envelope configuration. The pleated cartridge arrangement uses a paper-like fiber in either a cylindrical or panel configu- ration. It features extremely high efficiency on light concen- trations. Earlier designs employed cellulose-based media.
Today, more conventional media, such as polypropolene or spun-bonded polyester, are frequently used.
The variable design features of the many fabric collectors available are:
I. Type of fabric (woven or non-woven).
2. Fabric configuration (bags or tubes, envelopes, car- tridges).
3. Intermittent or continuous service.
4. Type of reconditioning (shaker, pulse-jet, reverse-air).
5. Housing configuration (single compartment, multiple compartment).
At least two of these features will be interdependent. For example, non-woven fabrics are more difficult to recondition and therefore require high-pressure cleaning.
A fabric collector is selected for its mechanical, chemical, and thermal characteristics. Table 4-\ lists those charac- teristics for some common filter fabrics.
Fabric collectors are sized to provide a sufficient area of filter media to allow operation without excessive pressure drop. The amount of filter area required depends on many factors, including:
1. Release characteristics of dust.
2. Porosity of dust cake.
3. Concentration of dust in carrier gas stream.
4. Type offabric and surface finish, if any.
5. Type of reconditioning.
6. Reconditioning interval.
7. Air flow pattern within the collector.
8. Temperature and humidity of gas stream.
Because ofthe many variables and their range of variation, fabric collector sizing is ajudgment based on experience. The sizing is usually made by the equipment manufacturer, but at times may be specified by the user or a third party. Where no experience exists, a pilot installation is the only reliable way to determine proper size.
The sizing or rating of a fabric collector is expressed in terms of air flow rate versus fabric media area. The resultant ratio is called "air to cloth ratio" with units of cfm per square foot offabric. This ratio represents the average velocity of the gas stream through the filter media. The expression "filtration velocity" is used synonymously with air to cloth ratio for rating fabric collectors. For example, an air to cloth ratio of 7: 1 (7 cfm/sq ft) is equivalent to a filtration velocity of7 fpm.
Table 4-2 compares the various characteristics of fabric
collectors. The different types will be described in detaillateL Inspection of Table 4-2 now may make the subsequent dis- cussion more meaningful. The first major classification of fabric collectors is intermittent or continuous duty. Intermit- tent-duty fabric collectors cannot be reconditioned while in operation. By design, they require that the gas flow be inter- rupted while the fabric is agitated to free accumulated dust cake. Continuous-duty collectors do not require shut down for reconditioning.
Intermittent-duty fabric collectors may use a tube, car- tridge, or envelope configuration of woven fabric and will generally employ shaking or vibration for reconditioning.
Figure 4-7 shows both tube and envelope shaker collector designs. For the tube type, dirty air enters the open bottom of the tube and dust is collected on the inside of the fabric. The bottoms of the tubes are attached to a tube sheet and the tops are connected to a shaker mechanism. Since the gas flow is from inside to outside, the tubes tend to inflate during opera- tion and no other support of the fabric is required.
Gas flow for envelope-type collectors is from outside to inside; therefore, the envelopes must be supported during operation to prevent collapsing. This is normally done by inserting wire mesh or fabricated wire cages into the enve- lopes. The opening of the envelope from which the cleaned air exits is attached to a tube sheet and, depending on design, the other end may be attached to a support member or canti- levered without support. The shaker mechanism may be lo- cated in either the dirty air or cleaned air compartments.
Periodically (usually at 3- to 6-hour intervals) the air flow must be stopped to recondition the fabric. Figure 4-8 illus- trates the system air flow characteristics of an intermittent- duty fabric collector. As dust accumulates on the fabric, resistance to flow increases and air flow decreases until the fan is turned off and the fabric reconditioned. Variations in air flow due to changing pressure losses is sometimes a disadvantage and, when coupled with the requirement to periodically stop the air flow, may preclude the use of inter- mittent collectors. Reconditioning seldom requires more than two minutes but must be done without air flow through the fabric. If reconditioning is attempted with air flowing, it will be less effective and the flexing of the woven fabric will allow a substantial amount of dust to escape to the clean air side.
