-nozzle piping is made
to swing out of the way or to be easily disconnected. Or,
the clean-air plenum is subdivided and valved so that
each subdivision can be isolated and pulsed by a single
solenoid valve.
   Where the bags must  be  serviced  regardless of
weather conditions, the clean-air plenum can be de-
signed as a walk-in housing requiring only one door,
but only on collectors fitted with an air nozzle over each
bag. Where bags and retainers are clamped from below
the tubesheet, the  doors must  be located on  the  side
walls of the unit  so that bags are within  30 in. of the
opening to allow proper takeup of clamps.
   If adjacent walls or equipment interfere with proper
location of a  side  door, the available doors must be
used, necessitating  the removal of a group of bags in
order to reach those furthest from the door.
   Bags suspended from shaking bars, with their open
ends  fastened to  a  bottom  tubesheet,  are  usually
grouped on each  side of a narrow walkway within the
clean-air compartment that is reached through a single
door  in  the wall. During rebagging, the  most remote
bags must be attached and  tensioned. Then, work must
progress toward the walkway.
   Envelope-type  bags on metal spacer frames are in-
stalled in horizontal rows, with their open ends clipped
to the vertical slots of  a clean-air plenum, and their
closed ends hooked to a support frame. Several  rows of
bags are located in tiers, one above the other. All bags
in a compartment are accessible from two doors—one
leading to the clean-air plenum, the other leading onto
a walkway in the dusty-air chamber.
   Long, large-diameter bags that are open at both ends
are usually fastened to a bottom 'tubesheet that is acces-
sible through  a side door at  the bottom of each com-
partment, .and to the top  tubesheet that is accessible
through one or more doors in the roof of  the dusty-air
inlet plenum. The side doors lead  to walkways between
the bags. Some designs limit the number of rows of bags
on either side of the walkway in order to improve access
for clamping and to avoid damage to the bags.
   All doors should  be loosely hinged, and clamped or
latched in such a  manner that all  surfaces of the gasket
are sealed airtight.  Gaskets should  be  closed-pore
sponge  rubber (40 Durometer hardness) that is ce-
mented  into a confining recess  around  the door or
around its frame.  The rubber and its cement should be
compatible with each other and with the dust to which
they are exposed. Latches or clamps should be so de-
signed that they can be opened manually  by operators
without using a wrench or special tool. The frames of all
doors should extend beyond any external  insulation so
that they can be flashed properly. The inside surface of
vertical or sloping doors on the dusty side  of the collec-
tor should be flush with adjacent  walls in order to
prevent dust buildup on ledges.
   Platforms, railings and ladders should  be furnished
for reaching all  elevated  equipment and all  access
doors. These should be built in accordance with appli-
cable safety codes. Rooftop equipment should be acces-
sible for servicing from within the peripheral  hand-
railing.  Ladders should  have safety cages starting  7 ft
above floors or platforms, and should be equipped with
                                        CHEMICAL ENO&Elikflc APRIL 97
                                                                1979
                                                103

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                                                  BACHOUSES

                                                 55      60      65      70
                                                 Slope sheet angle, a or B , deg.
                                                                         Slope sheet
                                                                           angle et
                                                                            65"
                                          Slope sheet
                                           angle
                                             80"
                           *VKl-***^tW"^9V^.-*^--?W^TSK^*<^'&
                          oppers having flat slope sheete
grab rails and, for greater safety, with narrow, nonskid
treads in place of rungs. Platforms should be made from
serrated  grating so that dust cannot accumulate and
slipping  is prevented.

