Electrostatic precipitators pressure drop

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The power consumed to operate a wet electrostatic precipitator is much less than that required by most other methods of control. There are four areas in which power is consumed (1) electrostatic power, (2) fan power, (3) insulator heating power, and (4) pump power. The total electrostatic power input required for operation is 0.8 to 1.0 kW/1,000 ft of collection area. A comparable piece of equipment is a venturi scrubber with 50-in.wg pressure drop. The power required for this installation would be 6 to 7 kW/1,000 cfm. This would mean that approximately seven times the power would be needed to achieve the same amount of cleaning with a venturi scrubber as opposed to using a precipitator. 

Electrostatic precipitators are made in a very wide range of sizes and will handle gas flows up to about 50 m3/s. Although they operate more satisfactorily at low temperatures, they can be used up to about 800 K. Pressure drops over the separator are low. 

An electrostatic precipitator is a gas-solid separator in which particles suspended in a gas stream are charged and removed by electrostatic force. Since the separation force is directly applied to the particles without the need of accelerating the gas phase, an electrostatic precipitator usually requires much less hydraulic power than other separation systems. Hence, electrostatic precipitation is widely recognized as an important technique of gas-solid separation. This technique is characterized by low pressure drop, relative high... 

Electrostatic precipitator 0.01-10 Require low gas flow velocities, very low pressure drop, very large units (>3000 m /min)... 

Electrostatic precipitation is also very efficient for retention of very fine particles, as long as these are not highly electrically conductive. The structural features of these units dictate that they are only cost effective for particulate and aerosol emission control for low pressure drop clean-up of very large volumes of gas hence their extensive use for the treatment of combustion gases of fossil-fueled power stations. Next generation developments have been reviewed [27]. 

Electrostatic precipitation is perhaps the most versatile and cost effective of all particulate collecting devices and can be applied to any process where there is a need to remove solid particulate and mist of fume sized particles from the gas stream, whether it be for recovery or pollution control duties. It can be designed to deliver any efficiency for any gas flow rate and temperature and has a low pressure drop and a life span of more than 20 years. 

Many types of particulate collection devices are available commercially (see Table 53.2). Each operates on a different principle for accomplishing removal of particulates from the gas stream. Four basic types are common in drying systems (1) the drying vessel itself (in the case of vessel dryers), (2) cyclones, (3) bag filters, and (4) wet scrubbers. Electrostatic precipitators (ESPs) are not used widely in drying installations in spite of their low-pressure drop and high collection performance. The initial cost of purchase and construction is high. For this reason, the emphasis in Section 53.3 will be on the three most widely used devices, e.g., cyclones, fabric filters, and wet scrubbers. For a concise discussion of various types of solid-gas separation equipment and guidelines for selection of dust collectors,... 

Conventional Wet Scrubbers. Wet scrubbers are used for final particulate control in all currently operated coal gasification systems except those that operate at atmospheric pressure. (Low-pressure gasification systems use electrostatic precipitators.) High operating pressure allows for operation at reasonably high pressure drops, making scrubbers very small, efficient, and inexpensive. Scrubbers also remove ammonia, chlorides, and other trace components from the gas. The scrubber blowdown water is subsequently steam stripped, and the stripped gas sent either to the Claus plant or an incinerator. 

Obviously, if pressure drops on the order of 6 to 8 kPa are acceptable from a process point of view, an 8 or 13-unit multicyclone (or some number in between) could prove to be a viable option for the task at hand. If not, other types of separation equipment may have to be considered such as a baghouse, an electrostatic precipitator, or a wet scrubber. Nevertheless, even if the pressure drop across the cyclones were acceptable, one would still need to consider the long-term wear implications associated with operating the cyclones at velocities in the range of 35 to 40 m/s. If the solids being processed are not especially abrasive and/or if they are sufficiently flne in size, it may be possible to operate at these velocities. If not, erosion-protective liners may need to be installed. Some bare-metal multicyclone systems have been observed to operate for many years at velocities of 70 to 85 m/s while processing several tons per day of rather abrasive sand-like particles that were under about 25 fim in size. 

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