Part lI Wastegas Engineering 9 Control of Primary particles 9.1 Wall Collection Devices The first three types of control devices we consider--gravity settlers, cyclone separators, and electrostatic precipitators--all function by driving the particles to a solid wall, where they adhere to each other to form agglomerates that can be removed from the collection device and disposed of. Although these devices look different from one another, they all use the same general idea and are described by the same general design equations 9.1.1 Gravity Settlers a gravity settler is simply a long chamber through which the contaminated gas passes slowly, allowing time for the particles to settle by gravity to the bottom. It is an old, unsophisticated device that must be cleaned manually at regular intervals. But it is simple to construct, requires little maintenance, and has some use in industries treating very dirty gases, e.g, some smelters and metallurgical processes. Furthermore, the mathematical analysis for grav ity settlers is very easy; it will reappear in modified form for cyclones and electrostatic precipitators 9. 1. 2 Centrifugal Separators We have spent considerable time on gravity settlers because it is easy to see what all their mathematics as outlet mean. But they have little practical industrial use Diry because they are ineffective for small particles. If nlet are to use them or devices like them. we must find a ubstitute that is more powerful than the gravity force they use to drive the particles to the collection surface cs and mechanics books usually show tha centrifugal force is a pseudoforce that is really the result of the body s inertia carying it straight while some other force makes it move in a curved path. It is convenient to use this pseudoforce for calculational centrifugal forces acting on particles can be two orders of magnitude larger than the gravity forces. For this mason centrifugal particle separators are much more useful than gravity settlers Collected How does one construct a practical centrifugal particle Rotary solids collector? There are many types, but the most successful is sketched in Fig 9.1 Fig. 9-1 Schematic of a cyclone separator It is universally called a cyclone separator, or simply a cyclone. It is probably the most widely used particle collection device in the world. In any industrial district of any city, a sharp-eyed student can find at least a dozen of these outside various industrial plants A cyclone consists of a vertical cylindrical body, with a dust outlet at the conical bottom. The gas enters through a rectangular inlet, normally twice as high as it is wide, arranged tangentially to the circular body of the cyclone, so that the entering gas flows around the circumference of the cylindrical body, not radially inward. The gas spirals around the outer part of the cylindrical body with a downward component, then turns and spirals upward, leaving through the outlet at the top of the device. During the outer spiral of the gas the particles are driven to the wall by centrifugal force, where they collect, attach to each other, and form larger agglomerates that slide down the wall by gravity and collect in the dust hopper in the bottom There are many other variants on the centrifugal collector idea, but none approaches the cyclone in 9-1
9-1 Part II Wastegas Engineering 9 Control of Primary Particles 9.1 Wall Collection Devices The first three types of control devices we consider--gravity settlers, cyclone separators, and electrostatic precipitators--all function by driving the particles to a solid wall, where they adhere to each other to form agglomerates that can be removed from the collection device and disposed of. Although these devices look different from one another, they all use the same general idea and are described by the same general design equations. 9.1.1 Gravity Settlers A gravity settler is simply a long chamber through which the contaminated gas passes slowly, allowing time for the particles to settle by gravity to the bottom. It is an old, unsophisticated device that must be cleaned manually at regular intervals. But it is simple to construct, requires little maintenance, and has some use in industries treating very dirty gases, e.g., some smelters and metallurgical processes. Furthermore, the mathematical analysis for gravity settlers is very easy; it will reappear in modified form for cyclones and electrostatic precipitators. 9.1.2 Centrifugal Separators We have spent considerable time on gravity settlers because it is easy to see what all their mathematics mean. But they have little practical industrial use because they are ineffective for small particles. If we are to use them or devices like them, we must find a substitute that is more powerful than the gravity force they use to drive the particles to the collection surface. Physics and mechanics books usually show that centrifugal force is a pseudoforce that is really the result of the body's inertia carrying it straight while some other force makes it move in a curved path. It is convenient to use this pseudoforce for calculational purposes. At even modest velocities and common radii, the centrifugal forces acting on particles can be two orders of magnitude larger than the gravity forces. For this mason centrifugal particle separators are much more useful than gravity settlers. How does one construct a practical centrifugal particle collector? There are many types, but the most successful is sketched in Fig 9.1. It is universally called a cyclone separator, or simply a cyclone. It is probably the most widely used particle collection device in the world. In any industrial district of any city, a sharp-eyed student can find at least a dozen of these outside various industrial plants. A cyclone consists of a vertical cylindrical body, with a dust outlet at the conical bottom. The gas enters through a rectangular inlet, normally twice as high as it is wide, arranged tangentially to the circular body of the cyclone, so that the entering gas flows around the circumference of the cylindrical body, not radially inward. The gas spirals around the outer part of the cylindrical body with a downward component, then turns and spirals upward, leaving through the outlet at the top of the device. During the outer spiral of the gas the particles are driven to the wall by centrifugal force, where they collect, attach to each other, and form larger agglomerates that slide down the wall by gravity and collect in the dust hopper in the bottom. There are many other variants on the centrifugal collector idea, but none approaches the cyclone in Fig. 9-1 Schematic of a cyclone separator