Dust hoppers and dust  discharge
  The material removed from the bags may be dropped
directly into a bin or silo if the baghouse  is used for
venting  that  bin  or silo.  Otherwise, the material is
dropped  into a collecting hopper that is shaped to direct
the  flow  to  the dust  outlet usually fitted  with an
airlock-type discharge valve.
  On  multicompartment units,  each  compartment
should be fitted with its own dust hopper, to permit its
isolation during cleaning or maintenance. To eliminate
a multiplicity of airlock discharge valves, the bag com-
partments  may discharge into  a common V-shaped
hopper equipped with a  screw conveyor that delivers
the dust to a single discharge valve.  Such a screw con-
veyor must operate at the baghouse pressure. Therefore,
104
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                                         CHEMICAL ENGINEERING APRIL 9. 1979

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 the conveyor must be equipped with effective seals at
 the  ends of the trough  and at the bearings, which
 should be outboard of the hopper. The conveyor must
 also be able to handle the collected dust at the proper
 rate.
   The flow properties of the collected dust should be
 known,  since they may  be quite  different than  the
 properties of the material from which they originated.
 The slope  angles of the dust hopper may have to be
 steeper than those of the storage silos of the  original
 material because (1) fine dusts tend to pack more read-
 ily than coarse materials,  (2) the dust may  contain
 moisture picked up in the conveying air or formed by
 condensation within the collector, (3) collected material
 should be removed as rapidly as it is deposited on  the
 hopper walls. Steep slope angles increase the overall
 height of the baghouse and may limit the space availa-
 ble for installing a discharge valve.  However, failure
 to provide the proper slope will create problems after
 installation.
   A conical hopper attached to a cylindrical baghouse
 presents no problem other than the increase in height
 required to  obtain a steep slope.  Attachment  of a
 square, rectangular or conical hopper to a square or
 rectangular baghouse requires study—especially of the
 valley angles formed by the slope sheets of inverted
 pyramidal hoppers, or by transitions to conical hoppers.
 These valley angles should be at the minimum value
 required to obtain material flow. If the wall-slope angles
 to the horizontal are known, the valley angles  may be
 obtained from Fig. 9, or from the formula from which
 the chart is derived:

              cot2 0  = cot2 a -f cot2 ft

 where 6  is the valley angle, and a and (3 are the slope
 angles between the horizontal and Wall A and Wall B
 of the hopper, respectively, as shown in the sketch in
 Fig.  9.
   Provision of the proper valley angle at an intersection
 does not ensure material flow, because moist materials
 can bridge across the intersection. To minimize bridg-
 ing,  the intersection should be  formed to as  large a
 radius as possible  when joining the flat  slope-sheets
 internally.
   On  baghouses cleaned  by reverse-air  blast (using
 compressed  air), the slope-sheet'angles can be reduced
 by a few degrees because the air blast helps  to propel
 the collected powder down the slope.
  Materials may still  hang up even though the proper
slope angles are provided on dust hoppers, because of
process upsets, changes in  moisture  content, etc. To
ensure flow, mounting pads should be installed on the
slope-sheets  for  attaching mechanical or pneumatic
vibrators for inducing flow. The vibrators should  be
placed at the centroid of each surface. The flow induc-
ers should be operated only when needed.  Cylindrical
hoppers require only one pad, located at the centroid of
the projected surface.
  The dust hopper and its supports should be designed
on the basis that it could become full of material and
may also have to support a surcharge. This situation
may arise if the baghouse becomes  full  of material
because of a broken bag, a malfunction of the dust-
 discharge valve,  or stoppage of  a  downstream dust
 conveyor.

