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DATE
[YYYY.MM.DD]
2008-11-03
TITLE
MECHANICAL COLLECTORS
MAIN TEXT
Exerpt from chapter 25.3 of the 2004 – ASHRAE Handbook – HVAC Systems and Equipment
Regulations and Monitoring 25.1
PARTICULATE CONTAMINANT CONTROL 25.2
Mechanical Collectors 25.3
Electrostatic Precipitators 25.7
Fabric Filters 25.10
Granular-Bed Filters 25.14
Particulate Scrubbers (Wet Collectors) 25.15
GASEOUS CONTAMINANT CONTROL 25.17
Spray Dry Scrubbing 25.18
Wet-Packed Scrubbers 25.18
Adsorption of Gaseous Contaminants 25.24
Incineration of Gases and Vapors 25.26
AUXILIARY EQUIPMENT 25.28
Ducts 25.28
Dust- and Slurry-Handling Equipment 25.29
OPERATION AND MAINTENANCE 25.29
25.3
Setting Chambers
Particulate matter will fall from suspension in a reasonable time if the particles are larger than about 40 /µm. Plenums, dropout boxes, or gravitational settling chambers are thus used for the separation of coarse or abrasive particulate matter from gas streams.
Settling chambers are occasionally used in conjunction with fabric filters or electrostatic precipitators to reduce overall system cost. The settling chamber serves as a precollector to remove coarse particles from the gas stream.
Settling chambers sometimes contain baffles to distribute gas flow and to serve as surfaces for the impingement of coarse particles. Other designs use baffles to change the direction of the gas flow, thereby allowing coarse particles to be thrown from the gas stream by inertial forces.
The fractional efficiency for a settling chamber with uniform gas flow may be estimated by
The superficial velocity of gases through the chamber is determined from measurements of the volumetric flow of gases entering and exiting the chamber and the cross-sectional area of the chamber. This average velocity must be low enough to prevent reentrainment of the deposited dust; a superficial velocity below 60 fpm is satisfactory for many materials.
Typical data on settling can be found in Table 5. Because of air inclusions in the particle, the density of a dust particle can be substantially lower than the true density of the material from which it is made.
Inertial Collectors
Louver and Baffle Collectors. Louvers are widely used to control particles larger than about 15 µm in diameter. The louvers cause a sudden change in direction of gas flow. By virtue of their inertia, particles move away from high-velocity gases and are either collected in a hopper or trap or withdrawn in a concentrated sidestream. The sidestream is cleaned using a cyclone or high-efficiency collector, or it is simply discharged to the atmosphere. ln general, the pressure drop across inertial collectors with louvers or baffles is greater than that for settling chambers, but this loss is balanced by higher collection efficiency and more compact equipment.
Inertial collectors are occasionally used to control mist. In some applications, the interior of the collector may be irrigated to prevent reentrainment of dry dust and to remove soluble deposits.
Typical louver and baffle collectors are shown in Figure 1.
Cyclones and Multicyclones. A cyclone collector transforms a gas stream into a confined vortex, from which inertia moves suspended particles to the well of the cyclone's body. The inertial effect of turning the gas stream, as used in the baffle collector, is used continuously in a cyclone to can improved collection efficiency. Cyclone collectors are often used as pre-cleaners to reduce the loading of more efficient pollution control devices. Figure 2 shows some typical cyclone collectors.
A low-efficiency cyclone operates with a static pressure drop from 1 to 1.5 in. of water between its inlet and outlet and can remove 50% of the particles from 5 to 10 µm. High-efficiency cyclones operate with static pressure drops from 3 to 8 in. of water between their inlet and outlet and can remove 70% of the particulates of approximately 5 µm.
The efficiency of a cyclone depends on particle density, shape, and size (aerodynamic size Dp, which is the average of the size range). Cyclone efficiency may be estimated from Figure 3.
The parameter Dpc, known as the cut size, is defined as the diameter of particles collected with 50% efficiency. The cut size may be estimated using the following equation:
At inlet gas velocity above 4800 fpm, internal turbulence limits improvements in the efficiency of a given cyclone. The pressure drop through a cyclone is proportional to the inlet velocity pressure and hence the square of the volumetric flow.
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