breadth of application. These devices are simple and almost maintenance-free. Because any medium-sized welding shop can make one, the big suppliers of pollution control equipment, who have test data on the effects of small changes in the internal geometry, have been unwilling to make these data public. The same basic device as the cyclone separator is used in other industrial settings where the goal is not air pollution control, but some other kind of separation. When it is used to separate solids from liquids it is generally called a hydroclone. A cyclone called an air-swept classifier is attached to many industrial grinders. It passes those particles ground fine enough, and collects those that are too coarse, returning them to the grinder 9.1.3 Electrostatic Precipitators(ESP) If gravity settlers and centrifugal separators are devices that drive particles against a solid wail and if neither can function effectively(at an industrial scale) for particles below about 5 Ix in diameter, then for wall collection devices to work on smaller particles, they must exert forces that are more powerful than gravity or centrifugal force. The electrostatic precipitator(ESP) is like a gravity settler or centrifugal separator, but electrostatic force drives the particles to the wall. It effective on much smaller particles than the previous two devices The basic idea of all esps is to give the particles an electrostatic charge and then put them in an electrostatic field that drives them Das.-colection to a collecting wall. This is an suppon one type of ESP, calleda two-stage precipitator, charging and collecting are carried out in of the esr this type, widely used in building air conditioners, is sometimes called an electronic air filter. However Dirty gas for most industrial applications the two separate steps are carried Corona dis part of the ESP. The charging E function is done much more the collecting function. and the size of the esp is largely determined by the Collected dust collecting functio Ground Dust renoved from Fig 9.2 shows in simplified form a wire-and-plate ESP with two Fig. 9-2 Diagrammatic sketch of a simplified ESP with two plates four plates. The gas passes between the vires, and one low channel Industrial-size ESPs have many such plates, which are electrically channels in parallel grounded (i.e, voltage =0) Between the plates are rows of wires, held at a voltage of typically -40 000 volts The power is obtained by transforming ordinary alternating current to a high voltage and then rectifying it through some kind of solid-state rectifier. This combination of charged wires and grounded plates produces both the free electrons to charge the particles and the field to drive them against the plates. On the plates the particles lose their charge and adhere to each other and the plate, forming a"cake. The cleaned gas then passes out the far side of the precipitator as shown in Fig. 9.3 9-2
9-2 breadth of application. These devices are simple and almost maintenance-free. Because any medium-sized welding shop can make one, the big suppliers of pollution control equipment, who have test data on the effects of small changes in the internal geometry, have been unwilling to make these data public. The same basic device as the cyclone separator is used in other industrial settings where the goal is not air pollution control, but some other kind of separation. When it is used to separate solids from liquids it is generally called a hydroclone. A cyclone called an air-swept classifier is attached to many industrial grinders. It passes those particles ground fine enough, and collects those that are too coarse, returning them to the grinder. 9.1.3 Electrostatic Precipitators (ESP) If gravity settlers and centrifugal separators are devices that drive particles against a solid wail, and if neither can function effectively (at an industrial scale) for particles below about 5 Ix in diameter, then for wall collection devices to work on smaller particles, they must exert forces that are more powerful than gravity or centrifugal force. The electrostatic precipitator (ESP) is like a gravity settler or centrifugal separator, but electrostatic force drives the particles to the wall. It is effective on much smaller particles than the previous two devices. The basic idea of all ESPs is to give the particles an electrostatic charge and then put them in an electrostatic field that drives them to a collecting wall. This is an inherently two-step process. In one type of ESP, called a two-stage precipitator, charging and collecting are carried out in separate parts of the ESR This type, widely used in building air conditioners, is sometimes called an electronic air filter. However, for most industrial applications the two separate steps are carried out simultaneously in the same part of the ESP. The charging function is done much more quickly than the collecting function, and the size of the ESP is largely determined by the collecting function. Fig 9.2 shows in simplified form a wire-and-plate ESP with two plates. The gas passes between the plates, which are electrically grounded (i.e., voltage = 0). Between the plates are rows of wires, held at a voltage of typically -40 000 volts. The power is obtained by transforming ordinary alternating current to a high voltage and then rectifying it through some kind of solid-state rectifier. This combination of charged wires and grounded plates produces both the free electrons to charge the particles and the field to drive them against the plates. On the plates the particles lose their charge and adhere to each other and the plate, forming a "cake." The cleaned gas then passes out the far side of the precipitator as shown in Fig. 9.3. Fig. 9-2 Diagrammatic sketch of a simplified ESP with two plates,four wires, and one flow channel. Industrial-size ESPs have many such channels in parallel
Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic rapper that strikes a vertical or horizontal blow on the edge of the plate. Through science,art,and experience designers have learned to make rappers that cause most of the collected cake to fall into hoppers below the plates. Some of the cake is al ways re-entrained thereby lowering the efficiency of the system. If the collected particles are liquid, e.g, sulfuric acid mist, they run down the plate and drip off. For liquid droplets the plate is often replaced by a circular pipe with the wire down its center. Some ESPs(mostly the circular pipe variety) have a film of water flowing down the collecting surface, to carmy the collected particles to the bottom without rapping There are many types of ESPs Support frarnes for Fig. 9.3 shows one of the Rapper systems discharge clectrodes Transfurmcr-rectifier sets most common in current u in the United States. Gas flo High-voltage insulators is from right to left. The gas enters at the right through 30s which the flow spreads from the much narrower duct distribution plate that across the entrance face of the converging nozzle on the left side(not shown) ma a uniform flow at the outlet and then reduce the that of the outlet duct the whole interior of the structure elect Particle collecting plates, the cutaway show Discharge electrodes only one set of plates and discharge electrodes. The ig.9-3 Cutaway view of a large, modern ESP showing the various parts. of rigid frames with many In this design the wire discharge electrodes have been Replaced by rigid frames with many short, pointed stubs short, pointed stubs, which serve the same function as the in Fig9.2. TH collecting surfaces are made of sheet metal sections with vertical joints that tend to trap the particles. Each pair of plates, along with the discharge electrode between them, acts like the single channel in the simplified version of an ESP shown in Fig 9.2. The rappers strike the supports for the discharge electrodes and the collecting plates at regular time intervals to dislodge the cake of collected particles. The multiple power supply transformer-rectifier sets supply DC current at m -40,000 V to the discharge electrodes. The collected particles, dislodged from the plates by tl rappers, fall into the particle collecting hoppers, from which they are automatically removed to storage. The drawing shows some of the structural steel frame and enclosure of the esp and the handrail on its top, but not the internal seals that hinder the gas from flowing around the area of the collecting plates Each point in space has some electrical potential V. If the electrical potential changes from place to place, then there is an electrical field, E= V/x, in that space. If we connect two such points with a conductor, then a current will flow. This v is the voltage we are all familiar with, and e is its gradient in any direction; the units of E are V/m In a typical wire-and-plate precipitator, the distance from the wire to the plate is about 4 to 6 in or 0. 1 to 0.15 m. With a voltage difference of 40 kV and 4-in. spacing, one would assume a field strength of 40 kv/0. 1 m=400 k V/m. This is indeed the field strength near the plate. However, all of the electrical flow that reaches the plate comes from the wires, and the surface area of the wires
9-3 Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic rapper that strikes a vertical or horizontal blow on the edge of the plate. Through science, art, and experience designers have learned to make rappers that cause most of the collected cake to fall into hoppers below the plates. Some of the cake is always re-entrained, thereby lowering the efficiency of the system. If the collected particles are liquid, e.g., sulfuric acid mist, they run down the plate and drip off. For liquid droplets the plate is often replaced by a circular pipe with the wire down its center. Some ESPs (mostly the circular pipe variety) have a film of water flowing down the collecting surface, to carry the collected particles to the bottom without rapping. There are many types of ESPs; Fig. 9.3 shows one of the most common in current use in the United States. Gas flow is from right to left. The gas enters at the right through an inlet diffuser (not shown) in which the flow spreads out from the much narrower duct to the perforated gas distribution plate that distributes the gas evenly across the entrance face of the precipitator. A similar plate and converging nozzle on the left side (not shown) maintain a uniform flow at the outlet and then reduce the cross-sectional flow area to that of the outlet duct. The whole interior of the structure is filled with discharge electrodes and collecting plates; the cutaway shows only one set of plates and discharge electrodes. The discharge electrodes consist of rigid frames with many short, pointed stubs, which serve the same function as the wires in Fig.9.2. The collecting surfaces are made of sheet metal sections with vertical joints that tend to trap the particles. Each pair of plates, along with the discharge electrode between them, acts like the