Document pkGvw6KzdjEaKbeeGzYz2JO6

528. Chapter 28 , ' 1945 Guide ELECraiC^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 Fig. 1. Diagrammatic Cross-Section of Electrostatic Precipitator ionizing field, and some 5000 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 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 so effective; so the current is usually rectified by means of vacuum tubes. The trans former and tubes are collectively termed the power pack. 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 leaving 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. Air Cleaning Devices 529 Electric predpitatorsYare available- irTtioth" automatic and nonTautomatic 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, 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 impingement filter. An arrangement of sliding contacts maintains the necessary electric circuits. 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 air flow and dividing it by the area of the duct connection to the cleaner in square feet. Cleaners are often rated at a face velocity in the range of 250 to 500 fpm. ' The resistance of an air cleaner to air flow is usually measured in inches water. The resistances of filters when new and clean and when operated at rated capacity are generally available from the manufacturer (see Catalog Data Section). The ability of air cleaners to clean air is called the efficiency or the arrestance, and may be denoted by the symbol E. The efficiency of an air cleaner differs -with the size and nature of the dust on which the cleaner operates. Obviously, large particles and lint are more easily captured than minute particles which are small in all dimensions. The efficiency of an air cleaner, algebraically expressed, is: where Oi = amount of dust per unit volume in uncleaned air. = amount of dust per unit volume in cleaned air. Several methods have been investigated for evaluating Di and D\. The particle count method is no longer used for efficiency evaluations except in rough field measurements or in investigation of filter perfor mance on specific and comparatively large particles such as pollen. Dust particles can be captured on microscope slides by means of one of the various kinds of impingement devices^ The process is useful if an inspec tion and analysis of dust is desired, but particle counting is not sufficiently precise for evaluating the efficiency of a cleaner operating on a hetero geneous dust. The weight method of evaluating efficiency has. found wide utility and was recognized by the American Society of Heating and Ventilating Engineers and incorporated in a code4. For this test, a known weight of a prepared dust is injected into air supplied to the filter and the quantity of dust in the cleaned air is determined by extracting and weighing the dust from a known volume of the cleaned air. Dust extraction from the air is accomplished by drawing the air through a porous crucible or thimble by means of a high vacuum. The dust-spot or blackness test for cleaner efficiency was developed at the National Bureau of Standardsi*. The test consists of drawing samples ,,, Standard Code for Testing and Rating Air Cleaning Devices Used in General Ventilation Work (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 225). `A Test Method for Air Fdters, by Richard S. Dm (A.S.H.V.E. Transactions, Vol. 44. 1938. p. 379).