Draft
8 ENFORCEMENT WORKSHOP ON
PLANT INSPECTION AND
1 EVALUATION PROCEDURES
0)
P
VOLUME VI
8 CONTROL EQUIPMENT OPERATION
I AND MAINTENANCE - FABRIC FILTERS
CO
(0
c
o
7E
m
IN
O>
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT
OFFICE OF GENERAL ENFORCEMENT
WASHINGTON, D.C. 20460
-------
REFERENCE MATERIAL FOR ENFORCEMENT WORKSHOP
ON PLANT INSPECTION AND EVALUATION PROCEDURES
Volume VI
Operation and Maintenance of
Fabric Filters
Compiled by
PEDCo Environmental, Inc.
505 S. Duke Street
Durham, North Carolina 27701
Contract No. 68-01-4147
PN 3470-1-CC
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Enforcement
Division of Stationary Source Enforcement
Washington, D.C. 20460
July, 1979
-------
FOREWORD
The following document is a compilation of selected
technical information and publications on the evaluation of indus-
trial air pollution control equipment operation and maintenance
practices. The reference manual is intended to be an instructional
aid for persons attending workshops sponsored by the U.S. Environ-
mental Protection Agency Regional Offices.
n
-------
TABLE OF CONTENTS
Page No.
Volume VI: Operation and Maintenance of Fabric Filters
VI-1. Fabric Filtration Systems Design, Operation
and Maintenance, S.A. Riegel 1-1
VI-2. Baghouses - What to Know Before You Buy,
S.A. Riegel, R.P. Bundy, C.D. Doyle, Pollution
Engineering, May 1973, pp. 32-34. 2-1
VI-3. The User and Fabric Filtration Equipment,
Donald H. Rullman, Journal of the Air Pollution
Control Association, Vol. 26, no. 1, January 1976,
pp. 15-31.3-1
VI-4. Ins and Outs of Gas Filter Bags, J.C. Walling,
Chemical Engineering, October 19, 1970. 4-1
VI-5. Baghouses: Separating and Collecting Industrial
Dusts, Milton N. Krauss, Chemical Engineering,
April 9,1979, pp. 94-106 5-1
111
-------
VI-1.
FABRIC FILTRATION SYSTEMS
DESIGN, OPERATION AND MAINTENANCE
by
STANLEY A. REIGEL
10001 Briar
Overland Park, Kansas 66207
1-1
-------
INTRODUCTION
This report will give you an overview of the design,
operation, and maintenance of air pollution control systems
using fabric filters (i.e., baghouses) as the control device.
Both engineering and cost factors will be discussed.
TYPES OF SYSTEMS
Baghouses are used in two fundamentally different types
of systems:
a) Dust control systems which may be described as a
system which is not really critical to a plant's continued
operation or to the immediate health or wellbeing of workers
in the plant or the general public. Dust control is used to
collect dust particles from the air, say in a woodworking
shop or in material transportation. Dust collectors usually
handle low volumes of air. High temperatures, continuous on-
line cleaning, long life fabrics, corrosion and other factors
usually are not considerations in their design and operation.
b) Process gas systems, on the other hand, are critical
to the production process itself. They may be called upon to
operate twenty-four hours a day, 365 days a year. They often
handle extremely large gas volumes and encounter high tempera-
tures, highly abrasive materials, sub-micron particles, high
dust loadings and corrosive agents. The real definition, how-
ever, is not necessarily that it is a heavy duty system—
rather that if it shuts down, the plant shuts down. For ex-
ample, in an asphalt plant if the baghouse system is not opera-
ting properly, the plant may not be able to achieve adequate
production rates. If the baghouse has to be shut down, pro-
duction also comes to a standstill. Also, where toxic gases
are present, operation of the process baghouse is critical.
In a plant where lead oxide is generated, for example, there
would be a hazard to workers if the baghouse malfunctioned.
Because of these factors, process baghouses are maintained in
a different manner from dust collectors. They are inspected
more freguently--where a dust collector may be inspected once
a month, a process baghouse may be inspected once a week. Al-
so, the inspections will be more thorough.
Process baghouses also must normally be more closely
monitored, and monitoring equipment may be more sophisticated
than on a dust collector. Where a visual check of the stack
of a dust collector usually is sufficient, a process gas bag-
house may have automatic stack monitors. Pressure gauges
usually will be inside the plant, at a monitoring station,
1-3
-------
rather than outside, attached to a baghouse leg.
Process gas baghouses also may represent a considerably
higher investment than dust collectors, which makes a good
maintenance important. However, due to increasing knowledge
about the effects of inhaling various particulates in the air,
the differences in maintenance of the two systems may be be-
coming less significant.
TYPES OF BAGHOUSES
No other air pollution control device is subject to as
wide a variety of modifications as is the baghouse. Baghouses
are classified according to; the type of fabric used; where
they collect the dust—either on the inside or outside of the
bags; the type of cleaning mechanism used—shaker, reverse
air, compressed air or sonic cleaning; or whether they are
continuous automatic or intermittant in operation. A baghouse
can be a combination of these classifications, such as inside
bag collectors, continuous automatic, shaker cleaning on a
process gas stream.
Inside Bag vs. Outside Bag Collectors
1. Inside bag collectors
Bag are attached both at the top and at the bottom of inside
collectors. See figures 1 and 2. Dust enters the hopper,
and the division between the clean and dirty air plenum is
near the top of the hopper, with the majority of the housing
being clean air side. Air flow is from the inside to the out-
side of the bags. It is necessary to enter the housing to
change bags.
2. Outside bag collectors
Typically, bags are suspended from the top of the housing.
Only the top portion of the housing is the clean air plenum,
with the major portion of the housing and hopper being on the
dirty side. Most outside collectors are designed so bags can
be inspected and removed from the top without entering the
housing. Since air flows from the outside to the inside of the
bags, they must be supported from collapse by cages on the in-
side of the bag.
Intermittant vs. Continuous/Automatic Baghouses
1. Intermittant baghouses
Intermittant baghouses are used only in applications in which
the production machinery is shut down at regular intervals, say
1-4
-------
Stainless
Steel
Clamp
-Cell PJate
V7777A \ZZZ7,
Cell
Plate
1
/
Cuff with
• Spring Steel—•—
Band
V/Z2
THIMBLE CONNECTION
SNAP BAND CONNECTION
Figure 1 Bag-cell plate attachments
-------
Rocking Motion
Shaker Bar
Tension Nuts
Bag Cap
Clamp
Figure 2.. Typical shaker arrangement
1-6
-------
during lunch periods, breaks, or at the end of the working
shift. The unit must be sized so it can operate without
cleaning for the time interval necessary.
2. Continuous automatic baghouses
These are units which clean their bags automatically during
operation at pre-set intervals. With the exception of pulse-
jet baghouses, continuous cleaning units must be compartmented
so a section of the baghouse can be dainpered off during the
cleaning cycle. In pulse-jet units, the compressed air blast
interrupts the air flow through the bag for the instant the
bag is being cleaned, and no damper valves or compartmentation
is necessary (although the effect is the same).
Types of Cleaning Mechanisms
1. Shaking
The most common type of inside collector is the shaker bag-
house. See figure 2. Shakers can range in size from small,
intermittant cleaning dust collectors with a few bags that are
shaken manually through a hand crank arrangement, to huge
compartmented systems with bags up to 40 ft. long and over a
million square feet of filtering area. The hopper of a shaker
baghouse is covered with a sheet of steel with holes in it
called a cell plate. See figure 1. Bags are fastened to the
cell plate and, at the top of the housing, to the shaker me-
chanism. Each row of bags is joined to a rod and each rod to
the shaker framework. Each shaker assembly consists of a
motor, eccentric, connecting arms and bearings. As the rod
rotates, a mechanical shaking is initiated which actually
shakes dust off the insides of the bags, where it then falls
into the hopper. Shaker baghouses normally utilize woven
fabrics and operate at low air-to-cloth ratios.
2. Reverse air
The reverse air baghouse is basically the same in design as
the shaker and operates similarly. The sole difference is
in the cleaning mechanism. Insteam of mechanical shaking of
the bags, a fan is used to reverse the air flow from insie-
outward to outside-inward. The result is that clean air is
backwashed into the dirty air plenum, through the bags from
the clean side to the dirty side, blowing collected particles
free. Reverse air cleaning is more gentle than shaking and
is used primarily with delicate fabrics. Bag construction is
similar to that of shaker systems; however, metal rings must
be sewn into the bags to keep them from collapsing during
cleaning cycles.
1-7
-------
3. Combination reverse air and shaker
Some baghouses may be designed for both shaker and reverse air
cleaning. During the cleaning cycle, the bags are first col-
lapsed with the reverse air fan, then gently shaken.
4. Pulse-jet
A pulse-jet baghouse uses short bursts of compressed air to
clean its bags which are attached only at the top to a tube
sheet. See figure 3. Over each row of bags is a compressed
air manifold with an orifice directed down toward the top of
each bag. During the cleaning cycle, an extremely short burst
of compressed air flows from the top to the bottom of each bag,
creating a shock wave that pushes the fabric out away from the
cage, knocking dust particles loose. Although several rows of
bags may be pulsed simultaneously, each bag receives its own
individual burst of compressed air.
The cleaning particles from the bag in a pulse-jet collector
is not accomplished by the backflow of air through the bag.
In fact, a typical pulse puts no more than 0.5 cubic feet of
air back through each bag; up to 2 cubic feet with induced
air flow from a venturi. Compared to the square feet of cloth
area in a typical bag, the effect is negligible. What acutally
cleans the bag is the sudden expansion from a shock wave,
which moves from the top to the bottom of each bag. Most
pulse-jet baghouses are manufactured with some type of venturi
over each bag, which most manufacturers consider necessary.
However, recent venturi design seems to have become more of a
marketing gimmick than a proper engineering design, since tests
have shown that all the different configurations manufacturers
placed over their bags made little difference in real cleaning
action. -Moving-horns, d±ffusers, exotic venturi shapes, para-
bolic velocity stacks, hyperboloid diffusers and even nothing
at all have been used over bags.
Although pulse-jet baghouses can be compartmented for certain
applications, compartmentation is not normally necessary.
Pulse-jet systems use densely felted bag materials to achieve
a high level of filtering efficiency. They also operate at
air-to-cloth ratios significantly higher than inside bag col-
lectors, so the baghouses are usually much smaller in size for
equivalent gas volumes.
Most pulse-jet baghouses are designed with hinged top doors
for inspection and maintenance of bags and electrical and
pneumatic systems, so there is no need to enter the baghouse
itself for routine maintenance.
-------
SLOTTED
VENIUW
MIKES
\
\
(( I f
If
Nl
/
/ I'f
— — - — — • — i /!
. / /
:' WlRS MS
ZINC
IXJ6C SWEET
xvvv
^
I I
I I
ATTACHMENT CG.TAJL - ALPHA SERJE^S
1-5
-------
5. Plenum pulse
A variation of the pulse-jet is the plenum pulse baghouse.
Basically, it operates in the same manner and also uses com-
pressed air or reverse air to clean the bags. The difference
is that the clean air plenum is compartmented into sections.
Each of these sections, when isolated by a damper, becomes a
semi-compartment and is pulsed all at once during cleaning.
ONe large burst of high pressure air, then, cleans several rows
of bags.
Plenum pulse baghouses may be equipped either with compressed
air or with high pressure blowers.
6. Blow ring
A reverse air variation in which a ring travels up and down the
length of the bag blowing air back through the bag as it moves,
These are very seldom used anymore although at one time they
were popular because they were the only type of baghouse that
permitted high air-to-cloth ratios that did not infringe on the
pulse-jet patent held by Midro.
TYPES OF BAGS AND BAG MATERIAL
Cloth Construction
1. Woven
Most commonly used with inside bag collectors. Fibers are woven
on conventional looms into specified fabrics. The type of
weave will vary depending on the application, and also, thread
size and type can be altered as necessary.
2. Felted
Most commonly used with outside bag collectors. Normally, a
light, loosely woven fabric, called a scrim, is used as a base
and fibers are needled into this base to form a dense fabric.
Bag Construction
1. Bag
Common name for all filters, but specifically, means a tube
shaped filter. This is the most common type of construction
and is used with both inside and outside collectors.
2. Envelope
Flat unit sometimes used with outside bag collectors. These
units offer the advantage of the maximum amount of cloth in a
1-10
-------
minimum space, but internal velocities are higher.
3. Cartridge
These are relatively new in design and resemble an automotive
air filter in that the cloth is pleated to increase filtering
area. So far they have been limited in application to low
temperature dust collectors.
Cloth Material
At least for process gas systems, the single most important
design decision is the cloth material to use. High tempera-
tures, corroosive chemicals, and small abrasive particles com-
bine to make the wrong decision very expensive. Bags that
were expected to last for three years may last only three months
(or even three weeks) if the wrong decision is made. Table 1
gives the properties of the most commonly used bag materials.
MECHANISMS OF FILTRATION
When a baghouse is started up the first time with brand new
clean bags it is a very inefficient device. In fact it leaks
like a seive. Clean bags are not good filters. The following
mechanisms are important because they make the bags get dirty;
these collection mechanisms are only important until the bag
builds up a "filter cake" of collected particles. After the
bags have been in operation a.while and the filter cake has
developed the bags are no longer, strictly speaking, the fil-
ters. Rather the filter cake of collected materials is itself
the actual filter and the bag merely supports it.
1. IMPACTION—the particle carried by an airstream collides
with a fiber and has enough inertial force so it doesn't de-
flect along with the airstream.
2. STRAINING--the particle is larger than the opening between
fibers and is trapped between them.
3. DIFFUSION--random movement of fine particulate causes par-
ticles to avoid following gas stream lines around the filter.
4. INTERCEPTION—a particle tends to follow airstream lines,
but as it passes the fiber, it comes close enough so that one
side impacts with the fiber.
5. AGGLOMENTATION—cohesion of one particle to another, and
adhesion of particles to fibers so that, in effect, the target
size is increased.
6. ELECTROSTATIC ATTRACTION—fiber has an opposite charge from
particles.
1-11
-------
TABLE 1.
^ > >TJ x-v T3 2! >TJ H OOZ2
FIBER PHYSICAL PROP. §§§§§!§ P & § P
O f > O t*l X *Tl O C/5 O *
•O M S T> C/5 PO 25 Z
OOMOH O * *
E = Excellent ^ w «< po K
G = Good
A = Average
F = Fair
P = Poor
Max. Cont. Op. Temp. Of
Abrasion Resistance
Toughness
R = Recommended
S = Satisfactory
N = Not Recommended
0 = No Information
285
A
A
225
E
G
CHEMICAL
n>
275
G
E
375
G
E
RESISTANCE
w
25
PI
170
E
A
450
P
F
500
P
P
180
F
A
200
A
E
TO REAGENTS
MINERAL ACIDS
Aqua Regia
Chromic
Hydrochloric
Hydrofluoric
Phosphoric
Sulfuric
S
R
R
R
R
R
S
N
N
N
N
N
S
N
S
S
S
S
S
S
S
N
N
S
N
S
R
N
R
S
R
R
S
R
R
R
R
R
R
R
R
R
R
R
R
N
R
R
R
N
N
N
N
N
S
N
N
N
S
S
S
S
S
ORGANIC ACIDS
Acetic
Benzoic
Carbolic
Formic
Lactic
Oxalic
Salicylic
R
R
R
R
R
R
R
S
N
N
S
S
R
S
R
R
S
R
R
R
R
S
S
N
S
S
N
S
R
R
0
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
N
S
S
S
S
S
S
N
BASES
Ammonium Hydroxide
Calcium Hydroxide
Potassium Hydroxide
Potassium Carbonate
Sodium Hydroxide
Sodium Carbonate
S
R
S
S
S
R
S
R
S
R
S
R
N
R
S
S
S
R
S
R
S
R
S
R
R
R
S
R
S
R
R
R
R
R
R
R
S
S
N
N
N
N
S
R
R
R
R
R
N
N
N
N
N
N
*USED INFREQUENTLY - Glass, Polypropylene, -Nomex, and Polyester account for
the vast majority of baghouses.
1-12
-------
TABLE 1 (continued)
^nr/nj.»_.ftjLi i\to.i_o irtiiv-.c.
TO REAGENTS
R = Recommended
S = Satisfactory
N = Not Recommended
0 = No Information
*s
o ?o
3 ^
o r1
T3 M
0 O
^
n>
~
0
f
H«
J^
M
0
*
33 0
o r1
O M
•o en
0 H
M W
^ ?d
ro
O
E9
^
hd
O
f
*"0
50
O
w
Z
M
M
£H
o
z
g
en
en
o
o
h*3
o
z
*
o
o
*
SALTS
Calcium Chloride
Ferric Chloride
Sodium Acetate
Sodium Benzoate
Sodium Bisolfite
Sodium Bromide
Sodium Chloride
Sdoium Cyanide
Sodium Nitrate
Zinc Chloride
R
R
R
R
N
R
R
R
R
S
N
S
R
S
S
R
R
S
S
N
R
R
R
R
R
R
R
R
R
R
S
S
S
S
S
R
R
S
S
S
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
S
N
S
S
R
N
N
S
S
N
R
N
R
R
R
R
R
R
R
S
S
N
S
R
S
S
R
S
S
N
OXIDIZING AGENTS
Bromine
Calcium Hypochlorite
Chlorine
Fluorine
Iodine
Ozone
Peracetic Acid
Potassium Chlorite
Postassium Permanganate
Sodium Hypochlorite
Sodium Clorate
Sulfur Trioxide
S
R
S
S
R
R
S
R
N
S
0
S
N
S
N
N
N
0
S
S
S
S
0
N
S
R
S
S
R
R
0
R
R
R
0
R
0
0
0
• o
0
0
S
S
0
0
0
N
R
R
R
R
R
S
R
R
N
R
S
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
N
R
R
R
R
R
R
R
R
S
S
S
N
R
0
0
S
S
S
R
N
N
N
N
N
N
N
N
N
N
N
N
N
1-13
-------
BAGHOUSE SIZING
Baghouses do not operate according to fractional efficiency
curves. In other words, a baghouse cannot be designed to yield
a given collection efficiency no matter how much information is
available on the process gas flow, particle concentration, par-
ticle size distribution, etc. A designer of a baghouse system
knows that the collection efficiency of the baghouse will be
very high (and the outlet concentration to atmosphere will be
very low) but he has no idea precisely what it will be.
Baghouse systems are designed from experience on the same
or similar applications.Most often the choice of what parti-
cular type of baghouse to use (i.e., inside vs. outside; shake
vs. reverse air vs. pulse; etc.) is based on one of the fol-
lowing:
a) The manufacture only builds one kind.
b) Although the manufacturer builds several types,
he has had good results with a particular type
(conversely, he may have had terrible results
with a particular type and will never design
it into a system again on this application).
c) The customer specifies a particular type be-
cause either he personally has had good results
with it or because his industry has, more or
less, standardized on it.
The fundamental design parameter around which the baghouse
is sized is the air-to-cloth ratio. Size is related to air-to-
cloth ratio by the following simple equation.
NBS =
(1)
AC
where :
NBS = Net baghouse size - square feet of filter area.
Q = Gas flow rate through the baghouse - actual cubic
feet of gas per minute.
AC = Air-to-cloth-ratio - feet per minute.
The actual baghouse is usually larger than the calculated
net for compartmentized baghouses because an extra compartment
must be added for cleaning and often an extra is also added for
on-line maintenance. The gas flow rate is usually calculated or
measured and the air-to-cloth ratio is selected by the designer
and depends on the type of baghouse and the application. Air-
to-cloth ratio is never calculated, it is selected. The air-
to-cloth ratio range commonly used for the various types of
baghouses is given below:
1-14
-------
1) Outside bag collectors using felted bags - (6 to 12)
2) Outside bag collectors using woven bags - (approx. 4)
3) Shaker - (2 to 3.5)
4) Reverse air - (1.5 to 2.5)
5) Combinations of above vary widely.
OPERATION
The two most important operating parameters for any bag-
house are the emission level and the pressure drop.
1. EMISSIONS. A major operating parameter of any baghouse is
that of the emission level. Either the system meets emission
standards or it doesn't. Generally, a properly operating bag-
house, under most conditions, should have no visible emissions
(with the exception of steam). Excessive emissions will be
caused anytime the integrity of the interface between the clean
and dirty air plenum is broken. In other words, damaged or
worn out bags, or broken welds or other structural damage to
the tubesheet, cell plate or housing.
2. PRESSURE DROP. This is the resistance the gas stream en-
counters as it flows through the baghouse. The greater the
resistance, the more power is required to force the air through
the system. If resistance is too great, there will not be
enough power to move the air and ventilation at the source will
not be adequate. Each baghouse is designed to operate at a
certain pressure drop or range of pressure drops. Pressure
drop is usually measured in inches of water. The correct pres-
sure drop is whatever it was designed to be; in some cases this
may be 2" and in others 10" - 12". During normal operation, if
the baghouse is functioning properly, the pressure drop should
remain fairly steady. If it begins to rise or drop suddenly,
or gradually creeps up during the day, that is an indication of
a problem.
Competent operation of a baghouse will, coupled with good main-
tenance, insure low emissions at an acceptable pressure drop
(assuming an adequate system design). Competent operation means
proper initial start-up, proper routine start-up, and shut-down,
and proper monitoring.
Initial Start-up
When a baghouse system is first installed, it is essential to
conduct a pre-start-up inspection, not only of the baghouse it-
self but of the entire system, including such items as motor
rotation, electrical functions, calibration of all indicating
equipment, etc. This procedure should be followed even if the
unit was factory assembled.
1-15
-------
a) Inspect the baghouse interior for debris. Pieces of
metal, nuts, bolts, welding rod pieces, etc., can destroy bags,
screw conveyors and even fan blades.
b) Inspect baghouse interior for leaks. A baghouse should
be air tight. Even pinpoint holes can cause future problems.
An easy method for checking leaks, before initial start-up,
is to go inside the baghouse, close the doors, extinguish the
lights and look for "stars". (NOTE: Always follow recommended
safety procedures when entering a baghouse).
c) Check bolted together sections to be sure all bolts
are in place and tightened down.
d) Check bag connections for tight clamps.
e) Check bag tensioning on inside collector systems.
f) Check gaskets on all doors.
g) Adjust cleaning mechanism as recommended by manufac-
turer.