single channel in the simplified version of an ESP shown in Fig. 9.2. The rappers strike the supports for the discharge electrodes and the collecting plates at regular time intervals to dislodge the cake of collected particles. The multiple power supply transformer-rectifier sets supply DC current at m -40,000 V to the discharge electrodes. The collected particles, dislodged from the plates by the rappers, fall into the particle collecting hoppers, from which they are automatically removed to storage. The drawing shows some of the structural steel frame and enclosure of the ESP and the handrail on its top, but not the internal seals that hinder the gas from flowing around the area of the collecting plates. Each point in space has some electrical potential V. If the electrical potential changes from place to place, then there is an electrical field, E = V/x, in that space. If we connect two such points with a conductor, then a current will flow. This V is the voltage we are all familiar with, and E is its gradient in any direction; the units of E are V/m. In a typical wire-and-plate precipitator, the distance from the wire to the plate is about 4 to 6 in., or 0.1 to 0.15 m. With a voltage difference of 40 kV and 4-in. spacing, one would assume a field strength of 40 kV/0.1 m = 400 kV/m. This is indeed the field strength near the plate. However, all of the electrical flow that reaches the plate comes from the wires, and the surface area of the wires Fig. 9-3 Cutaway view of a large, modern ESP showing the various parts. In this design the wire discharge electrodes have been Replaced by rigid frames with many short, pointed stubs
ii much lower than that of the plate; thus, by conservation of charge, the driving potential near the wires must be much larger. Typically it is 5 to 10 MV/m. (The first person to utilize this fact was presumably Benjamin Franklin, who invented the sharp, pointed lightning rod When a stray electron from any of a variety of sources encounters this strong a field, it is accelerated rapidly and attains a high velocity. If it then collides with a gas molecule, it has enough energy to knock one or more electrons loose, thus ionizing the gas molecule. These electrons are likewise accelerated by the field and knock more electrons loose, until there are enough free electrons to form a steady corona discharge In a dark room this discharge appears as a dim glow that forms a circular sheath about the wire. The positive ions formed in the corona migrate to the wire and are discharged. The electrons migrate away from the wire, toward the plate Once they get far enough away from the wire for the field strength to be too low to accelerate them fast enough to ionize gas molecules, the visible corona ceases and they simply flow as free electrons As the electrons flow toward the plate, they encounter particles and can be captured by them, thus charging the particles. Then the same electric field that created the electrons and that is driving them toward the plate also drives the charged particles toward the plate The typical linear velocity of the gas inside an ESP is 3 to 5 ft/s, much lower than that in a cyclone The typical pressure drop is 0. 1 to 0.5 in. H20, again much less than in a cyclone. The pressure drop in the ducts leading to and from the precipitator is generally more than in the ESP itself. The ESP industry is now well established. Standard package units are available for small flows (down to the size of home air conditioners), and large power plants have precipitators costing up to $30 million Wet ESPs are more complex, and the collected particles are not in the convenient form of a dry powder. But for the final 5% cleanup these problems seem a modest to pay for the greatly improved collection efficiency. Another approach is to make the final 5% collection in a filter, as described next. Sometimes the ESP-filter combination is more equivalent-performance ESP or filter 9.2 Dividing Collection Devices Gravity settlers, cyclones, and ESPs collect particles by driving them against a solid wall. Filters and scrubbers do not drive the particles to a wall, but rather divide the flow into smaller parts where they can collect the particles. In this section we shall first consider the two types of filters used in air pollution control, surface filters and depth filters. Then we shall discuss scrubbers The public often refers to any kind of pollution control device as a filter, giving the word filter the meaning "cleaning device. Technically, a filter is one of the devices described in this section Other devices(e.g, the"biofilters"described in Chapter 10)are not truly filters. Engineers must live with the difference between the technical meaning and that used by nonprofessionals 9.2.1 Surface Filters Most of us have personal experience with surface filters, as exemplified by those in a coffee percolator or a kitchen sieve. The principle of operation is simple enough; the filter is a membrane (sheet steel, cloth, wire mesh, or filter paper) with holes smaller than the dimensions of the particles to be retained Although this kind of filter is sometimes used for air pollution control purposes, it is not common because constructing a filter with holes as small as many of the particles we wish to collect is very difficult One only needs to ponder the mechanical problem of drilling holes of 0. lu diameter or of weaving a fabric with threads separated by 0. 1 u to see that such filters are not easy to produce. It can be done on a laboratory scale by irradiating plastic sheets with neutrons and then dissolving away the neutron-damaged area. The resulting filters have analytical uses but are not used for industrial air pollution control (although they are used industrially to filter some beers and other products, removing trace amounts of bacteria) hough industrial air filters rarely have holes smaller than the smallest particles captured, they often act as if they did. The reason is that, as fine particles are caught on the sides of the holes of a filter, they tend to bridge over the holes and make them smaller. Thus as the amount of collected particles increases, the cake of collected material becomes the filter, and the filter medium (usually a cloth) that originally served as a filter to collect the cake now serves only to support the cake, and no longer as a filter. This cake of collected particles will have average pore sizes smaller than the diameter of the particles in the oncoming gas stream, and thus will act as a sieve for them The particles collect on the front surface of the growing cake. For that reason this is called a
