Particle collection

Type of scmbber Pressure drop. Pa Minimum collectible particle dia, tlm ... 

Low resistivity can sometimes be a problem, if the resistivity is below lO" n-cm, the collected particles are so conductive that their charges leak to ground faster than they are replenished bv the corona. The particles are no longer electrostatically pinned to the plate, and... 

These inertial effects become less important for particles with diameters less than 5 /rm and for low wind velocities, but for samplers attempting to collect particles above 5 p.m, the inlet design and flow rates become important parameters. In addition, the wind speed has a much greater impact on sampling errors associated with particles more than 5 fim in diameter (4). 

The collection technique involves the removal of particles from the air stream. The two principal methods are filtration and impaction. Filtrahon consists of collecting particles on a filter surface by three processes—direct interception, inertial impaction, and diffusion (5). Filtration attempts to remove a very high percentage of the mass and number of particles by these three processes. Any size classification is done by a preclassifier, such as an impactor, before the particle stream reaches the surface of the filter. 

Alfalfa dehydration is carried out in a direct-fired rotary dryer. The dried product is transported pneumatically to an air cooler and then to a collecting cyclone. The collected particles are ground or pelletized and then packaged for shipment. The major atmospheric emission from the process is particulate matter, which is controlled by baghouses. Odors may also be a problem, but they disperse rapidly and are no longer a problem at distances of over 1 km. 

Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with electrostatic precipitators. Fabric filters therefore may be good candidates for collecting fly ash from low-sulfur coals or fly ash containing high unburned carbon levels, which respectively have high and low resistivities, and thus are relatively difficult to collect with electrostatic precipitators. 

The intermittent-flush, parallel-plate precipitator is built in either cylindrical or square housings and uses intermittent spraying to remove collected particles from the electrodes. The sprays are usually located just before or just after the precipitation fields. These sprays must not be applied at low pressure if the field strength is not to be interrupted. The intermittent-flush units are used on blast furnace applications. 

A very important parr of the gas-deatimg process is the removal of the collected particles from the cleaning system. This should be as controlled as possible in order to avoid particle reenrrainmenr to the gas flow. This can be accomplished in the case of liquid particles such as acid fume or tar or oil smoke. olid particles are normally removed by periodic rapping of discharge and collection electrodes. Solid particles can also be removed with the aid of water, as is done in wet electrostatic precipitators. 

Two-stage precipitators consist of separate sections for particle charing and collection. Particle charging is realized with corona wires between grounded metal plates. The collection of particles rakes place in a system of parallel plate electrodes of opposite polarities. Two-stage precipitators are typically used in indoor air cleaning and light industrial applications. 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

Particle Collection Particles in the extracted partial volume flow are retained in the collector filter. Tlie particle mass emitted is determined by the weight difference of the filter before and after the collection. Factors crucial to the measuring precision and the smallest measuring range of particle concentration are ... 

Impactor, cascade An instrument consisting of stages for producing successively increasing air velocities for collecting particles by size range. 

Size-selective sampling Industtial hygiene sampling methods that collect particles with a specific range of aerodynamic diameters. 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

Sediment deposition. Horizontal surfaces collect particles primarily by sedimentation deposition, sometimes referred to as fallout . These can provide valuable information relating to exposure for horizontal surfaces in the field such as water and ground. Most studies measuring spray performance within the application area, and/or outside this area by drift, include horizontal collectors. The most common types of such collector include flat papers, cards and plates. Water- and oil-sensitive papers have been widely used for looking at the uniformity of spray coverage, coefficient of variation, droplet densities and approximate droplet size within a spray block. 

Figure 8.30 Top, saapling train, for collecting particles from stack gases. Bottoa, dilution tube used for monitoring and collection of gas- and particle-phase vehicle eiilssions.

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