Pressure drop electrostatic

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

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. 

Equations 5.37, 5.38 and 5.39 for solid velocity and pressure drop ate applicable only in the absence of electrostatic charging of the particles. Many materials, including sand, become charged during transport and cause the deposition of a charged layer on the surface of the pipe. The charge remains on the earthed pipeline for long periods but can... 

Smeltzer, E. E., Weaver, M. L., and Klinzing, G. E., Pressure drop losses due to electrostatic generation in pneumatic transport, Indust. Engrg. Chem, Proc. Des. Dev., 21 390-394 (1982)... 

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. 

Another technique is gaining interest because of the ease of regeneration and improved flow characteristics (small and constant pressure drops). Instead of physically trapping a pollutant in its pores, the technique involves direct attachment of the contaminant molecules to the sorption material, usually a polymer. All molecules are composed of a number of atoms with a confluence of electrons spinning around them in what is called an electrostatic field or electron cloud. The cloud, however, is not necessarily uniformly distributed. 

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... 

It is recognized that within the range where drag reduction occurs, the solids concentration is so dilute that the averaged distance between particles is usually 10 or more particle diameters. Therefore, under this flow condition, interparticle effects can be neglected. Consider the case of a fully developed horizontal pipe flow with negligible electrostatic effects. From Eq. (11.6), the pressure drop depends only on the wall friction, as given by... 

Solids of group C are very fine-grained, cohesive powders (e.g. flour, fines from cyclones, and electrostatic filters) that virtually cannot be fluidized without fluidization aids. The adhesion forces between particles are stronger than the forces that the fluid can exert on the particles. Gas flow through the bed forms channels extending from the grid to the top of the bed, and the pressure drop across the bed is lower than the value from cq 1. Fluidization properties can be improved by the use of mechanical equipment (agitators, vibrators) or flowability additives such as Aerosil. 

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