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 9-19-1 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