9-4 ii much lower than that of the plate; thus, by conservation of charge, the driving potential near the wires must be much larger. Typically it is 5 to 10 MV/m. (The first person to utilize this fact was presumably Benjamin Franklin, who invented the sharp, pointed lightning rod.) When a stray electron from any of a variety of sources encounters this strong a field, it is accelerated rapidly and attains a high velocity. If it then collides with a gas molecule, it has enough energy to knock one or more electrons loose, thus ionizing the gas molecule. These electrons are likewise accelerated by the field and knock more electrons loose, until there are enough free electrons to form a steady corona discharge. In a dark room this discharge appears as a dim glow that forms a circular sheath about the wire. The positive ions formed in the corona migrate to the wire and are discharged. The electrons migrate away from the wire, toward the plate. Once they get far enough away from the wire for the field strength to be too low to accelerate them fast enough to ionize gas molecules, the visible corona ceases and they simply flow as free electrons. As the electrons flow toward the plate, they encounter particles and can be captured by them, thus charging the particles. Then the same electric field that created the electrons and that is driving them toward the plate also drives the charged particles toward the plate. The typical linear velocity of the gas inside an ESP is 3 to 5 ft/s, much lower than that in a cyclone. The typical pressure drop is 0.1 to 0.5 in. H20, again much less than in a cyclone. The pressure drop in the ducts leading to and from the precipitator is generally more than in the ESP itself. The ESP industry is now well established. Standard package units are available for small flows (down to the size of home air conditioners), and large power plants have precipitators costing up to $30 million. Wet ESPs are more complex, and the collected particles are not in the convenient form of a dry powder. But for the final 5% cleanup these problems seem a modest price to pay for the greatly improved collection efficiency. Another approach is to make the final 5% collection in a filter, as described next. Sometimes the ESP-filter combination is more economical than an equivalent-performance ESP or filter. 9.2 Dividing Collection Devices Gravity settlers, cyclones, and ESPs collect particles by driving them against a solid wall. Filters and scrubbers do not drive the particles to a wall, but rather divide the flow into smaller parts where they can collect the particles. In this section we shall first consider the two types of filters used in air pollution control, surface filters and depth filters. Then we shall discuss scrubbers. The public often refers to any kind of pollution control device as a filter, giving the word filter the meaning "cleaning device." Technically, a filter is one of the devices described in this section. Other devices (e.g., the "biofilters" described in Chapter 10) are not truly filters. Engineers must live with the difference between the technical meaning and that used by nonprofessionals. 9.2.1 Surface Filters Most of us have personal experience with surface filters, as exemplified by those in a coffee percolator or a kitchen sieve. The principle of operation is simple enough; the filter is a membrane (sheet steel, cloth, wire mesh, or filter paper) with holes smaller than the dimensions of the particles to be retained. Although this kind of filter is sometimes used for air pollution control purposes, it is not common because constructing a filter with holes as small as many of the particles we wish to collect is very difficult. One only needs to ponder the mechanical problem of drilling holes of 0.1μ diameter or of weaving a fabric with threads separated by 0.1 μ to see that such filters are not easy to produce. It can be done on a laboratory scale by irradiating plastic sheets with neutrons and then dissolving away the neutron-damaged area. The resulting filters have analytical uses but are not used for industrial air pollution control (although they are used industrially to filter some beers and other products, removing trace amounts of bacteria). Although industrial air filters rarely have holes smaller than the smallest particles captured, they often act as if they did. The reason is that, as fine particles are caught on the sides of the holes of a filter, they tend to bridge over the holes and make them smaller. Thus as the amount of collected particles increases, the cake of collected material becomes the filter, and the filter medium (usually a cloth) that originally served as a filter to collect the cake now serves only to support the cake, and no longer as a filter. This cake of collected particles will have average pore sizes smaller than the diameter of the particles in the oncoming gas stream, and thus will act as a sieve for them. The particles collect on the front surface of the growing cake. For that reason this is called a
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 design
9-5 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