The filtration velocity for large intermittent-duty fabric collectors seldom exceeds 6 fpm and normal selections are in the 2-4 fpm range. Lighter dust concentrations and the ability to recondition more often allow the use of higher filtration velocities. Ratings are usually selected so that the pressure drop across the fabric will be in the 2-5 "wg range between start and end of operating cycle.
With multiple-section, continuous-duty, automatic fabric collectors, the disadvantage of stopping the air flow to perm it fabric reconditioning and the variations in air flow with dust cake build-up can be overcome. The use of sections or com-
Generic Trade Name
Names Fabrics" Continuous Intermittent Dry Heat Moist Heat Abrasion Shaking Flexing Mineral Acid Organic Acid Alkalies Oxidizing Solvents
Cotton Cotton 180 G G F G G P G F F E
::
Q. c
Polyester Dacron(1) Fortre(2) Vl
- §:
"'IVycron(3)
-<
Kodel(4) ::
"'
Enka 0'.
Polyester(3) 275 G F G E E G G F G E
[
o·
Acrylic Orlon(1) ::
Acrilan(6) Creslan(7) Dralon T(8)
Zefran 275 285 G G G G E G G F G E
Modacrylic Dynel(10)
Verel(4) 160 F F F P-F G G G G G G
Nylon Nylon
(Polyamide) 6,6(1,2,6)
Nylon 6(5,11,12) 225 G G E E E P F G F E
Nomex(11) 400 450 E E E E E P-F E G G E
Polymide P-84(18) 500 580 E P G G E P-F G F G
E
Polypropylene Herculon(13) Reevon(14)
Vectra(15) 200 250 G F E E G E E E G G
Teflon Teflon
(flurocarbon) TFE(1) 500 550 E E P-F G G E E E E E
Teflon
FEP(1) 450 E E P-F G G E E E E E
Expanded Rastex 500 550 E E P-F G G E E E E E
PFTE
Vinyon Vinyon(16)
Clevylt(17) 350 F F F G G E E G G P
Glass Glass 500 600 E E P P F E E F E E
Fiberglass Fiberglass(19) 550 550 E E P P G G G G E G
*E - excellent; G
=
good; F=
fair; P=
poor** Registered Trademarks
(1) Du Pont; (2) Celanese; (3) Beaunit; (4) Eastman; (5) American Enka; (6) Chemstrand; (7) American Cyanamid; (8) Farbenfabriken Bayer AG; (9) Dow Chemical; (10) Union Carbide; (11) Allied Chemical; (12) Firestone; (13) Hercules;
(14) Alamo Polymer; (15) National Plastic; (16) FMC; (17) Societe Rhovyl; (18) Lenzing; (19) Huyglas
partments, as indicated in Figure 4-7, allows continuous operation of the exhaust system because automatic dampers periodically remove one section from service for fabric recon- ditioning while the remaining compartments handle the total gas flow. The larger the number of compartments, the more constant the pressure loss and air flow. Either tubes or enve- lopes may be used and fabric reconditioning is usually accom- plished by shaking or vibrating.
Figure 4-8 shows air flow versus time for a multiple-section collector. Each individual section or compartment has an air flow versus time characteristic like that of the intermittent collector, but the total variation is reduced because of the multiple compartments. Note the more constant air flow characteristic of the five-compartment unit as opposed to the three-compartment design. Since an individual section is out of service only a few minutes for reconditioning and remain- ing sections handle the total gas flow during that time, it is possible to clean the fabric more frequently than with the intermittent type. This permits the multiple-section unit to handle higher dust concentrations. Compartments are recon- ditioned in fixed sequence with the ability to adjust the time interval between cleaning of individual compartments.