Routine Start-up and Shut-down
On any process other than the most simple dust collection opera-
tions, a specific start-up procedure should be provided by the
manufacturer. If hot, moist gases are to be filtered, pre-
heating will raise the interior temperature above the dewpoint
and reduce chances for destructive condensation. A baghouse
can be pre-heated either by heaters in each compartment or by
recirculating gas through the system and heating it, or by
starting a process on some fuel that will not cause dust or
acid dewpoint (such as gas or oil firing a boiler before switch-
ing over to coal firing). Pre-heating will vary with the appli-
cation as to both time and temperature necessary to avoid con-
densation. «'
After routine shutdown, keep the fan running long enough after
the process has been shut down to purge corrosive gases from
the system. Also, it may be important to run the cleaning cycle
long enough to clean most of the dust from the fabric so that
you start with clean bags the next shift. It may also be im-
portant not to do this so you start with an adequate filter cake.
Routine Monitoring
As mentioned before, the two major indicators of a baghouse's
performance are its collection efficiency and its pressure drop.
As long as pressure drop is satisfactory, the proper amount of
air is moving through the baghouse; and as long as the air is
clean, the system is doing what it is designed to do. Simply
speaking, someone needs to visually check the baghouse's pres-
sure gauge and stack at regular intervals.Continuous stack
monitors and pressure drop recorders can be used where necessary.
Careful monitoring and reporting on a dust collector is not con-
sidered as essential as it is on a process baghouse. But the
operator should monitor and keep records on any system as if he
1-16
-------
were sure that one day he would have a major problem and will
have to call the baghouse manufacturer for assistance. If the
manufacturer has no idea of the history of the system, it is
not uncommon to have to rebag the entire unit and then watch
it fail again simply to monitor what happens as it fails so the
problem can be corrected. Routine inspection, therefore, should
always involve routine recording and accurate record keeping.
Special monitoring may be necessary in certain high tempera-
ture applications. There are heat sensitive materials, for
example, which can be placed on bags and which will change
color if the bag is exposed to excessive temperatures.
One good monitoring technique is to draw up a gridwork of the
baghouse and plot bag failures on this grid. It should also
be noted where, on the bags, failure usually occurs, such as
top, bottom, or middle, and the nature of the failure, whether
it is, say, a burn, a hole, or a rip. Typical monitoring and
indicating devices include:
--Pilot lights to show that the baghouse is operating
properly. Pilot lights show what motors are opera-
ting, which compartments are off or on-line, which
rows of bags are being pulsed, frequency of pulsing,
etc.
--Opacity meters which can show even a slight drop
in filtering efficiency which would not be detect-
able by the human eye.
--Pressure drop indicators such as magnehelic gauges
or manometers to show any change in pressure drop
during operation. Recorders may also be used for
a permanent record of pressure drop.
--Temperature indicators and/or records to show
when meximum operating temperatures are reached.
--Gas flow meters indicate the amount of air
moving through the system.
--Corrosion chips can be placed at strategic
points in the dirty air stream. These should
be made of the same metal as the baghouse and
should be inspected and measured regularly to
determine if corrosion is a serious problem.
MAINTENANCE
Here we are concerned with maintenance of the baghouse and
several important system components. Bag maintenance, per se,
is discussed later.
-------
Routine Maintenance
The key to a good maintenance program is record keeping. The
hallmark of a good routine maintenance program is no breakdowns.
1) Establish proper inspection intervals for all moving
mechanical assemblies (and bags) based on manufacturer's
recommendations. A typical scheme is listed in Table 2.
2) Keep a record of all inspections and maintenance work
performed. A typical speciman is shown in Figure 4.
Troubleshooting
The key to good troubleshooting is smart maintenance men and
adequate spare parts on hand. The hallmark of good trouble-
shooting is getting back in production fast after a breakdown.
A list of suggested spare parts is given in Table 3 and a
troubleshooting guide is shown in Table 4.
BAG MAINTENANCE
Bag maintenance is divided into two distinct categories:
1) location and repair of individual bag failures, and 2) com-
plete bag changeout of an entire unit.
Location and Repair of Individual Failures
On a process baghouse, individual repair requires the most
amount of time. In an ideal baghouse, all bags would last their
required time and then all would fail on the same day. This,
of course, never happens (unless there is a fire in the bag-
house) . There are variances in fabric quality, in bag manu-
facturing techniques, tolerances, gas flow distribution inside
the baghouse, areas of greater and lesser condensation pro-
blems, variations in the bag cleaning mechanism, etc. Any one
or any combination of these and other factors can cause bags to
fail prematurely.
It is typical to find any baghouse experiencing some small num-
ber of failures during the first month or two of operation.
These failures generally are due to manufacturing or installa-
tion defects. Later on there will be a period of few bag fail-
ures until you begin to reach the end of the operating life
of a set of bags. Then occasional failures will show up, fol-
lowed by an increasing rate of failure to the point where it
becomes expeditious to change out all of the bags.
The first indication that bag failures have occurred can come
through routine inspection or by noticing a visible emission
1-18
-------
TABLE 2
PERIODIC MAINTENANCE
Daily
1. Check pressure drop.
2. Observe stack (visual or with opacity meter).
3. Walk through system listening for proper operation (audible
leaks, proper fan and motor functions, bag cleaning systetn,
etc.).
4. Note any unusual occurrence in the process being ventilated.
5. Observe all indicators on control panel.
6. Check compressed air pressure.
7. Assure that dust is being removed from system.
Weekly
1. Inspect §iscrew conveyor bearings for lubrication (do not
lubricate).
2. Check packing glands.
3. Operate all damper valves (isolation,^ by-pass, etc.).
4. Check compressed air lines, including line oilers and filters
5. Check bag cleaning sequence to see that all valves are
opening and closing porperly.
6. (Inside bag collectors) Spot check bag tension.
7. (High temperature applications) Verify accuracy of tempera-
ture indicating equipment.
8. Check pressure drop indicating equipment for plugged lines.
Monthly
1. (Shaker) Check all shaker mechanism moving parts.
2. Inspect fans(s) for corrosion and material buidl-up.
3. Check all drive belts and chains for wear and tension.
6. Check all hoses and clamps.
1-19
-------
TABLE 2 (continued)
7. Check accuracy of all indicating equipment.
8. Inspect housing for corrosion.
Quarterly
1. Inspect baffle plate for wear.
2. Thoroughly inspect bags.
3. Check duct for dust build-up.
4. Observe damper valves for proper seating.
5. Check gaskets on all doors.
6. Inspect paint.
7. Check screw conveyor flighting.
Annually
1. Check all bolts.
2. Check welds.
3. Inspect hopper for wear.
1-20
-------
Figure
H'
I
to
Company
Check
Needs
Good Attn.
Serial No.
(
Structural-Bolts
Ladder Assembly
Airlock
Drive Assembly
A. Gear Reducer
Drive Shaft Align (
Coupler Shaft (
Bearings (
Belts (
Sheaves (
Serial No. (Motor)
H.P.
B.
C.
D.
E.
F.
G.
II.
I.
3.
Transfer Screw^ssy.(
Fan
A. Serial No.
B. Model No.
R.P.M.
Sheave Size
D. Make _
E. Sheaves ( )
F. Sheave Diameter ( )
G. Shaft Diameter ( )
H. Series of Belts
1. Shaft Diameter
J. Make
. ( ) Water Trap ( )
. ( ) Air Regulator ( )
. ( ) Bin Indicator ( )
. ( ) Magnehelic ( )
. ( ) Magnehelic Tubing ( )
. ( ) Baffle Wear ( )
( )
( )
( )
Item
15. ( )
16. ( )
17. ( )
18. ( )
19. ( )
20. ( )
21. ( )
22. ( )
23. ( )
2*. ( )
25. ( )
26.
27.
28.
29.
30.
Baghouse Inspection Report
Application
Date
Name
Check
Top Door Hold Down
Straps
Top Door Leaks
Manifold Pipes
Anchored
Manifold Pipes Holes
Manifold Pipes Center
Over Venturi
Venturi Properly
Seated
Cages Properly
Installed
Bag Clamps „
Bags-Visolite
Bags-General Appear.
Service Module
A. Wire Connections
Terminal Box
B. Wire Connections
C. Diaphram Valves-
Leaks
D. Solenoid Valves
Operating
E. Hoses
-------
TABLE 3
SUGGESTED SPARE PARTS
Following is a list of spare parts that should be kept on hand.
Quantities of parts will vary as to manufacturer's suggestion
and the type of process.
- Bags
- Bag support cages (reverse pulse and plenum pulse)
- Bag clamps
- Seals and caulking material
- Solenoids
- Diaphragms
- Timer components
- Baffle plates or wear plate sections for baffle
- Bag connecting rods (shaker and reverse flow)
- Tensioning springs (reverse flow)
- Belts for shaker mechanism (shaker)
- Motor for shaker mechanism (shaker)
- Fan belts
- Spare bearings and gasketing for all mechanical
components
1-22
-------
TABLE 4
TROUBLESHOOTING GUIDE
The following chart lists the most common problems which may be found in a
baghouse air pollution control system and offers general solutions to the
problems. There are a number of instances in which the solution is to
consult the manufacturer. This may not be necessary in plants that have
sufficient engineering know-how available.
Where the information applies to a specific type of baghouse, the following
code is used:
RP Reverse Pulse
PP Plenum Pulse
S Shaker
RF Reverse Flow
Symptom
Cause
Remedy
High Baghouse
Pressure Drop
Baghouse undersized
Bag cleaning mechanism
not adjusted properly
Compressed air pressure
too low (RP.PP)
Repressuring pressure
too low (RF)
Shaking not strong
enough (S)
Isolation damper valves
not closing S,RF,PP)
1-23
Consult manufacturer
Install double bags
Add more compartments
or modules
Increase cleaning
frequency
Clean for longer
duration
Clean more virorously
Increase pressure
Decrease duration and/
or frequency
Check dryer and clean
if necessary
Check for obstruction
in piping
Speed up repressuring
fan
Check for leaks
Check damper valve seals
Speed up shaker speed
Check linkage
Check seals
Check air supply on
pneumatic operators
-------
TABLE 4 (continued)
Symptom
Cause
Remedy
Low Fan Motor
Amperage/Low
Air Volume
Bag tension too loose (S)
Pulsing valves failed (RP)
Cleaning timer failure
Not capable of removing
dust from bags
Excessive re-entrainment
of dust
Incorrect pressure reading
High baghouse pressure
drop
Fan and motor sheaves
reverse
Ducts plugged with dust
Fan damper closed
Tighten bags
Check diaphragm
Check pilot valves
Check to see if timer
is indexing to all
contacts
Check output on all
terminals
Condensation on bags
(see below)
Send sample of du3t
to manufacturer
Send bag to lab for
analysis for blinding
Dry clean or replace
bags
Reduce air flow
Continuously empty
hopper
Clean rows of bags
randomly, instead of
sequentially (PP,RP)
Clean out pressure taps
Check hoses for leaks
Check for proper fluid
in manometer
Check diaphragm in gauge
See above
Check drawings and re-
verse sheaves
Clean out ducts and
check duct velocities
Open damper and lock
in position
1-24
-------
TABLE A (continued)
Symptom
Cause
Remedy
System static pressure
too high
Dust Escaping At
Source
Fan not operating per
design
Belts slipping
Low air volume
Ducts leaking
Improper hood design
Dirty Discharge
At Stack
Bags leaking
Bags clamps not sealing
Failure of seals in
joints at clean/dirty air
connection
Insufficient filter cake
Measure static on both
sides of fan and re-
view with design
Duct velocity too high
Duct design not proper
Check fan inlet configu-
ration and be sure even
air flow exists
Check tension and adjust
See above
Patch leaks so air does
not by-pass source
Close open areas around
dust source
Check for cross drafts
that overcome suction
Check for dust being
thrown away from hood
by belt, etc.
Replace bags
Tie off bags and replace
at later date
Isolate leaking compart-
ment if allowable with-
out upsetting system
Check and tighten clamps
Smooth out cloth under
clamp and re-clamp
Caulk or weld seams
Allow more dust to build
up on bags by cleaning
less frequently
Use a pre-coating of dust
on bags (S,RF)
1-25
-------
TABLE A (continued)
Symptom
Cause
Remedy
Bags too porous
Send bag in for permea-
bility test and review
with manufacturer
Excessive Fan
Wear
Excessive Fan
High Compressed
Air Consumption
(RP.PP)
Fan handling too much
dust
Improper fan
Fan speed too high
Build-up of dust on
blades
Wrong fan wheel for
application
Sheaves not balanced
Bearings worn
Cleaning cycle too
frequent
Pulse too long
Pressure too high
Damper valves not
sealing (PP)
Diaphragm valve
failure
1-26
See above
Check with fan manufac-
turer to see if fan is
correct for application
Check with manufacturer
Clean off and check to
see if fan is handling
too much dust (see above)
Do not allow any water in
fan (check cap, look for
condensation, etc.)
Check with manufacturer
Have sheaves dynamically
balanced
Replace bearings
Reduce cleaning cycle
if possible
Reduce duration (after
initial shock all other
compressed air is wasted)
Reduce supply pressure
if possible
Check linkage
Check seals
Check diaphragms and
springs
Check pilot valve
-------
TABLE k (continued)
Symptom
Cause
Remedy
Reduced Compressed Compressed air consump-
Air Pressure (RP,PP) tion too high
Restrictions in piping
Dryer plugged
Supply line too small
Compressor worn
Premature Bag
Failure -
Decomposition
Moisture in
Baghouse
Bag material improper
for chemical composition
of gas or dust
Operating below acid
dew-point
Insufficient pre-heating
System not purged after
shut-down
Wall temperature below
dew-point
Cold spots through
insulation
Compressed air intro-
ducing water (RP.PP)
Repressuring air
causing condensation
(RF.PP)
See above
Check piping
Replace dessicant or
by-pass dryer if allowed
Consult design
Replace rings
Analyze gas and dust and
check with manufacturer
Treat with neutralizer
before baghouse
Increase gas temperature
By-pass and start-up
Run system with hot air
only before starting
process gas flow
Keep fan running for
5-10 minutes after
process is shut down
Raise gas temperature
Insulate unit
Lower dew-point by keeping
moisture out of system
Eliminate direct metal
lines through insulation
Check automatic drains
Install aftercooler
Install dryer
Pre-heat repressuring air
Use process gas as source
of repressuring air
1-27
-------
TABLE 4 (continued)
Symptom
Cause
Remedy
High Screw
Conveyor Wear
High Airlock Wear
Material Bridging
in Hopper
Frequent Screw
Conveyor/Airlock
Failure
High Pneumatic
Conveyor Wear
Screw conveyor under-
sized
Conveyor speed too high
Airlock undersized
Thermal expansion
Speed too high
Moisture in baghouse
Dust being stored in
hopper
Hopper slope insufficient
Conveyor opening too small
Equipment undersized
Screw conveyor misaligned
Overloading components
Pneumatic blower to fast
Piping undersized
Measure hourly collection
of dust and consult
manufacturer
Slow down speek
Measure hourly collection
of dust and consult
manufacturer
Consult manufacturer to
see if design allowed
for thermal expansion
Slow down
See above
Remove dust continuously
Re-work or replace hoppers
Use a wide flared trough
Consult manufacturer
Align conveyor
Check sizing to see that
each component is capable
of handling a 100% de-
livery from the previous
item
Slow down blower
Review design and slow
blower or increase pipe
size
1-28
-------
TABLE 4 (continued)
Symptom
Cause
Remedy
Pneumatic
Conveyor Pipes
Plugging
Fan Motor
Overloading
Air Volume Too
High
Reduced Compressed
Air Consumption
(RP.PP)
High Bag Failure-
Wearing Out
Elbows too short radius
Overloading pneumatic
conveyor
Slug loading of dust
Moisture in dust
Air volume too high
Motor not sized for
cold start
Ducts leaking
Insufficient static
pressure
Pulsing valves not
working
Timer failed
Baffle plate worn out
Too much dust
Cleaning cycle too
frequent
Inlet air not properly
baffled from bags
Shaking too violent (S)
1-29
Replace with long radius
elbows
Review design
Meter dust in gradually
See above
See below
Dampen fan at start-up
Reduce fan speed
Provide heat faster
Replace motor
Patch leaks
Close damper valve
Slow down fan
Check diaphragms
Check springs
Check pilot valves
Check terminal outputs
Replace baffle plate
Install primary collector
Slow down cleaning
Consult manufacturer
Slow down shaking
mechanism
-------
TABLE A (continued)
Symptom
Cause
Remedy
High Bag Failure-
Burning
Repressuring pressure
too high (RF)
Pulsing pressure too
high (RP.PP)
Cages have barbs
(RP.PP)
Stratification of hot
and cold gases
Sparks entering baghouse
Thermocouple failed
Failure of cooling device
Reduce pressure
Reduce pressure
Remove and smooth out
barbs
Force turbulence in duct
with baffles
Install spark arrestor
Replace and determine
cause of failure
Review design and work
with manufacturer
1-30
-------
or an increased reading on a stack monitoring device. At this
time, it is necessary to inspect for bag failure.
In a compartmented baghouse, it is possible to monitor the
stack while isolating one compartment at a time. Through this
technique, any compartment which has experienced significant
bag failures can be discovered easily because stack emissions
will be reduced when the compartment in question is turned off.
In non-compartmented baghouses, or if no single compartment is
responsible for the majority of emissions, it may be necessary
to check the entire unit for failed bags. There are no dif-
ferent approaches used to locate failed bags - the major dif-
ference being in whether it is necessary to enter the unit to
look for failed bags.
There are three basic ways to search for leaking bags: 1) hunt
for the hole itself, 2) hunt for an accumulation of dust which
can be related to a nearby hole, and 3) use some detecting
device.
Inside collectors experience their most frequent bag failure
near the bottoms of the bags. If this is the case, an accumu-
lation of dust on the cell plate or near the failure sometimes
will be visible and relatively easy to spot, provided the fail-
ure is near the walkway. If the failure is higher up on the
bag, it will be more difficult to find because dust will be dis-
tributed over a wide area of the cell plate. In this case, it
is necessary to inspect the entire circumference and length of
every bag.
With top bag access on outside collectors, it is normally very
difficult to see the exact failure; however, in many applica-
tions a dust accumulation on the tubesheet or on the blow pipe
running over a failed bag will be readily noticeable.
The difficulty in finding a failed bag will vary with the ap-
plication and type of material being filtered.
One of the newest and most effective techniques that has been
found to locate failed bags is to inject a quantity of flore-
scent or phosphorescent 'dust into the baghouse and then inspect
the clean air plenum with a black light. The dust from even
very small leaks is easily visible as it glows under the black
light.
On inside collectors it is necessary to scan the entire length
of the bag with the black light to pinpoint the failure. On
outside collection units the florescent powder will be drawn
through the hole in the bag and will be easily visible either on
the venturi or around the blow pipe over the venturi.
1-31
-------
Use of this florescent powder technique is also effective in
spotting broken welds or other leaks in the baghouse tubesheet,
cell plate or housing.
In the past it was considered good maintenance procedure to
immediately replace a failed bag with a new one. Recently,
however/ it has been found that a new bag in the vicinity of
old ones will be forced to take a higher percentage of the
dust loading because the resistance is lower than that of the
"seasoned" bags. It was not uncommon to replace a bag one
month and then the next month to find the very same bag once
again needing replacement.
Also, individual one at a time bag replacement is quite costly
in man-hours on a per bag basis. More recently, it has become
an accepted technique to simply seal off the failed bag until
a significant number of bags have been sealed that the pressure
drop increase, at which time all the sealed off bags are re-
placed simultaneously. In most cases, 2% to 3% of the bags
(sometimes even up to 10% or more) can be plugged before no-
ticing a change in pressure drop.
On inside collectors if the failure has not occurred too close
to the cell plate, the bad bag can be cut, tied in a knot and
stuffed inside the cell plate. If the failure is too close to
the cell plate, it is advisable to plug the hole. In top ac-
cess outside collection baghouses, a plug is simply placed
over the failed bag.
There are commercially available plugs, although a plate of
steel with gasketing material will also work. The gasketing
material should be made out of the same type of material as the
bag. When using homemade plugs, it should be noted that they
must be heavy enough to resist the pressure differential,
otherwise the seal will be knocked out of place.
It is important on inside collectors to replace a plug failed
bags as soon as possible because of the "domino effect"in which
a hole in one bag will cause one to appear in an adjacent bag
and then that bag will cause a hole to appear in another bag
and so forth. >
It is important to keep track of the rate of failure of indivi-
dual bags. As the rate of failure begins to rise, it is possible
to predict the end of bag life and schedule complete changeout
of all bags at a convenient time. This is less costly than
changing individual bags, say month by month.
Complete Bag Changeout
1. Baglife
lr-32
-------
Depending upon a variety of factors, sooner or later, probably
within two to five years, the original bags will wear out. At
that time a complete changeout is required.
If inadequate bag life is obtained through no fault of the opera-
tor, it may be possible to charge the manufacturer due to
either an express or implied warranty.
a) Express warranty. Several years ago baghouse manu-
facturers uniformly refused to give express bag life
warranties. Now the opposite is true and virtually all
manufacturers will give at least a pro-rated warranty.
b) The UCC, which is law in every state (with the pos-
sible exception of Louisiana) will give the operator
the benefit of the implied warranties of "merchantibility"
and "fitness for a particular purpose" unless they are
specifically disclaimed by the manufacturer in writing.
"Merchantibility" means that the baghouse is of good
quality and "fitness for a particular purpose" means
that if the operator makes his intended use of the
baghouse known to the manufacturer and relies on his
expertise, the manufacturer must supply a baghouse
"fit" for that "purpose".
2. Changeout
When bag replacement is necessary, first de-bag the collector
and save all the useable parts such as cages, Venturis, and
clamps. It may be desirable to vacuum out the baghouse to
provide a more comfortable and safer working environment. Care
should be taken when installing new bags to avoid tears, punc-
tures and rips in the bags. Also, all clamps, seals, gaskets,
etc., should be checked after rebagging. When putting bags
on cages, the bags should not be twisted, as damage could occur.
