-------
volume may be obtained by closer folding, pleating, or packing. Possibly
new designs that simplify cloth replacement will be introduced and higher
"•" entrance and exit velocities will be developed for those baghouse systems
i; not limited by particle abrasion. At the present time, however, we
J must continue to examine filter configurations on the basis of: (1)
panels vs tubes or bags; (2) upward vs downward flow; (3) inside vs
!, outside collection; and (4) length, diameter, and length/diameter ratio.
£'
> 3.2.1 Pane1 vs Tube FiIters
!
' About 20 percent of the collector models available use flat
panels, the rest using either tubes or bags. The advantages of one type
over the other seem to depend on the application, since both are widely
used. One major manufacturer of panel type filters cites these advan-
'' tages:
3
(1) The flat filter gets 20 to 40 percent more cloth per ft of
| collector volume.
" (2) The air is moving more slowly when it enters the panel, than
air at cylindrical bag entrances, hence abrasion is less.
(3) A set of panels can be inspected more easily since both sides
of the cloth on every panel can be seen.
(4) A single panel can be changed more easily than a single tube,
although it may be easier to completely rebag the tube col-
lector once it is cleaned out.
(5) Dust getting through any leak in the cloth piles up under the
panels where it is more easily removed than dust piled around
the lower ends of tubes.
(6) There are more choices of inlet and outlet locations on a
panel baghouse than on a bag type,where inlet must be at the
bottom and outlet must be at the top (or vice versa).
(7) The panel may be ghaken or pulsed more uniformly than a bag
» which is shaken or pulsed from one end only.
(8) In the event fibrous materials are encountered, the panels
can be brushed down on the dirty side, whereas the dirty
side of the bag may be inaccessible. If the dust bridges
between the panels, every other one can be removed.
-------
A manufacturer of bag or tube filter equipment might counter with the
following opinions:
(1) Since bags can be very long, more cloth can be installed or
otherwise serviced per ft^ of floor space.
(2) In most installations, abrasion is not a problem and when it
is it can often be alleviated by suitable fabric design.
(3) It is easier to repair a leaky bag in place without removing
it than a panel.
(4) A single tube can be changed more easily than a single panel.
(5) Dust piled around the base of the tube is often the quickest
way to locate a leak.
(6) Unless the baghouse inlet and outlet are located logically
from the standpoint of air distribution and dust settling,
expensive baffles or plenums are necessary. Standard inlet
and outlet locations reduce costs.
(7) Shaking a baghouse takes less power and is quieter than
shaking a panel collector.
(8) The spaces between panels, being smaller, are more apt to clog
with coarse fibrous matter or in the event of a high surge of
dust than the relatively open bag or tube.
The views of both manufacturers obviously have merit in certain in-
stances, and have to be considered against the collector application of
interest.
3.2.2 Upward vs Downward Flow
Traditionally, the collector inlet was located at the bottom
of the baghouse where the heaviest dust settled immediately to the
hopper, thereby minimizing fabric abrasion and extending the period
between fabric cleanings. Since the compartment or entire baghouse was
shut down for cloth cleaning, the dust never had to fall against an up-
ward gas flow. Furthermore, downflow required the use of an extra tube
sheet and perhaps even more expensive, the use of an extra bottom plenum
(see Figures 3.3b and 3.3f). Thus, the upward flow design was logical,
s imple and leas t expensive.
-------
• x/
The introduction of reverse jet cleaning, and with it con-,
tinuous on-line operation, brought in the extra tube sheet and intro-
duced downward flow to encourage the dust to fall during filtration.
Otherwise, the dust mi^ht collect again on the nearby cloth. In fact,
this may happen even with downward flow, especially near the bottom of
the tube where the downward air velocity approaches zero. The four
reverse jet models available today all use downward flow.
It would seem that the same principle of downward flow for
continuous filtering would apply to pulse cleaning as well, but in fact
less than 10 percent of the approximately 30 pulse type models use down-
ward flow. The contradiction is partly explained by the theory that
pulse cleaning removes the cake without breaking it up as much as the
reverse jet does, so the dust falls as larger pieces. Also, when the
pulsed air is directed downward, as it usually is, it tends to carry
the dust downward although only briefly.
Because the settling velocity and primary flow velocity are
additive in downward flow, all sizes of particles tend to travel farther
in downward flow than in upward flow before being caught by the fabric.
Particles in downward flow have some probability of falling completely
through the filter tube to the hopper without being caught. This is
true of all particle sizes, whereas in upward flow only particles
larger than a size determined by the tube entrance velocity can escape
the fabric (Section 3.2.4.2; for most filter applications this size is
of the order of 3|jm.) Consequently, it would appear that the choice of
upward or downward flow might reduce the average cake weight, depending
on the particle size distribution. Dusts predominately larger than the
limiting size might be more suited to upward flow filtration. Smaller
dusts,if filtered downward,might result In slightly lighter cake weights.
A consideration that probably overrides the minimization of
cake weight is the distribution of particle sizes along the filter
surface and the effect this has on dust deposit permeability and subse-
quent local filtering velocities. Both large and small particles would
-------
be distributed more evenly over the length of the tube in the case of
downward flow. This should result in a more uniform use of the entire
filtering surface than in upward flow.
Another important consideration is that the upward flow col-
lector is generally less expensive, having one less tube sheet or plenum.
It is harder to adjust the tube tension with two tube sheets. In addi-
tion, with downward flow there is dead air in the hopper which increases
danger of condensation.
Some panel types are distinctly upward or downward flows,
in which case the above considerations apply, even though the panels
may be small in dimension. However, most panel-type filters tend to
use horizontal flow, that is, the inlet and outlet are approximately at
the same height on the baghouse.
These are the considerations in selecting the direction of
flow. To summarize, although the upward flow collector is slightly less
expensive initially, the downward system should give slightly the better
filtering performance and lower power requirement. However, the data
available are insufficient to quantitatively confirm this conclusion.
3.2.3 Inside vs. Outside Filtering
While this is not a consideration in most panel filters, bag-
type equipment offers the choice of filtering on either the inside or the
outside of the cylinder as was shown earlier in Figure 3.1. Of course,
reverse jet equipment provides inside filtering, as does all but one
model of downward flow equipment. About 60 percent of all upward flow
equipment is the inside filtering type. Clearly, the choice has nothing
to do with the fabric which can be sewn with either side out. An impor-
tant advantage to inside filtering is being able to enter the filter
compartment for inspection and maintenance during operation. Also, in-
side filtering does not usually require the use of supporting mesh which
can increase maintenance difficulties. However, inside filtering tends
to involve more fabric flexure during cleaning.
-------
3.2.4 Length, Diameter and Length/Diameter Ratio /
Most filter bags are 5 to 12 inches in diameter and 5 to 40
times the diameter in length. The choice of diameter and length does
affect filter performance, but the dimensions are more traditionally
determined by their effect on the initial cost of the bags and that of
the bagliouse.
3.2.4.1 Tube Diameter.- Filter cloth is usually woven in
standard widths,and in general, bag diameters are constrained by the
available widths of fabrics as woven. One common size is approximately
38 or 39 inches wide, from which two 5 or 6 inch diameter bags can be
' ; i
obtained, allowing for the necessary overlap at seams. For certain
applications, an 11*5 or 12 inch diameter glass fabric bag is the most
economical size from the available 38 inch wide glass cloth. A few bag-
houses are designed for use with 7 or 8 inch diameter bags. This size
is probably based upon a 54 inch wide cloth from which two bags can be
obtained from a single width. Wool felts, used in reverse jet baghouses,
are generally either 9, 10 or 18 inches in diameter.
The diameter of the filter bags also influences the size of
the baghouse. For example, about 1,750 square feet of filtering area
can be provided in about 80 square feet of floor area by using 6 inch
diameter by 10 foot long bags. If 12 inch diameter bags were used in-
stead, they would need to be about 14 feet long to provide the same fil-
tering area in the same floor space, though 12 inch diameter bags can
easily be obtained 20 feet long or more when there is adequate head room.
This (12 in. x 20 ft.) would result in a baghouse having about 2,500
2
square feet of filtering area in the same floor space (80 ft ).
While using a smaller diameter increases the filter capacity
per unit occupied floor area, there are disadvantages associated with
small bags. Small bags may bridge across, particularly in cases of un-
usually coarse collected dust or extreme surges in dust loading. If the
hopper plugs, any bag can become filled at the bottom with collected
-------
dust and the smaller bag is less easily emptied afterward. Smaller bags
require a larger number of clampings, tension adjustments and inspections
for the same cloth area, but they are easier to handle during maintenance
and repair.
3.2.4.2 Length-Diameter Radio and Tube Entrance Velocity. -
The air (and dust) velocity at the entrance of a cylindrical filter tube
is given by:
Ve - Vf x 4 (
For example: Filtration velocity = 3.0 FPM, Vf
Length/Diameter ratio = 25, L/D
Entrance velocity » 300 FPM, Ve
This velocity is typical of many collector installations.
In upward flow, the greater the entrance velocity the more
large particulate will be carried into the tube, (The largest particle
that can be lifted by an air current of given velocity and viscosity is
discussed in Chapter 2). Thus, the greater the entrance velocity, the
faster the dust deposit increases. The permeability of the deposit may
be expected to be diminished by the larger particles, however, as dis-
cussed in Section 2.4.8.
The upward velocity will decrease along the tubular filter
to zero. Thus, the size of particles that can be lifted (in upward flow)
decreases along the tube. This results in a partial distribution of par-
ticle sizes and a consequent variation in deposit permeability and weight
along the tube. These variables are affected by the entrance velocity
and the size distribution of particles entering the collector.
The cost of power required as a consequence of entrance head
loss is negligible, being less than one dollar per KCFM-year at 300 FPM.
However, in the case of abrasive dust, the bag may be scoured and excess-
ively worn near the entrance tube sheet, (Attempts to define the poten-
tially abrasive entrance velocity have not been successful; see Section
4.5.1).
-------
Length/diameter ratio affects the swaying stability of the
vertical tube. Bags should not rub together either during filtering or
during the cleaning cycle. Thus, the higher the ratio the greater the
advisable bag separation, partly offsetting the floor-saving advantages
of long tubes. Also, the lengthwise seam of the bag will stretch dif-
ferently than the rest of the cloth, making the high length-diameter
ratio bag bend as the tension or dust load changes.
The short stocky tube may shake more easily than the thin
longer bag, but the stresses at the shaking end may still be higher.
There is no accepted way to generalize this at present, as amplitude,
frequency, dust weight and several cloth parameters are involved. Also,
the relation between fabric stressing and fabric life is not clear.
Length/diameter ratios are usually between 5 and 40, but more
typically between 10 and 25. In purchasing, the choice of length/diameter
ratio seems to be mainly determined by whatever equipment is available,
and this is more dependent on other considerations.
3.2.4.3 Tube Length.- In addition to its relation to dia-
meter, tube length is also limited in other ways:
(1) The tension along the length of the bag increases as a
function of height, starting with whatever tension is
being applied at the bottom. It may be so large at the
top as to pull holes in the cloth at the cuff seams or
otherwise damage the fabric, in cases of heavy dust de-
posits .
(2) The longer the bag, the more expensive it is per square
foot, because of the difficulty of handling and sewing
a long bag. The choice of long bags requires a more
elongated dust collector for the same fabric area and
since elongated housings require more siding and stronger
structural members, the collector cost will generally in-
crease.
(3) Some cleaning mechanisms are length limited, notably air
pulse and shaking, wherein sufficient, energy for the en-
tire length of the tube must be applied at one end to the
detriment of that end. The energy requirement increases
faster than length, in general.
(4) Continuously cleaned dust collectors require the removed
dust to fall to the hopper during filtration. Instead
-------
of falling, many particles must be caught again on the
fabric. This results in relatively heavy deposits near
the bottom of the bags. The longer the bags, the heav-
ier this deposit will be. In a similar sense, intermit-
tently cleaned collectors require a pause after cleaning
to allow the dust to settle to the hopper. The shorter
the bags, the sooner the collector compartment can be
returned to service.
(5) Different thermal expansions between cloth and baghouse
require latitude in the bag fastenings or shorter bags,
or both. For example:
Resulting
Diff. Length Stress Change Tension
Steel-Glass: .00042 ft/ft 4.2 psi 2.5 Ibs
of bag
(6) Very long bags may require two or more people to install
and inspection and other maintenance may also be difficult.
(7) The baghouse may be ceiling limited if it is to be in-
stalled indoors.
(8) One cl&irn is made that bag surface scouring increases,
due to sliding of the loosened deposit downward along
the bag surface. Scouring may assist the cleaning pro-
cess, but it may also damage the fabric surface fibers.
It is not clear which in general, is the more important.
A review of bag lengths in a number of specific filter in-
stallations has indicated the use of slightly longer bags where smaller
particles are involved. There is no clear physical reason for this.
Recent air pollution control applications are chiefly concerned with small
particles. There is also apparently a slight trend in the fabric filter
industry toward longer bags and tubes, which may be coincidental.
As will be seen from a review of typical costs of fabric
filtration (Chapter 7), plant overhead and especially the cost of plant
*Assuming the fabric expands as a solid; per 100°F.
4
**Assuming tensile modulus of 10 psi and no bag fastening latitude;
per 100°F.
***Assuming a 6-inch diameter bag and .03-inch thickness; per 100 F.
-------
floor space are very important factors. It is entirely possible that
two collectors may be intended for almost identical service, the only
difference being that one will be used where floor space is plentiful and
free, and the other where floor space will contribute as much as 10 per-
cent to the annual cost of operation. One would prefer to select equip-
ment with longer bags in the second case.
3.2.4.4 Panel Geometry.- The average velocity entering a
space (W) between two filter panels of distance L from entrance to closed
end is given by V = V,. x 2 (L/W) . For example, if the filtering velo-
city is 2 FPM and L and W are 3 ft and 1.5 inches respectively, the
average entrance velocity is 96 FPM. It is generally less than for bag
and tube filters and consequently, many of the above comments relating to
entrance velocity do not apply. Furthermore, as the flow is often hori-
zontal, there may be more settling. There should be less abrasion of
the fabric surface because of the lower velocities.
In the normal interest of saving floor and plant space,
panels will be designed as close together as possible. This is subject
to the need to inspect and replace the cloth; clogging of the space
between the panels; touching during cleaning, such that the dust cannot
fall or such that one panel abrades against the next; entrance abrasion;
.and structural requirements. Most panel spacings are between one-half
and three inches and equipment using oval bags or envelopes have inside
dimensions on about the same scale.
Panel length and height vary from a few inches to a few feet.
In addition to the above limitations, the manufacturer's choices are
determined by such things as the width of the original bolt of cloth;
the need to maintain tension over the panel; the ease of installation
and replacement of panels; and thoroughness of cleaning, which depends
on cleaning methods.
-------
3.2.5 Other Configurations
Panel type filters vary from single flat fabric sheets to
oval envelopes with only one end open, really more like a bag than a
panel. A standard item with Pangborn In the multi-tube Filter <• lament,
two panels sewn together at Intervals so that when Inflated six cylin-
drical filter elements result, fastened together side by side. This
approach packs more fabric in a given collector volume, but may increase
maintenance costs, since a failure of one tube requires maintenance or
replacement involving all six. Also, tension adjustment of such a large
filter element may be slightly more difficult.
Reported in some of the early literature, and perhaps still
in common use in Europe, is the conical filter element which tapers from
a circular bottom of ordinary diameter to an apex at the top of the bag-
'jouse. This design should give nearly constant upward velocity the
entire length of the element and ,therefore, a more uniform particle size
deposition. The elements can also be packed more closely together with-
out danger of inter-abrasion, as long as they are not made too long,
except that space must be provided for their maintenance just as with
any filter element. Tapered elements are relatively expensive to con-
struct. A thin cone may bridge across the inside and be difficult to
clean. The shake dynamics of a cone are no doubt substantially different
than for a cylinder.
A few other novel fabric filter configurations are discussed
in Section 3.5
3.3 CLEANING MECHANISMS
Except for studies of specific air pollution control problems, most
development toward better fabric filter equipment appears to go into
improved methods of removing the accumulated dust deposit from the
fabric. As a result, we have the variety of cleaning mechanisms already
mentioned in this chapter. Just as the dust particles cohere to one
-------
another, the deposit cake adheres to the surface fibers of the cloth, or
in the case of deep or napped surfaces, among the surface fibers. The
problem is to remove the desired amount of the deposit from the fabric
quickly and uniformly without either:
• removing too much of the residual deposit which greatly improves
the collection efficiency at start-up on a subsequent filtering
cycle, for woven fabrics
. damaging the cloth, or using too much power, either of which can
be a substantial part of operating cost
• excessively dispersing the removed dust particles, as these would
probably then have to be re-filtered.
The standard cleaning methods are listed in Table 3.1 along with a
number of superficial characteristics frequently, although not always,
associated with those methods. More quantitative discussion of some of
these characteristics (efficiency, cost, filtration velocity, etc.) can be
found elsewhere in this handbook. The cleaning principles are discussed
here.
The time required for completion of the cleaning process is an im-
portant consideration in comparing cleaning methods. This time should
be much shorter than the time between cleaning periods, which as shown
in Section 2.4 is determined by filtering velocity, dust loading, pres-
sure drop tolerances and the "K" value of the dust. For a high dust
loading, the cloth might have to be cleaned very often - perhaps every
30 seconds, for example. For this, a manual cleaning process would be
out of the question. Likewise, a shaking process which might require
1 minute or more just to accomplish the cleaning, would mean that at
least two thirds of the fabric in the system would be out of use at all
times. On the other hand the reverse jet method accomplishes the
cleaning quickly, but is limited in repetition by the slowness of the
carriage. Pulse cleaning is very fast, but also has comparative dis-
advantages. It seems that every cleaning method in use has at least
one advantage over each of the others, and conversely.
-------
TABLE 3.1
COMPARISONS OF CLEANING METHODS1
10
-p-
Cleaning Uniformity
Method of
Cleaning
Shake
Rev. Flow,
no flex
Rev. Flow,
Collapse
Pulse-com-
partment
Pulse-bags
Reverse jet
Vibration,
rapping
Sonic assist
Manual flex-
ing
Ave.
Good
Ave.
Good
Ave.
V . Good
Good
Ave.
Good
Bag
Attrition
Ave.
Low
High
Low
Ave.
Ave . -Hi
Ave.
Low
High
Equipment Type
Ruggedness Fabric
Ave.
Good
Good
Good
Good
Low
Low
Low
--
Woven
Woven
Woven
Felt,
Woven
Felt,
Woven
Felt,
Woven
Woven
Woven
Felt,
Woven
Filter Apparatus Power
Velocity Cost Cost
Ave .
Ave.
Ave.
High
High
V.High
Ave.
Ave.
Ave.
Ave.
Ave.
Ave.
High
High
High
Ave.
Ave.
Low
Low
M.Low
M.Low
Med.
High
-High
M.Low
Med.
--
Dust Max. Sub-
Loading Temp.** Micron
Efficiency
Ave.
Ave.
Ave.
High
V.
High
High
Ave.
--
Low
High
High
High
Med.
Med.
Med.
Med.
High
Med.
Good
Good
Good
High
High
V. High
Good
Good
Good
These value judgments do not permit comparison of performance aspects, only of methods.
**
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3.3.1 Shake
Bags are most commonly shaken from the upper fastening.
Several combinations of horizontal and vertical motion are used. The
bags may all be fastened to a common framework which moves horizontally.
The. frame may have slight additional upward or downward swing, depending
on the linkage holding the framework. The framework may instead be
oscillated vertically. Alternatively, the bags may be attached in rows
to a rocking shaft. In this case the location of the point of attach-
ment with respect to the center of rotation determines whether the motion
is predominantly horizontal or vertical.
Rarely are the bottoms of the bags shaken instead of the
tops. Panel filters on the other hand, are usually shaken top and
bottom, that is, the entire filter bank moves from a single drive point.
The shake amplitude is usually designed into the equipment and may be
anywhere from a fraction of an inch to'a few inches. The frequency of
shake is usually several cycles per second and can often be adjusted to
obtain the most suitable fabric motions. Most shaking is essentially
simple harmonic or sinusoidal motion. When it is, the peak acceleration
may be easily computed; it is usually from 1 to 10 g s.
During the shake, the filtering should be stopped. Other-
wise, the dust will work through the cloth, reducing the efficiency and
possibly damaging the cloth by internal abrasion. In some equipment,
the flow is slightly reversed during shaking, both to prevent penetration
and to aid in cake removel. Still more elaborate, but possibly more
effective, cleaning procedures involve a series of alternate flows and
shakes which take time, but are claimed to give a gentler net treatment
to the cloth, plus more uniform and thorough cleaning.
In a typical cycle, the inlet flow to the compartment is
first dampered off by an automatic timer and valve mechanism. If neces-
sary, the outlet vent is also closed (Figure 3.4). In the absence of an
air lock between adjacent hoppers it may be necessary to close a damper
-------
0
*
1
•
'V
»
k
\
f
/ Jv
f/
1_
. t*,.«
c*.s. -•
• •
;"*A
UvJ ^
Filtering —^
Outlet
Shut
Cleaning
Remaining Compartments
Reverse Flow
Option
Time
Figure 3.4. Shake Cleaning Process and Associated
Pressure Cycles.
-------
there to prevent the intrusion of dirty air from hoppers still operating.
There should be zero forward pressure across the fabric during shaking,
since otherwise dust will work through the fabric. The timer starts the
shaker motor and the bags are shaken horizontally at 1 to 5 cps with an
amplitude of up to 2 inches; that is, the shaker imparts accelerations
i
on the order of a few g s. Shaking continues for 10 to 100 cycles.
Then the timer may start a small flow of clean reverse air using an
auxiliary blower or a secondary duct-and-damper system for 10 to 20
seconds. The shaking may be repeated, this time during the small re-
verse flow. Finally, the cleaning is stopped and after pausing to
allow the dust to settle, the inlet and outlet dampers are opened and
the compartment begins its filtering again. The entire cleaning cycle
may take from 30 seconds to a few minutes. Some installations do not
return the compartment on line until the next one is ready to be cleaned,
thereby achieving a fairly steady overall flow through the baghouse
system at the expense of some over-capacity.
The pressure across the compartment during the cleaning
cycle is sketched in Figure 3.4. If there are only a few compartments
in the baghouse system, shutting one down will increase the flow and
pressure drop across each of the others, the amount depending on the
fan curve. The pressure across the baghouse while the above cleaning
is performed is also indicated in Figure 3.4. This increase in filtering
velocity and pressure across the fabric in the other compartments can be
expected to have some effect, adverse but probably slight, on the filter
cakes forming in those compartments.
The mechanics of cleaning by shake have apparently not been
studied in any detail. The cloth is flexed to some extent and the cake
is thereby cracked or loosened from the cloth fibers. Shaking produces
inertial stresses at the cake-cloth interface, both shear and tensile,
and if these exceed the adhesive strength, then some of the dust cake
falls off. It is reported that a fine balance of bag tension, shake
frequency and shake amplitude is necessary in order to obtain the "s"
-------
shaped wave in the moving bag giving best results. On the other hand,
simple manual shaking equipment seems to work with a minimum of science,
although careless manual cleaning is hard on the fabric. The amount of
shake energy transmitted along the bag decreases from a maximum at the
shaken end (usually the top) to a smaller value as the energy is absorbed
along the bag. A heavy residual dust cake,not removed by the cleaning
process (usually in the case of sticky fine dusts), changes both the
mass and the flexibility of the bag and must affect the rate of energy
absorption locally. Further, the added residual dust weight accumulates
upward and increases the tension toward the top of the bag, affecting
the shape of the shake "wave" for better or for worse.
Shake equipment has few limitations in application. As
noted above, the dust loading time cannot be too short. The dust should
be fairly easily removable from the cloth as it is easy to damage the
cloth by over-shaking. This is especially true of glass bags. Although
it was at first believed that glass could not be shaken, the introduc-
tion of certain glass finishes has made this possible on some dusts which
separate fairly easily from glass fibers. Shake baghouses are popular
for both very small systems (manual shake) and very large continuous
sys 'cems.
Two problems in particular are often reported with shake
equipment. First, in an effort to keep manufacturing costs down, a
number of shaker mechanisms are underdesigned. They wear and as
they do, they shake less effectively. The bags load up or the operator
intensifies the shaking, and the mechanism then destroys itself unless
given an inordinate amount of maintenance. Second, as the free end of
the bag is shaken, the cloth must flex extensively at the fastened end,
resulting in fiber-fiber abrasion. This may explain the common occur-
rence of bag failures within the first couple of feet of bag. These
entrance end bag failures may also be related to dust deposit weight,
which is often heavier at the lower end of the bag and may result in a
higher rate of cleaning energy absorption in this region. Another
possible factor is particle entrance abrasion.
-------
3.3.2 Reverse Flow - No Flexing
If the dust releases fairly easily from the fabric, a low-
presoure reversal of the flow may be enough to loosen the cake without
flexing or mechanical agitation. To minimize flexural attrition of the
fabric, it is supported by a metal grid, mesh or rln^s, and is usually
kept under some tension (Figure 3.r>). The support is usually on the
clean side of the tube or bag, although dirty-side support can help to
keep the sides of the bag or the panels sufficiently apart to allow the
cake to fall to the hopper. Some filter equipment relies solely on low
pressure flow reversal, while other models use it in conjunction with
another method, shaking:for example, or use the much higher reverse
pressures of pulse or jet cleaning.
There are several ways of accomplishing flow reversal. In
addition to the standard dampers on each compartment, each one can have
its own reversing fan. A few models have a traveling apparatus that goes
from bag to bag or from panel to panel, blocking off the primary flow
Figure 3.5. Reverse Flow Cleaning.
-------
and introducing some air in the reverse direction.with a secondary
blower (Figures 3.3e and 3.3g). Perhaps a simpler method is to take
advantage of a suction on the dirty side, or a relative pressure on the
clean side, without using another blower as sketched in Figure 3.6. If in
using this cleaning system it becomes necessary to increase the suction
or pressure being used for cleaning by partially closing off the inlet
or outlet primary duct, then to avoid the added power consumption, it
may be better to use a small secondary fan or fans.
Any flow volume reversed through the filter must be refil-
tered. This means that in addition to taking cloth out of the system
for cleaning, this cleaning method increases the total air flow in the
remainder of the system. The net increase in air/cloth ratio is nor-
mally 10 percent or less, but this may cause a large (perhaps 40 percent)
increase in power consumption. This is partly through a decrease in
cake permeability.
' The pressure across the compartment and baghouse will be
similar to that sketched in Figure 3.4. However, some equipment users
believe repeated reverse flows introduced suddenly to "pop"
the cloth are beneficial, in which case the pressure-time cycle will be
different.
Reverse flow equipment with a minimum amount of cloth flexing
finds the same wide range of general applications as shaking equipment,
the one main requirement being that the dust release easily from the
fabric. Thus, felts are not cleaned by low pressure reverse flow.
Disadvantages of reverse flow equipment include the in-
creased filtration velocity (or alternatively a proportionally larger in-
stallation), the costs of any additional fans, ducts or dampers, and
when necessary the cost of a support grid. This grid tends to be a
nuisance when it comes to changing the bags or panels and considerable
effort has gone into its design. The grid or rings may chafe against
the cloth unless the cloth is so tight it can't move, which introduces
-------
Optional, to avoid
temperature changes
(Pressure)
F: Compartments filtering
R: Compartment being cleaned by dampered
control from suction side of system.
Figure 3.6. Schematic for Reverse Flow Cleaning
During Continuous Filter Operation.
-------
tension problems, or unless the cloth is fastened to the support, as for
example, rings sewn to the tube. Even such hard fibers as glass can
apparently fail by chaffing. Chaffing is such a problem that the sup-
ports are occasionally done away with at the risk of excessively flexing
the cloth.
3.3.3 Reverse Flow With Collapse
Even though cloth flexure is detrimental, especially when
the fabric is penetrated by grit, flexure is frequently the preferred
means of cloth cleaning. It is sometimes used in conjunction with other
methods. The cleaning method excludes panel type filters and felts,
but cloth bags having the dust cake on the inside are often cleaned this
way. The cylinder is often reported as collapsing for some reason into
a cloverleaf pattern (see Figure 3.5). The collapse is not 100 percent,
however, or the cake could not fall to the hopper. There should be just
enough reverse flow to crack and shear the cake until it falls off. It
may then be carried to the hopper by the reverse air current, depending
on collector design.
With this kind of cleaning there is an optimum tightness of
the cloth, said to be somewhat between 25 and 100 Ibs for a standard
11.5 inch diameter glass bag. More tension than this is hard on the
fastenings, or prevents the bag from collapsing enough. Less tension
results in too sharp a cloth bend at the tube thimble where the cloth
is fastened (see Figure 3.5). The tension of a suspended bag can be
automatically changed during the cleaning cycle if necessary, by adjust-
ing the position of the hanger frame.
The collapse method of cleaning uses essentially the same
damper, fan and ducting equipment as the method of reverse flow without
collapse. Just as repeated shaking is standard practice, the bag or
tube may be collapsed more than once. It may also be "fluttered" by
lightly and rapidly pulsing the reverse air to give a shaking effect,
-------
at some risk of damage to the cloth. Such flow changes may also trans-
mit pressure pulses to adjacent compartments and damage the cake struc-
ture there.
Chief disadvantages of collapse cleaning are flexural and
chaffing attrition of the cloth, and some lack of control of cleaning
intensity. For instance, if the cake fails to come off sufficiently
and begins to stiffen the cloth, there will be still less cleaning and
consequently, extra flexing in adjacent areas and faster wear. There is
little one can do to increase cleaning intensity, although one can re-
peat the process or change the rate of flexure.
3.3.4 Pulse Cleaning
This method attempts to overcome several of the difficulties
associated with other methods of cleaning. In this kind of equipment a
sharp pulse of compressed air is released in the vicinity of the fabric
giving rise to some combination of shock, fabric deformation and flow
reversal. Depending on the design, and there are over 25 models of
pulse cleaned equipment. The result is the removal of the dust deposit
without more than a brief interruption of the filtering flow. The fabric
receives a minimum of flexural wear and the filter installation is
smaller because the fabric is in use practically all the time. Also,
for reasons on page 3-46, most pulse equipment utilizes felt rather
than cloth. With felt the filtration velocity can be typically 3 to 4 times
that used in shake or reverse flow equipment, so the pulse filter in-
stallation is smaller.
The main distinction of pulsed equipment is the brief clean-
ing time, typically around one-tenth of a second. The very low ratio of
cleaning time to filtering time makes pulsed equipment uniquely useful
at very high dust loadings, up to several hundred grains per cubic foot
on large particles. Thus, this equipment is widely used for pneumatic
conveying systems, and it also has application at moderate and low dust
concentrations.
-------
The earliest designs of this kind, as shown in Figure 3.7,
use open top bags with a pulse nozzle in or just above the top of each
bag. A timer controlled solenoid valve releases 60 to 120 psi air
through the nozzle for a small fraction of a second. As the air pulse
moves down the bag, it may draw in other air and the combination tends
to bulge the bag. In one variation the filtering is on the inside of
the bag and the dust is partly removed by reverse collapse, but then a
pulse similar to the one shown above snaps the bag open and completes
the dust removal. Note that this forward pulse cleans without appa-
rently plugging the fabric. A second variation in pulse cleaning design
is the compartment pulse, in which several bags or envelopes are served
by the same nozzle located in a plenum at one end of the group; or, the
entire compartment is served by one or more nozzles. For example, at
least one model injects a reverse air blast into the compartment inlet,
which temporarily stops the flow and reverses it,causing partial col-
lapse of the filter elements.
Filtering
Pulse
Cleaning Wave
Figure 3.7. Reverse Pulse Cleaning.
-------
These variations in clean vs. dirty side pulsing and inside
vs. outaidv'-the-bag pulsing, raise the question of the mechanism or
imichnniHtroi uf dust removal. Sovrrnl imThaniBtiw nppt-ar poHHlhlt- «lppiMi«l-
Ing on dt'
1. The bag or tube stretches au the air balloons through it,
cracking and blowing off the cake, (inside pulses only).
2. The air pressure change, even after expansion and dilu-
tion, is still long enough and intense enough to cause
a flow reversal which blows off the cake, (clean side
pulses only).
3. The inertia of the air mass pulls a vacuum behind it which
pulls off the cake, (dirty side pulses only).
4. The shock of the pulse accelerates the fabric away from
the dust cake, even though the distance traveled by the
fabric is very small, (probably dirty side pulses only).
5. The fabric and dust accelerate outward together, but only
the fabric has the elasticity to snap back into shape.
Outside dust is left behind; inside dust is thrown off
at the end of the snap-back, (inside pulses only).
Among the approximately 25 pulse-type collectors the pressure-time func-
tion varies considerably from a true shock to little more than a quiet
reversal of flow, so it is entirely possible that several of these
mechanisms apply.
Efficiency of use of compressed air is important because the
electric power needed to run the compressor can equal that for the pri-
mary fan. Some models claim improved efficiency by using Venturis sur-
rounding the pulse nozzles as was shown earlier in Figure 3.3.i. These
entrain a certain amount of additional air which expands the bag, causing
the collected dust to drop off the outer surface. The entrainment may
also set up a shock wave which accelerates or flexes the filter cloth.
-------
It is expeciaily interesting that only one of the approxi-
mately 25 available pulse-cleaned models uses downward flow. Apparently,
the dust is able to fall through the upward flow to the hopper despite
upward velocities typically 3 to 4 times those in shaker and reverse
flow equipment. Evidently the pulse-removed deposit falls in large
pieces, but even so, a large quantity of dust must be redeposited on
nhe fabric. Possibly, these large pieces are beneficial in acting as
filter aid, but on the other hand they increase the total deposit on
the filter.
The components of a pulse cleaning filter include an air
compressor, a storage or surge tank, piping, solenoids and nozzles,
and some models use Venturis and fabric support gridwork as well. Few,
if any, ducting dampers are needed along with their associated controls.
As there are no moving parts required, the pulse method has an advantage
in terms of maintenance.
Just as in shake cleaning, in which all the cleaning energy
must be applied at one end of the bag, so in pulse cleaning the effect-
iveness of cleaning decreases with length of bag. This is evident in
the short bags typically used in pulse equipment, rarely if ever over
ten feet in length. Since there is little fabric motion, the bags or
tubes can be packed slightly closer without interchaffing. Even so,
large installations still occupy more floor space than types with 30- to
40-foot bags.
Both the intensity and frequency of pulse cleaning are
easily adjusted, but there is little control of top to^bottom uniformity
of cleaning. Woven cloths tend to be over-cleaned by pulsing, resulting
in excessive leakage following cleaning, whereas felts are more easily
maintained at a satisfactory level of residual dust deposit over the
entire bag length with excellent cleaning efficiency.
-------
There are several other reasons for using felts rather than
woven cloths, although in a few rare cases tightly woven fabric is used.
The dimensional properties of felts may make them more amenable to pulse
cleaning involving "ballooning". Bag inlet velocities are high in pulse
equipment and felts having softer surfaces may withstand more abrasion
than cloths. Felts form a more porous dust cake having lower specific
resistance, so they can be cleaned less often than cloths for the same
pressure build-up. Equipment using felt can be smaller because higher
filtering velocities are possible. Finally, felts can be used even
though they are initially more expensive, because there is little mech-
anical wear of the fabric in pulsed equipment.
Like other types of equipment pulsed cleaning has its dis-
advantages. It is limited in temperature to around 450 F at present,
because felt materials are not available for higher temperature. High pulse
pressure can damage the fabric by over-stretching it. Felts tend to
plug in depth rather than blind. They may have to be cleaned rather
than discarded because of their greater value. Felt, compressed air,
power and the compressor are all relatively expensive so that economic
balances are different for pulse equipment. As a result, equipment
sizes and shapes are different. Pulsed equipment may be best for some
applications and simply not economical for others.
3.3.5 Reverse Jet Cleaning
Introduced in the early 1950's, this method of cleaning
became known for its high filtering efficiency and high filtration velocities,
It was the first method to give felted materials a good, uniform cleaning.
It did this by using a small-volume jet of moderately pressurized air
spaced at moderate time intervals. Figure 3.3f shows how this can be
done. A carriage carrying the jet rings moves up and down the filter
tubes, driven originally by chains and sprockets, or by cables con-
trolled by limit switches. On the carriage a slotted ring close to the
fabric surrounds each filter tube. Most of the jet air goes through the
felt toward the dirty side, blowing the dust out of the deep felt surface.
-------
J
Since the four reverse jet models on the market are based on
the original Hersey patents, the models are much the same. One type
uses adjustable segmented rings around the tubes, another uses a flat —*
plenum through which are cut holes about the same diameter as the tubes.
The traveling carriage is easily designed for either cylindrical or
rectangular baghouses. Tube lengths are in the range of 6 to 30 feet.
i
In all models the flow is downward and outward. For this reason, the j
collector can be open sided, but as reverse Jet equipment is often used
on toxic materials because of its high efficiency, health codes usually j
require that the filtered gases be contained until exhausted outdoors.
There are times when a very low dust concentration must be
filtered, e.g., valuable or toxic dusts. At such times the reverse jet
collector is an early consideration, not only because of high efficiency, i
but because of high air capacity, typically 10 to 30 FPM depending -*
mainly on particle size. The time required for cake buildup can be as
long as a month, depending on concentration. To prevent useless running ~->
of the carriage the limit switches may be over-ridden either manually
or by a pressure switch. Once the carriage starts to move, it travels '
at its normal rate of a few feet per minute. It pauses between trips
for cake buildup, thus lengthening the mechanical life of the equipment. I
_j
Conversely with moderate dust loadings, the carriage usually
runs continually. On higher loadings it can be speeded up to a limited
extent if necessary. Note that in continuous operation the pressure
drop across the compartment is not quite steady, since compared with the j
midpoint of the tubes the end of the tube is first cleaned too soon and
i '
then cleaned late on the next pass. How much this causes the primary ,
flow through the system to fluctuate depends on rate of pressure buildup, —'
rate of carriage travel, the fan curva, etc.
Reverse jet equipment offers control of both intensity and —i
frequency of cleaning, and even offers the possibility of adjusting the ,
i
intensity according to carriage position if necessary, although there ^
-------
I
L
is no report of this being done. The cleaning is practically uniform
> from top to bottom, the more so since the deposit tends to be uniform
'— due to downward flow. For these reasons the felt should last longer
0 without plugging than with any other method of cleaning. Unfortunately,
<_ these advantages arc partly offset by wear problems in both the felt
and the moving parts.
*
L.; Reverse jet equipment is well suited to fine particles,
expecially if these cohere well. Otherwise, there is considerable re-
[__ entrainment when the deposit is removed and the collector operates at a
higher pressure drop. This equipment is also suited to coarse particles,
! especially non-abrasive ones.
Regarding wear, it is good design policy and almost necessary
! to have as much of the moving mechanisms as possible outside the com-
partment. Even the clean side of the compartment sooner or later gets
] dusty, if not from seeping dust then from perforated fabric. When this
****
happens the mechanism tends to wear and, eventually, to stick and jump.
The limit switches may become fouled and unless fail-safe the drive
chains will break, cocking the carriage and probably ruining the fabric
tubes. Once fouled it is very hard to get the mechanics back into the
L. original clean and unworn condition. Consequently, the frequency of
maintenance increases.
i
L. In a number of installations a hard deposit of dust and
fiber bits has built up on the jet rings over a. period of time so close
^_ to the cloth as to cause abrasion. This deposit can wear through the
fabric unless removed.
^ The air required to clean the fabric is usually supplied by
a high-speed blower at pressures of 5 to 20 inches. The volume required
is roughly 5 percent of the primary filtering flow which increases the
*_
filtration velocity only slightly.
-------
3.3.6 Vibration and Rapping Cleaning
Although several methods of cleaning involve motion and even
fluttering of the fabric, higher frequency agitation with little cloth
travel can also be effective. In vibration and rapping, either trans-
verse or longitudinal elastic waves travel over the fabric, accelerating
the fabric surface through displacements that are usually small. The
inertia of the dust cake causes stresses at the cake-fabric interface
which detach the dust cake from the fabric. Since it is easier to
vibrate or rap a compact group of taut filter elements, envelope filters
are the most commonly rapped type.
This method of cleaning is especially successful with deposits
of medium to large particles adhering relatively loosely. Since larger
particles are typically from low or moderate temperature sources, high
temperature fabrics are not usually used. Woven fabrics rather than
felts are used because they clean less expensively and even paper filter
material can sometimes be used.
The vibrating mechanism should be located outside the housing
to minimize dusty abrasion. At least one end of the frame supporting the
filter envelopes is floating and attached to an eccentric arm or vibra-
tor. If the cleaning is to be done manually, a common rod may protrude
or the housing may be opened to strike each filter frame.
The compartment is shut off during the vibrating or rapping.
If continuous filtering is necessary, several compartments can be used
with sequential automatic cleaning. The length of the vibrating time
varies. However, as with shaking, about 100 vibration cycles should be
sufficient. Longer vibration is probably not as detrimental to the
cloth as in shaking. The power requirement is low and the cloth life
can be several years barring heat, chemical, or other problems.
For larger particles, for which this equipment is especially
suited, the filtration velocity can be higher than with other types of in-
termittent equipment. Therefore, vibration/rapping equipment is compact.
-------
With the larger particles, higher pressure drops across the cloth can be
tolerated without as much danger of blinding the cloth or blowing the
__ cake through the cloth. For this reason small, light, unit-type inter-
mittent equipment may operate safely without attention for relatively
i> long periods, even without instrumentation. Eight hours is a typical
operating period between cleanings. Even though the cost of filtering
power at higher pressure drops across the small collector is high, it is
fr" frequently only a small part of the plant process cost.
The main disadvantage of this kind of equipment is probably
*"" the relatively few dusts for which it can be used. If the equipment is
located in a confined space it can be noisy.
i
! 3.3.7 Sonic Cleaning
Agitation frequencies still higher than those used in vibration
I and rapping have been attempted with ultrasonic and sonic cleaning methods.
' Although these frequencies are known to slightly improve the preagglomera-
i ^^
i tion of a few fine dusts, they have not, on the whole, been very effective
! in fabric cleaning. Lower sound frequencies are used, however, with success
] *- in a, few installations. The Fuller Company (Dracco) has installed sonicly
i
I cleaned equipment in cement plants for over ten years. They also used a
^_ successful combination of reverse flow-collapse plus sonic cleaning in
2
Bethlehem Steel's open hearth furnace baghouse about 1964-65. Usually,
' sonic cleaning must be supplemented by some other method.
i The frequency band used( can be estimated from the sound, des-
i cribed as low and mixed like a fog horn or railroad whistle. The cloth
i '
J motion is not apparent, but the vibration can be felt by touch. At the
! Bethlehem installation the sound was generated by compressed air, with
i — " 2
I „ less than 300 SCFM needed for 50 horns serving about 80,000 ft of cloth.
The pressure required was not determined. Following reverse flow and
i
~~ collapse the horns gave three five-second blasts, after which the collapse
and horn blasts were repeated a second time.
-------
In all reported installations of sonic cleaning (cement and
open hearth fumes), glass cloth has been used, at filtration velocities
of 2 FPM or less. However, it was claimed in 1959 that use of sonic
cleaning would reduce the normal cloth pressure drop of 4 inches by 25
to 50 percent, raising the possibility of a higher filtration velocity.
Sonic cleaning costs have been reported as lying between
those of reverse nir and shaking systems. The compressed air equipment
is apparently less than for pulse cleaning equipment. However, sonic
cleaning has never achieved the popularity of most of the other cleaning
methods, for reasons which have not been reported.
3.3.8 Manual Cleaning
Any of the automatic cleaning cycles can be initiated man-
ually at convenient times, as between shifts. An even simpler method
of cleaning is to approach each bag or panel and remove the cake by
hand as in beating a rug. The method has no hardware to wear out and is
as reliable as maintenance can be. However, the cloth itself generally
wears rapidly and endures fewer cleaning cycles than other methods.
This method is only practiced on smaller dust collectors
having, at most, only a few filter elements. They can be brushed in
place, or thumped to knock the cake loose. Alternatively, they can be
removed and turned inside out for more thorough cleaning.
When the more expensive fabrics blind or plug, well before
their other qualities are depleted, they are reclaimed by various
methods such as vacuum cleaning, dry cleaning, or laundering. An advan-
tage here is that weak spots may ;be detected and repaired before a hole
in the fabric actually occurs. A certain amount of skill goes into re-
moving dust without damaging the fabric. Manual cleaning of any large
amount of ordinary cloth is not economical.
-------
3.4 CONSTRUCTION AND MATERIALS
Fabric filter manufacturers are rarely able to guarantee the per-
formance of their equipment, because performance depends so much on how
the equipment is installed and used. Any manufacturer can, however,
guarantee the quality of his dust collector as sold. As a check on
quality, the manufacturer has standard specifications for the production
of the equipment. These specifications include a large amount of detail
for construction and materials, an example of which is presented in
Appendix 3.2.
3.4.1 Housing
External framework and configuration design is available in
three types, the choice depending mainly on moisture condensation con-
siderations. The open-sided pressure design (inside filtering) is
cheapest and used mainly below 135 F. The closed pressure type is next,
and followed by the slightly more expensive closed suction installation.
Above 160°F, the equipment is usually closed and insulated. The cost
increases with the area and thickness of metal used, which depends on
the pressure the walls must withstand. Cost will also reflect any neces-
sary weather protection, insulation, etc., as well as gas corrosiveness
and temperature.
Most small unit collectors are assembled at the factory,
usually welded, while larger units may be either assembled at the
factory or on location. The largest unit that can presently be shipped
by railcar is approximately 10' x 40' x 12' high; consequently many
larger designs are assembled from standard preassembled modules by
either welding or bolting on location. It is difficult in field assembly
to make good pressure or vacuum-tight seals between panels, modules and
flanges and seal quality is a chief complaint among fabric filter users.
To create a strong structure and provide good seals against
weather and condensibles, under the large deformations due to changes
-------
in temperature and pressure, some variations in construction are neces-
sary. Several styles of housing joints and sealants are indicated in
Figure 3.8. For steel, one manufacturer uses galvanized 14 gauge sheet,
others use 10 gauge welded hot-rolled steel. Much heavier material can
be used, up to one-quarter inch or more. The open hearth FF installation
at Bethlehem Steel used 3/16 inch for walls, roof and partitions, 1/4
inch for floor plates and 5/16 inch for hoppers and inlet plenum (Fuller-
2
Dracco design) . In passing, let it be noted that competition forces
many manufacturers to underdesign unless otherwise instructed (see
Section 8.3).
Materials other than steel may be used for housings; for
example, cement plant collectors are often made of concrete and one
fabric filter manufacturer has a design for precast reinforced concrete
panels. An aluminum company stated it feels that aluminum is definitely
the best all-round material to use. Corrugated asbestos cement paneling
is often used for exterior roofing and siding, with the interior walls
and partitions made of steel. Various other composite panels can be
used. Insulation can be sandwiched in the paneling or added to the
outside at factory or later as needed. The main precautions in selec-
tion of material are corrosion and changes in temperature that cause
thermal stresses in the seals and even in the bags.
-------
fSealant) !
Vertical ) !
Siding Joint)*
Horizontal
Siding Joints
mi
(Insulated)
U
[frfWTi
. y r|r
» B
Roof Joint
u
1
1
!
i *
Siding
J
tj
Panel
Stiffeners
i^^_
I
1
Main
Frame
i
Figure 3.8. Examples of Some Styles of Fabric Filter Compartment Joints,
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3.4.2 Hopper and Disposal Equipment
Hopper material In usually similar to that used for the hous-
ing with similar joints and seals. The inside may be treated to minimize
adhesion of the dust. Hoppers are commonly available with either 45 or
60 slope depending on the need to conserve height or to provide adequate
sliding, as affected by the plugging properties of the dust.
Most hoppers are under a few inches of pressure or vacuum.
Consequently they require some kind of valve at the bottom to let the col-
lected dust out without admitting air to reentrain dust and redeposit it.
Automatic filter equipment usually uses a rotary valve consisting of a
sealed paddle wheel as depicted in Figure 3.9. These valves rotate at
about 5 RPM, and are typically 6 to 12 inches in diameter. Thus they
handle around 2 CFM of collected material. If the solids flow is higher,
the valve can be run faster or more or larger valves can be used.
For low solids flow,and for many non-automatic installations
requiring periodic attention,sliding gates may be used. Unless the hopper
is under practically zero pressure these gates are used only when the com-
partment is shut off. When the hopper is under no pressure a gate may not
be necessary. A stocking perhaps 6 feet long reaches from the hopper
bottom to the dust bin. Likewise, the rotary valve or gate seldom dis-
charges with an open cascade of dust. A stocking is used or more commonly
the solid flow enters a conveyor, usually screw-type.
If for reasons of compartment cleaning or maintenance the com-
partments must be completely isolated then the rotary air lock valves are
located between the conveyor and the hoppers. Otherwise it may be less
expensive to install the conveyor directly on the hoppers and use one valve
at the end of the conveyor. The air slide, bucket, and belt methods of
conveying powdered materials are normally used only for high solids flow.
There are .however, times when the screw conveyor doesn't work as with
gummy dusts and those that lock up under pressure.
-------
\ /
J L
, i r
i i i
r, p
j
1
Chain
Drlva
4
^
c
);.
<
—
D;
r
— —
hj
i
Full
H«lf-skeleton
Skeleton
Rotary Valve
Manual Sliding C«ten
Trip Gatf
(Automatic or Mcnual)
Dlgcharge Duck
(Knotted, Clamped, etc.)
Valve #1
I
Drive
L.
Valve #2, etc.
Screw Conveyors
Discharge
Rotary
Valve
Figure 3.9 - Types of Hopper Discharge Equipment
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3.5 EXTENSIONS OF FABRIC FILTRATION EQUIPMENT
In addition to the standard FF designs already discussed, from time
to time novel filter equipment arouses interest among those acquainted
with the limitations of standard equipment as indicated earlier in Chap-
ter 1. The patent literature records a fascinating history of design
variations, of which only a small percentage are now in practice. Be-
sides these efforts to control dust, fabric filtration has demonstrated
other capabilities including control of gases and perhaps odors, control of
mists, and ultrafiltration for very finu particulates in atmospheric dusts.
3.5.1 Variations in Standard Fabric Filter Designs
3.5.1.1 Bourdale Rotary Filter.- The filter media surrounds a central
shaft and rotates "until such time that the cake of dust has such a mass
that its centrifugal force exceeds the fan suction, when it flies off the
surface and settles down into the hopper." This is a compact unit in use
on a wide variety of dusts up to 210 F. It is made by Wheelabrator Alle-
vard of Paris.
3.5.1.2 Tower Collector.- Two tiers of glass cloth bags are used, one
above the other, in a 60 foot or higher cylindrical chamber. The flow is
downward; cleaning is by sonics. Although the brochure of the company
that produces this configuration is not specific, the bags are probably
butted end to end on an intermediate tube sheet to relieve the tension that
would otherwise be extreme on a single 60 foot bag.
3.5.1.3 Self-Emptying Bag.- In one such design up to 8 bags, each
about 6 ft by 1.5 ft dia., are held at the top by a guy cord during filter-
ing. For cleaning the cord is released and under ;the weight of the dust,
the bag sinks through the lower tube sheet, turning itself inside out and
cracking off the cake. In another design used in a copper mine in Zambia
the bags are held up by inflation pressure, and their top is guided up
and down by a taut wire through a grommet in the top center of the bag.
Dust dumped as the bag inverts itself is later washed away by a stream
Q
through the inlet plenum.
-------
3.5.1.4 Granular Bed Plus Screen.- This equipment uses pulses to
clean a combination sand bed plus filtering screen, as sketched in Fig-
ure 3.10. Large particles form a cake on the screen(s), and fine particles
SCREEN
SAND
Filtering
Cleaning
Figure 3.10. Ducon Sand and Screen Filter Cleaned by
Back Flow.
are caught in the sand(s). "The channels in the filter element are not
completely filled with sand. Thus, during the cleaning cycle, short blasts
of high-pressure air expand the beds. The air dislodges filter cake from
the outer screen and restructures the granular material into a filtering
9
bed essentially the same as the original." The Ducon Co. says the device
can operate at filtering velocities of 30 to 40 fpm at temperatures
well above 1000 F and on sticky dusts.
3.5.1.5 Foreign Equipment.- Filtration and Separation, a British
publication, regularly lists developments in dust control equipment in
Great Britain and Europe. Much of this equipment is slightly different
-------
from U. S. Designs although several U.S. manufacturers have subsidiaries
in Europe, and this handbook survey of available equipment styles could
be extended to include European equipment.
3.5.2 Control of Gases and Odors
Several experimental studies have shown that finely divided
absorptive or adsorptive granular materials aerosolized into the gas stream
entering the fabric filter, can effect the removal of certain gaseous com-
ponents. The combination of good powder control with high specific surface,
and long, intimate gas-powder contact especially on the fabric, are advan-
tages offered by fabric filtration equipment over most other equipment.
Most recently the removal of up to 98.4% of SO- by sodium bicarbonate has
been demonstrated at a coal-fired power plant by Air Preheater Co. under
NAPCA contract. Fly ash and N02 are also removed at the same time.
The optimum equipment design for gaseous control would probably not be much
different from present filter designs, except for the system for introducing
and possibly recirculating the powdered material. This addition is esti-
mated to be a small part of the fabric filter system cost.
The addition of particle aggregates to a filter cake should
reduce power costs by reduction of the cake porosity. The partial recir-
culation of agglomerates from the collector hopper seems attractive but
there is no report of this practice. It would appear that agglomerate
recirculation in conjunction with absorptive powder recirculation would be
attractive in numerous possible gas and odor control applications.
Similarly, although as far as is known absorbing additive in
fabric filter equipment has not been used to control odors, it is a likely
prospect. Granular charcoal, catalysts, or fritted impregnated material
could be introduced to and removed by the filter, regenerated in special
closely confined equipment, and then recirculated.
3.5.3 Control of Mists
It is widely observed that the fabric in an ordinary fabric
filter collector must be kept dry or it will rapidly plug with dampened dust.
-------
This may be reversible, however, as shown by pilot plant experience at a
Southwest Portland Cement Company plant some years ago using silicpnized glass
fabric, collapse without shake cleaning, and cement kiln effluent normally
about 500°F: "...During subsequent months of testing, the bags at times were
saturated with water from condensation during normal variations of atmos-
pheric and kiln conditions (wet-process) and were even deliberately wetted.
In all cases, rising temperature dried the bags and, at the next collapsing
cycle, they discharged readily and returned to normal pressure drop. In-
spection of the fabric showed that no blinding had occurred, and the lack
of deterioration suggested an entirely satisfactory life of at least one
,,11
year."
Other filter beds, more open than normal filter fabrics, are
used for large particle demisting in a number of processes. These are
usually self draining, but if they are of intermediate porosity and there
is danger of clogging they can be kept open with water sprays. One such
study obtained effective removal of submicron TiO. particles plus around
12
90% removal of sulphuric acid mist, by irrigating synthetic felts.
Filtration was at 70 to 165 FFM, the spray rate was around 10 gal. per KCFM,
and felts fairly similar to felted fabric filtration materials were used.
The pressure drops encountered were 8 to 10 inches of water and were nearly
independent of filter velocity, possibly because the air permeability of
the wet felt depended on the amount of water remaining in the felt. Pro-
jected initial costs of a complete 56,000 CFM system were cited as $2.40
per CFM plus minor installation costs.
These examples of effective particulate control with fabric at
conditions below dewpoint may point the way to solution of condensation
problems in fabric filtration, enabling cooler and more economical operation
without fabric blinding aid plugging. There are many possibilities for
research and development in this area including the opportunity for coin-
cidental wet-process odor control. Although the field is not a new one by
any means, new synthetics and changing economics make it a viable and
inviting challenge.
-------
3.5.4 Ultrafiltration
The high efficiency made possible by filtering with a dust
cake is used to obtain very low effluent concentrations of toxic and radio-
active dusts, bacteriologicals, and atmospheric dust. Very low concentra-
tions of liquid or tarry particles may also be controlled. Aside from a system
for introducing the filter aid, the equipment is much the same as that for
any fabric filter system. Since the fabric is cleaned only very infre-
quently, however, perhaps once a year or longer, the cleaning mechanism and
dust disposal equipment can be minimal. A high fabric packing density can
be used without concern of chaffing. Maintenance is low and the equipment is
available for operation with a number of filter aids.
3.6 REFERENCES FOR CHAPTER 3
1. Wheelabrator Corporation, Pustube Collectors. Bulletin No. 565-E
page 10 (1967).
2. Herrick, R.A., Olsen, J.W., and Ray, F. A., Oxygen Lanced Open
Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
Jnl. A.P.C.A. 16: 1, 7 (January 1966).
3. Sonics Clean Dust Collector Bags Soundly, Chemical Week.
(14 November 1959).
4. Adams, R. L., High Temperature Cloth Collectors, Chem. Engg.
Progress. 62;4, 66 (April 1966).
5. Bourdale Rotary Filter Dust Collector, Filtration and Separation.
170 (March 1969).
6. Fuller Company, Collecting Hot Dust and Noxious Fumes with Dracco
Glass-Cloth Dust Collectors. Bulletin No. DCB-1B (1967).
7. The Ducon Company, Dueon Dust Control. Equipment. Bulletin No.
F-9069, (1969).
8. Flannel Bags Key to New Dust Filter, Engg. and Mining Jnl. 166:12,
98 (December 1965).
9. Chem. and Engg. News. 57 (15 December 1969).
10. Liu, H., and Chaffee, R., Evaluation of Fabric Filter as Chemical
Contractor for Control of S02 from Flue Gas, Final Report to NAPCA,
Contract PH22-68-51, by Air "Preheater Co., Wellsville, N. Y.
(31 December 1969).
-------
11. Hot Kiln Gases in Glass Cloth Bags, Pit and Quarry. 80,
(October 1958).
12. Morash, N., Krouse, M., and Vosscller, W. P., Removing Solid and
Mist Particles from Exhaust Gases, Chem. Engg. Progress. 63.:3, 70
(March 1967).
-------
CHAPTER 4
FABRIC SELECTION
4.1 INTRODUCTION 4-3
4.2 MATERIALS 4-3
4.2.1 Natural Fibers 4-4
4.2.2 Man-Made Fibers 4-7
4.2.2.1 Acrylic Fibers 4-7
4.2.2.2 Modacrylic Fibers 4-9
4.2.2.3 Nylon, Polyamide 4-10
4.2.2.4 NomexR Nylon 4-12
4.2.2.5 Olefin 4-12
4.2.2.6 Polyester 4-15
4.2.2.7 Teflon Fluorocarbon 4-18
4.2.2.8 Vinyon 4-20
4.2.2.9 Glass 4-20
4.2.2.10 Other Man-Made Fibers 4-22
4.2.3 Physical and Chemical Properties 4-22
4.3 YARN PRODUCTION 4-28
4.4 YARN PROPERTIES 4-29
4.4.1 Threads and Yarns 4-31
4.4.2 Yarn Twist 4-31
4.4.3 Yarn Number 4-32
4.4.4 Simple and Plied Yarns 4-34
4.5 FABRIC PRODUCTION 4-35
4.5.1 Weaving 4-35
4.5.2 Felting and Needle Punching 4-41
4.5.3 Fiber Additives, Yarn Treatments, and Fabric 4-42
Finishes
4.6 FABRIC PHYSICAL CHARACTERISTICS 4-46
4.6.1 Abrasion Wear 4-46
4.6.1.1 Sandblast or Surface Scour 4-48
-------
4.6.1.2 Internal Chafing 4-50
4.6.1.3 Internal Abrasion 4-52
4.6.2 Flexibility 4.52
4.6.3 Strength 4-55
4.6.4 Permeability 4-58
4.6.5 Dust Deposit Release 4-59
4.7 AVAILABLE FABRICS 4-59
4.8 REFERENCES 4-60
-------
CHAPTER 4
FABRIC SELECTION
4.1 INTRODUCTION
Satisfactory performance of a fabric filter on a specific applica-
tion requires selection of a fiber material compatible with the gas-
particle environment, and a fabric design appropriate to dust collector
geometry and collector cleaning requirements. Fiber, yarn, and fabric
parameters influence the ability of the fabric to collect dust at a, reas-
onable pressure drop. !A brief summary of developments in fiber, fabric,
treatments, and applications has been presented in Chapter 1. Man-made
fibers have been developed over the past 30 years with superior resistance
to physical and chemical environmental requirements. While traditional
fabrics of cotton and wool are still employed for many fiber applications,
the impact of man-made fiber fabrics has extended the range of application
of fabric filters to a much broader gas cleaning market. Continuing de-
velopments in fiber and fabric technology, as in dust collector design,
can be anticipated.