One variation of this design is the low-pressure, reverse-air collector which does not use shaking for fabric recondition- ing. Instead, a compartment is isolated for cleaning and the tubes collapsed by means of a low pressure secondary blower, which draws air from the compartment in a direction opposite to the primary air flow. This is a "gentle" method of fabric reconditioning and was developed primarily for the fragile glass cloth used for high-temperature operation. The reversal of air flow and tube deflation is accomplished very gently to avoid damage to the glass fibers. The control sequence usually allows the deflation and re-inflation oftubes several times for complete removal of excess dust. Tubes are 6-11 inches in diameter and can be as long as 30 feet. For long tubes, stainless steel rings may be sewn on the inside to help break up the dust Table 4-2. Summary of Fabric-Type Collectors and Their Characteristics
cake during deflation. A combination of shaking and reverse air flow has also been utilized.
When shaking is used for fabric reconditioning, the filtra- tion velocity usually is in the 1-4 fpm range. Reverse-air collapse-type reconditioning generally necessitates lower fil- tration velocities since reconditioning is not as complete.
They are seldom rated higher than 3 fpm. The airto cloth ratio or filtration velocity is based on net cloth area available when a compartment is out of service for reconditioning.
Reverse-jet, continuous-duty fabric collectors may use en- velopes or tubes of non-woven (felted) fabric, pleated car- tridges of non-woven mat (paper-like) in cylindrical or panel configuration, or rigid elements such as sintered polyethyl- ene. They differ from the low-pressure reverse-air type in that they employ a brief burst of high-pressure air to recondition the fabric. Woven fabric is not used because it allows exces- sive dust penetration during reconditioning. The most com- mon designs use compressed air at 80-100 psig, while others use an integral pressure blower at a lower pressure but higher secondary flow rate. Those using compressed air are generally called pulse-jet collectors and those using pressure blowers are called fan-pulse collectors.
All designs collect dust on the outside and have air flow from outside to inside the fabric. All recondition the media by introducing the pulse of cleaning air into the opening where cleaned air exits from the tube, envelope, or cartridge. In many cases, a venturi shaped fitting is used at this opening to provide additional cleaning by inducing additional air flow.
The venturi also directs or focuses the cleaning pulse for maximum efficiency.
Figure 4-9 shows a typical pulse-jet collector. Under nor- mal operation (air flow from outside to inside), the fabric shape will tend to collapse; therefore, a support cage is required. The injection of a short pulse of high-pressure air induces a secondary flow from the clean air compartment in
INTERRUPTABLE OPERATION INTERRUPTABLE OPERATION CONTINUOUS OPERATIONS
Light to Moderate Loading Heavy Loading Any Loading
Fabric Reconditioning
Requirement Intermittent Continuous
Type of Reconditioning Shaker Shaker
I
Reverse Air Reverse Pulse - (High Pressure) (Low Pressure) Pulse Jet of Fan Pulse Collector Configuratioil Single Compartment Multiple Compartment Single Compartment
with inlet or outlet dampers for each
Fabric Configuration Tube, Cartridge or Envelope Tube or Envelope I Tube Tube or Envelope Pleated Cartridge
Type of Fabric Woven Woven Non-Woven (Felt) Non-Woven
Air Flow Highly Variable Slightly Variable Virtually Constant Virtually Constant
Normal Rating 1 to 6 fpm 1 to 3fpm
I
1 to 3 fpm 5 to 12 fpm <1 to 7 fpm (filtration velocity, fpm)
/ ~ Motor driven vibrator
r;"AA
Clean
r Baffle
air
'l'UI31': TYPE ENVELOPE TYPE
/J~-=:Ji. - ·ir ~:~~~
'~ij]ijm~ f C2~ ~CO~3~, ~4~m~~~
Screen rapping I
J
Clean air side ,mechanism - I Three paslt.an outlet va,ves
Comportments 1,2, and 3
~~ ~:
under air load. Compallment ~
4 closed off for fabric cleaning.
MULTIPLE SECTION CONTINUOUS AUTOMATIC