If bolt-on clamps are used to hold bags in place, it should be
noted that pneumatic wrenches are available in the $100 to $200
price range. One pneumatic wrench will pay for itself not only
in time saved during the first bag changeout, but in assuring
that all clamps are properly tightened down.
There are new mechanized bag changeout procedures today whereby
an entire compartment of bags can be withdrawn all at once and
a new pre-bagged cartridge installed through the use of a crane,
usually mounted on top of the system. The old cartridge is re-
moved to a remote location where it can later be re-bagged at
a convenient scheduled time and held in readiness for use on
another compartment of the system as the need arises.
COST
What is the total cost of owning, operating, and maintaining
1-33
-------
a baghouse system? Figure 5 attempts to give a breakdown of an
answer to this question that would satisfy both an engineer and
an accountant (if such a thing is possible).
Some equations are presented below that predict (probably
within plus or minus 25%) the annual cost of operating and main-
taining a typical large baghouse system using a reverse flow
baghouse. The annual operating cost a reverse flow baghouse
is composed of:
1. Main fan power cost.
2. Reverse flow fan power cost.
3. Dust handling system power.
4. Direct operating labor.
ID Fan Power Cost
Te ID fan power cost for a baghouse is given by Equation 2.
FPC = l 17 . 10"4 . Q . AP . AH . PC (2)
Ef
where:
FPCB = Fan power cost attributable to baghouse, 1978 dol-
lars/year,
Q = Gas, flow rate, acfm,
AP = Operating pressure drop, kn. (probably about 6"
W.G.),
AH = Annual hours of operation, hr/year
PC = Local cost of electrical power, dollars/kWh
(see Table 3), and
Ef = Efficiency of ID fan, fraction,
= 0.6 approximately.
Reverse Flow Fan Power Cost
The annual cost incurred to clean bags via the reverse flow
fan is given by Equation 3.
, , . 10~4 . GCA . AP . AH . PC (3)
(NCN + 2)
where:
BCRR = Annual bag cleaning cost for reverse flow baghouse,
1978 dollars/year,
GCA = Gross cloth area, f t.2,
AP = Operating pressure drop, in WG (probably about
6" W.G.),
AH = Annual operating hours, hr/year,
PC = Local power cost, 1978 dollars/kWh, and
1-34
-------
I
(-0
Operation
Material
Utility
Consumption
Total Use and Ownership Cost
Maintenance
Material
Consumption
Capital
Charges
Depreciation
Financial Charges
Investment
Tax Credit
Miscellaneous
Charges
Taxes,
Insurance,
Contingency
Overhead
Absorption
Figure J>- Cost Breakdown
-------
NCN = Net number °f baghouse compartments (total number
of compartments less one for cleaning and one for
maintenance) .
Dust Handling System Power
The annual power cost to run the dust handling system is given
by Equation 4.
ASHp = 0.435 ' PC * DB • AH (4)
where :
ASHp = Annual power cost to run system, 1978 dollars/year,
PC = Local power cost, 1978 dollars/kWh,
DB = Dust burden, ton/hr, and
AH = Annual hours of operation, hrs/year.
Annual Operating Labor
The annual labor and maintenance cost, other than bag replace-
ment cost, is given by Equation 5.
OLRB = AWF (0.249 GCA° * 966+0 . 089 GCA+5.850) (5)
where:
OLRjj = Annual direct operating labor, 1978 dollars/year,
AWF = Area wage factor (see Table 3), unitless, and
GCA = Gross cloth area, ft2.
Maintenance Costs
The cost of maintaining a baghouse is divided into:
1. Direct labor (other than replacing bags) .
2. Parts and materials (other than replacement bags).
3. Direct labor to replace bags.
4. Bag cost.
Direct Labor (other than replacing bags)
MLB = 0.04 • GCA • AWF (6)
where:
= Annual direct maintenance labor (other than bag re-
placement) , 1978 dollars/year,
GCA = Gross cloth area, f t2 f ancj
AWF = Area wage factor, unitless.
1-36
-------
Parts and Materials (other than replacement bags)
MMB = 0.04 • GCA (7)
where: ,
MMB = Baghouse annual maintenance material cost, 1978
dollars/year (other than bags) , and
GCA = Gross cloth area, ft2.
Bag Replacement
Bags represent a substantial portion of the total baghouse
system investment and they must be replaced periodically as
they wear out. This section deals with the cost of bag re-
placement and focuses on:
1. Material cost, i.e., the cost of the replacement bags
themsleves.
2. Labor cost, i.e., the direct labor necessary to install
the replacement bags.
3. Frequency, i.e., the number of times the bags must be
replaced over 30 years or, in other words, in-service
bag life.
Material Cost
The cost of a set of replacement bags is given by Equation 8.
RBCR = 0.78 • GCA (8)
where:
= The cost of a set of replacement bags, 1978 dol-
lars/year, and
GCA = Gross cloth area, ft2.
Equation 8 is based on one foot diameter by 35 ft. long fiber-
glass bags. The equation can be used for other bag materials
by biasing with information contained in Table 5.
Labor
t
The labor required to change a set of bags in a reverse flow
baghouse is given by Equation 9.
BRLR = 0.021 • GCA • AWF (9)
where:
1^37
-------
BRLR = Bag replacement labor, 1978 dollars,
GCA = Gross cloth area, ft2, and
AWF = Area wage factor, unitless.
1-38
-------
TABLE 5
APPROXIMATE COST OF REPLACEMENT BAGS
1977 Cost
Material (dollars)
Nylon (5.3 oz/yd2) 0.64/ft2
Sewn in ring 2.00 each
Polyester (7 oz/yd2) 0.31/ft2
Sewn in ring 1.30 each
Fiberglass (silicon/graphite finish; 0.26/ft2
9 oz/yd2)
Sewn in ring 1>25 each
Fiberglass (10% PTFE finish; 9 oz/yd2) 0.42/ft2
Sewn in ring 1.50 each
Top caps (mild steel, 12 in. dia.) 2.80 each
Stainless steel clamps 1.75 each
-------
TABLE 6
COSTS THAT VARY WITH LOCATION
State
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada .
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
$
kWh
0.0322
0.0354
0.0451
0.0315
0.0371
0.0346
0.0450
0.0529
0.0433
0.0457
0.0489
0.0232
0.0460
0.0320
0.0435
0.0382
0.0281
0.0356
0.0335
0.0470
0.0479
0.0475
0.0349
0.0445
0.0381
0.0194
0.0297
0.0330
0.0387
0.0564
0.0366
0.0783
0.0326
0.0371
0.0435
0.0314
0.0209
0.0461
0.0458
0.0332
0.0326
0.0270
Area
Wage
Factor
0.89
1.79
1.00
0.78
1.06
1.03
1.01
1.11
0.84
0.81
1.00
1.03
1.12
1.19
1.15
0.96
1.01
1.06
0.81
1.08
0.93
1.33
1..07
0.76
1.02
1.27
0.97
1.05
0.82
1.04
0.80
1.02
. 0.74
0.92
1.20
0.95
1.13
1.05
0.80
0.77
0.87
0.85
1-40
-------
TABLE 6 (continued)
State
$
kWh
Area
Wage
Factor
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wyoming
0.0364
0.0277
0.0362
0.0432
0.0143
0.0342
0.0358
0.0221
0.99
0.93
0.84
0.85
N/AVAIL
1.10
1.11
1.03
1-41
-------
VI-2.
BAGHOUSES
WHAT TO KNOW BEFORE
YOU BUY
S. A. Reigel
R. P. Bundy
C. D. Coyle
Standard Havens, Inc.
Copyright (c) 1973 by Pollution Engineering.
Reprinted with permission from the May, 1973
issue.
2-1
-------
By S. A. REIGEL, R. P. BUNDY and C. D. DOYLE, Standard Havens Inc., Glasgow, Mo.
With (he advent of stricter emission standards designed!
to produce the national quality standard oil 75pg/m'
of paniculate, only the most efficient removal devices
will be suitable. The baghouse traditionally yfotds high
removal afficieraces (99.9 + percent).
A baghouse is a large metal box divided into two
functional areas. The first of these, the dirty air plenum,
may be a part of the baghouse proper or it may take
the form of a distribution manifold. The function of the
dirty air plenum is to distribute the fouled gas evenly
to the filtering elements or bags. The baghouse hopper,
which is the receptacle for collected material, is part
of the dirty air plenum. The clean air plenum is that
part of the baghouse where recombination of the air
circuits from each of the individual bags takes place.
The interface separating the clean air and dirty air
plenums is the filtering media or bags.
There are three important mechanisms involved In
removing particles from a flowing gas stream using
filter media: impaction, interception, and diffusion.
These three mechanisms which are responsible for
collision of a particle on a target, are of paramount im-
portance when the baghouse is first brought on line
and the bags are clean. After a short time, however, the
bags become caked with a layer of dust and this dust
cake actually becomes the filtering medium. No matter
how vigorously the bags are shaken, collapsed, or
pulsed, a residual dust cake is retained after each on-
line cleaning. The bags act primarily as a matrix to
support the dust cake.
Baghous© Types
All baghouses operate in basically the same way:
dirty gas is ducted to the unit where it is filtered by
cloth tubes or bags. This filtering action is extremely
efficient and results in virtually 100 percent of the
entrained particulate remaining in the unit on the bags.
The bags must be periodically purged of this collected
material. The method used for this cleaning charac-
terizes one type of baghouse from another.
All baghouses are either intermittent or continuous
automatic. Intermittent baghouses are designed to be
cleaned after the unit has ceased filtering, say at the
end of the work day. Intermittent baghouses cannot
be cleaned while on line, and thus are limited to low
dust loadings or infrequent operation. They have the
distinct advantage of being low priced. Continuous
automatic baghouses, on the other hand, are more
expensive but are able to operate 24 hours a day
without rest and can handle high dust loadings.
Baghouses are characterized and identified accord-
ing to the method used to remove collected material
from the bags. This Is accomplished in a variety of
ways, including shaking the bags, reversing the direc-
tion of air flow through the bags, blowing a jet of air
on the bags from a reciprocating manifold, or rapidly
expanding the bags by a pulse of compressed air.
The bags in shakoir-ltype baghouses are supported
by a structural framework. The structural framework
is free to oscillate when driven by a small electric
motor, Fig. 1. Periodically, on a timed basis, a damper
isolates a compartment of the shaker baghouse so that
no air flows. The bags in that compartment are then
shaken for approximately a minute during which time
the collected dust cake is dislodged from the bags. The
dust falls into the hopper for subsequent removal.
Rovers® flow baghouses are equipped with an auxili-
ary fan that forces air through the bags in the direction
opposite to filtration. This backwash action collapses
the bag and fractures the dustcake. When the bag is
reinflated by being brought back on-line, the fractured
dustcake is dislodged into the hopper. If the unit
operates under suction (the main fan is located on the
"clean" side of the baghouse) reducing pressure in the
baghouse may elimate the need for an auxiliary fan.
®8 baghousss incorporate a jet case or
manifold that surrounds each bag. The manifold travels
the length of the bag in a constantly repeating cycle.
As it passes over the surface of the bags, a jet of high
pressure air issues from orifices in the manifold and
blows the dust cake off the bags.
In recent years ravers© pulse baghousas, Fig. 2,
have enjoyed a rapidly increasing use. This design
utilizes a short (usually less than 100 milliseconds)
pulse of compressed air through a venturi, or diffuser,
directed from the top to the bottom of the bag, Fig. 3.
This primary pulse of air aspirates secondary air as
it passes through the venturi. The resulting air mass
violently expands the bag and casts off the collected
dust cake. A modification of this technique is the pres-
surized plenum type of cleaning. In this case an iso-
lated compartment above several rows of bags is sup-
plied with pressurized air. The change in pressure
differential across the bags when the damper is oper-
ated causes the bags to flex and cast off the dust cake.
The fundamental criterion used in applying any bag-
house to any application is the "air-to-cloth ratio,"
defined as the ratio of actual volumetric air flow rate
to net cloth area.
whera: AC = Air to cloth ratio, fpm
Q = Volumetric air flow, acfm
cloth area, sq. ft
2-2
-------
" AC Is equal to the superficial face velocity of the
air as it passes through the cloth. Shaker and reverse
air baghouses normally operate at an alr-to-cloth ratio
of from 1 to 3, while reverse pulse baghouses operate
at about 3 to 6 times this range. Units which are clean-
ed with a compartment off-line must be outfitted with
an extra compartment in order to keep a minimum net
cloth area on line at all times. In extreme cases, this
can double the size of the baghouse.
A second important operating characteristic is the
"filter drag" given by:
S =
A P •
V
where: S = Filter drag, in. H2O/fpm
AP = Pressure drop across filter, in. H2O
V — Superficial face velocity, fpm
As this equation indicates, baghouses do not obey
the fan law, which slates that the pressure drop varies
as the square of the volumetric air flow. Virtually all
other devices obey this law; Figure 4 "shows the varia-
tion of filter drag during a cycle varying from "just
cleaned" to "just before cleaning."
Filtering Bags
The most important components of any baghouses
are the filtering elements, or bags. Perhaps less is
known about predicting their in-service performance
than about any other component In general, bags
are either woven or felted. Woven bags are used in
shaker and reverse air baghouses; felted bags are
used in reverse pulse, plenum pulse, and jet case
baghouses.
Woven bags are usually fgrnished with a weight of
5 to 10 oz per sq yd and a permeability (acfm passing
through one sq ft of cloth with pressure differential of
0:5 in. H2O) of approximately 10 to 30 icfm. Felted
bags are heavier and much fuzzier, weighing 10 to 20
oz per sq yd. They also have a permeability of approxi-
mately 10 to 30 acfm.
A common problem with many man-made fibers is
their tendency to elongate under load and shrink at
higher temperature. This lack of dimensional stability
may cause a reduction in cleaning efficiency or pre-
mature mechanical failure of the bags. To avoid or
minimize these problems, many fabrics are heat-set.
Spun fabrics may also be singed or sheared for reduc-
ing the surface hairs in order to present a smooth
surface to the dust cake, thus making it easier for the
dust cake to dislodge during the cleaning cycle. Many
other surface finishes and mechanical operations may
be performed on the cloth to accomplish specific aims.
Resin finishes provide a smoother surface; graphite
finishes may be of value in eliminating build-ups of
static charge; napping a fabric may help improve
collection efficiency. Resins or graphite are used to
improve abrasion resistance.
Limitations
Although baghouses operate at the highest collection
efficiency level (99.9 + percent) all is not perfect. For
a given application baghouses will probably be one of
the more expensive solutions and will probably require
the most space for installation (see Table). The cost
of maintaining such facilities as settling ponds or pro-
viding chemical additives in wet systems may, however,
offset this price disadvantage. Baghouses will gener-
ally require much less power to achieve high efficiency
operation than water scrubbers and, of course, have
no water requirements.
The highest maintenance component of a baghouse
is the bags. The bags represent 20 to 40 percent of the
equipment cost and probably have an average life of
18 to 36 months. This means that the unit will be re-
bagged from three to seven times assuming a 10 year
amortization of the unit.
i
. There are several causes for bag failure—blinding,
caking, burning, abrasion, chemical attack, and aging.
The circumstances that can cause the above to occur
are varied—some due to normal operation, and some
due to misapplication or improper operation. Blinding
occurs when dust is captured within the bag material
and the cleaning mechanism is unable to dislodge it.
Either the cleaning .mechanism is not powerful enough,
or the nature of the dust is such that it can readily enter
the fabric. Obviously blinding is the result of either
misoperation (the cleaning mechanism is not powerful
enough) or misapplication (the nature of the dust is at
fault).
Caking is the formation of a solid mat of self-adher-
ing dust on the dirty air side of the bag which cannot
be removed by the normal cleaning mechanism of the
dust collector. The most common cause of caking is
the presence of water droplets in the gas stream which
causes mud to form on the bags. This later dries into
a hard cake. The water droplets can be caused by the
Fig. 1 Installation of shaker mechanism in shaker-type ' Fig. 2 Venturi and manifold in reverse pulse-type baghouse.
baghouse.
2-3
-------
Rulo-of-Thumb Costs of Typical Collectors ol
Standard Mild Steel Construction
Type of collector
Dollars per cubic foot per minute
Yearly
maintenance
Equipment Erection and
cost cost repair cost
Mechanical collector
0.07-0.25 0.03-0.12 0.005-0.02
Electrostatic
precipitator
0.25-1.00 0.12-0.50 0.01-0.025
Fabric filter
0.35-1.25 0.25-0.50 0.02-0.08
Wet scrubber
0.10-0.40 0.04-0.16 0.02-0.05
malfunction of water spray cooling equipment, or more
commonly, from condensation.
Water, in the form of condensate, is a common
cause of malfunction. If the temperature of the hot
humid gas falls below the dew point temperature in
the baghouse, water will condense, combine with the
collected dust and blind the bags with mud. The
gross gas temperature does not have to reach the
dew point for this phenomenon to occur: only the metal
skin of the baghouse need be below this temperature.
For this reason many baghouses are insulated to re-
tard heat transfer and maintain skin temperature above
the dew point.
Two types of burning can destroy bags. Baghouses
have a rather low temperature limitation ranging from
180 to 550 F. Consequently, on those applications
generating high temperature off-gas, there is a prac-
tical limitation of using those fabrics able to withstand
at least 400 F. Even then, a very reliable means of gas
cooling, such as evaporative cooling, is required, (POL-
LUTION ENGINEERING, Nov./Dec. 1970). Even when
the gas temperature is closely controlled, there is the
possibility of a hot spark reaching the bags and burn-
ing a hole.
Abrasion is a natural phenomenon resulting from
handling dust-laden gases. Some wear on bags occurs
from direct impaction, but wear is greatly increased
1
1
-J
c
TOTAL CYCLE REPEATED
( TO ATTAIN EQUILIBRIUM ^
INTERVAL
OFCAKE
REPAIR
+RFSCUA
*t>RAG \
*> D W
DEPOSITION OF HOMO-
GENOUS DUST MASS
to
d
«-
START Of
NEXT CYCLE
FILTERED DUST MASS. W (GRAINS/FT2)
How to Buy • Baghous*
If you have an air pollution or dust control problem In
your plant and are considering purchasing a baghouse, here
are some do's and don'ts:
DO: Write specifications that Include volume, tempera-
ture, dust and water content of the gas stream.
Specifications should clearly state the application.
DON'T: Specify crucial parameters such as air-to-cloth
ratio. No one knows the manufacturer's equipment
like the manufacturer. Trust him to make the cor-
rect decisions based on experience and require a
performance guarantee.
DO: State the applicable code requirements and per-
formance expected.
DON'T: Request a bag-life guarantee; most manufacturers
won't give one.
DO: Spell out peripheral equipment you want quoted
with baghouse; i.e., fan, motor, screw conveyor,
etc. If you want a turnkey job, require that the equip-
ment price be broken out separately from the erec-
tion price.
DON'T: Require detailed proposal if "budget" figures will
do.
DO: Expect the manufacturer to explain his features and
evaluate them as they pertain to your application
and plant operations,
DON'T: Arbitrarily request changes to standard design.
"Specials" cost much more and require fantastic
lead time.
DO: Check with the manufacturer's customers—they
know his ability to perform.
DON'T: Expect to visit an installation identical to the one
you're considering. No two operations are alike.
Also, improvements are being made daily that may
make what you buy better than what is in the field.
DO: Require factory assembly and prebagging if at all
possible. This will be less expensive and cause
fewer headaches than doing It at your plant.
when dust strikes a bag tangentially. Elimination of
abrasion is impossible, but proper design of a bag-
house will retard this cause of bag failure by channel-
ing the air in the least destructive way, and/or by
holding gas velocity to a minimum.
The destruction of a bag by chemical attack is
generally due to misapplication or misoperation. Pro-
cesses which generate gases at elevated temperature
frequently produce water vapor and acid radicals
which, at a given temperature (the acid dew point), will
react and form an acid vapor. The acid dew point
is dependent upon the concentrations of the con-
stituent in the gas, and is therefore very difficult to
predict.
Bags also tend to wear out from aging. After hun-
dreds of thousands of cleaning cycles, the fabric
weakens and will ultimately fail. The occurrence is as
unavoidable as human deterioration, but, like humans,
the better the bag is cared for, and the better the ori-
ginal equipment design, the longer it will last 5S
Fig. 3 Fitter drag cycle.
-------
VI-3. THE USER
AND
FABRIC FILTRATION EQUIPMENT
Copyright @ 1976 by Air Pollution
Control Association. Reprinted with
permission from the Journal of the
Air Pollution Control Association/
Vol. 26, No. 1.
That fabric filters offer unmatched opportunities for attaining op-
timum control of particulate emissions is generally conceded. It
is also accepted, however, that difficulties arise In predicting
and achieving consistency of performance when seemingly in-
significant changes occur in processing conditions, in baghouse
design, in maintenance, or in operation.
The following package of information draws on field experi-
ence to address this unpredictability and to examine specific
causes of success and failure. Several papers, and portions of
papers, were selected from those presented at the second
APCA specialty conference on The User and Fabric Filtration
Equipment, held in Buffalo, New York in October 1975. The pa-
pers which follow are authored by baghouse "users" in various
categories of industry; the one exception is a condensed version
of the keynote paper by a "manufacturer" which was designed
to bridge the gap between the first and second conferences.
The entire second conference will be reported in a Proceedings
whose availability will be announced in JAPCA.
3-1
-------
Baghouse Technology: A Perspective
Donald H. Rullman
American Air Fitter of Canada, Ltd.. Montreal. Quebec
As presented at the Second Conference on The User and Fabric
Filtration Equipment, this keynote address served to bridge the
two year period between the first and second conferences. In this
condensation, the author discusses the current state of the Indus-
try, and points out some of the user's responsibilities to the goal of
achieving regulation compliance In a cost-effective manner.
The dialog of the First Conference on The User and Fabric
Filtration Equipment, held in Buffalo in 1973, and this
Second Conference two years later comes down to two basic
elements:
—proper design of system and equipment
—living with the results.
The Clean Air Act Amendments of 1970 provided three
methods for mandating air pollution control for stationary
sources.
1) Setting of primary and secondary air quality stan-
dards,
2) Setting standards for hazardous air pollutants, and
3) Setting new source performance standards.
