surface filter One may visualize this situation with a screen having holes 0.75 in(1, 91 cm)in diameter. We could collect a layer of Ping-Pong balls easily on this screen. Once we had such a layer, we could then collect cherries, which, by themselves, could pass through the holes in the screen but cannot pass through the spaces between the Ping-Pong balls. Once we have a layer of cherries, we could put on a layer of peas, then of rice, then of sand In that way we could collect sand on a screen with holes 0. 75 inch in diameter. In typical industrial filters the particles are of a wide variety of sizes, so they do not go onto the screen in layers, but all at once. The effect is the same; very small articles are collected by the previously collected cake on a support whose holes are much larger than the smallest particles collected The theory of cake accumulation and pressure drop for this type of device is well-known from industrial filtration The two most widely used designs of industrial surface filters are shown in Fig 9. 4 and 9. 5 on pages 285 and 286. Because the enclosing sheet metal structure in both figures is normally the size and roughly the shape of a house, this type of gas filter is generally called a baghouse. The in Fig. 9 shake-deflate filter, consists of a large number of cylindrical cloth bags that are closed at the top like a giant stocking, toe upward. These are hung from a support. Their lower ends slip over and are clamped onto cylindrical sleeves that project upward from a plate at the bottom. The dirty gas flows into the space below this plate and up inside the bags. The gas flows outward through the bags, leaving its solids behind. The clean gas then flows into the space outside the bags and is ducted to the exhaust stack or to some further processing ust be some way the cake of particles that accumulates on the filters. Normally this is not done during gas-cleaning operation the baghouse is taken out of the gas stream for dustrial bagh flow has been switched off, the bags are shaken by the support to loosen the collected cake. A weak flow of gas in th reverse direction may also be added to help dislodge the cake, thus deflating the bags. The cake falls into the hopper at the bottom of the baghouse and is collected or disposed of in some way Often metal rings are sewn into filter bags at regular Clean intervals so that the bag will only partly collapse when the flow is reversed, and a path will remain open for the dust to fall to the hopper Because it cannot filter gas while it is being cleaned, a Pulse.a shake-deflate baghouse cannot serve as the sole pollution control device for a source that produces a continuous low of dirty gas. For this reason, one either uses a large enough baghouse so that it can be cleaned during periodic Cage shutdowns of the source of contaminated gas or installs several baghouses in parallel. Typically, for a major continuous source like a power plant about five baghouses will be used in parallel, with four operating as gas cleaners during the time that the other one is being shaken and cleaned. Each baghouse might operate for two hours and then be cleaned for 10 minutes. at all times one baghouse would be out of service for cleaning or waiting to be put back into service. Thus the baghouse must be 9-5 Typical industriai 9-5 baghouse of the pulse-jet design9-5 surface filter One may visualize this situation with a screen having holes 0.75 in. (1,91 cm) in diameter. We could collect a layer of Ping-Pong balls easily on this screen. Once we had such a layer, we could then collect cherries, which, by themselves, could pass through the holes in the screen but cannot pass through the spaces between the Ping-Pong balls. Once we have a layer of cherries, we could put on a layer of peas, then of rice, then of sand. In that way we could collect sand on a screen with holes 0.75 inch in diameter. In typical industrial filters the particles are of a wide variety of sizes, so they do not go onto the screen in layers, but all at once. The effect is the same; very small particles are collected by the previously collected cake on a support whose holes are much larger than the smallest particles collected. The theory of cake accumulation and pressure drop for this type of device is well-known from industrial filtration. The two most widely used designs of industrial surface filters are shown in Fig 9.4 and 9.5 on pages 285 and 286. Because the enclosing sheet metal structure in both figures is normally the size and roughly the shape of a house, this type of gas filter is generally called a baghouse. The design in Fig. 9.4, most often called a shake-deflate filter, consists of a large number of cylindrical cloth bags that are closed at the top like a giant stocking, toe upward. These are hung from a support. Their lower ends slip over and are clamped onto cylindrical sleeves that project upward from a plate at the bottom. The dirty gas flows into the space below this plate and up inside the bags. The gas flows outward through the bags, leaving its solids behind. The clean gas then flows into the space outside the bags and is ducted to the exhaust stack or to some further processing. For the baghouse in Fig. 9.4 there must be some way of removing the cake of particles that accumulates on the filters. Normally this is not done during gas-cleaning operations. Instead the baghouse is taken out of the gas stream for cleaning. When the gas flow has been switched off, the bags are shaken by the support to loosen the collected cake. A weak flow of gas in the reverse direction may also be added to help dislodge the cake, thus deflating the bags. The cake falls into the hopper at the bottom of the baghouse and is collected or disposed of in some way. Often metal rings are sewn into filter bags at regular intervals so that the bag will only partly collapse when the flow is reversed, and a path will remain open for the dust to fall to the hopper. Because it cannot filter gas while it is being cleaned, a shake-deflate baghouse cannot serve as the sole pollution control device for a source that produces a continuous flow of dirty gas. For this reason, one either uses a large enough baghouse so that it can be cleaned during periodic shutdowns of the source of contaminated gas or installs several baghouses in parallel. Typically, for a major continuous source like a power plant, about five baghouses will be used in parallel, with four operating as gas cleaners during the time that the other one is being shaken and cleaned. Each baghouse might operate for two hours and then be cleaned for 10 minutes; at all times one baghouse would be out of service for cleaning or waiting to be put back into service. Thus the baghouse must be Fig. 9-4 Typical industrial baghouse of the shake-deflate design Fig. 9-5 Typical industrial baghouse of the pulse-jet design