Document 2Ldnx8ovgqXk3zev97By2GX6

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1943 and acts as an adhesive when the cleaned plates next pass through the air stream. The plates forming the filtering medium .or curtain usually overlap each other and due to their special-shape many small air passages are formed between them: These air passages turn abruptly one or more times in order to give the impingement effect. An electrically driven rotating device is usually supplied with an auto matic filter. The device may be set to move the curtain periodically or a special switch, actuated by pressure drop, may be used to govern its motion. Such a switch will cause the gear to move the Curtain when the resistance of the filter to air flow becomes excessive and will stop it when the resistance becomes sufficiently low. In operation, the resistance of an automatic filter will remain approxi mately constant as long as proper operation is obtained. A resistance of % in. W. G. at a face velocity of 500 fpm is typical of this class. DRY AIR FILTERS As the name implies, adhesives are not used, on dry. air filters. The media in such filters are usually fabrics or fabric-like materials.. Media of wool felt, cotton batting (both glared and unglazed), celulose fiber and other materials have been used commercially. The medium in a filter of this class is usually supported by a wire frame in the form of pockets or V-shaped pleats in order to increase the area exposed to the passage of air. A 2 ft square unit may contain from, 15 to 30 sq ft of medium. Dry air filters are likely to have a comparatively high lint-holding capacity on account of the large area of medium used. Wool felt media are troublesome to clean when impregnated with greasy dust and they are too expensive to discard frequently. Both vacuum cleaning and dry cleaning have been used for reconditioning wool felt filters. ELECTRIC PRECIPITATORS . The fact that a particle exposed to an.electric field will assume a charge and migrate toward one of the electrodes has been utilized for some years in boiler plants as a means of smoke abatement. More recently, means were developed whereby the phenomenon could be employed in air cleaning in connection with air conditioning without generating ozone in intolerable quantities. The air stream in a precipitator passes first through a relatively high-tension electric field, known as the ionizing field and then through a secondary field where the precipitation of the dust occurs. The arrangement is as shown in Fig. 1. - In a typical case, a potential of 12,000 volts may be used to create the ionizing field, and some 500.0 volts between the plates upon which the precipitation of dust occurs. These voltages, which are capable of shock to personnel similar to that of a spark plug, necessitate some safety measures. A typical arrangement provides means: for automatically making the unit inoperative when a door to the precipitator is opened. To resume operation the procedure necessitates closing the door-and turning an electric switch, the latter of which should be located at a reasonable distance from the equipment. The voltages necessary for the 568 CHAPTER 29. AIR CLEANING DEVICES operation of the precipitator are usually obtained from'an alternating current building service line by means of a step-up transformer. Precipi tation with alternating current is possible but is not nearly as effective; so the current is -usually rectified by means of vacuum tubes. The trans former and tubes are collectively termed the power, pack. Electric precipitators are available in both automatic and, non-auto matic types: The plates of non-automatic precipitators are commonly coated with a light oil as an adhesive. Cleaning is accomplished with a water hose and, for this reason, the bottom of the equipment is made water tight and provided with a-drain. In One automatic type,1 precipita tion units are mounted on chains and are alternately dipped in oil and exposed to the air stream with an action similar to that of an automatic charged particle . Fig. 1. Diagrammatic Cross-Section of Electrostatic Precipitator impingement filter. An arrangement of sliding contacts maintains the necessary electric circuits. - Only a very .small amount of electric energy is necessary to operate an electric.precipitator and the resistance to air flow through the,device is practically negligible. Some care, is necessary, in arranging the duct approaches on the entering and leaying sides of precipitators to assure that the air flow is distributed uniformly over the cross-sectional area. The efficiency, of the precipitator is sensitive to-air velocity and the device itself has much less tendency to rectify, the air stream than filters, which have much higher resistances. . . PERFORMANCE AND TESTING The rating of an air cleaner is the air flow for which it is'designed expressed' in cubic feet per'minute. Face velocity is defined as the average velocity of the air entering the cleaner, and it is determined by taking the airflow and dividing it by the area of the duct connection to the cleaner -in square, feet. Gleaners-are often rated at a face velocity in-the range 569