Ambient air quality standards have been established for
six pollutants.
1) Sulfur dioxide
2) Paniculate matter
3) CO
4) Photochemical oxidants
5) Hydrocarbons
6) Nitrogen dioxide
Of these, primarily particulate matter is controllable by
bag filtration. However, in 1973, of the 23 industries for
which new source performance standards had been set or
were under consideration, 17 had a particulate matter stan-
dard.
Updating, as of June 1975, the Environmental Protection
Agency has promulgated 22 new source performance stan-
dards. An additional 16 have been proposed and 55 are cur-
rently being developed. A spokesman from EPA informed
me recently that their objective is to issue 22 standards per
year.
What the EPA tells us, essentially, is that they have de-
termined, fairly accurately, the scope of the problem by in-
dustry and by process, and are continuing to apply research
emphasis in measurement and filtration of fine particu-
lates. They have expressed an opinion that there is sub-
stantial evidence that fine particulate can be controlled in
a cost effective manner. That of course is why we are here.
3-2
How do we design the process, the system and the bag-
house to be compliant in a cost effective manner?
Throughout those presentations it will be evident that
there is still—and perhaps always will be—a mixture of art
and science in the application and performance of fabric
collectors. Let's review briefly where we stand today. What
is happening in this "arty science" of fabric filtration?
On the large collector scale there is considerable activity
in yet another major production area of the integrated steel
mill. That is in "hot metal operations," or those operations
involved in moving, tapping and handling molten iron from
the blast furnace to the basic oxygen furnace shop. Specific
applications include:
—cast house
—torpedo car reladling
—desulfurization, in the ladle or in a torpedo car by sev-
eral processes
—BOF secondary ventilation which involves fugitive
emissions control at the BOF during charging and tap-
ping.
Japanese steel producers have the bulk of experience in
these areas—particularly cast, house ventilation. Their
major installations, including BOF secondary emission con-
trol, are virtually all structural, reverse air collectors with
polyester fabric and 12 in. diameter bags. Two such collec-
tors will also be installed soon in England. Several BOF
secondary ventilation collectors have been installed in the
United States. These are shaker collectors. A similar instal-
lation is also being installed in Mexico and the first such
unit will be installed in Canada within a year.
We are well aware that the metric system is being in-
creasingly used in North America. If you travel into
Canada now, the reported temperature is given in degrees
Celsius. In fact, snow depths in Canada this year will be re-
ported in centimeters. Having lived in Montreal for a year
it would seem to me that the kilometer might be a better
unit of measure for snow fall.
In the metric system the 5 in. diameter bag becomes 125
mm, the 8 in. bag becomes 200 mm and the 12 in. hag be-
comes 300 mm. For the most part these bags are inter-
changeable with their North American counterparts.
While this is a North American conference, there is no
doubt that the European and Japanese producers can teach
us a thing or two with respect to certain applications, for
instance, ferroalloy applications. In North America two
basic collectors have been applied successfully on ferroalloy
and electrometallurgical processes. These are the glass or
Nomex fabric reverse air unit and the Nomex equipped
shaker collector. In Europe the predominant baghouse in
use on these applications is a glass reverse air collector, and
I understand they are going a step further. Other than for
ferrosilicon furnaces, many ferroalloy furnaces can be vir-
tually completely enclosed extracting a relatively small gas
Journal of th« Air Pollution Control Association
-------
TROUBLE SHOOTING GUIDE*
Trouble
Possible cause
Remedy
Loss of Air Flow
Insufficient air How,
lack of dryer capacity
or puffing at burner
Excessive air flow,
poor fuel consumption
or low temp.
Dusty Stack
Fan stopped
1) Cleaning cycle too long
2) Low compressor pressure
3) Low compressor pressure
4) Excessive dust load
5) Excessive Tiring rate
6) Mud plugging
7) Oil or carbon coating
8) Damper closed
9) Duct leaks
10) Fines system leaks
11) Plugged precleaner or duct
12) Plenum bypassed (reverse
air or plenum pulse types).
1) Cleaning cycle too short
2) Damper open
3) Inadequate firing rate
1) Holes in bags
2) Poor bag seating
3) Housing leaks
4) Bleed through
Check motor interlock circuit or
for tripped overload.
Check for broken or slipping belts.
Decrease cycle time and cycle timer.
Check comp., if ok, check blast
time.
Check for sticking blast valves or
broken lines.
Reduce feed rate or use cleaner
aggregated.
Check moisture removed vs cap.
chart and reduce feed rate.
Clean bags, maintain internal col-
lector temperature above dew pt.
Clean buys, clean burner and check
for proper air/fuel ratio.
Check damper position.
Weld and repair.
Check airlock seals and housing
discharge.
Clean out build-up.
Check plenum valves and bypass
switches.
Increase cycle timer.
Check damper position.
Check burner, if ok, reduce feed
rate.
Replace or repair.
Repair bag mounting device.
Repair leaks between bag section
and clean air plenum.
Replace or repair worn bags, in-
crease cleaning cycle. Also reduce
air flow to minimum to reduce
operating air/cloth ratio.
* "Special Problems with Fabric Filters on Hot Mix Asphalt Plant Dryers,
pany, Charlotte, NC.
1 W. Con Proctor, Rea Construction Corn-
volume off the process. Whereas this application had pre-
dominantly used scrubbers, the idea now is to use a com-
bustion chamber followed by a heat exchanger—with or
without heat recovery—and a relatively small reverse air
glass collector.
There are also municipal and commercial incinerator in-
stallations in Switzerland which have proved that the bag-
house is a viable option to the precipitator in this applica-
tion. Virtually every form of bag cleaning is in use in this
application with apparently no significant data indicating
one achieves better results than another. Since in most in-
stances incinerators actually discharge relatively low sulfur
levels Nomex can be used. Chlorides are the big problem;
therefore these units are well insulated. Also, considerable
attention has been given to material handling systems on
such collectors.
In the cement and rock products industry, the fabric col-
lector continues to be an alternative to the precipitator in-
certain applications previously considered as exclusively
belonging to ESP. The increased use of direct reduced pel-
lets in steel producing arc furnaces should augment lime
consumption, and there is an increasing number of success-
ful lime kiln fabric collector installations.
We should also say a word or two about collector con-
struction. In our 1973 conference, several manufacturers
discussed the merits of modular versus structural collector
construction. I do not intend to repeat those discussions
but I do wish to stress that both methods are widely ac-
cepted today. One has not eclipsed the other and will not.
Both techniques can give equivalent years of suitable en-
closure for the filtering system. Two years ago it would
have appeared that the modular unit would soon displace
the structural design for all small and medium range collec-
tor sizes. But then the cost of fabricated steel started mov-
ing higher and higher. Already having a weight disadvan-
tage the fabrication cost of modular units soon began to
outweigh the remaining erection cost advantage.
Under certain conditions the modular collector has a
higher equipment cost (than a structural unit) but a more
than compensating lower field erection cost. However, the
cost of fabricated plate can in certain cases destroy that ad-
vantage. Thus, responsible manufacturers today do, and
must, investigate more than just the technical factors we
have reviewed before making a final decision on whether to
recommend modular or structural.
Recent advances in fabric filtering technology involve
making the selected fabric collector more dependable, con-
ceivably more efficient, and easier to maintain as this de-
vice is applied to an increasing number of applications.
However, as the baghouse is in each case simply a compo-
January 1076 Volume 26, No. 1
3-3
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
nent of a system—albeit the major component—advance in
system design technique is truly important to the success
of a baghouse installation.
The User's Responsibility
Before I close I would like to discuss briefly a serious
matter—the user's or purchaser's responsibility toward
proper analysis of process, proper specification of system
and collector, and ultimate selection of manufacturer.
To the user's accounting department, purchase of an air
pollution control system undoubtedly represents a major,
non-return, capital expenditure except where there is sig-
nificant reclaim value in the collected material.
To the user's management, this situation also represents
a high capital outlay with increased operating costs. But it
also represents far more—risk. Risk that the equipment
will not perform, and perform dependably, and risk that
the letter of the law will not be met.
To the engineering and operating department of a com-
pany contemplating purchase of pollution control equip-
ment, the assignment to select the proper system and
equipment still occasionally represents a very real dilem-
ma. Unless a portion of the engineering department is spe-
cifically assigned continued responsibility for air and water
pollution control, too often involvement in these problems
presents new technologies with new jargons, and there is
inevitably the pressure of a tight time schedule.
This is normally the right situation to choose a qualified
consultant However, with or without this consultant, the
baghouse manufacturer has a serious responsibility to de-
termine what he can about the process and to inform the
user. The user has the responsibility to listen, to study, to
inspect installations, and to confer with other users of such
equipment.
With some fear of being misunderstood we can say that
the primary purpose of the steel envelope, shell, or building
around the filtering system (by which we mean bags, bag
suspension members, bag cleaning devices, instruments,
dampers, and material handling) is simply to provide an
optimal working environment to allow this filtering .system
to function properly. Design of collector housings entail
normal building design disciplines considering wind, snow
loads, seismic conditions, and the like. But occasionally it is
difficult for a manufacturer to get a potential user to men-
tally open the door of such a structure and fully and equal-
ly analyze the filtering system—that portion of the struc-
ture that has entailed far more design research and years of
practical engineering. In such cases, final analysis can too
easily dwindle to cost per ton of fabricated steel.
Fortunately such situations are becoming increasingly
rare. Often the deliberate, technical approach taken by
users and consultants in our work is impressive and highly
respected. The continuation of conferences such as this,
play a large part in allowing each of us, from our own per-
spective, to establish that necessary technical dialog.
Mr. Rullman is manager, Engineered Systems Depart-
ment, American Air Filter of Canada, Ltd., 400 Stinson
Blvd., Montreal, Quebec, Canada H4N 2G1.
Innovations in Ferroalloy Baghouse System Design
R. Neil Paylon
Airco Alloys, Niagara Falls. New York
System design used by a typical ferroalloy producer I* reviewed,
with special emphasis on shaker cleaning of Nomex fabric In large
structural baghouses. The use of air curtains at the furnace hoods,
duct size venturl for flow measurement, and other techniques «re
described to permit evaluation of Individual bags as well as com-
plete compartment* of bags. A unique hook-up of magnehellc
gauges provides an Instant assessment of the performance of
each compartment to assist operators In achieving optimum oper-
ation. A critique on waste heat recovery, gas coolers, and Induced
draft fans on dirty gas service is Included.
Airco Alloys, a Division of Airco, Inc., is the second largest
producer of ferroalloys in the United States. This paper
deals with the application of large filter fabric collectors to
collect fume from submerged arc electric smelting, open-
hooded furnaces. At the present time, Airco has three bag-
house systems controlling five furnaces, with an additional
three systems under construction for six other furnaces.
There are also two submerged arc furnaces equipped with
electrostatic precipitators, and one covered furnace with a
wet scrubber. This paper will describe the philosophy of,
and experience with, the components of the air pollution
systems which are in use or under construction. Following
3-4
Journal of the) Air Pollution Control Association
-------
this, we will go into the evolution of some of the unique fea-
tures of our systems which are at the heart of filter opera-
tion, and the reason for the title of this paper.
Ferroalloy furnaces producing fume considered most dif-
ficult to collect are from silicon metal, 50% to 75% ferrosili-
con, and ferrochrome silicon. These fumes are largely sili-
con dioxide, in submicron sizes, reported to average less
than 0.3 microns. It is the collection of these fumes that
will be discussed.
Thus far, we have used only two shaker type cleaning
methods; namely, fiberglass with deflation air and gentle
shake, and combination Nomes with deflation air and vig-
orous shake. Another popular cleaning method which is
used in our industry, both here and abroad, is reverse air
through ringed glass bags. Although glass filter media have
advantages of a 500° F plus operating temperature and
lower capital cost, the air-to-cloth ratio is limited to 2 to 1
or less. Thus far, Airco Alloys has elected to use the combi-
nation Nomes filter cloth and shaker mechanisms that per-
mit a 2.75 to'l ratio at equivalent bag pressures. Bag pres-
sures up to 16 to 18 in. WG on this type of application are
not uncommon. All bags are a standard llVj in. diameter by
30 ft-6 in. high. In order to cool gas from 500°F or mora
down to 375°-400°F, we use forced convection, tubular,
air-to-gas cooling. We prefer to put an investment in cool-
ers instead of more bags. Our experience with glass and
Nomex has shown good bag life but fewer bag failures with
Nomex. Perhaps the most significant difference is the in-
creasing filter drag of glass using gentle shake, compared
with the stable filter drag of Nomex.
Our experience with conventional dust handling by screw
conveyors, bucket elevator, and live-bottom storage hopper
has been satisfactory. We did have an initial bad applica-
tion of a vertical screw conveyor which taught us the value
of a dependable dust removal system. The collected fume is
being slurried in concrete misers and hauled away as lend
fill. One mixer truck has been able to keep up with about
20 tons/day of fume, hauling a distance of one mile, and
washing out the bowl after each load and scouring out with
crushed stone or slag at least once a day.
Furn® Mood Ventilation
The type of hood venting used on our ferroalloy furnaces
is an open hood approximately the same diameter as the
furnace, with a 5 ft vertical opening between the furnace
and hood to allow charging and stoking equipment to go
right up to the furnace. Thus, the hoods are upward of 30 ft
in diameter and about 10 ft high, with one or more offtake
ducts leading to stacks, and then to air pollution control
systems. (See Figure 1.) Sufficient ventilation must be pro-
vided to prevent an outflow of gases from over the furnace
into the furnace building. Starting in 1971, Airco Alloys has
used air curtains to help seal in the gases and to comple-
ment the use of vertical sliding doors in some cases. Al-
though the primary object at that time was to eliminate the
use of doors to reduce the required ventilation flow into the
furnace, the full evaluation of this technique has been diffi-
cult without accurate and continuous flow measurements.
What has been accomplished, unquestionably, is an eco-
nomical way to dispose of furnace taphole fumes
amounting to approximately 50,000 cfm, depending on the
size of the furnace. Building drafts make open hood venti-
lation difficult, requiring a face velocity through the venti-
lation space of at least 250 ft/min. In principle, the air cur-
tain jets blow downward and inward to the furnace, elimi-
nating inflow for 2 to 3 ft below the bottom of the hood
skirt. If the jet velocities are carefully controlled, the in-
duced secondary flow of room air is minimal, and any
culation outward from the air curtain is immediately re-
entrained into the horizontal inflow entering the furnace.
As a measure of our confidence in the air curtain tech-
nique, Airco has built these devices around the hoods of
three furnaces and is building three more in both new and
existing furnaces. Another ferroalloy producer has built
one unit with the likelihood of adding three more on exist-
ing furnaces. Low capital cost is possible, since the device is
an annular duct or ducts built either inside or outside the
hood skirt and requires only an additional \ in. static pres-
oura, over and above the hood and duct Icasae. The flow can
ba provided by in-duct vane asial fans.
1. Furnace
2 Hood
3. Tap
4. Offtaho
5. Fallout
6. Flow venturi
7. Coolers
8. Cooling air
9. Fallout
10. Suction monifold
U. 1.0. fans
12. Dampers
13. Baghouse
14. Dust elevator
15. Dust silo
16. Emergency dampor
Plgrro H. Fabric ffto oyotcm.
107® Volumo 23, Wo. 1
3-5
-------
1 om 2 4
Flguro 2. Variation In flunl otach tompcraturoo.
6 8 10 Wcon 2
Timo of doy
In our recently designed offtake ductwork from the hood,
we are using at least two offtakes with fall-out chambers in
which a change of direction and a reduction in velocity are
incorporated. Since the offtake gas temperatures will be of
the order of 600° to 700°F, we are using automatic control
dampers to limit large excursions of temperature in the
ductwork. An indication of temperature fluctuations in two
equal ducts under a single fan suction is shown in Figure 2.
In the latest design, we use a single damper, controlled by
ratio control, in an effort to keep both offtake temperatures
equal in value. This avoids the complication involved in
having separate temperature control dampers in each off-
take, since upon furnace shut down, the dampers will close
and transfer fan pressure from the bags to the suction oids
of the baghouoe, inviting & disastrous duct collapse.
(0°)
Full open
\ dampor
Aforaoton
On a dual single inlet fan installation, severe fan wear oc-
curred on blade wear plates and at the junction of the
blades with the back and side plates. The fan wheels did
not originally have weldments or wear plates on the back
and side plates. On the scroll, which was originally % in.
thick, up to % in. wear had occurred. It was necessary to
make costly wheel replacements on both fans. For future
installations on dirty gas service, these wear points should
be adequately protected and backed up by regular inspec-
tions for wear.
Para Characteristics
I.D. fans in this industry are usually controlled by inlet
box dampers. In selecting fans, we have found that serious
losses of static efficiency occur when the fan system pres-
sures are overestimated. We have also found that fan man-
ufacturers sometimes use overly optimistic factors for esti-
mating fan brake horsepower, based on percent of full load
volume. With radial tip fan wheels, we find that the prom-
ised horsepower savings are not achieved.
Figure 3 illustrates that the static efficiency at the rated
capacity can drop from 69% to 55% if the system pressure
changes from 30 in. to 24 in. In this example, 4 fans rated at
30 in. SP would cost $150,000 more per year in power costs
than if properly rated in 24 in. SP. From our experience
with several manufacturers, we would recommend that pro-
spective purchasers of industrial fans should ask for perfor-
mance curves at various inlet damper positions, and not ac-
cept the customary horsepower reduction factors given by
the fan manufacturer.
20 30 40
Flow, cfm H 10,000
Ptgao 8. EWc« o4 btot teatgy on ten offtetency.
Waste heat recovery from ferroalloy furnaces is not prac-
ticed to date in the U.S. To recover waste heat, the furnace
must be closely hooded to restrict excess room air from
tempering the combustion gas temperatures. We have con-
tinued to use open-hooded furnaces for several reasons, in-
cluding easier stoking, safer operation, and the ability to
change products to meet market demands. To be economi-
cally feasible, waste heat recovery requires that a user of
steam, hot water, or warm air is close by and can use the
heat throughout the year.
Studies made by Airco Alloys in 1972 indicated that fur-
nace offgas temperatures of at least 1000°F (538°C), would
be required to generate up to 150,000 Ib/hr of steam at 400
psig and 650°F from two of our largest furnaces located at
Calvert City, KY. Steam turbines would drive two large
I.D. fans and exhaust 100 psig steam for use in a nearby
chemical plant. In subsequent tests on one of these fur-
naces, offgas temperatures up to 800°F were obtained, but
ventilation was marginal and the existing air-cooled hood
would not withstand these temperatures. The furnace con-
struction also made such high operating temperatures too
risky, and so a maximum offtake temperature of 650°F
(343°C), was arbitrarily selected. It was decided to build
forced convection coolers, of a type first used in 1971, ns
this design fitted into the available site, was self-cleaning,
required little maintenance, and had only 2 in. of pressure
drop. The cooling air fans were selected for 100°F ambient
temperature, and have inlet louver dampers controlled by
cooler outlet temperatures. Two fans in parallel are used
for each pair of coolers.
Elsewhere in the industry, radiation-convection trom-
bone typs coolero are being used and are reported to be of
3-6
-------
less expensive construction. We believe that this design has
more pressure drop, requires greater site space, and lacks
outlet temperature control to the extent that it could over-
cool in winter and undercool in summer.
Similarly, evaporative cooling was rejected because of
possible acid dewpoint corrosion in the ductwork and bag-
house, requiring adequate insulation of these parts. Finally,
the possibility of sulfuric acid dewpoint in the Nomex bags
would make glass filter media a better selection, but this
requires lower air-to-cloth ratio and, thus, more filter bags.
As previously stated, we prefer an investment in coolers to
reduce baghouse size, and the number of operating parts.
Baghouse Maintenance
Contrary to our original expectations, bag replacement
cost is not the major maintenance cost in our structural
baghouses. Rather, it is the dust handling from compart-
ment hoppers through to the trucking of the mixed paste.
Plugups in the conveyor to elevator transfer point are com-
mon, and there are periodic replacement of conveyor bear-
ings and seals around dampers and conveyor drives. We
prefer to use poppet type dampers for their tight closing
characteristics, and prefer not to use them in horizontal po-
sitions due to failure of supporting rollers. Next, we have
sought better access to the dampers, and this required a
new structural design of the baghouse substructure. Final-
ly, we have gone to extra large compartments in order to
limit the number of compartments to 12 on new systems.
We have found that the larger compartments not only re-
duced the number of operating valves, conveyors, and rota-
ry valves to be maintained, but there also was a 10% reduc-
tion in cost. There should also be less danger of re-entrain-
ment of dust from the hopper when inlet and deflation
dampers open.
Gas Flow Measurements and Bag Evaluation
The primary variable in the dirty air side of baghouses is
flow. There must be sufficient ventilation to capture the
paniculate. This aspect of air flow measurements has been
lacking, and in the past, there has been a substitution of
temperature and pressure values, which are of secondary
use in indicating baghouse performance. We will be mea-
suring flows in the following manner in our latest baghouse
project.
Total System Flow
Main gas flow will be measured by a duct size venturi
with a design AP of 1.5 in. WG and a pressure loss of 0.25
in. WG. The flow differential will be transmitted to a 30-
day strip chart.
Bag Flow Measurement
Individual bags in a single compartment can be com-
pared by a simple test apparatus, with results as shown in
Figure 4. These are multiple venturi tubes, which are
mounted within the dust compartment and aligned with
the test bags and directly below the tube sheet. We have
been using a 4 bag cluster, of which at least one is a stan-
dard service bag identical to those used in the other com-
partments. The calculation of flow through test bags, as de-
termined from the velocity head measurements, can be
compared with the calculated flow through the standard
bag or bags, which in turn can be compared with the total
flow to the baghouse passing through X compartments of
bags identical to the standard test bags.
Compartment Flow Measurement
By measuring the pressure drop across the inlets of each
compartment in a baghouse, and knowing the total flow
from a flowmeter or velocity traverses, the changing and
relative flows into each compartment at any time can be
calculated. This permits a ready tool for evaluating the ef-
fect of changes in bags from one compartment to another,
different shaker cycles per minute, or malfunction in any
compartment which may be seriously affecting the flow ca-
pability of that compartment. Bag filter drag coefficients
from a furnace operation on a certain product can be used
to estimate system pressures in a future installation. Bag
pressures are the major pressure components in the calcu-
lation of system losses for I.D. fan selection.
