An electric filter is illustrated in Figure 17.8. The principle of operation is that when air is passed between a pair of oppositely charged conductors it becomes ionised if the voltage difference between the conductors is sufficiently large. Both negative ions and positive ions are formed, the latter being in the larger quantity. By contact with the dust particles mixed with the airstream, the charge of the ions is shared with the dust. In this way, about 80 per cent of the dust particles passing through the ionising field are given a positive
Charge and the other 20 per cent a negative charge. The ionising voltage used varies somewhat, and to achieve a given efficiency of ionisation smaller air velocities can be used with smaller voltages. However, typical ionising voltages are from 7800 to 13 000.
The electrodes which form the poles of the ionising unit mentioned above consist of alternate small diameter (of the order of 25 |im) tungsten wires and flat metal plates, spaced 25 mm or so apart. The intensity of an electric field is a function of the curvature of the charged conductor producing it, hence the small diameter of alternate poles. The tungsten wires are positively charged, the plates representing the negative electrodes being earthed.
The charged particles of dust which leave the ionising unit then pass through a collecting unit. This consists of a set of vertical metal plates, spaced about 15 mm apart. Alternate plates are positively charged and earthed and attract the negatively and positively charged dust particles, respectively. The voltage difference across the collecting plates is about 6000 or 7000, although at least one manufacturer is currently offering an electrical filter which uses about 8000 volts across the electrodes of both the ionising and collecting sections.
One of the main advantages claimed for electric filters is low maintenance cost, when compared with other filters of similar efficiency but lower capital cost. Although the low maintenance cost is debatable and if a comparison is made between filters on a basis of their owning and operating costs, the result is largely at the choice of the analyst, it is very necessary there should be automatic washing of electric filters if maintenance cost is to be minimised. To assist in the retention of the dust on the collecting plates, some manufacturers arrange for the plates to be coated with an oil. This may mean that a detergent is necessary as well as a water supply when washing is carried out. It also means that fresh oil must be put on the plates after the washing. A variation on this is to use a mixture of oil and detergent together so that when washing is done, usually with cold water, respraying with
Fig. 17.8 An electric filter.
Oil also provides the detergent necessary for the next wash. One manufacturer uses no detergent but asks that hot water be used for washing. Oil is not used in this case. One feature of washing common to all filters is that water at high pressure is required, gauge pressures of 2 to 3 bar being necessary. The method of washing is either from fixed standpipes attached to the appropriate reservoirs of water and oil or from a traversing nozzle system, attached to the reservoirs by flexible piping. The pump which feeds the water to the nozzles usually runs at 28 rev s-1 and is noisy, because of the requirements of a large head.
An insect screen is necessary upstream of the filter, and it is also necessary to provide either a pre-filter or an after-filter.
A pre-filter is preferred because it relieves the filtration load on the electric filter which follows. However, if it is used it must be washed automatically when the rest of the filter is cleaned, otherwise there is an increased maintenance problem. If an after-filter is used, its duty is merely to ensure that the filter fails safe. Neither the pre-filter nor the after-filter need be very elaborate; a simple screen of closely woven nylon cloth is often enough. If the
Filter is not washed regularly at suitably short intervals, the thickness of the dust-covering on the collection plates increases and, after a while, dust starts flaking off and is carried into the conditioned space. Hence another need for the after-filter. The ideal then is an automatically washed pre-filter and a simple after-filter.
Another type of electric filter ionises the air in the usual way and then retains the charged dust particles in a thickening layer on the collector plates. The dust is then swept off by the airstream as large agglomerated flakes which are retained in a storage section downstream consisting of an automatic, dry fabric, roll filter. Atmospheric dust-spot efficiencies of 90 per cent are achieved.
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