a direction opposite to the normal air flow. Reconditioning is accomplished by the pulse of high-pressure air which stops forward air flow, then rapidly pressurizes the media, breaking up the dust cake and freeing accumulated dust from the fabric.
The secondary or induced air acts as a damper, preventing flow in the normal direction during reconditioning. The entire process, from injection of the high-pressure pulse and initia- tion of secondary flow until the secondary flow ends, takes place in approximately one second. Solenoid valves which control the pulses of compressed air may be open for a tenth of a second or less. An adequate flow rate of clean and dry compressed air of sufficient pressure must be supplied to ensure effective reconditioning.
Reverse-jet collectors normally clean no more than 10% of the fabric at anyone time. Because such a small percentage is cleaned at anyone time and because the induced secondary flow blocks normal flow during that time, reconditioning can take place while the collector is in service and without the need for compartmentation and dampers. The cleaning inter- vals are adjustable and are considerably more frequent than the intervals for shaker or reverse-air collectors. An individual element may be pulsed and reconditioned as often as once a minute to every six minutes.
Due to this very short reconditioning cycle, higher filtration velocities are possible with reverse-jet collectors. However, with all reverse-jet collectors, accumulated dust that is freed from one fabric surface may become reintrained and redepo- sited on an adjacent surface, or even on the original surface.
This phenomenon of redeposition tends to limit filtration velocity to something less than might be anticipated with cleaning intervals of just a few minutes.
Laboratory tests(41) have shown that for a given collector design redeposition increases with filtration velocity. Other test work(42) indicates clearly that redeposition varies with collector design and especially with flow patterns in the dirty air compartment. EPA-sponsored research(43) has shown that superior performance results from downward flow of the dirty air stream. This downward air flow reduces redeposition since it aids gravity in moving dust particles toward the hopper.
Filtration velocities of 5-12 fpm are normal for reverse-jet collectors. The pleated cartridge type of reverse-jet collector is limited to filtration velocities in the 7 fpm range. The pleat configuration may produce very high approach velocities and greater redeposition.
4.3.3 Wet Collectors: Wet collectors, or scrubbers, are commercially available in many different designs, with pres- sure drops from 1.5 "wg to as much as 1 00 "wg. There is a corresponding variation in collector performance. It is gener- ally accepted that, for well-designed equipment, efficiency depends on the energy utilized in air to water contact and is independent of operating principle. Efficiency is a function of total energy input per cfm whether the energy is supplied
to the air or to the water. This means that well-designed collectors by different manufacturers will provide similar efficiency if equivalent power is utilized.
Wet collectors have the ability to handle high-temperature and moisture-laden gases. The collection of dust in a wetted form minimizes a secondary dust problem in disposal of collected material. Some dusts represent explosion or fire hazards when dry. Wet collection minimizes the hazard;
however, the use of water may introduce corrosive conditions within the collector and freeze protection may be necessary if collectors are located outdoors in cold climates. Space requirements are nominal. Pressure losses and collection ef- ficiency vary widely for different designs.
Wet collectors, especially the high-energy types, are fre- quently the solution to air pollution problems. It should be recognized that disposal of collected material in water without clarification or treatment may create water pollution prob- lems.
Wet collectors have one characteristic not found in other collectors - the inherent ability to humidify. Hum idification, the process of adding water vapor to the air stream through evaporation, may be either advantageous or disadvantageous depending on the situation. Where the initial air stream is at an elevated temperature and not saturated, the process of evaporation reduces the temperature and the volumetric flow rate of the gas stream leaving the collector. Assuming the fan is to be selected for operation on the clean air side of the collector, it may be smaller and will definitely require less power than if there had been no cooling through the collector.
This is one of the obvious advantages of humidification;
however, there are other applications where the addition of moisture to the gas stream is undesirable. For example, the exhaust of humid air to an air-conditioned space normally places an unacceptable load on the air conditioning system.
High humidity can also result in corrosion of finished goods.
Therefore, humidification effects should be considered before designs are finalized. While all wet collectors humidify, the amount of humidification varies for different designs. Most manufacturers publish the humidifying efficiency for their equipment and will assist in evaluating the results.
Chamber or Spray Tower: Chamber or spray tower collec- tors consist of a round or rectangular chamber into which water is introduced by spray nozzles. There are many vari- ations of design, but the principal mechanism is impaction of dust particles on the liquid droplets created by the nozzles.
These droplets are separated from the air stream by centrifugal force or impingement on water eliminators.
The pressure drop is relatively low (on the order of 0.5-1.5
"wg), but water pressures range from 1 0-400 psig. The high pressure devices are the exception rather than the rule. In general, this type of collector utilizes low-pressure supply water and operates in the lower efficiency range for wet
collectors. Where water is supplied under high pressure, as with fog towers, collection efficiency can reach the upper range of wet collector performance.