S 4
2
Shaking frequency
155 cpm
32 min. cycle
* Bagsll.W"x30'-6"
Combination Nomex
P.M. 1607
S*
Fiberglass
W.W.C. 425-04
-Collecting-
1&4 7 8 245 9 10 3&6 11 12 1&4
Compartment number
Figure 4. Test bag performance.
January 1976 VoJuma 26, No.
3-7
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
Compartment Monitoring
By connecting the manometer or differential pressure
gauges, as shown in Figure 5, the effectiveness of each com-
partment can instantly be monitored. This is quite impor-
tant when you consider that an operator rarely can afford
to spend an hour or so watching all compartments isolate,
clean, and return to service. The causes of the assumed loss
in flows may be: inoperative inlet, reverse air or reinflation
dampers, leaky dampers or loss of shaking and/or reverse
air flow. Defective compartments have been known to stay
on-line for weeks at a time because the deficiency was
never detected during casual daily inspections.
Automatic Air Purging
In an effort to maintain the operation of the compart-
ment flow monitors by preventing dust plugging of the
measuring lines, we are installing automatic purging valves
between the manometers and the measuring points. These
small valves are cam driven, and isolate each gauge for a
few seconds while they apply a 50 psig air supply pressure
to the measuring line. The purge timer starts the purge
cycle after a preset interval, and the cycle is stopped by a
limit switch on the camshaft.
To low pressure -7
side of gage /
To high prev.ure
side of gage
Inlet plenum —
Instrument air
SOpsig
5"-0-5"
magnehelic gage
020"
magnehelic
gage
Atm.
-0*3-
T
Instrument air
50 psig
r
3 way
solenoid valve
I \—Cam-operated 4-way air purge valves-^-/ |
Figure S. Typical compartment monitoring system.
Mr. Payton is with Airco Alloys, a Division of Airco, Inc.,
Box 368, Niagara Falls, NY 14302.
Review of Baghouse Systems for Boiler Plants
M. J. Hobson
Acres American Incorporated, Buffalo, New York
The principle of fabric filtration has only recently been accepted
for fossil fuel fired boiler partlculate emissions control. Information
on design criteria Is, therefore, limited. The paper provides data on
the few Installations where baghouses have been Installed and
successfully operated. The general conclusion Is that a strong re-
lationship exists between air to cloth ratio and bag life.
Fabric filtration is accepted as a standard method for par-
ticulate removal from the majority of industrial process
emissions. One application that is just beginning to achieve
commercial acceptance is the control of fossil-fueled boiler
emissions.
Until now, the reasons for choosing other emission con-
trol systems for boilers have been:
• High cost compared to less efficient, but acceptable, con-
trol methods;
• High flue gas pressure loss compared to other control
methods;
• Acid attack problems;
• Unacceptably short bag life;
• High totalized costs compared to switching to a cleaner
fuel;
• Possibility of high temperature excursions and flamma-
ble gas ignition.
In recent years, particulate emission regulations for both
industrial and utility size boilers have tightened to the
point that fabric filtration is now a serious competitor to
3-8
Journal of tha Air Pdutton Control Association
-------
electrostatic precipitation and wet scrubbing on moot in-
stallationR. The alternative of switching to a cleaner fuel is,
in most cases, no longer viable due to high price and alloca-
tion priorities.
It is important, therefore, to reexamine the application
of fabric filtration to boilers and determine whether those
remaining potential problems have been solved by recent
developments.
This paper was prepared to document the experience of
those few experimental and commercial installations of
boiler plant baghouses.
Boiler Flue Gas Characteristics
Flue gas characteristics, such as gas composition and
temperature, depend on the capacity of the boiler unit, the
type and extent of heat recovery equipment, the type of
fuel firing equipment and the fuel analysis. The flue gas
from industrial installations fired by spreader stokers is
usually in the temperature range 300°-400°F with high ex-
cess air and large participate size.
Large industrial and utility installations are pulverized
coal fired with flue gas temperatures in the range 250°-
350°F and particle size distribution ranging from 50 to 0.5
^m, with the bulk of the fly ash between 2 and 16 »/m.
Plant operation can vary considerably. Many small in-
dustrial boilers shut down in the summer or operate at a
low load. Larger industrial and utility units usually operate
continuously at a fairly high capacity factor.
As many designers and operators are aware, acid attack
on the gas side of flue gas systems is often proportional to
the frequency of unit shutdowns and the extent of low load
operation.
Pilot Studies
A considerable number of fabric filter pilot tests have
been performed on various boiler flue gas systems. These
results have been documented in previous technical papers
and I do not propose to recapitulate the results in detail.
The pilot tests showed that bulk woven fiberglass and
Teflon felt fabrics can provide the performance required on
most boiler fly ash removal applications. Both fabrics are
L
resistant to acid attack and abrasion and are sufficiently
impervious to give satisfactory dust removal efficiency.
Woven fiberglass was found to operate well at air to cloth
ratios between 2 and 4. When using Teflon felt air to cloth
ratios can be higher.
Commercial Installations
Data were collected from 10 installations of baghouses on
boilers. Four of these are power utility plants, the remain-
der are industrial boiler plants supplying steam for process
and heating. One of the power utility plants burns heavy
fuel oil, all the remainder burn solid fuel.
Each baghouse installation will be reviewed for design
data and operating experience. This information is given in
brief tabular form in Tables I, II, and III.
Southern California Edlton, AlamHo* Unit 3
Number 3 boiler at Alamitos Plant is an oil- and gas-
fired unit of 2,305,000 Ib/hr capacity. For 6 months of the
year, residual oil is fired which, until 1965, caused a visible
stack plume. A program of pilot tests showed that fabric
filtration could be utilized for heavy oil-fired boiler stack
Table I. Boiler data.
Case
no. Plant and location
1 Alamitos, CA
2 Sunbury
Shamokin, PA
3 Carborundum
Niagara Falls, NY
4 Amal. Sugar
NYSSA, OR
5 Amal. Sugar
Nampa, 10
6 Colorado UTE
7 Holt wood, PA
8 Winston-Salcm, NC
9 Winston-Salem, NC
10 du Pont
1 1 du Pont
Fuel
No. 6 Oil
1.5-1. 7% S
0.8% S anthracite
and up to 6% S
petroleum coke
(75/25 mixture)
Bituminous coal
7% ash 2.3% S
Coal
Coal
0.6-1. 9% S coal
12% ash
Anthracite, fines
and petroleum coke
0.8% S Coal
0.8% S Coal
Coal
Coal
Fuel firing equipment
Tangential tilting
return flow burners
Spreader
Spreader
Spreader
Pulverized coal
Spreader
Spreader
Spreader
Spreader
Plant capacity
on baghouses
2,305,000 Ib/hr
175Mw
75,000 Ib/hr
200,000 Ib/hr
150,000 Ib/hr
3 units each
132,000 Ib/hr
50% of 80 Mw unit
75,000 Ib/hr
40,000 Ib/hr
January 1976 Volume 26. No. 1
3-9
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
Table II. baxhousc desixn data.
Case no. Flue gas temp. ° F
1 258
2 325
3 350
4 310
5
6 310
7
8 ' 300-350
9 300-350
10
11
Gas now acfm
820,000
220,000
191,800
145,000*
86.000
200,000
38,000
23,000
gr/icf entering
baghoute
2.0
1.53
0.6
0.3
Air/cloth
ratio cfm/ft*
6.5
2.0
3.5
3.56
2.36
3.35
8.5
8.3
4.48
2.32
Clolh
cleaning; method
Deflation
& reverse air
Deflation
£ reverse air
Reverse air
Deflation
& shake
Deflation
&.• reverse air
Deflation
& shake
Deflation
& shake
Reverse air
Reverse air
Pulse-jet '
Reverse air
Suction or
pressurized
P
S
S
S
S
a-
s
s
s
Fabric type
Fil>«TKlass60IST
Filter Kluss
Tcllon coated
Fiberglass
Fiberglass
Fiber Klass
Fiberglass
Fiberglass
26-ciz fiberglass
26-0/ fiberglass
Teflon felt
Fiberglass
Teflon coated
a in 8 modules.
gases provided that an alkali is injected ahead of the bag-
house to neutralize SO.i. In 1965, a large horseshoe shaped
baghouse was installed at the base of the stack of No. 3
Unit at the Alamitos Station.
The air to cloth ratio is 6.5:1 when one compartment is
out for cleaning. The outside diameter of the baghouse is
90 ft, the height 80 ft and the depth 23 ft.
The alkali additive is injected at the outlet of the air
heaters. Flue gas temperature is 258°F and the bags are
cleaned about once per hour by deflation and reverse air.
The baghouse equipment has passed through five phases
of modification. Operation of the baghouse has steadily im-
proved due to the many equipment modifications made.
Bag failures are now reduced to less than 1% over a 5
month period. The stack plume is clear.
Sunbury, Pennsylvania (P.P.AL.)
Each of the two boiler units at the Sunbury Station have
two baghouses installed. The two boiler units generate a
total of 175 Mw and are fired with a mixture of anthracite
and petroleum coke. The anthracite has a sulfur content of
0.8% and the coke sulfur content can be as high as 6%.
The gas flow to each baghouse is 220,000 acfm and the
dust burden is 2 gr/fts at a temperature of 325°F. The bag-
houses were supplied by Western Precipitation and have
an air to cloth ratio of 2. Bag cleaning is by deflation and
reverse air and the I.D. fan is located on the dirty side. The
original mechanical collectors were retained ahead of the
baghouses. The mechanicals were modified to reduce the
pressure drop.
Of the 5040 bags, only 50 were replaced after over 2 yr of
operation and many of these failures were caused by im-
proper installation. The pressure drop for the baghouse
and ductwork is less than 5 in. water gauge. It is planned to
replace the bags every 2 yr.
Discharge loadings from the baghouse were measured to
be 0.017 gr/acf.
Carborundum Company, Niagara Fate, New York
The boiler plant is fired with coal and oil. One of the coal
fired units was fitted with a baghouse 6 yr ago and it has
operated for 9 mo/yr ever since. Numerous modifications
have been made to the baghouse and the boiler firing
equipment has also received attention. The last 18 months
of operation have been very successful ,from the viewpoint
of hag life.
The 75,000 Ih/hr boiler is fired with 7% ash, bituminous
coal with a sulfur content of 2.3%. The firing equipment is
a spreader stoker. No mechanical collectors are installed
ahead of the baghouse. The gas temperature entering the
baghouse is 350°F. The bags are fiberglass with an air to
cloth ratio of 3.5. The bag cleaning method is reverse air
and the baghouse is operated at negative pressure. The
cloth resistance is 6 in. maximum and the bag failures ex-
pressed as a percent per year of operation amount to 0.3%.
Amalgamated Sugar .
At one plant of Amalgamated Sugar in Oregon, a 200,000
Ib/hr coal fired spreader stoker has operated since 1973
with a fiberglass baghouse. The gas flow to the baghouse is
191,800 acfm at 310°F. The air to cloth ratio is 3.56. The
bag cleaning method is deflation and shake. The baghouse
is operated at a suction. No significant operating data were
available on operating experience from this installation.
Amalgamated Sugar
At another plant of Amalgamated Sugar, an 8-module
baghouse of Western Precipitation design is installed on a
150,000 Ib/hr boiler. The gas flow is 145,000 acfm and the
air to cloth ratio is 2.36. The bags are fiberglass cleaned by
deflation and reverse air. The baghouse is operated under
suction.
Colorado UTE
The Colorado UTE Electric Association has three 13 Mw
boilers, each with mnximum evaporative capacity of
132,000 Ib/hr. The boilers are fired by spreader stokers
consuming low sulfur coal with 12% ash.
Wheelabrator-Frye baghouses were installed on all boil-
ers. The first unit commenced operation in early 1974. The
flue gas temperature is 310° F and dust burden at the bag-
house inlet is 1.53 gr/scf. Fiberglass fabric bags are in-
3-10
Journal of the Air Pollution Control Association
-------
stnllod with nn air to cloth ratio of 3..'lf> when five of the nix
compartments are in operation (i.e., when one compart-
ment is cleaning or down for maintenance).
Pressure drop across the collector (headers, house, and
fahric) is maintained at 4.2 in. water gauge by initiation of
the cleaning cycle. Cleaning normally occurs every 3-4 hr.
To inhibit SO.-i corrosion, a heating system is installed
which circulates hot gas when the haghouse is down, during
startup and during low load conditions.
The dust collection efficiency is high; discharge concen-
trations are less than 0.003 gr/acf on all three units.
After some initial problems relating to bulk density of
the fly ash, frequency of emptying the hoppers, and flue
gas distribution, bag life is exceeding 1 yr of operation and
2 yr plus are expected.
Holtwood Station (P.P.&L.)
The boiler plant at the Holtwood Station is a 75 Mw unit
fired with anthracite fines and petroleum coke. The fuel is
pulverized and 50% of the flue gases discharge to a Wheela-
brator-Frye baghouse. The other 50% pass through a wet
scrubber.
The gas flow to the baghouse is 200,000 acfm at full load.
The dust burden entering the baghouse is not available,
but it is estimated to be approximately 4 gr/acf. The bags
are fiberglass with deflation and mechanical shake
cleaning.
Little operational data are available but the discharge
concentration was measured at 0.007 gr/acf.
Hanes, Wlnston-Salem
The boiler plant consists of two spreader stoker-fired
boilers; 1—75,000 Ib/hr, 1—40,000 Ib/hr. The fuel fired is
low sulfur coal and the dust burden entering the baghouses'
is 0.6 gr/acf for the 75,000 unit and 0.3 for the 40,000 unit.
These concentrations are low because the original mechani-
cal collectors were retained in service.
Two Dustex baghouses were installed after a program of
pilot testing. The baghouses were provided with I.D. fan8
and stacks and a draft control system to allow the existing
equipment to function without modification.
High air to cloth ratios of 8.5 and 8.3 were selected for 26
oz woven fiberglass fabric bags. The gas temperature enter-
ing the baghouses is maintained between 300° and 350°F
by bypass and control dampers.
Bag life on the 40,000 unit was poor due to abrasion at
the cage wire contact points. It was better for the 75,000
unit, possibly due to the coarser particle size distribution,
which resulted in a lower cloth resistance.
Gas cleaning performance was high. A discharge concen-
tration of 0.006 gr/scf was obtained in tests. Woven Teflon
and Gore Tex (microporous PTFE membrane on a woven
teflon grid) bags are now under test in this installation.
du Pont Plant
Four baghouses were installed at a 6 unit boiler plant of
E. I. du Pont de Nemours and Company, Incorporated to
arrive at source emission compliance for the overall plant.
Both pulse-jet and reverse air cleaning systems were select-
ed to compare performance. The pulse-jet design was fitted
with Teflon felt bags; the reverse air design was fitted with
Teflon coated fiberglass bags. The original mechanical col-
lectors were retained ahead of the baghouses. Air to cloth
ratios are 4.48 for the pulse jet and 2.32 for the reverse air.
During 12 mo of operation, only one bag failed in the
pulse jet units and that is reported to be due to contact
with the wall. The baghouse efficiency is 98.4%. In series
with the mechanical collector, the overall efficiency is
99.8%. The fiberglass bags have a life of 1-1/2 to 2 yr.
Table III. Baghouse operation experience.
Case no.
1
2
3
4
5
6
7
8
9
10
11
Months of
operation
23
30
54
14
20
4
9
9
12
%Bag
failures/year
2.2"
1.25
0.3
150
800
0.03
Clotha resis-
tance in. W.G.
5.7
5C
6.0
5.0
4.2
8.0
9.0
gr/scf leaving
baghouse
Clear
Stack
0.017
N.A.
N.A.
0.003d
0.007d
0.006
Comments
System extensively modified in five phases over 3 yr.
Alkali injection ahead of baghouse
Mechanicals ahead of baghouse
No mechanicals in series
No mechanicals ahead of baghouse
Heating and hot gas circulation during shutdown
Bags fail at cage wire- contact
Mechanicals ahead of baghouse
Mechanicals ahead of baghouse
a Maximum at full load.
b In last two phases.
c 1m hiding ductwork.
i f.
January 1076 Volume 26, No, 1
3-11
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
10
8
.9
e 6
I
o 4
.01 .1 1 10 100
% bag fiilures/yr
Figure 1. Trw relationship between air to doth ratio and bag falurw*.
1000
Conclusions
Although the data available on boiler plant baghouses
are scant, the information reviewed above indicates that
satisfactory results can be obtained with careful design and
intelligent boiler operation.
The relationship between air to cloth ratio, cloth resis-
tance and bag replacement rate is also evident even though
these factors are strongly influenced by the large variation
in boiler equipment, flue gas systems, fuel characteristics
and operating conditions.
Mr. Hobson is with Acres American Incorporated, Buffa-
lo. NY.
Glass Bag Filters for Lime Kiln Exhaust
James R. Gage
Bethlehem Steal Corporation, Bethlehem, Pennsylvania
The Information presented In this paper l» directed toward those
concerned with specifying and operating larger size fiberglass bag
*
filter houses. Information on emissions, bag (He, Instrumentation,
and maintenance Is presented from an operating Installation. The
solutions to particular problems of short bag IHe, one due to erratic
start-up and the other due to coarse dust are presented and dis-
cussed. The bag filter collectors are satisfactory for these applica-
tions and a bag IHe of 22 to 34 months Is being realized. Proper
hopper Inlet arrangement, hydraulic dampers, and reverse air draft
control are believed to be partially responsible for the good opera-
tion and bag life.
Many press releases announcing the installation of an in-
dustrial fabric filter dust collector will describe its function
as similar to an ordinary household vacuum cleaner. Well,
some bag filters do serve such cosmetic needs but others
are not that simple. The purpose of this paper is to explore
the inner workings of a large baghouse as it operates to con-
trol the paniculate emissions of a lime kiln exhaust. Useful
information is presented on cleaning cycles, extending bag
life and avoiding operating problems.
Background
In 1969 it became necessary to abandon efforts to collect
our lime kiln exhaust particulates with a low energy wet
scrubber. An estimated 125 tpd of dust was then .being
emitted from one kiln of which 40 tpd was high quality
burned lime dust. This dust was highly reactive with water
and produced a lime mortarlike deposit on all equipment
interiors. In addition, the high alkalinity of the scrubber
water would have required the added installation of a large
acid gas neutralizing system. Electrostatic precipitators
were seriously considered as a solution but the hij;h calci-
um dust was too resistive to collect without excessive col-
lector size and electrical power costs. Therefore, a fiber-
glass type baghouse was selected for both installations, one
for a single large 650 tpd lime kiln, the other a common
baghouse for four smaller lime kilns of 700 tpd total pro-
3-12
Journal of the Air Pollution Control Association
-------
duction. The characteristics of the gas stream for these two
applications are shown in Table I. The large kiln emits 180
tpd of baghouse dust, the four kilns emit a total of 150 tpd.
The presence of sulfur trioxide in the four kiln baghouse
exhaust is due to different kiln combustion controls and in-
ternal heat exchangers, which create a leas reactive dust
than at the large kiln. All the kilns use the same coal. Also
the stone feed is the same chemical analysis with some
kilns burning a finer size. The single kiln baghouse oper-
ates continuously except for minor kiln repairs every 6 to
12 months. The four kiln baghouse runs continuously ex-
cept for repairs to the main I.D. fan every 12 to 15 months.
Baghouse Construction Features
These baghouses were viewed as a part of the kiln pro-
cess equipment, therefore specifications were developed to
emphasize good operation and reliability. Plant visits and
literature searches were made to develop background
knowledge in writing the purchase specifications. Figure 1
shows the cross section of the haghouses purchased. The
large kiln bnghouse has 14 modules, the four kiln unit h-is
16. The lower exterior steel siding prevents chilling of the
screw conveyors and hoppers which are not insulated. Both
systems include I.D. fans with inlet box dampers to provide
automatic draft control for the kilns. Both systems were
provided with generously sized fans and baghouses to elim-
inate the process restrictions due to inadequate draft.
System Controls
As special features to maintain proper and continuous
operation, the specifications included an inlet pressure re-
corder, a hydraulic control system for the dampers and a
reverse air fan draft control system.
dusty air from the compartment hopper, collapsing the
bags. This further volume is delivered to the filtering areas
through the inlet duct causing an additional pressure rise.
A typical cycle is illustrated in Figure'4. By watching this
pressure trace, the general area of the trouble can usually
be discovered for the compartment being observed. The ac-
tion of the tipping valves is also a good indicator of the ac-
tual dust removal during the cleaning cycle. A rising pres-
sure trace with regular lines would indicate a change in the
process volume as shown in Figure 5. It is necessary to ser-
vice the pressure tap weekly by disconnecting the hose to
the pressure transmitter and blowing compressed air back
through the piping to the duct. Otherwise the tap will clog,
giving an erratic trace as shown in Figure 6. The informa-
tion from the pressure rise can be used to estimate the
flows in the compartments. Table II shows the typical re-
sults of the 14 compartment baghouse filtering 160,000
acfm with bags 8 months old.
Inlet Pressure Recorder
This provides a visible record of the internal functioning
of the baghouse. A pressure tap located on the I.D. fan out-
let duct senses baghouse inlet pressure which is transmit-
ted to a continuous strip chart recorder at the baghouse
control panel. With all compartments working well, the
pressure trace is a regular series of evenly sized vertical
lines showing the normal rise and fall caused by the
cleaning cycle as shown in Figure 2. The pressure trace will
thus show if the baghouse is operating well without a tedi-
ous inspection. However, if there are missing lines or no-
ticeably shorter vertical lines as in Figure 3, the corre-
sponding compartment is not doing its share of the filter-
ing. A direct check on that compartment can be made by
observing the pressure trace on its next cleaning cycle. The
inlet valve, while closing, should cause a pressure rise as the
flow increases to the remaining compartments. Next the re-
verse air (or collapse air) valve should open and remove hot
Table I. Gas stream characteristics.