 Outdoor installations
   Since many  exhaust  systems  draw air from within
 warm buildings, or handle hot materials from processes
 or handle materials containing moisture, condensation
 is a possibility within the  baghouse. This can occur on
 baghouses located indoors or outdoors, particularly in
. winter. Some manufacturers' guarantees are automati-
 cally voided if condensation occurs inside the unit.
   In such situations (especially outdoors), the baghouse
 should be insulated. Insulation  should be of sufficient
 thickness to prevent condensation at the lowest outdoor
 temperature recorded in  a  given  location.  All  doors,
 dust hoppers and projecting stiffeners should also be
 insulated. Outdoor insulation should be protected with
 aluminum or stainless-steel jacketing, applied so as to
 shed water. Flashings of the same material should be
 formed and fitted around all access openings. In some
 cases, removable  covers containing the insulation may
 be fitted over doors to permit easy access.
   If a baghouse does not require a  separate enclosure, a
 watertight housing should  be  provided around dust
 hoppers, dust-discharge  valves, and other dust-recovery
 equipment located below the baghouse to permit access
 for  servicing  during  bad weather.  Hinged  or  easily
 removable weathertight hoods should  be furnished to
 protect all external operating mechanisms.
   All electrical  conduit  to  timers  and  actuators
 mounted on the baghouse should be watertight.  Pneu-
 matic actuators, solenoid  valves, shaking mechanisms,
 and air piping to them, must also be insulated or traced,
 to prevent condensation of moisture and its subsequent
 freezing, which  will prevent operation of the accessories.
   If baghouses handling hot gases are shut down for the
 weekend, they become especially susceptible to conden-
 sation problems that prevent startup. Such units should
 be equipped with  an independent heater and a pressur-
 izing fan designed to maintain  the  normal operating
 temperature within the unit during shutdown. A grav-
 ity-closed damper on the discharge of the main exhaust
 fan will permit retention of the  heated air. Baghouses
 operating- under  pressure  should have a weathertight
 cover or an automatic backdraft damper at the  air
 outlet to prevent  intake of cold  air during shutdowns.

 Protection against fire
   When  hazardous materials are  handled, protection
 must .be  provided to eliminate  or minimize hazards.
 Some materials may only require fire protection; others,
 fire and explosion protection.
  . The best procedure for determining the hazards in
 handling a particular dust with unknown characteris-
 tics is to  submit test samples to  the  fire-insurance un-
 derwriter's laboratory or to a commercial testing labo-
 ratory that has facilities for conducting:
   /.  Combustion  test—Ignition  by match  flame  or
 Bunsen burner  to determine whether  the  material is
 self-extinguishing, or whether the flame will spread.
   2.  Spontaneous-combustion  test—Exposing  material  to
 high temperature for  a  prolonged period  (several
 hours). The material is contained in  a crucible im-
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                                        CHEMICAL ENGINEERING APRIL », 1979
                                                 105

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                                                    BACHOUSES
 merscd  in a hot salt bath.  Moist materials should be
 tested at their usual moisture content, and again after
 drying.
   3. Autoignitiun-teinperalure test—Same as the preceding
 test, but raising the temperature continuously to deter-
 mine the temperature at which ignition occurs. Evapo-
 ration of water from a moist sample may exclude air, so
 that the autoignition point could be on the high side.
   4. Dust-explosion test—-Material is placed in a  bomb
 having a variable vent opening, mixed with compressed
 air, and then ignited by an electric spark. The pressure
 developed is measured and  reported for two  condi-
 tions—(a) unvented, and (b) for a specific ratio of vent
 area to bomb volume. Materials known to be explosive
 should also be tested for rate of pressure rise in addition
 to the pressure developed from  an explosion.
   Where possible, any baghouse subject to  fire  or  ex-
 plosion  should be  located  outdoors,  and away from
 important equipment.
   Fire protection may be provided by a built-in sprin-
 kler system. The sprinkler heads would be rated  about
 25° (F) below the temperature  limit of the bags if no
 other protection were available; and 100° (F) above this
 limit, if early-warning protection were  installed. The
 sprinklers would be connected to a building sprinkler
 system.  The heads are usually located where they can
 cover most of the bag area. Additional heads may be
 needed to cover the dust hoppers. Because of difficulty
 in placing heads  in  some baghouses, assistance of the
 fire underwriter's engineering department should  be
 obtained.
   Opening of the sprinklers would be indicated by the
 water-flow detector and alarm on the building sprinkler
 system.  However, a local alarm  to warn system opera-
 tors may also be required. The local alarm would be a
 fire detector within the baghouse—the detector  being
 set at the same temperature as the sprinkler head. Pro-
 vision must be made to dispose of the water released by
 the sprinklers. When the  baghouse is  outdoors, a
 nonfreeze sprinkler system will be needed. A dry system,
. using air in the sprinkler lines, is expensive and requires
 considerable  maintenance.  A  system filled  with  a
 nonfreeze  solution, may foul the bags when the  heads
 are released. Hence, the bags may require replacement
 even if they are not burned.
   Several alternatives are available for sprinklers:
   1. An inert-gas (Halon) injection  system may  be
 installed. When actuated, it will exclude or reduce the
 oxygen  content in  the baghouse so  that combustion
 cannot be sustained. The sensor is a rate-of-rise temper-
 ature detector, set at the temperature limit of the bags,
 that operates a control unit to electro-explosively rup-
 ture a diaphragm  to release the  inert gas into  the
 baghouse. The gas  storage vessels are located outside
 the collector. If the system is actuated, there is no mess
 to clean up except char from the fire. The control unit
 can also initiate other functions such as closing damp-
 ers, stopping fans and sounding alarms.
   2. A steam-smothering or a water-deluge system that
 is actuated manually or  by a rate-of-rise temperature
 detector uses open nozzles, located in the same manner
 as sprinkler heads.
   3. A manually operated  deluge system can be de-
                 vised for collectors having reverse-jet air manifolds by
                 connecting a water header to the air manifold pipes at
                 the ends opposite the air header. When water is turned
                 on, the air pulses speed up wetting of the bags. The
                 timer can be temporarily switched to pulse all nozzles
                 simultaneously, and pulse time could be increased to
                 obtain  faster water distribution.

