Standard wet collectors are used to collect many types of mists. Specially designed electrostatic precipitators are fre- quently employed to collect sulfuric acid or oil mist. Even fabric and centrifugal collectors, although not the types pre- viously mentioned, are widely used to collect oil mist gener- ated by high speed machining.
4.6 GASEOUS CONTAMINANT COLLECTORS
Equipment designed specifically to control gas or vapor contaminants can be classified as:
1. Adsorbers 2. Thermal oxidizers 3. Direct combustors 4. Catalytic oxidizers
4.6.1 Absorbers: Absorbers remove soluble or chemi- cally reactive gases from an air stream by contact with a suitable liquid. While all designs utilize intimate contact between the gaseous contaminant and the absorbent, different brands vary widely in configuration and performance. Re- moval may be by absorption if the gas solubility and vapor pressure promote absorption or chemical reaction. Water is the most frequently used absorbent, but additives are fre- quently required. Occasionally other chemical solutions must be used. Packed towers (Figure 4-14) are typical absorbers.
4.6.2 Adsorbers: Adsorbers remove contaminants by col- lection on a solid. No chemical reaction is involved as adsorp- tion is a physical process where molecules of a gas adhere to surfaces of the solid adsorbent. Activated carbon or molecular sieves are popular adsorbents.
4.6.3 Thermal Oxidizers: Thermal oxidizers, or after- burners, may be used where the contaminant is combustible.
The contaminated air stream is introduced to an open flame or heating device followed by a residence chamber where combustibles are oxidized producing carbon dioxide and water vapor. Most combustible contaminants can be oxidized at temperatures between 1000 and 1500 F. The residence chamber must provide sufficient dwell time and turbulence to allow complete oxidation.
4.6.4 Direct Combustors: Direct combustors differ from thermal oxidizers by introducing the contaminated gases and auxiliary air directly into the burner as fuel. Auxiliary fuel, usually natural gas or oil, is generally required for ignition and mayor may not be required to sustain burning.
4.6.5 Catalytic Oxidizers: Catalytic oxidizers may be used where the contaminant is combustible. The contami- nated gas stream is preheated and then passed through a catalyst bed which promotes oxidation of the combustibles to carbon dioxide and water vapor. Metals of the platinum family are commonly used catalysts which will promote
oxidation at temperatures between 700 and 900 F.
To use either thermal or catalytic oxidation, the combusti- ble contaminant concentration must be below the lower ex- plosive limit. Equipment specifically designed for control of gaseous or vapor contaminants should be applied with caution when the air stream also contains solid particles. Solid par- ticulates can plug absorbers, adsorbers, and catalysts and, if noncombustible, will not be converted in thermal oxidizers and direct combustors.
Air streams containing both solid particles and gaseous con- taminants may require appropriate control devices in series.
4.7 UNIT COLLECTORS
Unit collector is a term usually applied to small fabric collectors having capacities in the 200-2000 cfm range. They have integral air movers, feature small space requirements and simplicity of installation. In most applications cleaned air is recirculated, although discharge ducts may be used if the added resistance is within the capability ofthe air mover. One of the primary advantages of unit collectors is a reduction in the amount of duct required, as opposed to central systems, and the addition of discharge ducts to unit collectors negates that advantage.
When cleaned air is to be recirculated, a number ofprecau- tions are required (see Chapter 7).
Unit collectors are used extensively to fill the need for dust collection from isolated, portable, intermittently used or fre- quently relocated dust producing operations. Typically, a single collector serves a single dust source with the energy saving advantage that the collector must operate only when that particular dust producing machine is in operation.
Figure 4-15 shows a typical un it collector. Usually they are the intermittent-duty, shaker-type in envelope configuration.
Woven fabric is nearly always used. Automatic fabric clean- ing is preferred. Manual methods without careful scheduling and supervision are unreliable.
4.8 DUST COLLECTING EQUIPMENT COST
The variations in equipment cost, especially on an installed basis, are difficult to estimate. Comparisons can be mislead- ing if these factors are not carefully evaluated.