4 Kilns
Large kiln
Inlet temperature
Inlet (lust loading
Dew point
SO, in outlet
SO, in outlet
Inlet volume
Outlet temperature
460° F
8 Kr/acf
13U"F
15 ppm
225 ppm
158, 000 acfm
340° F
490° F
lOgr/acf
145QF
None
25 ppm
160.000 acfm
380°F
suspension -v 1
grating ^
{, — Insulating
\ __ Fiberglass
bags (60)
11 >/." dia
31' long
Gather up
conveyor
i
V Fiberglass baghouse cross section.
January 1876 Volume 26, No. 1
3-13
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
Figure 2. Normal pressure trace with aH com-
partments functioning.
The volume control valve provides a 10 sec stroking time
for all the cylinders without resorting to any individual cyl-
inder speed control valves. An additional volume control
valve would he helpful, one for the reverse air dampers and
the other for the inlet air dampers. These central control!)
allow a fine degree of cleaning cycle tuning which would
not be practical using individual damper controls.
The slow damper stroking minimizes the rate of volume
change in the process during the cleaning cycle and pre-
vents the bags from reinflating too rapidly after their col-
lapse. This helps the I.D. fan to maintain control of the fir-
ing hood draft which is important in providing the proper
kiln combustion atmosphere. This hydraulic feature for
control is thus some compensation for the baghouse being a
variable draft loss device in contrast to the steady state op-
eration of either precipitators or scrubbers.
There are serious consequences for ignoring a problem
compartment. Eventually a module of hags which are on
line but are not being cleaned will fill up with a dust load
until the hanger springs fail dropping the bags. Their filter-
ing load is also transferred to the remaining compartments,
shortening the overall bag life. If the total cleaning mode
fails, the baghouse will build up pressure drop until it in-
terferes with the process. At this point the bags are well
loaded with dust. If either the main I.D. fan is shut down or
one compartment is cleaned normally, following the resto-
ration of the cleaning cycle, the whole baghouse dust load
will drop into the screw conveyors. This should be antici-
pated in the dust removal system design. If not, the han-
dling equipment may stall or flood causing further prob-
lems.
Hydraulic Control System
An hydraulic damper control system was chosen to pro-
vide repeatable damper motions at controlled speeds. The
hydraulic power-pack was located next door to the bag-
house control panel. Copper instrument tubing bundles
each with eight % in. tubes were run thru an 8 in. under-
ground conduit pipe to the baghouse interior. The system
includes a variable volume 5 gpm 500 psig pump, a volume
control valve and V4 in. direct solenoid operated hydraulic
valves. The cylinders (2 in. dia X 8 in. stroke) are located
along an interior walkway and drive through a pipe exten-
sion of the rod to the damper arms overhead. All cylinder
rods are retracted when the compartment is on line.
Figure 3. 14 compartment baghouee wRh one
compartment malfunctioning.
4.0
o
$
3.5
•& 3.0
EXPANDED
PRESSURE
TRACE
0 10 20 30 40 SO 60
Cleaning cycle time, seconds
Figure 4. Variation of baghouse Inlet prewure during cleaning
cycle.
Reverse Air Draft Control System
This system adjusts the reverse air manifold draft by
damping the fan performance to suit the variable pressures
to be expected at the baghouse inlet. Draft is maintained at
2.5 in. of water using a control damper in the duct operated
by a 25 sec/100" rotary actuator. The control instrument is
on the baghouse panel where the draft fluctuation can be
observed during the cleaning cycle. A fan ammeter is also
helpful in discovering any reverse air damper malfunctions.
For convenience, the dedusting line from the dust elevator
and storage bin is connected upstream of this control
damper. This provides some flow to prevent controller
wind-up should the reverse air dampers be unusually gas
tight.
One aim of the iystem is to prevent the collapsed bags
from enduring the full reverse air fan suction if damper
timing or operations are maladjusted. It also provides for a
constant cleaning effort even though the baghouse inlet
pressure may be rising or falling requiring more or less fan
static pressure to effect the cleaning.
The draft pickup tap on the manifold rarely plugs in
contrast to the baghouse inlet pressure tap.
3-14
Journal of trw Air Pototion Control Association
-------
Figure 5. Pressure trac« with changing pro-
cess volume.
Bag Collapse Adjustment
The baghouse cleaning cycle has been adjusted to pro-
vide a very gentle bag collapse. Primarily the reverse air ac-
tion is interrupted at a time when the bag is nearly col-
lapsed but well before the bag is drawn into the full star
shape and drawn tightly down towards the tube sheet col-
lar. This type of cleaning cycle is successful both due to the
precision of the hydraulically-operated dampers and to the
controlled supply of reverse air volume. It is noteworthy
that this type of cycle has been in use for the past 9 months
and spanned the end of the life of one set of bags and the
start of the new set of replacements.
This operation was instituted to try to correct two of the
worst problems in fiberglass bag life under reverse air
cleaning.
1. The severe stresses that occur at the bottom of a fully
collapsed bag causing premature failure at the maxi-
mum flexure points.
2. Overcleaning of the bottom few feet of the bag during
the maximum collapse of the cleaning cycle.
Operating the baghouse under this type of cleaning ac-
tion should provide benefits for any type or style of bags,
and allow the maximum possible bag life based on failure
due to flexure.
Baghouse Performance
Figure 7 shows emission concentrations versus bag ser-
vice life based on emission tests conducted on the exhaust
stacks of both baghouses. As expected, it indicates an in-
crease in emissions over the life of the bags.
BagLIf*
Figure 8 shows that there is some correlation between
lower filter rate and longer bag life. The net air quantity as
used here includes an estimated 10,000 acfm of reverse air
volume. Bag life is the point in the life cycle where bag fail-
ures become so numerous that it is more economical to
rebag the whole baghonse. No attempt was made to keep
track of individual bags and their average life. The bag-
house with the longer life enjoyed a normal 2 to ,T in. water
pressure drop whereas the other was normally 4 to 5 in.
water. However, the first set of bags for the large kiln were
not included in the data as they were replaced after 6
months of operation. This is an unusually short life and a
result of a combination of start up problems. The following
corrective measures were taken and a resultant improve-
ment to a 22 mo life was realized.
.015
-010
O 005
\
Pin holes found ^
m a tew bags
o
DUST
EMISSIONS
Single point sample
I
I
0 6 12
Months in service
Figure 7. Increase In dust emissions with service lite.
18
Figure 6. Pressure trace with plugged sensing
tap.
1. The cooling tower outlet temp was reduced from 550° to
510°F and the shutdown interlocks reset from 600° to
550° F.
2. The cooling tower water spray atomization system was
improved to reduce and later eliminate the bleed in air
cooling which had been inflating the total volume.
3 Doghouse hoods were installed over the reverse air con-
nections at the hoppers in order to reduce clogging of
the reverse air ducts. Figure 1.
4. The cleaning program system was revised to give damp-
er operations in sequence. This insured that the cleaning
cycle could function properly even if there were small
amounts of dust in the ducts slowing down the damper
strokes, which were also adjusted to 10 sec, to give maxi-
mum power and minimum bag popping during reinfla-
tion.
5. The slipping damper operator arms, which caused
damper malfunctioning, were corrected.
6. The system was adjusted to provide a gentle collapse of
the bags.
January 1076 Volume 26, No. 1
3-15
-------
THE USER AND FABRIC FILTRATION EQUIPMENT
Table II. Cleaning cycle description.
Start of cycle
Ihlct valve closet
Rev. valve opens
Kcv. valve closes
Inlet valve opens
Initial drag
Final drag
Ba^house
3.3 in. W.C.
3.5 in. W.C.
3.8 in. W.C.
3.5 in. W.C.
3.:.' in. W.C.
2.1 in. W.C./fpm
1.3 in. W.C./fpm
Volume
9,000 acfm
None
(-) 14,000 acfm
None
13.700 acfm
25
2.0
| 1.5
i
• Large kiln
• 4 kilns
1
LIME KILN
BAG LIFE
J_
I
0 12 24 36
Bag life, months
Figure 8. Effect of net fitter rate on bag life.
48
Moisture and Corrosion
After 3 years of operation, the extended walls have pre-
vented dust or mud buildup in the screw conveyors or bag-
house hoppers, even though these portions are not insulat-
ed, and there are no vibrators on the hoppers. The side wall
insulation has prevented any stiffness or acid salting in the
bags near the exterior walls. The baghouse modules interior
steel is scale free and in good condition. The 316 stainless
steel outlet stacks and H.R.S. manifolds are also in good
condition. The ground level access doors have been re-
moved and a louvre installed above the inlet duct to pro-
vide some air circulation for cooling.
Cyclone Precleaners
The coarseness of the dust has not caused wear on the fi-
berglass bags even though precleaners were not incorporat-
ed, although a dropout effect is obtained in the down flow
cooling tower. However, precleaning is a problem at one of
the two other Bethlehem Lime Plants and at a preheater
type lime kiln plant in Nevada.1 At the latter, two kilns
each with 10 ft dia cyclone precleaners provided very good
bag life after 18 months of operation. A third kiln, with a
multitube precleaner converted to a dropout box, experi-
enced unusually bad bag life during the same period. Plans
were to reinstall the multituhes at the earliest opportunity.
Bethlehem's problem occurred in collecting dust from
the lime cooler portion of a rotary hearth (or doughnut
shaped) kiln at one of two different locations. The first in-
stallation is a quite successful reverse pulse type cleaner
operated at 4 fpm, 5 to 6 in. water, 240° F while the second,
a modular shaker type unit was spectacularly unsuccessful
when operating at a net 2.5 fpm, at 2 to 3 in. water, 200°F.
Table III shows the dust size for all three type units. The
dust from the fiberglass baghouse is as coarse as the hopper
sample from the shaker. The dust from the successful pulse
collector is coarser than the shaker sample collected at the
tube sheet after passing through the broken bags. The
coarse material found on the tube sheet was blamed for the
bag failures and was removed by installing a retrofit cy-
clone precleaner. The shaker baghouse had, by then, oper-
ated 12 months with daily bag replacements totaling
$10,000/mo in direct bag and labor costs. The bag life has
been good since the cyclone installation.
However, it should also be noted that both the successful
fiberglass baghouses and the successful reverse pulse bag-
house have an inlet arrangement conducive to good dust
settling in the hopper. They have an overhead duct with
round 90° elbows from the duct into the upper portions of
the hopper. The down comer elbows promote dust migra-
tion to the outside or bottom of the elbow where it can drop
into the hopper without being swept upward with the air
flow into the bags. The unsuccessful modular shaker and
the fiberglass baghouse in Nevada do not have this same
feature and instead must rely on precleaners. The shaker
has an opposite mounted inlet duct with side outlets com-
ing directly into the hoppers at a point too low in the hop-
per as it was chosen to clear the cross bracing. The manu-
facturer's literature for the Nevada fiberglass baghouse
shows a square tee connection feeding opposite compart-
ments which would also be a very turbulent and non-segre-
Table III. Dust size comparison.
Unit
Source
Topsi/c
-100 Mesh
-200 Mesh
-325 Mesh
—5 micron
Baglifc, mo
Fiber
Klass
Hopper
20 Mesh
99%
88%
10%
N.A.
22-34
Reverse
pulse
Hopper
1 ti Mesh
28%
6%
N.A.
N.A.
18+
Shaker
Tube sheet
1 0 Mesh
27%
17%
11%
N.A.
Hopper
100 Mesh
98%
83% •
63%
15%
0-2
Cyclone
and shaker
Hopper
3L'5 Mesh
All
All
1%
72%
11 +
3-16
Journal of th« Air Pollution Control Association
-------
14 TIPS FOR FIRE PROTECTION OF FABRIC COLLECTORS*
^ ....... ..,._.. i i«nr •>• .mi mini i mi i •MMOTtM a iiiiimMWlNI. W (MtJIMIIP V00.
ti. I :se of only noiicomlnistible materials in construction of the collector.
7. Fabric filters should he. of low hazard materials. Where combustible media must be used, flame retardant treat-
ment must be provided.
H. K.ibnc fillers should be in metal enclosures with provision made for denning the filters.
9. Collectors should be constructed so as to eliminate ledges or other points of dust accumulation. Hopper bottoms 1
should be sloped and the discharge system designed to handle maximum flow.
10. The collector should be designed to handle the maximum possible operating pressure to which it might be subject-
ed. ' I
11 Clean-out doors or panels should be provided for interior cleaning of collectors.
. ]'£. Kxplosion relief venting should be provided.
];?. Coili••! tors should be located outside of buildings unless the interior room has an exterior wall that incorporate*, ox-
pli'sinn relief venting, with other walls, floor and ceiling designed to withstand a pressure of 1'.'• psi. The room
should be equipped with automatic sprinkler protection.
14. To protect ngainst damage, in addition to explosion relief \enting or design of a collector to withstand maximum
peak explosion pressure, the following features should be provided: -f
a. Kxplosion suppression system.
OR
h. Collector should be designed to operate using an inert or oxygen deficient gas as an operating medium.
''/?
c. Ci 11 lectors containing combustible parts should be equipped wit h nutomnl ic sprinkle is.
^s ... .. ....^J
* "Fire Protection for Fabric Collectors," Richard A. Gross, Factory Insurance Association, Buffalo, NY.
** Recommendations to protect collectors can be derived from loss experience. Also, NFPA Pamphlet #66 contains valuable guid-
ance regarding collectors used in air conveying system* which can be applied to collectors used in other applications.
gating arrangement. We are now more aware of this impor-
tant feature and intend to review carefully the manufactur-
ers' design of this function in the future. The buyer should
be especially aware of standardized modular units built for
a variety of applications for the lowest cost, for a preclean-
ing cyclone retrofit is an expensive correction.
Conclusions
The fiberglass baghouse has proven to be a satisfactory
dust collector for the exhaust of these lime kilns. We expect
to continue or to improve upon the bag lives experienced to
date. Above all, the units have provided good collection of
the dust satisfying state emission limits. Interruptions to
January 1876 Volunw 26, No. 1
the kiln operations have been nominal and the extra draft
provided has improved production on all the kilns to the
limit of other equipment in the kiln system.
References
1. David J. Krohn, "U.S. Lime Division's dust abatement efforts
whip pollution problem," Pit & Quarry (May 1974).
Mr. Cage is in the Mining Division of Bethlehem Steel
Corporation. Bethlehem, PA.
3-17
-------
VI-4
INS AND OUTS
OF
GAS FILTER BAGS
by
John C. Walling
Joy Manufacturing Co,
Copyright (c) 1970 by Chemical Engineering.
McGraw-Hill. Reprinted with permission from
the October 19, 1970 issue.
4-1
-------
Operation & Maintenance,
Ins and Outs
Of Gas
Filter Bags
Here is a guide to selecting the synthetic-fiber bag cloth and the bag-cleaning technique
that could most efficiently and economically handle your gas filtering problems.
JOHN C. WALLING, Joy Manufacturing Co.
Until recently, the so-called natural fabrics-cotton,
wool and silk—were all that were available for
gas filtration, despite their limitations with respect
to resistance to temperature, chemical attack, abra-
sion and wear. The development of glass-fiber cloth
capable of withstanding continuous temperatures
up to 600 F. made cloth practical in gas filtration
applications from which it was once precluded.
Synthetic Fibers for Dust and Fume Control
Acrylics—Synthetic fibers for filter cloth are made
from materials having a molecular structure that is
naturally, or which can be made crystalline. The
proper orientation of this crystalline structure during
the extrusion process gives the fiber the necessary
tensile strength. The most suitable member of the
acrylic family of plastics with regard to crystalliniry
is acrylonitrile, which yields an oriented fiber having
a tensile strength in the neighborhood of 40,000 psi.
An acrylic fiber by definition is one containing a
minimum of 85% acryonitrile. Acrylic fibers are
available in the U.S. under such names at Creslan
(American Cyanamid Co.), Orion (Du Pont Co.),
and Acrylan (Chemstrand, div. Monsanto Co.). The
maximum recommended gas temperature is about
275 F. Fibers of 100% acrylonitrile resist a some-
what higher temperature. Acrylic fibers exhibit ex-
cellent resistance to hot, acid atmospheres.
Poll/amides (nylon)—Polyamide resins are naturally
crystalline materials that can be quenched and cold
worked to yield a tough, oriented fiber having a
tensile strength of about 80,000 psi. Nylon fiber
is available from American Enka Corp., Allied Chem-
ical Corp. and Firestone Tire & Rubber Co. (Nylon
6), Celanese Corp., Du Pont Co. and Chemstrand
(Nylon 6.6). It is suitable for gas temperatures to
about 250 F. This is lower than that for many of
the other synthetics. However, the excellent abra-
sion resistance of nylon makes it very suitable for
filtering abrasive dusts when temperature is not a
factor. It resistance to alkalis is good under most
conditions, but its resistance to mineral acids is poor.
Nomex (nylon)—A. special nylon capable of con-
tinuous service in gas temperatures as high as 450
F., Nomex's (Du Pont) resistance to fluorides makes
4-3
-------
OPERATION AND MAINTENANCE . . .
Properties of Synthetic Fibers for Gas Filtration
Material Acrylic
Maximum temperature (dry). °F..... 275
Abrasion resistance Good
Resistance to acids Good
Resistance to alkalis Fair
Tensile strength, psi 40.000
Nylon Nomex Teflon Polypropylene Polyester
250 450 500 225 275
Excellent Good Poor Good Good
Fair Good Excellent Good Good
Good Good Excellent Fair Good
80,000 80,000 20,000 110,000 80,000
Cost/sq. ft. of surface Moderate Moderate High
High
Low
Moderate
:iai!iBiBinraui«»™iEi™i»im
it superior to glass in many high temperature appli-
cations. It resists acids and alkalis better than
conventional nylon. It is, incidentally, the material
used in the Apollo space suits.
Fluorocarbon.1 (Teflon)—Fluorocarhon resins have
unique properties because of the presence of fluorine
atoms in the molecular structure. They are prac-
tically inert to chemical attack. In addition, the
temperature resistance of the fluorine-carbon bond is
such that fluorocarbons retain their properties at
temperatures higher than any .other carbon-chain
plastic. Fluorocarbons are crystalline in nature, and
suitable for filler applications. Teflon is produced
in multifilament and monofilament form. The multi-
filament material can be used continuously in gas
stream temperatures up to 500 F. Its main disad-
vantages are its high price and poor resistance to
abrasion.
Polypropylene—\ polymeric resin of the polyolc-
fin family, polypropylene's qualities are similar to
polyethylene except that its heat resistance is higher.
The action of the catalyst induces a crystalline
structure that makes polypropylene very suitable in
the fiber form. Its tensile strength is in the neighbor-
hood of 110,000 psi.
Polypropylene fiber offers the combination of low
price with high resistance to acid and alkali attack.
However, it should not be exposed continuously to
gas temperatures higher than 225 F. Its abrasion
resistance is good.
Polyesters—Certain of the long-chain polyester resins
can be converted to fiber form by cold working after
extrusion to produce an enforced crystalline struc-
ture. Their tensile strength averages about 80,000 psi.
Polyester fibers (Dacron, Fortrel, Vycron, etc.) are
a popular synthetic filter material having good all-
around resistance to chemical and abrasive attack,
combined with gas temperature tolerance up to
275 F.
Manufacturers of synthetic cloth filters can suggest
finishing techniques—such as heat setting, scouring,
resin treating, calendering and napping—appropriate
to the application that will improve bag life, dimen-
sional stability, permeability and ease of cleaning.
Evaluating Cleaning Methods
A comparison of the gas-cleaning efficiencies of
bag filter cloths is not basically important, because
almost all commercial filter materials remove 99+% of
the dust and fume from gas streams. All else being
equal, the greatest variety in design is found in
the technique by which the filter bags are cleaned.
This is an important factor because it not only affects
the size of the baghouse, but also its cost, mainte-
nance requirements and dependability.
For a specific filter material, the size and cost of
a bag filter depends primarily on the area of filter
surface required and the amount of space available
for filtration within the filter housing. The basic
parameter of haghouse si'/c vs. capacity is the air-to-
cloth ratio, i.e., the total cu. ft./min. of gas filtered
divided by the total sq. ft. of filter cloth in the filter.
Most of the factors that affect the air-to-cloth
ratio are linked to application, such as gas temper-
ature, dust loading, filter cloth material and dust
characteristics. However, a factor that bears directly
on the ratio is the percentage of bags out of service
for cleaning at any one time. Another is the amount
of flexing and creasing imposed on the filter cloth
by the cleaning mechanism.
In recent years, new filter-cleaning techniques
have been developed that allow significant increases
in the air-to-cloth ratio without affecting the life
of the filter cloth, filter dependability, or mainte-
nance requirements. These advances allow a cor-
responding decrease in filter area and filter space
requirements for the same application and capacity.
In the case of some of the new high-performance
synthetic fabrics (which are costly on a square-
yard basis) high air-to-cloth ratios are essential if
first cost is to remain low enough to make their use
economically practical,
Popular Cleaning Methods
Backwash (Fig. 1)—The principle of removing
collected material from a filter medium by back-
washing is probably as old as filtration itself. In
4-4
-------
OPERATION & MAINTENANCE . . .
Filtering cycle Cleaning cycle
Filtering cycle Cleaning cycle
Shaker
BACKWASHING removes dust by collapsing bags—Fig. 1 SHAKER cleaning requires isolation of bags—Fig. 2
Filtering cycle 4 Cleaning cycle
Compressed
air headers
Filtering cycle
Cleaning cycle
Ejector
retracted
Ejector
in position
Pressure
waves
plus
backwash
EJECTOR cleans bags with a short burst of air—Fig. 3 IMPROVED EJECTOR (patented) also backwashes—Fig. 4
4-5
-------
the case of ban-type industrial gas filters, filter cake
is discharged partially or completely by collapsing
the hags during the backwash cycle. The resulting
flexing and creasing of (he filter material is the
major cause of bag deterioration in this design. In
addition, the section of the housing containing the
bags being backwashcd is necessarily out of service
during this cycle. These two factors limit the air-to-
cloth ratio of backwash-type filters to from 1:1 to
3:1, depending on the severity of the application.
Shaker (Fig. 2)—In some applications, the filter
cake can be discharged by simply isolating the
bag from the gas stream and shaking it, either
manually or automatically. This method reduces bag
flexing and extends bag life, compared to the back-
washing technique. In some cases, both backwashing
and shaking are used simultaneously. Air-to-cloth ratios
for shaker-type bag filters range from as low as 1:1 to
as high as 5:1.