                 Protection against  explosion
                   Explosion protection may be provided by installing
                 an inert-gas suppression system or explosion doors.
                   The  suppression .system is  similar  to  that for fire
                 protection, except that a highly sensitive pressure detec-
                 tor initiates action of the control unit. The pressure
                 setting  is determined from data for the rate of pressure
                 rise obtained during the bomb  test. The baghouse may
                 be isolated so as to contain the inert gas without loss by
                 the use of fast-acting inlet 'and outlet dampers, or by
                 shutting down the exhaust fan and rapidly closing an
                 outlet damper.
                   The  inert-gas suppression system detects the pressure
                 rise caused by an explosion, and acts in milliseconds to
                 stop pressures from reaching values that would rupture
                 the bag housing. Because of the short duration of an
                 explosion, and the dependence on proper functioning of
                 all control equipment, a backup system of explosion
                 doors may be  desirable.
                   Explosion doors are generally installed in baghouse
                 walls on the dusty-air side. They  may be hinged  doors
                 with special,  spring-loaded,  quick-releasing latches.
                 They may be rupture panels made of frangible materi-
                 als such as aluminum foil, lead sheets, polyethylene or
                 Teflon. They may be  laminates  of metal and plastic
                 designed to rupture along prescored lines to obtain full
                 opening of the panel. They may be loose doors held in
                 place by their own weight or by suction.
                   Latched or  loose explosion  doors on  outdoor bag-
                 houses   must have • rain  hoods  built  around  them.
                 Weighted  doors, unattached  by  any other  means,
                 should  have chains connected to them, which are fas-
                 tened to an internal structural member,  so that  the
                 doors do not become missiles when blown open.
                   Every explosion is rapidly followed by an implosion
                 that can be as damaging  to the structure as the explo-
                 sion. Explosion doors that are pivoted or hinged should
                 be designed to prevent automatic reclosing. Such  doors
                 should  be reclosed manually.
                   The concluding installment of this two-part series on
                 baghouse collectors, to appear  in the issue of Apr. 23,
                 will deal with the selection and testing of filters, meas-
                 uring performance,  and acceptance  testing of bag-
                 houses. A reprint notice for both parts will be carried at
                 the end of the article in that issue.
                                                   Steven Dtmatoi, Editor.
                 The author
                 Millon N. Kraus, J2 Chris Ave,, Hillsdalc, NJ 07642, recently retired from
                 Colgate-Palmolive Co., Jersey City, N.J., where he was supervisor of project
                 engineering. Previously, he worked as a supervisory marine engineer in
                 design at the New York Naval Shipyard and became a licensed marine
                 engineer with the U.S. Maritime Service. He also served as division
                 engineer for dairy processing plants and as mechanical superintendent for a
                 power-plant construction firm. He has a B.M.E. from Pratt Institute.  He is
                 a licensed professional engineer in New York and New Jersey, and a
                 member of ASME and Tau Beta PL
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