The purpose of this chapter is to present information on fiber and
fabric properties important to their application for industrial gas
filtration. In addition, pressure drop during fluid flow through fabrics
and dust deposits has been considered in Chapter 2; effects of fabric
structure on filter performance has been discussed in Chapter 6; and fabric
costs in dust collection have been considered in Chapter 7. In what fol-
lows, a number of terms unique to the textile industry are used, and these
are defined in Appendix 4.1.
4.2 MATERIALS
Production of textile fabrics begins with the basic structural unic,
a single fiber. Fibers are obtained from traditional natural vegetable
or animal sources, such as cotton (cellulose) or wool (protein). Fibers
are also produced from man-made or modified natural organic polymeric
-------
materials through modern chemistry and chemical engineering. Industrial
filtration fibers are indicated in Table 4.1 by generic group manufacturer,
2 3
and Trade names. A more complete index has been compiled by Dembeck.
The internal molecular orientation and crystallite structure of the fiber
determines its basic physical and chemical properties. End use fiber
characteristics are substantially controlled by additives and treatments.
Within a generic group, polymers, co-polymers, and homo-polymers may be
produced. For all these reasons, there are differences in physical prop-
erties and chemical resistance for each manufacturer's product in each
2
group.
Fibers are produced as staple (relatively short lengths (1 < length <
10 inches long), as monofilament (single long continuous filament L » 10 in.)
and as multifilament (many continuous parallel filaments). Natural fibers
are used for industrial filtration staple. Man-made filtration fiber is
used in both staple and filament lengths. Cross sections of typical nat-
ural and man-made fibers are shown in Figure 4.1. Fiber microscopy such
as this is a useful technique in textile identification and filtration re-
search, as discussed in various references. '
4.2.1 Natural Fibers
Cotton and wool fibers are commonly used in fabric filter dust
collectors. Cotton fibers are elongated single hollow cells, often
flattened (Figure 4.1), with characteristic irregular lengthwise twists.
Cotton fibers from different varieties of plants vary in length from 1/2
to 2 inches, and in width from 12 to 25um.
t
Sheep wool also varies with specie and with geographical loca-
tion. The surface of a wool fiber is made up of flat scales (cuticle)
which overlap like shingles. These scales are spaced approximately 20 ^m
apart along the fiber. They are very important in determining the mechanical
properties of wool fiber assemblages, and probably influence dust collec-
tion and release properties as well. Wool fibers are 10 to 70 urn in
diameter, and 1 1/2 to 15 inches in length. They are almost uniformly
cylindrical except for the ends which are tapered.
-------
TABLE 4-1
MANUFACTURERS AND TRADE NAMES OF INDUSTRIAL FILTER FIBERS**
ACETATE AND TRIACETATE
Calanesa.-"Acetate". "Arn.l"*
DuPont--"Acele"*
EaetsMn Chemlcal--"Estron"*
r.M.C Corp., American Vlscoss Dlvlslon--"Avlsco"*
ACRYLIC
American Cyanamld--"Creslan"*
Chsmstrand--"AcrHen"*
Dow Cheulcal--"Zafran"*
DuPont--"Orlon"*
HOOACRYUC
KastlMn Chenical--"Verel"*
Union Carbide--"Dynel"*
POLYAHIDBS
Allied Cheolcal--"CaproUn 6"*
American Enka--Enka nylon 6
Cheast rind--Chens t rand 6.6
DuPont--nylon 6.6 "Nomex"* nylon
Nylon Induetrles--Celanese Nylon
Firestone Synthetic Flb*r<--Flr«tton« nylon 6
U.S. Rubb*r--nylon 6.6
V«etr«--nylon 6.6 nylon 6
POLTOTMS
AMrlcaii Knka Corporation- -EnV« Polyeiter
B««untt Plb«r«--"Vycron 2.5"*
DuPont--"D»cron"*
EMMn Chalc*l--"Kodel 2.4"*
Flb«r bidu«crl« (Cel«nen)--"Portrel"*
Ktotlon--"Cry*t*l Milt"*
Vectr«--Poly««ter
POLYBTHYLBNE
Alun Folywr Corpor«tlon--"R«evon"»
American Th«m>pl«itlc PToduct«""A««rfll"*
Dawbarn--"DLP"*
Flraiton* Synthetic Ptb«n--Plre«ton« Polyethylene
G.P. Chealcal>--"Gerfll"*
Induatrlal Pla»tlc--"Tuff-Llta-L"*
Vectra--"Wynene"*
Vogt Manufacturlng--"Vopl«ii"*
POLYPROPYLENE
Alamo Polymer Corporation--"Reevon"*
American TharoopUitlc Producte--"ABerfll"*
Dawbarn--"DLP"*
Flraatone Synthetic Flbare--Plreatone Polypropylene
Herculee Powdero'o"Herculon11*
Industrial Ple»tlc--"TuMLlca-P"*
Vectra --Polypropylene
Voft Menutecturlng--"Voplei"*
RAYOU VISCOSE ,
Aaarlcan Enka--"3up.r«nka"*, Fiber 700"
Beaualt Fibers--Beavmlt Rayon
Celaaa«e--Celaneee Rayon, "Portlian"*
Courtauldj North Anerlcen--"Plbro"*
F.M.C. Corporation, Anarlcan Viscose Dlv.--"Avlsco"*,
"R«ytl«x"*, "Super Raytlex"*, Fiber 40"*, "XL"*
Industrial Rayon--"Lekroeet"*, "Tyron"*, "VlUwyte"*
SARAM
Dov CheMlcal--"Rovana"*
Firestone Synthetic Fibers--"Valon"*
Vectre--Saren
TBTLOW
DuPont--"Teflon"*
vniYOH
F.N.C. Corporation, American Viscose Division--
"Avlsco Vlnyon HH"*
Rhodl* Aceta (France)—"Rhovyl"*
Vogt Manufacturln(--Voplax Vlnyon
CLASS
FaeBO--Fa»co Fiber
Johns ManvlUe--nber Glass
Owene-Cornlng-Flbercles Corp.--"Fiberflas"*
Plttabursb Plata Claes--Flber Class
CerssElc, Aabeatoa, Metal i
COTKM AMD WOOL
Available as natural fibers
* Registered Trade Mark
** from J.P. Stevens, Inc., Ref. 2
-------
(a) Cotton (X280)
(A) Acrylic
(AcrilanR) (X410)
(b) Wool (X410)
(e) Modacrylic
(DynelR) (X410)
(c) Polyacrylonitrile
(Orion*) (X380)
(f) Polyamide (Nylon)
(X380)
(g) Polyester (hj Vinyl (Vinyon) (1) Giant (X150)
(DacronR) (X380) (X380)
Figure 4.1. Cross Sections of Filtration Fibers, (From Harris, Ref.4 ,)
L.f,
-------
4.2.2 Man-Made I-'ihers
"Certain (man-made) fibers offer outstanding resistance to
acids, others to alkalies. Most are impervious to mildew. Some....
have high temperature resistance so that....operations such as cooling
of exhaust gases can be reduced or eliminated. Because of their inherent
toughness and resistance to abrasion and other environmental conditions,
many....offer substantial overall savings due to increased service life
and reduced equipment down-time. The initial cost of fabrics made from
(man-made) fibers may be greater than fabrics made from natural fibers.
However, the increased service will make them the most economical in the
long run A careful choice of filter or dust collection fiber for each
2
application can pay dividends in efficiency and economy."
While the chemical processes used to manufacture the various
polymeric materials used as filtration fibers may differ, ' ' the fibers
348
themselves are made by somewhat similar processes. ' ' The liquid material
is spun (wet, dry, or extrusion) to mono- or multifilament, stretched to
orient the constituent molecules, then processed into yarn strands; and
by spinning, texturizing or through-tow and staple, to yarn.
9 10
4.2.2.1 Acrylic Fibers.- An acrylic fiber is designated '
as "A manufactured fiber in which the fiber-forming substance is any
long chain polymer composed of at least 857<> acrylonitrile units ",
CH
(-CHo- V -) , the remainder frequently being a copolymer such as vinyli-
L CN n
dene chloride to provide dye sites. Process steps in the production of
acrylic fibers are discussed in References 4 and 7. Among other acrylics
commonly used in fabric filtration are Orion and Acrilanv Both are some-
what dumbell-shaped in cross section (Figure 4.1) and their surfaces are
irregularly striated to various degrees. Acrylic fiber diameters are
about 15 to 35 (am.
Characteristics, properties, and available producer's forma
are given in Table 4.2. The acrylics offer a good combination of
"....abrasion resistance and resistance to heat degrada-
tion under both wet and dry conditions. An outstanding
-------
TABLE 4-2
CHARACTERISTICS, PROPERTIES, AND FORMS OF ACRYLIC FIBER FOR
INDUSTRIAL FILTRATION*
HEAT RESISTANCE:
Dry Heat; Inferior to "Teflon" and
Nomex" nylon, which are outstanding
in this respect. Inferior to poly-
esters, but superior to nylon and
natural fibers.
Moist Heat; Rated below "Teflon"
and "Nomex" nylon, but considerably
superior to polyesters, nylon, rayon
and natural fibers.
CHEMICAL RESISTANCE:
Acids: Satisfactory resistance to
most mineral or organic acids.
Superior to polyamides, polyesters
and cellulosic fibers but less resistant
than other synthetics.
Alkalies; Inferior to most fibers
in this respect, except natural
protein fibers, silk and wool.
Oxidizing agents; Fair to good
resistance to most oxidizing
agents. Superior to polyamides,
polyethylene and natural protein
fibers.
Organic Solvents; Excellent
resistance to most common organic
solvents. Superior to modacryl-
ics polyethylene, polypropylene
vinyon and protein fibers. .
PHYSICAL PROPERTIES:
Tenacity and Elongation:
Filament; Dry and wet tenacity
ranges from 3.8 to to 4.8 g.p.d.
Dry and wet elongation ranges
from 13 to 23.
Staple; Dry and wet tenacity
ranges from 1.8 to 3.5 g.p.d.
Dry and wet elongation ranges
from 20 to 557,.
*from J. P. Stevens & Co., Inc. (Ref 2)
Specific Gravity; 1.12 to i.18
Abrasion Resistance; Good, but
less resistant than polyamides,
polyesters, and polypropylene.
Superior to "Teflon."
FORMS AVAILABLE:
DuPONT "ORLON"
Staple; 1.5 to 16 denier/filament.
Cut lengths 3/4" to 5%". (Can be
converted to spun yarns of many
sizes).
Tow; 2 to 10 denier/filament.
All of the above are available in
bright, semi-dull and black.
CHEMSTRAND "ACRILAN"
Staple; 1 to 15 denier/filament.
(Can be converted to spun yarns
of many sizes).
Tow; 2 to 8 denier. Available in
high shrink form.
AMERICAN CYANAMID "CRESLAN"
Multifilament; Deniers of 75,150
and 200. Semi-dull only. Special
industrial 166 denier—107. shrinkage
in boiling water.
Staple; 1.5 to 15 denier/filament.
Bright and Semi-dull. Regular and
high shrink. (Can be converted to
spun yarns of many sizes). :
FARBENFABRIKEN BAYER AG. "DRALONG T"
Multifilament; 200-800 denier, bright.
Staple; 2-10 denier, white, dull and
doped-dyed light green for identifica-
tion purposes. (Can be converted to
spun yarns of many sizes).
RHODIACETA "CRYLOR"
Multifilament; Denier of 100 to 225.
Staple; Net yet .available in U.S. Market
DOW CHEMICAL "ZEFRAN"
Staple; 3 and 6 denier per filament
cut 2 inches (Can be converted to spun
yarns of many sizes).
-------
characteristic is the ability of acrylics to withstand
a hot acid atmosphere, making this fiber a good choice
in the filtration of exhaust gases. Acrylic fabrics are
used for dust collection in the manufacture of ferrous
and non-ferrous metals, carbon black, cement, lime and
fertilizers.
"Other dry filtration or dust collection applications
include the drying of raw flour, sand and coal, mining
and ore dressing. Wet filtration applications include
the manufacture of dyestuffs, paint, varnish, solvents,
storage batteries and mineral oil; also galvanizing
and copper ipining. Acrylic filters function well in
the presence of steam, as in heat exchangers.
". . . . .Acryl.lc fibers made especially for industrial
use, such as "Dralon T" and "Crylor," are available
as homo-polymers, composed of 100% acrylonitrile
units. The homo-polymers offer a good hydrolytic
resistance, and are recommended for temperatures of
up to 284°F, while copolymers are recommended for
temperatures of up to 248°F. Dow Chemical Company's
new acrylic fiber is a homo-polymer and will operate
in the higher temperature range recommended for homo-
polymer acrylic fibers. Wet performance of Dow's new
acrylic is maintained to be superior to other acrylic
fibers."2
9
4.2.2.2 Modacrylic Fibers.- Modacrylic fiber is designated
as "...a manufactured fiber in which the fiber-forming substance is any long
chain. .. polymer composed of less than 85%, but at least 357» by weight of
P
acrylonytrile units..." For example, Dynel is a 40-60 acrylonitrile-
vinyl chloride mixture. The cross sections of Dynel fibers resemble dumb-
4
bells, and their surfaces appear uneven.
Although few gas filtration applications use modacrylics, the
one most used for filtration and dust collection is manufactured by
Union Carbide Chemicals under the trade name 'Dynel'. 'Verel' (Eastman
Chemical Products) is also an important available modacrylic fiber.
(Other producers are indicated in Ref. 3.) "Modacrylics have good chemi-
cal and abrasion resistance generally, offer excellent dimensional
stability and are unaffected by many acids and alkalies, even at high
concentrations. Water has no adverse effect on the fiber which retains
-------
oyer 95% of its strength when wet. Dry and moist heat degrade modacrylic
mere than the other fibers discussed in the brochure, with the exception
2
of vinyon." Characteristics and properties of modacrylic fiber are given
in Table 4.3.
9
4.2.2.3 Nylon, Polyamide.- Nylon fiber is designated as....
"a manufactured fiber in which the fiber-forming substance is any long
chain...polymer having recurring amide groups ...(-(J-NH-)...as an integral
part of the polymer chai.n." Table 4.4 indicates the basic polymer in the
several types of nylon currently available. Nylon fibers are cylindrical,
as shown earlier in Figure 4.1, with smooth surfaces devoid of markings.
The fibers are uniform in diameter (10 ^m and upward) and appear round in
cross section. Continuous filaments and staple fibers (1 to 5 in. long)
are produced.
"Three types of nylon are available for filtration:
Nylon 6,6 (DuPont, Celanese and Chemstrand), Nylon 6
(Allied Chemical, Enka and Firestone) and 'Nomex1
nylon (DuPont).
" 'Nomex' nylon differs considerably from the other
types, and therefore is treated separately.
"Nylon 6,6 and nylon 6 have similar characteristics,
except that the latter has a lower melting point and
somewhat poorer heat resistance at extreme tempera-
tures. Possibly for this reason, and because nylon
6,6 was introduced in this country before nylon 6,
the former is used more widely in filtration. Where
elevated temperature is not a factor, however, it
would seem that the two types would give comparable
service.
"Because of nylon's high abrasion resistance, it is
used in filtration of abrasive dusts or wet abrasive
solids at low temperatures. Nylon's good elasticity
makes it ideal for conditions where continuous flex-
ing takes place. This fiber is a good choice in wet
filtration applications, at low temperatures and in
an alkaline atmosphere, for example in dyestuffs.
Fabrics of nylon provide good cake discharge."2
The general characteristics and properties of nylon fibers in use for
filtration are indicated in Table 4.5.
-------
TABLE 4.3
CHARACTERISTICS, PROPERTIES AND FORM OF MODACRYLIC
FIBER FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:
Dry Heat; Shrinkage starts at 250 F
but can be heat stabilized at higher
temperatures below the stiffening
state. For prolonged use, 180°F
should be considered maximum.
Surpasses acetate, polyethylene,
saran and vinyon in heat resistance,
but inferior to other fibers. This
fiber will not support combustion.
Moist Heat; As with most fibers,
moist heat has more effect on the
fiber than dry heat.
CHEMICAL RESISTANCE:
Acids; Little effect even at high
concentrations for most mineral and
organic acids, including aqua regia,
chromic acid, nitric acid, phosphoric
acid and sulfonic acid. Can be used
in the presence of moderate concen-
trations of nitric and sulfuric acids.
Adversely affected by phenol and high
concentrations of acetic acid.
Alkalies; The fiber has good alkaline
resistance under most conditions.
Oxidizing Agents; Excellent resistance
to nearly all oxidizing agents.
Organic Solvents; Softens or
dissolves in warm acetone and
some other ketones, otherwise
would be considered good for
organic solvents. Not affected
by dry cleaning solvents or gas-
oline.
PHYSICAL PROPERTIES:
Tenacity and Elongation; Depending
on type 2.4 to 3.0 g.p.d. dry
and wet tenacity. Dry and wet
elongation ranges from 32 to 39%.
Specific Gravity; 1.30 to 1.37.
Abrasion Resistance; Good, but
inferior to polyamides, including
"Nomex" nylon, polyesters and
polypropylene. Superior to
"Teflon."
FORMS AVAILABLE:
Staple Only;
EASTMAN ("VEREL") 3-24 denier.
(Can be converted to spun yarns of
many sizes).
UNION CARBIDE ("DYNEL") 2-24 denier.
(Can be converted to spun yarns of
many sizes).
from J. P. Stevens & Co., Inc., Ref. 2
-------
TABLE 4.4
CHEMICAL COMPOSITION OF POLYAMIDES
Nylon-4 Pyrrolidone
5 Valerolacturn
6 Caprolactum
6T Hexamethylene terephthalamide (aromatic)
7 Heptanoamide
8 Caprylamide
9 Amino-nonanlc acid
11 Amino-undecanoic acid
12 Lauryl lactum from butadiene
66 Hexamethylene adipamide
68 Hexamethylene-diamine and suberic acid
610 Hexamethylene diamine and sebacic acid
MXD-6 Metaxylylene adipamide (aromatic)
j^
4.2.2.4 Nomex Nylon.- Notnex Nylon is a proprietary aromatic -
polyamide linked structure developed by DuPont for applications requiring
good dimensional stability and heat resistance. "Nomex can be used at
temperatures at which other fibers melt. Unlike glass, it is resistant
to fluorides and has good abrasion and flex resistance. It has a wide
range of filtration applications, including the cement industry, carbon
black, non-ferrous metals and steel."^ Characteristics, properties, and
producer forms are indicated in Table 4.6.
it
4.2.2.5 Olefin.- An olefin fiber is designated as a
manufactured fiber in which the -fiber-forming substance is any long chain.
polymer composed of at least 85% by weight of ethylene, propylene, or
9
other olefin units..." Polypropylene is one olefin used for industrial
gas cleaning applications.
"The production of polypropylene fiber varies among
the manufacturers.... to achieve certain properties,
such as dyeability, light stability, heat sensitivity,
and shape of the filament cross section. The pro-
-------
TABLE 4.5
CHARACTERISTICS, PROPERTIES, AND FORMS OF NYLON FIBER FOR FILTRATION*
HEAT RESISTANCE:
Dry Heat: Up to 250°F. nylon has
reasonably good dry heat resistance.
It is superior to acetate, modacrylic
and saran. However, the other
synthetics are superior to it.
Moist Heat; Nylon performs adequately
in moist heat at temperatures ranging
up to 225°F. However, its high initial
tensile strength and abrasion resistance
will make it a preferred choice in a
number applications.
CHEMICAL RESISTANCE:
Acids; Most mineral acids cause
degradation and partial decomposition.
Soluble in formic acid.
Alkalies; Good resistance to alkalies
under most conditions. In this respect
nylon is better than acrylics but
not as good as the olefins.
Oxidizing Agents; High concentrations
and temperatures may cause complete
degradation.
Organic Solvents: Withstands common
organic solvents very well. Some
phenolic compounds cause solubility.
PHYSICAL PROPERTIES:
Tenacity and Elongation: Both nylon
6 and nylon 6.6 are produced in a wide
range of strengths.
Filament; Dry and wet tenacity ranges
from 4.0 to 9.2 g.p.d. with dry and
wet elongation running from 16 to 42%.
Staple; Dry and wet tenacity ranges
from 3.5 to 7.2 g.p.d. with dry and
wet elongation running from 16 to 50%.
Specific Gravity; 1.14.
Abrasion Resistance; One of nylon's
outstanding characteristics is
abrasion resistance and in this
respect nylon is superior to all
other fibers.
FORMS AVAILABLE:
NYLON 6.6:
Mpnpfilament; 7 to 30 denier
Multlfilament; deniers of 20,30,40,
50,60,20,80,90,100,140,200,260,400,
420,520,630,780,800,840,1050,1260,
1680,
(NOTE: The underlined deniers are
those that are used most often).
Staple; 1.5 to 18 denier/filament.
(Can be converted to spun yarns
of many sizes).
Tow; 3 to 18 denier/filament
Above forms available in bright,
semi-dull, dull and black.
NYLON 6:
Monofilament; 15 to 20 denier
Multifilament; deniers of: 30/40/
50/70/100/140/200/840/1050/1260/
2100/2400/2500/3150/3360/4200/
5000/7500/10,000/15,000
Staple; 2.0 to 15 denier/filament.
(Can be converted to spun yarns of
many sizes).
from J. P. Stevens & Co., Inc. Ref. 2
-------
TABLE 4.6
CHARACTERISTICS, PROPERTIES, AND FORMS OF NOMEX
FILTRATION*
NYLON FIBER FOR INDUSTRIAL
HEAT RESISTANCE:
Dry Heat; "Nomex" nylon does not melt,
but at temperatures above 700°F degrad-
ation sets in rapidly. In dry heat,
up to and including 450°F., this fiber
may be used satisfactorily, as long as
there is no acid dew point problem.
Moist Heat; Small amounts of water
vapor at elevated temperatures, and
in intimate contact with water or
saturated steam, "Nomex" nylon exhibits
a progressive loss in strength. How-
ever, it withstands these conditions
much better than Nylon 6.6 or many
other fibers.
CHEMICAL RESISTANCE:
Acids; Withstands both mineral and
organic acids much better than nylon
6.6 or nylon 6 but not as well as
polyesters and acrylics.
Alkalies; Excellent resistance to
alkalies at room temperature (better
than polyesters and acrylics)
but degraded by strong alkalies at
elevated temperatures.
Oxidizing Agents; Like nylon 6.6
or nylon 6, "Nomex" is degraded by
oxidizing agents.
Organic Solvents; Highly resistant
to most hydrocarbons and many
other organic solvents.
PHYSICAL PROPERTIES;
Tenacity and Elongation; Dry
tenacity: 5.5 g.p.d. Wet tenacity:
4.1 g.p.d. Dry elongation: 18%
Wet elongation: 15%.
Specific Gravity; 1.38
Abrasion Resistance; Superior to
acrylic fibers and about equal
to polyesters and nylon 6.6.
FORMS AVAILABLE:
Multifilament; Deniers of 100,200
and 1200.
Staple; 2 denier/filament (can
be converted to spun yarns of
many sizes).
Both filament yarn and staple
are available in natural (off-
while), international orange
and olive green.
* J. P. Stevens & Co., Inc., Ref. 2
-------
' duction of polypropylene fiber results in a
relatively economical and inexpensive fiber be-
cause the basic substance from which it is made
is propylene gas--a by-product of petroleum dis-
tillation. The most costly aspect is the manu-
facturer's initial investment in research and
plant establishment."8
"Polypropylene combines the virtues of lightness,
high strength and excellent resistance to most
acids and alkalies with the important added at-
traction of low cost. Having the lowest density
of any synthetic fiber in filtration and dust
collection, polypropylene offers the greatest
cloth,yield per pound of yarn and is one of the
most economical synthetics.
Polypropylene fibers are manufactured under such
trade names as "herculon" (Hercules), "Reevon"
(Alamo Polymer) and "Vectra" (National Plastic
Products).
The sleekness of the fiber allows for a fabric
providing good cake discharge and resistance to
blinding. Since its moisture absorption is vir-
tually nil, polypropylene is a good choice for an
application such as dye production, where pigment
changes can be made with only a light wash be-
tween batches.
"Polypropylene has been used successfully at
165°F in the greige, and in the heat set stage
at up to 250°F."2
Characteristics, properties, and producer forms are given in Table 4.7.
4.2.2.6 Polyester.- A polyester fiber is designated as
t
"...a manufactured fiber in which the fiber-forming
substance is any long chain...polymer composed of
at least 85?0 by weight of an ester of dihydric
alcohol and terephthalic acid (p - HOOC - C,H, -
COOH) "9 *
The general characteristics of this material, which is a commonly used
dust collection fabric particularly in the cement industry, are presented
D
in Table 4.8. The Dacron polyester fibers are round (see Figure 4.1),
but variation in shape of the spinnaret holes will affect the appearance
-------
TABLE 4.7
CHARACTERISTICS, PROPERTIES, AND FORMS OF POLYPROPYLENE FIBER
FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:
Dry Heat; Polypropylene has the
lowest heat resistance of all the
synthetics except the modacrylics,
and loses tenacity in direct propor-
tion to increases in temperature.
It should be remembered, however,
that the fiber's very high initial
tenacity will leave a generous margin
for many applications.
Moist Heat; Since polypropylene is
non-hygroscopic, its heat degradation
characteristics are essentially the
same under moist conditions as they
are under dry conditions.
CHEMICAL RESISTANCE:
Acids; Very good resistance to both
mineral and organic acids. Attacked
at high temperatures by nitric acid
and chlorasulfonic acid.
Alkalies; Generally good, except
poor resistance to sodium and
potassium hydroxide at high tem-
peratures (above 200op) at high
concentrations.
Reducing Agents; Good resistance to
most reducing agents.
Organic Solvents; Good resistance to
most organic solvents. Exceptions
are ketones, esters, aromatic and
aliphatic hydrocarbons at high
temperatures. Soluble at 160UF
in chlorinated hydrocarbons.
PHYSICAL PROPERTIES:
Tenacity and Elongation;
Filament; Dry and wet tenacity
4.8 to 8.5 g.p.d. with dry and
wet elongation 15 to 30%
Staple; Dry and wet tenacity
ranges from 4.5 to 6.0 g.p.d.
with dry and wet elongation 15
35%.
Specific Gravity; 0.90 to 0.91
are of the lowest specific gravities
of any fiber.
Abras ion: Re s istance; Excellent
abrasion resistance wet and dry.
FORMS AVAILABLE:
Monofilament; 6 to 12 mils round
and flat.
Multifilament; 165 to 4000 denier.
Staple; 1.5 to 15 denier. (Can
be converted to spun yarns of
many sizes)
Available in natural and solution
dyed colors. ,
from J. P. Stevens & Co. Inc., Ref. 2
-------
TABLE 4.8. CHARACTERISTICS, PROPERTIES AND FORMS OF POLYESTER
FIBERS FOR INDUSTRIAL FILTRATION*
HEAT RESISTENCE:
Dry Heat: Polyesters are not compar-
able to "Teflon" and "Nomex" nylon,
which have usually high heat degrad-
ation resistance, but they are
superior to most other synthetics
in this respect, though subject to
hydrolytic degradation under
certain circumstances. However,
polyesters are used for filtra-
tion and dust collection under
conditions of wet heat, particu-
larly where the initial yarn
strength exceeds minimum re-
requirements sufficiently to
allow for hydrolysis. Below
350 F there is little shrinkage
of polyester fiber.
CHEMICAL RESISTANCE:
Acids; Good resistance to most
mineral and organic acids except
high concentrations of nitric,
sulfuric and carbolic acids.
Alkalies; Good resistance to
weak alkalies and moderate resis-
tance to strong alkalies at low
temperatures. Strong alkalies
at high concentrations and temper-
atures dissolve polyesters.
Oxidizing Agents; Good resistance
to most oxidizing agents.
Organic Solvents: Excellent to
most organic solvents, but
unsuited for some phenolic
compounds and affected by
cyclohexanone at 313°F.
PHYSICAL PROPERTIES:
Tenacity and Elongation:
Filament; Dry and wet tenacity
ranges from 4.4 to 7.8 g.p.d.
Dry elongation from 10 to 2570.
Water has little effect on either
the strength or elongation of
polyester fibers.
Staple; Dry and wet tenacity ranges
from 2.2 to 5,5 g.p.d. Dry and
wet elongation from 18-50%.
Specific Gravity; 1.35 - 1.38.
Abrasion Resistance; Polyester
fibers have excellent abrasion
resistance ranking next to nylon
in this respect.
FORMS AVAILABLE:
Multifilatnent; Deniers of 30, 70,
100, 140, 150, 220, 250, 420,
440, 840, 880, 1000, 1100, 1680,
and multiples of 1100 up to 16,500.
Staple; 1.5, 2.25, 3.0, 4.5, 6.0
8.0, 15. (Can be converted to
spun yarns of many sizes).
Direct Spun Yarn; From 4s/I to
30s/l.
*From J. P. Stevens & Co., Inc., Ref. 2.
-------
and mechanical properties of the fiber, as will the extent to which the
fiber is drawn. The usual diameters are 10 to 30 urn. The following
quotation provides some additional information on this material.
"Polyester fibers are manufactured under such trade
names as "Dacron" (DuPont), "Fortrel" (Fiber Indus-
tries /Celanese) , "Vycron" (Beaunit), "Kodel" (East-
man Chemical Products) and Enka Polyester (American
Enka Corporation). They can be woven into filter
and dust collection fabrics affording very good
resistance to chemicals, abrasion and dry heat
degradation, plus excellent dimensional stability."^
P
4.2.2.7 Teflon Fluorocarbon.- The characteristics, prop-
erties and forms of these fibers are summarized in Table 4.9 and described
in the following quotations.
"Teflon - Dupont's trade name for their fluorocarbon
fibers - is available in two forms. The multi-fila-
ment yarns are made from the homo-polymer polytetra-
fluoroethylene (TFE). The monofilament yarns are
produced from the co-polymer of tetrafluoroethylene
and hexafluoropropylene (FEP). Both polymers are
composed of long-chain molecules in which all of the
available bonds are completely saturated by
fluorine. These carbon-to-fluorine bonds are ex-
tremely strong, resulting in fibers which are excep-
tionally stable to both heat and chemicals. Teflon
fiber has no known solvents except certain perfluor-
inated organic liquids at temperatures above 570°F.
The fiber is inert to concentrated mineral acid,
organic acid, alkalies, oxidizing agents and
organic solvents at elevated temperatures.
"The fiber remains flexible and non-brittle from
minus 100°F to plus 550°F. Teflon withstands
prolonged exposure at 450°-500°F without degra-
dation. Above this temperature some decomposi-
tion results but even at 500°-550°F decomposi-
tion is slow. The gaseous decomposition products
evolved at high temperatures are highly toxic
and must be removed from the work areas through
adequate ventilation.
"Teflon fibers have a very low coefficient of
friction resulting in excellent cake discharge.
This fact, coupled with its chemical inertness
and resistance to dry and moist heat degrada-
-------
TABLE 4.9. CHARACTERISTICS, PROPERTIES AND FORMS OF TEFLONR
FIBER FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:
"Teflon" TFE can be used
o.
Dry Heat;
in continuous service up to 500"F.
"Teflon" FEP can be used in continuous
service up to 450 F.
Moist Heat; Due to zero moisture
absorption, these fibers withstand
moist heat temperatures about as
well as dry temperature.
CHEMICAL RESISTANCE:
Acids; Inert
Alkalies: Inert
Oxidizing Agents; Inert
Organic Solvents; Inert
The only substances known to react
with these fibers are alkali metals
fluorine gas at high pressure and
temperature, and chlorine trifluoride.
This is the most chemically resistant
fiber produced.
PHYSICAL PROPERTIES:
Tenacity and Elongation:
Filament and Staple; Dry and wet
tenacity for type TFE is 1.6 g.p.d
and for type EEP is 0.5 g.p.d.
Dry and wet elongation is 15%.
Specific Gravity; 2.1
Abrasion Resistance; Teflon
fibers are inferior in abrasion
resistance to the acrylic, nylons
and polyester fibers, but are
superior to glass fibers.
FORMS AVAILABLE:
"TEFLON" TFE
Filament Natural Brown
100/200/400/1200 denier.
2400 up to 26,400 denier plied
yarns
Filament Bleached White
225/450/1350 denier.
2700 up to 29,700 denier plied yarns
Staple Natural Brown
6.67 denier/filament
Staple Bleached White
7.50 denier/filament
Staple available in cut lengths
of 0.5" up to 5.0" in 0.5" incre-
ments (Can be converted to spun
yarns of many sizes).
"TEFLON" FEP
Monofilaments (clear, colorless)
3/5/8/11/16/20/32/50/60 mils.
from J. P. Stevens & Co., Inc., Ref. 2
-------
tion, makes Teflon suitable for filtration and
dust collection under severe conditions. Its
major weakness is abrasion resistance in which
Teflon is inferior to all other synthetic
fibers except glass. The high price of Teflon
fiber limits its use in the filtration field.
However, for uses under extreme temperature
conditions, Teflon may prove to be most
economical in the long run. It would further
be expected that as the demand for Teflon in-
creases and production expands, the price of
Teflon will decline."2
9
4.2.2.8 Vinyon.- Vinyon fiber is designated as ..."a manu-
factured fiber in which the fiber-forming substance is any long chain...
polymer composed of at least 8570 by weight of vinyl chloride units (-CH- -
CHC.0-)1.1 The characteristics, properties and producer forms of the fiber
are presented in Table 4.10. The following quotations summarize these
characteristics.
"Characteristic of vinyon fibers is the dumbbell
shape of their cross sections....Fibers with an
occasional twist are observed. Both continuous
filaments and stable fibers (1-5 in.) occur in
trade, the width of all types being 16 to 18 u."^
"Vinyons are made of 100% polyvinyl chloride by
the French Societe Rhovyl, and as a copolymer
of polyvinyl chloride and polyvinyl acetate by
the FMC Corp., American Viscose Division.
Vinyons offer very good resistance to most
chemicals, even in highly concentrated form.
They can be made into fabrics providing extremely
smooth cake discharge, and are used for filtra-
tion of air and (numerous liquids)."2
:
4.2.2.9 Glass.- Glass fiber is a product of fusion, a non-<
crystalline silicate analogous to other fiber polymeric materials, (-SiO,-)
Selected silica sands, limestone, soda ash, and borax or other ingredients
are melted at about 2500 F and the mixture is extruded through spinnarets.
The resulting filaments may be drawn while still molten and later twisted
and plied into filament yarn. Or, the extruded glass may be drawn and
broken by jets of compressed air into staple of lengths 8 to 15 inches.
-------
TABLE 4.10. CHARACTERISTICS, PROPERTIES, AND FORMS OF
VINYON FIBER FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:
Dry Heat; The maximum working
temperature for Rhovyl's type
"Clevyl T" is 350°F., provided
the fabric is clamped to prevent
shrinkage.
Moist Heat; Due to vinyon's low
water absorption the effects of
moist heat are similar to those of
dry heat.
CHEMICAL RESISTANCE:
Ac ids; Excellent resistance at
room temperature to mineral acids,
including hydrochloric, nitric,
and sulphuric acids, aqua regia
and organic acids with the exception
of carbolic acid.
Alkalies; Very good resistance to
alkalies such as potassium hydroxide,
sodium hydoxide and ammonium
hydroxide.
Organic Solvents; Dissolved by
ketones, and partially dissolved
or softened by esters and ethers.
Certain other organic solvents
cause swelling at certain temper-
atures. In general, this fiber should
not be considered for use with
most organic solvents other than
mineral oil, aliphatic hydro-
carbons, alcohols and glycols.
PHYSICAL PROPERTIES:
Tenacity; Filament and Staple:
"Clevyl T" (Dimensionally stable
to 212°F) Staple 1.7 to 2.0 g/d
"Type 55" (557, shrinkage at 212°F)
Filament 2.7 to 3.0 g/d.
Abras ion Res is tance; Not as resis-
tnat as polyesters or nylons, but
comparable to the acrylics.
Specific Gravity; 1.34-1.38
FORMS AVAILABLE:
RHOVYL STAPLE:
"Clevyl T" (Dimensionally Stable
to 212°F) 3.5/5/8/15 d/fil.
"Type 30" (30% shrinkage at 212°F)
3d/fil.
"Type 55" (55% shrinkage at 212°F)
1.8/3.0/6.4 d/fil.
RHOVYL FILAMENT:
"Type 55" only: 75/100/200/400
800/1600 denier.
FMC VINYON H.H. STAPLE ONLY
Deniers of 1.5 to 5.5 staple
lengths of 1/2 inch to 2 inches
but not in all deniers. (Can
be converted to spun yarns of
many sizes)
From J. P. Stevens & Co., Inc.,Ref. 2
-------
The fibers are then treated with a lubricant which is of great importance
in the durability of the eventual fabric. Following drying, the fibers
are processed much like the more conventional fibers.
Glass fiber photomicrographs are shown in Figure 4.1. The
fibers are perfectly round with very smooth and structureless surfaces.
Diameters range from 5 to 16 urn for most textile fibers. Characteristics
and forms of glass fibers are given in Table 4.11.
4.2.2.10 Other Man-Made Fibers.- Other generic organic fiber
materials that could be used for industrial gas filtration include acetate
and triacetate (cellulose acetate), rayon (regenerated cellulose), manu-
factured rubber, saran (vinylidene chloride), and vinal (vinyl alcohol).
They appear to offer no significant advantages for industrial gas filtra-
tion in most instances, although saran has some application in spray
tower mist elimination. (Other unclassified fibers are summarized in
Ref. 3, pp. 218-221).
Several fiber materials and finishes are currently available
in pilot plant quantities having potential for high temperature applica-
tions or control of electrostatic effects in fabrics. Some metals and
3
ceramics having potential filtration uses are summarized in Appendix 4.2.
R R
Fiberfrax , and Brunsmet , have been laboratory tested and found satis-
factory for filtration at high temperatures ( > 1000 F), but require further
development in filtration systems.
4.2.3 Physical and Chemical Properties
Relative physical and chemical properties of filter fiber
materials are summarized in Tables 4.12, 4.13 and 4.14;. Table 4.13
shows general quantitative physical properties and a qualitative estimate
2
of chemical, heat, and abrasion resistance. Where both high and low
tenacity fibers are produced, the range of strength and breaking elonga-
tion is given. Since many fibers are produced in both filament and staple
fibers, the general physical properties are shown separately for each
form. Fiber resistance to specific chemical compounds is indicated in
-------
TABLE 4.11
CHARACTERISTICS, PROPERTIES, AND FORMS OF GLASS FIBERS
FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE
CHEMICAL RESISTANCE
PHYSICAL PROPERTIES
FORMS AVAILABLE
Fabrics made from glass yarns actually gain in
strength as the temperature rises from room
temperature to 400°F. From that point, strength
and flexibility decrease.
The recommended operating temperature is 500 F
with surge limits up to 600°F
Glass is resistant to acids of ndrmal strength
and under ordinary conditions. It is attacked
by hydrofluoric, concentrated sulphuric, and hot
phosphoric acids. Overall resistance to acids
is slightly above average.
Hot solutions of weak alkalies will also attack
glass. Overall resistance to alkalies is poor.
Operating a glass baghouse at or below the dew
point can be particularly damaging if acid
anhydrides or metallic oxides are entrained in
the gas stream. Fluorides and the oxides of sul-
fur are particularly damaging to glass.
Glass is considered to be incombustible because
it is completely inorganic. In addition, it
has a low coefficient of linear expansion and
hence is dimensionally stable. Although glass
has an extremely high tensile strength it has
poor flex-abrasion resistance.
Various chemical treatments to the glass fabric
improve the flex-abrasion characteristics of
glass bags.
Filament
Filament and Textured
Filament and Spun
From Albany Felt Co., Ref. 11.
-------
TABLE 4.12. RELATIVE PROPERTIES OF MAN-MADE FIBERS*
Resistance to
Acids"
"Teflon"
Polypropylene
Vinyon
Modacrylics
Acrylics
Polyesters
"Nomex" nylon
Nylon 6.6 & 6
Rayon
Resistance to
Alkalies
"Teflon"
Polypropylene
Vinyon
Modacrylics
Nylon 6.6 & 6
"Nomex" nylon
Polyesters
Rayon
Acrylics
Resistance
to Oxidizing and
Reducing Agents
"Teflon"
Polypropylene
"Nomex" nylon
Modacrylics
Polyesters
Acrylics
Nylon 6.6 & 6
Vinyon
Rayon
tensile
Strength
Nylon 6.6 & 6
Polyesters
Polypropylene
"Nomex" nylon
Rayon
Acrylics & Modacrylics
Vinyon
"Teflon"
Resistance to
Abrasion__(Wet_& DryJ
Nylon 6.6 & 6
Polypropylene/Polyesters
"Nomex" nylon
Acrylics & Modacrylics
"Teflon"
Rayon
Vinyon
J-9.
Dry Heat
"Teflon"
"Nomex" nylon
Polyesters
Acrylics /Rayon
Nylon 6,6
Nylon 6
Polypropylene
Modacrylics
Vinyon
Resistance^ to
Moist Heat
"Teflon""
"Nomex" nylon
Acrylics
Nylon 6.6 & 6
Rayon
Polyesters
Polypropylene
Modacrylics
Vinyon
Max. Recommended
For Continuous
"Teflon"
"Nomex" nylon
Polyesters
Acrylics
(homopolymers)
Acrylics
Rayon
Nylon 6; 6
Nylon 6
Polypropylene
Modacrylics
Price Relationshi
Service
,o
5001-
450°*
300°*
284°
275°
275°
250°*
250°*
225°
180°
(Highest to Lowest)
STAPLE FILAMENT
"Teflon"
"Nomex" nylon
Glass
Nylon 6.6 & 6
Polyesters
Acrylics
Modacrylics
Vinyon (Rhovyl)
Polypropylene
Viscose Rayon
"Teflon"
"Nomex" nylon
Acrylics
Modacrylics
Polyesters
Nylon 6.6 & 6
Glass
Vinyon (Rhovyl)
Polypropylene
Viscose Rayon
* These fibers are subject to hydrolysis when exposed in hot, wet atmospheres
in varying degrees. Polyesters degrade to the greatest extent. "Nomex"
nylon next, then nylon 6, and nylon 6.6 least.
The following is quoted from DuPont's NP-33, "Properties of 'Nomex1.11
"At elevated temperatures, "Nomex" fiber in intimate contact with water
or saturated steam exhibits a progressive loss in strength with water
vapor. (Nylon 6,6 completely deteriorates in less than 100 hours under
the same conditions)"
** Based on prices per pound at time of publication (1965 est) - See Chapter 7.
From J. P., Stevens and Co., Inc., Ref. 2.
-------
I o
TABLE 4.13
SUMMARY OF PHYSICAL AND CHEMICAL PROPERTIES OF INDUSTRIAL FILTER FIBERS
-O
I
///***// //
* / / / / / / / / /
FILAMENT i
Breaking Tenacity gpd (Dry, \\ '« ' " " « ^ "^i
B,eak,ng Tenacity gpd, We,, °] 3| 38 40 J° 79 * ' ^ 7 J
Breakmg E.onoa.ion « s , Dry, » « » 1< |3 « 18 ; J° ;
Breaking Elongalic. • , , We,, » « 23 '« Jf » '5 £ \
STAPLE
Breaking Tenacity gpd, Dry, {* *° ». 3° » » 38 ":»j
Breaking Tenacity gpd, We,, ?8 J« J« » J* *J ^ 4'° ! *| !
Breaking E,onga,,o,.^Dry, » » « » ; » | » « " ' «
Breaking Elongation -., We,, S S S » M » « '"'.£'
Specific Gravity 1 33 1 18 1.18 1 12 1 15 i 1.14 . 1.30 1 14 ; 1 38 • 1.38 '
Mai Recommended Operating f ; 175 275 275 275 284 .. 275 : 180 250 ! 450 300 i
i i i '• ' • i
Resistance to Abrasion G : G G G G G G E E ( E ;
Resistance to Dry Heat FjG.G G'GjGJFG :E!G!
:••?•• ' ' " : -H -..».-.. , , ,-
Resistance to Moist Heal F ; G G G E-GiF G E'Fl
Resistance to Mineral Acids • p '• G G G'G^GG'P F G
Resistance to Organic Acids • P ' a G G G : G G F E G!
Resistance to Alkalies P|F F F.FiFiGG GiGi
Resistance to Omdizing Agents FG;GGG:GG F |Q Q|
Resistance to Solvents !F-E EE'EiE'GE E;Ei
f * J?
///////
S / / f f f 3
: i
44 4.6 ; 1 0 4.8 ! 1.5 ' '
78 ! 54 70 . 8.5 , 57 23
44 46 1.0 4.8 : 07 : •
7.8 • 5.4 70 8.5 36 23
10 19 . 10 15 9 15
25 25 80-30-30 30
19 19 20 15 l» "5
25 - 25 80 30 40 25
22 ' 25 53 45 15 i 0
40;55'S4 6.0 46 15
2.2-25 53 4507 10
40 5 5 5.4 6.0 3.6 , 1 5
18 24 . 32 15 9 15
50 : 45 40 35 , 30 25
18 : 24 32 15 14 15
50 45 40 ' 35 40 25
1.38 i 138 : 1-38 ' .92 . 90 ; 1.52 170
• • r- | :
300 I 350 j 300 ; 150 225 | 275 ; 1.60
• 1 ' ' !
E'EJE,G;E.,GG
GiGJG'FlG.GF
F 1 F 1 F F F • G i F
GjGJG-G'E'.PiG
1 j r- -t ! !
GJG:G,G EiGjG
G G G:GjE|FjG
1 ! " 1 :
G G | G . P ; G ; F F
E:EiE;GJ'GJGjG
/ f f f
/ / oj /
; i i
• 6 C? 60
30 73 j
• 6 07 39 j
30 47 1 i
•5'23 ;
•c.
•i '2 25 j
•C J
t
16 07' i 1
30 '
• 6 37 1
30 i ;
15 40 I
60 ^
15 40
60 i 1
210 1 34 254 . 1
500 350 ; 550 , 200
I | j—
F , G ' P | F .
'ill
E : P i E | F L
E P ' E : F ,
E E \ E j P
E . a: E| e|
E G P ! G |
« F E ! F j
E| F E j E i
// /
i i
i
: 3B i
i : ;
i -
i i
i 1T
,
i- -i- '
30 '• ' 10 !
19 { M7 i
J3 ! OB j
i.| ,6,
3 ! ! 25 1
7 i j 35 ;
3 i i 25 ;
7 J 50 i
50 ; 1 25 i 1 32 !
J25 ; 175 j 200 !
,, » *
! i
. i J
G ; G j G
G F i F !
!
G F 1 F j
P ! F ' F
-4— j
GIF F
G ! P P
F i P P
j
EG F
-------
Table 4.14. (Additional detailed engineering test data on fiber proper-
2 12
ties is presented in Appendix 4.3. ').
"It is possible that a fiber indicated as being
fair within a general chemical class (in Table
4.13) may be rated as good for a particular chem-
ical within the general grouping. Each fiber has
its own strong and weak points and must be evalu-
ated on its own merits."2
The evaluation of the resistance of fibers to chemical reagents pre-
sented in Table 4.14 is based on liquid filtration requirements. However,
the data presented in the table have direct applicability to dust filtra-
tion as the time, temperature, chemical concentration, type of polymer,
treatment of fabric, etc., all have their effect on final selection of a
filter or dust collection medium. For these reasons, tests should be made
under operating conditions, as a composite table cannot cover all possible
conditions to which a filter or dust collection fabric may be subjected.
The maximum temperature, degrees Fahrenheit ( F), as enumerated in
Table 4.14 is the maximum temperature that should be employed normally
for dust collection or liquid filtration of dilute solutions. A fiber
marked R (recommended) can be expected to usually withstand most conditions
reasonably well. S (satisfactory) fibers should be considered only for
low concentrations and moderate temperatures. N (not recommended) fibers
can be expected to give poor results under any conditions.
"Chemical composition of synthetic fibers greatly
affect their chemical resistance. This is true
within a generic family as well as from one type
of fiber to another. Chemical resistance of a
co-polymer fiber is usually lower than that of a
pure homo-polymer base fiber. The co-mono-mers
used to form "links" are particularly sensitive
to external effects....
Using acrylic fibers as an example, in acidic or
oxidizing solutions, in dry or humid air be-
tween 275°-350°F, the difference in behavior be-
tween homo- and co-polymers becomes very important.
The homo-polymer is superior. The influence of
composition of the basic acrylic polymer has
little effort when considering organic snlvpnts,
weak acids, most mineral salt solutions and
cleaning or bleaching agents. In alkaline media
up to intermediate concentrations and tempera-
-------
TABLE 4.14
RESISTANCE OF FIBERS TO CHEMICAL REAGENTS
BtaB. Twv. QapMi FaWriwn * n 3
laanjuu. *ooa
Aqua R«0i« N
Pho»pho«C JS j
Sulfunc IN i
Tmilf|rrT. agp, |
Acetic S
6«ruO»C N
Carbolic N
Formic N
Lactic S
Diane R
Salicylic N
Aoimoniitfn HydrovidB S
Catciwn HydronMM N
Potautum Hytfroiida N
Pobmnm Carbonate N
Sodnm Hydrouda N
MLTC
CateiuaiCMDrida 8
FavricCMorUa N
SodHMAcaMa N
SodMi BanzoaH R
SodwmBroaMa 8
SediMmCMonda R
SodwmCyankla S
SottwnlMraia S
SoawMSrffaii S
£ncCMorid» S
ajKMUjna 8
CMortoa S
FluoMW N
IT • i <
T8;JT5 7TJ
i *
R -R-nr
s s s
s s is
s s s
m?
sl
si
s
ni
j
5
11
S
•oaooiaaoitfo-xoaoD jiupw:r%it«o.uDii?ojsu!2aoi2is IISDOD
4-4—1- :.,..,::;']•; j
,isU|s:R . s s ..R:R!R;RI..«:S!«;
Ring's B'R'R'R-^ ~K~ R • 5 ; R | a ! R 1 R ' s '
,!•'. s ..M-.:,-. sVR'sls^J..:
n is i « ;P S:SiR,R s R R;R.N:R R N N
4 f . . j . . . \ , . »— , ; •
1 i i j I i i i ; 1 1 :
-M !— I— I tlii.il
.|.|.!.L.!.vJ.4i^r^
S!N!R S'NJR S!R:S SIN'N
! i i i i 1 j 1 i ; i 1 i I i
* /-*/ '//
*/// * // *•/ [I ff// /////./
//if IIffft / e r i///_//1 if
-------
Cures, both types of acrylic polymers behave in
a simiLnr manner. As alkaline conditions become
more severe the homo-polymer again shows super-
iority.
Under weak or intermediate conditions of chemi-
cal concentration and/or temperature, generic
family only will need to be considered. As
severity of conditions increase, then special
fibers within a generic type become important
considerations. "Tailor-made" fibers are being
manufactured within a generic family. "Nomex"
nylon will withstand 450°F whereas nylon 6 or
6,6, the common types of nylon, are useful up
to 250°F."
"In the acrylics, "Crylor" and "Dralon T" can
be used at higher temperatures than most other
acrylics because of their higher acrylonitrile
content. All manufacturers of synthetic fibers
are striving to produce better fibers for fil-
tration and dust collection."
4.3 YARN PRODUCTION
The standard definition of a yarn is as follows:
"Yarn: a generic term for a continuous strand
of textile fibers, filaments or material in
a form suitable for knitting, weaving, or
otherwise intertwining to form a textile
fabric. Yarn occurs in the following forms:
a. A number of fibers twisted together
b. A number of filaments laid together without
twist
c. A number of filaments laid together with
more or less twist
d. A single filament,...monofilament
e. A narrow strip of material such as paper
cellophane, or metal foil...
Varieties include single yarn, plied yarn,
cabled yarn, cord, thread, fancy yarn."
Yarns formed of short staple fibers are called spun yarns. Yarns
may also be formed of a twisted bundle of individual fine monofilaments
(typically several hundred, each monofilament of order 10 ^m diameter), in
which case the yarn is called mult ifilament or filament. Single or mono-
filament yarns (e.g. as in monofilament fishing line, 5 to 10 Ib. test)
are not as widely used in fabrics for industrial gas filtration as multi-
-------
Multifilament yarns are made directly from individual monofilaments,
frequently as they are extruded, in which case the desired number of spin-
narets is used. Following varying amounts of thermal and mechanical
molecular crystallite orientation, a finish lubricant may be added and a
slight twist is given the yarn to maintain multifilament order. This low-
twist basic yarn is then coverted to the specific yarn form required for
the desired fabric design either by the fiber producer, by an intermediate
processor, or by the fabric manufacturer. Man-made fiber staple may be cut
frommultifilament yarn. Staple yarns are made by the fiber yarn or fabric
manufacturer by parallelizing the short fibers and twisting them together
for the desired strength.
The method or "system" of yarn production is determined by the type
of fabric desired for dust collection. The major fabric types are pre-
sented in Table 4.15 based on information provided by Albany Felt Company.
A variation of the woolen system is the worsted system, in which the
fibers are given additional parallelizing treatment, or combed, before
being spun. As a result the worsted yarn is smoother and of finer quality
(Figure 4.2). The type of spinning machine as well as the amount of
combining influences the fiber orientation, and thus has an influence on
the amount of free fiber available for dust filtration.
4.4 YARN PROPERTIES
Yarns of staple fibers can be made with various fiber lengths and
various fiber diameters. The diameter of the yarn itself can range from
relatively fine to comparatively heavy and thick. The amount of twist
is also variable. All yarns composed of staple fibers must possess
sufficient twist to hold the fibers in place. The amount of twist in
staple yarns also depend upon staple length, shorter staple requiring
higher twist for adequate weaving strength.
Filament yarns are smooth and even unless they have been deliberately
formed in an uneven manner for novelty effects. They may be thick in
diameter and heavy, gossamer sheer and light, or of any intermediate
weight and diameter.
-------
TABLE 4.IS
MAJOR FABRIC SYSTEMS *
Woolen System Spun Fabrics
Thli terminology relates to « manufacturing system and not Just wool flb«ra.
Synthetic can be woven on the 'woolen system1.
Woolen System Spun Fabrics are made with soft, lofty, open, heterogeneous,
low twlat yarns, which result In a fabric with many small pores for high capacity
and efficiency. This type fabric Is Ideal for vary flna dust up to and Including
coarse grades ....
This type construction was designed primarily for the Shaker type baghousa.
Cotton System-Spun Fabrics
This terminology relates to a manufacturing system and not Just cotton fib-
ers. Synthetic fibers can be woven on the cotton system.
In this type fabric, the yarns are smaller, tighter and of high twlat. Be-
cause of this, the yarn count is Increased to prevent duat leakage and the result-
ing fabric is generally lighter in weight and offers less capacity than a woolen
system spun fabric.
This type fabric could be used for a Shaker type baghouse.
Filament Fabrics
Filament filtration fabrics are characterized by slick yarns without any pro-
truding fiber ends along the yarn and therefore the surface of the fabric Is very
smooth .
Since this type fabric Is completely without a surface "cover" or "nap", the
yarns are packed quite closely together, even tighter than a cotton system fabric
of equal permeability.
This results In a fabric which has excellent release characteristics, but
limited capacity.
This type construction was designed for the Reverse-Air type baghouse.
"Combo" Fabrics (Spun- filament)
This type fabric Incorporates some of the advantages of both the woolen system
fabric and the Filament fabric. In this unique design, the filament yarna ere
manufactured in the lengthwise or warp direction of the bag whereas the woolen
system spun yarns are manufactured in the crosswise or filling direction.
The "combo" fabrics are a so-called one sided construction whereby the fila-
ment yarna are predominant on one side, giving the Inherent advantage of cake re-
lease. The spun yarns are used to add strength and to cover the Inters tlcea of
the yarn and thereby reduce pore sice. /
This fabric was designed primarily for Shaker baghouses, however, it also has
limited use in Reverse Air Collectors.
The "combo" fabric offers a good balance of capacity and cleanabllity where
plugging problems are encountered.
Needled Fabrics
This type fabric la manufactured by mechanically interlocking layere of fibers
by inserting a multitude of reciprocating barbed needles. Needled fabrics should
uae a woven base fabric (sometimes referred to aa a scrim) for strength and stability.
This type fabric offers excellent capacity and wear with the reverae-Jet and
pulse jet type collectors. '
From Albany Felt Company, Ref . 11
-------
Figure 4.2. Diagram of Fiber Lay- a. Woolen Yarn (note, Fibers Usually Lay
in Much More Random Manner); b. Worsted Yarn (Ideal Fiber Lay).
(From Joseph, Ref. 14).
4.4.1 Threads and Yarns
Thread and yarn are basically similar. Yarn is the term usu-
ally applied when the assemblage of fibers is used in the construction
of a fabric. Thread is the product used to join pieces of fabric together
in the construction of textile products. Thread is frequently of plied
construction. It is fine, even, and strong. Several types of thread are
available on the market: there are simple ply threads, cord threads,
elastic threads for special use, monofilament threads of the man-made
fibers, and mult ifilament threads.
4.4.2 Yarn Twist
As the staple fibers or filament fibers are formed into yarns,
a certain amount of twist is added to hold the fibers together. The
amount of twist is measured by the number of turns per inch. The more
turns per inch the stronger the yarn becomes, up to a point. Beyond the
optimum,which varies depending upon fiber content, staple length, size
of the yarn and appearance desired, the yarn will become somewhat brittle
and tend to break easily. According to Gurley yarns with low twist
have less than 5 turns per inch; medium twist yarns from 5 to 20 turns
per inch; and high twist yarns from 20 to 30 turns per inch. It has been
suggested that optimum ranges are 3 to 6 turns per inch for filament fiber
yarns and 10 to 20 turns per inch for staple fiber yarns.
-------
The direction of twist is also important. Yarns may be
twisted either with a right-hand twist or a left-hand twist. The right-
hand twist is called a Z twist, while the left-hand twist is an S twist,
as in Figure 4.3. Various effects can be obtained by combinations of
yarns ot: different twist direction, and the durability of yarns may be
increased by efficient plying of S and Z twist single yarns.
4.4.3 Yarn Number
Yarn number is a measure of linear density. Direct yarn num-
ber is the mass per unit length of yarn; indirect yarn number is the length
per unit mass of yarn. Yarn number is frequently called yarn count in the
indirect system. To some extent the yarn number is an indication of the
diameter when yarns of the same fiber content are compared.
Figure 4.3. Diagram of S and Z Twist in Yarn. (From
Joseph, Ref. 14).
Over the years various methods of determining yarn number
have been developed. Cotton yarns have been numbered by determining the
weight in pounds of 840 yard hanks, or, aiore frequently, the number of
840 yard hanks required to weigh one pound. For example, if 840 yards
weigh one pound, the yarn number is Is; if it requires 30 such hanks to
weigh one pound, the yarn is a 30s. A heavy yarn would be Is; a medium
yarn is considered a 30s, while a very fine yarn might be a 160s.
-------
Woolen system yarn is measured by the number of 300-yard hanks
per pound while worsted system yarn is measured by the number of 560-yard
hanks per pound. Man-made fiber yarns are usually measured using the denier
*
system. The denier is equal to the weight in grams of 9000 meters of yarn.
Recently the textile industry has considered among other systems
the Tex numbering system for all fibers. A kilometer of yarn is weighed in
grams and the weight becomes the yarn number. In this system the larger the
number the heavier the yarn and, conversely, the smaller the number the
finer the yarn. Note that:
Tex = Wt in grams
1000 meters of yarn ^ '
and
9 Tex = 1 denier (4.2)
The denier or Tex may be used to determine monofilament fiber size and
approximate yarn diameter from the appropriate fiber material density.
j^
Consider for example a Nomex nylon yarn stable fiber (see
Table 4.6, for forms available) of 2 denier/filament (2/9 tex). The
diameter of this fiber is obtained as follows. Since
f\
T D L p. = Wt in grams, (4.3a)
J Df (9 x 10 x 10 cm)(1.38 gm/cm3) = 2 grams (4.3b)
from which
Df = 14.3um
The fiber density, Pf, was obtained from Table 4.13 (also see Appendix
4.3).
*
This system dates back to early Roman history where a coin, the
"Denier" (~ 0.5 gm) was used as a medium for buying and selling silk.
-------
p
A 100 denier multifilament Nomex nylon yarn (Table 4.6) would
be composed of approximately 50-2 denier filament ends. The yarn dia-
meter would be of the order
(1-0 I Df2 N- f Dy2 (4.4)
where 1-e = a. = the packed fraction, depending upon twist and yarn tight-
ness
Df = the individual filament diameter
N = the number of filaments in the yarn
D = the yarn diameter
y
from which (assuming 1-e = 0.6), D & 14.3 v30 % SO^m, or approximately
the thickness of a human hair.