4.8.1 Price Versus Capacity: All dust collector prices per cfm of gas will vary with the gas flow rate. The smaller the flow rate, the higher the cost per cfm. The break point, where price per cfm cleaned tends to level off, will vary with the design. See the typical curves shown on Figure 4-16.
4.8.2 Accessories Included: Careful analysis of compo- nents of equipment included is very important. Some collector designs include exhaust fan, motor, drive, and starter. In other designs, these items and their supporting structure must be obtained by the purchaser from other sources. Likewise, while
Table 4-3. Dust Collector Selection Guide
Collector Types Used in Industry
Concen- Particle Dry Cen- Low-Volt Hi-Volt
Operation tration Sizes trifugal Wet Fabric Electro- Electro- See Note 1 Notew Collector Collector Collector static static Remark No.
CERAMICS
a. Raw product handling light fine S
a a
N N1
b. Fettling light fine- S S
a
N N2
medium
c. Refractory sizing heavy coarse N S 0 N N
3
d. Glaze & vitro enamel spray moderate medium N 0 0 N N
CHEMICALS
49
a. Material handling light- fine- S 0 0 N N
4
moderate medium
b. Crushing, grinding moderate- fine- 0 S 0 N N
5
heavy coarse
C. Pneumatic conveying very fine-
a
Sa
N N6
heavy coarse
d. Roasters, kilns, coolers heavy mid- 0 0 0 N N
7
coarse
COAL, MINING AND POWER PLANT
49
a. Material handling moderate medium 0 S
a
N N8
b. Bunker ventilation light fine S S 0 N N
9
C. Dedusting, air cleaning heavy medium- S 0
a
N N10
coarse
d. Drying moderate fine N 0
a
N N11
FLY ASH
a. Coal burning--chain grate light fine S S 0 N 0
12
b. Coal burning--stoker fired moderate fine- S S 0 N 0
coarse
C. Coal burning--pulverized rnoderate fine S
fuel S 0 N 0
13
d. Wood burning varies coarse S S 0 N S
14
FOUNDRY
a. Shakeout light- fine N 0 0 N N
15
moderate
b. Sand handling moderate fine- N
medium 0 0 N N
16
C. Turnbling mills heavy medium- N S
a
N N17
coarse
d. Abrasive cleaning rnoderate- fine- N S
a
N N18
heavy medium
GRAIN ELEVA TOR, FLOUR AND FEED MILLS
49
a. Grain handling light medium 0 S 0 N N
19
b. Grain dryers light coarse S S 0 N N
20
C. Flour dust moderate medium 0 S 0 N N
21
d. Feed mill moderate medium 0 S 0 N N
22
METAL MELTING
50
a. Steel blast furnace heavy varied N 0 S N S
23
b. Steel open hearth moderate fine- N 0 S N S
24
coarse
C. Steel electric furnace light fine N S 0 N S
25
d. Ferrous cupola rnoderate varied N 0 0 N
26
e. Non-ferrous reverberatory varied fine N S 0 N N
27
f. Non-ferrous crucible light fine N S
a
N N28
METAL MINING AND ROCK PRODUCTS
a. Material handling rnoderate fine- N
medium 0
a
N N29
b. Dryers, kilns moderate medium- 0 0 0 N 0
30
coarse
C. Rock dryer moderate fine- N
rnedium S S N S
31
d. Cernent kiln heavy fine- N
medium N 0 N S
32
Collector Types Used in Industry
Concen- Particle Dry Cen- Low-Volt Hi-Volt
Operation tration Sizes trifugal Wet Fabric Electro- Electro- See
Note 1 Notew Collector Collector Collector static static Remark No.
