For large throughput rates, plate type precipitators are almost exclusively used. These are of two type horizontal flow or vertical flow type-the former being preferred most even though it has a serious drawback in the amount of floor space occupied. Horizontal flow types are grouped into sections, arranged in series, so that the gas leaving one section enters the next one down the line, through powered individually.
Adjacent sections, normally housed in a single casing, have gaps to accommodate the suspension frames and rapping mechanisms for the discharge electrodes, and also afford access for the installation inspection, maintenance and repairs of the interior fittings. The air gap provides extra electrical safety in cases when consecutive sections are fed at widely differing voltage levels.
The discharge and collecting electrodes are invariably of varying shapes and designs and each vendor comes out with a new design in order to survive in market. However, it may be said safely that these new designs are prompted by commercial rather than technical considerations. Some of the shapes of the collecting and discharge electrodes are presented in Figs. 9 and 10 respectively.
The collecting electrodes are so designed as to limit dust retirement by shielding the dislodged material both from the gas stream and electric field. These electrodes may be fully enclosed (e.g. pocketed), semi enclosed (e.g. trough type) or flat plates provides with baffles.
The discharge electrodes are normally wire or point types. Wire electrodes are commonly of circular, square, angled or asteroidal section, although some hexagonal or star types are also used. They are mostly made of nickel-chromium alloys or alloy steels, though plain iron electrodes are also in use.
Fiakt design of helically coiled wire with eyes at both the ends for suspension in its freedom to expand and contract and good response to rapping. Point electrodes are in general made of strips, either flat or reinforced by having one or both of their edges bent over or pressed into various corrugated patterns. The strips are punched or silt at regular intervals; the strips formed by this punching are bent outward, and constitute the points at which the discharges are concentrated.
The collecting electrodes are rapped individually, in a cycle which should fulfil two key requirements:
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(i) That every electrode must be rapped once in a cycle, and
(ii) Adjacent electrodes should never be rapped successively.
In different sections, electrodes arc rapped at different frequencies and with different intensities, selected to match the rate of the deposit build-up in the respective section.
Clearly, for proper operation, there is a relationship between the rapping interval, the number of electrodes to be rapped, and the duration of a full sequence. The current rapping interval is mostly established by trial and error when the unit is commissioned into service. Sometimes, intermittent rapping is preferred.
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Four types of rappers are normally supplied by the vendors.
These are:
(i) Electromagnetic impulse, either single or multiple,
(ii) Electric vibrators,
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(iii) Pneumatic impulse, and
(iv) Mechanical hammers.
Of these, mechanical hammers are frequently very effective.
In most circumstances, the flow changes up-stream of the unit have maximum effect on the reduction of collection/particulate off the collection electrodes. Once a flow mal-distribution occurs, the situation continuously deteriorates, because low velocity portions of the gas stream deposit dust at the points where flow direction changes.