Breaking tenacity (c.f. Table 4.13 or Appendix 4.3) in grams/
denier can be converted to stress from the fiber size and breaking weight
P
applied. For the 2 denier Nomex nylon staple fiber above, breaking ten-
acity (dry) is 5.3 grams/denier, or 10.6 grams. Stress at rupture is
thus determined as follows:
Stress % Force/area
Stress = 10.6 x 980/(14.3 x 10* ) n/4 = 6.5 x 10 dynes/cm or about
9.5 x 104 psi.
4.4.4 Simple and Plied Yarns
Yarns that are even in size, have an equal number of turns
per inch throughout and are relatively smooth, are called simple yarns.
A simple, single yarn is the simplest assemblage of fibers suitable for
operations such as weaving and knitting. These yarns may be made from
any of the fibers and by any of the basic systems.
A simple-ply yarn is composed of two or more simple-single
yarns plied or twisted together. In naming a ply yarn the number of
singles used precedes the word "ply." For example, if two singles are
used, it would be called 2 ply. Typical plied and cord yarn are shown in
Figure 4.4.
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a-
4.5 FABRIC PRODUCTION
4.5.1 Weaving
The production of a flexible textile fabric for use in filtra-
tion systems involves weaving; rarely are completely non-woven materials
such as paper used. Most felts used in filtration are first woven, then
given further treatment. Figure 4.5 shows the major weaves used in non-
felted filter fabrics. Woven fabrics are formed by (interlacing yarns at
right angles on a loom, after which the raw or "greige" fabric may be
further treated. While there are many patterns of interlacing, the fabrics
in most common useage in gaseous filtration are classed generally as twill
and sateen (satin). Plain weave fabric is also sometimes used. The engi-
neering technology of these and other weaves is discussed in detail else-
16 ' 8
where and the following quotation from Potter and Corbman summarizes
the fabric production process:
(a) Photograph of Cord, Composed
of Four Plies, Each Ply Com-
posed of 7 Single Yarns.
(b) Four Ply Yarn, Slightly Mag-
nified, Showing Fibers Sep-
arated in One of the Single
Yarns Forming the Ply.
.
Figure 4.4. Plied Yarns. (From Joseph, Ref. 14),
-------
(a) Plain Weave, Showing Loose or Open Construction
and Close Construction, Such as Muslin.
(b) Twill Weave. "This drawing shows
a three-shaft twill-two warp yarns
in each repeat. This is a right-hand,
filling-faced twill because the
diagonal moves from the upper right
down to the lower left, and more
filling than warp appears on the
face of the fabric. It is also re-
ferred to as a one up and two down
twill (1) because the warp goes over
one and under two filling yarns.""
(i) Warp Floats are
Seen Interlacing
Every Eighth Filling.
(ii) Warp Floats are
Seen Interlacing
Every Fifth Filling.
(iii) Filling Floats
are Seen Interlacing
Every Fifth Warp.
(c) Satin Weave, Showing (i & ii ) Warp-Face and (iii) Filling-Face
Construction.
Figure 4.5. Weaving Styles for Filtration Fabrics. (From Ref. 8).
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PREPARATION FOR WEAVING
"In the weaving operation, the lengthwise yarns that
form the basic structure o£ the fabric are called the
warp. The crosswise yarns are the filling, also re-
ferred to as the weft or the woof. The filling yarns
undergo little strain in the weaving process. In pre-
paring them for weaving, it is necessary only to spin
them to the desired size and give them the amount of
twist required for the type of fabric for which they
will be used.
Yarns intended for the warp must pass through such
operations as spooling, warping, and slashing to pre-
pare them to withstand the strain of the weaving
process. These operations do not improve the quality
of the yarn. In spooling, the yarn is wound on larger
spools, or cones, which are placed on a rack called a
creel. From this rack, the yarns are wound on a warp
beam, which is similar to a huge spool. An uninter-
rupted length of hundreds of warp yarns results, all
lying parallel to one another. These yarns are un-
wound to be put through a starch bath called slashing,
or sizing. The slasher machine covers every yarn with
a starch coating to prevent chafing or breaking during
the weaving process. The sized yarns are passed over
large steam-heated copper cylinders that remove the
moisture and set the size. They are then wound on a
final warp beam and are ready for the loom.
TWILL WEAVE
A distinct design in the form of diagonals is charac-
teristic of the second basic weave, called the twill.
Changes in the direction of the diagonal lines pro-
duce variations, such as the herringbone, corkscrew,
entwining, and fancy twills The values of the
twill weave include its strength and drapability.
The diagonally arranged interlacings of the warp
and filling provide greater pliability and resili-
ence than the plain weave. Also, twill fabrics are
frequently more tightly woven and will not get dirty
as quickly as the plain weave, though twills are
more difficult to clean when they do get soiled.
The yarns" are usually closely battened, making an
especially durable fabric. Twill weaves are there-
fore commonly used where strong construction
is essential.
In the twill weave, the filling yarn interlaces
more than one warp yarn but never more than four,
as strength would be sacrificed by so doing. On
each successive line, or pick, the filling yarn
moves the design one step to the right or to the
left, thus forming the diagonal. Whichever the
-------
direction of the diagonal on the face of the
fabric, the design runs in the opposite direction
on the reverse side.
When the direction of the diagonal starts from the .
uppcT left-hand side of the fabric and moves down
toward the lower right, it is called a left-hand
twill. When the direction of the diagonal starts
from the upper right-hand side of the fabric and
moves down toward the lower left, it is called a
right-hand twill. Although there is no advantage
of one over the other, the direction of the diagonal
can aid in the recognition of the face of the fabric.
The steepness of the diagonal can indicate strength
and durability in the fabric. In order to obtain
a steep twill, more warp yarns must be used than
filling yarns. And since warp yarns have a higher
twist and are stronger than filling yarns, the
steeper the twist the stronger the fabric is
likely to be.
Twill weaves are named according to the number of
harnesses required to make the design. A three-
of four-harness twill is frequently used. The
word "shaft" may be substituted for "harness,"
as in three-shaft or four-shaft twill.
Twill weaves are also classified as even or
uneven according to the number of warp and fil-
ling yarns that are visible on the face of the
fabric. The even twill, for example, shows an
equal number of warp and filling yarns in the re-
curring design, such as two over and two under.
This pattern makes what is called a four-shaft
twill, and it requires four harnesses.
Most twill weaves are uneven. An uneven twill
may show more warp than filling yarns in the re-
curring design; this is called a warp-face twill.
If more filling yarns than warp yarns show on
the face, the weave is called a filling-face twill.
Warp-face twills are generally stronger than fil-
ling face twills because the stronger warp yarns
on the face of the fabric can take more abrasion
and wear. Warp-face twills generally have much
more warp than filling yarns; consequently, such
fabrics hold their shape better and drape better
due to the warp's greater twist and resilience.
Twills are described in terms of the interlacing
of the warp yarns over and under the filling yarns.
An uneven four-shaft twill, for example, that has
three warp yarns riding over one filling yarn is
referred to as a three up and one down, or ly.
On the other hand, a three-shaft twill that has
one warp yarn riding over two filling yarns is
referred to as a one up and two down, or "z-
-------
SATIN WEAVE
In the basic construction, the satin weave is
similar to the twill weave but generally uses
from five to as many as twelve harnesses, pro-
ducing a five- to twelve-shaft construction. It
differs in appearance from the twill weave be-
cause the diagonal of the satin weave is not
visible; it is purposely interrupted in order to
contribute to the flat, smooth, lustrous surface
desired. There is no visible design on the face
of the fabric because the yarns that are to be
thrown to the surface are greater in number and
finer in count than the yarns that form the reverse
of the fabric. The satin weave may have a warp-
or filling-face construction.
Warp-Face Satin Weave. Warp-face satin is woven
so that the warp may be seen on the surface of
the fabric. For example, in a five-shaft con-
struction, the warp may pass over four filling
yarns and under one; in a twelve-shaft construc-
tion, the warp may pass over eleven filling yarns
and under one. Since the warp lies on the sur-
face and interlaces only one filling at a time,
the lengths of warp between the filling are called
floats. These floats lie compactly on the surface
with very little interruption from the yarns going
at right angles to them. Reflection of light on
the floats gives satin fabric its primary charac-
teristic of luster, which appears in the direction
of the warp.
The long floats found in the satin weave might be
considered a disadvantage because they represent
a minimum of interlacings, and therefore a poten-
tial weakness in the fabric. Furthermore, to
increase the smoothness and luster of the fabric,
the yarns are given a minimum of twist and are
therefore relatively weak. The longer the float,
the greater the chance that the surface of the
fabric will snag, roughen, and show signs of
wear
Satin-weave fabrics drape well because the weave
is heavier than the twill weave, which, in turn,
is heavier than the plain weave. More harnesses
are used for satin weave, thus compressing a
greater amount of fine yarn into a given space
of cloth. This compactness gives the fabric more
body as well as less porosity
Filling-Face Satin Weave. The filling-face
satin weave is also called the sateen weave;
however, this sometimes causes confusion because
-------
some cotton and rayon fabrics are also identified
a.s .sali'c'ii. In thi.s construction, the- Tilling
yarn lies on the surface of the fabric as it
pauses regularly over and under the warp yarns.
For instance, a filling yarn may pass over four
warp yarns and under one. The floats are con-
sequently made up of the filling yarns, and the
luster appears in the filling direction.
"On the conventional loom, the warp yarns
that are to run lengthwise in the fabric are
wound on a cylinder called the warp beam, which
is at the back of the loom. The warp also ex-
tends to a cylinder called the cloth beam, which
is at the front of the loom and on which the
fabric is rolled as it is constructed. Sup-
ported on the loom frame between these two
cylinders, the warp yarns are ready to be inter-
laced by the filling yarns that run in the width
of the cloth, thus producing the woven fabric.
In any type of weaving, four operations are
fundamental. They are performed in sequence
and are constantly repeated The essential
parts of the loom are: warp beam, cloth beam,
harness or heddle frame, shuttle, and reed. These
parts perform the following operations.
Shedding—raising warp yarns by
means of the harness or heddle
frame
Picking—inserting filling
yarns by means of the shuttle
Battening—pushing filling
yarns firmly in place by
means of the reed
Taking up and letting off-
winding the finished fabric
on the cloth beam and re-
leasing more of the warp
from the warp beam.
CLASSIFICATION OF WEAVES
The manner in which groups of warp yarns are
raised by the harnesses to permit the inser-
tion of the filling yarn determines the pattern
of the weave, and in large measure the kind of
fabric produced. Weave patterns can create var-
ying degrees of durability in fabrics, adding
to their usefulness and also to their appear-
ance. In a simple weave construction, consisting
-------
of the filling going under one warp and over the
next, two harnesses are needed: one to lift the
odd-numbered warp yarns, and a second to lift the
even-numbered warp yarns. More than two harnesses
are required for advanced weaves, and as many as
forty for figured weaves
PLAIN WEAVE
The plain weave is sometimes referred to as the
tabby, home-spun, or taffeta weave. It is the
simplest type of construction and is consequently
inexpensive to produce. On the loom, the plain
weave requires only two harnesses. Each filling
yarn goes alternately under and over the warp yarns
across the width of the fabric. On its return, the
yarn alternates the pattern of interlacing. If
the yarns are close together, the plain weave has
a high thread count, and the fabric is therefore
firm and will wear well."8
4.5.2 Felting and Needle Punching
The felts used in fabric filtration in their early stages of
production are also woven, but subsequent steps completely change the
character of the material from that of a woven fabric. While felts can
be made simply by matting fibers together and by other non-woven
methods (see below), the use of a woven base fabric called a scrim greatly
increases the strength and stability of the fabric.
The production of a felt depends on whether its fibers are
naturally binding. Because woolen fibers are scaley, and also shrink
when exposed to heat and moisture, a woolen scrim shrinks when mechani-
cally worked in warm water in the presence of certain lubricants and
chemicals. The identity of the separate yarns tends to be replaced by
a more homogeneous character. The material becomes felt-like in density,
stiffer and thicker. Minor amounts of synthetic fibers added to the
woolen yarns can modify the properties of the felt thus produced.
To further increase the homogeneous character of the felt
surface, it may be napped. This has always required the use of teasels,
woody, thistle-like parts for a weed plant, the barbs of which pluck
fibers from the surface of the felt. When enough nap has been raised in
this way, it may be singed or otherwise trimmed to the desired thickness.
-------
Need It1 punching is a method of combining two or more layers —'
of liber into a 1'elt-likc fabric. Usually one layer is a scrim for
strength, while the other(s) may consist of fibers of almost any descrip- _
tion or combination. Thus considerable control over separate properties
of the finished material is possible. For example, the scrim may con-
tribute the desired dimensional stability while the top layer'contributes
the ideal properties for dust control. The surface layer might be 100%
dacron, the scrim 1007o nylon, for example.
The technique used in needle punching is to prepare a scrim '
and a batting separately. The batting may be formed by carding, by air-
lay of fibers, or by other random web-forming equipment. The batting
is generally of the same order of weight or lighter than the scrim. •—'
It is unrolled or otherwise spread over the scrim. Needles having for-
ward barbs are punched from the batting side into or through the scrim, _j
and the batting fibers thus laced into the scrim remain behind when the
needles are withdrawn. Production is at the rate of about 10 FPM.
Variations in the needling process include needle angle, number of
repetitions, two-sided needling, etc. When a shrinkable scrim is used,
the needled material may later be felted in various ways to produce a
still more dense and uniform material. :
_j
Non-woven production methods include resin bonding, wet bond-
ing (paper-like materials), spun bonding (while the fibers are still
tacky from their extrusion stage), heat bonding, chemical bonding, spray "^
n 1 f
bonding, and stitch bonding (a sort of knitting within a matt). '
Nearly all fibers used in fabric filtration can be used in non-woven —'
fabrics. Because non-woven fabrics can often be produced more rapdily
than by weaving, it appears that filtration fabric or even filter ele- _,
ments might eventually be produced by non-woven methods.
4.5.3 Fiber Additives, Yarn Treatment, and Fabric Finishes _J
Natural fibers are produced with an outer molecular film; cot-
ton cellulose fibers are covered with a wax-like adhesive; and wool pro- __
tein fibers contain oils, fats, and waxes from glandular secretions dur-
ing growth. Both may contain other agricultural chemicals added to pro- '
tect or preserve the fibersto aid in the harvesting process, e.g., in-
-------
secticides, defoliants, etc. Wool will have much of the grease removed
in a washing, scouring or solvent extraction process. Additional oil
(animal, vegetable, or mineral) may be provided as a lubricant for spin-
ning. The production of fabrics from fibers and yarns Involves many
chemical, physical and mechanical processes. ProporLlca of the fiber,
yarn, and fabric are continually modified through treatment and additives
to meet the requirements of production machinery.
Each additive or treatment may result in some residual mater-
ial attached to individual fibers that ultimately affects its strength
in service or its ability to act as a substrate for filtration. The
potential combination of additives, treatment or finishes is nearly
limitless. Prediction of fabric performance in service (pressure drop,
efficiency, life) on a given application is presently empirical through
service testing, and thus the science of fabric treatments for improved
performance is relatively undeveloped. Furthermore the addition of
materials to improve filtration performance, particularly life in service
is often proprietary. Many techniques are relatively new, and not subject
to competitive or comparative testing procedures. The entire field of
the role of finishes and additives in gas filtration requires further
analysis before analytical generalizations can be produced for optimiza-
tion studies.
Each of the generic man-made fiber materials discussed above
is a complex chemical structure with varying physical properties and
surface characteristics.
These polymeric materials frequently contain one or more of
the following additives:
• Plasticizers reduce flow viscosity or temperature
in melt spinning the resin, such as DOP in PVC;
and are also employed to improve low temperature
flexibility.
Solvents are used in wet spinning as with acrylic
plastics or rayons; and in coating, adhesives, etc.
. Organic peroxides are used as polymerization initia-
tors or for cross-linking reactions in thermoplastic
materials to transform them into thermosets.
-------
Antioxidants are added to reduce oxldative deter-
ioration during manufacture, processing or storage,
and to provide heat protection especially among
vinyls.
These agents may bt; required in varying amounts.
Flow-control agents may be required to control melt
viscosity and transformation of the polymer to a
stable form for end-use.
Colorants, pigments, and delustrants include powered,
colloidally dispersed or dissolved materials added to
provide color or reduce brightness.
./Flame retardants are employed to provide fire protection
for flammable polymers.
Stabilizers are used to import thermal stability or
mechanical protection during processing and to protect
the mix against changes induced by other additives,
e.g. by neutralization of contaminants, residues,
or impurities.
Ultraviolet absorbers are added to reduce UV absorption
by the polymer or to quench molecular reactions, in
order to limit physical degradation of the plastic ex-
posed to sunlight.
. Antistatic agents may be applied as a coating to ex-
ternal fiber surfaces or added internally; the agent
acts as a hygroscopic material to assist the charge to
leak away (make the fiber electrically conductive) and
may modify the charging process, reverse the sign of
the charge, or promote dissociation of ionic material
present on the fiber.
Other additives, filters, and processing aids may be
employed as viscosity depressants, parting agents,
emulsifiers, coupling agents, internal or external
lubricants, or adherents.
Many compounds ,are used in varying amounts to assist in manu-
facturing and processing operations and to provide stability, compatibility,
and other desirable end-use properties. Since filter fiber is a small
proportion of the total annual market for man-made fiber textiles, mater-
ials used in production and processing are developed, produced, and used
primarily for the larger market applications, rather than for fabric fil-
tration objectives. Each producer's fiber, while generically the same
-------
(i.e., same basic polymer), will be produced differently and will have
different additives which may affect dust particle-fiber attachment, de-
posit formation, and stability. The reduction of these phenomena to
engineering design parameters in filtration will be difficult and complex.
The yarn may be treated as well as the fiber. Yarn treatments
now include surface addition of lubricants, antistatic agents, and var-
ious mechanical operations such as attenuation or stretching, heat set-
ting, and bulking or texturizing. Warp yarns are subjected to greater
strain during weaving and may be sized or coated to prevent chafing and
breakage.
Fabric finishing includes those processes to improve appear-
ance or serviceability of the fabric after leaving the textile machine
(greige goods). Cotton and wool fabrics are usually cleaned (washed or
scoured) and bleached, and may be chemically treated to provide water-
proofing, mothproofing, mildewproofing, or fireproofing. Mechanical •
finishes applied to cotton and wool include singeing, napping, shearing,
felting, or shrinking. Synthetic fabrics are usually heat set to re-
lax internal stresses in the yarns set up during weaving. This causes
shrinkage, although it enhances dimensional stability for subsequent
exposures to temperatures below the setting temperature. Additional mois-
ture protection (water repellents) and antistatic agents may be applied.
Glass fiber fabrics are subjected to high temperatures
(~10 F) to relieve yarn twisting and weaving stresses and to heat clean
the weaving size. Glass fabrics are usually lubricated with silicones,
graphite and other proprietary finish agents to reduce fiber-fiber abra-
sion resistance during filter cleaning. Graphite finishes of the order
of 5% by weight have been indicated to provide extended service life of
glass fiber fabrics at temperatures < 500 F, as illustrated in Figure 4.6.
Largely as a result of improved finishes, the use of glass fiber fabrics
for applications at temperature > 275 F has increased greatly. Competi-
tive fabrics now include NomexR (< 425°F) TeflonR (< 450°F), and newer
R R
man-made fibers (Brunsmet , Fiberfrax , and Polyimides). Improved lubri-
cants and finishes for glass fiber fabrics to provide longer life at
higher temperatures (> 600 F) are under continuing development.
-------
M
*
VI
Ol
u
(0
3
T3
C
W
800
700
600
500
400
300
200
100
1 1 i i i
O Legend:
\ & - Commercial ulieone fim*
~ 1 O - Commercial silicon* plut
\ tjrjphite finish
\
\
\
.\
\
\
\
\
\
\
\
0\
\ 0
*v^ 1 — "1^2* ~
i i i i i
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
CO
01
X
01
CO
C
O
200
300 400 500 600 700 800
Figure 4.6. Effect of Graphite on Glass Fabric Tempera
ture Endurance. (From Ref. 18).
4.6 FABRIC PHYSICAL CHARACTERISTICS
4.b.l Abrasion Wear
The mechanical abrasion of fabric is only one of several kinds
of wear, other kinds including chemical and thermal degradation, handling,
etc. As Table 4.16 indicates, a fabric could conceivably experience many
kinds of wear at one time or another during its lifetime of manufacturing
and use. Most of these types of wear are terms from popular usage and con-
sequently are ill defined. For present purposes the term "abrasion" will
be defined and used as an eroding away of fabric fibers or fiber surface
material, through moving contact between (a) the fiber and dusjt particles
or (b) adjacent fibers.
-------
TABLE 4.16
SOME TYPES OF MECHANICAL WEAR IN FABRICS
Fatigue Migration
Sliding Slippage
Rubbing Breaking
Galling Cutting
Scoring Grinding
Snagging Plowing
Picking ; Penetrating
Flexing : Rupturing
Welding Shearing
Compression Catching
Plucking Bending
Fretting Polishing
Fracture Chaffing
Material science describes the forms of abrasion as either two-
body or three-body abrasion. Fiber-to-fiber rubbing or fiber-to-particle
collision (sand blasting) are two-body systems. Interstitial dust particles,
caught and chaffing under pressure between adjacent fibers or yarns, repre-
sents three-body abrasion. The mechanics of the two processes are somewhat
related, however, and involve the relative sizes of the bodies, the hard-
nesses or relative hardnesses, the pressure between the bodies, the coef-
ficient of friction and the nature of the motion between them. The laws
of material abrasion are nevertheless only partially determined because of
other effects which are more difficult to evaluate such as humidity, rate
of oxidation of freshly exposed surfaces, smoothnesses of particles, any
accumulation of freshly abraded particles, etc.
The laws of material abrasion are undeveloped, and fabrics,
being of complex geometry and having extremely varying stresses and strains,
are especially difficult to analyze. Neither can fabrics be analyzed in
detail for abrasion during their fabric filtration performance, as abrasion
-------
hi usually reported simply as "holes" in the filter element (Section 8.3).
Kv«-n tliouKh i-t appears that abrasion represents up to 25% of fabric failures
llu-rc- JM insufficient data relating to particle hardness, coefficient of
I rl.(-tlon, ft cetera, to establish the rules of filtration fabric abrasion.
Manufacturers of filter fabrics have little technology for
the abrasion resistance of their fabrics, although they do measure the
resistance of fabrics to abrasion by standard testing procedures. Typi-
cally a swatch of the fabric is rubbed back and forth over a rough surface,
sometimes in the presence of grit,until the fabric becomes perforated. The
fabric abrasion resistance is rated according to the time required for per-
19
foration. Other methods include sandblasting, and flexure or tension
20
during rubbing. Thus it appears that fabric manufacturers as well as
users rely on an evaluation of the destruction of the fabric without attemp-
ting to determine the basic cause or causes of failure. Considerably more
21 22 23
analytical approaches have been taken by Backer ' and Hamburger , as
10 24 16
reviewed by Kaswell ' and Hearle
4.6.1.1 Sandblast or Surface Scour.- Particles entering a fabric col-
lector filter element are apt to strike the fabric surface with sufficient
velocity to abrade and remove portions of fibers, eventually perforating the
fabric. This is analogous to the process called sandblasting, and although
sandblasting involves perpendicular impact while the fabric is struck at a
25
shallow angle, the mechanics should be related. Raleigh studied perpen-
dicular sandblasting. He assumed that the rate of wear is related to the
maximum strain produced in a surface by an impact. This strain is propor-
tional to
-. V N/'jJ
Y
where V is the impact velocity, p the density and Y the tensile modules
of the impacted material. To the extent that this strain exceeds the
tensile strain limit of the material, bits of the material will chip out.
*
Another test that is frequently used is to abrade the fabric against it-
self with a rolling crease. Cycles to failure are an indication of re-
sistance to flexure and two-body abrasion.
-------
KvidenLly this applies better to hard, brittle materials than to plastic
ones, where, the mechanism is more probably plowing than chipping.
Zcnz and Othmer reported a general agreement that 90 sandblast
crrosion is proportional to
-LV3
where L is sand loading or concentration, although the exponent was reported
to vary from study to study between 3 and 6. The exponent of 3 can be shown
to be consistent with the assumptions that wear rate (depth/time) is propor-
tional to both the particle impact rate (number/time) and the average parti-
cle kinetic energy. In the above case it is assumed that the mechanism is
plastic plowing, rather than brittle chipping.
Fabric abrasion using sandblast equipment has been studied experimen-
27
tally. Parker found with approximately 60 mesh granules that harder abra-
sive (circa silicon carbide hardnesses) and higher blast pressures shortened
fabric life; for example:
Pressure (psi): 30 40 50 60
Life (seconds): 100 54 35 23
Unfortunately Parker did not state the rate of sand flow which may or
may not have varied with blast pressure.
Generally Parker found the rate of removal of fiber was constant over
the life of the fabric, although it slightly accelerated during the life of
2 24
heavier (8-12 oz/yd ) fabrics. Kaswell also indicated an approximately
linear relationship between fabric strength loss and the time of operation
of the abrasion equipment.
Types of surface abrasion test machines other than sandblast are more
widely accepted, and their data may be related to filter element scouring.
24
Kaswell reviewed these methods and a large number of studies of fabric
surface abrasion made chiefly by the garment industry. He credits Backer
22
and Tanenhaus with an examination of fourteen fabric properties that re-
late to abrasion resistance:
-------
Geometric Aspects
1. Area o£ contact between fabric and abradant
2. Local pressures developing on specific yarn points
3. Threads per inch
A. Knuckle height
5. Yarn size
6. Fabric thickness
7. Yarn crimp
8. Float (knuckle) length
9. Yarn conesiveness
10. Congressional resilience
11. Fabric tightness
12. Cover factor
Abrasion Aspects
1. Direction of abrasion
2. Magnitude and direction of tensions developed during abrasion
In conclusion it appears that fabric surface scouring is related to
dust loading and impact velocity, and to particle sharpness and hardness.
Fabric designed with a dense surface (level; few knuckles) should retain
its strength longer. Nap, pile or sacrificial surface fibers will extend
the life of the fabric A soft fiber material, flexible fibers, and re-
silient yarn and fabric will also reduce the rate of surface fiber degradation.
4.6.1.2 Internal Chaffing.- The rubbing together of two fibers as the
fabric is flexed is also two-body abrasion. Glass fibers are notoriously
weak in this respect. Originally glass fabric had almost no durability;
crossed glass fibers apparently became nicked due to chaffing or due to
dissolution at the point of contact and then snapped easily. However when
silicone and other lubricants were added, their life extended almost in-
definitely in comparison.
When a yarn is bent its elementary fibers slip past one another, un-
less they are bonded together or under a large lateral pressure from twist-
ing or weaving. In the latter cases the yarn is beam-like in rigidity and
liable to rupture in tensile failure if bent. More normally, fibers slip
to some extent as the yarn is bent; and also adjacent yarns tend to slip
as the fabric is flexed. Thus the shaking or collapse cleaning of a filter
fabric is associated with myriad tensile stresses and fiber slippages with
chaffing.
-------
To a first approximation, the strain associated with a bending fabric
is (ft = •« R) where t is the fabric thickness and R the radius of bend.
Also approximately, the amount of slippage (distance) between fibers of
diameter D in a yarn undergoing bending strain 6 Is 1 ~ £D. , or
J ~ ^f (4.1)
R
A precept of two-body abrasion is that the volume of material removed during
28
a rubbing of distance 1 is
;
where k is a wear, coefficient, L is the: loading between bodies, and H the
hardness of the body being abraded, (v and L may alternatively be expressed
per unit area). It appears that k depends on several aspects of the materials
and their geometry, but is frequently of order unity.
29
Rabinowicz discusses four common types of two-body wear, any of which
may apply during the fabric deformations active in dust collectors:
• Adhesive wear. One surface adheres so strongly to
a second because of atomic bonds at their contact
that on sliding, a portion of one is torn loose.
• Gouging wear. A proturberance on one surface
plows a groove in the second. It appears that k
(wear coefficient) may be of the order of the
28
protuberance size in microns.
. Corrosive wear. Corrosive films that normally tend
to resist further corrosion may wear away, exposing
fresh surface to corrosive attack.
• Fatigue wear. Tensile stresses trailing the sliding
body may crack the supporting surface, leading to
chipping of fragments from the supporting surface.
It appears from these brief reviews that chaffing inside the fabric
may be reduced by minimizing the:
-------
. number of fabric deformations (cleaning cycles)
. degree (radius) of deformation (cleaning intensity)
• tightness of the fabric weave
• softness of the fiber material
• fabric thickness
. fiber diameter
• fiber roughness
• interfiber friction coefficient.
Table 4.17 lists frictional properties for several fibers. These affect
both the rate of wear between fibers and the stiffness of the yarns, to be
discussed below.
4.6.1.3 Internal Abrasion.- Many dust particles are abrasive in con-
tact with the fibers ordinarily used in filtration fabrics. Since most
industrial dusts and fumes include some particle of sizes substantially
smaller than the fiber diameters (10-30um) there is great likelihood of
penetration between adjacent yarns and adjacent fibers. Here the fine
particles may be trapped by interfiber pressures, and as the fabric flexes,
they cut and gouge the fibers. This is three-body abrasive wear. This
wear also follows Eqn. .(2) as far as can be determined, except that k may
28
be an order of magnitude smaller than for two-body wear
The evidence for wear of a fibrous structure due to imbedded grit is
scant, despite the claims of carpet cleaning services and the manufacturers
of laundry detergents. Soiling of carpets and garments has received much
24
attention according to Kaswell but primarily from the standpoint of color
changes rather than mechanical degradation. Apparently dust filtration
fabrics have not been tested for degradation due to imbedded particles. While
reports from filter operation indicate a general weakening of the fabric
with time, thermal degradation, surface scouring and other mechanisms are
more readily shown to be the causes than imbedded particulates.
4.6.2 Flexibility
The flexibility of a filtration fabric is important for at
least two reasons: Removal of the dust deposit may be improved by flexing
-------
I
Ln
U)
TABLE 4.17
FIBER FRICTIONAL PROPERTIES*
THE COEFFICIENTS OF FRICTION OF VARIOUS AIR-DRY TEXTILE
FIBERS AND FILAMENTS IN COMMERCIAL CONDITION
Rubbing Surfaces
Nylon/nylon
Very fine
6 denier
27 denier
Bristle
Viscose rayon/viscose
rayon
Acetate rayon/acetate
rayon
Cotton/cotton
--from sewing thread
--from cotton wool
Wool/wool
from tops
commercially scoured
coarse, "clean"
Coefficient of
a
Friction
0.14
0.15
0.23
0.6 (0.5-0.8)
0.19
0.29
0.29
0.57
u2 = 0.38
Hi = 0.24
u2 = 0.49
Ul = 0.20
u2 = 0.42
ul =0.25
Fiber Daimeter(u)
18
28
62
500 approx.
30 (variable)
41 (variable)
18 (variable)
20
18
26
STATIC COEFFICIENTS OF FRICTION OF FIBERS
(Average Values)
Wool
With-scale
Anti-scale
Wool in water
With-scale
Anti-scale
Cotton
Jute
Viscose rayon .
Acetate
Nylon
Saran
Terylene
Steel
0.11
0.14
0.15
0.32
0.22
0.46
0.43
0.56
0.47
0.55
C.58
. 0.29
In the case of wool (il is the coefficient of friction in the direction of the scales (root-to-tip)
and n2 is the coefficient of friction in the direction against the scales (tip-to-root).
-------
of the fabric substrate and conversely, such flexure may cause i'abric de-
gradation as noted above. Thus, fabric flexibility may be both necessary
and harmful. Since the. role of flexibility in fabric filtration is not
well established, only the principles of fabric flexibility are stated here.
A single fiber flexes beam-like according to the well-established
(4.3)
24 •> /
relation ,3 4
_ _
y ~ 48YI ' ~ 64
where t, is the span length and y is the center span deflection under
s
center load W. Y is the elastic modulus and I the moment of inertia of
the fiber which as expressed above is specifically for a circular fiber.
A parallel group of N fibers with zero friction between them
would deflect 1/N as much, under the same load conditions as the single
fiber. On tho other hand with high friction the bundle would deflect as
2
a single beam, and the deflection would be approximately 1/N as much as
ior the single fiber. A yarn of parallel fibers, having intermediate
2
friction, would deflect between 1/N and 1/N as much as given by Eqn. 4
for only one fiber.
A twisted yarn deflects somewhat more easily, but the exact
expression for deflection is highly involved. The amount of inter-fiber
friction in a yarn is proportional to the coefficient of friction and to
the pressure between fibers. This pressure results from' twisting, and it:
usually changes with bending. One simple measure of the amount of friction
in a yarn is the amount of permanent set it acquires when bent; a perfectly
frictionless yarn would normally recover elastical-ly.
When woven, each yarn contributes its own stiffness to the
fabric. The yarn stiffness will generally be increased however by in-
creased inter-fiber pressures from bending the yarns and packing them
together. Thus a section of fabric N' yarns wide will generally be more
-------
than N' time as stiff as N' single yarns. Extenuating factors in calcu-
lating fabric stiffness are slippage vs. friction between the yarns, the
thickness of the fabric, and the position and the straightness of yarn
through the fabric.
Fabric flexibility as a property of the fabric can be designed
via any of the above factors. Other design variations include texturized
(teased) yarns, lubricants or bonding agents, fibers of several diameters
or lengths, and a variety of finishing treatments including napping and
calendering. Many fabrics are designed to be more flexible in one direc-
tion than in the other. This opens the possibility in filtration fabrics
of using relatively delicate fibers or yarns in one direction for collecting
the dust deposit, and using more flexibly durable yarn in the other direc-
tion for removing the deposit.
Flexing of filtration fabric is of necessity associated with
interfiber tensions and frictional chaffing. Any grit present may cut
the fibers during chaffing (Section b). In addition all fibers undergo
some molecular fatigue wj Li. repeated flexing.
4.6.3 Strength
Although fabric breaking strength is not usually an important
parameter in filtration, per se, it is frequently specified because it is
one check on the quality of yarns and fibers in the fabric. Strength is
also one indication of the aging a fabric has undergone. Filtration fabric
rarely tears in use unless the filter element has been sewn in such a way as
to concentrate the tension, i.e. at a cuff seam. Tensile modulus is more
closely related to the performance of the fabric since it determines the
distribution of tension over the filter element and thus the distribution
of cleaning energy in mechanical cleaning.
Fabric strength is related to fiber strength or denier, defined
as the weight in grams of 9000 meters. Appendix 4.3 lists the denier
strengths (i.e. weights) and other fiber mechanical properties of a variety
of fibers. Strengths and moduli are also indicated in Figure 4.7.
-------
Tvpicul Mlri'MH Mlriiin curve for n textile liber. (Dillmi,
.1 II.. Inil. I.UK Clinn 44: 21 IT), lil.V.M
I FiMrflM
t Collo* SO/1
1 Ctlwctl
4 H T
5 Canon it/I
Sow Mon»n N
20 V««wi NOMU IWIiflMlMM
ti
(2 own.
25 C«M Report No. M, Office of the QoarUrmMUr
General.
The immediate elastic recovery is determined by extrapolating to sero load the initial straight-line portion of the recovery
lieally defined M: 100 - (ii
Delayed recovery is the recovery which take* place slowly and is ma
ivcovery (%) - permanent aet (%)).
Permanent a*t ia mtaeursH after retnoving the •train and allowing the sample to recover for
Figure 4.7. Stress Strain Curves for Fibers*
* From Harris, Kef. 4
-------
'flu- strength of a yarn can obviously not be greater than the
summed strength of its fibers and actually the yarn strength is practi-
cally always less. This is for two reasons: first, the central-most fibers
in the yarn are nearly straight and tighten first; hence they break before
the outer spiraling fibers reacli full load. Second, part of the tension
in the outer fibers goes into pressure directed into the yarn rather than
into longitudinal tension; hence a tightly twisted yarn may be a very weak
one in tension. In the-case of staple fibers which must be twisted to ob-
tain yarn strength, there is a twist giving maximum yarn strength; this
maximum depends on fiber properties number of fibers, etc.
If a typical fiber in the yarn is examined, it will lie at a
twist angle to the yarn axis as depicted in the following sketch :
The stresses (cr) in yarn (y) and fiber (f) are related by
2
,) = n cos •!> (4. A)
Thus the strength of the fiber contributes less strength to the yarn.
Fiber and yarn strains are also related by a similar but more complex
relationship, since the yarn has a widely ranging fiber packing depending
*
on fiber geometry within the yarn. Consequently even though the fiber
tensile modulus is determined (See Figure 4.7 and Appendix 4.3), the ex-
pression for the yarn's tensile modulus, that is, the ratio of yarn stress
to yarn strain, is not algebraically simple.
The process of weaving further modifies the contributions of
yarn strength and tensile modulus to the fabric. The yarn is generally
weakened by being bent around orthogonal yarns, depending on its twist and
the sharpness of the bend. The tensile modulus of the fabric is generally
increased by the weaving process over that of the unwoven yarns, depending
* Loose twisted yarns may elongate considerably before the fibers become
tightened, in analogy to the members of a folding gate. Small elastic
strains in homogeneous compressible media can be described by Poisson's
ratio, but this approach is misleading in fibrous structure applications
where the geometric deformations are often large.
-------
on the tightness of the weave. Consequently the fabric is less resilient in
tension than a parallel assemblage of the same number of fibers, that is, it
absorbs less strain energy before rupture. Kaswell cites the importance of a
fabric's elongation-recovery properties in numerous industrial applications.
Strength and modulus arc further complicated by dependencies on
humidity, mechanical fatigue, and of course the manufacturer's processing
and finishing variables. Backer and other at MIT and elsewhere ' have
done much to relate fabric and fiber physical properties, and much of this
developing science may be useful in obtaining improved filtration fabrics.
4.6.4 Permeability
The resistance to flow of gases through porous materials has
been discussed in Chapter 2. The permeability of filtration fabrics is so
decreased by the residual dust deposit that the permeability of the clean
fabric appears to have little to do with its use.
The objective in fabric design is to maintain a highly permeable
residual dust and fabric combination, while yet passing a minimum amount of
dust. Toward this end the pores through a fabric must be closely controlled.
They must not exceed a certain bridging diameter. (Section 2.3.2). If the
pores are too small they will cither plug or pass too little gas, which dis-
tributes itself according to the square of the pore diameter. In an ideal
filtration fabric probably all the fabric pores should be the same size,
the size depending on dust properties, etc.
Pore size and thus fabric permeability are dependent on cloth
24
design structure. Kaswell indicates reductions in permeability with in-
creasing pick count and also with pick diameter as would be expected with
the decreasing pore sizes. Filling twist was found to have a greater effect
on fabric porosity and permeability for a given fabric than any other con-
structional variable; the tighter the filling yarn the more permeable the
fabric. This would be expected for any fabric passing more air between than
through the yarns.
Pore size must not be visualized as simply the distance between
crossed cylinders, however. For all but filament yarns there will be nume-
-------
rous Cibor ends protruding into the pores, and in the extreme ease (napped
or felted materials) the pores will be primarily between fibers rather than
between yarns. In such cases the uniformity of spacing is equally important,
but the means of achieving it are less dependent on weave. Yarn texturizing
and post-weave surfacing treatments contribute much to the permeability of
the clean fabric, and perhaps also to the residual dust deposit permeability.
4.6.5 Dust Deposit Release
The ability of the fabric to release the deposited dust will
dopond on the mode and intensity of cleaning and also on the adhesive.
character of the fabric. Cleaning and fabric adhesion are discussed in
Chapters 2 and 6. The way in which fabric construction relates to deposit
release has not been determined, but is presumed to depend partly on the
electrical resistances of selected fibers. Resistance is seen to depend
on humidity, which is independently known to have a marked effect on fil-
tration fabric performance (Section 2.2.2)
4.7 AVAILABLE FABRICS
There are at least 50 U.S. manufacturers of dust filtration fabrics
or filter elements for dust collectors. The list of manufacturers in Appen-
dix 4.4 resulted from a 1969 survey of nearly 200 companies believed to have
interests in the kind of filtration fabrics used in dust and fume collectors.
The list is representative of U.S. fabric manufacturers and suppliers,
but undoubtedly does not include all such firms, and may not accurately
represent the interests of every firm listed. For example a firm although
not specifically mentioning a fabric product in its brochures is often able
to supply the product on short notice. Other firms prefer to specialize in
certain fibers, filter element types, etc.
Chapter 7 discsses the purchase costs of filter elements, filter fabric,
and typical fibers in some detail. As indicated in Chapter 1, the market
for dry filtration fabrics is estimated at $15 to 30 million annually, about
half the fabric going into new collection equipment and half replacing fabric
which has worn out.
-------
The distribution of fabric manufacturers, while not analyzed in detail,
may be assumed similar to the distribution of filter equipment manufacturers.
That is, the typical fabric manufacturer has one half to one million dollars
in sales, and the largest manufacturers have sales of several million
annually.
4.8 REFERENCES FOR CHAPTER 4
1. J. J. Press, Man-made Textile Encyclopedia, Textile Book Publishers,
Inc., Div., Interscience Press Inc., New York (1959).
2. J. P. Stevens & Co., Inc., Selecting Fabrics for Filtration and Dust
Collection. Bulletin, New York (1961 est).
3. A. A. Dembeck, Guide to Man-Made Textile Fibers and Textured Yarns
of the World, 3rd Ed., The United Piece Dye Works, New York, (1969).
4. M. Harris, Ed., Handbook of Textile Fibers, Textile Book Publ. Inc.
(Interscience) New York (1954).
5. A. N. J. Heyn, Fiber Microscopy; A Textbook and Laboratory Manual,
Interscience Publishing, New York, 1954.
6. J. M. Preston, Modern Textile Microscopy, Emmott & Co., Ltd., London,
1933.
7. H. Bunn, Ind. Eng. Chem.. 44: 2128 (1952).
8. M. D. Potter and B. P. Corbman, Textiles; Fiber to Fabric, 4th Ed.,
McGraw-Hill Book Co., New York (1967).
9. P. L. 85-897, 85th Congress-Second Session, Textlie Fiber Products
Identification Act. 15 U. S. Code 70, 72 Statutes 1717 (1960).
10. E. R. Kaswell, Wellington Sears Handbook of Industrial Textiles,
Wellington Sears Co., Inc., N. Y. (1963).
11. Albany Felt Company, Dry Filtration Manual, Technical Bulletin,
Industrial Fabrics Div., Albany, New York., (1968).
12. W. E. Morton and J. N. S. Hearle, Physical Properties of Textile
Fibers, The Textile Institute, Butterworths, London, (1962).
13. American Society for Testing and Materials, Standards on Textile
Materials, Part 24, p. 39 (1965).
14, M. L. Joseph, Introductory Textile Science, Holt, Reinhart & Winston,
Inc., (1966).
-------
15. M.H. Curley, Man-Made Textile Encyclopedia, New York: Textile B<«ok
Publishers, p. 229, (1959).
16. J.W.S. Ik-arlr, P. Grosberg, and S. Hacker, Structural Mechanics of
Kiln-fa, Yarns, and Fabrics, Vol. 1, Wiley-Intcrscience, N.Y., (1969).
1.7. Modern i'lastics Encyclopedia, 45: 14A, (1968).
18. T.W. Spalte, J.K. Hagan, and W.F. Todd "A Protective Finish for Glass-
Kibcr Fabrics", Chcm. Eng'g. Prog. 59:4, 54 (April, 1963).
I1). J.A. Sal vatorr. A Study of. the Influence of Selected Paramo tors on
tin- Work to Abrade a Fabric Using a Modified Touting Device, M.S.
Thesis, Lowell Technological Inst., Lowell, Mass. (1968).
20. E.H. Allard, Wet and Dry Abrasion Resistance of Substrates as
Measured by the Stoll Abrader with the. Flex Element, K. S. Thesis,
Lowell Technological Inst., Lowell, Mass. (1969).
21. S. Backer, "The Relationship between the Structural Geometry of a
Textile Fabric and its Physical Properties; II. The Mechanism of
Fabric Abrasion, Textile Research Jnl.. Q, 453 (1951).
22. S. Backer and S.J. Tenenhaus, "The. Relationship between the Structural
Geometry of a Textile Fabric and its Physical Properties; III. Textile
Geometry and Abrasion Resistance", Textile Research Jnl., 21, 635 (1951)
23. W.J. Hamburger, "Mechanics of Abrasion of Textile Materials", Textile
Research Jnl. . 15_, 169 (1945).
24. E.R. Kaswell, Textile Fibers, Yarns, and Fabrics, Reinhold Publ. Co.,
N.Y., (1953).
25. Lord Rayleigh, "The Sand Blast", Nature. 9_3, 188 (1914).
26. F.A. Zenz and D.F. Othmer, Fluidization and Fluid-Particle Systems,
Reinhold Chem. Engg. Series, Reinhold Publ. Co., N.Y., (1960).
; : i
27. E.P. Parker, Abrasion Resistance of Substrates as Measured by the
Sand Blast Method, MS. Thesis, Lowell Technological Inst., Lowell,
Mass. (1967).
28. E. Rabinowicz, Department of Mechanical Engineering, Massachusetts
Inst. of Technology, Cambridge, Mass., Unpublished Notes, (1964).
29. E. Rabinowicz, "Wear", Scientific American. 206:2. 127 (1962).
30. W.D. Freeston, M.M. Platt, and M.M. Schoppe, "Mechanics of Elastic
Performance of Textile Materials; Part XVIT: Stress-Strain Response
of Fabrics under 2-Dimensionai Loading", Textile Research Jnl...
37.: 11, 948 (1967).
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CHAPTER 5
ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS
TABLE OF CONTENTS
5.1 DESCRIPTION OF PROCESS EFFLUENT TO BE FILTERED 5-4
5.1.1 Gas Flow 5-5
5.1.2 Gas Properties 5-6
5.1.3 Dust Flow 5-6
5.1.4 Dust Properties 5-6
5.1.5 Variability in Aerosol Composition 5-7
5.1.6 Emission Requirements 5-7
5.2 DUST COLLECTOR DESIGN • 5-8
5.2.1 Pressure Drop 5-8
5.2.2 Air/Cloth Ratio 5-9
5.2.3 Cleaning Mechanism and Fabric 5-11
5.2.4 Cloth Area 5-12
5.2.5 Cloth Life 5-13
5.2.6 Housing Configuration 5-13
5.2.6.1 Number of Compartments 5-13
5.2.6.2 Fabric Arrangement 5-14
5.2.6.3 Compartment Structure 5-14
5.2.7 Capital Cost Estimates 5-15
5.3 FAN AND DUCTING DESIGN 5-16
5.3.1 Ducting Layout 5-16
5.3.2 Ducting Costs 5-17
5.3.3 System Pressure Drop 5-18
5.3.4 Fan Selection 5-19
5.3.5 Minimizing Fan and Ducting Costs 5-22
5.4 PERIPHERAL EQUIPMENT, INSTRUMENTS, AND CONTROLS 5-22
5.4.1 Particulate Pre-Conditioning Equipment 5-22
5.4.2 Gas Pre-Conditioning Equipment 5-23
5.4.3 Instrumentation ' 5-25
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TABLE OF CONTENTS (Continued)
5.4.4 Control Equipment 5-25
5.4.5 Dust Disposal Equipment 5-26
5.5 FINAL SYSTEM DESIGN 5-26
5.6 PROCUREMENT AND RESPONSIBILITY 5-28
5.7 REFERENCES FOR CHAPTER 5 5-30
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CHAPTER 5
ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS
The design of a fabric filter system is similar to most engineering
design assignments in that there is no single rigorous approach to the
solution of the problem at hand. Many parameters must be identified and
their inter-relationships understood. Placing an important variable in
proper perspective whether it be, for example, the fabric cost or gas
stream dew point, represents a key step in the design process. The pri-
mary purpose of this Chapter is to highlight the many factors that must
be considered in designing the overall system. Since many of these inter-
relationships are discussed in greater detail in other sections, a very
general procedure for selecting and/or designing the system components
is presented here, with the emphasis placed upon the pitfalls and expec-
ted problem areas.
The overall performance of the filter system will only be as good
as that of the poorest functioning component. Therefore, only those
well experienced with fabric filter equipment should assume the respon-
sibility for system design, especially of new equipment and new appli-
cations. The replacement of existing equipment is somewhat easier since
one does have some' practical guidelines. In general, the first decision
that one must make is whether to do the job in-house, or to contract for
a "turn-key" job in which the entire system is provided by an outside
firm. In either case, the best available consultation should be sought
and the purchaser should retain the responsibility for complete descrip-
tion of the process effluent to be filtered. Once the above groundwork
is established, the experienced engineer is presented with a tractable
design problem which he can undertake in the conventional iterative
manner and which involves the following main design steps:
1. Define the effluent -- mass flow rate, dust properties, gas
properties, and process variations with time.
2. Approximate the collector design
3. Approximate the fan and ducting needs.
4. Based on (2) and (3) select peripheral equipment, instruments,
and controls.
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5. Repeat (2) and (3) to minimize the estimated total system cost. ,-
6. Review for alternative effluent control methods; budget; and
procure. '
The first three steps can be carried out approximately without the cost
data for individual system components by using the guidelines set forth
in this handbook. In later stages, specific apparatus must be evalu-
ated, in conjunction with equipment supplier support, to arrive at real-
istic estimates of initial costs. '•'
Operating, maintenance and overhead costs are also essential con-
siderations in preparing design specifications. Even the accounting
practices of the purchasing company may decide the relative attractive-
ness of one system over another, e.g., the depreciation of a low first ~'
cost, higher maintenance system versus that of a higher capital cost in-
stallation. Total annual cost is probably the best criterion to use (see _,
Chapter 7). Other criteria of performance that are difficult to assign
a cost to, but which affect the overall quality of the system, include
..'•y
the emission level of the equipment, its reliability, simplicity of
operation, etc. Although the latter factors are difficult to quantity
the design engineer must take them into account.
5.1 DESCRIPTION OF PROCESS EFFLUENT TO BE FILTERED
The definition of the problem, which is common to any engineering
design effort, constitutes the basis for its effective solution. In the
case of fabric filtration, the crux of the problem is to define as com-
pletely as possible the process effluent properties. The minimum infor-
mation required is listed in Table 5.1. So important are these data ~
that, in almost all cases, preliminary stack samplingishould be perfor-
med and, in some instances, tho entire operation should be simulated on - _^
a bench or pilot scale with experimental dust generation and filtration
facilities.
If a process is characterized by variations in gas flow and/or gas
particulate composition, the equipment must operate at peak loads with- ,
out media plugging, as well as at reduced flows where condensation may
occur. Any potential future increase in effluent loading should be con-
-------
sidered in estimating design capacity, since an initial overdesign is
often less costly than subsequent reconstruction.
It appears highly advisable to prepare a brief summation as out-
lined below in which the key properties of the effluent and the control
requirements are Listed. Most fabric filter manufacturers submit a
similar questionnaire to potential customers prior to quoting on new
equipment.
TABLE 5.1
EFFLUENT AND FILTERING REQUIREMENTS
1. Process Effluent
(a) Gas flow:
Average:
Maximum:
Temperature:
Water Content:
Other Constituents:
(b) Dust flow:
Average:
Maximum:
Six.e distribution:
Size: <1 <5 <20 -50 <80 <95
%: __ _ __ _ _
Particle density: »
Bulk density:
Est. range of K2:
Other properties:
3. Exhaus t
2. System
(a) Preferred location -
(in) (out)side
(b) Space limitations, if any
(c) Ambient weather
Range:
Temperature:
Snow, water, wind loads
(d) Weight requirements
<9Q um (e) Cost considerations
(a)
(b)
(c)
(d)
Particulate level:
Gaseous req'ts:
Visibility req'ts
Preferred exhaust location:
(in) (out) side
Distance from collector:
5.1.1 Gas Flow
Determine the volume of gas emitted by the dust generating
process prior to any corrective adjustments of temperature or dew point.
-------
Should there be temporal variations, the maximum, minimum, and average flows
should be estimated. The cost of the fabric filter system will be approx-
imately proportional to the volume of gas emitted by the process. There-
fore, it is imperative that one establish what gas volumes will obtain
when practicable process changes and closure or all unnecessary vents
of leak points are considered. One should anticipate, and be prepared to
defend, process changes that constitute minor expenses relative to the
savings achieved through reduced gas handling capacity.
5.1.2 Gas Properties
Determine the temperature and pressure of the carrier gas
stream and estimate its approximate water content. Identify any abnormal
gaseous constituents such as acid vapors, toxic and/or corrosive fumes,
combustible or explosive materials, condensibles, etc. Determine whether
composition and/or concentration vary significantly with time, particu-
larly during process start-up or shut-down operations.
5.1.3 Dust Flow
Determine the weight (mass) rate of dust or fume generation
by the process, again making certain that the quantity has been minimized
as much as possible by process adjustment. Variable load conditions,
particularly peak values, must be considered in determining filter capa-
city if overloading or plugging is to be avoided. Standard procedures
for measurement of effluent properties are described in several test
manuals (for example, see reference 1).
5.1.4 Dust Properties
The better the system designer understands the properties of
the dust particles (see Chapter 2), the easier his task of designing the
filtration system becomes. Minimal information for developing a functional
system, however, must include a characterization of the particles in
terms of mean or median diameter and,if possible, the distribution of
sizes. A knowledge of effective densities for discrete and bulk par-
tides as well as an estimated permeability (K») for the dust is also
useful in establishing filtering conditions.
-------
The value i>£ (K?), which is thought to reflc-ct tlie integrated
effoctfl of particle dimensions; e.g., length to diameter ratio, cohesive-
ness between particles, dust cake rigidity, and spatial arrangement of
particles in the dust cake, is a valuable design tool. On the other
hand, individual measurement of the factors which presumably determine
the K- are not as yet sufficiently understood to make their quantifica-
tion possible. Additional dust properties for which no strict quantita-
tive definition is currently made, but which constitute important in-
puts for system design are: the softness or stickiness of the dust as a
function of temperature or humidity; abrasiveness; agglomerating charac-
teristics; "seeping" tendency; adhesion of the dust cake to the fabric.
5.1.5 Variability in Aerosol Composition
Allowance should be made for the fact that.even without inten-
tional modification of the gas temperature or the particle size proper-
ties, there may be radiation cooling, moisture leakage into the gas
stream, agglomeration of the smaller particles and/or sedimentation of
the larger particles, or other changes during transit through the system.
Therefore, one must attempt to define the aerosol as it enters the filter
unit.
5.1.6 Emission Requirements
The degree of particulate control which must be attained
with the overall filter system should be determined early in the design
process. This will usually be stated as a maximum tolerable weight
emission rate rather than as a system efficiency. The requirements may
also specify other factors that must be considered, e.g., toxic gases,
odors, or visibility of particulate or steam plumes.
It must also be decided whether the filtered effluent can
be discharged directly to the outside environment (with the attendant
problems of heat loss, make-up ventilation and visible exhaust) or re-
leased within the building. In the latter case, the problems of heat,
materials toxicity, nuisance and/or hazard in the event of filter rup-
ture, and noise take on added importance because of confinement.
-------
5.2 DUST COLLECTOR DESIGN
The design criteria applying to the collector itself will, in turn,
affect those for other system components. During the early design phase,
however, it may not be possible to predict the interrelationships be-
tween design criteria and costs. Therefore, until realistic trade-offs
can be established among collector size, fan requirements, and hoods and
ducting, cost estimation within approximately + 50 percent is acceptable.
As a subsequent aid to cost optimization, it may be helpful to construct
a table by which one can estimate how the collector cost will increase
with respect to an increase, say 10 percent, in any significant variable:
^
•rl
8
r~*
t>
M
0)
4J
r-l
•rl
A 10% increase in this variable: V
0)
M
s
cd
4)
(X
0)
H
T
0)
M
3
M
0)
£
U
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ri a
,a o
P
01
M
C cd
•rl M
O 01
Qu O«
si
DT
H
01
4J
Q)
•rl
Q
Ol
r-l
U
•H
4J
a
D
P
r-l
g
r-l
PM
a
01
3
4-1
H
Q
$
•rl
1
Q
.J
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I
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tance
w
•rl
M
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U
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8.
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K2
Will add this much to the
INITIAL COLLECTOR COST: ($)
ANNUAL SYSTEM COSTS: ($)
The effects of selected variables on the annual operating and maintenance
costs may also be estimated,as Indicated, since the collector design has
a strong affect on the annual costs (Chapter 7).
5.2.1 Pressure Drop
Estimate the average pressure differential across the filter
media and deposited dust layer during normal operating conditions.
Although the value selected may be somewhat arbitrary, several practical
considerations, e.g., collector strength under pressure or vacuum, fan
power requirements, and dust cake mechanics, point to a few inches of
water as the optimum pressure drop. Lacking any better design guides,
-------
3 inches of water is acceptable as a typical value. On the other hand,
the use of high velocity filtration, felted fabrics, or the presence of
a sticky or low porosity dust cake often require that optimum pressure
drop be of the order of 8 to 10 inches of water. Pressure loss through
the collector alone, exclusive of the media drop, is usually small com-
pared to that of the loaded fabric. The pressure loss associated with
the duct, hood and stack system will probably be in the same range as
that for the collector with fabric.
The pressure drop through the combined fabric and dust layer
can be treated as an independent variable in the sense that the design
engineer can exercise considerable control over the cleaning mechanism.
By increasing the intensity and/or frequency of cleaning it is possible
in some cases to reduce pressure drop to levels approaching those of the
clean fabrics. If this concept is carried too far, however, the collec-
tion efiiciency may be lowered, the fabric itself damaged, and the power
costs for driving the cleaning mechanism increased to prohibitive levels.
Thus, the selection of the optimum operating pressure loss becomes a
matter of trade-offs based upon engineering judgement and field trials.
Since the final operating pressure loss may not necessarily conform to
the original design point, it is not practical to over-refine the pre-
liminary estimates of average and peak pressure drops.
5.2.2 Air/Cloth Ratio
2
This ratio (CFM of air filtered per ft of cloth filter area)
is very important in determining collector performance. The ratio (or
its equivalent, filtering velocity) is discussed throughout this handbook
in connection with dust deposit characteristics, collector configurations,
collector efficiency and pressure drop and maintenance requirements, to
mention some principle effects related to air/cloth ratio. Ratios in
current use range from less than 1:1 to more than 20:1. The choice de-
pends on cleaning method and fabric, and on characteristics of the par-
ticles .
There is no precisely determinable ratio for a given applica-
tion as the choice also depends on estimates and trade-offs, such as
-------
initial collector cost and recurring power costs. Consequently, there is
no precise analytical method for determining the best air/cloth ratio,
although Chapters 2 and 6 indicate several approaches to the construction
of analytical models. Instead, it is customary to select ratios based
on similar previous experience, that is, ratios that have been.proven on
similar combinations of cleaning method, fabric, and dust.
Each dust collector manufacturer has guidelines for the selec-
tion of air/cloth ratio, based on his experience with a variety of appli-
cations. These guidelines vary from manufacturer to manufacturer, largely
as a result of differences in equipment. Four such guidelines are sum-
marized in Appendix 5.1 for a shaking bag collector, a glass cloth col-
lector employing principally reverse flow plus flexural cleaning, a
reverse jet collector, and a reverse pulse collector. These are typical
of the guidelines that manufacturers have made publicly available.
Normally, these guidelines should enable estimates to within at least
25 percent of the optimum design ratio. In unusual cases,and for a more
exact estimate, consultation with an experienced manufacturer is advis-
able. Frequently, in new applications a pilot study has been used to
determine the best air/cloth ratio. Such studies can be misleading,
however, unless they accurately model the proposed equipment and use a suit-
able aerosol.
There is an optimum air/cloth ratio for each set of filter
system design parameters: i.e. dust to be filtered, configuration of filter
system, cleaning mechanism to be employed, fiber material to be utilized and
configuration of the fiber media. However, for a given set of the above
parameters, the total system cost versus air/cloth ratio relationship is
rather flat near the optimum ratio. Thus there is a tendency to minimize
initial costs by selecting an air/cloth ratio toward the high end of the
range. On the other hand it is frequently reported that, with lengthy
operation at two different filtration velocities, the lower of the two
filtration velocities results in lower operational costs. Thus one must
carefully weigh the traditionally cited advantages of lowered air/cloth
ratio (i.e. lower power costs, decreased maintenance and higher collection
efficiency) against the larger initial capital costs associated with
-------
increased collector size and the penalty for its space occupancy. Further,
as pointed out earlier in Chapters 1 and 2, it is anticipated that with
advancing fiber, fabric media and cleaning technology the optimum
range of air/cloth ratios, for any set of design and operating parameters,
will tend to increase.
5.2.3 Cleaning Mechanism and Fabric
The selection of the cleaning mechanism arid the filter fabric
are best made together, since both items ate closely related (Section
3.3). For example, felted fabrics are almost exclusively cleaned by
pulse or reverse-jet air, whereas most woven fabrics are cleaned by
other means. Of the relatively few choices of fiber media (approximately
eight, see Section 4.2), most will be eliminated for reasons such as
poor temperature and/or corrosion resistance or excessive cost. Of the
several cleaning mechanisms used in filtration systems, only two or
three will meet the specific requirements for a given installation: i.e.,
high, low or moderate dust loadings; continuous or intermittent operation;
ease of removal of dust from the fabric; small floor area; minimal pressure
drop; high efficiency; etc.