e, Cement grinding moderate fine N N 0 N N 33
f, Cement clinker cooler moderate coarse 0 N 0 N N 34
METAL WORKING 49
a, Production ~indin~ light coarse 0 0 0 N N 35
scratch brus ing, a rasive cut off
b, Portable and swing frame light medium S 0 0 N N
c, Buffing light varied S 0 0 N N 36
d, Tool room light fine S S S N N 37
e, Cast iron machining moderate varied 0 0 0 S N 38
PHARMACEUTICAL AND FOOD PRODUCTS
a, Mixers, grinders, weighing, light medium 0 0 0 N N 39
blending, bagging, packaging
b, Coating pans varied fine- N 0 0 N N 40
medium
PLASTICS 49
a, Raw material processing ~ee comments under 0 S 0 N N 41
hemicals)
b, Plastic finishing light- varied S S 0 N N 42
moderate
c, Extrusion light fine N S N 0 N
RUBBER PRODUCTS 49
a, Mixers moderate fine S 0 S N N 43
b, Batchout rolls light fine S 0 S S N
c, Talc dusting and dedusting moderate medium S S 0 N N 44
d, Grinding moderate coarse 0 0 0 N N 45
WOODWORKING 49
a, Woodworking machines moderate varied 0 S 0 N N 46
b, Sanding moderate fine S S 0 N N 47
c, Waste conveying, hogs heavy varied 0 S S N N 48
Note 1: Light: less than 2 gr/fe; Moderage: 2 to 5 grlft3; Heavy: 5 gr/fe and up,
Note 2: Fine: 50% less than 5 microns; Medium: 50% 5 to 15 microns; Coarse: 50% 15 microns and larger.
Note 3: 0
=
often; S=
seldom; N=
never,Remarks Referred to in Table 4-3
1, Dust released from bin filling, conveying, weighing, mixing, pressing 11. Difficult problem but collectors will be used more frequently with air forming, Refractory products, dry pan and screen operations more pollution emphasis,
severe, 12, Public nuisance from boiler blOW-down indicates collectors are
2, Operations found in vitreous enameling, wall and floor tile, pottery, needed.
3. Grinding wheel or abrasive cut-off operation. Dust abrasive. 13. Large installations in residential areas require electrostatic in addition 4, Operations include conveying, elevating, mixing, screening, weighing, to dry centrifugal.
packaging, Category covers so many different materials that recom- 14, Cyclones used as spark arresters in front of fabric collectors, mendation will vary widely, 15, Hot gases and steam usually involved.
5. Cyclone and high efficiency centrifugals often act as primary collectors 16, Steam from hot sand, adhesive clay bond involved, followed by fabric or wet type. 17. Concentration very heavy at start of cycle,
6, Cyclones used as product collector followed by fabric arrester for high 18. Heaviest load from airless blasting due to higher cleaning speed, over -all collection efficiency, Abrasive shattering greater with sand than with grit or shot. Amounts 7. Dust concentration determines need for dry centrifugal; plant location, removed greater with sand castings, less with forging scale removal,
product value determines need for final collectors, High temperatures least when welding scale is removed.
are usual and corrosive gases not unusual. 19, Operations such as car unloading, conveying, weighing, storing, 8. Conveying, screening, crushing, unloading, 20. Collection eqUipment expensive but public nuisance complaints be- 9, Remove from other dust producing points, Separate collector usually, coming more frequent.
10, Heavy loading suggests final high efficiency collector for all except 21, Operations include conveyors, cleaning rolls, sifters, purifiers, bins
very remote locations, and packaging,
Remarks Referred to in Table 4-3 (continued) 22. Operations include conveyors, bins, hammer mills, mixers, feeders
and baggers.
