The hopper and discharge device design including geometry, size, dust storage capacity, number, and location are important so that dust is removed on a routine basis. A well-designed dust hopper is sloped (usually 60°) to allow dust to flow freely to discharge devices. It includes access ports and strike plates to help move dust that becomes stuck. Dust should be only temporarily stored in the hopper and removed periodically by the discharge devices to prevent it from backing up into the ESP where it can touch the plates, possibly causing a cell to short out. In addition to the amount of fly ash present, there are a couple of special considerations to keep in mind when ESPs are used on coal-fired boilers. First, the amount of fly ash in the flue gas can vary depending on what type of coal is burned and the ash content of the coal. Coal having a higher ash content will produce more fly ash than coal having lower ash values. Consequently, the discharge device must be designed so that the operator can adjust the frequency of fly ash removal. Second, hoppers need to be insulated to prevent ash from "freezing," or sticking, in the hopper.
Finally, emission regulations in terms of opacity and dust concentration (grain-loading) requirements will ultimately play an important role in the final design decisions. Electrostatic precipitators are very efficient; collection efficiency can usually be greater than 99% if the ESP is properly designed and operated.
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