By a process of elimination, therefore, a review of past
successful filtering performance will usually show that only a few
cleaning mechanism - fabric combinations are compatible and sufficiently
attractive to warrant economic evaluation. The time required for
cleaning also determines the choice of cleaning mechanism. This time
should be a small fraction of the time required for dust deposition,
since otherwise too large a fraction of the fabric will be out of ser-
vice for cleaning at any given time. It is common with shake cleaning
equipment, for example, to have a cleaning-to-deposition time ratio of
the order of 0.1 or less. Applying this criterion, having a ten compart-
*It is not intended here to exclude or impugn the merits of a
hitherto untried combination. The design approaches set forth in this
chapter, however, are based upon successful precedent or at least qual-
itative findings which lend some substance to cost estimates. If it
is desired, or is absolutely necessary to consider some novel combination,
one should view the problem as a research and development effort and not
as a routine fabrication process. The path that should be followed will .
be more apparent once preliminary cost estimates have been prepared,
based upon conventional design approaches.
-------
ment baghouse would mean that one compartment is out of service at all
times. Therefore, the choice of cleaning mechanism affects system size
as well as fabric life, maintenance, etc.
5.2.4 Cloth Area
The amount of fabric in actual use at a given time is found
by dividing the estimated total flow entering the collector by the sel-
ected air/cloth ratio. The volume flow rate of the effluent entering
the collector will not necessarily be the same as that discharging from
the generating process, owing to temperature changes, the added volume
of vaporized cooling water, and dilution air which may be added deli-
berately for cooling purposes or accidentally by air leakage. The latter
factor may contribute to a significant flow increase in systems operating
under large negative pressures. In filter applications involving a vary-
ing flow, some judgement is required to decide whether to size the
equipment for the peak flow, the average flow, or for some intermediate
point. It is again necessary to seek a compromise between the increased
cost of larger equipment and the increased cleaning cost, and the possible
risk of fabric damage associated with short term, high pressure drops.
Except for certain systems that are operated intermittently,
e.g., a few hours on line followed by cleaning only during down time,
most filtration units will require reserve fabric capacity to allow for
off-line cleaning, inspection and maintenance. Since it is common prac-
tice to isolate temporarily defective filter units until it is convenient
to replace them, additional reserve capacity may be required. The total
fabric area to be installed can be estimated by multiplying the area to
be in actual use at any time by the term
(1 + TR+F) (5.1)
where (T^) is the time ratio (cleaning to deposition) discussed in the
K
preceding section and (F) the fraction of the fabric area expected to be
out of service at any time due to replacement, inspection, or mainten-
ance operations. Judicious timing of the above procedures in relation
to peak flow periods may, however, reduce the multiplying factor.
-------
Costs for Hie installed fabric media as a function of unit
size may be estimated from data given in Section 7.3.
5.2.5 Cloth Life
Fabric deterioration often results from the combined assault
of several factors (Chapter 8), rather than from any single effect such
as thermal erosion, mechanical stress through repeated flexure, chemical
attack, abrasion, etc. All possible modes of failure should be con-
sidered during the preliminary design phase. Again, previous experience,
especially that relating to similar fabric-cleaning-dust applications,
may be the best and only guide. Extrapolating from experience, one might
estimate that the reduction in fiber life through thermal erosion might
double for a 20 F rise in temperature, or that the mechanical attri-
tion rate might double when the frequency of cleaning is doubled.
Generally, it should be possible to estimate fabric life
within a factor of two in situations where no direct experience can be
cited. If performance data are available, estimated reliabilities may be
upgraded to perhaps + 20 percent, which is the order of dependability
of the best pilot plant data. Having established a reasonable estimate
of fabric life, one can then reach an annual cost figure for fabric media.
5.2.6 Housing Configuration
5.2.6.1 Number of Compartments. - Selecting the number of
separate compartments for a fabric filter installation is a relatively
easy decision. The basic information required is the allowable variation
in gas flow with respect to process or plant ventilation, the availabi-
lity of sizes of commercial units (compartments or filter house modules)
and the expected frequency of maintenance. In small collectors, indivi-
dual compartments may contain as little as 100 square feet of fabric sur-
face, although collectors as large as approximately 50,000 CFM capacity
may also have only one compartment. Multiple compartments of almost any
size may be chosen, subject to availability. With the exception of re-
verse jet and pulse jet units, at lease one compartment will be out of
service during the cleaning cycle. It may also be necessary to provide
-------
.1
additional compartments for emergency, extended maintenance, or unexpec-
ted increases in process effluent.
5.2.6.2 Fabric Arrangement. - Fabric filter media in panel,
tube, and bag form represent the most commonly used industrial configura-
tions. Although the selection of filter geometry may occassionally be
a matter of preference, the type of fabric and the cleaning method
usually dictate the configuration. Panel filters, for example, cannot
be cleaned by flexure, and reverse jet cleaning requires open ended
tubes. Other important considerations are discussed in Section 3.3.
Panels, tubes, or bags are commercially available over a
broad range of dimensions as discussed in Section 3.1. Once the require-
ment for fabric surface per compartment is determined, it remains to
decide what combination of filter length, diameter, and spacing will be
the least expensive. This is too often interpreted as a maximum filter
packing and a rather compact filter housing, i.e., a container with no
dimension much greater than twice the smallest dimension. Closely packed
filter elements tend to wear against one another, and make inspection and
maintenance difficult (Section 8.3). Compact housings incur a greater
cost for plant floor space than taller units. Recently, one manufacturer
has introduced super-long bags (—60 ft), partly to conserve floor
(3)
space . Compact housings may, however, be more suitable in outside
locations where land use costs less than the erection of tall structures.
Other considerations in selecting the fabric configuration are
downward vs. upward flow and inside vs. outside filtering. The ease and
the frequency of fabric replacement, and the uniformity of flow distri-
bution and associated danger ;of condensation in stagnant air pockets are
also considerations.
5.2.6.3 Compartment Structure. - Although many kinds of
fabric collectors can, in principle, be operated either under negative
or positive pressure, the larger units are often custom designed for one
condition or the other to minimize costs. Least expensive is the instal-
lation needing no housing at all, in which the particulate is collected
on the interior surfaces of a positive pressure bag system. The danger
of cooling below the dew point is an important consideration, particu-
-------
larly in a suction housing where infiltration leakage often occurs unless
a more expensive gas-tight design is adopted. Thermal excursions of the
collector during startup and shutdown can effect the sealing character-
istics of critical gasketed connections, depending on the structural
materials. Interconnection of compartments via ducting or hoppers and
their isolation, especially during the cleaning cycle, are important
considerations (Section 3.4). The generalized rules for designing the
compartment structure are not well defined. Construction materials are
likewise not generally prescribed. The most common housing material is
steel, as discussed in Chapter 3. The choice of material depends on the
nature of the dust and gas mixture and their flammability, corrosiveness
etc. Lacking a more specific guideline, one would do well to follow pre-
cedent in selecting materials of construction, metal gages and dimensions.
5.2.7 Capital Cost Estimates
A detailed discussion of initial and annual cost factors is
presented in Chapter 7. The cost of the baghouse, including its amorti-
zation schedule and plant overhead costs,should now be estimated based
upon the tentative equipment selections. The designer should now devote
primary attention to initial installation costs with emphasis upon how
these initial costs will contribute to the total annual cost of the fabric
filter system operation, in accordance with specific company costing pro-
cedures. At this time, sufficient information has been acquired to com-
plete the working estimate guide at the beginning of Section 5.2.
Some of the design decisions cited above may have been based
upon very limited data. Consequently, one should review the compilation of
preliminary estimates, particularly those reflecting choice of fabric,
cleaning mechanism, and average pressure drop through the filter media.
The objective should be to describe a collector that will be within ~10
percent costwise of the optimum design. It is not advisable, however, to
finalize the design until other components of the overall system have
been selected and evaluated with respect to their possible influence on
collector design.
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5.3 FAN AND DUCTING DESIGN
The design and selection of filter system components, other than the
collector discussed previously, also require some iterative
approaches during the planning phase. Fortunately, the alternatives are
fewer, and in most cases, the design features are simpler. The main
trade-off area is between the pressure drop through the ducting and stack i
systems (an inverse function of duct diameter) and the fan size and cost. —'
In many cases, the pressure drop through the fabric filter unit alone
will largely determine fan size. _j
5.3.1 Ducting Layout ,
The positioning and amount of ducting are determined mainly
by the expected locations of effluent source, collector, fan, and venting
point. Ordinarily, minimum duct lengths and diameters will be used un- —'
less the ducting is also intended to provide radiant cooling of the gases
prior to filtration, or to function as settlement chambers to remove the _J
larger particles. The first step in design is to establish the locations
and lengths of main, branch and riser ducts showing types of junctions '
and bends.
The distance from the dust collector to both the dust source !
and the point of discharge of the filtered effluent, must be considered
with regard to duct costs and space availability. The cost of plant floor i
space may be a reason for locating the collector outside the plant, other
possible reasons being ceiling limitations and safety. On the other hand, i
the outside collector must usually be protected against weather and insu- ~
lated against temperature changes. Also, not to be overlooked in locat-
ing the collector are space clearances for, and accessibility to; the nee- _J
essary ducting; crane or hoist requirements, especially for fan repair;
the weight of the collector; the feasibility of removal and disposal of
the collected dust; and plant insurance costs.
In laying out the duct plan, the locations of cleanout doors, '
•••*«••
dampers, dilution valves and any auxiliary cleaning equipment, i.e., dry
inertial collectors, scrubbers, or cooling towers should be taken into '
-------
consideration. Whenever possible, all ducting should be located so as
to be easily installed and maintained.
It is important to use as little horizontal ducting as pos-
sible and to select diameters which allow for complete transport of the
dust load to the collector. Sharp bends and abrupt changes in dimensions
should also be avoided to prevent dust accumulation. Since the above
approach suggests high velocity systems, it should be recognized that
power costs also increase with higher duct velocities and that abrasive
dusts can cause rapid duct erosion, particularly in bends. Useful velo-
city guidelines have been set forth in Table 2.42(b) and in various engineer-
ing manuals to aid in this aspect of system design.
Duct sizes through the system are estimated by the recommen-
ded transport velocities and the calculated volume flow rate through each
section. The latter value is computed from the source flow, corrected
for any added volume attributable to dilution air, and adjusted to the
temperature and pressure conditions at the location of interest in the
duct system. In cases of fluctuating flow, it is recommended that the
ducting be sized for some intermediate flow between average and peak
loads.
The ducting sections located downstream of the collector may
be sized somewhat larger, owing to absence of settlable particulate in
the gas stream. The final dimensions should represent a balance between
the increased cost for larger duct size and the lower pressure drop
(power cost).
5.3.2 Ducting Costs
Standard steel ductwork is normally used, unless there is
danger of corrosion, abrasion, adhesion, high temperature, thermal dis-
tortion, need of insulation, or unusual surges in pressure or in tempera-
ture. Aluminum, glass, plywood, etc., can sometimes be used as substitutes
for steel. While the duct material may be suggested by the collector man-
ufacturer, the ducting is normally made up and installed locally.
Cost estimates or quotations are readily obtained as soon as
diameter, length, and guage specifications are available (Section 7.1.3).
-------
One must consider the coats of dampers, ports, Tee sections, elbows,
flanges or other special components, keeping in mind the fact that fit-
ting or adapting the ducting to the above parts may involve more cost
than that for assembling straight vertical or horizontal runs.
5.3.3 System Pressure Drop
The total system pressure drop is that of the combined losses
in the duct and the fabric filter unit. Ducting losses vary approximately
as the square of the gas velocity and can be readily calculated by standard
formulas or from tabular or graphical data (see Section 2.5). Pre-
sure drops for non-linear shapes (elbows, Tee's, reducers, etc.) are
usually expressed in equivalent length of straight duct of the same dia-
meter. In conventional practice, one traces the largest branch (usually
from the most remote source of dust generation), noting the temperature
and gas flow through each succeeding section up to the collector inlet.
A similar procedure is followed from the collector exit to the point
where the filtered effluent is discharged to the inside or outside atmos-
phere. Ordinarily, the total pressure drop associated with the ducting
alone will be in the range of 3 to 6 inches of water, although other
values may apply in some circumstances.
One then adds to the estimated pressure drop through the
ductwork, the collector pressure drop and that of any other component of the
system, e.g., a centrifugal collector. The result will be the net pre-
sure to be supplied by the fan when ambient pressures are the same at
system inlet and outlet. Should the ambient pressure at the system in-
let exceed that at the outlet, the net pressure requirement for the fan ,
is decreased by this difference.* The opposite applies if the pressure
gradient is reversed.
*The kinetic energy or velocity pressure retained by the air leaving
the fan may be deducted from the fan static pressure requirement, except
when the air is charged to a large stagnant space, e.g., exhausting the
fan outdoors. The kinetic deduction is frequently overlooked, however,
on the justification that by omitting it, a safety factor is added. See
the excellent discussion on fan selection in "Fan Engineering" by the
Buffalo Forge Company(6).
-------
5.3.4 Fan Selection
Of the types of fans commonly used in industrial ventilation,
centrifugal fans are most often chosen for the primary flow in fabric
filtration systems. These are described below and in Figure 5.1:
1. "Forward-curved blade types; A multi-bladed, "Squirrel Cage"
wheel in which the leading edges of the fan blades curve toward
the direction of rotation. These fans have low space require-
ments, low tip speeds and are quiet in operation. They are
usually used against low to moderate static pressures such as
encountered in heating and air conditioning work. Not recom-
mended for dusts or fumes that would adhere to the short curved
blades, causing unbalance and making cleaning difficult.
2. Straight or Radial-blade (paddle wheel, long shaving wheel):
The "workhorse" for most exhaust system applications, they are
used for systems handling materials likely to clog the fan
wheel as the name indicates. Such fans usually have a medium
tip speed and a medium noise factor and are used for buffing
exhaust, woodworking exhaust or for applications where a heavy
dust load passes through the fan.
3. Backward blade type; The type in which the fan blades are in-
clined in a direction opposite to the fan rotation. This type
usually has a high tip speed, provides high fan efficiency and
has non-overloading characteristics. Except in case of direct
driven arrangements, non-over-loading feature is over emphasized
in exhaust ventilation work as the exhaust system acts as a
load limiting orifice to make overloading of any exhauster motor
from variations in system conditions improbable. Blade shape is
conducive to buildup of material and fans in this group should
be used only on clean air containing no condensible fumes or
vapors."(7)
The first consideration in fan selection is which of the numer-
ous fan types is best suited to the estimated air flow and pressure re-
quirements. Reference to manufacturers' data depicting the performance
of various types over a range of speeds and delivery volumes will estab-
lish this. Another consideration in selecting the fan type is whether
it is to be used on the clean side of the filter or on the dirty side
where maintenance will probably be higher. (See also the discussion of
types of collector construction, Section 5.2.6.)
Although several fan sizes of a specific type can meet the de-
sired pressure-volume requirement by changing the speed, only one size
-------
\
BACKWARD CURVED BLADES
i
I
VOLUME - CFM
STRAIGHT OR RADIAL BLADES
VOLUME-
FORWARD CURVED BLADES
VOLUME-CFM
Figure 5.1. Some Fabric Filter System Centrifugal
Fans. (From Ref. 7).
-------
will perform at hits peak efficiency. Therefore, there may be no parti-
cular economy in selecting a smaller size unit of lower cost, since opera-
tion at a higher speed will lend to proportiately greater power consumption.
On the other hand, a slight oversizing of the fan will result in lower
fan speeds and probably less maintenance without a serious drop in ef-
fiency. Extra fan capacity is a safety factor against future process
expansion or unexpected peak flow requirements which the fan must be capable
of handling. Another consideration in fan selection is the reliability re-
quirement: Can a fan outage be tolerated, or should more than one fan be
installed?
The process of fan selection from the several trial sizes in-
volves the following steps. First, choose a maximum rated capacity,
roughly 20 percent more than the required capacity. Determine from the
table or fan curve the power and speed requirements at the required pres-
*
sure and flow, and from these data, estimate the installed fan and motor
costs and maintenance costs. Using an acceptable amortization rate, per-
haps ten years and the local cost of power, compute the total annual cost
of operating this particular fan size. Note that the cost of the drive,
mountings, and motor may easily double the cost of the fan alone.
Note that one is limited to available motor speeds if a direct
drive is to be used. Direct drives consume less power, need less main-
tenance, and present no belt slippage problems. On the other hand, many
small and moderate sized fans are belt driven, both for reasons of initial
cost and to permit future speed adjustments.
This initial cost estimate for fan and associated equipment is
probably adequate for the early phase of the system design process. To
minimize fan costs, however, the above computation should be repeated for
slightly larger or smaller fans until a minimum cost is reached. It may
become apparent before undertaking this step that a change in fan style
*Fan test data is usually based on standard temperature and pressure,
i.e., standard air density. If the fan is to handle non-standard
density gas, different curves will be used.
-------
or size should be considered, due to revisions in collector design pres-
sure drop and in ducting requirements. Having determined the fan arrange-
ment of minimum cost, again consider whether there is a sufficient mar-
gin of capacity and of reliability.
5.3.5 Minimizing Fan and Ducting Costs
Preliminary estimates of ducting requirements and costs should
be reviewed. It may be determined, for example, that a small increase in
duct diameter, say 5 percent, will allow use of a smaller fan with a net
lowering of overall cost; that is, the lower operating and capital costs
for the fan assembly overrides the increased initial costs for the duct-
ing. In this example, the decrease in pressure drop through the duct
system would be of the order of 20 percent based upon the inverse fourth-
power relationship between pressure loss and duct diameter. Having es-
tablished a revised value for duct pressure drop, new criteria are avail-
able for resizing the fan and motor drive. If the combined cost of the
duct and fan system decreases appreciably as a result of this reassess-
ment, the process should be repeated until a minimum point is identified.
It also is recommended that the above cost analysis be exam-
ined in terms of the key variables affecting the initial and annual costs
for the combined fan-ducting system. Again, a tabular array of the system
cost increase,as a function of change in the major design variables,con-
stitutes a useful working tool.
5.4 PERIPHERAL EQUIPMENT, INSTRUMENTS, AND CONTROLS
In earlier sections of this chapter, a preliminary design approach
has been outlined for the main components of the fabric filter system.
As the few remaining system parts are less dependent upon the primary
system dimensions, they should now be selected so that the overall fabric
filter system design can be finalized (Section 5.5).
5.4.1 Particulate Pre-Conditioning Equipment
The pre-filter particulate treatment process, if any, may be
designed for: (a) the enhancement of particulate agglomeration, thereby
precipitating out of the gas flow some material prior to deposition on
the fabric and/or improving the resistance properties of the dust cake,
-------
(b) the prc-separation of coarse particles by gravity sedimentation or
inertial techniques, or (c) the injection of additives to produce a dust
cake sufficiently dense to provide high efficiency collection of low con-
centrations of fine material, or alternatively additives to produce a
more porous dust cake with lower resistance.
Use of any of the above techniques may require reappraisal
of preliminary cost estimates for the fabric filter system for the fol-
lowing reasons (see Section 2.4.8 for details):
(a) Change of deposit permeability and hence cycle time,
due to alteration of particle size distribution.
(b) Change of inlet loading.
(c) Change of cake removability or residual permeability.
(d) Change in properties of the collected material such as
bulk density and total volume.
(e) Changes in gas stream properties.
One must balance the economic advantages of changes in dust properties
against the costs of installing and operating the pre-conditioning equip-
ment. Of course, if the treatment process is used to elevate collection
efficiency to levels satisfying pollution control regulations, the equip-
ment costs become a necessary investment.
5.4.2 Gas Pre-Conditioning Equipment
Hot process effluents are usually partly cooled before fil-
tering to reduce the flow volume which, in turn, decreases the filter
fabric area requirement. Lowering the gas temperature toward the dew
point often extends fabric life and may permit the use of less expensive
or more durable materials. Prior to entering the collector, the process
effluent may also be altered through combustion, absorption, chemical
reactions or humidification, any of which may influence the design of
other fabric filter system components. Temperature conditioning, a com-
mon treatment, is usually accomplished by the methods described in
Table 5.2. In other situations, it may prove more economical to filter
the gas essentially at process effluent temperature,or even at increased
temperature, provided that the temperatures do not exceed the upper limit
for the fabric.
-------
Table 5.2
METHODS OF TEMPERATURE CONDITIONING
Radiatton-Conyoction Coo1ing (long, uninsulated inlet ducts)
Advantages: Lowest flow volume of the three methods
Smoothing or damping of flow, temperature, pressure
or other surges or peaks in the process effluent
stream
Saving of heat (building space heating).
Disadvantages:
Cost of extensive ducting
Space requirements of ducting
Possibility of duct plugging by sedimentation.
Evaporation (by water injection well ahead of the filter)
Advantages:
Disadvantages:
Low installation cost, even with automatic controls
Capability of close and rapid control of temperature
Capability of partial dust removal and/or gas control
via scrubbing.
Danger of incomplete evaporation and consequent
wetting of the filter or chemical attack of the
fabric or filter
Increased danger of exceeding the dewpoint and
increased possiblity of chemical attack
Increased steam plume visibility, a hazard near highways
Possible increase in volume filtered.
Dilution (by adding ambient air to the process effluent stream)
Advantages:
Disadvantages:
Lowest installation cost, especially at very high
initial temperatures.
Substantial increase in total filtering volume
Automatic control of both temperature and filtering
velocity is not possible
Uncontrollable intake of ambient moisture, dust, etc
without prior conditioning of the dilution air.
* See also Ref. 8.
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Somewhat related to the three cooling methods outlined in
Table 5.2 are the special systems for emergency protection of the equip-
ment. These include:C02 devices that automatically release high-pressure
gas to cool the effluent, should its temperature exceed a critical pre-
set limit; fail-safe dampers held in position by set-point melting links;
burst-out panels to prevent duct or collector rupture due to pressure or
vacuum excursions; and various types of alarm systems. Such equipment
can be reliable and relatively inexpensive compared to the cost of re-
placing a collector system. Rapid-cool equipment can also serve in shor-
tening the time required to take equipment out of service. Heating equip-
ment may facilitate start-ups by preventing condensation.
5.4.3 Instrumentation
Instrumentation which provides a continuous record and/or
direct display of the important factors describing the overall filtration
process is an essential part of a good fabric filter system. The cost is
relatively low (~ a few percent of the total system investment) and the
judicious selection of instrument type and function may permit the use of
less expensive fabrics or materials of construction, e.g., ordinary sheet
steel rather than a special alloy. A discussion of recommended instru-
mentation is presented in Section 7.1.5. The measurement of those system
characteristics that affect its overall function, e.g., temperatures,
pressures (absolute and differential), primary gas and dilution air flow
rates, water rates, dew point levels, and possibly continuous monitoring
of stack oftluont concentrations, all enhance the probability of succes-
ful system operation. Any or all these devices can be designed to alert
the operator through appropriate warning systems (horn, alarm bell, flash-
ing lights) when abnormal conditions arise.
5.4.4 Control Equipment
Solenoid valves, damper actuators, timers, etc., associated
xtfith the filter need not be considered here, since they are usually in-
cluded in the basic filter price. Auxiliary blowers such as those for
reverse cleaning may or may not be included and air compressors for pulse
-------
cleaning arc seldom included as part of the filtration unit. Special
ducting dampers that must open and close rapidly or provide high leak
tight integrity (^ 0.1 percent leakage under adverse temperature, pres-
sure and corrosion conditions) should be specified in the system design.
5.4.5 Dust Disposal Equipment
The physical and/or chemical properties of the dust collected
in the hopper require appraisal at this point. Although the disposal tech-
nique is usually independent of the primary system operation, some pre-
treatment of the process effluent may contribute significantly to the
ease of handling of the collected dust. For example, the injection of
an inert mineral dust might reduce the tackiness of some resinous mater-
ials to the point where bridging problems in hoppers or plugging in screw
conveyors could be reduced, and the overall system economics improved.
The final selection of hopper design, hopper outlet valving,
vibration equipment (if needed), screw conveyors, chutes, pneumatic con-
veyors, and other ancillary equipment will depend upon several factors.
These may include the angle of repose of the dust, its bulk density and
the volume and/or weight to be handled. Another factor is the tendency
of the dust to flow freely or to bridge, agglomerate or become sticky
under mechanical stresses (stirring, rapid motion, vibration) or under
changes in temperature or humidity. One must consider whether the hopper
is to operate under positive or negative pressure, and also what labor
requirements are necessary for dust handling, i.e., whether the dust is
to be reprocessed, sold or dumped as waste. Several types of disposal
equipment are discussed in Section 3.4.2.
5.5 FINAL SYSTEM DESIGN
Having established preliminary design features for the individual
V
parts of the system, it is now necessary to integrate these into a com-
patible system. This process calls for more engineering judgement and
experience than that required for the preliminary design phase. In addi-
tion, the final planning should be geared to management's philosophy on
the relative importance of initial costs and total annual costs.
-------
Summary guideline tables outlined in previous sections should be re-
examined to determine whether overall costs can be lowered by adjustments
in any of the principal variables. Should any appreciable differences in
costing be observed, design optimization procedures should be repeated at
this time. Since the basic types of equipment required are now apparent,
appropriate manufacturers' equipment lines should be reviewed for prepar-
ation of specifications. It is highly advisable to consider standard
items when possible. Quality, expected service life, ease of repair and
delivery times, as well as initial cost will be evaluated. Where possible,
preliminary cost estimates or final quotations may now be obtained.
Before fixing the design specifications, a final search should be made for
unique or extenuating factors that could possibly influence performance
or total costs, such as safety features, noise, general plant ventilation,
insurance, relocation of system components, tax rebates, etc. One should
also re-examine future needs with respect to plant expansion or process
changes. If there remain some questionable design areas,or if certain
design aspects are controversial, it may be advantageous to seek the pro-
fessional opinion of some competent outside person(s) or agency. Other-
wise, final design and cost estimates can be submitted for approval and
procurement.
No further design or cost changes of significance are expected.
However, it may be possible to trim the overall costs a few percent by
A;.
the following analytical procedure. Let the total annual cost (C) be
expressed as the sum of: power costs (P); all costs related to fabric life
including materials and labor (F); annual distributed initial costs in-
cluding interest, taxes and any other item that is directly related to
initial installed cost (I); and fixed costs not considered elsewhere (X)
C=P+F+I+X (5.2)
Select a key design parameter, such as filter velocity (V), and introduce
it into each of the terms of this equation where applicable. In each
term, assign to the parameter its appropriate exponent (Table 5.4)."
-------
Power, for example, is approximately proportional to V . This means that
if a slightly different velocity (V) were chosen, the revised estimate
2
of power cost (P1) would be approximated by P' = P (V'/V) . Substitution
of the design parameter into the other terms of the equation yields the
following expression for a revised total cost (C1)
C- = P(V'/V)2 + F(V'/V)~°-5 + ICV/V)"1 + X (5.3)
That is, the result of changing from the design velocity previously
selected to a new value can be estimated by the expanded equation. Cost
factors P, F, I, and X have already been determined by careful design pro-
cedures. To estimate the effect of a 5 percent increase in filter velo-
city, the ratio (V'/V = 1.05) is used, and (C1) is readily evaluated.
By few repetitions of this process, the velocity that will give the
minimum total cost may be estimated. If this new estimate is appreciably
different from the original design value, a re-examination of the system
design is called for.
By this process, the effect of minor changes in any design parameter
may be estimated. For parameters having both positive and negative
exponents in Table 5.3 (filter velocity, Ave. Cloth Ap, etc.) an optimum
design value may be estimated. Note, however, that the parameter exponents
listed in Table 5.3 should be accepted as estimates only, owing to the
limited field and laboratory data available from which they derive. With
respect to trends, i.e., direct or inverse relationships, and weak or strong
dependency of a given cost category, the exponents cited in Table 5.3 may
be interpreted as tentative typical values. These exponents will not
apply uniformly to all systems. One is also not constrained to use the
precise grouping of cost factors cited here.
5.6 PROCUREMENT AND RESPONSIBILITY
The final responsibility for the design, fabrication, procurement
of materials, approval of specifications, performance testing, mainten-
ance, and the overall performance of the fabric filter system should fall
within one department and preferably with one individual of decision
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Table 5.3
APPROXIMATE OPTIMIZING EXPONENTS OF COSTS'
PARAMETER
Filter Velocity
Temperature
Particle Size
Ave. CLoth Ap
Cleaning Intensity
Flow Volume
Loading
Humidity
POWER
+2
+1
--
+1
-1
+1
+ .5
+--
CLOTH-RELATED
-.5
-1-2.75
-2
--
+1
+1
+1
+--
INITIAL
-1
+1.75
-1
-1
-1
+1
+.5
- —
Note: Fixed costs (X) are invariant by definition.
*
May vary from above, values according to circumstances.
insufficient information.
Blanks indicate
making capability. Although several departments or divisions within a
company may contribute their services, any joint sharing of responsibi-
lity should be avoided to minimize oversights, schedule conflicts, or
other committee type problems.
Following receipt of quotations for system components and installa-
tion costs, one should verify compliance with specifications, including
the warranty aspects of all components. This is equally important when
a turn-key package has been purchased. Although there will be occasions
when completely defensible design changes may be proposed by equipment
suppliers, one should not expect any extreme deviations in dollars or
design with a carefully executed program.
Fixing the responsibility for on-line fabric filter system perfor-
mance cannot be overstressed, especially with smaller installations.
Frequently, a manufacturer is blamed for faulty equipment operation,
whereas the problems have actually arisen because of user abuse and neg-
lect. This has led to dissatisfaction and experimentation in some plants,
-------
sometimes leading to the purchase of several fabric ftlt.er designs to
handle one type of process effluent. This makes maintenance routines
unnecessarily complicated. When dissatisfaction has resulted in purchase
from several different manufacturers, one cannot expect to receive inter-
ested customer service from any of them.
As a final point, few if any manufacturers will guarantee the per-
formance of fabric filter equipment, because its function is highly sen-
sitive to process effluent changes as well as to the quality of the in-
stallation job and subsequent maintenance. All of these are difficult
to document. Perhaps one of the best procurement policies is to request
the manufacturer to provide and guarantee installation and startup as
part of a package. He should also provide a set of guidelines for
operational routine and maintenance.at no cost, and he may provide
training for the men who will use the new system.
5.7 REFERENCES FOR CHAPTER 5
1. American Society of Mechanical Engineers, Determining the Properties
of Fine Particulate Matter: Power Test Codes. PTC-28, New York (1965),
2. The Buffalo Forge Company, Guide to Shaking Bag Aeroturn Selection.
Buffalo, New York, September 1968.
3. The Fuller Company, Collecting Hot Dust and Noxious Fumes with
DraccoR Glass-Cloth Dust Collectors, Bulletin DCS-IB (July 1967).
4. R. Frey and T. Reinauer, New Filter Rate Guide, Air Engineering.
(30 April 1964).
5. F. A. Zenz and D. F. Othmer, Fluidization and Fluid-Particle
Systems. Reinhold Pub. Co. (I960).
6. The Buffalo Forge Company, Fan Engineering. 6th Edn., Buffalo, NY,
(1961).
7. American Conference of Governmental Industrial Hygienists, Indus-
t;rial Ventilation, 10th Edn., Lansing, Michigan (1968).
8. P. W. Spaite, D. G. Stephan and A. H. Rose, High Temperature Fabric.
Filtration of Industrial Gases, J.A.P.C.A. 11, (5) 243, (May 1901).
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CHAPTER 6
FABRIC FILTER PERFORMANCE
6.1 INTRODUCTION 6'3
6.2 LABORATORY PERFORMANCE OF CLEANED EQUIPMENT f-9
6.2.1 Bench Scale Performance 6-9
d.2.1.1 Pressure Drop - Time Relationship 6-9
6.2.1.2 Dust Collection Efficiency 6-10
6.2.2 Single Bag Performance, Pilot Scale Tests 6-13
6.2.2.1 Pressure Drop - Time Relationship: 6-13
Basic Concepts 6-13
Liquid Filtration Analogy 6-17
6.2.2.2 Dust Profiles 6-20
6.2.2.3 Analysis of Mechanical Shaking 6-30
Residual Drag 6-33
Filter Capacity 6-38
6.2.2.4 Effects of Fabric Structure 6-49
6.2.2.5 Effects of Humidity 6-62
6.2.2.6 Effects of Velocity 6-6*
Hopper Fallout 6-6V
Particle Size Stratification 6-70
Deposit Consolidation 6-72
Particle Penetration 6-74
6.2.3 Single Compartment Performance 6-75
6.2.3.1 Shake-Type Collector 6-76
Light Dust Loading 6-76
Filter Aid 6-78
Periodic Shaking 6-80
Normal Dust Loading 6-81
6.2.3.2 Other Single Compartment Studies 6-83
Hopper Fallout 6-83
Particle Size Stratification 6-84
Deposit Consolidation 6-84
-------
6.3 LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT 6-85
6.3.1 Basic Pressure Drop Equations 6-85
6.3.2 Performance of a Multicompartment Collector 6-90
6.3.2.1 Studies with Light Dust Loadings 6-91
Basic Media Performance 6-91
Further Studies of Filter Aids 6-92
6.3.2.2 Studies with Heavy Dust Loadings 6-94
' Frequency and Number of Raps 6-94
Reverse Flow Air 6-96
Inlet Dust Loading ' 6-96
Comparison of 5 Fabrics 6-9'J
6.4 LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLEC- 6-99
TORS
6.4.1 Reverse-Jet Filter (Mersey Type) 6-99
6.4.2 Pulse-Jet Collector 6-101
6.5 FIELD PERFORMANCE 6-101
6.6 REFERENCES 6-105
-------
CHAPTER 6
FABRIC FILTER PERFORMANCE
6.1 INTRODUCTION
The determination of fabric filter performance includes the specifi-
cation of pressure drop, collection efficiency, component life, and costs
during operation, in terms of dust, gas, and fabric parameters. Pressure
drop with a given dust, gas, and fabric is determined by collector filter-
ing velocity, by the method or mechanisms of cleaning, and by the amount
of cleaning. These same factors simultaneously influence efficiency, life,
and operating cost. Every fabric filter system in service can, in principle,
he described by one or more analytical statements relating pressure drop to
dust and gas flow rates, dust-fabric resistance, and cleaning mechanism
operating parameters. This Chapter considers pressure drop performance
and its variation. Data are also presented on fabric filter efficiency
as effected by operating parameters. Efficiency, cost, and life have not
been reduced to analytical relationships as has pressure drop and, as a
consequence, overall optimization of performance across the several vari-
ables has not been reported. Supplementary cost data are contained in
Chapter 7, and maintenance, service life, and failure modes of components,
principally fabric, are discussed in Chapter 8.
There, are approximately 100 fabric filter models commercially avail-
able in the U.S. (Chapter 3), each available in a range of sizes. These
differ principally in the type and arrangement of fabric used and in the
method of cleaning the fabric to control operating pressure drop. The unit
operation of all these models can be represented, as indicated in Figure
6.1, by a system into which the particles and gas flow, and to which power
is applied to produce external observable changes in gas and particle flow
rates. (Additional related system inputs include costs and additional out-
puts include component life considerations.) With proper quantitative
statements about the input parameters shown on the left, the output vari-
ables may be estimated with reasonable assurance. Not all the factors
indicated are equally important, and in specific instances many may be
relatively unimportant to the total performance of the system.
-------
POWER
I
FLUID FLOW -
Temperature I
Density I
Viscosity .
Moisture I
Chemical Species I
Inert
Acid
Alkaline
Corrosive
Condensible
Flow Rate, Average
Rate of Change of Flow
Flow History
PARTICLE FLOW
Concentration
Particle Size
Size Distribution
Shape
Density & Packing Char.
Bulk Density on Fabric
Density Changes with Pres
Surface Characteristics
Moisture Adsorption
Adhes ion
Electrostatic Properties
Oxidation
Sintering or Bonding
Chemical. Species
Inert
Ac id
Alkaline
Corrosive
FABRIC FILTER
S YSTEM
I
V
FLUID FLOW
PARTICLE FLOW
PRESSURE DROP
COLLECTION EFFICIENCY
POWER EFFICIENCY
COSTS
Capital
Operating
Maintenance
Repair
COMPONENT LIFE AND FAILURE ELEMENTS
Operating History
Fabric
Seams
Supports
Mechanisms
ANCILLARIES
Dust Handling System
Control
Other Systems
Figure 6.1. Parameters Controlling and/or Describing The
Performance of Fabric Filter Systems.
-------
TheHe external system parameters are determined by the functioning
of the internal components of the fabric filter, as illustrated in
Figure 6.2. Each of the components listed can exist over a range of
sizes, shapes, operating variation, and arrangements within the collector.
Table 6.1 presents a summary of the effects on performance of fabric fil-
ter internal component configuration and operational functioning.
This chapter discusses the relationships of the external performance
of fabric filters to the form and functions of the internal components,
insofar as this information is available. Performance relationships are
highly dependent on properties of the particulate material described in
Chapter 2. Chapter 4 contains fabric characteristics which, in conjunc-
tion with the deposited dust, also influence performance. Details of the
internal configurations of alternate collector systems are discussed in
Chapters 3 and 5.
Laboratory testing of fabric filters under controlled conditions has
provided much basic knowledge and handbook data for collector design and
interpretation of performance. Laboratory or pilot plant research and
development studies require a simulant dust for the process to be modeled;
dust feeding and redispersion apparatus; flow, pressure, temperature,
and humidity measuring instrumentation; appropriate duct work; stack
sampling equipment; and other special equipment for measurement and
analysis of performance parameters. Most major fabric filter manufacturers
have development laboratories available for trials on specific dusts,
including particle size measurement instrumentation, one or more commercial
filters set up for test, fabric evaluation test rigs at bench scale, dust
generation apparatus, etc. Data developed from these sovirces is usually
proprietary. However, summary charts, tables or nomographs are often pre-
sented to aid in selection of appropriate fabric area for specific filter
applications (Section 5.2).
Discussed below are the data available on filter performance in
terms of internal component configuration and function and input para-
meters. The basic types of collectors for which comprehensive labora-
tory data are available include:
-------
INTERNAL COMPONENT CONFIGURATION AND
CHARACTERISTICS
1. Fabric
A. Size of Fabric Element, L.D.
B. Fabric Design, Weave, Arr.
C. Fabric Structure, Nap
D. Strand Structure, Arr.
E. Yarn, Fiber, Fibril Size, Arr,
F. Fabric Materials, Service-
ability
G. Manufacture, Finishes,
Lubricants
H. Permeability; clean;
in-service
I. Fabric Weight
J. Fabrication, Sewing
2. Collector
A. Compartments
B. Headers, Dust Inlets
C. Fabric Attachments
D. Construction Materials
E. Hoppers, Dust Outlets
Cleaning Mechanisms
A. Mechanical
B. Gasdynamic
C. Combined, Sequencing
D. Coupling Characteristics
Performance Re lat ionsh ip.s
A. Local Pressure Drop
B. Local, Efficiency
C. Residual Drag, Local.
D. Deposit Drag, Local
E. Local Velocity
F. Local Particle Concen-
tration
G. Changes in above as re-
lated to internal con-
figurations, external
system inputs, and
microscopic processes.
Figure 6.2. Fabric Filter Internal Component Parameters.
-------
TABLK (). I
EFFECTS ON PERFORMANCE OF FABRIC FILTER INTERNAL
COMPONENT CONFIGURATION
Component
Typical
Configurations
Effects, Remarks
Housing
Dusty
Gas
Inlet
Class-
ifier
Dust
Hopper
Rectangular,
cubical,
cylindrical
Top, bottom,
tangential
Rectangular
chamber, one
module without
cell plate or
other internal
components
Inverted
pyramidal,
conical, see
Appendix 6.1
On outside filtering fabric configura-
tions, the gas approach velocity is de-
termined by housing size, relative to
space occupied by filter medium; housing
acts as settling chamber in this case, to
reduce dust concentration reaching fabric,
especially for the larger size particles;
and may amount to > 50% reduction in dust
load to fabric at C > 5 gr/ft3.
Inlet at bottom, hopper, or on side usually
followed by baffle to prevent direct jet
of dusty gas on fabric; inertial effect of
baffle reduces the dust concentration
reaching the fabric, particularly for larger
particles (> 50 urn); tangential inlet en-
hances this effect; top inlet designs with
inside filtering fabric configuration tend
to have less stratification of particle
size on fabric; bottom inlet designs have
larger particles depositing nearer bag in-
let, produces wear, scour, and abrasion
problem on the fabric with some dusts; in
large multicompartmented designs, inlet
header must be properly proportioned for
flow-pressure balance.
Classifier reduces dust load to fabric by
settling and inertial turn effect, espec-
ially for the larger more abrasive particles;
usual large empty chamber adds little or no
increased pressure drop; same function can
be accomplished with inertial skimmer or
cyclone in smaller space but with greater
pressure drop requirement.
In conjunction with bottom inlet location
and inlet baffle, hopper acts as inertial
skimmer or gravity settling chamber; sized
hopper relative to gas flow determines
particles inertially removed; dust build-up
in hopper can reduce effective flow to
bottom entry bags, hopper dust flow
-------
TAHLK 6.I (Continued)
Contponont
Typical
Configurations
Effects, Remarks
Cleaned
03 S
Outlet
Fahri c
Arrange-
ment
Fabric
Arrange-
ment and
Suspens ion
Tube
Sheets,
Thimbles
Cleaning
Mechanisms
Side, top
Bag, tented
top, open
bottom
Sleeve, open
both ends
Sleeve, open
top, over
wire frame
Multibag
Envelope
S leeves-clamp,
spring insert
Closed ends-
hook, buckle,
cap
Envelope-
button, hook
(Numerous)
problems, bridging, etc., associated with
high humidity (> 70%) and greater adhesion
(see Ch. 2); recent studies on hopper de-
sign and dust flow indicated in Appendix
6.1; bin vent and silo filter applications
may discharge directly to storage without
hopper; hopper must be air-tight when under
suction to prevent redispersion of collected
dust to fabric.
No effect of configuration on performance
except insofar as outlet gas flow related
to back flow air utilized for cleaning;
large multicompartment units require careful
design of outlet header to assure even in-
let flow distribution.
Fabric cotifiguration and utilization de-
termined local dusty gas velocity which in
turn controls Local dust-fabric resistance;
fabric arrangement relative to cleaning
method and mechanism determines both resid-
ual drag and effective drag; too much fabric-
crowded into housing becomes difficult to
service, thereby limiting maintenance and
producing poor performance, i.e. dust leaks
and accelerated deterioration of fabric and
other components.
Tension and tension changes affect cleaning
distribution, and indirectly, fabric por-
osity. Installation and ease of maintenance.
Wear often results at stress points.
Spacing of elements affects wear, aero-
dynamics and sedimentation, and maintenance
ease. Thimbles can cause wear.
See Sections 3.3, 8.8.
-------
. Single compartment intermittently cleaned designs
. Multicompartment periodically cleaned designs
. Continuously cleaned designs
(Gradations between available commercial designs tend to make these cate-
gories a matter of degree.)
Most laboratory tests are directed primarily at pressure drop,
and at ways to control, pressure drop within reasonable limits by
changes in operation, usually changes in cleaning. Less data on the
field performance of fabric filters in specific applications are avail-
able, due to the usually limited range of variation of parameters. Per-
formance from such pilot plant or full scale tests is discussed in a
final section of this chapter.
6.2 LABORATORY PERFORMANCE OF INTERMITTENTLY CLEANED COLLECTORS
Intermittently cleaned collectors are designed and operated on a
relatively light duty cycle. Sufficient fabric area is furnished, in
conjunction with low dust concentration, so that the pressure drop in-
creases slowly over a time period in the order of hours. The collector
fan (exhauster) is shut off and some form of shaking or vibration is used
to clean the fabric. Typically, the filter may operate on its dust
source over a full shift, and be shut down for cleaning when the produc-
tion machinery is idle. Most intermittently cleaned collectors operate
with a filtering velocity in the vicinity of 2 to 3 fpm with dust
loadings less than about a grain per cubic foot. They are usually em-
ployed for relatively coarse dusts on dust control, venting, or nuisance
applications.
6.2.1 Bench Scale Performance
6.2.1.1 Pressure Drop-Time Relationship.- Specific dust-fabric
filter resistance coefficients (K«') discussed in Chapter 2 have been ob-
^ 2
tained in many cases with bench scale apparatus on the scale of 1 ft
1 2
of fabric. ' The pressure drop increase for a stated operating time
may be estimated (see Chapter 2) by the following equation:
& Ap(t) = KZ' cVt/7000 (6-O
-------
where C. - inlet dust concentration (grains pur ft'), V = filtering vel-
ocity (ft/min) and t - time (min) . Experimental values of K«/ determined
l>y Williams et al., Tor a constant filtering velocity and a single
(cotton) fabric-, were presented earlier in Table 2.37. They were shown
to be a function of particle material and approximate particle size, in
accordance with the expected influence of these parameters on deposit por-
osity.
2
Later studies by Snyder and Pring demonstrated that K ' is
a function of dust properties and type of yarn or fabric (amount of free
fiber surface available to the dust deposit), as shown in Tables 6.2a and
b. Their data indicated that K,,' is a resistance coefficient specific for
the dust and fabric. The quantitative separation of thfe effects of fabric
from the dust properties in the specific resistance coefficient was not
attempted. Effects of dust particle size and shape, and fabric yarn or
nap on the resulting pressure drop increment are shown in Figures 6.3a, b
and c. Snyder and Pring's pressure drop - deposit curves are seen to be
non-linear with a marked upward curvature evident in several instances.
Typical K»' values ranged from 234 for freshly-formed MgO fume on napped
D
Orion to 7 for petroleum coke fines on spun staple Orion napped both sides
Napping resulted in about a 10% reduction in K«', but usually resulted in
a higher residual dust deposit after shaking.
6.2.1.2 Dust Collection Efficiency.- Collection efficiency
was found to range from 84°X,' for MgO fume to greater than 99.47 lor petro-
leum coke. However, the Snyder and Pring^ data are of limited value as
they do not represent typical industrial usage. Fabrics were not utilized
for long periods to achieve equilibrium priming or aging, and cleaning
parameters were not quantified. The principal value of their study was
to direct attention to the effects of free available fiber on K,.', and
to the non-linear variation of K2' with deposit weight. They also pro-
vided experimental validation of the predicted effect of «as viscosity
on pressure drop.
-------
TABLE 6.2a
PROPERTIES OF VARIOUS ORLON FILTER FABRICS'
Dust: fine petroleum coke
Results: average of 5 cycles
Dust Residual
Cloth Weight Perme- Loading Dust, Collect-
Oz./Sq. ability Grams/ Grams/ ing
Yd. New Used Sq.Ft. Sq.Ft. Efficiency
Napped filament 3.9 35 10 29.00 4.18 99.91
Orion, 3/1 twill
Knit Orion, 7.6 85 58 15.40 4.58 99.70
napped
Orion spun 7.5 60 33 18.21 1.48 95.19
staple
Orion spun 9.0 100 50 19.95 2.15 99.64
Filtra-
tion
Constant ,
K '
K2
17.1
12.5
15.0
7.4
staple, napped
both sides
Orion spun fiber-
stock, 3/2 twill
4.9
110 62
17.96
0.99
93.42
13.0
From Snyder and Pring, Ref. 2.
TABLE 6.2b
FILTRATION CHARACTERISTICS OF NAPPED AND UNNAPPED
SIDES OF ORLON*
Orion: 1 oz. napped filament, 76 x 72 count, 3/1 twill
Results: average of 4 runs
Fume: freshly generated magnesium oxide
Napped side
Unnapped side
Used
Perme-
ability3
4.2
8.1
Dust
Loading
Grams/
Sq.Ft.b
12.0
13.8
Residual
Dust ,
Grams0
15
4.5
Collection
Efficiency
%
89.4
84.0
K '
K2
234
251
Permeability after shaking, cu. ft/min./sq. ft. at 0.5 inch w.g.
Corresponds to weight of dust removed by shaking, grams/sq.ft.
c
Dust remaining on cloth after shaking.
t
From Snyder and Pring, Ref. 2)
-------
o
CN
re •
.5*
Figure 6.3a. Values of Ap vs.
Deposit Weight (LTV) in filter-
ing fine petroleum coke dust.
Figure 6.3b. Values of Ap vs.
LTV covering dust generated in
abrasive biasing of steel paint
drums on high twist, unnapped
Orion with an extremely low fiber
surface area per square foot and
fiberstock Orion.
eo-~ 400—soa~ *6o—
COO 2000 MOO 4000 SOOO
TOO 600 BOO
LTV GRAINS /fT*
Figure 6.3c. Effect of particle
size and shape on filter resistance
of cotton sateen cloth. Silica gel
dust, 43% less than 10 microns;
Scale dust, 27, less than 10 microns;
Limestone dust, 32% less than 48
microns.
(From Snyder and Pring, Ref. 2).
-------
6.2.2 Single Bag Performance, Pilot Scale .Testa
6.2.2.1 Pressure Drop-Time Relationship
Basic Concepts. - When textile fabric is used as a filter for
dusty gas in the form of a bag, tube, or envelope, and then cleaned repe-
titively, an equilibrium pressure drop-time behavior is observed. After an
2
initial priming or aging period (which.may range from < 10 hours to > 10
hours for certain dusts and fabrics), the pressure time response appears as
illustrated in Figure 6.4. The pressure-time trace will then be repeated
cyclically, provided that the gas and dust flow rates are maintained con-
stant and if the cleaning energy patterns, durations, etc., are applied
uniformly. The pressure-time curve depends upon the gas viscosity, particle
properties, (size, shape, surface phenomena), fabric properties, (fiber,
yarn, weave, finish, nap) and operating characteristics of the cleaning
mechanisms. If the flow of dust to the fabric filter is stopped at any
point, and a pressure-flow curve is determined using cle^an air, it will be
found that
Ap - S
(I}"
(6.2)
n
10
»
* 9
I
w
e a
°~
§ 4
Du*t:
tutlcli 111* Mi'-
Du«t Coocutratlo*
a - gralna/ft*
Fabric Warn cc, vt, AT?,
|: Typa a«?Utudt (raq. duratioa
Incrtuad pr«iur« loi> ciuM
1>)F lncrt.it u K,' .t hljhtr
vtloclty 2
2.1 x 5 - A.9 In.HjO. calculattd prtilurt drop
•t iqual dtulcy, at
hlghtr flov ratt
- Kfftctlvt prtnurt drop
-Rttldual pr«iure drop
v • 3.0 fpm
"' '
?0
32 36
40 44
TIMl, minutet a V, lbi/ft
Figure 6.4. Pressure Response in Constant Flow Rate Gas Filtration
(Constant Gas and Particle Flux). (From Hnt^wardl and
Durham, Ref. 3).
-------
where n -- 1 as long as viscous flow obtains (for a granule Reynolds
number < 10). The property S or "drag" is a variable function of
fabric, flow, and dust parameters as described in Chapter 2; it is
analogous to the resistance in a purely resistive electrical cir-
cuit described by Ohm's Law, i.e., Voltage = Resistance x Current.
4 5
The drag, S, may thus be defined ' as
(6.3)
for small Reynolds numbers, which is the case of most fabric filtration.
S is a property of the filter and applies to any filter area. If S varies
from area to area, due to variations in permeability for example, then
the overall effective drag S is given by the following, again analogous
to the electrical case of resistances in parallel:
j: <6'4>
where the a. are the incremental component filter areas and the S. are
the local drags.
The use of the drag concept allows direct comparison of filter
media from one filtering situation to another, regardless of filtering
6"
velocity or unit size.
The Darcy permeability, K , which defines the intrinsic per-
meability of the filter medium in terms of its structural form, is analogous
to electrical conductivity. It is related to the specific resistance, K9 ,
of the filter medium by the relation
K (l-e)pp S ' (l-c)pp -
The ratio, W/S, provides another measure of cake permeability in terms of
the density of filter deposit per unit area, W and the resistance to air-
flow per unit velocity or drag S. The latter description of permeability
is merely the reciprical of the previously cited specific resistance, K«.
It should be noted that the permeability is an intensive property
-------
(independent of mass) whereas the drag, S, is an extensive property (mass
dependent) that relates filter resistance to flow rate. While the drag
will always increase as additional material is deposited on the filter,
permeability may increase, decrease, or remain constant over the same
period, depending upon flow, fabric, and dust interactions.
Iltil izing concepts of filter drag and areal density of deposit,
the typical pressure-time response during equilibrium operation of a
fabric filter element shown in Figure 6.4 can be represented as a drag-
density relationship as presented in Figure 6.5. Several typical features
FILTERED OUST MOSS, VMgrtlm/ft*)
Figure 6.5. Schematic Representation of Basic
Performance Parameters for Fabric
Filters.
of performance are illustrated. The total drag of the filter medium in
service is the sum of the drag produced by the fabric with its irreduc-
ible residual deposit plus the drag of the dust deposit added during
the filtration cycle.
All practical cleaning methods so far developed result in
some nonuniformity in the effectiveness of cleaning obtained in different
areas of a filter cloth. Because of this, when filtration is resumed on
a recently cleaned cloth area, the rate of change of drag of the medium
will vary in the early part of the filtering cycle. The curve in Fig-
-------
urc (>.r> indicates that cleaning leaves a nonhomogeneous and discontinuous
surface- that is subject to rapid increases in resistance when a new de-
posit is first being formed. Curve segment A represents the increase
during this part of the cycle. After the discontinuities have been
largely eliminated by preferential flow of the dust-laden gas through low-
resistance areas, a relatively uniform deposit has been formed. Sub-
sequent deposition of dust generally results in a linear increase in re-
sistance to fiow as dust accumulates, curve segment B. Efficiency of the
filter is lowest during the early part of the cycle while the discontin-
uities are being repaired.
This relationship will hold for any dust-fabric combination
but the shape of the curve may be significantly altered by differences
in the cleaning or the nature of the fabric. Highly effective cleaning
will lower the residual drag value, S , and poor cleaning will increase
it. For that part of the cycle represented by curve segment B where the
homogeneous deposit is formed, the specific properties of the dust tend
to control the rate of increase of resistance. The permeability, K, of
the dust-fabric combination will be reflected by the slope of the curve.
These general relationships relating to pressure loss through the medium
in a filtration process apply whether the area being considered is a
single bag or many bags in a single compartment, so long as all of the
area is put in service at one time.
Because of the dominant importance of pressure drop in
equipment of this type, the drag of hhe filter medium must be given
primary consideration. Filter drag, however, is but a single element of
the total pressure drop that determines fan and power requirements. In
practice, the total pressure drop to be overcome may, because of duct
losses, be twice that attributable to the filter medium alone. The total
system will contain sections with gas in turbulent as well as viscous flow,
so that total pressure drop will vary with flow exponentially by some
power between 1.0 and 2.0 instead of directly with flow as it does for
the Ap across the medium alone.
The residual drag, S , is determined by the properties of
the dust and the fabric,in conjunction with the operation of the cleaning
-------
mechanism. Cleaning energy input, as distributed through accolor.ition,
frequency, and duration, do fines the amount of dusL removed down to some
irreducible minimum value. The effective drag, S , is defined as that
value of the draj; obtained from extrapolation of the linear portion of
the S-W curve to zero W, at the ordinate. For practical purposes, the
filtration cycle appears as:
S(W) = |£ = Se + W/K (6.6)
where S is some function of S , fabric, and dust parameters. The actual
shape of the S-W curve in region A has not been presented; instead it
is usually approximated by this equation.
In terms of the specific dust-fabric filter resistance co-
efficient ,
K2' = 7000/K (6.7)
2
providing W is calculated as grains of dust/ft of fabric. Then the
pressure-time relation is generally of the form
Ap(t) = SgV + K2' C^V2 t/7000 (6.8)
when the S-W curve is linear from the extrapolated value of S . The form
of the S-W relation is not always linear, and seems to depend on the amount
of fiber available at the fabric surface for dust holding, as will be dis-
cussed below.
Liquid Filtration Analogy.- Because investigations of liquid
filter performance have been treated in much greater detail in the tech-
nical and engineering literature, it is of some value to examine their
applicability to fabric filtration. A typical liquid filter utilizing a
fabric, as applied in the Chemical Process Industry, operates at constant
pressure drop and the flow rate (or total flow volume) is allowed to de-
crease until a fixed volume has been treated. The filter is then stopped,
drained, and the resulting cake (deposit) is removed by manual or semi-
automatic means, which may involve the physical removal of the fabric
system from the main flow housing. In rotary pressure or vacuum filters,
the deposit and removal steps are continuous and automatic. In the manu-
-------
nl paper Liu' rake formed from a slurry of wood fibers is contin-
•7
nous I y withdrawn as product. The use of. rapid sand filters (2-'J gpni/l'L")
for water purification (in Sanitary Engineering) involves operation of a
fixed hod of granules as a filter for fine participates until the deposit
storage capacity of the bed is exceeded, whereupon the filter is then
backwashed.
In each of these areas there are extensive investigations and
analyses of the filtration process under a condition of variable deposit
and variable flow or pressure drop. However, the following reasons
preclude the direct utilization of these data to the processes of fabric
filtration of gases, even when the same filtration substrates are employed;
1. The adhesion forces between small particles at a
fibrous or granular substrate are several orders of
magnitude smaller in liquids. Adhesion forces for
particles in air or gases are typically of the order
of 1 dyne (Chapter 2, Table 2.10) for sizes in the
range of 1 to 100 urn, and are dependent upon humidity,
capillary condensation, and surface effects such as
contamination, roughness, electrostatic charge,
etc. Table 6.3 indicates that adhesion forces in
liquids are. typically less than 10"^ dynes. They
depend upon van der Waals forces operai ive at
molecular dimensions (~- 10~8 Cm).
2. Pressure drop in liquid filtration is generally
much greater than in fabric filtration of air
and gases. Typical terminal pressure drop in
fabric filtration is less than 15 inches of
water, i.e., less than 1/2 psig. Pressures
used in liquid filtration in the chemical pro-
cess industries are generally greater than
10 psi (rotary vacuum filter) and may exceed
100 psi (plate or frame type). Rapid sand fil-
ters typically backwash at 3 to 5 psi.
3. The combination of lower adhesion forces and
greater pressure drop are believed to have sub-
stantial effects on the characteristic of the
deposit in liquid filters as contrasted to de-
posits likely to occur in fabric filtration of
dusty gas. In liquid filtration through fab-
rics, one typically observes a deep cake of
order of several millimeters thick, much
thicker than the fabric media. Calculated
values of deposit thickness in fabric filters
-------
(Table 2.36) are generally less than 1 milli-
meter. Measurements of deposit thickness in I'abric
filtration have not been reported.
The irreducible residual deposit pressure drop
in fabric filters, measured by Sg or Sr as shown
in Figure 6.5 is a major portion of the total oper-
ating pressure drop in every cycle. In the case
of liquid filtration, Ruth (see Appendix 6.2)
and other investigators have shown that the
pressure drop of the filter medium at the be-
ginning of the filter cycle is of little sig-
nificance in the resistance of tho medium nflor
T
a cake has formed.
TABLE 6.3
ADHESION OF VARIOUS PARTICLES TO SUBSTRATES OF
VARIOUS MATERIALS IN WATER
(for F = 7 x 10"5 dyn)*
Substrate Material
Glass
Steel
Bronze
Glass
Paraffin
Particle
Material
Quartz
Graphite
Quartz
Graphite
Graphite
Quartz
Quartz
V"
11.0
7.5
11. 0
7.0
7.0
5.0
5.0
•v 7 **
V /0
0
20
33
40
40
76***
0***
*
(From Zimon, Ref. 8).
7 = fraction of particle numbers remaining attached after
application of indicated force.
For F, = 1.6 x 10 dyn.
det
The structure of the deposit formed at the surface of a
fabric dust filter is markedly influenced by relative humidity and ad-
hesion, and electrical characteristics of the dust and fabric. The aggre-
gates formed are probably very filamentous and chainlike for fine part-
icles, although more compact, similar to the liquid filtration case, for
larger dust particles. For these reasons, deposit formation in dust
-------
filtration probably differs from that in liquid filtration. Liquid fil-
tration concepts such as complete pore blockage, standard blocking, cake
filtration, and the like, are very likely to be inappropriate models for
gas filtration phenomena. While analogies between liquid and gas filtra-
tion are attractive to consider, especially the concept of cake formation
in the latter case, they require further investigation before their utility
can be evaluated. A summary of pertinent studies in the field of liquid
filtration applied to fabric filter technology is given in Ret". 3.