23. Primary dry trap and wet scrubbing usual. Electrostatic is added where maximum cleaning required.
24. Use of this technique declining.
25. Air pollution standards will probably require increased usage of fabric arresters.
26. CAUTION! Recent design improvements such as coke-less, plasma- fired type, have altered emission characteristics.
27. Zinc oxide loading heavy during zinc additions. Stack temperatures high.
28. Zinc oxide plume can be troublesome in certain plant locations.
29. Crushing, screening, conveying involved. Wet ores often introduce water vapor in exhaust air.
30. Dry centrifugals used as primary collectors, followed by final cleaner.
31. Industry is aggressively seeking commercial uses for fines.
32. Collectors usually permit salvage of material and also reduce nuisance from settled dust in plant area.
33. Salvage value of collected material high. Same equipment used on raw grinding before calcining.
34. Coarse abrasive particles readily removed in primary collector types.
35. Roof discoloration, deposition on autos can occur with cyclones and less frequently with high efficiency dry centrifugal. Heavy duty air filters sometimes used as final cleaners.
36. Linty particles and sticky buffing compounds can cause pluggage and fire hazard in dry collectors.
dust storage hoppers are integral parts of some dust collector designs, they are not provided in other types. Duct connec- tions between elements may be included or omitted. Recircu- lating water pumps andlor settling tanks may be required but not included in the equipment price.
4.8.3 Instal/ation Cost: The cost of installation can equal or exceed the cost of the collector. Actual cost will depend on the method of shipment (completely assembled, sub-assem- bled or completely knocked down), the location (which may require expensive rigging), and the need for expensive sup- porting steel and access platforms. Factory installed media will reduce installation cost. The cost can also be measurably influenced by the need for water and drain connections, special or extensive electrical work, and expensive material handling equipment for collection material disposal. Items in the latter group will often also be variable, decreasing in cost per cfm as the flow rate of gas to be cleaned increases.
4.8.4 Special Construction: Prices shown in any tabula- tion must necessarily assume standard or basic construction.
The increase in cost for corrosion resisting material, special high-temperature fabrics, insulation, andlor weather protec- tion for outdoor installations can introduce a multiplier of one to four times the standard cost.
A general idea of relative dust collector cost is provided in Figure 4-16. The additional notes and explanations included
37. Unit collectors extensively used, especially for isolated machine tools.
38. Dust ranges from chips to fine floats including graphitic carbon. Low voltage ESP applicable only when a coolant is used.
39. Materials vary widely. Collector selection depends on salvage value, toxicity, sanitation yardsticks.
40. Controlled temperature and humidity of supply air to coating pans makes recirculation desirable.
41. Plastic manufacture allied to chemical industry and varies with opera- tions involved.
42. Operations and collector selection similar to woodworking. See Item 13.
43. Concentration is heavy during feed operation. Carbon black and other fine additions make collection and dust-free disposal difficult.
44. Salvage of collected material often dictates type of high efficiency collector.
45. Fire hazard from some operations must be considered.
46. Bulking material. Collected material storage and bridging from splin- ters and chips can be a problem.
47. Dry centrifugals not effective on heavy concentration of fine particles from production sanding.
48. Dry centrifugal collectors required. Wet or fabric collectors may be used for final collectors.
49. See NFPA publications for fire hazards, e.g., zirconium, magnesium, aluminum, woodworking, plastics, etc.
in these data should be carefully examined before they are used for estimating the cost of specific installations. For more accurate data, the equipment manufacturer or installer should be asked to provide estimates or a past history record for similar control problems utilized. Table 4-4 lists other char- acteristics that must be evaluated along with equipment cost.
Price estimates included in Figure 4-16 are for equipment of standard construction in normal arrangement. Estimates for exhausters and dust storage hoppers have been included, as indicated in Notes 1 and 2, where they are normally furnished by others.
4.9 SELECTION OF AIR FILTRATION EQUIPMENT Air filtration equipment is available in a wide variety of designs and capability. Performance ranges from a simple throwaway filter for the home furnace to the "clean room" in the electronics industry, where the air must be a thousand times as clean as in a hospital surgical suite. Selection is based on efficiency, dust holding capacity, and pressure drop. There are five basic methods of air filtration.
4.9.1 Straining: Straining occurs when a particle is larger than the opening between fibers and cannot pass through. It is a very ineffective method of filtration because the vast majority of particles are far smaller than the spaces between fibers. Straining will remove lint, hair, and other large parti-
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