Ejector (Fig. 3)—Both backwash- and shaker-type
bag filters suffer from the same limitations: (1)
frequent cleaning cycles that cause rapid deteriora-
tion of the filter material, and (2) a large proportion
of the filter must be removed from service during
the cleaning cycle. The ejector principle is a recent
development that is suitable for many applications,
and is capable of operating at air-to-cloth ratios up to
15:1 with reasonable material life.
Contrary to the conventional backwash or shaker
arrangement, gas flows from the outside to the
inside, and the bag shape is maintained by means
of a wire frame (Fig. 5). A compressed air no/./.le,
usually augmented with an ejector tube, is positioned
above each bag outlet. A short burst of compressed air
creates a pressure wave that travels down the bag
and pneumatically inflates it as it passes. This action
discharges the filter cake from the bag.
Gas flow through the bag effectively stops during
the period this pressure wave exists, even though
the bag is "onstream" as far as the system is con-
cerned. There is therefore no need to isolate sections
of the filter for cleaning. Theoretically, each bag
can be blown individually. As a practical matter,
Cleaning
FRAME retains bag shape in ejector cleaning—Fig. 5
however, each row of bags is usually piped to the
same compressed air header, and blown simultane-
ously.
Improved Ejector (Fig. 4)—The conventional ejector
design depends on a controlled flexing of the filter
cake to break and dislodge it from the filter cloth.
In many cases, this is inadequate for proper cleaning.
Unforeseen conditions-such as moisture condensation
on the filter surface, electrostatic attraction between
the cloth and the filtered material—will cause the
filter cake to "ride" with the cloth as it flexes, in
which case the ejectors arc ineffectual and the filter
must be taken out of service.
The improved design mechanically seals off each
bag as it is blown and causes a positive backwashing
action that, in addition to the flexing action of the
pressure wave, thoroughly removes hard-to-dislodge
filter cakes. This development greatly extends the
application range and reliability of the ejector-type
filter, while retaining its other advantages.
Maintenance and Access
The failure of even a small proportion of the
filter bags in an industrial gas filter seriously com-
promises its efficiency. Present day concern for pollu-
tion makes it imperative that broken filter bags be
replaced promptly. A well designed filter must there-
fore allow quick and easy access for the removal
and replacement of individual bags.
Conventional filters depend on walkways within
the filter housing for access to individual bags. These
walkways take up valuable filter space. In addition,
manhandling filter bags into place within the re-
stricted confines of the housing is expensive and
time consuming.
This problem can be resolved by an arrangement
in which the bags and bag cages can be inspected
and removed from outside the filter housing. Dag
replacement becomes a simple matter of (1) push-
ing aside the ejector, (2) releasing three hold-down
bolts, (3) lifting the old bag out, (4) dropping the
new bag in, (5) refitting the the hold-down bolts
and (6) repositioning the ejector.
A complete filter can be rebagged by this method
in a fraction of the time formerly required, and
access walkways within the housing are also elim-
inated. •
John C. Walling is a senior
applications engineer with
the Western Precipitation Div.
of Joy Manufacturing Co.
(1000 W. Ninth St., Los Ange-
les. Calif. 90054). A graduate
of University of Washington,
where he received a B.S. in
mechanical engineering and
a B.A. in business administra-
tion, he joiiied Western Pre-
cipitation after a stint with
American Standard Co. His
background includes 10 years
of experience in gas handling
and cleaning, mass transfer,
and air and water pollution
control systems.
4-6
-------
VI-5.
BAGHOUSES:
SEPARATING AND COLLECTING
INDUSTRIAL DUSTS
by
Milton N. Krauss
Copyright c 1979 by Chemical Engineering,
a McGraw-Hill publication. Reprinted with permission
from the April 9, 1979 issue, pp. 94-106.
-------
Baghouses:
separating and
collecting
industrial dusts
The selection and operation of baghouse collectors is mainly
an art. Here is practical information on filter fabrics and
cleaning; baghouse design, operation and servicing; and
methods for protecting the system from Jire and explosion.
Milton N. Kratts
Q The separation of dusts from industrial-gas streams
may often be done by using filters made of natural or
synthetic fibers. These filter elements are bag-shaped,
and placed in structural enclosures called baghouses.
In addition to supporting the filter' elements, the
baghouses contain baffles for directing airflow into or
out of these elements, equipment for cleaning the fabric,
and a hopper for collecting and discharging the dust.
Baghouses are used to control dust nuisance when-
ever dust-laden air must be discharged to the atmos-
phere, or to, recover valuable dust 'from a process
venting system. Sizes may range from small bin-venting
filters to large multicompartment filters that receive
dust from an extensive system of exhaust ducts.
Dust filtration and collection is an art. The sizing and
selection of dust filters are based on (a) past experience
and (b) actual tests that use specific fabrics at specific
dust-to-air loadings. Because of the many types of col-
lectors and filter fabrics, combined with numerous mar
terial properties that affect filtration, the selection of a
filter should be made by actual test at the design load,
using the least-costly fabric for the requirements.
Separation principle for filters
Dust filtration in baghouses is accomplished by pass-
ing the dust-laden air through a filter fabric that is
formed into cylindrical tubes or oblong bags. As the air
passes through the fabric, the dust is retained on the
surface and in the interstices of the yam strands, build-
ing up a filter cake that also acts as a filter medium. In
order to reduce the resistance to airflow (so as not to
affect the flowrate), the filter cake must be periodically
dislodged. The method" of cleaning the fabric is the
basic difference between various filters.
The greater the area .of filter cloth provided for a
given dust loading on a specific baghouse, the longer it
takes for a filter cake to build up that will affect the
airflow. In comparing the area of filter fabric offered by
different vendors, we should consider the different
methods of cleaning the fabric. The criterion should be
a constant pressure drop across the fabric for a specified
airflow rate and a specified dust loading, rather than
the term "air-to-cloth ratio."
If checked dimensionally, this term is the face veloc-
ity of the air (ft/min) through the effective area of the
fabric for a given type of filter. The air-to-cloth ratio
only has significance when comparing the performance
of a particular manufacturer's line of baghouses when
handling different materials. The cleaning method es-
tablishes whether a baghouse is suitable for continuous
or intermittent service.
Properties of filter fabrics
In order to function properly, a filter fabric must
have the following properties:
Permeability—The fabric must be sufficiently porous to
94
5-1
CHEMICAL ENGINEERING- APRIL 9, 1979
-------
permit a satisfactory flow of air. The permeability of a
fabric is generally stated as the clean airflow in
(ft3/min)/ft2 of fabric, at a pressure differential of
0.50 in. water column, as determined by the Frazier
test.* However, the Frazier permeability alone is no
guide to fabric selection because materials having the
same permeability may not have the other characteris-
tics required to successfully filter a specific material.
Also, the filter cake formed on the fabric surface will
reduce airflow.
Mechanical strength—The fabric must resist the tensile
forces caused by the operating pressure differentials, by
mechanical shaking during cleaning, and by pulsing
during reverse airflow. It must withstand abrasion
where it is clamped to tubesheet ferrules, where it is
supported on metal grids or retainers, and where it is
subjected to the impact of the filtered materials.
Solids retention—Fabric construction must be open
enough to prevent the accumulation of fines in the
interstices of the yarn, yet tight enough to prevent fines
from blowing through the fabric. This may seem
anomalous, but the filter cake on the surface of the
fabric also acts as a filter medium and narrows the
interstices.
Corrosion resistance—The fabric must resist attack and
weakening due to chemical action between the fibers
"ASTM Standard D-737, Standard Method of Teat for Air Permeability of
Teatile Fabrics, American Soc. for Testing and Materials, Philadelphia, Pa.
and the filtered materials, especially if moisture is pres-
ent due to condensation.
Heat resistance—From some processes, the fabric must
resist high-temperature exhaust gases. Each fabric has a
definite temperature limit beyond which it will tend to
disintegrate.
Cleanability—The fabric must have a surface texture.
that is conducive to rapid release of the filter cake
during cleaning. This, too, may seem anomalous, since
the surface must also retain the cake. However, the
forces involved during cleaning are usually the airflow
in the opposite direction to normal airflow, and me-
chanical flexing of the fabric. The fabric should have a
high rate of electrostatic-charge dissipation so as to shed
charged dust particles.
Dimensional stability—The fabric must resist stretching
or shrinking that would affect its permeability. Many
untreated fabrics tend to increase in permeability after
laundering.
The properties of commonly used filter fabrics are
shown in Table L
Fabric construction
A knowledge of the construction of filter fabrics will
indicate how well the preceding property requirements
can be met. However, final selection should be made by
test and by an economic study that weighs first cost
against probable maintenance expense. All fabrics will
5-2
CHEMICAL ENGINEERING APRIL 9, 1979
95
-------
BACHOUSES
eventually blind when handling fine materials, or ma-
terials that crystallize or polymerize when embedded in
the fabric. It is better to replace all cloth in a filter at
regular intervals (as determined by experience or by
periodic permeability tests) than to operate the unit
until airflow is drastically reduced and other problems
are created.
Filter fabrics may be felted or woven. Felted fabrics
are composed of fibers that are compressed under high
pressure and are relatively thick. The pressure enmeshes
the fibers, and in so doing forms a strongly bonded
sheet of material having complex labyrinthine inter-
stices through which air can pass in three directions.
Synthetic monofilament fibers cannot form a strong
bond between themselves and are usually pressed over a
woven sheet of scrim fabric for strength. Wool fibers are
naturally barbed and have a stronger bond than syn-
thetics. Most felts can absorb moisture due to capillary
Properties or cornmorvfilter fabrics
•"~- •' -
action between the fibers—even though the fibers
themselves are moisture resistant. .
A simple test for relative absorption of moisture is to
stand equal lengths of fabric on edge in ajar containing
a shallow layer of water. After several minutes, the
samples should be withdrawn and the height to which
the water has risen in each sample noted.
The thickness of felt provides for maximum dust
impingement. The many changes in direction due to
lateral airflow traps the finest particles. Inspection (a)
by ultraviolet light of the cross-section of felt fabrics
that have handled dust containing fluorescent materi-
als, and (b) by microscopic examination of the cross-
section of felts handling other types of dust, shows that
the particles rarely penetrate through' more than one
third of the felt thickness. While there is a reduction in
air permeability, this is not as much as would occur in a
woven fabric having the same quantity of trapped dust
per square inch of filter area.
Felted fabrics are used for'maximum product recov-
ery where filtered air is recirculated or discharged into a
work space. Felts are primarily used in collectors having
a high face velocity for airflow (i.e., high air-to-cloth
ratio).
Woven fabrics are composed of twisted yarns that are
woven into geometric patterns having various spacings
between the yarns, and with a specific surface finish
that is designed to retain or shed a filter cake, depend-
ing upon the application. The permeability of these
fabrics depends on the type of fiber, the tightness of
twist and the size of the yarn, the type of weave (geo-
metric pattern), the tightness of weave (thread count),
and the type of surface finish. Correct coordination of
all these factors for a successful filter application cannot
be done by abstract analysis. The fabric must be tested
in a dust collector considered suitable for a particular
application on the basis of operational and economic
factors. Woven fabrics have a low ratio of weave open-
ings to yarn area and require a surface filter cake to
retain the dust. This limits the face velocity for airflow
96
CHEMICAL ENGINEERING APRIL 9. 1979
-------
Shaker
motor "\
Eccentric Bap suspension ^Partition
rod / bar /
Reverse-air
damper in
normal
position ^
To exhaust fan
Compartment
in service
Level indicator _•
for dust
Reverse-air
damper in
cleaning
^position
To exhaust fan
"" Fresh-air damper
Top of silo-'"
---Compartment
-*--'"°~ ht»inn /•Ipanorl
to low values [1.5 to 3 (ft3/rnin)/ft2] in order to prevent
dust from blowing through the openings and destroying
the filter cake.
The permeability of woven fabric is very dependent
on its construction. A review of textile terminology will
prove useful in understanding the effects of yarn design
and fabric weaving on permeability. The yarn may be
made from:
1. Staple—the term for short fibers of cotton or syn-
thetic material—drawn into parallel strands and
twisted into yarn by spinning.
2. Continuous filaments of synthetic material, made
by extruding a solution through a finely perforated
nozzle and then spinning the filaments into yarn.
-The degree of twist of the yarn affects permeability. A
tight twist resists penetration by dust particles, and a
loose twist permits retention of dust, or "blinding" of
the yarn. Filament yarns are stronger than staple ones.
Spun yams have more flexibility than filament yarns
and more resistance to repeated flexing.
Weaving is the interlacing, at right angles, of a series
of yarns to form a textured fabric. The basic weaves are
plain, twill and satin (Fig. 1). In the plain weave, each
crosswise or filling yarn passes alternately over' and
under one warp or lengthwise yarn. Each warp yam
passes alternately over and under each filling yarn. This
type of construction makes the tightest weave; by vary-
ing the thread count, the weave may be made as open
and porous as required.
In the twill weave, the filling yarn passes under one
warp yarn, then over two or more warp yarns. In each
succeeding row, the filling thread moves one warp ei-
ther left or right, forming the diagonal lines that iden-
5-4
tify a twill weave. Twills have fewer interlacings than
plain weave and, depending on the thread count,
greater permeability.
In the satin weave, the filling yarn passes over one
and under four or more warp yarns in the next row.
This weave has few interlacings, spaced widely but
regularly. It has a smooth surface and greater permea-
bility than the other weaves. Cotton fabric in this weave
is known as cotton sateen. Regardless of weave, the size
of yarn and the number of warp and filling yarns per
inch (thread count) greatly affect permeability. A low
thread count will permit higher airflow rates at low
pressure drop, whereas a high count will trap more fine
particles and produce a higher pressure drop.
The woven fabric may have various finishes. Cotton
fabrics may be preshrunk to maintain dimensional sta-
bility; They may also be napped in order to hold a filter
cake. (Napping is the process of scratching the cloth
surface to raise the fibers and form a soft pile.) Syn-
thetic fabrics may be heat-set to obtain dimensional
stability and a smoother surface with uniform permea-
'bility. All fabrics may be silicone-sprayed to obtain
abrasion resistance, improved cake release, or low mois-
ture absorbency.
Many other fabrics are available for special applica-
tions. Selection of these should be made only after
determining that the common fabrics are not suitable
for the proposed service, and after discussing the appli-
cation with specialists in filter-media firms.
Example of performance
The following example illustrates the unpredictabil-
ity of fabric performance. Specifications for the original
CHEMICAL ENGINEERING APR|L 9_ ,979
97
-------
BACHOUSES
-2
-1
c
co
Cleaning cycle
;;• '.'Atmospheric pressure .'.j,-;.';: -i/^-:?-^',*:. *'z.-.~~'"."- ",', !; :'V"-i .
100
200
300 400
Time, s
500
600
700
bag material as furnished by a filter manufacturer are
compared to a replacement material supplied six years
later by the same manufacturer for the same filter and
the same service.
Fabric manufacturer
Material
Thread dia., in.
Thread count, yarns/in.
Weave
Permeability, (ft3/min)/ft2 20 to 25
(Frazier)
Original
ABC Co. .
Orion
filament
0.0085
70x68
Twill
Replacement
XYZ Co.
Orion
staple
0.024
90x60
Satin
43
Despite the apparent dissimilarity of these fabrics,
they gave the same performance. In this case, filtration
and pressure drop were evidently a function of the type
of filter cake built up on the fabric during service rather
than a function of the fabric.
The fabrics are formed into filter bags having config-
urations to suit the collector. The bags may be cylindri-
cal tubes open at both ends for attachment to top and
bottom tubesheets using band clamps or snap rings, and
fitted with anticollapse spreader rings to permit the
released dust to fall out. Or, the bags may be cylindrical
tubes closed at the top to permit suspension from a
system of shaker bars, and open at the bottom for
fastening to a tubesheet. Or, they may be cylindrical
tubes closed at the bottom and open at the top for
installation over a wire retaining-cage suspended from a
tubesheet.
The bags may be oblong—in one version, closed at
one end, open at the other for slipping over a retaining '
cage (like a pillowcase);'in another version, closed at
one end for top suspension, with several cylindrical
openings at the lower end for clamping to a tubesheet.
In any bag design, the following details should be
considered:
Thread used to sew the bag should be of the same
material as the bag, or compatible with it. The open
ends of the bag, which are attached to tubesheet ferrules
5-5
98
by clamps or snap rings, should have a reinforced cuff.
All hems and seams should have two rows of stitches,
with edges folded or treated to prevent fraying.
Spreader rings should be enclosed in a pocket that is
doubly reinforced inside and outside the bag.
The suspension end of hung bags should be doubly
reinforced and folded over to form a loop for hanging
on a wide suspension hook, or fitted with a metal grom-
met if hung from an eye bolt or hook bolt. Cuffs of
oblong bags that are hooked over support pins should
be fitted with metal grommets at the hook locations.
Where bags must be grounded, a copper wire should be
sewn the full length of the bag, with sufficient lead to
clamp to a tube ferrule.
Filler cleaning
The shape of the filter element, its method of support
and the normal direction of airflow through the filter
will determine the method of cleaning. Normal airflow
will hold the dust cake against the fabric. Stopping the
flow of air will tend to release the dust cake. Simultane-
ously flexing the element, or reversing the airflow, will
ensure release of the cake so that it can drop into a
hopper.
Flexing can be induced (1) mechanically, by motor-
driven crank mechanisms that oscillate closed-end tube
elements, or (2) pneumatically, by reverse airflow for
flow-through or closed-end tube elements.
On flow-through elements that collect dust on the
inside surface, spreader rings may be used to form nodes
for more-effective reverse-air flexing and to prevent
collapse of the elements. (A collapsed element would
prevent the dust from falling into the hopper.) On
closed-end tubular elements that collect dust on the
inside surface, a top-suspension tensioning device is
depended upon to prevent collapse of the elements
during cleaning and to hold their surfaces taut to form
the dust cake during normal airflow.
Envelope, oblong or tubular elements that collect
dust on their external surfaces are generally placed over
metal retaining cages or grids that keep the fabric taut
so as to retain the dust cake and maintain the internal
open area required for clean-air flow. During cleaning,
reverse airflow flexes the fabric away from the retainer
grids; restoring normal airflow whips the fabric
against it.
On collectors where normal airflow through the fab-
ric is not stopped, an extra-high reverse airflow is
needed to dislodge the dust cake. This may be done by
using high-pressure air jets or separate air-reversal
blowers.
Cleaning methods for baghouses
Commercial baghouses use one or more of the clean-
ing methods previously discussed. These methods may
be further described as follows:
Shaking only is used on unit filters having woven bags
where service can be interrupted long enough for clean-
ing—usually 3 to 4 min. The bags may be (1) envelope
type, mounted over a metal spacer, with the closed end
connected to a motor-driven shaker grid and tensioning
bar, and the open end fastened to a clean-air plenum
wall; or (2) tubular type, with the closed end suspended
CHEMICAL ENGINEERING APRIL 9. 1979
-------
from motor-driven shaker bars, and the open bottom
end fastened to a tubesheet for the entry of dust-laden
air. The cleaning cycle may be initiated by (a) a timer
set for a specific cleaning period, (b) a pressure-differen-
tial sensor, or (c) a manual stop-button connected to a
time-delay relay. In this method, the exhaust fan stops,
allowing the internal pressure to reach atmospheric,
and the shaker motor operates for a preset interval.
After cleaning, operation of the dust collector may be
restored automatically or manually, depending upon
the service.
Shaking with reverse airflow is used with either type of
bag described for unit filters. However, for continuous
service, the filter elements must be distributed among
two or more compartments, and the exhaust fan must-
remain in operation during the cleaning cycle. Each
compartment is fitted with its own shaker mechanism
and a set of reverse-air dampers. Each set comprises an
outlet damper, ducted to the exhaust fan, and a smaller
damper opening directly to atmosphere. A program
timer permits cleaning the bags in each compartment
successively, with the time interval between cleanings
adjusted to suit the service.
During cleaning, the timer activates the reverse-air
damper drive to close the clean-air outlet damper to the
fan, and simultaneously opens the atmospheric-air inlet
damper. Suction from the active compartments draws
air through the bags in the reverse direction. Limit
switches make sure the dampers are correctly positioned
and start the shaker mechanism. After the shake period,
the reverse-air damper drive opens the clean-air outlet
damper and closes the atmospheric-air inlet damper,
reactivating the compartment. During the cleaning
cycle, a certain amount of dust is picked up by the
reverse airflow and deposited on the bags in the active
compartments.
Fig. 2 shows a two-compartment filter used for vent- •
ing a silo receiving material from a pneumatic convey-
ing system. Fig. 3 shows the variation in bin pressure
during the cleaning cycle for this collector.
Reverse-airflow cleaning is used for all types of filter
elements (tubular or envelope, woven or felt materials,
dust inside or outside of the bag, elements distributed
between several compartments or all in one housing).
There are many arrangements for creating reverse air-
flow—all intended to enable continuous service. Several
common arrangements are:
1. A compressed-air nozzle at the clean-air discharge
of each bag injects high-pressure air into the tube in the
reverse direction from normal airflow in pulses of short
duration (1/10 to 1/25 s) for systems having closed-end
tubular bags supported on metal retainer cages sus-
pended from the clean-air plenum tubesheet. The pulses
are controlled by external timers that operate special
air-supply solenoid valves, each of which serves a group
of bags. The time between pulses is adjustable, but is
factory-set for the intended service. See Fig. 4.
2. A variation of this method is to divide groups of
bags (including the clean-air plenum) into zones by
means of internal partitions. The clean-air plenums are
manifolded into an outlet duct via poppet valves. Each
zone is equipped with a single pulse valve that supplies
compressed air to the group of bags. During the clean-
Upper Nozzle or Venturi
plenum orifice nozzle
'
Solenoid Compressed
valve air supply
/ atlOOpsig
\
Exhaust
outlet.
Collector
housing
Discharge
ing cycle, the poppet valve in the zone to be cleaned
closes, stopping airflow through the zone. The pulse
valve opens for 0.1 s, admitting a burst of air into the
plenum to clean the bags. The poppet valve then auto-
matically reopens, putting the cleaned zone back on-
stream. This operation is repeated in each successive
zone until all bags in the collector have been cleaned.