6.2.2.2 Dust Profiles.- Pressure-time data shown in Figure
6.4 and S-W curves given in Figure 6.5 represent a macroscopic average
over the total fabric surface for a single cylindrical filter tube (or
9
for many bags in parallel). Stephan, et al., have investigated local
interactions of dust, gas, fabric, and cleaning parameters at various
locations on a single bag. Local mass deposited (areal dust density)
was measured with a I'-gaging mass probe and local flow rate with a
velocity probe. These two instruments are shown in Figure 6.6a and b,
respectively. The improved mass probe containing a fi-source directed
toward an end window G-M tube is held adjacent to the filter bag. The
resulting signal is interpreted through calibration as mass per unit
area. The filter velocity probe measures air flowing through a section
2
of the bag. Both devices measure an area of 1 to 2 in. . Clean unused
22 2
fabric has an areal density of ~500 gr/ft , 10 oz/yd , or 0.07 Ib/ft ),
so that the dust deposits encountered in practice are 1 to 10 times the
2
weight of the fabric. Residual dust weights are typically < 0.1 Ib/ft .
Typical results obtained with the mass and velocity probes
on a 9-inch diameter by 60 inch long bag are shown in Figure 6.7. De-
posited mass is greatest at or near the center of the bag height (~ 800
2
gr/ft ). Filtering velocity is lowest in this same region ( < 1 fpm).
Figure 6.8 illustrates the variation of filtering velocity with dust de-
posit density over the bag. Values of the local relative specific dust-
fabric filter resistance coefficient (K') have been calculated (assuming
a uniform 1 in. H_0 pressure drop over the total bag) as shown in Figure
6.9. Dust resistance is lowest at the bottom (probably as a consequence-
-------
End window
G-M tube
detector
Personnel
dosimeter
Figure 6.6a. Improved Mass Probe,
-
1.83 in.
Air Flow
Aperture
Haystings-Raydi
thermal anemone
flowmeter
I Cellular foam rubber
baffle
c
Figure 6.6b. Filter Gas Velocity Probe.
-------
600 700 800
WALL DENSITY \q,t. tl ? )
HI.TCR VtLDCiry
Figure 6.7. Corresponding Mass and Filter Velocity
Profiles. (From Stephan, et al., Ref. 9)
U
o
200 400 600 800
LOCAL WALL DENSITY, groins/ ft2
Figure 6.8. Variation of Filtering Velocity with Dust
Deposit Density, from Figure 6.7.
-------
00
01
00
n)
01
TD
3
Operating cow) it tow:
9 x 60 In. b«|, cotton i«t**n,
fly «ih c««t duic
7000 x 1 In. HjO
W~x~V
-------
100
90
eo
TO
«>
t: 30
< 20
IO
0
o.a
_L
'o,n
I
I
30 4O 60 80 100
ZOO 300 400 600 800 K>00 2000 3OOO
Log Duit H»i (Cr«ln>/
Log Dust Drag (In. HjO/fpn)
2.0 3.0 4.0
Flgur. 6.10b. Developmont of Duel: Drag Prof 111 Through • nitration Plxlod
rfrom R«f. A)
Hot«« to Fltur* 6.10
Pllt.r tub.. v.r. 6 In. In dl«a«t«r by 63 In. long fabricated of atandanl cotton aatun (National
Flltar Madia Styla Ho. 74, 96 narp x 60 fin, 9.7 ot/yd?) having a ratad Frailar Poro.lty of
about 15 ft3/mtn. In tarn of raalitanc. and paraaablllty, tha fabric would ba ratal at Sg «
0.033 In H20/«pm and K - 14,300 (gralna/ft2)/(ln H20/£pm). Shaking action vaa vartlcal at
tha cop. Kith a 2-1/4 In. anplltud. at a fraquancy of 6.79 cpa for a duration of ona alouta.
Taata vara conduct.d at avaraga flltar valocltlaa ranging fron 0.75 to 7.5 fp», and duat coocan-
tratlona In tha rang. 1
-------
develops which Is maintained for the remainder of the filtration cyclo.
Hoth S and W profiles tend to become nearly flat at high mass deposit
values. Effects of particle elutriation and of velocity on dust-fabric
resistance (K~' ~ V) are no doubt factors in the departure of the pro-
Tiles from flatness. The variations in profile slopes near top and
bottom of the filters appeared in all experimental runs, indicating that
even when starting, with a perfectly flat profile, dissimilarities develop
along the bag length near the top and bottom of the bags. This investi-
gation indicated that filtration does not occur uniformly over the length
of a filter bag. Different quantities of dust are collected and local
filter velocities vary by factors greater than 2 in the initial part of
the filtering cycle at different locations on the tube. The correspond-
4
ing values of local permeability vary in space and time. These varia-
tions reflect the differences in cake removal by the pattern of cleaning
energy applied, the relative adhesion of different fractions of dust as
a function of cleaning energy applied, and the differences in dust struc-
ture over the length of the tube.
The approach to profile linearity is expected since areas of
low resistance will handle higher than average air volumes, and vice
versa. Consequently more dust is deposited on the low resistance areas,
less on the areas of high resistance, and a self-balancing system exists.
2
It is likely that the period 0,1 •- 0,2 (i.e., addition of 50 gr/ft )
corresponds approximately to the period of deposit repair and non-linear
K«'. Values for the effective average resistance coefficient for the
total bag structure, (K' = 7000/K = 7000 S/W) are shown in Figure 6.11
for each of the increments, 0,1 -+ 0,n. The resistance coefficient in-
creases rapidly during addition of the first increment of dust, then falls
to a relatively constant value through much of the remainder of the cycle.
The terminal value is somewhat higher, possibly because of dust deposit
compaction. Dust permeability profiles for another test series are
shown in Figure 6.12. Average K«/ increased from 2 to 12.7 during the
test. Effects of particle stratification are apparent at the top and
bottom of the bag, at the various intervals during filtration.
General conclusions drawn from these studies are:
-------
BltSI
•* «J
*J O
t-1 (VI
•^ X
u C
U> -
a y
•o c
01
O -H
•H O
O *M
£8
CO O
1.
0
l-'igure 6.11
09
10
At - Ap
in. H20
1.5 20 2.!
bAp, Dust Deposit Pressure Drop,
Variation of Average Specific Dust-Fabric Resistance Co
efficient During a Filtration Cycle.
1. The local specific dust-fabric filter resistance
coefficient is a variable with respect to both
location on a filter tube, and time '
dust deposited.
2. The dust mass and drag profiles reflect interac-
tions of particle size, flow, and structure
during deposit formation.
3. The average specific dust-fabric filter re-
sistance coefficient tends to rise during the
filtration cycle.
Usiag the same techniques of mass and velocity measurement,
the deposit characteristics may be studied during the process of cake
removal. Effects of shaking on dust mass and drag profiles are illus-
trated in Figures 6.13 a and b. Figure 6.13a shows a mass profile just
before cleaning, and after one and 20 minutes of shaking. In this par-
-------
Ave. Av«. AP. W, ,
K JC9, "H90 gr/ft*
0,1 "5577 2.4 To? 1060
0,2 991 7.1 1.0
0,3 671 10.4 2.0
0,1) 553 12.7 3.0 2290
200 900 400 600 800 1000 2000 3000 40OO 6000 10/300
DUST MASS PERMEABILITY (GRAINS / FT2)/(IN. H20 / f pm)
Figure 6.12. Decrease in Dust Mass Permeability through a nitration
Period (from Stephan ,et. al., Ref. 4)
ticular case, 89.5 percent of the dust deposit was removed by one minute
cf shaking and 97.2 percent by a 20-tninute shaking period. The reductions
in drag are illustrated in Figure 6.13b. The first minute of shaking re-
duced the pressure drop by 89.7 percent and the next 19 minutes reduced it
by only an additional 2.8 percent. However, dust removal itself is of
secondary importance to resistance decrease, which determines the pressure
drop after shaking. Before shaking, the cake had an average resistance of
2
7.4 inch H20/(lb/ft -fpm) and this value was slightly decreased to 7.2 by
one minute of shaking. After 20 minutes of shaking, specific resistance
was increased to 20. Continued shaking may reduce dust mass proportionally
more than resistance. Reduction in effective resistance is more meaning-
ful than quantity of dust removed in determining optimum lengths of clean-
ing periods.
-------
MIN CLI ANIN'.
ITRIOOI
I.I J . 1 1_J
0 4(1 bO m loo I'll'1 4illt 600
OUSI MASS -graint / ft2 t
Figure 6.13a. Fiffect of Cleaning on Residual Dust Mass
Profiles (From Stephan, et al., Ref. 4).
100
so
80
60 -
a
§ 30h
\~
_l
< ZO
10
0
r
B (20 MIN. CLEANING
°-' PERIOD)
EFFECTIVE
* 0.973 in
u ^0 _ (I MINI S^-O.IOO
(20 MIN.) sn,« 0.073
I .1 L
.Ol .02 04 .C6 .06 .1
S (I MIN. CLEANING
PERIOD)
M0.r -
. . . .
DUST MASS Drag
A .6 .a i.o
2.0
(Inches H.O/fpm)
Figure 6.13h. Kflucl of Cleaning on Residual Dra^ I'ro-
f Mi's (From Stephan, et al . , Re L'. 4).
Additional data on development of residual dust mas.; pro-
files during cleaning are shown in Figure 6.14. An essentially uniform
terminal dust mass of 1650 grains per square foot was reached under the
filtration conditions given in the Figure. The filters were then shaken
for 5 seconds (= "1st shake," consisting of 34 strokes) and the residual
profile was measured. Approximately 8870 of the terminal dust was removed
during this first period. Successive incremental shaking periods were
then conducted, and profiles were measured. Only 3"/, of the terminal
mass was removed in the second period. The profiles were progressively
flattened with further shaking.
-------
isi THROUGH 7tn SHAKE, EACH 5 SEC
8m THROUGH nth SHAKE, EACH 2 WIN
i;ih SHAKE, 5 MIN
Ulh SHAKE, IOMIN
TERMINAL Lf 30>r> H?0
AVG F,,.TER VELOCITY oiii/m."-
IV.ET DUST CONC , 75grom»';. (I
150 200 250
DUST MASS, groms/tq ft
JOO
J50
••00
Figure 6.14. Residual Dust Mass Variation with Cleaning Dura-
tion. WT = 1650 gr/ft . Shake, 6.75 cps. (From
Stophan and Walsh, Ref. 10.)
Stephan and Walsh also discuss aging, equilibrium residual
profiles, filtering velocity, and particle penetration into the fabric.
They emphasize that a non-uniform residual profile exists at the start
of every filtration cycle. Filtration never occurs uniformly over the
total filter surface. Different filter velocities, amounts of dust col-
lected, and deposit structure occur at various locations. They conclude
that data from laboratory bench-scale determinations of dust-fabric
resistance coefficients cannot be equated for design purposes to the
effective or average resistance coefficients of the same dust on full-
;ized filter tubes.
One practical implication of the Stephan-Walsh study:
"....concerns the differences in local cleaning
intensities which exist between very short clean-
ing periods and appreciably longer ones.... Filter
cleaning follows the law of diminishing returns -
i.e., each shaking stroke contributes less and
less, with the degree of cleaning approaching
asymptotically some limiting value for a given
set of cleaning conditions."
-------
"This.... suggests...the hypothesis that apprec- ^
iably shorter cleaning periods than are now common;v
in use may be employed without proportionately
reducing the length of the subsequent filtration
periods. Such a situation would have two distinct
advantages:
1. "Rate of filter wear, which will always —•
be related in some way to the total number
of cleaning strokes experienced by the
filter, could potentially be reduced. _
2. With the use of shorter cleaning periods
at more frequent intervals, increased
filter ratios - i.e., smaller equipment ""*
size - might well be possible." ^
This hypothesis is supported by the recent successful development of pulse- —
jet filters, which are continuously cleaned on-line by a short (< 1 sec)
burst of high pressure compressed air (100 psi) every 30 seconds or so.
Higher filtration velocities are used (~10 fpm), and fabric life appears
to be at least as long as in many shaken designs.
•^
6.2.2.3 Analysis of Mechanical Shaking - Mechanical shaking of
a single bag or compartment is discussed below. Analysis of multi-compart-
—s
ment shaking with and without simultaneous reverse air flow, and reverse-
jet and pulse-jet cleaning are considered in following sections.
The fabric is cleaned periodically to remove deposited material
by any of several possible means (Section 3.3), of which mechanical shaking
_/
is one of the more common. A minimum pressure differential at the start
of the filtration cycle can be achieved by vigorous cleaning to keep aver-
age residual accumulation and drag small (Curve A, Figure 6.15). However, ~"
too much energy applied to the fabric to remove accumulated dust shortens
fabric life and reduces collection efficiency (Curve B). There is, thus, _j
an economic optimum to be determined (Curve C), which depends on many pro-
perties of dust, fabric and cleaning.
Fabric cleaning can be generalized to considerations of force
applied to the deposited powder layer as a consequence of: (a) motion
induced in the fabric, or (b) interaction of the motion of cleaning gas
with the deposit and fabric. Removal forces produced on the deposit are
_/
resisted by adhesion (particle-to-fabric) and cohesion (particle-to-
-------
InCrnalCy and Duration of Clewing
Figure 6.15. Cost Analysis in Fabric Filter Cleaning
particle) forces within the deposit matrix. In principal, the force app-
lied to the deposit can be determined from consideration of the energy
applied during cleaning. In practice, cleaning forces are produced by a
configuration having fixed frequency and amplitude, or other characteristic
patterns of application, and deposit mass variations. The point of opti-
mum cleaning is produced by field adjustments of one or more of these
variables.
A comprehensive analysis of mechanical shaking in fabric fil-
tration has been reported by Walsh and Spaite . They studied the effects
ol shaker amplitude, (c ) shaker frequency (u>) and duration (number of
9
strokes, u>t) on residual drag (S ) and filter capacity (weight of dust col-
lected during the filtering cycle, W without exceeding some terminal fil-
ter drag, S ). Tube geometry, fabrics, shaking direction, etc., were held
-------
constant.. The pilot scale test unit is shown in Figure 6.16. bags were
installed with zero tension and zero slack. The range of variables inves-
tigated included V = 3 fpm, 0.05 < e < 2 in. , 5.2 < cu < 56 cps, and 5
""" S """* ~~
< o)t < 700 strokes. Maximum acceleration produced by the simple harmonic
motion is:
= (2 in> c (6.9)
where - - amplitude of the motion, 1/2 the peak-to-peak stroke length.
The muxJmum occurs at the limits of the shako motion. The range oJ'
accelerations studied was 0.72 <. ir v. 15.5 g's (10 < y physical limits as illustrated in Figure 6.17. Shaking
r
I ISLE
3 JNH
3
. Bi.C*£«S
&) 5 OJST FEECE"
£ MiC
» SHAKING MCOivSv
0 :uST CATCH |;NS
•N KACC*
1 1 OJST :ATCH iiNS C.J"-
i2 HOPPER OX"
.s :U*T CATCH
Fly /isli test dust;
-------
10
i
i
i
ut
O
a!
2°'
V»- LIMIT OF PR* i ICAL
AMPLITUDES («MO)
«'•
09 g'«
0 01
0 I
* u«i,h », S|i«lt» t) produce an initial reduction in residual drag,
S . A point is soon reached when increased duration produces little
further reduction in residual drag (S = S . ). The initial cleaning
r mm
-------
JUT ciiwmoHi;:
Ml.TUS i;nTWN 4ATKKN MhKIC
KI.Y ASII rrsr imsi, t..1 m rtii). • I.H. cm
MI.TKK Vr.U*:\T> - I-" 11'»> K
I INCH
I// INCH
I *x
\ x^
1
,\
1 ^~0
I |
11
^-.?S
A--
1
W'iLV!.1! IJ
/.inlllh |n I, In/
.M>»IU in./.In.;
I.I»H>'' In./nln.'
jo ,,,.t. ,,h,ik.-
'T~
20 4O 6O aO tOO I2O I4O ISO
I IWIMBtH OF STROKES),
180 200 210
Fi.j';uro b. L8. Effect of Cleaning Duration on Residual
filter Drag for Several Shaking Conditions,
(From Walsh and Spaite, Ref. 1.1.)
strokes accomplish most of the cleaning. A practical limit exists beyond
which increased shaking duration did not substantially decrease residual
drag. This limit, indicated at N in the figure, ... "was approximately
s
reached when oscillatory motion was developed over the entire filter tube.
Physically, N was the number of strokes required to produce significant
s
discontinuities in the dust mass structure so that ... drag was at a prac-
tical minimum because of low resistance areas." Maximum capacity was
achieved with a greater number of strokes. In either case, 15 seconds of
shaking was sufficient to achieve an effective minimum drag and maximum
capacity. Commercial fabric filters typically shake for 30 seconds or
longer.
The effect of shaker acceleration on minimum residu; I dv.ig
(S at N ) is shown in Figures 6.19a and b. These data indicate that
v min s
the minimum drag produced (the nearly horizontal portion of the curves
in Figure 6.18) is a function of the acceleration, as
mm
(7
(in. H20/fpm)
(6.10)
for 0.7 v.,v< 6 g's. For i» 6 g's, S . -^constant, independent of
~~~ •*-* • m i. n
-------
6
o
c
i
to
o
<
s
o a
u. o 6
0 4
111 : i u •. - COT i UN i*Ti t N r«b».c
,,i'M I.: uTHUH 0 'I > «S»i
I .LTI N vllOCi'T - 1 0 torn
rfRMINAl. FiLTf'NORAC- -^ ?.0m
Figure 19a.
o
6, S • constant, (0.19 for
fly alls on sateen weave cotton)
0.7 _. O < 6 g's
I
1.0 235 10
SHAKER ACCELERATION (da's)
20
Figure 6.19b. Minimum Residual Drag as a Function of Shaker Acceleration.
-------
The form of the S -cut: function appears to he relatively in-
dependent of the type of dust (Figure 6.20), within the limits of the
data available. Properties of the dust (particle size distribution,
shape, adhesion forces, density) and characteristics of the fabric (free
available fiber, weave, yarn, fiber material) as well as filtration
parameters will affect the S -tot values. Hard-to-clean dusts, or napped
fabrics, for example, will require greater shaking duration, but the
shape of the drag-duration curve would be. expected to be similar to those
shown in Figures 6.18 and 6.20.
The path followed by residual drag (S <* S . ), as a function
of cleaning duration,was analyzed using the data of Figure 6.18 and found
to be represented approximately by:
s*. ' s •} i
t r 3 . x 1
S - S . =4 los (au)t) ' 4 (6.11)
t mm v '
where S is the terminal drag prior to shaking, S is the residual drag
produced by the number of cleaning strokes (< N ), and S . is the mini-
mum drag obtainable with a given input acceleration (Equation 6.10).
Data points taken from Figure 6.18 (for cot < N ) are shown in Figure 6.21,
~~ S
The number of strokes (tut) required to reach minimum resi-
> . ) is a functioi
mm
Figure 6.22. For these data,
dual drag (S . ) is a function of shaker amplitude (e ) as indicated in J
Ns ' (a*>s = (6.12)
where ( is the shaker amplitude (1/2 the peak-to-peak total motion of
S
the shaker) .
In summary, residual drag appeared to be determined by
(number of strokes x peak acceleration), Equation 6.11. However, resi-
dual drags smaller than a certain limit were not obtainable, the limit.
depending on acceleration, Equation 6.10. At that acceleration, N
strokes (Equation 6.12) were required to reach the limiting drag.
-------
Figure 6.20.
CLEANING DURATION, UK, STROKES
Effect of Dust on Residual-Drag-Cleaning Dura-
tion (S -ot) Relationship. (From Walsh and Spaite , Ref. 11)
-H I tin A I Draft ••[!»
Figure 6.21. Effect of Acceleration and Shaking Dura-
tion on Residual Drag.
-------
W
01 3.0
0
4-1 CO
01
^ jC
tfl U I 0
01 C
CM M
-------
I.IOO
"U 1.000
U-
v>
z
< BOO
(C
S3
£ 600
o
2
3 *°°
-------
6.13) for a given input acceleration. Data points taken from Figure fa.23
(for o>t < N ) are shown in Figure 6.25.
— w
02 04 0.6 0.8 10
Q— FILTER CAWCITY RATIO
Figure 6.25. Effect of Acceleration and Shaking Dura-
tion on Residual Deposit.
The required number of strokes to achieve N is given
approximately by
«- ^w* I7s (6.15)
for 2e > 1 inch. Smaller amplitudes (2e < 1 inch) appear to arrive at
S S
N sooner than estimated by Equation 6.15.
S
In summary, Equation 6.13 indicates the maximum amount of
dust (W . that can be removed from the fabric by a given acceleration
input. Equation 6.15 indicates (approximately) the number of shaker
strokes (N ) required to reach the maximum dust removal. If the re-
w
quired number of strokes to reach maximum dust removal is not achieved
during the shaking cycle (cot < N ) an (
— w
removed (W) is given by Equation 6.14.
during the shaking cycle (cot < N ) an estimate of the amount of dust
— w
-------
Thf.sr results arc indicated in Figure 6.26. Data i'rom
i:tirvc C of Figures 6.18 and 6.23 have been plotted to indicate uhr
dust mass relationship through a single filtering and cleaning cycle.
These represent equilibrium conditions at constant filtering velocity
and dust concentration during the filtering cycle. The relationship
i'or drag produced by added dust mass has been discussed above and in
Chapter 2. For many dusts and fabric combinations, this is
SOO = l^li - !iZ + s - K2 ci vt + se (6.16)
V 7000 7000
2 3
where W is in gr/ft of fabric and C. is in gr/ft of gas. The interval
.shown at A in Figure 6.26 represents the process of deposit repair when
the rate of change of drag (resistance) is a decreasing function of dust
deposit. The interval S^-S is approximately 0.2 in.H.O/fpm. Vnlues
pf S -S are discussed below, for various dust, fabric, and cleaning
combinations. Tho corresponding value of W at the point where the
linear part of the S-W filtering curve actually starts is approximately
100 grains/ft . The filtering cycle (path A & B) can be approximately
represented by
K ' C, Vt
Upon reaching the terminal drag (S ), tho dusty gas flow is
stopped and shaking begins. Drag is reduced (path C) according to the
shaking mechanism inputs, as given in equations 6.10, 6.11 and 6.12.
(summarized in Table 6.4).
Equations 6.9 through 6.17 define the loop shown in Figure
6.26, approximately. For an assumed inlet loading of 5 gr/ft and a
2
filtering velocity of 3 fpm (15 gr/ft -min), the pressure drop-time
history of the filtering and cleaning cycles appear approximately as
shown in Figure 6.27 (from Figure 6.26). Equations and data presented
above provide an estimate of the transient conditions in filtering and
-------
1.0-
0-75
B
a.
O
CM
33
CO
M
o
o>
0.5-
Irreducible
Residual
Dust Mass
Held in
Fabric In-
terstices
as a Func-
tion of
Fabric
Dust Prop-
erties
Amount ob-
tained tron
shake -
cleaned
filter tube
weight
0.25-
Number
of
Clean-
ing Strotes
15 sec shake
Shaking Conditions
21=1 inch
a = 1.44g
a, = 5.25 cps
Curve C, Figs. W
6.21, 6.26 -max
New fabric
0
Equilibrium
Res idual
Interstitial
Dust Mass
200 400
Filtered Dust Mass, W, gr/ft
600
2
800
Figure 6.26. Fabric Filter Performance
With Intermittent Mechanical
Shaking (15 sec shake).
-------
TABLE 6.4
SUMMARY OF CLEANING EQUATIONS FOR FABRIC FILTER DRAG
AND DUST DEPOSIT DURING SHAKING WITHOUT AIR FLOW
6.9
6.10
6.11
a .
- (2
S . = ; 0.72 < « < 6, For a > 6, S . -» Constant (0.19)
a 1/2 ~ ~ rain
S - S
.
mm
S - S
.
t mm
6.12
(cot) =
6.13
6.14
W 810 (l-e"a)
max
W W
^ -f logfaeut)- f ; jj£- < 1
max max
6.15
= (a*)w« ; 2 es > 1 inch
s
cleaning cycles for various values of filter operating and cleaning
parameters. As an illustration of their utility, Figure 6.28 has been
constructed for a cleaning time of 5 seconds, keeping all other para-
meters constant (taken from the data shown in Figures 6.18 and 6.23).
Much less dust is filtered prior to reaching terminal drag. The corres-
ponding pressure drop history is shown in Figure 6.29. The number of
cleaning strokes used per hour for the 5 second cycle is 75, (Figure 6.28),
as compared to 108 per hour for the 15 second shake cycle, (Figure 6.26).
The amount of dust collected is 900 gr/ft -hr with either cycle.
In contrast, a typical pulse-jet collector operated under
-I
similar conditions of inlet concentration (fly ash, 5 gr/ft ) at a fil-
tering velocity of 8 fpm would clean each bag approximately 88 times per
hour, and each cleaning cycle (0.16 sec duration, 100 psi jet air) would
2
remove approximately 27 gr/ft of dust. Equilibrium pressure drop for
-------
1} tec
•hake •
cycle (80
•trokei)
Number of Cleaning Strokei/Hr
108
20 50
OmtATIM TME,
Figure 6.27. Filter Preieure Drop Hletory (Coniteat Velocity ead Duet Concentr«tloo).
this collector would bi1 effectively constant at 4.0 in. H_0. Amount of
2
dust removed would be 2400 gr/ft -hr. Short-cycle on-linp cleaning has
been developed rapidly by filter manufacturers during the past few years.
The number of cleaning operations per hour in these designs is approxi-
mately equal to or less than the number of cycles required in shaking
designs, so fabric life is probably at least as long. Most use felted
fabric having inherently high collection efficiency, so that short-cycle
cleaning does not adversely affect average collection. Dust handling
capacity is much higher per unit fabric area with on-line cleaning.
Walsh and Spaite discussed the effects of the duration of
cleaning cycle on filter capacityin terms of practical applications of
the research results presented above. They indicate:
"...that the influence of factors affecting the transmission
of motion may be directly related to N , and it has been
shown that longer shaking amplitudes might be desirable
since they favor the transmission of motion. A definitive
conclusion cannot be reached, however, until the influence
of amplitude, on filter wear is known."
-------
1.0
•O.75
rt
(X
O
CN
PC
0.50
t>n
0.25
m
New Fabric.
5 sec. shake
Shaking conditions
2e=l inch
c*=1.44g
W=5.25cps
Curve C.Figs. 6.21 &6.26
I
200
i
400
Equilibrium
Residual DnsL Mass
Filtered Dust Mass, W, gr/Et
2
Figure 6.28.
Fabric Filter Performance with Intermittent
Mechanical Shaking (5 sec).
"Another application of the data is related to reducing
filter wear for a given shake by minimizing the total number
of strokes applied over a period of time. The cleaning dura-
tion which will achieve this effect can be found by drawing
a line from the origin and tangent to the curve of a "capa-
city vs duration" relationship, as shown in Figure 6.30,,
The duration at the point of tangency will allow the most
dust to be filtered per shaking stroke, such that the total
-------
o
C
•H
Q.
O
t-l
•a
D
v.
-U
J-,
PL,
3.0
2.0-
Data 1'rom
Figure b.31
Assume C.=5gr/ft
V = 3 fpm
Stop
main
dusty
gas
t: 1 ow >
3
Number of cleaning strokes/hr
25 x 3 = 75
I
10
Operating time, rain.
20
0'
Kigurt- h.29. filter Pressure Drop History (Constant Velocity
and Dust Concentration.
number of strokes utilized over a period of time (say one
week), will be significantly less than the tptuJ number usod
with a duration of Nw strokes. The total amount of dust
collected over this period and maximum filter drag for each
cy.-le would be the same in both cases. For the illustration
shown, I he fotnl number of strokes required for operation at
this optimum condition For a 40-hour interval would ho approxi-
mately one-third the number required if N strokes wore used
for each cleaning period".„.(also see a similar comparison in
Figures 6.26 and 6.28 using 2/3 the number of strokes in going
to N vs N , respectively).
"In practice, operation unde? this condition probably would
produce a residual dust ...(deposit)... less uniform than that
after Nw strokes, so that significant aging (gradual increase
in residual drag with time) may complicate operation in this
-------
region...The extent of such nonuniformities can be seen by
considering local velocities of filtration as shown in Figure
6.31. These data were taken while determining the curve of
Figure 6.30 and indicate the degree of nonuniformity that
existed after several cleaning durations. For instance, the
profile after cleaning with 28 strokes shows velocities on
the order of five to six fpm through the upper portions of
the filter, while the profile after cleaning with 688 strokes
shows a uniform velocity of three fpm over the entire filter
surface. The former situation indicates that the shaking
motion was effective in reducing local filter drag only at the
top of the bag; hence, velocities there are higher than aver-
age. Repeated operation with this shake would result in a
gradual build-up of drag at the bottom of the filter and a
gradual increase in the effective; drag of unit. This type of
drag Increase would not occur when operating with a duration
of 688 strokes because the entire filter area would be affec-
ted by the shaking motion".„.(also see Section 6.2.2.2 above
for residual profile uniformity with duration of shaking).
"Figure 6.19a, which contains several curves representing
residual drag as a function of acceleration and terminal drag,
has certain practical implications. These data indicate that
in the region below the optimum acceleration, residual drag
was dependent on terminal conditions (see equation 6.11),
while in the region above the optimum acceleration it was
practically constant (see equation 6.12). In other words,
changes in conditions such as increased cycle times or in-
creased dust concentrations, which would increase the terminal
drag of a unit, will increase residual drag if the accelera-
tion of the shake is below optimum for the system. Also, if
a unit is operated on a time-cycle and the acceleration is
below optimum, both residual and terminal drags will increase
from cycle to cycle1'until the drag reduction during cleaning
equals the drag increase during filtration. This effect will
be in addition to any other normal increases associated with
an approach to equilibrium. Assuming that cleaning duration
is in excess of N , a reduction in residual drag can be
accomplished only by increasing acceleration. This type of
phenomena may explain the results reported by Lemke, et al.,
(12) (ZnO from hot dip galvanizing kettles), wherein the
residual drag of two pilot-plant filter units, operating on
a timed cycle, increased gradually over a period of time.
The increase was of such an extent that vigorous hand shaking
was periodically required to maintain reasonably low filter
pressure differentials. The vigorous shaking by hand, then,
was comparable to increasing the acceleration of the shaker.
It would seem possible, therefore, that a dependence of res-
idual drag on terminal conditions may be a general indicator
of insufficient acceleration, and that the region of opera-
tion (i.e., above or below the optimum acceleration) may be
determined by varying filtration times an^. monitoring the
effect of such changes on residual drag."•'•I
-------
000
eoo
z
4
a
4 400
a
A-MAKlMUM
/ AVtHACC
/
SLOPE
I I I I
TtuwWiLWWO • i OiN M,0/t»«
IDURATION FOR MAXIMUM CAPACITY PER SHAKING STROKE
* I I I I I I I I II
100
ZOO 300 400 500 6OO 700 800 900
CLEANING DURATION (NUMBER or STROKES)
1000 MOO
DETERMINATION OF DURATION FOR MAXIMUM CAPACITY
PER SHAKING STROKE
Figure (>..}0.
Determination of Duration for Maximum
Capacity Per Shaking Stroke. (From Walsh
;
' / '
/
/
*
PROFILE A
DURATION
OF
28
STROKES
1 1 1
1
-
—
/
f\
} '
(
\
1
'PROFILE B
DURATION
OF
344
STROKES
1 1 1
1 /I 1 1
PROFILE C
DURATION
OF
; 688
STROKES
I/ 1 1 1
t : » « e
FILTER VELOCITY
-------
Walsh and Spaite presented the following conclusions:
"1. A well-defined limit will exist beyond which increased
shaking with a given shaking stroke will not significantly
contribute to further cleaning (see equations 6.11 and
6.14).
2. Filter capacity and residual filter drag are a function
of shaker amplitude and the square of shaker frequency
(maximum acceleration of bag cap), but again a well-
defined limit exists beyond which increases in these
factors will not result in further cleaning (see equa-
tions 6.10 and 6.13).
3. Shaker amplitude has an influence on the number of clean-
ing strokes required for a given degree of cleaning.
This influence is independent of its effect on the maxi-
mum acceleration of the shaking stroke (see equations
6.12 and 6.15).
4. A relative measure of the influence of factors affecting
the transmission of shaking motion can be obtained by
considering cleaning durations required to produce mini-
mum filter drag.
5. For any given shaking arrangement there will exist a
cleaning duration which will produce the least bag wear over
a period of time." 11
There is a definite requirement for study of the effects of
these same shaker variables on fabric wear and bag life for optimization
of cleaning costs.
6.2.2.4 Effects of Fabric Structure. - There are more than a
dozen generic types of man-made fiber materials that can be used in fabric
for gas filtration, in addition to cotton and wool. Fiber, strand, twist,
yarn, yarn count, weave, and finish parameters are discussed in Chapter 4.
2
The initial studies of Snyder and Pring indicated an effect of fiber
material, fabric weave, and mechanical finish on filtration performance
(See Figures 6.3a, b, and c, and Tables 6.2a and b). More recent inves-
tigations of the effects of fabric structure in pilot scale systems have
13 11
been presented by Spaite and Walsh (two bag unit, Figure 6.19),
14 15
Durham and Borgwardt, et al. Other data have been presented in
Chapter 2.
13 R
Spaite and Walsh studied the performance of three Fiberglas
£
and two Dacron fabrics, woven from continuous filament yarns (specifica-
tions and photographs of test fabrics are given in Figure 2.26a).
-------
"Basic performance curves for the Fiberglas fabrics when
operated at a nominal velocity of 2 fpm are shown in Figure
6.32a. Each set of test bags were exposed over the entire
range of nominal velocities investigated (2 fpm to 10 fpm),
and two runs were made at low velocity to determine the
extent of changes in the residual medium during the tests.
The lower curve for each fabric was obtained after new bags
were operated at 2 fpm for 48 hours. The upper curve for
each pair shows results of a repeat run made at 2 fpm under
conditions that were identical, except that the cloth had
been exposed for one week to the higher filtration velocities,
The higher resistance after exposure is a reflection of the
effect of dust penetration into the fabric, or "aging".^
Figure 6.32a shows an increased effect of aging on the more
n
open Fiberglas fabric No. 1, as compared to No. 3:
"...the tighter weave, No. 3, exhibited a higher resistance
to flow when it accumulated a semi-permanent residual dust
mass. Much of the advantage in low residual drag shown
initially by the open fabric, No. 1, was cancelled out by the
effect of aging; ...dust cake permeability was affected, but
little by .aging.
The importance of changes in filter drag and filter resis-
tance coefficient is most evident when their combined effect
is shown as total resistance to flow. In Figure 6.32b, the
increase in filter pressure differential that occurs during
filtration is shown as a function of the mass of dust fil-
tered. These curves represent the pressure increase during
operation at velocities of 2 fpm, 4 fpm, and 6 fpm on the
tightest (No. 3) and loosest (No. 1) FiberglasR fabrics.
The filtration capacity of the fabrics, in terms of the mass
of dust than can be collected before a given differential
pressure is exceeded, depends on the fabric structure most
strongly at high filter velocities. At higher velocities
this dependency becomes greater as the mass deposited becomes
greater; at lower velocities it becomes less significant as
the total mass increases.
The effect of fabric structure and nominal velocity on the
specific dust-fabric filter resistance coefficient is shown
in (the upper pnrLion of) Figure 6.33. In every instance
the dust deposit creates an increase in 1'he specific resis-
tance coefficient as filtering velocity is initially in-
creased.
These changes in the resistance coefficient are substantial
over the range of velocities that would exist in multicompart-
mented units operating at nominal velocities of 2 to 3 feet
per minute. Thus, they are an important consideration in any
analysis of flow and resistance patterns in such an installa-
-------
WET GROUND MICA TFST OUST
NOMINAL VELOCITY (3) 2 Ipm
INLET CONCENTRATION (5) 5 GRAINS/FT
Dust Deposit Density, W (gr/ft^)
Figure 6.32a. Effect of Fiberglass Fabric Fill Count Variation
WET GROUND MICA TEST OUST
— FIBCRCI ASS N«3
flBfHGLASS N« I
Dust Deposit Density, W (gr/ft2)
Figure 6.32b. Effect of Fiberglass Fabric Construction
WET OAOUNO MICA TEST OUST
NOMMAL VELOCITY (g) t «»m
I^LET COWCENTWTK5N ® S ORA(HS/FTS
0 100 200 100 400 SOO tOO TflO *X
Dust Deposit Density, '.-.' (gr/ft^>
Figure 6.32c. Effect of DacronR Fabric Fill Count Variation.
(From Spaite and Walsh, Ref. 13)
-------
•o
4O
JO
« 20
1 -
I
U
i
(bottOB f««<). mlct
duit) J u»,
D R O.SM*
P
FIBERCUS
(top feed,
fly uh.
I) tin
COTTON SATEEN
(Cop (<«d, fly
-------
n
The basic performance curves for Dacron , shown in Figure
6.32c are similar to those obtained with FiberglasR. The.
filter drag at the end of the period of deposit repair was
again found to be higher for the tighter weave fabric,
Dacron B. The accelerated increase in initial drag for both
Fiberglas and DacronR indicates that such an effect is char-
acteristic of the continuous-filament yarns.
The effect of velocity on the specific resistance coefficient
for the mica dust deposited on Dacron followed the same gen-
eral pattern shown for Fiberglas^ in Figure 6.33 (upper
portion).
The increases in pressure differential across the filtering
medium during the filtering cycle are essentially the same
for both Dacron^ and Fiberglas^.: The previously discussed
similarity in the relationships for the individual components
of the pressure loss (i.e., loss across the initial dust
fabric medium and loss through the deposit...) are reflected
here in curves that are so nearly identical to those shown
for Fiberglas^ in Figure 6.32b that it is unnecessary to show
both sets.
A second measure of performance that must be considered when
fabric structure and velocity are varied is the amount of dust
discharged through the fabric. Most of this discharge occurs
in the early stages of filtration.
Figure 6.34 shows the manner in which dust discharge varied
as a function of nominal velocity and Fiberglas^ structure.
Each filtering cycle was terminated at a drag level of 1.5
in. H20/fpm. Because of this, some cycles were ended before
they could progress, substantially beyond the region of for-
mation or repair of the uniform filtering medium. Such tests
are designated by circles on the curves.
A most interesting aspect of Figure 6.34 is the curve which
shows dust discharged per cycle as a function of velocity for
FiberglasR No. 1. The data points numbered 1 through 7 repre-
sent the results of 7 consecutive tests of this fabric, each
made after 24 hours of operation at test velocity.
The efficiency was poor in early tests, but showed a marked
improvement, associated with the aging of this loosely woven
fabric. The interim runs at low velocitiep (points 5 and 7)
indicate that the improvement in efficiency tended to be per-
manent. During Run 7 the dust discharge changed from 11.8
grains/sq.ft. for the second cycle to 10.9 grains/sq. ft. for
the fifty-second cycle. Subjective estimates of filter effi-
ciency support these data: before aging, a number of pinholes
and high velocity air jets were in evidence over the surface
of the fabric; after aging, these largely disappeared. During
the tests, the efficiency of the fabric changed from 85.5
percent for Run J to 96.1 percent for Run 7'.*3
-------
The curves i.n Figure 6.34 show that dust discharge was approx-
imately proportional to velocity and inversely proportional to Tilling
yarn count.
"Conditions that developed in the filter enclosure were simi-
lar to those observed in industrial fabric filters. The bags
developed a slight dust layer on the exterior surface, and
some dust collected on the floor of the clean air side. The
weight of this material was not included in the dust discharge
data. The dust discharge data are probably higher than would
be encountered in actual use because of the intensive cleaning
employed for these tests.
•p
For the Dacron fabrics, the effect of nominal velocity on
dust discharge per cycle is shown in Figure 6.34b. A general
linear relation is again shown, but the relationship is dif-
ferent in that the Fiberglas* fabrics allowed greater dust
discharges at low velocities and Dacron^ generally showed a
lower rate of increase in dust discharge with velocity.
These differences might be attributable to a more even dis-
tribution of air flow through the DacronR fabric. An
indication of the distribution of areas available fui. high
air flow is obtained by considering light transmission
through the fabrics (Figure 2.26a).
The most striking point in the comparison of the two cloth
types is that the two materials with nearly equal clean-cloth
permeability values (Fiberglas^ No. 1 - permeability 15.84
and Dacron^ B - permeability 14.62) have different performance
characteristics. The difference is reflected in dust mass vs.
filter drag curves (Figure 6.32), and the efficiency curves,
(Figure 6.34). The comparisons demonstrate the impossibility
of predicting performance of an untried fabric from permeabi-
lity data and known performance data for a second fabric, even
when the same dust is involved. Thus, cloth permeability
per se seems to be of less importance to these filtration
parameters than the manner in which the fabric structure is
changed to alter permeability1^^
The effect of filtering velocity on the specific dust-fabric
filter resistance coefficient was also investigated by Borgwardt, et al
Their flyash data are shown in Figure 6.33 (lower portion). They found
that:
K2' ~ V372 (6.18)
for 1 < V < 4 fpm. The earlier data of Spaite and Walsh (Figure 6.33,
upper portion, mica dust 5-10 urn D x 0.5|im thick) are less well-
-------
N*l
WIT MOUNO MIC* TC8T OUST
TCNMNM. OHM $ 1.9 IN. M,0 / fpm
( ) INDICATE EXKMMENTM. WOW
0 MOttATC K.EEOMO THMUOHOUT CYCLE
FIKMLAM N»2
r 4 * * 10 it i4 it
Filtration Velocity, V (fpm)
Figure 6.34a. Effect of Nominal Velocity on Dust
Discharge for Fiberglass Fabrics.
Irt
I
40
~ li
UJ
d >o
»«T GROUND MCA TEST OUST
TERMINAL DRAG $ I S IN H.O / Ipm
• INDICATE BLEEDING THROUGHOUT CTCLE
OACRON A
o r « t < >o ie <4 K
Filtration Velocity, V (fpm)
Figure 6.34b. Effect of Nominal Velocity on Dust
Discharge for Dacron^ Fabric.
(From Spaite and Walsh, Ref. 13)
-------
behaved, but indicate that K- ~ Vn for n > 0 and 2 < V < 8 fpm. In each
of these studies fabric, dust, and velocity all affect K~. (See also
Section 2.4.8.4).
Figures 6.32(a) and (c) can be used to determine the unknown
interval (S - S ). The nonlinear portion of the S-W curves occurs during
e r 2
the period of dust deposition less than about 100 gr/ft . Approximate
values of the interval (S - S J are ~ 0.2 in.H 0/fpm.
14
Durham and colleagues have extended the above investigations
to nine additional woven fabrics indicated in Table 6.5. Six different
R R R
fiber materials were tested: Nomex , Teflon , Polypropylene, Orion -
R R
Acrylic (Microtain ), Dacron and Cotton. Yarn configurations included
continuous multifilament warp and fill, filament warp and spun fill, spun
T>
warp and fill, and napped (Dacron ) woolen system yarns-warp and fill.
Weaves and thread counts were different for each fabric, as was fabric
3
weight and clean air-flow permeability. Three grains/ft of flyash test
dust ( ~ 10 (am D ) was filtered at 4 fpm (constant mass flux 1.7 x 10
2 P
Ibs/ft min) in all tests (top entry, single 5.5 in. diam. x 66.6 in.
long bag, under suction). Cleaning was accomplished with a mechanical
reciprocating shaker attached to a flexible connection at the bottom of
the bag (2 e = 1.75 in.; co = 5.6 cps; duration, 0.5, 1, 3, and 5 min; cot
= 170, 340, 103, 1.7 x 103 strokes; ,1 = 2.85 g's). (See equations 6.10
through 6.15 for effects of shaking parameters on minimum residual drag
and maximum dust capacity per filtering cycle). Filter drag-deposit
density (S-W) curves are shown in Figures 6.35a, b, and c, and 6.36.
Fabrics have been grouped qualitatively by yarn free available fiber
area, according to the classification hypothesis indicated in Figures
2.69 a-d (Chapter 2). Four classes of fabric were identified according
to a qualitative estimate of free available fiber:
(a) Continuous multifilament warp and fill, Figure 6.35a
j^
1. Nomex
2. TeflonR
3. Polypropylene
* W is dust deposited on the fabric, as calculated from dust fed and
hopper fallout (see discussion below).
-------
TABLE 6.5
Fabric Filter Media Specifications and Performance with Constant Particle Flux
(1.7 x 10"3lbs/ft2-min)
Fabric
••bar
1
2
3
4
5
6
7
1
9
T5» —
Conpoelttoo
•0.1'
Teflon*
Poly-
propylene
*-.»
D«ro.»
Cotton
Orion*
ee
acrylic
Dae too*
Tarn
Fl leant
»arp & fill
FUnent
•arp & fill
filament
varp 6 fill
fllanent
varp;apun
fill
Pliant
uerp:»pun
fill
Spun varp
6 fill
Spun varp
4 fill
Spun varp
4 fill
Woolen
ayet.
Heave Count ,
threads /In.
3*1 96*78
tvlll
3,1 76r.il
tvlll
3*1 74*73
tvlll
3*1 95x58
tvlll
3*1 77*81
' tvlll
Sateen 97*63
SH-U
)*2 85x77
tvlll
2*2 39x35
tvlll
2*2 41*39
tvlll
Clean
Air Flov
FeraeeMllty
15-20
20-40
10-20
20-25
20-30
15-20
15-25
to
55-65
Clean
Drag
0.028
0.017
0.033
0.022
0.020
0.028
0.025
0.008
0.008
Fiber
Denalty,
../c.5
1.38
2.1
0.9
1.38
1.3*
1.50
1.14
1.14
1.38
fabric
Veltbt
o»/ys*
4.5
8.6
4.3
5.4
5.8
9.5
5.7
9.8
12.5
Teredeal
drag.S,
la.ayvfpaj
1.5
1.0
1.5
1.0
1.0
1.0
1.0
0.7
1.0
Drag.S ,
In.HjO'fi
0.40
0.15
0.21
0.22
0.05
0.41
0.20
0.05
0.09
tlve
" J"»- 1.
In.BjO/fpn
0.99
0.54
0.92
0.55
0.41
0.56
0.32
0.25
0.20—
t-s
0.59
0.39
0.71
0.33
0.36
0.15
0.12
0.20
0.11
Terminal
Bepotlt,
' It. /ft2
0.045
0.055
0.095
0.060
0.075
0.077
0.12
0.10
0.205
Storage
Capacity.
"t««
0.030
0.055
0.06)
0.060
0.075
0.077
0.12
0.14
0.205
Specific KiUtance
Coefficient,
(IWf?2)-'
11.5
8.7
6.2
7.8
7.7
5.8
5.8
4.8
2.4~*
»|lpiMl.a»e
0/1000 cfv. '
0.85
0.51
0.68
0.47
0.39
0.39
0.33
0.23
0.17
• - cfWft at O.S la. EjO. eatuved linear Of - V.
e* - Ubaay felt Hlcroula*
*** - latiaated from linear portion of S-W curve up to U < 0.1 lbe/ft , then t^' Increaalag to 8.5 at V - 0.2 lee/ft
•ote: n« eah teat duet. 10 u« dlam., C, - 3 graln»/ftT, » - 4 fp., cleaning tin. 180 aec. 5.5 1«. «la». « 66.6 In. lea* bag*.
-------
(b) Filament warp and spun fill, Figure 6.35b ^
4. Nomex
_ _ R
5. Dacron
(c) Spun warp and fill, Figure 6.35c
6. Cotton Sateen
7. OrlonR -'
T>
8. Acrylic (Microtain )
(d) Napped, Figure 6.36 _,
T>
9. Dacron
i
Effects of increasing amounts of free available fiber should —<
include reduction in K/ and increased amount of dust storage capacity
(W /S ). More free fiber (A -* D) should also reduce the amount of dust ^
deposit released for a given shaking cycle, An increase in o)t on class A
fabrics should produce little additional dust removal (AW small as 0) . /
o Dust storage capacity (Wt/St) increases with amount of free
available fiber (except for polypropylene, which is higher
than expected). Capacity varies from 0.03 to 0.2 Ibs dust/
ft of fabric per unit drag. ""*
i
-------
1.5
e
•a
o
c
00
n)
£
0)
c5
0.5
1. NomexR Filament
warp and fill
\3. Polypropy-
lene Fila-
ment , warp
and fill
Te.flonR Filament
warp and fill
V .t^ .
t 30 sec
0 ° O L80 sec
0 300 sec
cleaning time
1.5 '•
0
40 NomexR
Filament warp,
DacronK
Filament
warP. Spun
till
4- )0 HIM'
OO- 180 sec
0 300 sec
cleaning tinu1
_L
.02 .04 .06 .08 .10 .12
.02 .04 .06 .08 .10
T
(a) Dust Deposit Density, T7, lbs/ft2 (b) Dust Deposit Density, T7, Ibs/ff
1.5
a
•H
s
7. Spun OrlonR
6. Cotton Sateen
8, Spun
Acrylic
30 sec
180 sec
Q° 300 sec
cleaning time
i t i i
0 .02 .04 .06 .08 .10 .12 .14
(c) Dust Deposit Density, ft, lbs/ft2
Figure 6.35. Effect of Fabric on Filter Performance
(From Durham, Ref. 14).
-------
s 1.0
W
c
•H
c/1
u °5
(0
9. Napped Dacron^
+ 60 sec
o 180 sec
o 300 sec
cleaning time
'
1
Q02 Q04 0.06 0.08 O.I2 0.14 0.16 0.18 0.20 0.22 0.24
_ 2
Dust Deposit Density W, Ibs/ft
P
Figure 6.36. Effect of Shaking Duration on Dacron Fabric
Filter Performance (From Durham, Ref. 14).
o Amount of dust removed by an increase in shaking duration
(cot) is relatively lower with filament fabric (No. 1).
Duration increase from 30 to 300 sec (170 to 1.7 x 103
strokes) produced a 0.005 lb/ft^ increase in dust holding
capacity. Spun yarns (Nos. 5, 6, 3) tend to release dust
more slowly, an increase in duration from 30 to 300 sec.
producing a 0.015 lb/ft^ increase in capacity. Napped
fabric (No. 9) shows the largest effect of_increased
shaking duration, 60-300 sec. producing W of 0.03 Ib/ft
At_an inlet flux of 1.7 x 10"3 Ibs. dust/ft2-min, these
AW values correspond to an increase in operating time
between cleaning cycles of (Col. 4):
Fabric
Filament
Fil./Spun
Spun
Spun
Napped
No.
1
5
6
8
9
W
30-300
Time of
W
30-300
Time to
0.005
0.015
0.016
0.014
0.03
(60-300
2.9
8.8
9. it
8.2
17.6
sec)
min
'180
26.5 min
44.0
45.0
59.0
121.0
-------
V
o Increasing cleaning Duration from 30 sec. to 5 min. on fil-
ament fabric (No. 1) produces about 3 min. longer filter-
ing time in a 26 min. cycle, or no net improvement. The
filtering cycle is lengthened less than the amount required
for the additional shaking.
o Increasing cleaning duration from 30 sec. to 5 min. on
spun yarn fabrics (Nos. 5, 6, 8) produced about a 9 min.
longer filtering cycle in a 45 to 60 min. cycle, or about
15 to 20 percent longer filtering cycle.
o Increasing cleaning duration from 1 min. to 5 min. on the
napped fabric (No. 9) produced an 18 min. longer filtering
cycle (15 percent increase in filter operating time be-
tween cleanings).
Dust characteristics will modify the quantitative observations
above, but general trends shown are expected to be the same. For most
dusts, increased cleaning duration beyond 30 sec. or 1 min. will probably
not produce a net increase in filtering time greater than about 10 to 15
percent. The effect of increased cleaning time on reduction of fabric
life is assumed to be linear. A five to ten times increase in cleaning
duration (30 or 60 sec to 5 min) will probably reduce fabric life by a
similar factor. Assume fabric life is of order 5 x 10 shaking cycles.
Cleaning for 30 sec uses 170 cycles (5.6 cps). Let the filtering cycle
be 45 min long followed by a 30 sec shake and 30 sec dwell for settling.
Then every 46 min will require 170 shake cycles. Fabric life is esti-
mated as (5 x 10 x 46/170 x 60) 2.8 years. If the cleaning duration is
extended to 3 min (6 x, or 10 shaking cycles per cleaning cycle) the
filtering cycle may be extended about 10 percent or 49.5 min. The com-
plete cycle will consist of 49.5 min of filtering followed by 3 min shake
and 30 sec dwell for settling, or 53 min. Fabric life may be estimated
f O
as (5 x 10 x 53/10 x 60) 0.55 years, or about l/6th of the life obtained
with shorter cleaning duration. Pulse jet collectors cleaned on-line
every 30 sec (120 cycles/hr) would be expected to have an average fabric
life of order 5.2 years, under 'the above assumptions. None of these esti-
mates include effects of factors tending to reduce fabric life during fil-
tration, such as scour and abrasion, temperature, condensation, acid or
alkaline deterioration and other gas or particle factors.
14 —
Durham observed anomalies in S-W curves for different
shaking times for tests at different parts of the year. For example, in
-------
Figure 6.35a, Fabric No. 1, it was found that data produced for 3 min.
shaking did not agree with data for 30 sec and 5 min shaking obtained
several months later. Similar anomalies are evident in data for fabric
R ~
Nos. 6 (cotton) and 8 (Microtain ). The shift in the S-W performance-
curves arc positive (Nos. 1 and 8, to the right, higher W per units)
and negative (No. 6, smaller W /S ).
6.2.2.5 Effects of Humidity - Durham and Harrington
have shown that the apparent anomalies in performance just described
probably result from changes in ambient moisture content (Relative
Humidity, R. H.). Relative humidity was controlled between 20 «nd 60
percent R.n. In the same experimental configuration described above. ^
Resuspended fly ash was used as a test dust, and the 11 fab-
rics enumerated in Table 6.6 were tested. Increasing relative humidity
generally reduced overall resistance of the filter. Three responses were
calculated to identify the system resistance: effective drag, S , specific
/ e
resistance coefficient, K2, and terminal drag, S . (Figures 6.5 and 6.35
show typical values of these three parameters.)
Table 6.7 presents results of the effects of relative humi-
dity on K™, S and S . The specific dust-fabric filter resistance coeffi-
cient was significantly reduced by increasing the relative humidity. Effec-
tive drag exhibited no particular trend. In most instances, terminal drag
was reduced with increasing relative humidity. The specific resistance
Coefficient determines the rate of increase in resistance during the linear
portion of the filtering cycle to achieve any given value of terminal drag.
Thus, reduction in K« offers potential for increased bag life and reduced
maintenance (i.e., less frequent shaking required). Figure 6.37 illustrates
the effect of R.H. (20 < R.H. < 60 percent) on K_ for three acrylic fabrics
and for polypropylene. An increase in R.H. from 20 percent to 60 percent
reduced K' by a factor of order 2.
-------
ON
U)
TABLE 6.6
FABRIC CHARACTERISTICS
(From Durham and Harrington, Reference 16)
Fiber
Composition*
Nylon
Creslan
R
Dae r on
Polypropylene
Crylon
R
Dralon
OrlonR
Cotton sateen
Glass filament
Glass Combination
Glass texturized
Type
Warp
CF
CF
CF
CF
CF
CF
CF
S
CF
CF
CF
Yarn**
Fill
CF
CF
CF
CF
CF
CF
CF
S
CF
S
T
Thread Count ,
Yarn Denier threads/in;
Warp Fill Warp Fill
210 210 74
200 200 80
250 250 76
210 210 81
200 200 77
200 200 76
200 200 76
95
54
48
46
68
76
66
69
63
71
62
58
56
22
24
Fabric
Weave Thickness,
Pattern Mils
2 x
3 x
3 x
3 x
3 x
3 x
3 x
2
1
1
1
1
1
1
Twill
Twill
Twill
Twill
Twill
Twill
Twill
Satin
3 x
2 x
3 x
1
2
1
Twill
Twill
Twill
9
10
9
12
10
9
8
24
9
24
16
.4
.8
.1
.4
.2
.8
.7
.1
.6
.6
.5
Fabric
Weight
oz/yd
4
4
3
4
5
4
4
10
9
16
14
.1
.0
.9
.6
.1
.4
.3
.5
R R R
* Creslan acrylic, Amer. Cyanamid; Dacron polyester, Du Pont; Crylon acrylic, Crylon S.A.(Fr);
DralonR, Farberfabriken Bayer (W.Ger.); Orion acrylic, DuPont.
-------
TABLE 6.7
EFFECT OF RELATIVE HUMIDITY ON SPECIFIC RESISTANCE COEFFICIENT, EFFECTIVE DRAG, AND TERMINAL DRAG
(From Durham and Harrington, Reference 16
Specific dust-fabric filter
resistance in. H.O/ft-min
lb/ft2 L
Effective drag,
in. H 0/ft-min
Terminal drag,
in. H 0/ft-min
Description*
Relative humidity, %
20 30 40 50 60
Relative humidity,7=
20 30 40 50 60
20
Relative humidity, 7=
30 40 50 60
R
Nylon
Creslan
Dacron
Polypropylene
Ti
Crylon
T, -, R
Dralon
t>
Orion
Cotton
Filament glass
Combination
glass
Texturized
glass
11.2 7.6 6.4 3.9 2.6
9.9 9.6 7.8 5.6 4.0
8.4 7.7 7.2 4.2 3.0
9.6 7.8 7.8 5.0 3.6
6.4 3.8 4.2 4.2 2.3
9.3 5.2 4.2 4.1 2.3
6.8 6.5 4.3 3.8 3.4
8.0 7.5 7.3 5.8 5.4
5.7 5.6_ 6.5 6.2 5.3
5.1 4.3 4.4 3.7 3.2
0.24 0.23 0.32 0.32 0.30
0.12 0.14 0.23 0.30 0.29
0.69 0.61 0.63 0.60 0.62
1.05 0.98 0.99 0.94 0.95
0.12 0.12 0.18 0.21 0.32
0.45 0.40 0.38 0.37 0.32
0.78 0.82 0.74 0.59 0.61
0.46 0.48 0.45 0.45 0.49
0.53 0.55 0.48 0.63 0.68
0.25 0.24 0.20 0.18 0.20
7.7 7.0 6.4 5.5 4.3 | 0.49 0.45 0.40 0.41 0.42
J_
0.60 0.48 0.55 0.45 0.38
0.45 0.48 0.50 0.48 0.40
0.98 0.88 0.88 0.73 0.75
1.37 1.25 1.25 1.10 1.08
0.34 0.33 0.36 0.36 0.30
0.76 0.68 0.56 0.50 0.41
1.00 1.03 0.90 0.76 0.73
0.74 0.73 0.66 0.53 0.74
0.73 0.75 0.73 0.85 0.88
0.43 0.38 0.35 0.32 0.30
0.75 0.70 0.58 0.58 0.58
* See Table 6.6 for generic fiber description and manufacturer.
** Mass flux 1.7 x 10"3 Ibs/ft2-min, for 20 min. filtering cycle, W = 0.034 lbs/ft'
L
-------
a
01
u
•l-l
<4-l
41
U
c
id
•H C-J
VM 4J
•H VM
-------
TABLE 6.8
EFFECT OF RELATIVE HUMIDITY ON OUTLET DUST CONCENTRATION AND EFFICIENCY
(From Durham and Harrington, Reference 16)
Outlet dust concentration, grains/1000 ff3
Bag
Description*
Nylon
-, R
Creslan
R
Dacron
Polypropylene
CrylorR
Dralon
Orion
Cotton
Filament glass
Combination glass
Texturized glass
20
130
168
34
36
148
26
12
0.04
148.1
10.4
63.8
Relative humidity, °L
30 40 50 60
148
177
32
32
89
24
7.5
0.2
135.9
10.3
40.2
61
100
13.1
35
56
17
6.9
0
106.4
2.2
19.9
4.4
37
1.9
7.0
13
0.8
3.9
0
25.4
0.1
6.0
0.02
3.1
0.7
2.7
1.3
0.6
0.8
0
9.1
0.1
1.1
95
94
98
98
95
99
99
99
95
99
97
20
.62
.47
.86
.80
.12
.11
.59
.99+
.00
.65
.84
Efficiency, weight 7=
Relative humidity, 7=
30 40 50
95.02
94.02
98.95
98.96
97.29
99.23
99.75
99.99+
95.31
99.66
98.66
98.02
96.35
99.12
98.85
98.14
99.43
99.78
99.99+
96.55
99.92
99.31
99.86
98.78
99.94
99.78
99.56
99.98
99.87
99.99+
99.18
99.99+
99.80
60
99.99+
99.90
99.98
99.91
99.96
99.99+
99.97
99.99+
99.71
99.99+
99.96
Note: Inlet dust concentration C. =3.0 gr/ftj, fly ash, 4.0 urn median- diameter.
* See Table 6.6 for generic fiber description and manufacturer.