For conventional equipment, water requirements are rea- sonable, with a maximum of about 5 gpm per thousand scfm of gas. Fogging types using high water pressure may require as much as 10 gpm per thousand scfm of gas.
Packed Towers: Packed towers (see Figure 4-10) are es- sentially contact beds through which gases and liquid pass concurrently, counter-currently, or in cross-flow. They are used primarily for applications involving gas, vapor, and mist removal. These collectors can capture solid particulate matter, but they are not used for that purpose because dust plugs the packing and requires unreasonable maintenance.
Water rates of 5-1 0 gpm per thousand sefm are typical for packed towers. Water is distributed over V-notched ceramic or plastic weirs. High temperature deterioration is avoided by using brick linings, allowing gas temperatures as high as 1600 F to be handled direct from furnace flues.
The air flow pressure loss for a four foot bed of packing, such as ceramic saddles, will range from l.5-3.5 "wg. The face velocity (velocity at which the gas enters the bed) will typically be 200-300 fpm.
Wet Centrifugal Collectors: Wet centrifugal collectors (see Figure 4-11) comprise a large portion of the commercially available wet collector designs. This type utilizes centrifugal force to accelerate the dust particle and impinge it upon a wetted collector surface. Water rates are usually
2-5
gpm per thousand scfm of gas cleaned. Water distribution can be from nozzles, gravity flow or induced water pickup. Pressure drop is in the 2-6 "wg range.As a group, these collectors are more efficient than the chamber type. Some are available with a variable number of impingement sections. A reduction in the number of sections results in lower efficiency, lower cost, less pressure drop, and smaller space. Other designs contain multiple collecting tubes. For a given air flow rate, a decrease in the tube size provides higher efficiency because the centrifugal force is greater.
Wet Dynamic Precipitator: The wet dynamic precipitator (see Figure 4-12) is a combination fan and dust collector. Dust particles in the dirty air stream impinge upon rotating fan blades wetted with spray nozzles. The dust particles impinge into water droplets and are trapped along with the water by a metal cone while the cleaned air makes a turn of 180 degrees and escapes from the front of the specially shaped impeller blades. Dirty water from the water cone goes to the water and sludge outlet and the cleaned air goes to an outlet section containing a water elimination device.
Orifice Type: In this group of wet collector designs (see
Figure 4-12), the air flow through the collector is brought in contact with a sheet of water in a restricted passage. Water flow may be induced by the velocity of the air stream or maintained by pumps and weirs. Pressure losses vary from 1
"wg or less for a water wash paint booth to a range of3-6 "wg for most of the industrial designs. Pressure drops as high as 20 "wg are used with some designs intended to collect very small particles.
Venturi: The venturi collector (see Figure 4-11) uses a venturi-shaped constriction to establish throat velocities con- siderably higher than those used by the orifice type. Gas velocities through venturi throats may range from 12,000-24,000 fpm. Water is supplied by piping or jets at or ahead of the throat at rates from 5-15 gpm per thousand scfm of gas.
The collection mechanism of the venturi is impaction. As is true for all well-designed wet collectors, collection effi- ciency increases with higher pressure drops. Specific pressure drops are obtained by designing for selected velocities in the throat. Some venturi collectors are made with adjustable throats allowing operation over a range of pressure drops for a given flow rate or over a range offlow rates with a constant pressure drop. Systems are available with pressure drops as low as 5 "wg for moderate collection efficiency and as high as 100 "wg for collection of extremely fine particles.
The venturi itself is a gas conditioner causing intimate contact between the particulates in the gas and the mUltiple jet streams of scrubbing water. The resulting mixture of gases, fume-dust agglomerates, and dirty water must be channeled through a separation section for the elimination of entrained droplets as shown in Figure 4-11.
4.3.4 Dry Centrifugal Collectors: Dry centrifugal collec- tors separate entrained particulate from an air stream by the use or combination of centrifugal, inertial, and gravitational force. Collection efficiency is influenced by:
1. Particle size, weight and shape. Performance is im- proved as size and weight become larger and as the shape becomes more spherical.
2. Collector size and design. The collection of fine dust with a mechanical device requires equipment designed to best utilize mechanical forces and fit specific appli- cation needs.
3. Velocity. Pressure drop through a cyclone collector increases approximately as the square of the inlet velocity. There is, however, an optimum velocity that is a function of collector design, dust characteristics, gas temperature and density.
4. Dust concentration. Generally, the performance of a mechanical collector increases as the concentration of dust becomes greater.
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AMERICAN CONFERENCE OF GOVERNMENTAL INDUSTRIAL HYGIENISTS
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