without first removing the solids. There are many examples where that has been done successfully Obviously, if there is no good way to deal with the contaminated water stream, then the scrubber has merely changed an air pollution problem into a water pollution problem 9.3 Choosing the Collectors In choosing a primary particle collection device one must consider the size of the particles to be cleanings,and details of the nature of the particles. The following rules of thumb may be helpful. collected, the required collection efficiency, the size of the gas flow, the allowed time betwe 1. Small or occasional flows can be treated by throwaway devices, e.g, cigarette and motor oil filters, in which the collected particles remain in the device. Large and steady flows require collection devices that operate continuously or semicontinously, and from which the collected particles can be removed continuously or semicontinuously. a throwaway device may be used as a final cleanup device, e.g., a high-efficiency filter may remove the last few particles from the air flowing to a microchip production clean room 2. Sticky particles(e.g, tars) must be collected either on throwaway devices or into a liquid, as in film of flowing liquid There must be some way to process the contaminated liquid thus produce o a scrubber or cyclone, filter, or wet ESP whose collecting surfaces ntinually coated with 3. Particles that adhere well to each other but not to solid surfaces are easy to collect. Those that do the reverse often need special surfaces, e.g., Teflon-coated fibers in filters that release collected particles well during cleaning 4. Electrical properties of the particles are of paramount importance in ESPs, and they are often significant in other control devices where friction-induced electro-static charges on the particles can aid or hinder collection 5. For nonsticky particles larger than about 5 u, a cyclone separator is probably the only device to 6. For particles much smaller than 5 ix one normally considers ESPs, filters, and scrubbers. Each of these can collect particles as small as a fraction of a micron 7. For large flows the pumping cost makes scrubbers very expensive; other devices are chosen if ibl 8. Corrosion resistance and acid dew point (Sec. 7. 12)must always be considered 9.4 Sumi 1. Gravity settling chambers, cyclones, and ESPs work by driving the particles to a solid wall where they form agglomerates that can be collected. These three devices have similar design equations. 2. Filters and scrubbers divide the flow. They have different design equations from all collection devices and from each other 3. Both surface and depth filters are used for particle collection. Surface filters are used to collect most of the particles in a heavily laden gas stream. Depth filters are mostly used for the final cleanup of air or gas that must be very clean or for fine liquid drops, which coalesce on them and then drop off 4. To collect small particles, a scrubber must have a very large relative velocity between the gas being cleaned and the liquid drops. For this reason co-flow scrubbers are most often used. The venturi scrubber is the most widely used type of co-flow scrubber9-8 without first removing the solids. There are many examples where that has been done successfully. Obviously, if there is no good way to deal with the contaminated water stream, then the scrubber has merely changed an air pollution problem into a water pollution problem. 9.3 Choosing the Collectors In choosing a primary particle collection device one must consider the size of the particles to be collected, the required collection efficiency, the size of the gas flow, the allowed time between cleanings, and details of the nature of the particles. The following rules of thumb may be helpful: 1. Small or occasional flows can be treated by throwaway devices, e.g., cigarette and motor oil filters, in which the collected particles remain in the device. Large and steady flows require collection devices that operate continuously or semicontinously, and from which the collected particles can be removed continuously or semicontinuously. A throwaway device may be used as a final cleanup device, e.g., a high-efficiency filter may remove the last few particles from the air flowing to a microchip production clean room. 2. Sticky particles (e.g., tars) must be collected either on throwaway devices or into a liquid, as in a scrubber or cyclone, filter, or wet ESP whose collecting surfaces are continually coated with a film of flowing liquid. There must be some way to process the contaminated liquid thus produced. 3. Particles that adhere well to each other but not to solid surfaces are easy to collect. Those that do the reverse often need special surfaces, e.g., Teflon-coated fibers in filters that release collected particles well during cleaning. 4. Electrical properties of the particles are of paramount importance in ESPs, and they are often significant in other control devices where friction-induced electro-static charges on the particles can aid or hinder collection. 5. For nonsticky particles larger than about 5 μ, a cyclone separator is probably the only device to consider. 6. For particles much smaller than 5 ix one normally considers ESPs, filters, and scrubbers. Each of these can collect particles as small as a fraction of a micron. 7. For large flows the pumping cost makes scrubbers very expensive; other devices are chosen if possible. 8. Corrosion resistance and acid dew point (Sec. 7.12) must always be considered. 9.4 Summary 1. Gravity settling chambers, cyclones, and ESPs work by driving the particles to a solid wall where they form agglomerates that can be collected. These three devices have similar design equations. 2. Filters and scrubbers divide the flow. They have different design equations from wall collection devices and from each other. 3. Both surface and depth filters are used for particle collection. Surface filters are used to collect most of the particles in a heavily laden gas stream. Depth filters are mostly used for the final cleanup of air or gas that must be very clean or for fine liquid drops, which coalesce on them and then drop off. 4. To collect small particles, a scrubber must have a very large relative velocity between the gas being cleaned and the liquid drops. For this reason co-flow scrubbers are most often used. The venturi scrubber is the most widely used type of co-flow scrubber