(From Durham and Harrington, Ref. 16).
I ._ L ..
-------
probably tlve major factor in producing uniformly high effi-
ciency. Figure 6.38 illustrates the effects of relative hum-
idity on the outlet dust concentrations for Crylor^, CreslanR,
Orion**, and polypropylene fabrics.
Relative humidity also significantly affects the outlet par-
ticle concentration at various times during the filter cycle.
Figure 6.39 indicates an initial outlet particle concentra-
tion of about 1.5 x 1C)6 particles per cubic foot at the begin-
ing of a 20-minute filter cycle. As the filter cycle proceeds,
the particle concentration drops off much faster as the rela-
tive humidity increases. Curves similar to those in Figure
6.39 were obtained with all fabrics except cotton sateen.
Relative humidity affects the rate at which the interstitial
openings of the filter medium are bridged, the structure of
the deposit, and the filtration characteristics of the system.
The net effect of increasing the relative humidity of the
carrier gas was to improve the efficiency of the system."16
o I8°
<4-
g 160
I
2 140
Q
2 120
100
o
o
80|-
UJ
O
40|-
20
0
O Creslan Acrylic
• CrylorR Acrylic
Polypropylene
Acrylic
10
20 30 40
RELATIVE HUMIDITY, %
50
60
Figure 6.38. Effect of Relative Humidity on Outlet Dust Con-
centration.
(From Durham and Harrington, Rof. 16).
-------
10'
RELAXIVE HIIMIDITY, 7»
D 20
O 30
A 40
• 50
• 60
0
Figure 6.39.
8
10 12 14
FILTER TIME, min.
16
20 22
Relationship of Particle Concentration and Filter Time at
Various Relative Humidities. (From Ref. 16)
-------
These results are consistent with observations of increased
particle adhesion force at higher R.H. (c.f. Figures 2.17 and 2.19, and
Equation 2.15). The increased force should tend to produce a more open,
porous, and filamentous deposit having lower K' and better retention (less
particle migration and bleed). Humidity may also affect the size of the
particles collected on the filter.
It should be noted that the above effects were obtained only
with fly ash, while humidity had no apparent effect on resistance or col-
lection efficiency when using cement dust, pulverized limestone, or amor-
phous silica under identical test conditions. Clearly, further research
is needed to determine what aspects of fabric and particles affect, through
humidity, the resistance and collection efficiency of a filter system.
6.2.2.6 Effects of Velocity - Several aspects of filter per-
formance have been observed to depend upon total gas flow rate or average
filtering velocity. These include:
o Increase in specific resistance with velocity;
o Amount of dust depositing directly in the storage
hopper before reaching the filter (hopper fallout);
o Particle size stratification of dust passing to the
fabric because of fallout, and within the bag, as
velocity (flow) decreases throughout the length of
the bag;
o Deposit reorieptation or consolidation if velocity
increases during filtering cycle (deposit collapse);
o Particle penetration through thin or open areas on
the fabric surface (deposit puncture).
Hopper Fallout - Hopper fallout has been observed during
single bag and single compartment studies. A fallout factor, 7, has been
defined as:
_ amount of dust deposited in hopper, W - W _
amount of dust entering collector inlet, W ' or
w (6.19)
7 = 1 - ~
W
-------
That is, / i-s the fraction of the total dust entering the inlet of a
fabric filter deposited directly in the hoppers, without being filtered
14
by the fabric. Hopper fallout is reported in the fabric study of Durham
above, (but without 7). Values of 7 depend upon dust particle size and
shape, as related to hopper, inlet;, baffle, flow, and collector configura-
i. ion parameters. Typical values Tor y from fabric I'll Lei: cumpai LiueuL
studies (discussed below, see Figure 6.50) are 0.1 - 0.25 over a flow
3
range of four for either top or bottom bag inlet .
Particle Size Stratification. - Velocity also affects dust
resistance by altering the particle size distribution in the deposit.
The bulk flow of gas and dust is parallel to the filter surface and de-
creases in velocity from a maximum at the entrance of the bag to zero vel-
ocity at the other end. Filter arrangement also produces changes in par-
ticle size distribution by inertial separation and settling of larger par-
ticles as fallout. During bottom feed operation, a greater portion of
larger particles enter the bag as the gas throughput is increased (i.e.,
dust fallout decreases). Thus, the average particle size of the fallout
varies with filtration rate, as does the average particle size of the
filter deposit. Since the resistance coefficient is sensitive to the por-
osity of the cake and the surface-to-volume ratio of the particles, as
given by the Kozeny equation, size stratification will contribute to
changes in the resistance of the dust cake.
The effect of velocity on particle size statification along
the length of a single bag is shown in Figures 6.40 and 6.41. In these
tests, made on a single filter bag, samples of the dust cake were taken
from several different vertical positions along the bag. Figure 6.40 in-
dicates the effect of velocity on median particle diameter deposited at
four different elevations (of a 60 inch bag height). Just above the bot-
tom inlet (2 in.), median particle size varies from 8 to 15 ^m, as aver-
age filtration velocity is varied from 2.5 to 8.5 fpm (approximate upward
velocity at bag inlet, 109 to 327 fpm). Just below the top, most of the
dusty gas has been filtered, upward velocities are low, and median par-
ticle size varies from 5 to 13 um as average filtering velocity varied
from 2.5 to 8.5 fpm. The deposit at the top is composed of finer particles
(higher K~). Figure 6.41 shows the effect of filtering velocity on par-
ticle size distribution at the center of the bag. Higher gas flow supports
-------
POSITION ON BAG. Inch*! from bottom
2 •
16 .
27 *
58 «•
10 n
Figure 6.40.
Effect of Filtration Rate on Particle Size in Deposit at
four bag altitudes (Single bag filter unit, bottom feed)
(From Bongwardt and Durham, Ref. 3).
7.4 fp«
6.1 fpn
3.7 fpi" A
2.5 fp» «
10
IS 20 25
FAHXCU SIZE, «ietOM
30
35
40
Figure 6.41.
Effect of Filtration Rate on Particle Size Distribution in
deposit at Center of Bag (Single Bag Test Unit, Bottom Feed)
(From Borgwardt and Durham, Ref. 3).
-------
larger particles, the median particle size increasing from about 6um at
2.5 fpm to about 14 urn at 7.4 fpm. These figures show that elutriation
of the particles has occurred inside the bags and that the degree of elu-
triation depends upon rate of gas flow. Changes in particle size distri-
bution between different positions on the filter surface result in the
formation of non-uniform filter cakes. These phenomena indicate that the
specific resistance coefficient will depend upon filtration rate, through
effects on particle size.
Deposit Consolidation. - Deposit consolidation (collapse)
occurs when forces produced on and in the granular matrix exceed frictional
and adhesive forces supporting the particle aggregate structures. Forces
tending to produce consolidation arise from static pressure differentials
across the deposit (1 in. H_0 = 0.036 psi = 5.2 psf), viscous drag of the
fluid on the particles, kinetic energy imparted by depositing particles,
flow pulsations, or physical contact with the filter. If these forces
are greater than interfacial particle-particle-fiber forces, the structure
can shift to a more compact, stable orientation. Permeability is reduced,
and pressure drop rises .
Deposit consolidation and collapse has been observed experi-
4
mentally on both bench-scale apparatus and in a pilot-scale 2-bag unit.
In experiments with fly ash on bench-scale equipment, cake collapse was
produced by gradually increasing air flow through a dust cake supported
on a flat circular filter 1.5 inches in diameter. Deformation of the
matrix occurs in steps over appreciable intervals of filter velocity.
Deposit thicknesses were measured with a microscope having a graduated
micrometer fine adjustment. Irreversible compressions up to 50 percent
4
of the original deposit thickness were observed. (Actual deposit thick-
ness was not reported.)
In an experiment with a pilot-scale 2-bag unit, filters were
2
loaded to an average dust mass of 950 grains/ft at an average filter
velocity of 3.4 fpm as shown in Figure 6.42. The resultant pressure dif-
2
ferential was 3.4 in. H~0 (effective permeability 950 (grains/ft )/
-------
I
-si
UJ
i rtuuu
E
o.
t:
x^eoo
z
V}
Z cnn
'ERMEABILITYJJGRA!
A C
o <
o <
«- 200
en
(O
2
Q <
1 1 J II 1
** '* ** **** ^2^1st COLLAPSE, A P=b-9 IN. H20
\ 2. nd COLLAPSED pr 8.7 IN. H20
~~ ^>- J^^(t<^3rd COLLAPSE, A P=I04 IN. H20
•^B" O CT O w*^ H2U
_ \.,xAP=l5.8 1N.H20 _
NOTE:
DUST MASS CONSTANT AT 950 GRAINS/ft2,
CAKE DEPOSITED AT 3.4 fpm
KEY:
m m INCREASING Ap
o o DECREASING A P
1 II 1 1 1
324 6 8 10 12 U
AVERAGE FILTER VELOCITY (fpm)
Figure 6,42. Successive Deposit Collapse Observed On Pilot-
-------
(in. II 0/fpm) ). Filter velocity was lowered to about 0.75 fpm and then __,
raised to 5.9 fpm during which time permeability remained constant at 950
(K' =7.2). As filter velocity was gradually increased above 5.9 fpm,
however, permeability decreased rapidly to 785 (K_ = 9.9) at V = 6.5 fpm.
From 6.5 to 7.2 fpm, the permeability of 785 was maintained. Between 7.2 >
and 7.6 fpm a second collapse occurred, further reducing permeability to ~'
745 (K' = 10.1), a value which was maintained up to 8.1 fpm. At this point,
filter velocity was reduced to 5 fpm to substantiate that a new matrix —';
structure of lower permeability had been created. Two more distinct col-
lapses were observed as filter velocity was ultimately raised to 9.8 fpm, _,
and after each of these, filter velocity was lowered and then raised, to
demonstrate that a permanent consolidated structure had been produced in
each instance. After the final collapse, permeability had been reduced to
585 (K' = 11.9), or to about 60 percent of the permeability of the uncol- \
lapsed deposit. Filter drag was increased from 1.0 in. HoO/fpm for the uncol-
lapsed cake to 1.62 for the final collapsed matrix, or by more than 60 percent. ,
The same effect has been observed in single compartment and multicompartment ^
filters, as discussed below. These effects also depend upon R.H. in the gas,
and fabric parameters. ^J
Particle Penetration - Woven fabrics arc susceptible to pin-
hole flow leakage, a common phenomena with many types of porous media.
Deposit puncture, which occurs when small pinholes are opened through the
matrix, should be more prevalent on filament yarns or worn staple fabrics.
Local resistance is lowered and high flow occurs through the opening.
Local disintegration of the filter deposit is a more severe manifestation
of the application of external forces that cause deposit collapse. The •~f
puncture may be self-repairing, since increased flow through the area
brings more dust to the vicinity of the puncture. Thus, inertial deposi- _j
tion mechanisms are enhanced by the higher velocity. If the holes are
;
too large, however, penetration will continue. !
Deposit puncture has been confirmed experimentally as shown
in Figure 6.43. On a bench-scale flat filter paper it could not be in- _j
duced even at very high pressure differentials. However, when a cotton
sateen filter fabric was used as the support, puncture occurred as shown
in Figure 6.43. It is seen that the unpunctured cake had a drag of 1.0
-------
•$.
2.0
1.5
1.0
OS
NOTE: ORIGINAL DUST MASS APPROXIMATELY
1,000 groins/ft.2, CAKE DEPOSITED AT
5.3 fpm
BEFORE
•—• INCREASING AP
o—oOECREASING AP
AFTER PUNCTURE
, FABRIC RESISTANCE
10
20 30 40
FILTER VELOCITY, fpm
50
61
Figure 6.43. Deposit Puncture Observed on Bench-Scale
Filter. (From Ref. 4).
in. H-O/fpm. As filter velocity was raised above 10 fpm, drag began to
decrease sharply and continued to decrease, approaching a limit erf
approximately 0.47. Reduction of filter velocity to 10 fpm verified the
constant resistance of the punctured deposit. The pinhole punctures crea-
ted were easily visible by microscopic examination of the fabric against
a bright light.
An example of pinhole healing is described by Stephan, et.
4
al. The air velocity through the perforated area dropped from approxi-
mately 100 fpm to about 2.3 fpm during a few minutes of normal filtering.
6.2.3 Single Compartment Performance
The laboratory performance of single compartment intermit-
tently cleaned fabric filters is effectively represented by single bag
performance tests discussed above. Differences are associated with scal-
ing and in distribution of flow to each of the several bags.
-------
6.2.3.1 Shake-Type Collector - Billings, et. al., reported -"
p
laboratory performance data for a small Wheelabrator Dustube (2 #35A)
2 3
cloth tube collector (500 ft of fabric, nominal 10 cfm capacity). This _j
equipment is representative of several commercial collectors.
i
The test unit consisted of two identical steel chambers, each —I
containing 32-5" x 70" cloth tubes. In normal operation, one chamber fil-
tered the dusty air while the second was cleaned by mechanical shaking of _^
the cloth tubes. Operational time between cleaning cycles varied with dust
loading and desired pressure drop characteristics. Tubes were usually
shaken when they attained a pressure drop of 2 to 4 in. H^O.
As the number of compartments increases, different sections !
—^
may be shaken sequentially, but the proportion of bags not in operation
decreases so that large multi-chambered units operate at nearly constant j
, —i
pressure drop.
The objectives of the test program were as follows: _J
1. To evaluate the resistance-efficiency characteristics
with a variety of aerosols at different dust load-
ings and filtration rates. ~J
2. To compare laboratory results with those obtained
in the field, since only field results indicate
service life and the effect of maintenance pro-
cedures on filter performance.
3. To investigate the feasibility of this type of
unit for removal of low concentrations (e.g. toxic
materials such as Be, radioactive dusts, etc.)
from air and gas streams by means of augmentation j
with filter aids. —'
Efficiency and pressure drop tests were conducted in three j
operating modes: (a) performance of woven cotton medium with light dust
loadings and no bag shaking (i.e., basic cloth performance), (b) per- '•
_j
formance of asbestos-flocked bags with light loadings and no shaking, and
(c) performance with higher dust loadings and frequent, periodic shaking.
Light Dust Loadings.- Tests with dust loadings in the range
0.04 to 0.1 gr/1000 cu. ft. (atmospheric dust) illustrated initial fil-
6-76
-------
ter performance and provided basic information on cloth characteristics.
Figure 6.44 illustrates the efficiency and pressure drop increase as new
cotton bags slowly acquired a deposit of atmospheric dust. These results
were obtained during a period of 484 hours of operation on atmospheric
dust, but are presumed to illustrate what takes place during a few min-
utes when higher dust loadings are being filtered. Both efficiency and
100
FILTRATION RATE 3CFM/SPFT
SATEEN WEAVE COTTONBAM
KM X 68 THREADS PER INCH
i.s a
•MAINS OF OUST PCM SOU Aft E FOOT Of CLOTH
Figure 6.44. Efficiency and Pressure Drop; New Cotton Bags With Atmo-
spheric Dust, (prom Billings, et. al, Ref. 17).
pressure drop rise as a deposit forms but the rate of increase fails off
after only a few grains of dust have been deposited. (From observation
of industrial operations it is known that this increase would continue
for1 several weeks with the very low dust loadings found in ambient air.)
Over the period of this experiment penetration decreased 3 fold (from
27 percent to 9 percent) while pressure drop doubled (from 0.09 to 0.18
in.H20).
o
CM
(X
o
£
0)
CO
CO
-------
Filler Aid. - Figure 6.44 indicates that i.L takes many hours
of operation (at dust loadings less than 0.1 gr/1000 cu.ft.) to increase ^J
filter efficiency to 90 percent. For recovery of atmospheric dust,and
I
chemically toxic or radioactive dusts, a high initial efficiency is re- j
quired. This can be achieved with deep beds (up to 5 feet) of fibers or
granular solids, or with glass fiber paper filters which are discarded ;
when their resistance becomes excessive. However, operations with a
fabric filter on low dust concentrations can be simplified and mechanized ;
j
by using a filter aid. —'
Asbestos floats (fibers too small for other use) were dis- j
persed and filtered onto clean cloth of the same type as above. Table
6.9 indicates the effectiveness of this treatment in increasing the
collection efficiency of oil smoke (Diol 55, mass median diameter 1.2 urn). -J
Efficiency was measured optically.
—•
TABLE 6.9
EFFECTIVENESS OF FILTER AIDS FOR LOW PARTICULATE LOADINGS '•
Quantity of
Asbestos on
Filter Cloth
gr.V sq.ft.
0
32
50
82
154
182
194
Pressure Drop
(at 3 cfm/sq.ft.)
in H20
0.098
0.135
0.160
0.170
0.187
0.202
0.212
Smoke
Efficiency
per cent
20.3
44.9
52.7
84.7
96.4
99.4
99.9
This shows a very substantial gain in efficiency) 20.3 to i
99.9 percent) for a modest pressure drop increase (0.098 to 0.212 in.
H~0) . Since bag filters are usually designed for resistances of 3 to J
4 in. H»0 before shaking, flocked bags can be operated for many months
on low loadings before shaking of the bags and renewal of the flock.
i
-------
Table 6.10 indicates how efficiency decreased as the filter
aid was removed. Some asbestos fibers were held permanently by the
fabric; hence, after shaking, only a small smount of additional flock
restored the high efficiency characteristics of the filter.
TABLE 6.10
REDUCTION IN EFFICIENCY OF ASBESTOS FLOCKED BAGS DURING SHAKING
Time of Bag
Shaking
:Minutes
ob
0.5
1.0
2.0
3.0
5.0
7.0
10 0
15.0
20.0
30.0
40.0
.Pressure Drop
(at 3 cfm/sq. ft)
in. H20
0.212
0.208
0.204
0.196
0.182
0.180
0.176
0.171
0.167
0.166
0.166
Smoke
Efficiency3
percent
99.9
97.7
97.0
96.0
94.0
93.3
90.0
89 .0
88.4
85.3
83.0
83.0
a. 1.2 micron droplets and optical penetrometer
b. Flocked with 194 grains of asbestos per square foot of cloth
(rrom Billings, et.al, Ref. 17).
Table 6.11 lists the efficiency of plain and asbestos flocked cotton bags
for a variety of other aerosols and demonstrates the wide application
of the technique.
The quantity of asbestos required to produce high filtration
efficiency is dependent,to a large extent, on how uniformly the fibers are
dispersed on the cloth. With the flocking apparatus used a resistance
rise of only 0.1 to 0.2 in. H^O was adequate.
Commercial systems lor high efficiency performance with filter
aids have been discussed in Chapter 3 (see Figure 3.31). Wheelabrator
*
now uses a preliminary precoat , prior to addition of asbestos or other
*U.S. Patent No. 3,041, 808.
-------
TABLE 6.11
EFFICIENCY OF BAG COLLECTOR FOR VARIOUS AEROSOLS*
"— •• • ' " ' "• '
Aerosol
Atmospheric dust
Uranium trioxide
microsphcres
Copper sulfate
microspheres
Aluminum chloride
fume
Ammonium bi-
fluoride fume
Ammonium bi-
fluoride fume
Talc
Loading
gr./lOOO
ft3.
0.025
0.0079
0.86
166
15
15
0.1 -
6.0C
Median
Size by
Count
microns
0.5
0.8
0.9
0.6
0.5
0.5
1.4
Pressure
Drop
(at 3 cfm/sq.
in. H20
0.212
0.212
0.236
0.244
0.244
0.11
0.157 -
3.54
Bag
Treatment
ft2.)
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
None
None
Weight
Efficiency
per cent
99. Ob
99.9
99.1
99.2
99.7
74.0
99.9+
a. Sateen weave cotton bags, filtration velocity 3 cfm/sq.ft.
b. 85 percent efficient by discoloration test, 68.2 percent efficient
by particle count
c. Grains per cubic foot
(From Billings, et. al., Ref. 17).
fibrous filter aid, to promote release of the filter aid and any tarry
atmospheric constituents. The specific resistance coefficient for fil-
2
tration of atmospheric dust on filter aid is 16 (in. H20/fpm)/(lb/ft ).
Periodic Shaking.- Figure 6.45 indicates variations in pres-
sure drop during normal filtering and shaking operation. The cycle
includes a 15 minute filtration period, 2.5 minutes for shaking, and a
2.5 minute off-period for dust settling. There are some variations in
maximum and minimum pressures attained. For these thoroughly aged cotton
_J
-------
t»
14
t-t
~t*
o
CSl
«!.«
*
a
£'••
I"
«i.t
3
<0
S'-o
M
*.•
.•
<
.4
Q/C? • OUST LOADINt IN Oft*
• FILTCftlNa PERIOD
• SMAKIN9 PCNIOO
- acrM/so.Ft or COTTON •*• ,
TALC DUST (NYTAL 500) /
a>
•?;
/
f
>
!/
r
>
/
f
^
4
(
w1
O
«v
7
/
/
j
/
/
/
T
IMS
v»...
PE«
.... ,
H
>
/
/^
CUtIC FOC
SETTUN
CMKN)
/
*/
/
/
^
/
r
>T
0
V..
-------
10 20
30 40 50 60 70 SO 90 KX> HO 120 ISO 140 ISO ISO 170
FR.TMATION TIME.- MINUTES
2OO
Figure 6.46. Effect of Dust Loading on Rate of Filter Pressure Drop
Increase. (From Billings, et. al., Ref. 17).
greater than 99.9 percent in all cases. Pressure drop, when filtering
3 2
O.L gr/ft of talc at 3 cfm/ft , increased from 0.3 to 0.7 in. HLO in
2
30 minutes. At 1.0 gr/ft , the pressure drop increased to 1.5 in H-O at
the end of 30 minutes.
The outlet dust loading increased somewhat with inlet load-
ing, but collection efficiency remained above 99.9 percent. These data
indicate that while there are some differences in specific cases, the
general outlet dust concentrations from this type of collector using
3
cotton sateen fabric will probably be less than 0.1 gr/1000 ft (200 vg/
Data presented earlier in Table 6.8 for cotton sateen and fly ash tend to
confirm this general observation. Field performance data (Table 6.23)
have indicated similar outlet loadings. However, outlet loadings up to
3 3
180 gr/1000 ft (0.4 gm/m ) were observed with man-made filament yarns.
Relative humidity also affects filter efficiency as discussed above.
3
6-82
J
-------
TABLE 6.12
EFFICIENCY AND PRESSURE DROP OF "DUSTUBE" FILTER AT VARIOUS
LOADINGS OF "MICRONIZED" TALCa
Inlet
Loading
gr./cu.ft.
0.1
1.0
2.0
Pressure Drop
in. H20
0.3 to 0.7
0.3 to 1.5
0.5 to 3.0
Outlet
Loading ~
gr./lOOO ft .
0.012
0.034
0.070
Weight
Penetration
percent
0.012
0.0034
0.0035
Weight
Efficiency
percent
99.988
99.9966
99.9965
a. Mass median diameter 2.5 microns, geometric standard deviation 1.6,
b. Pressure drop increased from lower to higher value in 30 minutes.
c. Air to cloth ratio 3 cfm/ft.^; sateen weave cotton bags.
(From Billings, et. al., Ref. 17).
6.2.3.2 Other Single Compartment Studies.- The data discussed
below were obtained on multi-compartmented collectors operated in parallel
mode, (no compartment off-line for cleaning) such that effects studied
relate primarily to single compartment performance. The 3-compartment
3 18
pilot fabric filter system used for most of these studies, ' is shown
in Figure 6.47. Each compartment contained 8 cotton sateen bags 60 in. long
x 5 in. diameter.
Hopper Fallout.- Hopper fallout, 7, was defined in Equation
6.21 as the fraction of dust entering the inlet of a filter collector
that does not reach the fabric. Figure 6.48 shows values of 7 when fil-
tering upward and downward, at several gas volume through-puts. Average
2
air to cloth ratio ranged from 1.3 to 4.5 cfm/ft . in bottom entry, up-
flow operation:
. more material was carried into the filter bags as
velocity increased, and
. The fallout varied from 27% at 1.3 fpm to 8% at 4.5 fpm.
In top-entry, down-flow operation:
. the fraction falling into the hopper increased as
velocity increased, and
. the fallout varied from 8% at 1.3 fpm to 30% at 4.5 fpm.
-------
Figure 6.47.
Three Compartment Baghouse (From Robinson,
et. al., Ref. 18).
Lacking internal configuration dimensions, an analysis of flow velocities,
particle sizes, settling velocities and stratification has not been made.
Particle Size Stratification.- A particle size distribution
of fly-ash from hopper and filter deposits is shown in Figure 6.49 ob-
tained from a 30,000 cfm prototype 4-compartment field unit operated on a
18
pulverized coal-fired boiler. Hopper fallout varied from 8 to 16%.
Fallout sizes were considerably larger than those reaching the fabric.
Deposit Consolidation.- The effect of filtering velocity on
the specific resistance coefficient has been reviewed above (see Figure
6.42). Further experimental data on the three compartment collector
operated in parallel mode is shown in Figure 6.50. Dust was deposited
at low velocity (point a), the dust feed stopped, and the air flow in-
creased without further dust deposition. At points b and c the gas flow
6-84
o
J
u
-
i
— •
9
-------
§
u,
o
•-C
u.
Ab
10
35
30
i . -1
2C
15
K)
',
0
^ ( i I I I ' i '
Top Feed = top inlet, downflow, dust deposition inside
_ sleeve
Bottom feed = bottom inlet, upflow, dust deposit on in-
side of sleeve
TOP FEED DOWN
BOTTOM FEED UP FLOW . FLOW
. 9
+v* /
2^
/• ^^^ ^
9/. 4 ^*^
^^^ >,
»x^» •»•
of
•
-
-
H
\
-i
-
3-comp. collect
(8)5 Y. 60in. cotton
~ sateen bags/comp. *"
(collector fabric area,
. J 1 1 l l i ' 1 .
) .'00 100 600 800
100
Figure 6.48.
TOTAL GAS FLOW_, cfm
Fly Ash Fallout vs. Gas Throughput For Top and Bottom
Feed (Parallel Flow, Constant Rate Filtration) (From
Borgwardt and Durham, Ref. 3).
was decreased toward zero and raised again. The test shows that as flow
is decreased the filter permeability remains constant, but when flow is
increased the permeability decreases. The apparent compacting effect
may be due to a displacement of particles from the deposit at higher gas
flow and redeposition deeper in the filter matrix.
6.3 LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT
6.3.1 Basic Pressure Drop Equations
In discussing the performance of single compartment filters,
the parallel functioning of the separate filter elements was compared to
-------
1000
MRTCLC SIZE, MtorMw
Figure 6.49. Particle Size Distribution of Fly Ash (from
Robinson, et al, Ref. 18)
10
HOW INCREASING
FLOW DECREASING „
•>..—.—. •
(a)
_L
2 3 4
AVERAGE GAS FILTER VELOCITY (»). fpn
Figure 6.50. Dust Cake Compression (Pilot Baghouse, Parallel
Flow Operation) (From Borgwardt and Durham, Ref. 3)
-------
that of electrical resistances in parallel. The same analogy applies to
two or more compartments operating side by aide with the same differen-
tial pressure driving force. The equations are essentially unchanged if
the compartments are all shut down for cleaning at the same time. If
however, the compartments are taken off line one at a time for cleaning,
they operate at different cycle phasings, and the equations must account
for this.
When a just-cleaned compartment is returned to servi.ce, its
drag will be less than that of the other compartments (Figure 6.51).
Consequently the flow through the compartment will be greater (Figure 6.52),
This means that the rate of deposition of dust in this compartment will
be greater than in the other compartments, and the compartment drag will
increase more rapidly. The compartment will rapidly approach a level of
operation similar to the average of the other compartments.
The flow through the overall system, which also increases
when the cleaned compartment is first returned to service, gradually de-
creases as the drag in the separate compartments increases. When the
next compartment is removed from the system for cleaning, the overall
flow will decrease again. Thus the overall flow follows a cyclic pattern.
The variation in overall flow depends on the characteristic
curve of the fan driving the system and other system flow resistance
characteristics. As a consequence of overall flow variation, the dif-
ferential pressure across the separate compartments will also undergo at
least some variation. This is indicated schematically and described in
Figure 6.53.
Velocity variations may occur not only from compartment to
compartment, but also from filter to filter and along the length of a
filter, as discussed above in single bag performance analysis. Therefore
the use of nominal velocity to describe overall filtration is arbitrary,
since for a given nominal velocity many combinations of internal flow
distribution may exist.
-------
raw
•0
i
fc •«
3 4O
£
= «
A
II 1 1 1
1
•
">•*-•
- 1 5
J±t|-
tn fc
M
1 1 1 1
0.0 IX) 1.5
FILTER DRAG (INCHES
8.0
Figure 6.51. Instantaneous Filter Drag Profile for Six Compart'
ment Baghouse. (From Walsh and Spaite, Ref. 5).
10
£ •
"i T—r
—r
AVERAGE VELOCITY : 3jO
-------
c\j
CO
UJ
X
z
UJ
tt:
UJ
u.
u.
o
UJ
o:
:D
co
CO
UJ
a:
a_
TIME
Figure 6.53. Schematic Pressure Differential Curve for
Multicompartment Baghouse)(From Ref. 5).
Explanation: Time t.
Time
Time t
A compartment has just returned on-line
with a lowered drag; the system flow is
higher than usual, and the collector
pressure drop is lower than usual.
The next compartment has jus t been removed
for cleaning, and total cloth area is
less than usual. Thus filtering velocity
is slightly increased, and pressure diff.
has also increased. System flow has decreased.
The compartment has been cleaned, and is
returned on line. The system returns to the
same state as at Time tft, and recycles.
-------
Residual conditions are those which exist in a particular ^
compartment as it is put on stream after cleaning, and terminal conditions
are those which exist in a compartment just before it is taken out of _j
service for cleaning. The residual drag (set by cleaning method and dust-
fabric parameters) and terminal drag( set by fan and dust-fabric resistance
characteristics) establish overall limits for a particular installation;
therefore they are critical operating parameters. Variations in filter :
drag, in establishing differences in filter velocities, also influence
dust penetration into the filter, filter blinding, and the structure of
the dust cake. Drag itself is affected by these changes. Some of the
equations describing multicompartment system dynamics are developed in
Appendix 6.3, and are of a form perhaps best studied by computer tech- ->
niques.
6.3.2 Performance of a Multicompartmented Collector ^
Billings, et al. have reported a laboratory performance of j
a commercial multicompartment collector cleaned automatically on a time ~~*
*
cycle. The test unit consisted of four compartments, each containing (
2
eight bags 8 in. in diameter and 6 ft. long (100 ft of cloth/compartment), .^"
as shown in Figure 6.54. Dusty air entered the bottom of es<-h section,
passing upward and through the fabric tubes. Once every 5.2 minutes each
compartment was shut off from the fan by means of a damper, vented to
atmosphere, and the bags cleaned. Dust removal was accomplished by |
lifting the whole assembly about 1-1/2" and dropping suddenly.
t
In addition, outside air was drawn into the compartment by the
negative pressure of the hopper and inlet plenum, and passed through the
bags in a reverse direction to help remove dislodged dust. Reverse air
passed down the inside of the bags and through the inlet header to the
other compartments. This cycle was then repeated on another section.
Sateen weave cotton bags were normally used in this equipment at a fil- —'
2
tration rate of 10 CFM/ft .
Performance characteristics investigated on this collector V
were the effects on pressure drop and efficiency of changes in reverse ,
flow air, amount and frequency of rapping, dust loading, air velocity, __,
and aerosol particle material. Performance of five different bag'mater-
ials was also compared.
-------
c
-
OUTLET,
TO FAN
A. BACK FLOW
AIR OPENING
B. SHAKING
MECHANISM
C. AIR VALVE IN BACK-
FLOW POSITION
D. 8-6' BAGS PER
SECTION
Figure 6.54. Cloth Tube Filter Cleaned by Mechanical Rapping and
Back Flow Air. (From BilUa**, «t. «!., fcef. 17).
An adaption of the British Simon Suction Filter manufactured in the
U.S. by Entolater Div., Safety Car Heating and Lighting Co., New Haven,
Connecticut.
.-
c
6.3.2.1 Studies With Light Dust Loadings
Basic Media Performance.- The combination of rapping bags, as an
integral unit, to minimize distortion and the use of reverse air flow per-
mits the choice of several bag materials other than sateen weave cotton
cloth. Such fabrics are: (1) wool felt with its inherently higher effi-
ciency but less porous structure which requires reverse air for adequate
cleaning and (2) woven glass cloth capable of withstanding high temperature
but having poorer flexing characteristics. Several bag materials were tested
including heavy and light wool felt, woven glass cloth lubricated with a sil-
-------
R 17
iconc1, woven napped Orion , and sateen weave cotton. The basic pressure
clrop-efIficiency characteristics of these fabrics were evaluated on light
3 3
dust loadings (0.01 to 0.1 gr./lO ft ). No bag cleaning was employed
during these tests.
The pressure drop and efficiency of various fabrics is shown
in Table 6.13 for atmospheric dust and copper sulfate microspheres. On
atmospheric aerosol at 10 cfm/sq. ft., the fabrics may be rated in order
p
of increasing efficiency as: Orion .cotton, light wool, woven glass, and
heavy wool.
The last, two columns of Table 6.13 show the inlet loading and
weight efficiency determined with copper sulfate microspheres for each
of the fabrics shown in the first column. The clean cloth pressure drop
for each fabric is related to copper sulfate efficiency. The relationship
between weight collection efficiency and resistance for these five fabrics
is *
Wt. eff. = 98.5 (Ap)°-33, (6.20)
for Ap inches of water, at a filtration velocity of 10 fpm with a
19
copper sulfate particle size of 2.7 microns(MMD). Billington and Saunders
report the exponent of pressure drop as 0.43 for 13 air conditioning filters,
using a stain density measure of efficiency with particles in the same size
range. They also mention an exponent of 0.1 found by Bigg for weight effi-
ciency of fabric filters using particles in the range 25 to 100 microns.
Further Studies of Filter Aids. - To extend the knowledge of
performance of filter aids to other fabrics and higher filtration velocities
R
than those described in Section 6.2.2.1, Orion , cotton, heavy wool felt,
and woven glass were flocked with asbestos floats and tested with copper
sulfate aerosols at a filtration velocity of 10 fpm. Bags were not shaken
during the flocking process, nor during the testing with copper sulfate.
The results of these tests are given in Table 6.14. It is possible to in-
crease the efficiency of these materials to over 98 percent by the appli-
cation of asbestos floats as a filter aid.
2
The use of 200 gr/ft. of asbestos floats had been suggested
as an optimum amount in the study described in a previous section. To
-------
TABLE 6.13
FABRIC COMPARISONS WITH LIGHT LOADINGS3
I'abr it-
Light Wool
Felt
Orion,
Napped
Cotton,
Sateen
Weave
Heavy Wool
Felt
Woven Glass
Clean
Fabric
Pressure
Drop in H?0
0.07
0.10
. 0.29
0.34
0.56
Atmospheric
Inlet Loading
gr./lOOO
cu.ft.
0.184
0.130
K
0.045
0.220
0.058
Dust
Weight
Efficiency
percent
74.8
60.4
/>
35. 5°
85.4
81.9
Copper
Inlet
Loading
gr./lOOO
cu. ft.
0.89
0.81
1.02
1.01
0.90
Sul fate
Weight
Efficiency
percent
41.1
45.6
63.7
71.3
.
81.1
'Filtering velocity 10 cfm/sq. ft.
Believed to be experimental error.
£
Probably nearer 60 percent.
(From Billings, at. al., Ref. 17).
TABLE 6.14
FABRIC COMPARISONS USING ASBESTOS FLOATS AS A FILTER AID3
Fabric
Orion
Cotton
Dense Wool
Woven Glass
Pressure Drop"
in H20
Initial
0.16
0.36
0.36
0.81
Final
0.95
1.43
0.90
2.20
Copper Sulfate Weight
Efficiency
per
Initial
45.6
63.7
71.3
81.1
Final
98.56
98.95
99.28
99.60
3200 gr./ft2
Filtering velocity - 10 fpm
(From Billings, et. al., Ref. 17),
-------
confirm this on a different aerosol and at a different filtration rate,
glass bags were flocked in small increments, and the efficiency was evalu-
ated after each step with copper sulfate microspheres (Table 6.15). Each
increment of filter aid caused an increase in pressure drop and a decrease
in amount penetrating. Effluent loadings at a velocity equal to 3 fpm were
not appreciably different from those at 10 fpm.
Test 7 shows the increase in penetration when the unit was
shaken five times, i.e. five complete cycles or 20 strokes per compartment.
About 60 percent of the original material, by weight, was recovered from
tVie hopper at this point. The remaining asbestos on the bags then amounted
2
to about 80 gr./ft . Penetration and pressure drop at this time were dif-
ferent than during loading due to the uneven removal of the flock and chan-
neling of the flow through less flocked areas. Use of asbestos floats as
a filter aid permits control of penetration and pressure drop. The primary
aerosol particle concentration cannot be too high or frequent replacement
of the filter aid will be necessary (see Figure 3.31). The use of glass
bags with asbestos flock can substantially increase collection efficiency
for high temperature work in such applications as the final gas cleaning
stage in incineration processes. Field trials will be required to estimate
service life of the combination. Present applications of filter aids (1970)
also include use of granular reactants for recovery of gaseous contaminants.
6.3.2.2 Studies With Heavy Dust Loadings
Frequency and Number of Raps.- The standard cleaning cycle
consisted of four raps per section,per 5.2 minutes, in conjunction with
entering reverse flow air. Table 6.16 shows the variation of pressure
crop and penetration as the number of raps i's decreased. Efficiency in-
creased as pressure drop increased witn decreasing number of cleaning strokes,
An investigation of the effects of the frequency of rapping was
made. This was done by reducing the cycle time and the number of raps, in
direct proportion, to keep the number of raps per section constant over any
-------
f
TABLE 6.15
EFFECTIVENESS OF FILTER AIDS FOR LIGHT LOADINGS OF COPPER SULFATE MICROSPHERES'
Test
No.
1
2
3
4
4a
5
5a
6
6a
7
Filtration
Velocity
cfm/ft.2
10
10
10
10
3
10
3
10
3
10
Cumulative
Filter Aid
on Fabric
gr./ft.2
0
25
50
100
100
150
150
200
200
b
Pressure Drop Copper Sulfate
in H«0 Loading
gr./lOOO ft.
0.86
1.56
1.76
1.80
0.63
2.00
0.66
2.20
0.72
1.46
Inlet
0.92
0.79
0.87
0.88
1.41
0.88
1.68
0.80
1.62
0.80
Outlet
0.130
0.024
0.019
0.012
0.007
0.008
0.008
0.003
0.003
0.026
Weight
Penetration
percent
18.9
3.1
2.2
1.4
0.5
0.9
0.5
0.3
0.2
3.4
Weight
Ef f icienc
percent
81.1
96.9
97.8
98.6
99.5
99.1
99.5
99.7
99.8
96.6
a. Woven glass bags lubricated with silicone, no fabric cleaning during tests, except prior
to Test 7.
b. Recovered 60% from bags by rapping, five cycles = 20 strokes.
-------
TABLE 6.16
EFFECT OF DECREASING THE NUMBER OF RAPS IN THE STANDARD
CLEANING CYCLE ON I^RESSURE DROP
AND EFFICIENCY
No. Raps/
Section in
5.2 min.
cycle
4
'}
2
1
Average
Pressure Drop
in. H«0
3.2
3.6
4.5
5.7
Dust
Inlet
gr./ft3
0.85
0.60
0.60
0.91
Loading
Outlet
gr./lOOO ft3
1.68
0.94
0.63
0.49
Weight
Penetration
percent
0.20
0.16
0.10
0.054
Weight
Efficiency
percent
99.80
99.84
99.90
99.946
a. Sateen weave cotton bags; back flow air 100 cfm; total air volume
3000 cfm; fly ash test dust.
(From Billings, et. al., RcL. 1.7).
long time interval. Data presented in Table 6.17 indicate that several raps
followed by a long pause produced lower pressure drop than short cycling. The
rapping operation appeared to be more effective in removing dust than the
reverse air, the velocity of which was about 1 fpm, (see below).
Reverse Flow Air.- The amount of reverse flow air that enters a
compartment to aid in cleaning is often controllable. The above equipment was
modified to study the effect of reverse air volume. Results in Table. 6.18
indicate a 6-fold increase in filter penetration when air volume was in-
creased 5-fold. At the same time, pressure drop decreased about 50%.
Inlet Dust Loading - The average pressure drop increases
with inlet dust loading as shown in Figure 6.55. With fly ash, the relative
rate of pressure drop rise with dust loading decreases at higher loadings,
regardless of filtration velocity. Talc produces relatively higher pressure
drop at 2000 cfm and the rate of rise with dust load is higher than with
fly ash. These curves are similar to performance data for continuously
on-line cleaned reverse-jet and pulse-jet collectors.
-------
TABLE 6.17
KFFKCT OF VARIATION IN CLEANING CYCLE FREQUENCY ON PRESSURE DROP
OF A MULTICOMPARTMENT COLLECTOR*
Cycle No. of
Raps/Section
1
2
3
4
No. of
Minutes
1
2
3
4
Average Pressure Drop
in H20
Comp . I
5.8
5.4
5.0
4.8
Comp. IV
6.0
5.8
5.5
5.4
*Sateen weave cotton bags, back flow air 100 cfm, total air
volume 3000 cfm or 10 cfm/ft2, inlet loading 0.65 gr./ft3,
fly ash test dust. (From Billings, et. al., Ref. 17).
TABLE 6.18
EFFECT OF CHANGES IN REVERSE AIR VOLUME ON PRESSURE DROP AND PENETRATION*
Amount of
Reverse Air
cfm
51
f.Q
85
120
180
250
Average
in H90
3.1
9 Q
2.6
2.6
2.3
2.2
Dust
Inlet
gr/cu.ft
1.10
i in
1.10
0.94
0.88
0.94
Loading
Outlet
gr/1000 cu.ft.
1.3
4.3
4.1
4.6
7.2
Weight
Penetration
Percent
0.12
0.39
0.44
0.52
0.77
Weight
Efficiency
percent
99.88
99.61
99.56
99.48
99.23
*Sateen weave cotton bags; air volume 2000 cfm; 4/5 cleaning cycle; fly
ash aerosol; 1 fr/ft-1. (From Billings, et. al. , Ref. 17).
Table 6.19 shows the penetration of fly ash as the inlet
3
dust loading is increased from 0.40 to 13.0 gr./ft . Individual effluent
concentrations of each compartment are given in Table 6.20. Compartment I
contributed almost twice as much to the effluent as the others, probably
due to uneven distribution of flow in the unit.
-------
o
CM
CX
2
Q
CO
CO
01
0)
60
Cfl
H
01
ENTOLETER UNIT
CURVE AEROSOL OUANTITV RF AM
A FLY ASH 3000 CFM 120 CFM
8 • • 200O • 63 •
C - " tOOO • 20 •
D TALC 200O • 230 •
Figure 6.55.
INLET DUST LQADING-GR./FT3
Variation of Filter with Inlet Dust Concentration
(From Billings, et. al., Ref. 17).
TABLE 6.19
EFFECTS OF INLET DUST CONCENTRATION ON FLY ASH PENETRATION3
Dust
Inlet
gr . /cu. ft .
0.40
2.90
5.10
13.00
Loading
Outlet .,
gr./10J ft .
0.52
1.50
2.40
3.80
Weight
Penetration
percent
0.130
0.052
0.048
0.029
Weight
Efficiency
percent
99.870
99.948
99.952
99.971
a. sateen weave, cotton bags; back flow air 100 cfm; total air volume
1000 cfm or 3.3 cfm/sq.ft ; 4/5 cleaning cycle.
(From Billings, et. al., Ref. 17).
-------
Compar J son of five fabrics.- The basic performance of five fabrics
iltiM-ing light loadings of atmospheric dust and copper sulfate has been dis-
cussed above. These fabrics were also used to filter heavy dust loadings to
obtain comparative performance data. The pressure drop and penetration for
each fabric, used with the standard cleaning cycle, are given in Table 6.21
for fly ash and talc dusts. Fly ash required a lower pressure drop on all
fabrics. Fly ash penetration was also greater. Fabric order is similar in
n
each series, except for Orion fabric, which had the highest penetration on
fly ash and near the lowest on talc. This shift in order of penetration for
D
Orion may have been caused by humidity or electrostatic charge effects.
6.4 LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLECTORS
6 . A . 1 Reverse Jet Filter (Horsey Type)
Billings et al reported on laboratory performance tests of a
reverse- jet continuously on-line cleaned fabric filter (see Figure 3.3F).
Pressure drop and efficiency were found to vary with inlet dust loading,
filtering, velocity and cleaning ring operation parameters. Empirical
pressure drop performance equations were derived for test dust simulants.
For resuspended vaporized silica (particle size, order of 1 (Jim),
the equilibrium operating pressure drop (in. H^0)was found to be:
Ap = fo.31V(M + 1.5)°'19 + 800Q - Vi x 0.9s] ( RJoV°<18 (6.21)
L 1000 J \100/
where V = filtration velocity, 7 to 20 fpm
2
M = dust flux to fabric, 1 to 87 gr./ft - min, (=^7)
V . = velocity of reverse- jet air, 2000 to 8000 fpm
RJO = percent of time reverse- jet operates, 6 to 100%
g
The pressure drop equation for resuspended fly ash
16, cr >3) was found to be:
S
Ap =(b.047V(M + 14)°'35 + 4200 - Vrj x 0.70~| (W\"°>25 (6.22)
L J
1000 UOO
for 10 < V < 30 fpm, 1 < M < 76 fr/ft2 -min, 2200 < V < 4200 fpm and
7 < RJO < 100%.
-------
TABLE 6.20
INDIVIDUAL COMPARTMENT EFFLUENT CONCENTRATIONS WHEN FILTERING AN
INLET DUST CONCENTRATION OF 1.1 GRAINS PER CUBIC FOOT OF FLY ASH3
Compartment
(I farthest from
II
III
(IV nearest fan)
Overall
Dust Loading
Outlet ~
gr./lO ft .
fan) 2.2
1.3
1.2
1.2
1.2
Weight
Penetration
percent
0.19
0.12
0.11
0.11
0.11
Weight
Efficiency
percent
99.81
99.88
99.89
99.89
99.89
Pressure
drop
in H20
3.4
3.5
3.5
3.6
a. sateon weave cotton bags; back flow air 100 cfm; total air volume
2000 cfm or 6.7 cfm/sq.ft.; 4/5 cleaning cycle.
TABLE 6.21
COMPARISON OF FIVE FABRICS FILTERING HEAVY DUST LOADINGS
Fabric
Cotton
Light Wool
Dense Wool
Orion
Woven Glass
Orion
Cotton
Dense Wool
Light Wool
Woven Glass
Operating
Pressure Drop
in Ho^
5.5 A. Talc3
5.0
5.6
5.5
5.9
B. Fly Ash
2.4
3.0
2.7
2.6
4.7
Weight
Penetration
gr./10Jft .
0.990
0.740
0.340
0.330
b 0.063
5.60
1.40
0.30
0.26
0.12
Outlet
Loading-,
gr./10Jft .
99.9010
99.9260
99.9660
99.9670
99.9937
99.440
99.860
99.970
99.974
99.988
Mass median diameter 2.5 microns; geometric standard deviation 1.6;
filtration velocity 5 cfm/sq.ft.; inlet loading 1.0 gr./ft.;
4/5 cleaning cycle, 4-Compartment Unit.
b. Mass median diameter 16 microns; geometric standard deviation >3;
filtration velocity 10 cfm/sq. ft.; inlet loading 1.0 gr./ft .,
4/5 cleaning cycle, 4-Compartment Unit.
(From Billings, et. al., Ref. 17).
-------
Pressure drop for fine talc (M' = 2.5 |am, cr = 1.6) was
o x _ o
Lound to be:
Ap - fo.l6V(M + 1.3)°')0 + 2000 - Vl
L
1000
0.8~| /RJO\
-1 \100/
-0.14
(6.23)
for 7 <. V < 20 fpm, 1 < M <, 70 er/ft -min, 2000 < V < 8000 fpm, and
10 < RJO < 100%.
Average inlet and outlet dust concentration for 8 test aerosols are given
in Table 6.22. Efficiency varied with amount and magnitude of cleaning
6.4 2 Pulse Jet Collector
20
Dennis and Silverman reported on laboratory test of a
o
Micro-Pulsaire pulsejet continuously on-line cleaned collector (See
Figure 3.3H).
Equilibrium pressure drop for resuspended fly ash was given by:
Ap = 0.14 +
0.48 (Ci)
0.23
0.5
(6.24)
te\ t1
l\100/
For resuspended vaporized silica, pressure drop was found
to be
Ap = 0.14
1.75 (Ci)
0.27
WO
).5
(6.25)
for 0.06 < Ci < 1 gr/ft , and other parameters same as above. Weight col-
lection efficiencies were reported to be greater than 99.9% on all tests.
6.5 FIELD PERFORMANCE
Pressure drop and efficiency for several fabric filters tested in the
21
field were reported by Dennis et al . Data for intermittently cleaned
tube and screen type collectors is shown in Table 6.23. Average operating
efficiency was found to be greater than 99% in all tests. Field test re-
sults for reverse-jet collectors are shown in Table 6.24. Efficiency for
-------
TABLE 6.22
AVERAGE INLET AND OUTLET DUST CONCENTRATIONS FOR A VARIETY OF AEROSOLS
TESTED ON THE REVERSE-JET COLLECTOR3
Aerosol
Atmospheric Dust
it M
ii ii
Copper Sulfatc Microsphcres
ii ii n
it n n
Uranium Trioxide Microsph.
ti n ti
n it n
Talc
n
n
n
Vaporized Silica
n n
M n
n n
y n
U ii
M n
Fly Ash
n n
n n
M n
n n
V "
Alundum
Calcium Carbonate
n n
n n
Filtration
Velocity
ft/min
25
17
8
25
17
8
25
17
8
8
11
8
8
20
28
17
22
10
10
10
10
29
20
10
10
10
17
25
25
25
Average Dust Loading
gr./cu.
Inlet
3.2x10";?
2.1x10"?
3.3x10°
ISOxlO'c
250xlO~
470x10
640x10";!
770x10°
1800x10
0.14
0.16
2.0
8.7
0.06
0.09
0.22
0.87
1.07
1.21
1.26
0.37
2.1
4.0
6.8
11.2
13.4
2.0
0.0055
0.033
0.25
ft.
Outlet
0.6x10";?
0.2x10°
0.2x10
4.9x10";?
3.7x10";?
31x10
27x10"^
68x10°
120x10°
1.2x10";?
1.2x10°
3.9x10°
6.0x10
2.6x10",
4.9x10°
4.4x10°
5.8x10°
1.0x10°
41x10°
1.8x10"
8.6x10";?
153x10°
63x10°
86x10°
44x10°
61x10
13. 8x1 O"5
e
11.1x10 ^
17.3x10°
48xlO-5
Reverse-Jet
Velocity
fptn
b
b
b
b
b
b
b
b
b
2000
1620
2000
2000
2000
2000
4800
2000
4000
4000C
4000
5000C
4000
4000
4000
2000
3000
3000
b
b
1450
a. Single 18"x60" bag; AmericanPelt Co., #51002, 19 oz./sq,
b. No reverse-jet operation
c. Three 6"x60" bags; same as above.
(From Billings et al., Ref. 17).
yd.
-------
TABU 6.23
FIEU) TEST RESULTS FOR CLOTH SCREENS) AND TUBE (T) COLLECTORS CLEANED BY
MECHANICAL SHAKING*
o»
l~>
o
Test
and
Unit
13(T)b
16(T)
16a(T)d
17 (T)
18(S)°
19 (S)
20(S)
21 (S)
22(S)
23(S)
24 (S)
25(S)
Operation
Founding
Found ing
Founding
Founding
Granite
chipping
Founding
Truing &
shaping
grinding
wheels &
sticks
Rubber
Material
DUST DESCRIPTION
Inlet Outlet
Loading Median Size
Grains per Microns
Iron Scale
Si02
Bronze Scale
SiO,
Bronze Scale
SIO,
Iron Scale
sio2
Granite
Iron Scale
SiO,
Ai6
SiO,
Talc
Cu. Ft.
0.68
0.44
0.595
0.13
0.032
0.39
0.19
0.15
0.10
1.33
0.88
4.3
Mass
2.6
105
3.8
6.2
48
4.3
4.4
4.0
3.2
~
Count
0.63
0.82
0.71
0.46
0.47
0.56
0.51
0.57
0.45
0.62
1.3
Loading
Grains per
Cu. Ft.
0.000015
0.000069
0.000048
0.000013
0.000028
0.00063
0.000013
0.00011
0.000074
0.00003
0.0025
0.0064
Median Size
Microns
Mass Count
0.57
-
1.1 0.41
3.4 0.37
0.87 0.43
0.96 0.43
0.59 0.38
0.46
Air
Flow
Rate
rs.i.p.)
c£*a
7,000
560
1,850
2,000
7,020
18,200
13,900
32,000
32,000
4,520
32,000
7,000
CFM per
Sc. Ft.
Cloth
2.5
0.8
2.6
2.2
2.3
1.5
1.3
4.1
3.2
1.4
3.2
Pressure
Loss
Across
Collector
in. Water
1.5
5.0
0.3
2.2
3.0
1.9
1.2
2.9
4.1
1.7
3.3
Collection
Efficiency
by Weight
Per Cent
99.99
99.98
99.99
99.99
99.91
99.65
99.99
99.93
99.93
99.99
99.72
99.85
Manu-
fac-
turer
American
Wheela-
brator and
Equipment
Corporation
Sly
Corporation
Pangborn
Corporation
*STP » 70°F and 760 am Hg
Cloth tubes
°Cloth envelopes over «etal screens
TJnit 16 following replacement of worn-out bags
-------
I
I—
o
TABLE 6.24
FIELD TEST RESULTS FOR CLOTH BAG COLLECTORS CLEANED BY REVERSE-JET AIR
Test
and
Unit
26
27
28
29
30
31
32
33
34
35
35a
35b
36
Operation
Truing and
shaping
grinding
wheels and
sticks,
etc.
Metal
polishing
Crushing &
grinding
Found ing
Drying
Material
A/ -0
£. J
B,C
Polishing
rouge
BeO
BeO;Si02
Tapioca
Starch
DfST
Inlet
Loading
Grains per
Cu. Ft
0.130
2.260
0.650
3.10
3.65
2.44
0.075
0.0094
0.0048
0.14
0.029
0.0054
3.17
DESCRIPTION
Outlet
Median Size
Microns
Mass
18.0
7.2
8.5
10.0
3.3
-
2.3
9.8
1.2
-
-
-
7.7
Count
0.52
0.70
0.59
0.52
0.54
-
0.48
0.55
0.43
-
-
-
4.8
Loading
Grains per
Cu. Ft.
0.001
0.0009
0.0008
0.0009
0.0025
0.0011
0.0021
0.00022
0.00012
0.00052
0.00057
0.000022
0.00054
Median Size
Microns
Mass
0.59
1.1
0.75
0.72
2.0
0.74
1.8
1.2
0.56
-
-
-
1.1
Count
0.42
0.43
0.41
0.44
0.45
0.43
0.43
0.42
0.43
-
-
-
0.43
Air
Flow-
Rate
f C f p 1
' S • 1 . r . '
cfra
18,800
930
1,230
9,500
8,000
12,000
2,200
6,000
5,000
9,600
9,600
9,600
2,130
CFM
per
S,, Ct-
c . r L •
Cloth
17.0
5.5
7.2
32.0
26.0
53.0
46.0
32.0
27.0
12.0
12.0
12.0
11.3
Pressure
Loss Across
Collector
in. Water
4.0
3.5
3.5
3.2
2.9
3.1
3.6
6.4
3.6
2.4
2.4
2.4
4.4
Collection
Efficiency
by Weight
Per Cent
99.23
99.96
99.88
99.97
99.93
99.95
97.20
97.62
97.51
99.63
98.03
99.59
99.99
STP - 70F and 760 mm Hg
Note: All units manufactured by Turner and Haws Company ("Aeroturn")•
-------
3 /
inlet dust loadings ^ 0.1 gr/ft was found to be 97) to 98%, with filtering
velocities in the range of 25 to 45 fpm. These latter data have been attri-
buted to poor maintenance and possible bag wear, but indicate typical field
values under as-found maintenance conditions.
The literature gives pressure drop and efficiency data for a number
of fabric filter installations, along with at least partial descriptions
of the dust, gas, fabrics used, and operative detail. These data are sum-
marized insofar as reported in Appendix 6.4. Due to wide variation in con-
ditions of filtration, the data have not been analyzed. The data is indi-
cative of fabric filter field performances, particularly those of new and
unusual filter installations as these are the ones most frequently reported
in the literature.
The efficiency of fabric filters in collecting particles of various
sizes has not been reported. Preliminary laboratory tests on particle size
efficiency using monodispersed aerosols have been discussed in Chapter 2.
6.4 REFERENCES FOR CHAPTER 6.
1. C. E. Williams, T. Hatch, and L. Greenburg, Determination of Cloth
Area for Industrial Air Filters, Heating, Piping, and Air Condition-
ing, U, 259 (1940).
2. C. A. Snyder and R. T. Pring, Design Considerations in Filtration of
Hot Cases, Industrial and Engineering Chemistry 47, 960 (195'i).
3. R. H. Borgwardt and J. T. Durham, Factors Affecting the Performance
of Fabric Filters.NAPCA. PHS, USDHEW, AICHE, Annual Meeting Paper
(Nov. 1967).
4. D. G. Stephan, G. W. Walsh, and R. A. Herrick, Concepts in Fabric
Air Filtration, Am. Industrial Hygiene Assoc. Jour., 21, 1 (1960).
5. G. W. Walsh and P. W. Spaite, Characterization of Industrial Fabric
Filters,ASME Winter Ann. Meeting (Dec. 1960).
6. R. A. Herrick, Theory and Application of Filter Drag to Baphouse
Evaluation, Air Engineering, p. 18 (May 1968).
7. K. J. Caplan, ed., Air Pollution Manual, Part II...Control Equipment,
American Industrial Hygiene Association, Detroit, Mich., Section 5.3
(1.968).
B. A. D. Zimon, Adhesion of Dust and Powder, Plenum Press, N.Y., 116 (1969).
-------
6.4 REFERENCES (Continued)
<>. I). C.. Stophan, P. T. Bohnsl.av, R. A. HerricU, G. W. Walsh, and
A. II. RO.JO , Jr., A New Technique for Fabric Filter Evaluation,
Am. Industrial Hygiene Assoc. Jour. 19. 276 (1958).
10. D. G. Stephan and C',.W. Walsh, Residual Dust Profiles in Air Filtra-
tion, Ind. Eng. Chem. _52, 999 (1960).
11. G. W. Walsh and P. W. Spaite, An Analysis of Mechanical Shaking in
Air Filtration, J. Air Pollution Control Assoc. 12. 57 (1962).
12. E. E. Lemke, ot al., Air Pollution Control Measure for Hot-Dip
Galvanizing Kettles, J. Air Pollution Control Assoc. 10, 70 (I960).
13. P. W. Spaite and G-W. Walsh, Effect of Fabric Structure on Filter
Performance, Am. Indust. Hygiene Assoc. J. 24, 357 (1963).
14. J. Durham, Filtration Characteristics of Fabric Filter Media, NAPCA,
PUS, USDHEW, internal Technical Report (Feb. 1969).
1'3. R. H. Borgwardt, R. E. Harrington, and P. W. Spaite, Filtration
Characteristics of Fly Ash from a Pulverized Coal-Firec' Powe,:
Plant, J. Air Pollution Control Assoc. 18, (1968).
16. J. R. Durham, and R. E. Harrington, Influence of Relative Humidity
on Filtration Resistance and Efficiency. NAPCA, PHS, USDHEW, AICHE
63rd Annual Meeting paper, Chicago," 111. (November 1970).
17. C. E. Billings, M. W. First, R. Dennis, and L. Silverman, Laboratory
Performance of Fabric Dust and Fume Collectors, USAEC Report No.
NYO-1590-R, Harvard Air Cleaning Laboratory, Boston, Mass., Rev. Ed.
(Jan. 1961).
18. J. W. Robinson, R. E. Harrington, and P. W. Spaite, A New Method of
Analysis for Multicompartmented Fabric Filtration, Atmos. Envir. 1
495 (1967). ~
19. N.S. Billington and D. W. Saunders, Air Filtration, Jour. Institution
of Heating and Ventilating Engineers, (London), 15, 46, (1947)
20. R. Dennis and L. Silverman, Fabric Filter Cleaning by Intermittent
Reverse Air Pulse, ASHRAEJ, 43 (1962).
