Document 3J11eed0GN3LR3kogV7YQ1Yz3

836 CHAPTER 33 1957 Guide therefore, care must be exercised in arranging the duct approaches on the entering and leaving sides of the cleaners in order that the air flow may be distributed uniformly over the cross-sectional area. The efficiency of the electronic air cleaner is sensitive to air velocity,, and the device itself has much less tendency to rectify the air stream than filters which have higher resistances. In most systems, resistance is deliberately added in the form of a perforated plate, pre-filters or after-filters for the purpose of obtaining -- a uniform distribution of air. The resistance, including the baffles or after-filters, is constant and generally ranges from 0.15 to 0.25 in. of water at the usual velocities of 300 to 400 fpm. Such plates or filters, however, cannot compensate for defects in the duct layout ahead of the cleaner. Suitable screens of no coarser than 16 mesh should be installed across all Air Cleaning 837 means are employed. The dielectric filtering medium may consist of glass fiber mat, cellulose mat or other similar material, and is supported on or in contact with a gridwork consisting of alternately grounded and charged members, the latter usually being held at a potential of 12,000 volts d.c. An intense and non-uniform electrostatic field is thus created through the dielectrio medium. Airborne particles approaching the field are polarized and drawn toward filaments or fibers of the media. The general arrange ment illustrating this type of filter is shown in Fig. 2. The precipitator of this type offers resistance to air flow, when clean, on the order of 0.10 in. of water at 250 fpm velocity and, unlike the ionizing type, the resistance of the charged media type electronic cleaner rises as dust is accumulated on the media. Because of these characteristics the filter tends to equalize the air distribution over the face of the filter. Like Fig. 1. Diagrammatic Cross-Section of Ionizing Type Electronic Air Cleaner outside air entrances to prevent insects, leaves, bits of paper and similar material from entering the cleaner. Where lint is present in appreciable quantities, devices for lint removal should be installed ahead of the cleaner. Electronic air cleaners of the ionizing type are efficient, low pressure drop devices for removing fine dust and smoke particles. They are avail able in fixed collector-plate and moving collector-plate types. The fixed collector plates are often coated with a special oil as an adhesive. Cleaning is generally accomplished by washing the cells in place with hot water from a water hose or by means of fixed or moving nozzle systems. The bottom of the equipment is made water tight and provided with a drain. In one moving-plate type, the grounded elements on which the dirt collects are mounted on chains, and are alternately dipped in oil and exposed to the air stream. Charged Media Electronic Air Cleaners The charged media type air cleaner consists of a dielectric filtering medium, usually arranged in pleats as in typical dry filters. No ionization Fig. 2. Cross-Section Diagram of Charged Media Type Electronic Air Cleaner typical replaceable media mechanical filters, the charged media precipitator is serviced by replacing the filtering medium. The dielectric properties.of the media are impaired when the relative humidity exceeds 70 percent. AIR FILTER PERFORMANCE AND TESTING Air filters are generally rated in terms of the total air flow for which they are designed, expressed in cubic feet per minute. Face velocity is defined as the average velocity of the air entering the filter, and' it is determined by taking the air flow and dividing it by the area of the duct connection to the cleaner in square feet; Filters are often rated, at a face velocity in the range of 250 to 500 fpm. Resistance to'air flow is usually measured in niches of water column. The resistances of filters when new and clean, and when operated at rated capacity, are generally available from the manufacturer (see Catalog Data Section). A suitable allowance should be made in system design for increase of filter resistance due to the accumu lation of dust. The ability of air cleaners to clean air is called the efficiency or the arrest^ce, and may be denoted by the symbol E. The efficiency of an air