See Fig. 5.
3. A motor-driven plenum chamber receiving air
from a separate blower delivers reverse air to closed-end
oblong bags that are supported as in the previous ar-
rangement but not zoned. The plenum rotates and
blows reverse air into the clean-air discharge end of
each bag as its nozzles travel over the bags. Timing is
5-G
CHEMICAL ENGINEERING APRIL 9, 1979
99
-------
Solid
state
timer
Door
/ seal
Internal
hinge latch
Plenum
_ chamber
7 ,doors
Door
.retainer
Housing'
-'Internal partitions
determined by the speed of rotation of the plenum
chamber. See Fig. 6.
4. A counterweighted frame carries a series of blow-
rings that encircle each filter tube. The frame is driven
slowly up and down by chains supported from overhead
shafts and sprockets. Air is continuously supplied to the
blow-rings by an external blower or fan connected by a
hose, or a specially designed air-supply connection to a
manifold mounted on the blow-ring carriage. See Fig. 1.
5. A traveling air-plenum, moving horizontally, is
positioned over the clean-air discharge of each vertical
line of envelope-type filter bags. Diaphragms on the
traveling plenum seal off adjacent rows of bags. Reverse
air may be supplied in two ways: (a) atmospheric air
may be drawn through the filter bag in the reverse
direction from normal airflow by the operating suction
of the exhaust fan, or (b) clean air may be blown back
into the bags from a separate blower mounted on the
traveling plenum. See Fig. 8.
Design requirements for filters
Manufacturers offer standard designs for baghouses.
However, such designs may have to be radically modi-
fied to suit the user's requirements. These may be fixed
by the characteristics of the material entering the
baghouse, and by the operational needs of the duct
system to which the baghouse is connected. Before pre-
paring a specification for a baghouse, consideration
must be given to design details such as: housing con-
struction, physical location, servicing provisions,
method of operation, and protection from fire and
explosion.
100
CHEMICAL ENGIK
AND APRIL 9, 1979
-------
Blank covers are provision for future filter elements
Housing construction .
Baghouses may operate under positive or negative
pressures. Baghouses that receive air from a fluidized-
bed process or from a positive-pressure pneumatic con-
'veying system generally operate under positive pressure.
" The pressure is limited by the pressure drop across the
bags because discharge is directly to atmosphere.
. Most baghouses operate under negative pressure,
which imposes stringent design requirements for the
bag enclosure walls.
Although the pressure drop across a baghouse may be
3 to 6 in. water column, the negative pressure imposed
by the exhaust fan is determined by the duct-system
requirements that may be five to ten times those of the
pressure drop. If the air inlets to the baghouse are
blocked by powder or closed inlet dampers, the suction
may increase to values well above the normal operating
level. The user must determine 'the maximum suction
that may be imposed on the baghouse, and seek a unit
that is rated for this condition.
If the dust-air mixture can become explosive, the
baghouse walls must also resist a positive pressure that
is determined from dust-explosion tests. These fix the
vent area in the walls, so as. to limit pressure buildup.
The user must specify the allowable positive pressure for
baghouse design.
Cylindrical baghouses are simpler to design for high
suction or high pressure than are flat-sided enclosures.
Dust-laden
air inlet
Reverse-jet supply blower.
Blow-ring
carriage
drive
Blow-ring
carriage—
ounterweights
Blow-ring--
carriage
Clean-air
discharge
5-
CHEMICAL ENGINEERING APRIL 9. 1979
101
-------
BACHOUSES
Standard suction ratings, for cylindrical units range
from 15 to 160 in. water column, depending on diame-
ter. Rectangular or square baghouses have ratings that
range from 15 to 30 in. water column. These units
require external battens and cross-bracing to stiffen
them. Multicompartmented units have interior walls
that require special bracing to resist differential pres-
sures and stress reversal during cleaning operations.
Tubesheets are usually made of heavier plate than are
the walls, and are fitted or formed with stiffeners be-
cause the tubesheets not only serve to support the bags
and retainers but also act as walkways during tube
replacement. Distortion of tubesheets can cause top-
suspended bag retainers to touch each other and retain
dust. Some baghouses are only available as cylindrical
units; others may be square, rectangular or cylindrical,
depending on size and application.
Fabrication of baghouses
The enclosures for housing and supporting the bags
may be made in several ways:
/. Modular design—A complete, factory-assembled
5-9
unit consisting of bolted or welded components may
include the dust hopper, supporting legs, cleaning ac-
cessories, timer, internal baffles, bags, etc. Installation
only requires rigging into location atop a base to hold
down the legs before connecting to the inlet and ex-
haust systems, and utilities. A group of parallel-
connected units may be used to get additional capacity.
The parallel connections may be internal, using com-
mon clean-air or dusty-air plenums; or external, using
ductwork manifolds attached to inlet and outlet con-
nections on each module. Module sizes are limited to
those that can be transported over the road. The largest
(12 to 14 ft square in cross-section) would require a
low-bed trailer.
2. Factory-weldedsubassemblies—The bag housing, inlet
and outlet plenum chambers, dust hoppers, etc., are
separately fabricated to form the largest subassemblies
that can be shipped over the road. Connections to
adjacent components may be prepared for bolting or
welding at thejobsite. The method of assembly depends
on the materials of construction and location of the
baghouse. Field welding is precluded where bags are
102
CHEMICAL ENGINEERING APRIL 9, 1979
-------
already installed in the subassembly. The bag enclo-
sures may contain all the bags attached to their retain-
ers, and may have cleaning accessories factory-piped
and factory-wired.
3. Knocked down—All structural components are fab-
ricated as separate panels, with formed flanges for as-.
sembly in the field by bolting or by welding. This type
of construction is limited to very large baghouses fabri-
cated of special materials or located at jobsites where
access by crane is not possible. The flanged joints also
serve as stiffeners on the flat surfaces of hoppers, plenum
chambers, tubesheets and bag enclosures.
Materials of construction and unit size
The materials of construction and the sheer size of
some units may preclude complete factory assembly,
which is preferred in order to eliminate leakage and fix
responsibility.
Baghouses requiring hot-dipped galvanized surfaces
or internal linings of rubber, polyurethane or other
plastics for resisting corrosion or abrasion will need
bolted and gasketed joints because welding will destroy
the coating. Hot-dipped galvanized sections are limited
to sizes large enough to fit into available dip tanks.
Welding of galvanized surfaces is possible, but special
joint preparation and field coating the weld with com-
patible materials are required in order to maintain
corrosion resistance. Larger galvanized sections may be
welded by using lower-cost electrogalvanized sheet steel
instead of hot-dipped, in order to avoid the size limita-
tion imposed by dip tanks. However, special welding
preparation and coating of the weld will be required.
The weight of the zinc coating on electrogalvanized
sheets should equal that normally specified for hot-
dipped galvanizing (a coating of not less than 1 oi/ft2
on' each surface). Complicated surfaces such as tube-
sheets on baghouses requiring interior coatings may
have to be stainless steel or Hastelloy because coating
these surfaces may not be possible. All bolted joints
will require gasketing.or the application of a sealing
compound to the mating surfaces. These materials
should be compatible with the interior coating and the
dust entering the collector.
The large size of a unit may require fabrication and
shipment knocked-down in order to erect the baghouse
in areas inaccessible to a crane. Such areas could be (a)
building interiors, (b) yards surrounded by high build-
ings, or (c) roofs loaded with other equipment: In many
cases, a crane could lift the components to a peripheral
location, from where they could be trolleyed on a cable
to the final position.
Access for servicing
The location of access doors is principally determined
by the method of grouping the bags and attaching
them to their supports. The closed-end tubular bag and
its metal retainer suspended from the clean-air plenum
tubesheet is the most common design because it permits
the closest grouping of bags in a given space. Access is
through hinged doors located entirely across the clean-
air plenum, atop the collector. The bag and its retainer
are simply dropped through the tubesheet and fastened
to it by clamps, snap rings or twist locks, using the bag
material as a gasket. Internal air-nozzle piping is made
to swing out of the way or to be easily disconnected. Or,
the clean-air plenum is subdivided and valved so that
each subdivision can be isolated and pulsed by a single
solenoid valve.
Where the bags must be serviced regardless of
weather conditions, the clean-air plenum can be de-
signed as a walk-in housing requiring only one door,
but only on collectors fitted with an air nozzle over each
bag. Where bags and retainers are clamped from below
the tubesheet, the doors must be located on the side
walls of the unit so that bags are within 30 in. of the
opening to allow proper takeup of clamps.
If adjacent walls or equipment interfere with proper
location of a side door, the available doors must be
used, necessitating the removal of a group of bags in
order to reach those furthest from the door.
Bags suspended from shaking bars, with their open
ends fastened to a bottom tubesheet, are usually
grouped on each side of a narrow walkway within the
clean-air compartment that is reached through a single
door in the wall. During rebagging, the most remote
bags must be attached and tensioned. Then, work must
progress toward the walkway.
Envelope-type bags on metal spacer frames are in-
stalled in horizontal rows, with their open ends clipped
to the vertical slots of a clean-air plenum, and their
closed ends hooked to a support frame. Several rows of
bags are located in tiers, one above the other. All bags
in a compartment are accessible from two doors—one
leading to the clean-air plenum, the other leading onto
a walkway in the dusty-air chamber.
Long, large-diameter bags that are open at both ends
are usually fastened to a bottom 'tubesheet that is acces-
sible through a side door at the bottom of each com-
partment, .and to the top tubesheet that is accessible
through one or more doors in the roof of the dusty-air
inlet plenum. The side doors lead to walkways between
the bags. Some designs limit the number of rows of bags
on either side of the walkway in order to improve access
for clamping and to avoid damage to the bags.
All doors should be loosely hinged, and clamped or
latched in such a manner that all surfaces of the gasket
are sealed airtight. Gaskets should be closed-pore
sponge rubber (40 Durometer hardness) that is ce-
mented into a confining recess around the door or
around its frame. The rubber and its cement should be
compatible with each other and with the dust to which
they are exposed. Latches or clamps should be so de-
signed that they can be opened manually by operators
without using a wrench or special tool. The frames of all
doors should extend beyond any external insulation so
that they can be flashed properly. The inside surface of
vertical or sloping doors on the dusty side of the collec-
tor should be flush with adjacent walls in order to
prevent dust buildup on ledges.
Platforms, railings and ladders should be furnished
for reaching all elevated equipment and all access
doors. These should be built in accordance with appli-
cable safety codes. Rooftop equipment should be acces-
sible for servicing from within the peripheral hand-
railing. Ladders should have safety cages starting 7 ft
above floors or platforms, and should be equipped with
CHEMICAL ENO&Elikflc APRIL 97
1979
103
-------
BACHOUSES
55 60 65 70
Slope sheet angle, a or B , deg.
Slope sheet
angle et
65"
Slope sheet
angle
80"
*VKl-***^tW"^9V^.-*^--?W^TSK^*<^'&
oppers having flat slope sheete
grab rails and, for greater safety, with narrow, nonskid
treads in place of rungs. Platforms should be made from
serrated grating so that dust cannot accumulate and
slipping is prevented.
Dust hoppers and dust discharge
The material removed from the bags may be dropped
directly into a bin or silo if the baghouse is used for
venting that bin or silo. Otherwise, the material is
dropped into a collecting hopper that is shaped to direct
the flow to the dust outlet usually fitted with an
airlock-type discharge valve.
On multicompartment units, each compartment
should be fitted with its own dust hopper, to permit its
isolation during cleaning or maintenance. To eliminate
a multiplicity of airlock discharge valves, the bag com-
partments may discharge into a common V-shaped
hopper equipped with a screw conveyor that delivers
the dust to a single discharge valve. Such a screw con-
veyor must operate at the baghouse pressure. Therefore,
104
5-11
CHEMICAL ENGINEERING APRIL 9. 1979
-------
the conveyor must be equipped with effective seals at
the ends of the trough and at the bearings, which
should be outboard of the hopper. The conveyor must
also be able to handle the collected dust at the proper
rate.
The flow properties of the collected dust should be
known, since they may be quite different than the
properties of the material from which they originated.
The slope angles of the dust hopper may have to be
steeper than those of the storage silos of the original
material because (1) fine dusts tend to pack more read-
ily than coarse materials, (2) the dust may contain
moisture picked up in the conveying air or formed by
condensation within the collector, (3) collected material
should be removed as rapidly as it is deposited on the
hopper walls. Steep slope angles increase the overall
height of the baghouse and may limit the space availa-
ble for installing a discharge valve. However, failure
to provide the proper slope will create problems after
installation.
A conical hopper attached to a cylindrical baghouse
presents no problem other than the increase in height
required to obtain a steep slope. Attachment of a
square, rectangular or conical hopper to a square or
rectangular baghouse requires study—especially of the
valley angles formed by the slope sheets of inverted
pyramidal hoppers, or by transitions to conical hoppers.
These valley angles should be at the minimum value
required to obtain material flow. If the wall-slope angles
to the horizontal are known, the valley angles may be
obtained from Fig. 9, or from the formula from which
the chart is derived:
cot2 0 = cot2 a -f cot2 ft
where 6 is the valley angle, and a and (3 are the slope
angles between the horizontal and Wall A and Wall B
of the hopper, respectively, as shown in the sketch in
Fig. 9.
Provision of the proper valley angle at an intersection
does not ensure material flow, because moist materials
can bridge across the intersection. To minimize bridg-
ing, the intersection should be formed to as large a
radius as possible when joining the flat slope-sheets
internally.
On baghouses cleaned by reverse-air blast (using
compressed air), the slope-sheet'angles can be reduced
by a few degrees because the air blast helps to propel
the collected powder down the slope.
Materials may still hang up even though the proper
slope angles are provided on dust hoppers, because of
process upsets, changes in moisture content, etc. To
ensure flow, mounting pads should be installed on the
slope-sheets for attaching mechanical or pneumatic
vibrators for inducing flow. The vibrators should be
placed at the centroid of each surface. The flow induc-
ers should be operated only when needed. Cylindrical
hoppers require only one pad, located at the centroid of
the projected surface.
The dust hopper and its supports should be designed
on the basis that it could become full of material and
may also have to support a surcharge. This situation
may arise if the baghouse becomes full of material
because of a broken bag, a malfunction of the dust-
discharge valve, or stoppage of a downstream dust
conveyor.
Outdoor installations
Since many exhaust systems draw air from within
warm buildings, or handle hot materials from processes
or handle materials containing moisture, condensation
is a possibility within the baghouse. This can occur on
baghouses located indoors or outdoors, particularly in
. winter. Some manufacturers' guarantees are automati-
cally voided if condensation occurs inside the unit.
In such situations (especially outdoors), the baghouse
should be insulated. Insulation should be of sufficient
thickness to prevent condensation at the lowest outdoor
temperature recorded in a given location. All doors,
dust hoppers and projecting stiffeners should also be
insulated. Outdoor insulation should be protected with
aluminum or stainless-steel jacketing, applied so as to
shed water. Flashings of the same material should be
formed and fitted around all access openings. In some
cases, removable covers containing the insulation may
be fitted over doors to permit easy access.
If a baghouse does not require a separate enclosure, a
watertight housing should be provided around dust
hoppers, dust-discharge valves, and other dust-recovery
equipment located below the baghouse to permit access
for servicing during bad weather. Hinged or easily
removable weathertight hoods should be furnished to
protect all external operating mechanisms.
All electrical conduit to timers and actuators
mounted on the baghouse should be watertight. Pneu-
matic actuators, solenoid valves, shaking mechanisms,
and air piping to them, must also be insulated or traced,
to prevent condensation of moisture and its subsequent
freezing, which will prevent operation of the accessories.
If baghouses handling hot gases are shut down for the
weekend, they become especially susceptible to conden-
sation problems that prevent startup. Such units should
be equipped with an independent heater and a pressur-
izing fan designed to maintain the normal operating
temperature within the unit during shutdown. A grav-
ity-closed damper on the discharge of the main exhaust
fan will permit retention of the heated air. Baghouses
operating- under pressure should have a weathertight
cover or an automatic backdraft damper at the air
outlet to prevent intake of cold air during shutdowns.
Protection against fire
When hazardous materials are handled, protection
must .be provided to eliminate or minimize hazards.
Some materials may only require fire protection; others,
fire and explosion protection.
. The best procedure for determining the hazards in
handling a particular dust with unknown characteris-
tics is to submit test samples to the fire-insurance un-
derwriter's laboratory or to a commercial testing labo-
ratory that has facilities for conducting:
/. Combustion test—Ignition by match flame or
Bunsen burner to determine whether the material is
self-extinguishing, or whether the flame will spread.
2. Spontaneous-combustion test—Exposing material to
high temperature for a prolonged period (several
hours). The material is contained in a crucible im-
5--12
CHEMICAL ENGINEERING APRIL », 1979
105
-------
BACHOUSES
merscd in a hot salt bath. Moist materials should be
tested at their usual moisture content, and again after
drying.
3. Autoignitiun-teinperalure test—Same as the preceding
test, but raising the temperature continuously to deter-
mine the temperature at which ignition occurs. Evapo-
ration of water from a moist sample may exclude air, so
that the autoignition point could be on the high side.
4. Dust-explosion test—-Material is placed in a bomb
having a variable vent opening, mixed with compressed
air, and then ignited by an electric spark. The pressure
developed is measured and reported for two condi-
tions—(a) unvented, and (b) for a specific ratio of vent
area to bomb volume. Materials known to be explosive
should also be tested for rate of pressure rise in addition
to the pressure developed from an explosion.
Where possible, any baghouse subject to fire or ex-
plosion should be located outdoors, and away from
important equipment.
Fire protection may be provided by a built-in sprin-
kler system. The sprinkler heads would be rated about
25° (F) below the temperature limit of the bags if no
other protection were available; and 100° (F) above this
limit, if early-warning protection were installed. The
sprinklers would be connected to a building sprinkler
system. The heads are usually located where they can
cover most of the bag area. Additional heads may be
needed to cover the dust hoppers. Because of difficulty
in placing heads in some baghouses, assistance of the
fire underwriter's engineering department should be
obtained.
Opening of the sprinklers would be indicated by the
water-flow detector and alarm on the building sprinkler
system. However, a local alarm to warn system opera-
tors may also be required. The local alarm would be a
fire detector within the baghouse—the detector being
set at the same temperature as the sprinkler head. Pro-
vision must be made to dispose of the water released by
the sprinklers. When the baghouse is outdoors, a
nonfreeze sprinkler system will be needed. A dry system,
. using air in the sprinkler lines, is expensive and requires
considerable maintenance. A system filled with a
nonfreeze solution, may foul the bags when the heads
are released. Hence, the bags may require replacement
even if they are not burned.
Several alternatives are available for sprinklers:
1. An inert-gas (Halon) injection system may be
installed. When actuated, it will exclude or reduce the
oxygen content in the baghouse so that combustion
cannot be sustained. The sensor is a rate-of-rise temper-
ature detector, set at the temperature limit of the bags,
that operates a control unit to electro-explosively rup-
ture a diaphragm to release the inert gas into the
baghouse. The gas storage vessels are located outside
the collector. If the system is actuated, there is no mess
to clean up except char from the fire. The control unit
can also initiate other functions such as closing damp-
ers, stopping fans and sounding alarms.
2. A steam-smothering or a water-deluge system that
is actuated manually or by a rate-of-rise temperature
detector uses open nozzles, located in the same manner
as sprinkler heads.
3. A manually operated deluge system can be de-
vised for collectors having reverse-jet air manifolds by
connecting a water header to the air manifold pipes at
the ends opposite the air header. When water is turned
on, the air pulses speed up wetting of the bags. The
timer can be temporarily switched to pulse all nozzles
simultaneously, and pulse time could be increased to
obtain faster water distribution.
Protection against explosion
Explosion protection may be provided by installing
an inert-gas suppression system or explosion doors.
The suppression .system is similar to that for fire
protection, except that a highly sensitive pressure detec-
tor initiates action of the control unit. The pressure
setting is determined from data for the rate of pressure
rise obtained during the bomb test. The baghouse may
be isolated so as to contain the inert gas without loss by
the use of fast-acting inlet 'and outlet dampers, or by
shutting down the exhaust fan and rapidly closing an
outlet damper.
The inert-gas suppression system detects the pressure
rise caused by an explosion, and acts in milliseconds to
stop pressures from reaching values that would rupture
the bag housing. Because of the short duration of an
explosion, and the dependence on proper functioning of
all control equipment, a backup system of explosion
doors may be desirable.
Explosion doors are generally installed in baghouse
walls on the dusty-air side. They may be hinged doors
with special, spring-loaded, quick-releasing latches.
They may be rupture panels made of frangible materi-
als such as aluminum foil, lead sheets, polyethylene or
Teflon. They may be laminates of metal and plastic
designed to rupture along prescored lines to obtain full
opening of the panel. They may be loose doors held in
place by their own weight or by suction.
Latched or loose explosion doors on outdoor bag-
houses must have • rain hoods built around them.
Weighted doors, unattached by any other means,
should have chains connected to them, which are fas-
tened to an internal structural member, so that the
doors do not become missiles when blown open.
Every explosion is rapidly followed by an implosion
that can be as damaging to the structure as the explo-
sion. Explosion doors that are pivoted or hinged should
be designed to prevent automatic reclosing. Such doors
should be reclosed manually.
The concluding installment of this two-part series on
baghouse collectors, to appear in the issue of Apr. 23,
will deal with the selection and testing of filters, meas-
uring performance, and acceptance testing of bag-
houses. A reprint notice for both parts will be carried at
the end of the article in that issue.
Steven Dtmatoi, Editor.
The author
Millon N. Kraus, J2 Chris Ave,, Hillsdalc, NJ 07642, recently retired from
Colgate-Palmolive Co., Jersey City, N.J., where he was supervisor of project
engineering. Previously, he worked as a supervisory marine engineer in
design at the New York Naval Shipyard and became a licensed marine
engineer with the U.S. Maritime Service. He also served as division
engineer for dairy processing plants and as mechanical superintendent for a
power-plant construction firm. He has a B.M.E. from Pratt Institute. He is
a licensed professional engineer in New York and New Jersey, and a
member of ASME and Tau Beta PL
106
5-13
CHEMICAL ENGINEERING APRIL 9, 1979
-------
|