21. R. Dennis, G. A. Johnson, M. W. First and L. Silverman, How Dust
Collectors Perform, Chem. Engg.» 196, (February 1952)
-------
CHAPTER 7
ECONOMICS
7.1 INTRODUCTION 7-3
7.2 INITIAL COSTS 7-8
7.2.1 Initial Filter Cost 7-11
7.2.2 Initial Fan, Compressor Cost 7-17
7.2.3 Duct Costs 7-20
7.2.4 Dust Disposal Equipment 7-22
7.2.5 Instrumentation 7-23
7.2.6 Planning and Design Costs 7-27
7.2.7 Foundations and Installation 7-27
7.2.8 Start-Up Period 7-28
7.3 OPERATING AND MAINTENANCE COSTS 7-29
7.3.1 Power Costs 7-31
7.3.2 Labor Costs 7-32
7.3.3 Plant Overhead Costs 7-36
7.3.4 Collection System Returns 7-38
7.4 CLOTH AND BAG COSTS 7-39
7.5 ACCOUNTING COMPARISONS OF COSTS 7-44
7.5.1 Annual Distribution Method 7-45
7.5.2 Anticipated Cost Summation Method 7-46
7.6 ECONOMY IN FABRIC FILTER OPERATION 7-48
7.7 REFERENCES FOR CHAPTER 7 7-48
-------
\
\ CHAPTER 7
ECONOMICS
7.1 INTRODUCTION
Whether fabric filtration represents the optimum gas cleaning method
for a given set of performance specifications depends upon a detailed
cost analysis. The available data citing cost experience, however, are
limited and, in many cases, oriented towards specific operations. There-
fore, unless the design engineer is confronted with a very similar and
proven fabric filter application, he should prepare a written specifica-
tion and seek expert professional opinion. All cost factors associated
with the design, fabrication and installation of the fabric filter system
should be determined as accurately as possible before selecting the dust
control system. In those cases where the gas treatment equipment will be
of modest size or where it represents but a very small fraction of a large
capital investment, one can justify a less rigorous approach.
The intent of this chapter is to provide the design engineer with
practical guidelines for estimating the cost of various fabric filtration
approaches. A concept underlying this chapter is that several trade-
offs must be considered between size of components and operating para-
meters in order to optimize costs. For example, if low cost performance
and high efficiency were the only criteria, one could never have too
much cloth area for treatment of a fixed gas volume (Figure 7.1). There
will be some point, however, when the initial costs for the larger fil-
tration system will override the lower recurring costs of power, mainten-
ance, etc.
The cost data assembled here have been compiled from several sources;
e.g., equipment users, manufacturers of filter systems and ancillary
parts, and open literature surveys by many investigating groups. Pur-
chase costs cited for past years have been converted to their 1969 equiv-
alents in accordance with the Marshall and Stevens Index, Table 7.1.
Several partial cost analyses have appeared in recent years, includ-
f\ f\ i
ing those by Stairmand ' and Stephan, that relate the collection efficiency
-------
TABLE 7.1
MARSHALL AND STEVENS INDEX FOR UPDATING EQUIPMENT COSTS*
1945
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
Example :
M&S
Index
106
170
181
181
183
186
192
209
226
232
236
Equipment
$1460 in
1969
Factor
2.64
1.65
1.61
1.61
1.53
1.51
1.46
1.34
1.24
1.21
1.19
costing $1000
(early) 1969.
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
in 1955
M&S
Index
239
238
238
239
242
245
253
263
273
285
(296)
is estimated
1969
Factor
1.17
1.18
1.18
1.17
1.15
1.14
1.11
1.06
1.03
1.00
(0.95)
to cost
See Ref. 1, pg. 26-8; and Chemical Engineering. recent issues, last page,
Sli« ol collector (It" of fabric)
Figure 7.1. Trade-off in Costs Due to Collector Size, for
Fixed Gas Flow.
-------
of various types of particulate collectors )to control costs. In Fig-
ure 7.2, for example,the efficiency is compared to control costs as a
function of particle size. Although the costs quoted, which reflect
British experience, are about 50 percent lower than those reported in
this chapter, the relative impact of particle size upon cost is directly
applicable. Stephan's data, summarized in Table 7.2, gives purchase costs
and power requirements for a spectrum of participate col led ors i-tuvtivr
on various particle sizes.
In detailed cost analyses presented in this section, several appli-
cation-related variables will be shown to have an important bearing on
cost. Therefore, in referring to rather generalized summations of the
types shown in Figure 7.2 and Table 7.2, one must keep in mind that
individual component costs, frequency of replacing parts, amount of main-
tenance, and cost accounting techniques may vary widely from one instal-
lation to another. A reasonable and useful cost estimate can only be
prepared by examining in turn each contributing cost factor. Figure 7.3
indicates several important aspects typical of most fabric filter systems:
1.50
fr
M
CO
0)
1.00 T
- • .50
u
g
0
I 5 10 M M 60 80 90 95 98 99 99 5 99 9
Collection Efficiency (%)
Figure 7.2. Gas Cleaning Costs for Dusts
of various Particle Sizes , for
300,, 000 CFM and 8000 hrs/year.
(From Stairmand, Ref. 2.) (1968)
-------
TABLE 7.2
APPROXIMATE CHARACTERISTICS OF DUST AND MIST
COLLECTION EQUIPMENT1
iqulpmrat Type
A.
1.
C.
D.
B.
r.
Settling
Chamber*
1. Simple
2. Multiple
tray
IMF till Sep-
arators
1. Baffle
chamber
2. Orifice
Impact Ion
3. Louver type
4. Gas re-
versal
Cyc lone*
1. SlngU
2. Multiple
3. Machsnl-
cal
Filters
1. Tubular
2. Reverse
J«t
3. Envelope
Electric*!
Preclplta-
tor»
1. One-itege
2. Two-stage
Scrubber*
1. 8pr«y
tower
2. Jet
3. Venturi
4. Cyclonic
J. Inert 1*1
6. Packed
7. Mechani-
cal
Purcha** Smallest Pressure Power
Cost* Particle Drop Uaad*** „_ .
(»/c£«) Collected (in H20) KV . «~«r"
(micron*)** (1000 cfa'
0.1 40
0.2-0.7 10
0.1 20
0.1-0.4 2
0.1-0.4 10
0.1 40
0.1-0.2 15
0.3-0.7 5
0.2-0.7 5
0.3-2.3
-------
Fan &
Ducts
67
Electric
Power
Dust
Disposal
77.
Equipment
Installed
157,,
General
MaInt.
Cost of Capital
117,
Plant
Overhead
Fabric
Replace-
ment
Replace
ment
Fabric Pur
chases
107,
Typical Installed Cost, $2.38/cfm
Typical Annual Cost, $1.05/cfm-year
Figure 7.3. Fabric Kilter Annual Cost Distribution.
(From GCA Survey, 1969).
The cost of the collector is only a few percent of the total
cost; therefore, the collector cost taken alone is a poor
criterion to use in selecting the system.
Fabric replacement (labor plus material) represents about 1/5
of the total annual cost; therefore more serviceable fabric
may lead to substantial savings.
Fabric replacement cost is about 4 times greater than collector
cost; therefore the collector should be very carefully designed
to promote fabric life.
Labor costs are nearly 1/3 of the total annual cost, and are
twice as much as the initial costs. Therefore the equipment
should be designed to need a minimum of attention time.
-------
. Direct operating costs are half the total annual cost; therefore
even after the system is bought and installed, substantial sav-
ings may result from careful use.
An itemization of the principal costs associated with the installa-
tion and use of a fabric system and "typical" values for each cost are
given in Table 7.3. These costs and their normal apportionment as de-
picted in the "pie" diagram, Figure 7.3, are discussed in detail in the
following sections.
Two basic cost categories, initial and annual, must be considered.
The initial cost includes the planning effort, site preparation, in-
stallation and other activities required to place the system into rou-
tine, effective operation. The annual costs include not only those for
power, labor and replacement parts but also various plant overhead
charges and a certain amortized fraction of the initial cost. Methods
of combining these separate costs are discussed in Section 7.5.
7.2 INITIAL COSTS
The cost of purchasing, installing, and placing the fabric filter
system in routine service will usually include the following items:
Reference
Cost Item Typical Cost Range Section
($/CFM) "
Basic filter assembly with 0.3 - 10. 7.2.1
filter media
Fan, blower, compressor 0.05-1. 7.2.2
Ducting ($/ft) 2-50 7.2.3
Dust disposal equipment -- 7.2.4
Instrumentation 0.01-0.1 7.2.5
Planning and design -0.1- 7.2.6
Foundations and installation 0.1-5. 7.2.7
Start-up, training, and shake- -0.1- 7.2.8
down period
A more detailed check list of equipment and services considered as
capital cost items is given in Appendix 7.1. One should also prepare a
-------
TABLE 7.3
TYPICAL, COSTS OF FABRIC FILTRATION
I. Installed Cost - $2.38 per CFM
Planning and $0.10
design
F.O.B. baghouse 0.80
Freight 0.05
Fan and motor 0.25
Ducting 0.65
Disposal equip- 0.10
ment
Instrumentation 0.05
Foundation and 0.28
installation
labor
Start-up 0.10
Total: $2.38
II. Annual Cost - 0.77 per CFM per year
Electric power $ 0.12 (
Labor 0.30 \ ; .,
Plant overhead 0.25-- \ " ' . A, ,
01 L. t- n in ( Insurance, etc.:. 045
Cloth purchases 0.10 ^
Total: $ 0.77
III. Total Cost of Operation - $1.05 per CFM per year
Annual cost $ 0.77
15 yr. amortization 0.16
of the installed
cost
Interest on the 0.12
unamortized
portion of in-
stalled cost,
at 10%
Total: $ 1.05
(see text for explanation and variations)
-------
detailed flow (lliiv.ram so that no vital components will be overlooked
during the costing process, Most ot the items listed above represent ex-
penditures of funds that would be charged directly to the filter system.
Certain costs, however, may not be charged to the system, but incorporated
In the general plant overhead.
The total initial investment is amortized over a period of time,
usually 10 to 1.5 years, that is consistent with the expected equipment
life.* Since n fixed amount of capital is tied to the filter installation,
thus preventing investment elsewhere, one must charge this loss of use
of the capital to the project annually as interest. Both amortization
and interest distributions are discussed in Section 7.5. They are com-
bined with the annual operating and maintenance costs to establish the
effective yearly cost of owning and operating the filter system.
As a rough rule of thumb, the total initial investment cost includ-
ing material and labor will average about $2.50 per CFM of gas filtering
capacity, with an expected range of $1 to $7 from one installation to
another depending on severity of the problems. This overall average cost
is based mainly on the results of a survey of 40 fabric filter installa-
tions for which the ranges in size and cost are shown in Figure 7.4.
10
100
1000
10
Ifl5
10
SIZES, ft of fabric
^ • >
r
10
100
TOTAL INSTALLED COST, $/CFM
Figure 7.4. Filter Installation Cost Data. (Arrows Indicate
Median Installations, ~5000 ft2 and ~$2.40/CFM.)
This period varies; power plants anticipate a 30 to 35 year life
for most plant equipment, while 5 years may be used in corrosive chemical plants,
-------
The breakdown of these costs, which has been given previously in Table 7.3
and Figure 7.3, is discussed in the following sections. The average
Ligure is apportioned more or less evenly among F.O.B. basic filter cost,
fan and ducting costs, and other costs associated with the purchase and
installation of the system.
Initial fabric filter system costs for many diverse applications are
listed in Appendix 7.1. Although these data are not always specific
;ts to what equipment and labor costs were included, tho data should
onablo cost estimates to within 50 percent of actual, cost by the
experienced designer.
While the scale factor from F.O.B. collector cost up to installed
system cost is typically about three (Table 7.3),this value is obviously
highly dependent on the individual system. It will be shown below that
both the basic filter cost and the costs of ducting and installation can
be quite different depending on the application. Examples of initial
cost variations in the Iron and Steel industry provided by an experienced
contractor in the air pollution control equipment field are shown in
Table 7.4.
7.2.1 Initial Filter Cost
The filter assembly as shipped by the manufacturer normally
includes hopper, cleaning mechanism with associated fans or motors, and
one set of filter elements. The equipment may or may not be assembled,
and it may not necessarily include supporting structures, paint or protec-
tive coatings, and insulation. Thus "the initial filter cost" is not
entirely meaningful until all these factors are defined clearly. Since
users of filtration equipment often fail to specify in their publications
the above details on their initial costs, the data presented in this
survey are subject to unavoidable uncertainties in some areas.
Initial costs are indicated in Figure 7.5 for four common
types of fabric filter dust collectors; reverse jet, pulse jet, mechan-
ically cleaned multi-compartment bag collectors, and single compartment,
-------
TABLE 7.4
ESTIMATES OF CAPITAL COST BREAKDOWN''
1. Matt-rial breakdown for fabric filter systems (percent):
BOF
Sinter Plant
Windbox
Electric Open Shop Electric
Furnace Hearth Furnace
4
4
71
10
3
5
1
2
37
32
6
5
7
7
2
:«5
34
7
7
9
2
2
25
42
4
9
8
7
2
17
27
10
10
7
1
Foundat ions
Ductwork and
stack
Collector
Fan and motor
Structural
Electrical
Water treatment
and piping
Controls 2 443 5
2. Labor/Material ratio for various components of particulate con-
trol systems(at $5/hr):
Collector 0.35
Fan, motor and 0.15
starter
Stack 1.00
Ductwork 1.00
Steel 0.30
Foundations 1.30
Electrical 1.50
From Barnes, Ref. 25.
intermittently cleaned envelope collectors. These costs are as the col-
lector is normally shipped by the manufacturer, with freight paid to the
shipping point (free on board, or F.O.B.). Costs are given both as dol-
lars per square foot of fabric and as dollars per CFM of filtering capacity,
assuming whenever necessary a typical air/cloth ratio (filtration vel-
ocity). These costs are based upon a survey of filter users, manufac-
turers' data, and the technical literature. Certain generalizations may
be made from the information presented in Figure 7.5:
-------
10*
10*
10 KX> O KT 10" K
Reverse Jet Equip-
ment (15 fpm assumed)
10'
10*
I01
I I I 1 I
10 K» O* »4 I0» 10* "*
K> 100 10" 10* 10* 10* won*
Pulse Cleaned Equipment
(10 fpm assumed)
Note:
(a) All equipment without primary exhausters, but with secondary
fans, blowers, etc. used for cleaning.
(b) Symbol (x) indicates data points estimated using the assumed
aiWcloth ratio.
Symbol (T) indicates high temperature equipment.
Symbol (0) indicates miscellaneous data points.
(c) Numerals indicate data source:
(1) Kane, American Air Filter Co., Ref. 26.
(2) Wright, Dracco-Fuller Co., Ref. 27.
(3) Chetn. Engineering, (9 Feb. 1970).
(4) Dennis, Ref. 28. Compressors and cleaning air fans
included.
(5) US PHS, Ref. 29.
(6) A manufacturer's price list.
(d) All prices are updated, via Table 7.1, to reflect 1969
price levels.
Figure 7.5. Initial Fabric Filter Costs - 1969 Basis;
F.O.B.
-------
10
10
10
10
10
10 100
10'
10
WO
10*
\
DO
Compartmented Bag-Type
Equipment (3 fpm assumed)
CFM
10
100 10*
•0*
Unit Envelope-Type Equipment
(3 fpm assumed)
Note:
(a) All equipment without primary exhausters, but with secondary
fans, blowers, etc. used for cleaning.
(b) Symbol (x) indicates data points estimated using the assumed
ai*r/cloth ratio.
Symbol (T) indicates high temperature equipment.
Symbol (0) indicates miscellaneous data points.
(c) Numerals indicate data source:
(1) Kane, American Air Filter Co., Ref. 26.
(2) Wright, Dracco-Fuller Co., Ref. 27.
(3) Chem. Engineering. (9 Feb. 1970).
(4) Dennis, Ref. 28. Compressors and cleaning air fans
included.
(5) US PHS, Ref. 29.
(6) A manufacturer's price list.
(d) All prices are updated, via Table 7.1, to reflect 1969
price levels.
Figure 7.5. (Continued)
-------
1. Reverse jet and pulse jet collectors are about twice
as expensive as other types in terms of fabric area,
but are comparable on the basis of gas filtering
capacity. Intermittently cleaned equipment is least
expensive because it is designed for easier dust con-
trol applications and interruptable operations.
2. Costs per unit capacity increase sharply below about
3000 CFM, and approach levels as low as $.50 to $1.00
per CFM in large installations. For a preliminary
estimate, a cost of $.80 per CFM of filtering capacity
might be considered as typical, and the figure of $2.
per ft2 of fabric would represent a large number of
collectors to within 50 percent of the true value.
3. Although cost data from a single source falls on one
curve, costs from different sources appear to vary by
as much as a factor of 3 for various reasons cited
previously. This variation in cost is especially true
for bag-type equipment, which is designed for a wide
variety of temperature and particulate applications.
Part of the data in Figure 7.5 is shown in Figure 7.6 for
better comparison. This was published about 1965 by a prominent manufac-
27
turer.
The type of construction of the fabric filter assembly is
particularly important for bag-type units, and for large equipment the
unit cost depends more on construction features than on size. This var-
iability factor is shown in Table 7.5 for several collector configurations
designed primarily for high temperature applications. These cost esti-
30
mates include the baghouse,complete with the bag cleaning accessories,
and materials handling equipment to accommodate dust removal from collec-
tion hoppers. They do not include the primary air moving equipment nor
any costs for field installation, piping, ducting, or electrical wiring
or insulation.
Except for preliminary cost estimating, prospective purchasers
of filter equipment are advised to consult reputable manufacturers for
accurate costs. Purchasers are advised to specify quality of material
and construction, and to examine critically any bargain prices. Because
the trade skills involved in producing filter equipment are fairly modest,
the field is crowded. Manufacturers are often compelled to seek economies
-------
MCCHANICAL
tasRi
3- OOMIKNT WTICIINCV
LEGEND
CiNTNIfUML
6- HMH (NIMY-VtMTUMI
NMHMC
7-MTWWTTCNT
JCT -
DUTY
AOTO*UTIC MUBURE TYW
IOOO
O.H 0.60 a 78
COST/CFM
J
Figure 7.6. Cost Per CFM of 12 Different Dust Collector
Designs Compared to the Total Volume Handled
Per Minute. (To Convert to 1969 Estimated Price
Levels, Multiply Scale by Marshall and Stevens
Index 1.14.) (From Wright, Ref. 27).
-------
L
TABLE 7.5
L
APPROXIMATE COST FOR BAG-HOUSES OF THE INDICATED CONSTRUCTION
PRICE RANGE
$/SQ. FT. OF CLOTH
WITHOUT INTERNAL PARTITIONS:
Open pressure unit $1.20 to 1.40
Closed pressure unit 1.55 to 1.70
All welded unit 1.85 to 2.00
L WITH INTERNAL PARTITIONS:
Open pressure unit 1.35 to 1.50
! Closed pressure unit 1.80 to 1.85
L, All welded unit 1.95 to 2.20
L *
Based on 11 1/2-in. diameter x 30.-ft. long fiberglas filter
bags and a minimum of 40,000 sq. ft. of cloth area. Cloth
cleaning by repressuring. (From Adams, Ref. 30} 1969 cost basis,
that are sometimes not in the best long-term interests of the buyer. A
dollar saved initially may cost several dollars in subsequent operating
i and maintenance costs. Furthermore, equipment that is well designed for
one application may be a poor selection elsewhere. Rather than invest in
cheap equipment, it is advisable to consider,if necessary, the redesign or
modification of good quality standard stocked equipment. Although the
purchase cost may be doubled, the long-term costs may be greatly reduced.
It should be noted that about 15 percent of the operational problems en-
countered by fabric filter users are the result of equipment inadequacies.
7.2.2 Initial Fan, Compressor Costs
The purchase cost of fans or blowers suitable for fabric fil-
ter equipment, which includes motors, drives, and starters, varies in-
versely with system capacity as indicated in Figure 7.7. Purchase cost
also increases with the design pressure to be delivered by the fan although
this is not readily apparent in the figure because oC the limited data
from any one source. Cost is also determined by the style of the fan and
-------
' Si-SO/CFM
' Ol?! '"anufacturer-
, several < Pr°duct
verai sizes ft"
* *^ •
10
Figure 7.7.
Fan and Blower Costs, Including Motor, Starter, etc.
1969 Basis. Fan Design Pressure Indicated.
-------
its design speed. Generally, fans delivering 10,000 CFM or more will cost
between $.10 and .20 per CFM, fans delivering about 1000 CFM will cost
$.30 to .55 per CFM, and the cost of smaller fans increases sharply.
On the largest applications, the fan(s) may be custom fabri-
cated. The gas properties and type of service affect the trade-off be-
tween initial cost and annual costs? e.g., a smaller fan running at higher
speed will be slightly leas efficient, and will probably require more
maintenance than a larger fan operating at less than rated speed. This is
especially true if the fan is located on the inlet side of the filter,
where slower speeds are desirable if the dust: is abrasive or gummy. Fan
maintenance, which can be a big problem in some installations, may justify
a larger initial size (and cost). It may be desirable to oversize the fan
merely to allow for unforeseeable expansions of the collector or increased
process effluents. In addition, it may be advisable to oversize the fan
on collectors where the overall pressure differential fluctuates for any
reason, to minimize reduction of gas flow during periods of high resistance.
Fan and motor costs are approximately equal. A starter for
a 10 HP motor costs about $75, and the drive cost should not exceed $50.
Fan-motor sets are around 60% efficient, such that one air HP, e.g.
1000 CFM capacity at 6.35 in. water represents about 1.6 electrical HP.
Installation labor cost which is normally about 157= of materials cost and
foundation costs is not included above.
The cost of an air compressor for pulse-cleaned equipment,
pneumatically operated instrumentation, and dampers in some filter houses,
is normally included with the initial fabric filter cost (see Section 7.1.1
for pulse cleaned equipment). Otherwise a separate compressor must be
purchased at considerable expense, since the existing supply of shop air
is often inadequate with respect to volume or pressure, or is not con-
sistently available. For pulse cleaned collectors, the amount of com-
pressed air varies according to the cleaning mechanism design. A
typical design value is 2 SCFM per 1000 CFM of gas filtered, with delivery
pressures ranging between 75 and 100 psig. The following estimates have
been provided by compressor manufacturers specifically for continuous
operation with dust collectors:
-------
I'll IS I'll
Air,
SCFM
135
100
10
Supp ly
Pr 1-8 Hiiro ,
psig
100
100
100
Motor Power,
IIP
25
25
3
Total Cost,
$ F.O.B.
2700
2500
700
Approx. Initial
Cost, $ per CFM
filtered
.04
.05
.14
The cost is estimated to increase with the 0.28 power of the volume re-
quired. Field installation is estimated at 0.60 times the cost of the
compressor, and the ratio of installation labor to total materials cost
is estimated at 0.27.
7.2.3 Duct Costs
The cost of ducting depends on the gauge and diameter as well
as on the length and the material. It also depends on the complexity of
the system, i.e., the number of elbows, Tees, transition sections, etc.,
and the amount of fitting needed to adapt the ducting to existing flanges.
The ducting may require clean-out or inspection ports, dampers, or explo-
sion panels. The cost of installing the ducting will also depend on
location accessibility, as well as the method of joining and sealing the
duct components.
2
One manufacturer of small filter units (up to 1200 ft ) has
reported his experience in installing ducting systems which for present
32
purposes would be considered smaller than typical. He indicated an
average cost for installed ducting of $1.75 to 1.85 per pound of sheet
metal, depending on gage and diameter. This included both fittings and
straight piping on his medium and large jobs, but the cost was more for
his smallest jobs. For example, a connecting duct assembly drop for a
single lilter unit of this size averaged about $53. (1969 cost basis).
In the Chicago area one duct branch including headers cost about $250.
Using the figure $1.85 per pound, Table 7.6 gives the estimated installed
cost per foot for several gages and diameters: typically it is around
$10 per foot. In support of this Table, a Boston sheet metal contractor
independently estimated a cost of $5 per foot for a 100-foot system of
6" 18 gauge ducting including soldered joints and about 3 gates, installed.
Table 7.6 also lists the costs of typical duct fittings be-
fore installation. The cost of a fitting depends, of course, on the
-------
TABLE 7.6
APPROXIMATE DUCT COSTS
(a) Installed Duct
Systems
($ per foot)
Metal Gage
(U.S. Std.
6
14
16
18
20
22
24
26
30
Thickness
) (inches)
.203
.078
.062
.050
.0375
.0312
.025
.0188
.0125
(b) Duct Fitting
Duct Diameter (inches)
6 11 24 48
108. 216.
20.75 41.50 83.
8.25 16.50 33.00 66.
6.60 13.20 26.40 53.
4.90 9.80 19.60 39.
4.10 8.20 16.40 31.
3.30 6.60 13.30
2.50 5.00 10.00
1.65 3.30
**
Costs
($ per item, not installed)
Description
Straight pipe,
Same, with self
seal
Reducer
Flange
Tee, 45°
Gauge
per foot 20
24
-locking 24
26
20
20
20
Diameter (inches)
6" 12" 24"
2.40 3.90 6.50
.56 .95 2.97
1.50 2.80
1.18 2.24 7.60
$18. (12x6) $36. (24x12)
$23. (12") $35(24")
$33(12x12x8) $50(24x24x12)
1969 cost basis of $1.85 per pound for galvanized sheet steel, riveted
and soldered, including a nominal number of fittings.
T«
1969 Boston area prices; in lots of 10 fittings each.
-------
amount of work required to make it; typically the fitting costs 2 to 4
times as much as the same length of straight pipe.
31
Installation labor is estimated to be 85 to 100% of material
cost; that is, the cost of the installed ducting is about half material
and half labor. This rule of thumb varies with material and difficulty
of installation, of course.
Materials other than galvanized steel are sometimes used.
Estimates for shop fabricated, field erected ducting of aluminum, gal-
vanized steel, and stainless steel are $5.42, 8.00, and 15.12 per foot,
respectively, (no sizes or gauges given). These costs are further esti-
mated to increase With the 0.55 power of the size. These estimates pre-
sumably apply to the same ducting requirement and therefore should repre-
jf
sent thicker aluminum than steel, for structural reasons.
There is little in the filtration literature on actual duct
costs, but a survey of nine large filter installations averaging 50,000
CFM indicated the costs of the associated ducting to be $.60 per CFM
(range, $.25 to $1.00). This included applications of both high and low
temperature filtration, and both short and long reaches. In Iron and
Steel Industry applications the system ductwork cost is estimated at
2S
around 30% of the total initial material costs.
7.2.4 Dust Disposal Equipment
The purchase of dust handling and disposal equipment must be
compared against doing the job manually, but even so the equipment is
justified by the high cost of labor on all but the smallest dust flow
rates. The disposal equipment typically includes a rotary valve and
drive, one or more dust conveyors with drives, and a collection bin of
some kind. This excludes the hopper, which is usually provided as part
of the basic collector.
A good quality 8" dia. rotary air lock valve metering on the
order of 1 CFM costs $400 to $500 including motor and drive. Motor and
*
Aluminum sheet goes by Brown and Sharpe gauge numbers, while steel
goes by U.S. Standard gauge numbers. For approximately the same thickness,
use two B&S numbers lower than the U.S. Standard number.
-------
drive1 onch cost about $75 depending on size and detail. It may be pos-
sible to drive the valve from the conveyor shaft and eliminate the need
of a separate motor.
The most common type of dust conveyor in use is the screw con-
veyor consisting of a 6" to 15" diameter covered trough, a screw of 10
to 16 gauge carbon steel, end plates, motor and motor mount, drive, and
drive guard. The total cost is on the order of $40 per foot, depending
on design. The trough and screw alone cost approximately $25 per foot
(9" diameter, 14 gauge screw, 12 gauge trough, 2" screw shaft diameter,
30 ft. long). The cost of conveyor is about $450 including motor.
Normal power requirements are l/15th HP per foot of conveyor,
but nof all dusts flow this freely. A number of dusts become sticky as
the temperature drops, or pack together due to vibration or mechanical
churning; consequently many problems arise with screw conveyor plugging
and breakdage, and other types of dust conveying equipment are sometimes
used. Table 7.7 gives cost estimates for several such conveying methods.
The costs do not agree with the above estimates, and the data in
Table 7.7 is suspect for other reasons, but it may be useful in making
comparisons.
7.2.5 Instrumentation
The effective utilization of any fabric filter system,regard-
less of size, cannot be obtained unless certain basic measuring devices
are incorporated. Optimum collector performance depends upon the observa-
tion or control of several major variables such as gas temperature,
system pressure, fabric pressure drop, gas flow rate, humidity, particu-
late properties (mass concentration and size), dust levels in hoppers,
etc. The above listing of variables does not necessarily represent their
relative order of importance. In a system where accidental condensation
of moisture could produce rapid fabric blinding the sensing of dewpoint
might take priority. High temperature filtration, on the other hand, re-
quires that the temperature not exceed levels beyond the tolerance of the
fabric. In applications characterized by high and possibly variable dust
loadings, the maintenance of gas handling capacity depends upon the meas-
urement of fabric pressure loss. The number of separate sensing devices
-------
TABLE 7.7
REPORTED ESTIMATES OF CONVEYOR COSTS'
(per foot of length)
Type, Size Cost Size /ov
Belt 18 in. wide 460
24 550
36 640
42 720
48 770
Bucket (height)
30 tons/hr. ,,
t r\ • r- • \ 4- £j
(8 in.x 5 in.)
75 tons/hr. ,._
(14 in.x 7 in.)
120 tons/hr. _.«
(15 in.x 8 in.) ^
Roller, 12 in. wide 7
15 8
18 9
20 10
Screw, 6 in. dia 235
12 280
14 300
16 310
Vibrating, 12 80
in. wide
18 115
24 125
36 135
*Frotn Ref. 31.
Exponent
0.65
0.65
0.65
0.65
0.65
0.65
0.83
0.83
0.90
0.90
0.90
0.90
0.90
0.80
0.75
0.60
0.80
0.80
0.90
0.90
0.33
0.33
0,28
0.28
0.28
0.44
0.44
0.44
0.33
0.33
0.29
0.29
0.25
0.25
0.25
0.25
0.28
0.28
0.26
0.26
(1) Although reported as $/ft, some of these costs appear more reason-
able as total costs
(2) Cost increases with
(3) Installation costs
. 1969 basis.
equipment length
raised to this
include foundations, electrical,
powe r .
paint and fie]
labor, and add about 65% to purchase costs.
(4) Total installation labor is about 30% of total material cost.
-------
justified and their degree of sophistication are related to the size
and/or the capital investment associated with the fabric filter system.
Regardless of system size, pressure measuring devices in some form should
bo used and are usually supplied by the manufacturer. Simple draft gages
provide a measure oJ: fabric resistance (and magnitude of dust loading),
and give the static pressures at various points within the system. These,
in turn, can be related to gas flow rates and power consumption.
In a small system, observation of key pressures permits manual
adjustment of gas flows and actuation of fabric cleaning equipment,
either manual or motor driven. Similarly, reference to visual display of.
temperature indicates whether dilution air dampers or pre-cooling sprays
should be adjusted. In the large fabric filter system, however, re-
liance upon manual control becomes highly impractical and the sensing
systems are coupled directly to control locations for automatic operation.
A well instrumented system not only protects the investment and decreases
the chance of malfunction, but also enables the user to diagnose and
rectify many of the simpler operating problems without resort to outside
professional consultation. In any case, outside assistance can be employed
more effectively when the operating characteristics of the system are
defined and understood by the user.
Table 7.8 gives representative catalog costs of instrumenta-
tion types commonly used in fabric filter systems. Relatively more
expensive equipment is recommended for long-term rugged use, high tem-
peratures, abrasive conditions, corrosive gases, etc. The additional
cost of installing and adjusting the typical instrument is estimated to
be of the same order as the instrument price.
The instrument indicators can be grouped on a central control
panel, the cost of the panel running $200 to $500 per foot of panel de-
pending on the density of the instruments. This cost includes wiring and
pneumatic piping but excludes instrument cost. The cost of installed
instrumentation will vary from $.01 per CFM for a large baghouse to roughly
$.10 for a 1000 CFM unit.
Standard instruments should be used whenever possible for re-
liability and ease of replacement parts. Standardization will minimize
-------
TABLE 7.8
TYPICAL INSTRUMENTATION CATALOG COSTS - 1969
Variable, type
Temperature
Flue gas thermometer x
Dial-type thermometer x
Recording thermometer x
Sensing switches
Sensor 4- Rustrak recorder x
Thermocouple bridge + amplifier
Pressure
Draft gauge, inclined x
U-tube monomcter x
Sensing switches, min-max
Sensing switches, differential
Flow
Hand-held tube-type kit x
Low velocity mass flow,
electronic x
Time record clock x
Motor-type valve or damper
controller x
Purpose:
A: For indicating
B: For recording
C: For controlling
B
x
x
X
X
X
X
X
X
Catalog Cost
45 -
30 -
25 +
20
200
200
30
15
30
40
275
178
80
125
70
100
70
60
Humidity
In-line industrial hygrometer x
Recording hygrometer (Temp,humid.)
x x
Transducer, electronic x
Dew point sensor x
Mechanical Sensing
Lever-arm, mercury switch x x
Particle variables •«.
Continuous size monitor
Fabric hole detector x x
Mass Concentration meter x x
110
325
350
15
100
-------
maintenance costs which will be one of the factors traded off against the
initial costs.
7.2.6 Planning and Design Costs
A certain amount of time goes into planning the dust collect-
ing system, principally in the form of calculations and drafting but
also as meetings, procurement, travel, etc. For the experienced user
of fabric filter equipment this may not amount to much, especially if he
uses so many small collectors that he keeps them in stock. For inex-
perienced users and those designing collection systems for new applica-
tions, the time may range from 10 to 1000 man-hours or more. Large
systems justify careful planning, as do those that must perform with high
reliability, and those that involve a large risk of fabric damage.
It is customary to charge small amounts of staff time to an
overhead account, which means the cost will not be reckoned against the
filter system. The cost of larger planning programs may or may not be
added to the other costs of obtaining and installing the system. Never-
theless, the costs should be estimated and included in all comparisons of
alternative dust control systems, unless the planning is to be furnished
free by an equipment supplier.
Procedures for designing a filter system are discussed in
Chapter 5. Engineering of this kind will cost approximately $15 per
hour, including the normal overhead costs of maintaining an engineering
staff. Thus a typical planning cost for a good sized system may be $1500,
or on the order of $.10 per CFM of system capacity.
7.2.7 Foundation and Installation
A fabric filter system may be installed outside the main plant
building when no space is available within the main structure or when a
cost analysis shows that the outside location is on the whole less expen-
sive. Although the cost of outside site preparation is discussed at
1 31
length in the literature, ' the basis for cost breakdown is sometimes
cbscure, particularly between direct and indirect costs.
Since fabric filter units weigh from 0.25 to 1 ton per 1000
CFM capacity not including fans, motors and ducting, they require substan-
-------
Lial foundations and supporting members. These may account for 4 percent _^
ol" the installation for material costs and an additional 5 percent for
associated labor (Table 7.4).
Many fabric filter installations, even though themselves mod-
orntoly well scaled against weather, still are best housed for easier
maintenance in bad weather. A separate light weight building to house
2
the equipment is estimated to cost $1.50 to $5 per ft of floor area
based on reported costs for single-story garage and maintenance shop ^
shells. This includes foundation, structural frame, outside siding, in-
side partitions, and paint. The filter may alternatively be located — I
indoors, where it requires typically 1 sq. ft of floor space for each
50 CFM capacity. For this space a substantial annual charge is normally _
made (Section 7.2..3a). Outside or inside, the cost of installation, at
about $5 per hour wages plus 507* overhead, amounts to 30 or 40% of the
initial equipment cost on large equipment. It may be, however, 2 or 3
times higher in special cases. Itemized installation costs are discussed
in preceding sections and shown in Table 7.4. ~"J j
A frequent consideration is whether it is best to purchase
the filter assembled or to assemble it on site. The net cost to the
purchaser will probably be about the same; the cost of assembly at either
location is estimated at around 10% of the total on-line cost. Factory —
assembly is usually preferred from the standpoint of job quality unless !
unusual skills are available at the site. Railroads, however, have size _
* !
limits of approximately 9.5 x 12 (high) x 40 feet. The cost of painting
including protective coatings is estimated at 57, of the assembled cost, j
and the place at which the painting is done makes little difference to j
the purchaser. ;
7.2.8 Start-Up Period j
• ' i
Although not usually reckoned as part of the baghouse instal- • !
lation cost, there is in fact nearly always a period following installa-
r
*
Freight rates for steel vary with the amount shipped. Minimum car-
load rate in the Boston area requires at least 40,000 Ibs.
-------
tion when personnel are becoming accustomed to the equipment and unfore-
seen operating problems are being corrected. This period ranges from a
lew hours to over a year. Start-up cost is herein treated as an initial
rather than as an annual cost for convenience, even though it may extend
into the useful life of the equipment.
Analysis of 78 problems reported by the users of 26 fabric filters
of many types and sizes indicated that at least 20 percent of the problems
were related to becoming accustomed to the equipment or to faults in the
installation. Half of these start-up problems were connected with the
filter itself, such as poor seals between panels, bags located too close
together, or failure of the shaker mechanism due to weak design. Other
start-up problems are cited in Chapter 8. Whether from inadequate tlesign
or untrained use,, problems do often arise and they do cost the purchaser
money. Furthermore the installation of the equipment may require a tempo-
rary plant shutdown, which may be considered a start-up cost.
It is estimated that the start-up costs may on the average add 50% to
the normal maintenance cost for the first year, that is, amount to about
$0.10 per CFM. Start-up cost may be much higher, or on the other hand the
purchase of equipment known to be of superior quality, and the use of well
trained or experienced personnel, can much reduce the learning costs.
7.3 OPERATING AND MAINTENANCE COSTS
The costs of operating the fabric filter system from day to day and
keeping it in good working condition include:
\
Cost Item Typical Cost Range Section
Powe»-
Fan power for filtering $ .10 - .25 per CFM-yr 7.2.1
High pressure air for cleaning 0 - .25 "
Labor
Fabric replacement .02 - .20 " 7.2.2
General maintenance .02 - .20 "
Dust Disposal .01 - .15 "
Plant Overhead
Space, heat, lights, insurance etc. .05 - .50 " 7.2.3
-------
Operating and maintenance costs considered to be "typical" are indicated
in Tablo 7.3 and Figure 7.3, where together they make up $0.67 per CFM-year,
or about 2/3rds of the overall yearly filter cost. In addition there are
the costs of purchasing replacement fabric which are discussed separately
in Section 7.3, and various incidental costs discussed in the following
pages. To these are added a yearly amortized portion of the initial costs
of the equipment, installation, and capitol costs (Section 7.1) as dis-
cussed in more detail in Section 7.4.
Operational and maintenance cost data were available for about 50
*
fabric filter installations via both the literature and the GCA Survey .
These varied in size from 330 to 820,000 CFM, The mean operating and
maintenance cost as reported, including power, labor and fabric replacement
&
but excluding overhead , was $0.52 per CFM-year. Only 6 installations re-
ported O&M costs in excess of $1. per CFM-year as indicated in Figure 7.8.
Overhead is estimated at typically an additional $0.025 per CFM-yr.,
(Section 7.2.3).
10.
Figure 7.8. Total Operating and Maintenance Cost.
(As reported, including power, labor, and
fabric replacement, but excluding overhead)
(From GCA Survey, 1969).
The following indicates the difference in apparent costs of two
hypothetical plants, one of which meticulously includes overhead while
the other uses only direct costs:
* It cannot be determined precisely what indirect costs are in the figures
reported in the literature. Engineers are inclined to report only direct
expenditures, while others may include the costs of plant space, employee
fringe benefits, tax rebates for expendible items etc. GCA surveyed direct
costs by asking for task labor requirements in hours, prices as billed, etc.
7-30
-------
Power
Labor
Space
Heat
Other overhead
Fabric (see Section 7.4)
Annual O&M Costs ($/CFM-yr)
Direct Cost Only
$ .12
.21
.18
Including Overhead
$ .12
.38
.05
.06
.04
.09 (Tax rebate)
Total: $ .51 $ .74
Each of these costs varies widely from installation to installation.
7.3.1 Power Costs
The cost of power for pumping the gas through the filter is
perhaps the most easily determined of all the system costs; it is given
simply by
Power Cost = CQp/E
where C = cost of electricity
Q = volume flow rate
j5 = average fan pressure
E = fan-motor efficiency
The cost of electric power is typically $.012 per kilowatt hour,
depending on location and the type of industry. The power conversion
efficiency of fan-motor units is usually taken as 60%. The average pres-
sure at the fan is the only indeterminate factor and even this is partly
determined by the filter operator's control of the fabric cleaning process.
The average fan pressure is typically 5 to 10 inches of water (See Figure
7.7) and the portion contributed by the filter alone, indicated in Figure 7.9,
is typically 3 to 6 inches. Power cost for fan, blower, or compressor oper-
ation may be determined using Figure 7.10, which is based on the above equation
using a 60% electrical to air power conversion efficiency.
-------
(a) lie fore Filter Cleaning •
I ttl I J
_
oTi iTo * io7
(in. H20/FPM) •
(b) After Filter Cleaning
0.1
ltll.1.1 -1. .
i.oVio7
. H90/FPM)
(in. nn'
Figure 7.9. Typical Filter Pressure Drops.
(Assuming an average filter velocity of
3 FPM, median pressure drops are 7.5 and
4.5 inches of water, or 6 inches average.
(From GCA Survey, 1969.)
Figure 7.10 is based on full time operation; therefore, if for some reason
the filter system only operates 3 hours per day the actual power cost would
be one eighth as much.
7.3.2 Labor Costs
Labor skill requirements range from supervisory and instrument
repair categories down to unskilled labor for such tasks as dust disposal,
and these wages currently range approximately from $2.50 to around $6.00 per
hour. Considering the typical labor overhead of employee benefits, medical
coverage, administration etc. the net cost of average labor to the company
can be estimated at almost twice the actual wage.
Among about 30 fabric filter installations reporting labor data,
about .047 man-hours were required per'CFM-year, equivalent to about $0.21
per CFM-year in wages. Costs for specific filter installations are listed
in Appendix 7.2.1. These costs were reported to be distributed as in Table 7.9
among several tasks common to most filter installations. The task labor dis-
tributions are further shown in Figure 7.11. Fabric replacement is indicated
-------
50
10'
Inches of
Water,
Fan or
Blower
Diff.
Pressure
10 100
Cost, $/ye«r per 1000 CFM
1000
100 i
Pounds
per
sq.ia 10 .',
(psig)
Fan or •
Compressor
Pressure
3.0 c/kwh
Power Cost
10 100
Cost, $/year per CFM at Pressure
Figure 7.10. Air Power Coats. (Based on a 60% fan-motor
power efficiency and full-time operation.)
-------
TABLE 7.9
REPORTED LABOR DISTRIBUTION COSTS
Re
'X.
No. Reporting:
No. Practicing:
Mean Practice:
Median Practice:
Bag
>p lac ing
47
27
26
.084
$2.00/bag
General.
Maintenance
39
26
26
.083
$.045/CFM-yr
Dust Bag
Disposal &
13
27
13
.073
$2.00/ton
Cleaning
Repairing
1
27
4
Var.
—
Total
100
(30)
(30)
207**
(Units)
%
Count
Count
$/CFM-yr
* GCA Survey data. Reported wages, before overhead.
** The average of combined task costs was $.207/CFM-yr; the combined task
average cost is $.24/CFM-yr.
A. Replacement Labor, $/year D. Other Maintenance Labor,
B. Replacement Labor, $/bag A $/KCFM-year
C. Other Maintenance Labor, $/year E- Disposal Costs, $/year
F. Disposal Costs, $/ton
Figure 7.11. Reported Labor Costs, GCA Fabric Filter Systems Survey,
1969, (Wages, before Overhead)
-------
to absorb almost half the labor; on the average it takes about half an
hour to replace a filter element including preparation time, filter clean-
out time etc. Fabric lasts about one year on the average (Section 8.3).
Fabric filters typically require about 10 hours of attention per KCFM
per year for such things as inspection of the fabric, instrument adjustment,
fan servicing, etc., excluding the other labor categories shown and excluding
unscheduled repairs. (Unscheduled repairs are so variable that they were
not made a part of the survey, but may perhaps be estimated at 1 or 2 hours
per KCFM-year).
The purchase of dust handling and disposal equipment trades off
against doing the job manually. Among 13 fabric filter installations
reporting dust disposal costs the average direct cost was about $2. per
ton with variations from $12. per ton down. These companies had already
installed a reasonable amount of disposal equipment, and the $2./ton figure
represents the manual balance of the task. Most of these companies did
the work themselves, but about a third of them contracted the work out to
a local trucking firm.
Fabric cleaning and repairing is apparently practiced by only a
small percentage of filter users, usually those with expensive fabric
that tends to bind or plug frequently. One installation periodically sent
the bags out to be dry cleaned. Another found that ordinary cotton bags
could be patched in place quickly and inexpensively. (See Chapter 8).
These reported costs could be adjusted in two ways although as given
here they have not been adjusted. First, as mentioned above the cost of
labor overhead could be included as a factor of from 1.5 to 2, except for
labor contracted outside the company. Second, these costs could be ad-
justed for the fraction of the year the equipment was in actual use. For
example,200 hours a year spent in replacing fabric on a filter used only
8 hours per day might be interpreted as equivalent to 600 hours a year on
a full-time filter. However, there is no assurance that the fabric life
would remain the same, as if the fabric were being damaged by condensation
due to frequent shut-downs the fabric life might actually increase if the
operation went to full-time. Lifewise most other labor requirements cannot
-------
be .scaled with assurance. A scaling factor may if desired be obtained from _j
Appendix 7.2, where approximately two-thirds of the installations operated
i .
full-time and the other third operated an average of 227. of the time, giving
an average operation of 757« of the time and a scale factor of 1.33.
7.3.3 Plant Overhead Costs ^
Just as there is an overhead cost associated with the use of i
manpower, there is a cost associated with the use of the plant. This can __, :
include a variety oi" charges — Rentals, utilities, administration costs, .
taxes, etc., and the overhead cost factor depends to a large extent on ; ]
jraj |
the company's accounting practices. Some of these costs may go on the i
books against the. filter system whether or not they involve actual cash
expenditures, while other costs may never be compared with the filter oper- "~* j
ation even though they are directly related. The cost of space occupied by
the filter is apt to be one of the latter. The following presents a brief —•' '
summary of typical plant overhead costs per CFM-year:
Direct Expense Actual Expense '
Space $ 0 $ .055
*-**
Heat .15 .15
Insurance, taxes .045 .045 _
Other Var. Var.
Total: $ .195+ $ .25+
Variances in these costs are discussed briefly below.
7.3.3.1 Space - The filter housing and its peripheral equip-
ment should be considered as renting the plant floor space occupied, as
they prevent the space being used for something more profitable. In certain
circumstances the upper room space may also have value. On the other hand
equipment located in unused buildings or outdoors may not incur a space
2
cost. Plant space varies in value, but a typical rental value is $2.75 ft
year (1969). Rental cost may not include heat, light, water, sewer, taxes,
etc.
-------
The filter capacity packed into one square foot of
K base area depends on the filter height, the filtration velocity and
MM- duaiiing mechanism, and consequently the packing varies widely from
limtollntlon to installation. Figure 7.12 shows the capacity packing for
IM
140
110
100
•0
•o
40
to
Apparent Upper /o
Limit
Practiced
i o .
10 100 IOOO DOOO K)OOOO 10
MOW UMM.ITY (CfM)
Figure 7.12. Plant Floor Area Required per Filter Capacity.
(Based on Collector Dimensions plus 3 ft.
Perimeter Clearance.) GCA Survey Data.
30 installations surveyed. It appears there is an upper packing limit
in practice; for example, few installations of 10,000 CFM capacity would
2
exceed 80 CFM per ft although many would occupy the same amount of plant
floor area and filter less air. As several maintenance problems appear when
the fabric is packed too close together (Section 5.2.6) or when the air/
cloth ratio is made too large (Section 4.2.2) there may be good reasons
to avoid the apparent packing limit indicated in the figure.
Picking from the figure a typical capacity of 50 CFM/ft2
of floor, and the rental cost of $2.75/ft2-year, one obtains a typical
space cost of $0.55 per CFM-year, which is one of the overhead costs.
-------
7.3.3.2 Heat - Some baghouses exhaust unwanted heat to the
outdoors, but others waste heat, particularly in winter months. In the
northern half of the U. S., fuel usage costs approximately $.40 per CFM ~
per year when the air is exhausted outdoors. This is an annual average
based on a 154 hour week and an 8 to 9 month heating season. Over the
U. S. an average heat cost may be $0.15 per CFM-year. Other cost consider-
ations are the exhausting of air previously air conditioned, and the cost
-^*
or value of providing alternative ventilation by means other than the filter
system.
7.3.3.3 Insurance and Property Taxes - These combined are
estimated to cost about 1.870 of the installed cost of the baghouse, or
N»>
about $.045 per CFM per year.
7.3.3.4 Lights - Lighting should be installed around the
->
baghouse, but the annual cost is probably negligible.
7.3.3.5 Supply Inventory - Normally at least one set of bags
is kept on hand at all times, and spare fan parts etc. may also be stocked.
Annual inventory costs amount to approximately 10% of the inventory invest-
_/
ments.
7.3.4 Collection System Returns i
I
Offsetting the filter system annual costs in some cases is an
actual profit from operating the system. For example, particle -collection • j
i
may be a basic part of the manufacturing process, or the dust collected ""'
l
may have a high sale value as in many non-ferrous metals refining applications. i
The return on investment for a fabric filter installed to control *" I
a community air pollution factor is frequently difficult to asses. In- j
stallations which eliminate dust damage claims and expensive and time-consuming —' j
complaints or litigation will represent a direct economic return to the
user in terms of reduction of management problems. ^ • i
Since many dust induced diseases have an etiology requiring _,- \
several years exposure, and are non-specific, the utilization of fabric .^ :
filtration for health protection purposes usually makes excellent economical ;
sense. Other direct benefits from the reduction of visible but otherwise
-------
innocuous dusts include improved plant housekeeping and reduced maintenance
on plant machinery, improved visibility, increased employee morale, improved
product quality, etc.
7.4 CLOTH AND BAG COSTS
Bags fitted for a specific model of fabric filter are available from
a number of suppliers, usually including the manufacturer of the filter.
The prices depend very much on the cloth used; that is, on the kind of fiber,
the weave and weight of cloth, and on specific treatments given the cloth,
yarn, or fiber during manufacture to protect the fabric in use from fire,
rot, mildew, abrasion and so on. Table 7.10 indicates costs for a number of
fairly typical bags as purchased a set at a time. Also given for comparison
are some fabric and fiber costs. A typical bag, ready to be installed by
the purchaser's maintenance crew, costs $10; however, bag prices range between
$1 and $100. A breakdown of the cost of a typical bag in Table 7.11 indicates
that the bag cost increase accrues in making the fabric, and the rest in
making the bag from the fabric.
As basic as the purchase price of the bag is, it is only one factor in
the fabric-related costs of operating the filter system. Even more important
is the fabric life which depends on many things as discussed in other sec-
tions of this volume. By maximizing the length of time the fabric remains
useful, the fabric costs and related replacement labor costs measured in
dollars per year are minimized. Figure 7.13 indicates the results of a
survey of about 40 filter installations. As indicated, most fabric lasted
about one. year, with a resultant median cost of about $0.10 per CFM-year
ft
and a mean cost of $0.18 per CFM-year. Cotton and glass were the most
widely used fabrics reported in the survey.
Associated with the installation of every bag is a labor cost, typi-
cally between $1.50 and $2 per bag; this is discussed in Section 7.2.2,
Fourteen fibers used in fabric filtration are listed in Table 7.12
(See also Section 4.2.1). The approximate price of each fiber relative
to cotton is given along with its temperature limitations. Figure 7.14
rolato.s temperature and cost for some of the common fibers and shows why
glass is a popular fiber despite its mechanical durability limitations.
-------
TABLE 7.10
TYPICAL FILTRATION FABRIC COSTS
Basic
Material
Cotton
Wool
OrlonR
Dacron
Nomcx
Nylon
Fiberglass
TeflonR
i>
Fiberfrax
n
Brunsmet
Fiber
Cost/lb.
.40
(Wide Var.)
.66-. 80
1.40
2.50-6.00
1.00
.60
--
20.00
-^40.00
Woven Fabric
Cost/Yd
.41
1.77
1.01
— 'V
1.04
--
.70
.98-1.68
8.00
(8 oz)
38.00
--
Felted Fabric
Cost/Yd
--
3.97
4.82
4.82
11.50
--
N.A.
36.70(23 oz)
29.20(19 oz)
~'
--
*
Selected Retail Bags
Cost Lengths x Diameter
13.60
1.50
50. F
35. F
5.
3.
30.
22.
13.70
2.80
25. F
50.
27.
—
27.
20.
16.50
10.
9.63
2.80
75.
._
--
21' x 9"
5' x 5"
14' x 7"
7' x 12"
9.5' x 6"
13.5' x 5"
30' x 10"
25' x 11.5"
22. 5T x 12"
9' x 6"
5' x 5"
25« x 11.5"
20' x 8"
—
20' x 8"
25' x 12"
25' x 11.5"
22.5' x 8"
12' x 5"
6' x 7"
25' x 11.5"
—
--
* Note: Bag costs and fabric costs are not related, as
separate sources (References 9, 10,11, and GCA
(F) Felted fabrics
(R) Registered Trademark
the information is from
Survey, 1969).
-------
TABLE 7.11
COSTS OF TYPICAL BAG
Bag purchase cost:
Sizo:
2
Cost per ft :
Fabric cost:
Fabric cost per bag:
Fiber cost:
«- Fiber cost per bag:
Fiber-to-fabric factor:
Fabric-to-bag factor:
Typical Bag
$ 10.
20 ft2 ,
$ .50
$ 2. /yard
$ 4.40
$ .75/lb
$ 1.10
~ 4x
~ 2x
Range Available
$ 1- 100
1 - 100 ft2
$ 0.15 - 4.00
$ 0.40 - 40. /yard
$ 0.25.- 25.
$ 0.40 and up/lb
(wide variation)
.. ..
~0.5x - 5. Ox
Types of fabric used:
Cotton 33
Glass 33
Polyester (e.g. DACRONR) 15
Acrylic (e.g. ORLONR) 5
Wool 5
Other types 9
100
Fabric Life, months
1000
i. . * t
.01 0.10
Fabric Costs, $/CFM-
1.00
Figure 7.13.
Fabric Usage Reported and Costs.
(Costs as reported, before tax credit.)
(From GCA Survey, 1969, and miscellaneous
literature data.)
-------
TABLE 7.12
FIBER, TEMPERATURE RANGE, AND RELATIVE COST
Type (Typical Name)
Cotton —
Rayon, acetate —
Wool
p
Acrylic (OrlonR)
(Dynel )
Vinyls
Polyester (Dacron )
Polyethylene, ^
polyolefin (Polyfain )
Saran
Polyamide (Nylon^
- modified (Nomex )
Polypropylene
Poly-TFE (TeflonR)
Glass
Asbestos
Ceramic (Fibref rax' )
Recommended
Temp. Range:
Max. Contin.
225
250
275
240
—
325
250
500
500
600
2800
(160-190)
210
(180-235)
(200-275)
(150-180)
250
(250-280)
200
160
200
425
(225-450)
(450-550)
500
2300
Approximate
Cost
1
1.1
2.75
2.75
3.2
2,7
2.8
2.
2.5
2.5
8
1.75
30.
5.5
3.8
— 75
Metal
(Brunsmet )
-100
-------
0
6
4
2
20
I'8
u 16
14
12
10
8
6
4
2
0
Poly TFE
Glatt
100 200 300 400
UPPER TEMPERATURE
500
600 700
Figure 7.14.
Approximate Temperature Capability/Cost Rela-
tionship for Filtration Fabric Materials.
(Costs Relative to Cotton.)
-------
7.5 ACCOUNTING COMPARISONS OF COSTS
As discussed in earlier parts of this Chapter, fabric filtration
costs fall into two classes, initial or one-time costs, and periodic costs.
The variations in each from installation to installation can be many-fold
and each installation must be separately analyzed. Furthermore, as noted
earlier, certain costs, notably overhead items but also others, may or
may not appear as costs of the filter system even though they are related.
In evaluating the costs of fabric filtration one should consider how the
cost figures are going to be used in order to decide which of the hidden
costs to include, if any.
First costs (Section 7.1) may include:
Planning and preparation for installation
Equipment and parts purchasing
Installation
Start-up and training
Periodic costs (Sections 7.2, 7.3) may include;
Replacement fabric
Power
Labor and associated labor overhead
Plant overhead, taxes
Planned replacement parts
Unexpected repairs
Deductions or costs of such extenuations as
Sale value of dust collected
Effectiveness, reliability, convenience, etc.
Capital costs
First and periodic costs may be combined in either of two ways, as
an annualized distribution of first costs plus the annual charges, or as
a summation of future periodic costs plus the first costs. The first
treatment is the simpler; it is adequate for most cost estimating, and
has been Implied in earlier parts of this Chapter. The second is more
*
In order to compare one particulate control system with another, one
should attempt to reduce all criteria of performance to the same denominator,
such as dollars per CFM-year, or dollars per ton, etc. There is no
completely acceptable way of doing this. One may use the data of Stair-
mand (e.g. Figure 7.2) to assign a dollar value to a given collection
efficiency, but this must be modified in terms of the plant emission
requirement.
-------
precise; it is preferred by cost accountants because it enables a more
accurate comparison of alternative proposals. Each is discussed below
with examples.
7.5.1 Annual Distribution Method
To estimate the total average yearly cost of owning and using
the filter system, one can simply divide the total initial cost by the
expected life of the system, and add the result to the expected annual costs
of operation. This is called amortizing the investment using straight-line
depreciation; in this method the book value of the initial investment de-
creases steadily through the life of the system. Every year a fraction of
the investment is written off the book value, and accepted as a filter system
cost.
Since money is tied up in the investment, and since the same
money could have been used elsewhere to generate profit, the investor suffers
a loss of interest income as another cost. Using an interest rate of 10% and
$1000 invested, the first year title interest cost is $100. The second year,since
the investment has depreciated, say l/15th in anticipation of a 15 year life,
the remaining investment is $933, and the interest cost is $93.33. For the
average year the equipment will be worth $500 and the interest cost will be
$50; thus over the 15 year period a total of $750 will be acknowledged on
the books as interest cost. This total interest cost may be annually distri-
buted for convenience as simply $50 per year; it is equivalent to simply
(Investment) x (Interest rate) f- (2) per year. Note that even though the
interest may not be an actual cash outlay,unless the money was borrowed,
it is attributable to the filter system.
Example;
Total initial investment: $1,000 including materials, labor, freight,etc.
Cost of investment capital: 8%
Anticipated equipment life: 12 years
Anticipated yearly costs: $325 per year including power, maintenance,
fabric, plant space etc.
-------
Straight investment depreciation: $ 83.33 per year
Capital (interest) cost:
$1000 x .08 * 2 - 40.00 "
Yearly O&M costs: 325.00 "
Total estimated cost of owning and
operating the fabric filter system: $ 447.33 per year
or $5,380 Total.
Likewise the rate of depreciation need not be constant; other acceptable
methods include faster-than-straight line depreciation and slower-than-straight
line depreciation. Plant accounting policies may prefer any one of several
commonly used depreciation schedules, and income tax laws accept any of
several schedules. Unless the design engineer has a specific reason for
36
using another method, the straight-line depreciation method is simplest
7.5.2 Anticipated Cost Summation Method
Alternatively the anticipated future costs of using the equip-
ment may be back-computed to the present time and summarized, in terms of
present dollars.
Example based on the system just discussed:
Total initial investment: $1,000.
Capital (interest) cost:
$40./yr x 12 yrs 480.
O&M costs:
$325./yr x 12 yrs 3.000.
Total cost of owning and
operating the system: $ 5,380. or $447.33 per year.
This is of course equivalent to the annual distribution method, and equally
useful for estimating; but it is not as accurate as the following method.
An important principle in finance is that future expenditures are
greater than their present value. For example, suppose a cost of $404 is
anticipated at a time one year from now. By investing $372 at the present
time at a return of 8% the cost can be met when the time comes. Thus the
future expenditure is said to have a "present worth" of $372. All such
anticipated future expenditures including interest costs can be summarized
in present dollars. The preceding example may now be treated in the following
manner:
-------
Approx.
Year Book Value Interest O&M Total Yearly Cost
Present Value
of Costs at SI
0 $
1
2
3
4
5
6
7
8
9
10
11
12
Total:
1000
917
833
750
667
583
500
417
333
250
167
83
0
- __ _ _ _
$ 77 $ 325 $
70 "
63 "
57
50 "
43 "
37 "
30 "
23 "
17 "
10 "
3 "
$ 480 $ 3900 $
Total initial investment:
Present value of future costs:
___
404
395
388
382
375
368
362
355
348
342
335
328
4380
$1,000.
2,794.
_ - _
$ 372
339
309
281
255
232
211
191
174
157
143
130
$ 2794
Present value of system
lifetime expenses:
$ 3,794.
According to this example, $3794 invested now would pay for the same
equipment and its use as would the $5380 estimated by the previous methods.
The smaller figure in this case represents more accurately the expected
costs to the company, and this accounting method (or still more refined
methods) should be used whenever their increased complexity is justified.
This method should be used whenever two or more systems having
different initial/annual cost ratios or different lives are to be compared.
Consider for example two systems, the one described above and an alternative'.
system:
-------
lixamplt
System No. 1
Total initial investment: $
Capita] (interest) cost:
87,, 12 yrs
O&M costs: (for 12 yrs)
IJasos for comparison:
I) Initial costs only:
2) Annual distribution:
3) Present value of future costs
1000
480
3900
1000
5380
3794
System No. 2
$ 600
288
4512
600
5400
3660
Hardly anyone comparing two different systems would look only at their
initial costs. Using the method of annual distribution of initial costs —'
as a criterion of comparison would, in the above example, lead to the wrong
decision, as the more detailed present value method shows the second system ^
will cost the company less.
7.6 ECONOMY IN FABRIC FILTER OPERATION _,
Chapter 5 discussed ways to minimize costs in designing fabric filter
systems, and Chapter 8 will discuss guidelines for operating and maintaining _/
the system. Operating personnel should be aware that even though they may
never see the electric bill or the invoices for the replacement fabric, they
can do much to affect day-to-day costs. This applies to the personnel run-
ning the dust generating process as well as those close to the filter system. i
Temperatures can be kept down, minimum cleaning can be exercised, minimum air
can be filtered, etc. Many ways to keep costs down are discussed in other ,
parts of this handbook. —'
7.7 REFERENCES FOR CHAPTER 7 :
—'
1. R.H. Perry, C.H. Chitton, Kirkpatrick, Perry's Chem. Eng. Handbook.
McGraw Hill, 4th Ed.
2. C. J. Stairmand, Some Industrial Problems of Aerosol Pollution, Proc.
Rov. Society. 307:209 (1968).
i
3. C. J. Stairmand, Selection of Gas Cleaning Equipment. A. Study of Basis ~*
Concepts, Filtration Society Conference on Dust Control, Olympia,
London, Sept. 22-25 (1 Sept. 1969).
~j
4. D.C. Stephan, Dust Collector Review. Am. Foundrymen 's Sot-. Annual
Meeting, Philadelphia, Pa. (May 1960).
t'
-------
5. R. L. Adams, Application of Baghouses to Elec. Furnace Fume Control,
JAPCA 14:8. 299 (August 1964).
6. F.A. Bagwell, L.F. Cox and E.A. Pirsh, Design and Operating Experience
with a Filterhouse Installed on an Oil Fired Boiler, JAPCA 19;3. 149
(March 1969).
7. C. J. Lewis and B.B. Crocker, The Lime Industry: Problem of Airborne
Dust, JAPCA 19:1. 31 (January 1969). .
8. Organization for Economic Cooperation and Development, Air Pollution
in the Iron and Steel Industry, O.E.C.D., Great Britian(1963)
9. W. B. Harris and M. G. Mason, Operating Economics of Air Cleaning
Equipment Utilizing the Reverse Jet Principle, Ind. & Eng. Chem. 47:12,
2423 (December 1955).
10. B. P. Harrison, Baghouse Cleans 500° - Cement Kiln Gases, Air Engin-
eering. 14 (March 1963).
11. R. J. Plass, Comparison of the Cottrell Electrostatic Precipitator
and the Silicone Glass Bag Filter, Proc. Seminar on Electrostatic
Precipitation, Penn State U., University Park, Pa. (December 1960).
12. U. S. Senate, Com. on P.W., Hearings Before the Subcommittee on Air
and Water Pollution (18 May 1967).
13. J A. Fife, and R. H. Boyer, What Price Incineration Air Pollution
Control?. Metcalf and Eddy, Inc., Boston, Mass, (circa 1967).
14. J. H. Weber, The Impact of Air Pollution Laws on the Small Foundry,
62nd Annual Meeting, JAPCA. (June 1969).
15. W. Muhlrad, Baghouse Dust Collection of Brown Smoke from an Oxygen
Converter, Stahlund Eisen 82:22, 1579 (October 1962).
16. J. L. Smith and H. A. Snell, Selecting Dust Collectors, Chem. Engg.
Prog. 64:1 (January 1968).
17. R. A. Gussman, C. E. Billings, and L. Silverman, Open Hearth Stack
Gas Cleaning Studies. Semi-Annual Report to American Iron and Steel
Institute SA-17 (August 1962).
18. A. Little and B. W. Sutton, Industrial Air Cleaners - A Study of Cost
and Efficiency, Filtration and Separation. 109 (March 1967).
19. J. H. D. Hargrave and A. F. Snowball, Recovery of Fume and Dust from
Metallurgical Gases at Trail, B.C., Can. Min. and Met. Bull.. 366,
(June 1959).
20. T. Killman, Dust and Fume Control for Electric Furnaces, Texas Steel
Co., Fort Worth, Texas (May, 1969).
-------
21. National Asphalt Pavement Association, Guide for Air Pollution Con-
trol of Hot Mix Asphalt Plants, Nat'l. Asphalt Pavement Assoc.,
Riverdale, Md., Information Series 17.
22. R.L. Chamberlin, and G. Hoodie, What Price Industrial Gas Cleaning,
Proc. Intern. Clean Air Cong,. , (London) 133, (1966)
23. H. R. Crabaugh, A. H. Rose, and R. L. Chass, Dust and Fumes from Gray
Iron Cupolas-Control In L.A. County, Air Repair 4:3, 125 (Nov. 1954).
24. R.L. Chamberlin, and P.B. Crommelin, Economic Aspects of Air Pollution
Control for the World's Heavy Industries, presented at the First
World Congress on Air Pollution, Buenos Aires, Argentina (November 14-
21, 1965).
25. T.M. Barnes, A Cost Analysis of Air Pollution Control in the Inte-
grated Iron and Steel Industry. Battelle Memorial Institute for
NAPCA, Fed. Clearinghouse No. PB 184576 (15 May 1969).
26. J. M. Kane, Operation, Application and Effectiveness of Dust Collec-
tion Equipment. American Air Filter Co., Bull No. 270 P2, Louisville,
Ky. (1952).
27. R. J. Wright, Select Carefully - Dust Collectors Fit Different Needs,
Plant Engineering, 8 (circa 1965).
28. R. Dennis, E. Kristal, G. Peters, and L. Silverman, Laboratory
Performance of the Mikto-Pulsaire Collector, Air Cleaning Lab.,
Harvard University, Boston, Mass., Contract No. AT(30)-1:841 (June 1962).
29. PHS, USDHEW, Control Techniques for Particulate Air Pollutants.
NAPCA No. AP-51, Washington, B.C. (January 1969).
30. R.L. Adams, High Temperature Cloth Collectors, Chem. Eng. Progress 62:4,
66 (April 1966).
31. Chemical Engineering. 76:6, 132 (24 March 1970).
32. F.R. Chase, Application of Self-Contained Dust Collectors. The Torit
Corporation, St. Paul, Minn. (1963).
33. M.S. Peters, Plant Design and Economics for Chemical Engineers,
McGraw-Hill Book Co., Inc., New York (1958).
\
-------
CHAPTER 8
OPERATION AND MAINTENANCE
TABLE OF CONTENTS
8.1 OPERATION OF BAGHOUSE SYSTEM 8-4
8.1.1 Start-up 8-4
8.1.2 Routine Operation 8-5
8.1.2.1 Use of Instruments 8-5
8.1.2.2 Flow Variation 8-6
8.1.2.3 Cleaning Cycle 8-6
8.1.2.4 Changes in Operation 8-7
8.1.3 Shut-downs 8-7
8.1.4 Safety 8-8
8.2 MAINTENANCE OF BAGHOUSE SYSTEM 8-9
8.2.1 Hoods and Collection Points 8-10
8.2.2 Inlet Ducting 8-11
8.2.3 Blast Gate and Flow Control 8-11
8.2.4 Fans 8-12
8.2.5 Entrance Baffles 8-13
8.2.6 Hoppers 8-13
8.2.7 Bag Retainment 8-14
8.2.7.1 Thimble Sheets 8-14
8.2.7.2 Fastening 8-14
8.2.7.3 Tension 8-15
8.2.8 Filter Elements 8-16
8.2.8.1 Spare Stock 8-16
8.2.8.2 Installation 8-16
8.2.8.3 Inspection 8-17
8.2.8.4 Salvage of Filter Elements 8-18
8.2.9 Collector Housing 8-19
8.2.10 Specific Cleaning Mechanisms 8-19
8.2.10.1 Shake 8-20
8.2.10.2 Reverse Flow 8-20
8.2.10.3 Reverse Flow Plus Collapse 8-21
8.2.10.4 Pulse 8-21
8.2.10.5 Reverse Jet 8-22
-------
CHAPTER 8
OPERATION AND MAINTENANCE (Continued)
8.2.11 Rotary Valves and Conveyors 8-22
8.2.12 Instrumentation 8-23
8.3 ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION PROBLEMS 8-24
8.3.1 Types and Frequency of Problems Reported 8-25
8.3.2 Specific Applications Reporting Problem Types 8-28
8.3.3 Literature Survey of Maintenance Problems 8-29
8.4 REFERENCES FOR CHAPTER 8 8-33
-------
Chapter 8
OPERATION AND MAINTENANCE
A good fabric filter system can last 15 years and longer, providing
it has the continuing interest of its personnel. Those dust control
systems located in the center of plant activity, and those systems col-
lecting a valuable dust, are the ones that usually receive the most atten-
tion and need the least supervision. On the other hand the filter house on
the roof, out back, in a cold place, etc. and the filter that seems to
contribute nothing but trouble will not be popular and will require extra
effort.
A few principles for successful operation apply to any fabric filter
system:
1. Reduce operating and maintenance costs by selecting the most
suitable equipment in the first place. Study the operating,
the instrumentation, and the maintenance manuals before pur-
chasing. Get equipment of adequate quality.
2. Follow the manuals. Know what is in them, and why.
3. Know what is entering the filter system.
4. Treat the fabric with care at all times.
5. Keep the flow into the filter as low as possible, limited
only by the danger of reaching the dewpoint. If there is
one single way of minimizing operating and maintenance costs
this is it, according to the reports of many filter users.
Complete guidelines for the operation and maintenance of specific filter
models are available from most fabric filter manufacturers. This chapter
discusses upkeep problems and practices common to most equipment. Be-
cause both operating and maintenance personnel are involved in system up-
keep, these practices are separated in two sections, each of which may be
used as a general guide. The last section discusses fabric filter equip-
ment problems reported in a survey of about 50 different fabric filter
installations. The problems are analyzed for common causes.
-------
8.1 01'K RAT ION OF BAGHOUSE SYSTEM
Like any new cqviipment, the filter system has a start-up period which
can last a few minutes or many months, depending in part on how well the
system was planned. Afterward the system functions with much less atten-
tion. It is important to remember, however, that any change in the input
conditions will make at least some difference in the filter's operation and
require a period of special attention.
8.1.1 Start-up
When the new equipment is started for the first time, the fan
should be checked for correct direction of rotation and speed. The duct-
ing, collector housing, etc. should be checked for leaks. Gas flows and
pressures should be checked against the design specifications. Instruments
should then be checked for correct reading and calibration adjustments made
as necessary. Control mechanisms, and especially all fail-safe devices,
should be checked for operability.
At the first start-up of the system, and also whenever new
bags have been installed by the maintenance crew, the bags should be checked
after a few hours of operation for correct tension, leaks, and expected
pressure differential. Initial temperature changes or the cleaning cycle
can pull loose or burst a bag. It is wise to record at least the basic
instrument readings during this start-up period on new bags, for ready
reference and comparison during later start-ups.
It is not always specified, but it is generally a good idea to
start a new set of bags at a filter velocity lower than that to be normally
used. The reason for this is that new bags have a low flow resistance, and
if the fan is run at normal speed with no other flow resistance in the sys-
tern, the filter velocity will at first be quite high. As a result the effi-
ciency will tend to be low and dust particles may be driven into and plug
the cloth. In contrast, a low initial velocity will allow the first dust
particles to stay closer to the surface of the cloth, bridging over the pores
in the cloth and leaving the rest of it much more permeable. Then the velo-
city can be gradually increased until, after a small percentage of the normal
dust cake has formed, the normal filtering velocity can be utilized.
-------
In addition, when removal of the dust cake must be unusually
complete during every cleaning cycle, it may be worthwhile to begin fil-
tration at a lowered velocity at every cycle. This can be done manually
by gradually adjusting a blast gate, or if the procedure must be done
frequently or if precise control is needed, it can be accomplished via
automatic control equipment. Some filter equipment is specifically de-
signed for this.
During any start-up, transients in the dust generating pro-
cess and surges to the filter house are probable and ought to be anticipated.
Unexpected temperature, pressure, or moisture has often badly damaged a
new installation. In particular, running almost any indoor air or combus-
tion gases into a cold filter can cause condensation on thji_j*alls and
cloth, leading to blinding and corrosion. Condensation'in the filter-
house may void the manufacturer's guarantee. It can be avoided by pre-
heating the filter or the gas.
8.1.2 Routine Operation
Day-to-day use of a filter system requires frequent observa-
tion and occasional adjustments in order to determine and adhere to the
best overall compromise between performance, bag life, power costs, etc.
8.1.2.1 Use of Instruments - A single monometer used across the
filter cloth can provide a wealth ot information. It indicates the
permeability of the cloth, tells how heavy the dust deposit is before
cleaning, how complete the cleaning is, and whether the cloth is starting
to plug or blind. It tells what surges in velocity the dust deposit is
undergoing, and whether there is any flow through the cloth during the
cleaning cycle.
It is a good idea to post a list or a recording of the normal
differential pressures through one filtration cycle, as a means to quick
detection of later trouble. A high differential may mean:
•an increase of air flow
•the beginning of blinding of the cloth
•hoppers so full as to block off the bags
•condensation in the cloth
•cleaning mechanism inoperative
-------
while a low differential pressure may mean:
•the fan is slowing down, perhaps due to
belt slippage or fan motor problems
'broken or undamped bags
•plugged inlet ducting or valves closed
•leakage between sections of the filterhouse
The reading of the manometer should be easily visible from the plant
operating floor, and one man should have responsibility for checking it.
On an expensive system an alarm should sound if certain pressure dif-
ferential tolerances are exceeded.
Other instrumentation can be nearly as valuable. If it has
been installed it is probably there for a good reason and it should be
used. Transients frequently occur in pressure, flow rate, temperature,
and humidity. Operating personnel should know at once when an indication
is excessive and what the causes may be. A trouble-shooting manual, such
as shown in Appendix 8-1, should be kept handy to the filterhouse.
8.1.2.2 Flow. Variations - With multiple or variable dust pick-up
points there will be variations in flow rate and filtering velocity. All
the branches of the inlet ducting may be open or some may be shut off,
depending on plant activity. Also while one collector compartment is
down for its cleaning cycle, another may be down for bag changes or still
another for inspection. These system changes affect the flow through
the filter. Too much flow through too few bags amounts to an overload
or too high a filter velocity, leading to inefficient filtering, plugging
of the cloth, loosening or bursting of the bags, or unsatisfactory build-
ing ventilation. Too low a flow is a frequent cause of condensation. When
flow variations are anticipated, it is wise to have at least manual means
of flow control, and automatic control equipment is often justified.
8.1.2.3 Cleaning Cycle - As the cloth ages, adjustments in the
cleaning cycle may be advisable either in the amount or the intensity of
cleaning or in the length of the cleaning cycle. One tries to use as
little cleaning as possible so as to prolong the life of the fabric; but
one has to use enough cleaning to keep the differential pressure at an
-------
economic level. There is In principle a point of optimization, although
this may be difficult to locate in practice. Operating at Che point ot
minimum cleaning is indicated by & gradual build-up of differential pres-
sure, perhaps over a period as long as a few days. Then before the pres-
sure gets too high, a slight increase in cleaning action (frequency or
intensity or duration, as appropriate) should reverse the trend in pres-
sure. After a few cycles or a few hours the pressure should reach a suf-
ficiently low level to reduce the cleaning action. If this gradual fluc-
tuation in differential pressure is not observed, it may mean that the
fabric is being overc leaned, because the only way to be sure the lilter
operation is near the point of minimum cleaning is to continually operate
around it. Changes in- process or fabric condition will undoubtedly cause
the cleaning requirements to shift from week to week, requiring a continual
hunting for them. Normally the process will be sufficiently stable through
automatic control so that day to day observation and adjustment of the
cleaning cycle can be made manually by a skilled person in a few moments.
This procedure of hunting for the minimum cleaning requirement is normally
worthwhile on all except possibly the smallest filter systems.
8.1.2.4 Changes in Operation - The fiber is carefully designed
to operate with a certain flow rate, particle size and type, etc., and
any changes in these conditions, expected to exceed ten percent or so,
should be analyzed for their effect on the overall filter operation. This
should be an economic analysis by an engineer skilled in fabric filter
systems. If large changes are indicated or a large installation invest-
ment is involved, a short pilot study using,if necessary, borrowed pilot
filter equipment may be justified.
8.1.3 Shut-downs
The main precaution in shutting down the filter system is pre-
vention of moisture in the filterhouse. Condensation can appear through
the cooling of gases containing moisture, particularly combustion gases,
if they are not completely purged from the filter system and replaced
with drier air before the filter cools down. This can also happen with
-------
air at ambient moisture levels if the filter is in a colder location. It
can happen if weather leaks into the collector particularly during ex-
tended shut-down periods. To prevent condensation some plants purge their
systems carefully on shut-down and then seal the system off completely.
Others continue a flow of warm air through the filter during the shutdown,
which also helps prevent condensation when they start up again. A par-
tially sealed-off filter system can be lightly pressurized with reasonably
dry air to exclude seepage of damp air.
In addition to general maintenance during shutdowns, other
reported practices ^include cleaning corners and crevices of any dust
accumulations which might solidify during a prolonged shutdown; removal
of any material which might catch fire on contact with air;and removal of
the bags for storage under more suitable conditions.
8.1.4 Safety
The preceding portions of this chapter -mention a number of
precautions against system damage. There are of course any number of
possible accidents. Baghouse structures and ducting are usually ade-
quate to prevent metal fatigue or panel collapse during normal pressure
excursions. It may, however, be less expensive to include explosion
panels than to design the entire system to withstand a large surge of
pressure. A surprisingly large number of dusts are flammable and some
are spontaneously combustible. (A good test is to ignite a small conical
pile of the dust and observe the rate and amount of burning.) The danger
of fire in a high velocity air system containing cloth is apparent. Some
plants find that their ventilating hoods collect a surprising quantity of
combustible material like lunch bags and candy wrappers. Instrumentation
responding quickly to sudden temperature changes, such as an automatic
cooling system, may be justified in some installations.
Operating personnel should be accustomed to think "Hazard."'
whenever a fabric filter is being used on abrasive dust or toxic fumes
or gases. (Many dusts have some degree of abrasiveness which can make
trouble in other plant equipment unless controlled.) Safety codes in
such cases will not normally permit the recirculation of the filtered air
-------
to the building'but if the air is recirculated, any bag failure can be
serious. If the exhaust is to the outdoors then consideration has to be
given to providing make-up air for ventilation to the building, as no
sizable exhaust rate can be made up by leaks through doors and windows.
At times the closing of windows or doors may cut down the air flow through
the building so much that the dust pick-up hoods fail to ventilate ade-
quately. In certain weather the makeup air inlet may entrain some of the
outdoor exhausted gases unless precautions are taken.
The disposal of collected dust can be a problem. If the dust
is poured or stirred in the open or if there are leaks in the disposal
equipment some of the material will re-aerosolize; it does not take much
dust escape (~0.1 percent.') to offset the remarkably high efficiency
of the fabric filter. The procedures needed for disposal of radioactive
dust can use up the permissible exposure quotas of many men unless care-
fully planned.
8.2 MAINTENANCE OF BAGHOUSE SYSTEM
There is a wide tendency to regard the dust collector as a piece of
trivial equipment until something goes wrong with it, whereupon it becomes
despised as a troublemaker. Planned preventive maintenance is a better
policy, the amount depending on how much the plant has to lose in case of
a system failure. A few companies successfully contract out the dust col-
lector maintenance, some contracting the entire job and others only the
supervisory portion, leaving the routine maintenance to in-house men.
Timing of maintenance is important. A supervisor or mainten-
ance forman may spend two hours every day looking over a large system,
because to let things go any longer than this is, in his experience, in-
viting trouble. For example, one leaky bag can rapidly destroy the ad-
jacent bags unless detected. Other procedures may be best done once a
month, or every six months, depending on what they are.
Maintenance procedures will be based on the operating and
maintenance manuals furnished with most fabric filter equipment, and later
they will be supplemented by experience. Appendix 8.1 lists examples of
-------
maintenance procedures and troubleshooting lists extracted from Heveral such
manufacturers' manuals. Records of bag changes as well as non-routine
maintenance may prove invaluable later in pinpointing high-cost parts of
the collector system and for projecting costs of any proposed similar
equipment.
In the following paragraphs maintenance practices are sugges-
ted for the various parts of the system. While these are extracted from
reports of over 100 specific installations, some comments regarding equip-
ment apply generally. The one factor most responsible for high maintenance
costs seems to be excessive filter velocities. Another factor is loca-
tion; centrally located equipment receives more attention than isolated,
inaccessible equipment and as a result often gives better performance.
Similarly a single large system has fewer servicing points than a number
of small dust collectors and will cbst less to maintain, other things
being equal.
The system components should all be readily accessible. Lad-
ders, walkways, and cranes necessary for maintenance should be provided
at the time of installation. Lighting outside and if possible inside
the filterhouse should be installed. If the equipment is outdoors it
should be especially well protected, and the hoppers, disposal equipment
and fans should be enclosed to make servicing easier during bad weather.
Wherever there is possibility of dust accumulation or plugging there
should be access doors or cleaning ports. Doors, valves, bag clamps and
tension adjustments should all be serviceable by hand with a minimum of
tools, to save time.
8.2.1 Hoods and Collection Points
Check periodically for ill advised changes such as holes cut
in the hoods, more hoods added, ducts blocked off, or dust intakes moved
away from the dust source. Be sure any temperature sensor located in
the gas stream is where it will pick up a temperature truly representative
of the mixed gas.
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8.2.2 Inlet Ducting
This is a moderately large problem area Cor several reasons:
(a) Abrasion. Flat, heavy 'plates installed at bends may
increase elbow life. Alternatively soft, rubber-like
material may out-last metal.
(b) Corrosion. Insulate to reduce condensation. Ducting
made of plywood may be compatible with mildly corrosive
gases. Long ducts, such as "hairpin" cooling systems,
are particularly subject to corrosion unless condensa-
tion is avoided.
(c) Sticking. Some dusts may "paint, out" on the inside of
the ducts. If this is due to dampness or due to thermal
precipitation from hot gas to cold duct, insulation
should help. An increase in pressure drop from end to
end is an indication of plugging. While vanes in duct
turns can minimize pressure losses at the turns, they
may collect material.
(d) Settling. Install clean-out doors or air lances in
long horizontal runs if there is any danger of settling.
Frequent plugging may require a Vee-shaped duct with a
conveyor in the bottom of it, or a higher air velocity
(i.e., smaller duct) at the expense of an increased pres-
sure drop. If possible keep the. bottom of the ducting
straight; when tapers are necessary, put them in the
sides or top.
(e) Temperature surges. Peaks in gas temperature will not
be reduced or dampened appreciably by short lengths of
ducting. However, the removal of duct insulation or
the addition of a heat absorber like brick work or a
steam boiler in the duct system will help to dampen
temperature excursions. Otherwise a high-temperature
by-pass of the filterhouse or some other variety of
fail-safe equipment may be needed.
8.2.3 Blast Gate and Flow Control
Problems with flow control equipment are frequently reported.
Any valve which may have abraded, plugged, become moved, etc. should be
checked periodically. Especially the blast gate should have a positive lock
against vibrational moving. It should be accessible, not in a duct high
overhead, as it may have to be adjusted frequently.
-------
Filter compartment inlet dampers are a high maintenance item
and spare parts should be stocked. An indication of a bad damper seal is
a differential pressure across the cloth when there is supposed to be no ~
flow through the compartment. This is given as a common reason for short
bag life in shake-type systems. The valves on the clean side of the bag- —
house, if any, may also malfunction if the bags have been allowed to leak
extensively and the valves are fouled. Either air cylinders or motor
driven actuators for dampers and valves may be used, depending partly on
the required speeds of opening and closing. Motor driven actuators, al-
though slower, may be slightly more reliable.
Being able to isolate a compartment from the rest of the
system without shutting the system down can be invaluable to baghousc
maintenance. However, even the best valves may not be acceptable in all '
cases; for example if there is danger of carbon monoxide, internal main-
tenance may be prohibited except during plant shutdowns. :
8.2.4 Fans ~"
Fans and blowers are reported to be a large problem area,
particularly those located on the dirty side of the baghouse where material
can accumulate on the vanes and throw off the balance. Corrosion and
abrasion can also cause trouble. It is wise to anticipate and prepare for ~~
some fan maintenance in order to avoid long, expensive shutdowns. For
example, the fan should have access doors, electric disconnects, and —/
crane facilities for both ease and safety. Fan maintenance ia roduced
by installing rugged equipment, and a large slow fan will probably need
less attention than higher speed equipment. If one type of fan blade
does not seem to last very well, another type may be better (see Section 5.3).
Having two or more fans on the system may enable maintenance of one of them
without shutting down the system completely. Furthermore, seal maintenance
problems on the filterhouse may be reduced by having fans both before and -'
after the baghouse, enabling operating the house at nearly zero gauge pres-
sure. Condensation and corrosion in the fan may be alleviated with duct and _w
fan insulation; most fans come provided with drains in anticipation that
water will sooner or later get into the fan housing.
-------
The fan should be checked for direction of rotation periodically,
as even a fan running backwards moves some air. Air Clow and Can speed should
be measured, not just estimated, checking belt wear and adjusting the belt
tension as necessary. These checks can be combined with routine lubrication
procedures.
Vibration noise probably means the rotor is out of balance,
and/or the bearings are going bad. Abrasive or sticky material may easily
change the fan blade weight, and the faster the fan speed, the more serious
this is. Sometimes a relatively small adjustment in speed can temporarily
alleviate a vibration until such time as the fan can be shut down for repair.
One remedy for accumulation of material on the blades includes sand-blasting.
This is a matter of hours, if the rotor has to be removed, but abrasive
blasting systems have been installed in the fan housing to expedite the job.
8.2.5 Entrance Baffles
Good baffling can reduce maintenance in the filterhouse by
helping to distribute the gas flow and the dust load more evenly to each
compartment and to each bag. A baffle can protect the. fabric nearest the
compartment inlet from direct impact of abrasive particles, or it can pro-
tect the first couple of feet of each filter element from the same thing.
Used skillfully, baffles can either direct the largest dust particles
downward to the hopper or upward to the filter surfaces, thus in effect
controlling the flow resistance of the dust deposit. For any of these
purposes it might conceivably be advantageous to use baffles that are manu-
ally adjustable. On the other hand, baffles sometimes have to be removed
when they contribute to maintenance problems by accumulating dust or abrad-
ing too rapidly.
8.2.6 Hoppers
Bridging and backup in hoppers should be anticipated whenever
the dust is expected to have a high angle of repose (a steep angle of slide
when piled), a. low aggregate density, or a high particle length/diameter
ratio. Steep-sided and/or wide-bottomed hoppers will presumably have already
been installed. Further assistance may be obtained by antifriction coating
-------
the hopper surfaces, or by air-pulsed rubber liners in the hoppers. Vibra-
tors are often used to loosen the flow and if used, heavy low-frequency vi-
bration may be preferable. However, some dusts tend to lock up when vibrated,
especially when the dust is already deep in the hopper. If surges in dust
load are causing back-up, then over-running of the rotary valves will help.
Moisture introduced through a leaking rotary valve in a suction system can
cause sticking on the hopper walls. If condensation is occurring in the
hopper, insulation should help.
Sledge hammering is reported to be one method of freeing a
clogged hopper. A better method is the installation of poke holes in the
side of the hopper; these can be designed in the original equipment if
the need is anticipated. Any hopper will get better attention if it is
housed against inclement weather.
8.2.7 Bag Retainment
8.2.7.1 Thimble Sheets - One fairly common complaint is that the
bags are packed together so closely as to abrade against each other or
against the baghouse. Close bags can neither be inspected adequately
nor installed nor adjusted easily. If the bags are more than about three-
deep beside the walkway, or two-deep for large bags, the man cannot reach
from the walkway to do his job. Hence damage often results as he misplaces
his feet. In the event of high maintenance resulting from overcrowded bags,
some baghouse users take out part of the bags and block off the thimbles.
Unfortunately this overloads the rest of the bags. It would be better to
take out the old thimble sheet and seal in a better designed one.
The thimbles themselves are ideally seamless and blunt-lipped
to minimize stress points and chafe on the cloth.
8.2.7.2 Fastening - The method of fastening and terisioning bags is
one distinction between equipment by different manufacturers, and there are
many variations. Generally, a minimum of tools should be needed; clamps
should be both quick and finger-adjustable. By the same token, both hands
should be free for the work; that is, the man should not have to hang onto
-------
a ladder or loan far over as he works on the bag. Fastenings should be
secure, because as a taut bag is collapsed it becomes tighter and the fasten-
ing tends to slip. Also large changes in baghouse temperature can lead to
differential thermal expansion, bag tension changes, and possible slippage.
Sometimes -a bag pulls completely loose from the thimble, as indicated by a
dusty exhaust and an unusually low manometer pressure.
8.2.7.3 Tens ion - At present there is no general rule for how much
tension a bag should have for best overall performance. A very slack bag
can fold over at the lower cuff and bridge across or wear rapidly. Too
much tightness will damage the cloth and work against the fastenings.
Because of the seam, as the tube is tightened it may arc, banana-like,
enough to touch the next tube. Whatever tension the bag has when it is
installed will be increased by the weight of the. dust cake, especially at
the top of the bag. This can be an increase of several times the bag weight
with some dusts. Circumferential tension as from inflation may further cause
the tube to tighten lengthwise. The fabric properties may change as the bag
ages, and this may also change the tension. For all these reasons the tension
of the cloth needs to be checked from time to time, especially a few hours
after installation of the cloth.
Correct tension is mainly a matter of filter dimensions and
cleaning mechanism. Shake cleaning in particular seems to require a
unique combination of tension, shake frequency, and bag properties for best
results. The manufacturer's recommendation should be followed until there
is more pertinent experience to go on. One rule for obtaining adequate
tension is the two finger method; when two extended fingers are slipped
over the uninflated, flattened tube, it is claimed that the wrist should
just be able to rotate 90 degrees. This amounts to about one-half inch of
slack, not a precise amount but a very ready test. Bags that clean by
collapsing may be under the best tension when they take on a cloverleaf
pattern as they collapse. Correct tension is reported as being from 25 to
100 pounds for glass bags of one-foot diameter, enough to keep them suffi-
ciently open during cleaning to let the dust fall to the hopper.
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8.2.8 Filter Elements
*
In most filter systems the biggest part of the maintenance
program is related to fabric upkeep. The cloth has to be inspected regularly,
and it is replaced either on a preventive basis or on short notice as trouble
occurs. Some filter users are able to salvage some of the removed filter
elements, while others find salvage does not pay. While other parts of the
filter system may be maintained by the standard skills found in most plants,
filter elements need special handling at all times. Glass fabric,for example,
is especially fragile and ca,n be ruined by kneeling on it or dragging a tool
across it.
8.2.8.1 Spare Stock - At least a few replacement and.usually a
complete set of filter elements should be kept on hand, the quantity de-
pending on the expected bag life, the risk, and the delivery time. The
margin of safety and risk of shutdown are balanced against the costs of
storage and inventory, which are annually about 10 percent of the purchase
cost of the fabric stored. The spare filter elements should be labeled to
indicate type and quality, clearly enough to avoid any possible confusion
with other sizes, cloths, or manufacturers, and kept well separated from
used filter elements. Elements arc stored safely against mildew, larvae,
crushing, etc. usually on a first-in, first-out inventory system.
Filter elements can be purchased either from the equipment
manufacturer or from a firm that specializes in sewing various types of
filter elements (Section 4.6). On the one hand, it is a good idea to avoid
splitting the responsibility for filter system performance between the equip-
ment manufacturer and a separate bag supplier. On the other hand, most filter
users feel vulnerable if they are committed to getting all their stock from
one source. At least one filter user solves this problem by buying cloth and
sewing his own bags. Generally, the experienced firm supplies the most re-
liable products. Inexpensive filter elements often turn out to require a
lot of maintenance and be more expensive in the long run.
.8.2.8.2 Installation - In many filter houses the elements can be
installed by one manj but in the interest of a short down-time a crew of
two or three men usually do the job. (Not all types of equipment have to
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bo shut down for bag replacement; sec Section 5*2.) Practices differ be-
tween keeping old and new elements in separate compartments and mixing them
together, which is customary when accidents happen only to a small percen-
tage or when there is no problem about down-time. Keeping them separate
is better preventive maintenance, although it is more expensive since some
cloth will be discarded before its life is up. Keeping them separate is
preferred when filterhouse conditions are noxious, or when entrance to the
compartment takes a lot of preparation time. In either method records should
be kept showing date of maintenance, a description of the cloth installed,
and the location of the change by thimble number or compartment number.
8.2.8.3 Inspection - External maintenance inspection of the filter
house and system is usually performed daily, while the filter elements
themselves are typically inspected once a week to once a month. The appearance
of the air exhausted from the filter is not always a reliable indicator
of element condition; the inside of the filterhouse should also be seen.
Any dust on the floor of the clean side of the filterhouse
indicates faulty operation, and the location on the floor is often a quick
indication of which element has failed. More often, however, locating the
hole in the fabric is a difficult, time consuming job, sometimes because
the hole is hidden by other elements. Fortunately experience often shows
that most of the holes occur in certain repetitive areas of the bags.
Holes (or more important, thin places about to become holes) may sometimes
be located by running a fluorescent light tube or flashlight through the
filter element, perhaps while it is still in the filterhouse. Sometimes
on squeezing the bag or tube a puff of dust will show the location of
the hole.
It is important to repair holes as soon as possible after
they develop, of course, especially when abrasive dust is being filtered,
because a hole in one element can quickly cut a hole through the adjacent
fabric. If there isn't time to repair or change the perforated bag it
should be tied off until such time. The same applies to a seam failure;
use whatever quick remedy will permit continuing the operation until a
downtime can be scheduled.
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The inspection procedure at an 80,000 CFM 500°F installation
2
has been described in the literature and may be of interest: .
"At present an inspection of the bags while the baghouse
is operating requires a supervisor, the operator, and
three mechanics. The operator locks out one compartment
at the panel board, one mechanic closes the tipping gate
valve at the bottom of the hopper, and the other two
mechanics open the top and bottom inspection doors. The
supervisor enters the bottom of the compartment, inspects
the bags ?..;<•• general appearance, and any defective bags
are tied off or capped at the cell plate. The procedure
is reversed to put the compartment back in service, and
the next compartment is inspected. All 10 compartments
may be inspected and serviced in this manner in less than
four hours. Inspections are made about once a month. Bags
are replaced during furnace rebuilds, when the baghouse is cool."
8.2.8.4 Salvage of Kilter Elements - liags can be removed
for cleaning and repair if most of the cloth is still sufficiently valuable.
Patches can be sewn on, or sometimes applied with quick-setting glue
without removing the bag from the baghouse. Thermosetting and pressure
sensitive adhesives have been used to repair glass bags. Of course that
portion of the cloth surface patched is lost for filtration, and the
surrounding cloth takes a slightly higher load. Bags with holes all in
one end can have a new end sewn on.
There are a number of ways ot cleaning a removed bag, if this
is judged to be worthwhile. Dry methods include turning it inside out and
beating or brushing it, tumbling it in a drum, vacuum cleaning it, and using
jets of compressed air on it. These techniques may damage the cloth, es-
pecially glass cloth. For example, cloths plugged to the point of rigidity
may be actually broken by bending them. Wet cleaning methods may be more
practicable when a cloth has become blinded or plugged well before its me-
chanical life is up. In rare cases wet'cleaning of the bags inside the
filterhouse may succeed. Felts have been dry cleaned successfully. Un-
specified cloths were reported to withstand 6 to 8 washings before they had
to be thrown away. Consulting the fabric supplier or manufacturer might
well salvage a large cloth investment that has met with a partial accident.
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It is n°°d practice to install salvaged fabric in separate
compartments from new fabric in order to get full service from the. new
compartments.
8.2.9 Collector Housing
The importance of accessibility has already been discussed:
a central location, inclement weather protection, ladders, walks, and
lights are all conducive to good maintenance and will reduce maintenance
expenses. Also the interior of the collector should be readily accessible,
if possible during operation. High temperatures or noxious dusts may make
collector entrance hazardous or unpleasant and thus expensive. Respirators,
coveralls, gloves, etc. if needed for entrance, should be kept close, by for
emergency repairs.
Faculty seals are a frequent complaint in collector housings,
especially when the equipment has been assembled by an unexperienced crew.
Also, seals tend to be weakened by weather, heat and age. Seals should be
checked every six months to a year; one technique involves placing one man
inside the compartment at night with a light and another man outside. While
large leaks may be indicated by a manometer that does not zero when it is
supposed to, minor leaks as along a. thermocouple wire which can admit water
to begin a corrosion problem are harder to find. If necessary, the filter-
house panels and flange connections can be dismantled, cleaned up, and
reassembled, probably using both cemented gaskets and bolts. Seals around
the doors to the filu>rhouse must not be overlooked.
The doors should be openable by hand without need of special
tools; they should be large enough for easy entrance, fabric maintenance,
and periodic cleaning of the dust which inevitably penetrates the cloth to
the clean side of the compartment. Glass panels in the doors may be worth-
while for observing the filter elements in operation.
8.2.10 Specific Cleaning Mechanisms
Each of the standard mechanisms (see Section 3.3 and below)
requires some maintenance procedures not shared by the others. Generally
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the more elaborate the cleaning apparatus the more maintenance ot all types
needed. One should use a minimum of cleaning energy, if the cloth fails
to clean adequately then a small temporary increase in the cleaning process
may bring the pressure drop back into line. The process of blinding tends
to be an accelerating one, because as the free area of the cloth decreases
the fan pulls harder in attempting to maintain the flow. This increased
pressure drop in an already marginal situation must hasten the blinding
process. For the maintenance man, this means being alert and ready to adjust
the cleaning procedure as necessary. At the same time, however, all changes
in cleaning procedure should be fully cleared before they are. put into prac-
tice, because the cleaning process has far-reaching effects and a large part
of the filter system expense is related to the cleaning action. ~
8.2.10.1 Shake - Any wear in the shaking machinery results in a
"*••
lessening of the shaking action, and unless the trouble is recognized for
what it is, the shaking intensity may be stepped up until the mechanism __
destroys itself. Regular lubrication and avoidance of gritty dust in the
mechanism can best be achieved by having most of the shaker mechanism out-
«••*•
side the filterhouse.
If the cloth is not cleaning, check the shaker rack to be sure
*w
it is moving. (For doing this while the rest of the filter system is down,
a jog button located near the door of the compartment is useful.) A small
adjustment of shake amplitude or frequency may markedly change the propa- *"
gation of the shake wave along the cloth tube and improve the cleaning. If <
the cloth still won't clean sufficiently with a safe amount of shaking, it —•
may be necessary to reduce the filtration velocity or alter the particulate
characteristics for a few hours. The velocity should, of course be abso- ^
lutely zero or even negative duringrthe shake part of the cleaning cycle.
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Llie dust into the hopper. The rate of flow (or the back pressure) and the
timing need adjustment from time to time to keep the residual drag at an
economical lovel.
8.2.10.3 Reverse Flow Plus Collapse - Whenever the cloth is flexed
as part of the cleaning process, and wherever it flexes, there, is apt to bo
a high rate of wear indicated by a thinning and eventual perforation of the
cloth. This is especially common near the thimbles where the flexing is
3-dimensional. A decrease of air flow or an increase in cloth tension will
help to reduce flexing. Frequently cloth flexure is reduced by installing
rings inside the tube, sometimes sewing them into the tube. In the cases
of reverse pulse and panel filters, wire grids are generally used to back
up the cloth. While these do reduce flexure wear, they may also introduce
frictional wear between the cloth and the ring or grid.
Note that any type of mechanical wear of the fabric--abrasion,
flexure, or tensile wear--will be amplified by adverse environmental con-
ditions. The molecular structure of both synthetic and natural fibers
may be damaged by high temperatures, moisture, and/or chemical conditions,
thereby weakening the fibers and making them more susceptible to mechanical
wear. Thus in a particular case, lowering the filtration temperature may
alleviate a flexural wear problem. The converse can also apply. For example,
fibers are available for filtering at very high temperatures (nearly 2000 F)
but at present there are no practicable ways of cleaning them without ex-
cessive fiber flexure. In other words, reducing the mechanical demands on
a fabric may make it more tolerant of tough environmental conditions. Fa-
brics with different finishes, weaves, etc. may also be put on trial to cir-
cumvent a particularly troublesome mechanical or environmental problem.
8.2.10.4 Pulse - As there are almost no moving parts in the pulse
cleaned apparatus, hardware maintenance is certainly reduced compared to
other cleaning methods. However, the excessive use of cleaning air pressure
may balloon the bags so much as to weaken them by overstretching. If the
fabric is being damaged and cleaning cannot be diminished, one must try
another fabric or attempt to reduce the adhesiveness of the dust (Section 2.2).
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8.2.10.5 Reverse Jet - As there is a fair amount of mechanism
within the clean side of the baghouse, any penetration of the bags by dust
results in a wearing of the mechanism, gradual or rapid, depending on dust
abrasivencss. Beyond a certain point, unless the equipment is completely
overhauled the mechanism will not only destroy itself but the bags as well.
Thus this method of cleaning while popular in some applications is at other
times associated with fairly high maintenance.
Sometimes in reverse jet equipment a hard residue of felt
fiber and dust builds up on the cleaning rings so near the bags as to
begin to rub. This accretion can usually be sanded off, but may be prevented
by antifriction coatings or rings made of stainless steel. The cleaning
rings must stay smooth and level and at the prescribed distance from the
cloth. The cloth tubes must be kept taut. Equipment operability can
usually be prolonged by changing from continuous tn intermittent cleaning,
in which the carriage pauses for a period of time between trips. Here
again is the principle of using a minimum of cleaning.
8.2.11 Rotary Valves and Conveyors
These can be high maintenance items if the dust packs to-
gether or adheres to the paddle wheel or other surfaces. The intake of
moisture due to faulty seals is a frequent cause of sticking. Ordinarily
a good quality valve will give long service if it gets lubrication and a
check of its seals and clearances from time to time'. Anti-adhesion coat-
ings may be used to some advantage on the wheel blades. A glass window
in the valve (kept clean) is a popular method of telling whether the valve
is discharging normally. Kraus lists a number of engineering and main-
tenance practices for rotary valves used in pneumatic conveying.
*-\
Smeary dust material in a screw conveyor can directly cause
repeated plug-ups and breakdowns. Converting to a pneumatic conveying
system is not always an improvement, depending on the dust. Possibly dif-
ferent conditions of dust temperature or moisture will change the properties
of the dust; otherwise it may be necessary to use separate collection bins
instead of conveying to a central point.
-------
Air leaks in the rotary valve and sometimes air leaking along
the conveyor from one compartment to another can re-aerosolize dust and
overload the filter. Indications of this arc non-zeroing manometers and
imusuu I .flit rr ilr.a(.',s .
8. 2.. I2 Instrumentation.
The operability of fail-safe mechanisms and automatic control
instrumentation is very important to the safety of the filter cloth, and
it is usually up to a maintenance, man to be sure these are in working order.
He may advise the installation of more or better instruments, since most
manufacturers supply a minimum of instrumentation with their equipment in
the interest of economy. Good instrumentation often pays for itself,
however.
One thing to check after equipment installation is the loca-
tion ot all sensing instruments because a small difference in location can
be serious. For example, the wrong temperature may be measured if the gases
are not well mixed at the sensor location; or, a high gas velocity may give
an error in the static pressure sensed at one side of a differential mano-
meter. All instruments should be calibrated after installation, and re-
checked monthly for sensor location, leaks (manometers)., sticking, legibility,
etc.
A central panel for most or all instrument readout,as opposed
to scattered instruments,has been estimated to reduce instrument mainte-
nance costs by as much as 50 percent, as well as making operation of the
collector system more convenient. One should record instrument readings
over one normal operating cycle for use in checking and troubleshooting
later in the life of the equipment. The record should be posted beside
each instrument.
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8.3 ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION ;
During a 1969 survey of about 50 fabric filter installations in a
variety of industries the people most familiar with the day-to-day
upkeep of these filter systems were personally interviewed. These people
wore asked to assist with about 130 questions relating to the equipment
and its performance. The questions included:
- What are your principal causes of fabric failure?
- Have you tried other fabrics, and why are you using
the present one?
- Do you receive any complaints regarding the quality of
your filtered effluent?
- Do you have problems associated with fabric blinding?
- What, if any, are the major difficulties with with your filter
system?
- What aspects of performance or operation could be improved,
based on your experience?
- What suggestions would you make for improvements in design
or manufacture?
- What do you see as being the principle requirements for
research or development?
From the answers to these questions, nearly 100 different suggestions
for possible research and development investigation were obtained, as
discussed in another volume of the contract documentation. The surveyed
installations also reported a total of 112 operating problems which are
summarized in this section.
As considerable care was taken to distribute the survey across a
spectrum of filter applications, the operating problems reported here
may with some confidence be considered typical of most filter users.
The list does not include all possible problems because of the limited
size of the survey. There is furthermore a wide variation in problems
encountered from application to application, and from installation to
installation.
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Appendix 8-2 lists 43 installations which reported operational
problems, together with the applications data suspected of being related
to the problems, These are:
- dust source
- particle size
- air/cloth ratio
- maximum differential pressure
- temperature
- fabric material
- cleaning method
- particle abrasiveness
There are often other circumstances that also contribute to filter oper-
ating problems.
In addition to the 1969 survey of fabric filter users, the fabric
filtration literature reports numerous examples of operational and main-
tenance problems.
8.3.1 Types and Frequency of Problems Reported
Table 8.1 lists 23 types of operational problems encountered
by the installations surveyed. They are grouped into 5 causality cate-
gories as follows:
1. Fabric-dust interactions 9 types of 60 problems reported
problems
2. Filter element difficulties 2 " 6 "
3. Filter element-hardware 5 " 12 "
interactions
4. Collector design problems 5 " 27 "
5. System design problems 2 " 7 "
23 types of 112 problems reported
problems
The largest category of problems reported was the first: 60 different
fabric-dust related problems were reported. Of these the most frequent
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TABLE 8.1
TYPES AND FREQUENCY OF PROBLEMS REPORTED
1. Fabric - dust deposit interactions Frequency
a. interstitial deposit related 8
abrasion, wear
b. flexure wear failure 10
c. seeping 4
d. blinding 14
e. burning, heat 6
f. holes, pinholes, shot holes 6
g. hydroscopicity 4
h. condensation 5
i. deposited dust hardens, cake 3
tears, cracks bag
Subtotal 60
2. Fabrication failures not. particularly related to dust interaction,
mechanical
a. seams, sewing 2
b. tears at top 4
Subtotal 6
3. Design or maintenance failures related to tensioning, supports, rings,
collars, or cleaning device interactions
a. chafe on housing or other bags 3
b. tensioning, bags too loose 1
c. cage, wire, ring abrasion, 5
wear (also dust related),
support mechanism interact
(continued)
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TABLE 8.1 (Continued)
d. cleaning carriage bag wear 1
e. seals around cloth-metal collars 2
Subtotal T2
4. Collector design problems, internal, external mechanisms, incl,
auxiliaries, (ex. pipes, hoods)
a. unable to enter collector to service or 4
maintain during operation
b. hole detection problems or performance 3
effluent monitor
c. hopper dust sticking, holdup, screw 6
conveyors plug
d. internal mechanism wear 4
e. external mechanism wear, timer, shaker, 10
fan, bearings, doors, seals, wall failure
Subtotal 27
5. Dust collecting system design problems, external to collector
(incl. pipes, hoods)
a. piping, elbows, abrade 4
b. hood inlet control poor 3
Subtotal 7
Total: 112
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type was blinding: 14 problems of blinding of the fabric by the dust
were reported, as detailed in Table 8.1. Over 25 percent of all problems
reported were from blinding, flexure wear, and interstitial abrasion.
From this brief analysis one is able to anticipate that most
fabric filter maintenance will center on interactions between the fabric
and the dust. The dust may not collect in a convenient way on the fabric;
the dust may not remove easily from the fabric; the dust may damage the
fabric in some way. Being thus forewarned, maintenance labor may possi-
bly be minimized, for example, by running pilot tests before selecting
the fabric.
Appendix 8.3 compiles the data in Table 8*1 and Appendix 8.2
to indicate the kinds of application in which each problem type occurs.
For example ,in Appendix 8»3 one may observe that "fabric wear apparently
related to interstitial abrasion" was reported in:
- 6 installations with particle sizes over 10 microns, out of
7 reporting
- 5 installations with temperatures under 200 , out of 8 reporting
- 2 installations with very abrasive dust, out of 6 reporting
- 4 installations with cotton fabric, out of 8 reporting, etc.
While this breakout does not necessarily demonstrate cause-and-effect
relationships, the table may be useful in troubleshooting operational
problems, as it enables comparison with applications reporting similar
problems.
8.3.2 Specific Applications Reporting Problem Types.
Appendix 8.2 lists the problems reported by each of the various
installations surveyed. These installations are grouped by industrial
category, enabling for example, the observation that all of the (four)
combustion applications reported only problems related to fabric-dust
-------
interactions. Iron and steel applications also reported fabric-dust
problems, notably abrasion from fabric support hardware. Again, this
analysis does not necessarily demonstrate cause and effect relationships;
instead, the analysis is presented for comparison with the reader's ope-
rational experience.
8.3.3 Literature Survey of Maintenance Problems
A review of about 500 documents published during the last 10
to 15 years yielded a large number of comments related to problems asso-
ciated with operating and maintaining fabric filter systems. These in-
cluded comments based on experience with a single filter system, as well
as comments from users of many systems and from manufacturers usually with
many years of experience. Much of this information has already been woven
into the previous sections of this Chapter and is not repeated here. Some
of the literature, however, is so valuable in amount of detail, or in de-
scription of a specific installation, or in insight and experience, that
it is indexed here in Table 8.2. Most of these reports are from the Iron
and Steel Industry and the Nonferrous Metals Industry.
Three cases in point are described in Appendix 8.4. These are
fabric filter applications of sinter plant discharge, oil combustion,
and copper smelting. The original accounts make interesting reading, as
they demonstrate the range of problems associated with the development
of new filter applications. These accounts are typical of the better
literature describing filter system operation and maintenance.
-------
oo
o
TABLE 8.2
MAINTENANCE PROBLEMS AND PRACTICES REPORTED IN THE
FABRIC FILTRATION LITERATURE
Industrv* Reference
1 4,5
5 6
5 2
5 7
7 8
7 9
7 10
7 11
7 12
7 13
8 14
8 15
8 3
8 16
8 17
8 18
8 19
Bust/Source
Oil combustion
Carbon Black
Soap, etc.
Cleansers
Cement Plant
Cement
Cement
Glass Furnace
Various applies.
Coal
Cupola,
Foundry
Cupola
Open hearth
General (Steel)
Electric Steel
Furnace
Sinter strand
General (Steel)
Fabric
Glass
Glass
-
Cotton
Glass
Var
-
Var
-
Var
-
-
Glass
-
-
Glass
Glass
Problem**
lb,3b,3c
G,le
Misc.
Id
3b,li
G
G
lh,li,ld
lh,lb
le,lh
G
4e
3a,3c ,4c
Misc.
le
la,3b,ld
4c , 4e , Ih
Remarks
System and procedures development
for new application
General
Pneumatic conveying equipment
Also chemical attack reported
General
General discussion of fabrics etc.
General
Several applications discussed
Panel discussion
General
General manual
General
System and procedures development
Misc. applications at Bethlehem Steel
plants ; Nui^erous problems
Labor and material cost analysis for
one year
Discussion of system upkeep
Misc. applications at Jones & Laugh 1 in
Fluoride attack.
L_
-------
TABLE 8.2 (Continued)
f
Industry
9
9
9
9
9
9
9
9
oo 10
t
U)
G*
G :
G
G
G
G
G
G
Reference
20
21
22,23
24
25
26
27
28
29
30
31
32
33
34
35
36
1
Dust /Source
Uranium plants
Copper Smelter
Secondary Cu
Refining
Arsenic salts
Zinc Roaster
Smelter
Cu Smelting;
ZnO
Zinc galvanize
Air filtration
General
General
General
General
General
General
General
General
Fabric
Wool
OrlonR
Glass
Glass +
-
Glass
TeritalR
OrlonR
Cotton
Various
-
-
-
-
-
Felts
-
(Cotton)
Problem
Misc.
lb,3a
Misc.
lb,3d
li.la.ld,
2a
-
1-, 3-
Id
-
Id
G
G
Misc.
Misc.
3d
G
Misc.
Remarks
Numerous problems on 18 R.jet
collectors; costs
Detailed experience with several
installations
General discussion of system
experience
Russian plant experience
System and procedures development
Designed for low maintenance
Routine inspection described
Occasional manual cleaning required
Maintenance economics of low efficiency
filter equipment
Residual cake profiles
General discussion by system manu-
facturer
Inspection schedule, disposal
methods discussed
Abrasion, adhesion, bridging etc.
discussed
Filter sleeve maintenance and storage
Reverse jet equipment
Unit sized filters
General discussion by system manu-
37
General
facturer
Troubleshooting checklist, general
-------
TABLE 8.2 (Continued)
Industry
G
G
Reference
38
39
Dust/Source Fabric
General
General
Problem
G
Remarks
General upkeep of all types of
collectors
World-wide survey of maintenance
costs reported
* Industry Key:
1 - Combustion Processes
2 - Food and Feed
3 - Pulp and Paper
4 - Inorganic Chemicals
5 - Organic Chemicals
6 - Petroleum Refining
7 - Nonmetallic Minerals
8 - Iron and Steel, Foundry
9 - Non-ferrous Metals
10 - Miscellaneous
G - General discussion
** Problem lb, 3c, etc: For explanation, see Table 8.1.
G = General discussion of system upkeep experience.
-------
8.4 REFERENCES FOR CHAPTER 8
1. R. L. Clement, Selection, Application, and Maintenance of Cloth
Dust Filters, Plant Engineering, pg. 24, circa 1965.
2. R. A. Herrick, J.W. Olsen, and F.A. Ray, Oxygen-Lanced Open
Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
JAPCA 16:1, 7 Jan. 1966.
3. M. N. Kraus, Pneumatic Conveying, I + II, Chem. Engg., 167
(Apr.. 12, 1965) and 149 (May 10, 1965).
4. D.N. Felgar and W.E. Ballard, First Years Experience with Full-
Scale Filterhouse at Alamitos Generating Station, Electric World,
Circa 1966.
5. F.A. Bagwell, L.F. Cox and E.A. Pirsh, Design and Operating
Experience with a Filterhouse Installed on an Oil Fired Boiler,
JAPCA 19:3, 149 (March 1969).
6. I. Drogin, Carbon Black, JAPCA 18:4, 216 (April 1968).
7. Dust Collection at Oakite, Anonymous, Soap and Chemical Specialties,
157 (June 1965).
8. R.E. Doherty, Current Status and Future Prospects-Cement Mill Air
Pollution Control, Third Nat'l. Conf. on Air Pollution,Proc., PHS,
USDHEW (December 1966).
9. R.F. O'Mara, and C.R. Flodin, Filters and Filter Media for the
Cement Industry, JAPCA 9:2. 96 (August 1959).
10. G. Funke, Operation and Maintenance of Filter Collectors, Staub
(English Translation) £6:4, 25 (April 1966).
11. A.B. Netzley and J.L. McGinnity, Glass Manufacture, Air Pollution
Engineering Manual, USPHS 999-AP-40, Cincinnati (1967).
1-2. Anonymous, Panel Probes Dust Collection Problems, Rock Products,
76 (January 1965).
13. H.E. Soderberg, Considerations in the Selection of Dust Collectors
for Coal, Mining Congress J., 62 (October 1964).
14. American Foundrymen's Society, Foundry Air Pollution Control
Manual, Des Plaines, 111., 2nd Ed. (1967).
15. R.W. Mcllvaine, Air Pollution Equipment for Foundry Cupolas,
JAPCA 17:8. 540 (August 1967).
-------
16. H. M. Chapman, Experience with Selected Air Pollution Control In-
stallation at Bethlehem Steel, JAPCA 13:12, 604 (December 1963).
17. T. Killman, Dust and Fume Control for Electric Furnaces. Texas
Steel Co., Fort Worth" Texas (May 1969)"" "
18. T. A. Young, Gary Steel Works Experience with Dust Control at No. 3
Sinter Plant, Blast Furnace and Steel plant, 1057 (December 1968).
19. S. Vadja, Blue Ribbon Steel with Blue Skies, I&S Engr., 71 (August 1968). ~
20. W. B. Harris and M. G. Mason, Operating Economics of Air Cleaning :
Equipment Utilizing the Reverse Jet Principle, Ind. & Eng. Chem. —''
42:12, 2423 (December 1955).
21. D. J. Robertson, Filtration of Copper Smelter Gases at Hudson Bay _
Mining and Smelting Co. Ltd., Can. Min. and Met. Bull.. 326 (May 1960).
22. R, H. Graves, Discussion of Baghouse Design and Operation, Nat'i. i
Assoc. of Secondary Materials Industries, Inc., Pittsburgh, Pa. "^
(June 1967).
23. Anonymous, Bag Life Extended at Copper Refinery, Air Enge (18 Jan. 1969). J
24. V.G. Matsak, Smoke Purification Equipment, in USSR Literature, on
Air Pollution Related Disease. USPHS TT60-21475 141 (1960). i
' ' —- -H J-L-LJi-- - --!'_- - / _—1- -LT.J— ^J
25. L.P. Landucci, and R.E. Eyre, Pilot Plant Filtration of Zinc
Suspension Roaster Gases, Can. Min. and Met. Bull.. 703 (October 1962). i
—j
26. Anonymous, Bag Filter for Smelting Plant is Fully Automatic.Filtration
and Separation. 326 (July 1966).
—i
27. H.E. Schwartz, L.E. Kalian, and A. Stein, Controlling Atmospheric
Contamination in Smelting and Refining of Copper-base Alloys,
Air Repair £: , 5 (May 1955) . •
28. G. Thomas, Zinc-Galvanizing Equipment, Air Pollution Engineering
Manual. USPHS No. 999-AP-40, Cincinnati (1967). j
—/
29. D.G. Hill, An Evaluation of the Costs of Air Filters, Filtration
and Separation, 297 (July 1967).
30. D.G. Stephan and G.W.Walsh, Residual Dust Profiles in Air Filtration, *""'
Ind. and Eng. Chem. 52: 12, 999 (December 1960).
•"• I
31. R.L. Adams, How to Maintain Cloth-Type Collectors, Air Engineering, —'
20 (May 1963).
-------
32. W.O. Vcdcler, Filters Aid At-Source Control of Dusts, Chem. Eng..
140 (May 1951).
33. J.M. Kane, Guideposts Tell How to Select Dust Collecting Equipment,
Plant Eng,, , (November 1954).
34. R. Ashman, Filtration Fabric Selection, Power and Works Eng..
53 (February 1959).
35. K.J. Caplan, Current Applications of the Reverse Jet Filter Principle,
A.M.A. Archives. Ind. Hyg. Assoc.. 21; 200 (March 1960).
36. F.R. Chase, Application of_S_elf-Contained Dust Collectors, The
Torit Corp., St. Paul, Minn." (1963)7 " """ ""
37. S. Levine, What You Should Know About Dust Collectors, Rock Products,
53 (April 1965).
38. P. Staubung, Operation Maintenance of Dust Removal Equipment,
Wasser Luft Betr. (German) 12, 215 ( April 1968).
39. R.L. Chamberlin and P,B. Crommelin, Economic Aspects of Air
Pollution Control for ^he.World's Heavy Industries^ presented at
the First World Congress on Air Pollution, Buenos Aires, Argentina,
(November 14-21, 1965).
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