Document eoGO6QxdR45bB2w0Kydv2oJM

HEATING VENTILATING AIR CONDITIONING CUIDE 1943 from 250 to 500 fpm. The resistance of an air cleaner to air flow is usually measured'in inches water gage. The resistance 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 E= Di (1) Di = amount of dust per unit volume in uncleaned air. Di = amount of dust per unit volume in cleaned air. Several methods have been' investigated for evaluating Di and Z>2. 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 ari 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. i; 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 Standards?. The test consists of drawing samples of cleaned air and of uncleaned air through filter papers simultaneously. The ratio of the areas of paper through Which the air samples are drawn and the ratio of the amount of air drawn through the papers are adjusted during successive trials to yield spots of approximately equal blackness on the papers. The ratios of the areas arid of the volumes of the air samples are then indicators of the filters effectiveness. A special photo meter is provided for comparing the blackness or opacity of the papers by transmitted light. For tests of ordinary air filters by this method, a dust is injected into the air stream. The'dust'consists of precipitated smoke particles from a Cottrell precipitator used in a local power plant for smoke abatement. For tests of electrostatic air cleaners, no dust is `A.S-H.V.E. 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 Filters, by Richard S. Dill (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 379). 570 CHAPTER 29. AIR CLEANING DEVICES added to the air. Tests are commonly made with the dust existing in the air at the location of the installation on a clear day. Some specifications for this, type cleaner have required that dust spots of equal area shall1 be taken, and that the downstream spot shall not be any dirtier than the upstream spot when 10 times as much air is drawn through .it as is'drawri through the upstream spot. When this condition is met the cleaner is said to have an efficiency or arrestance of 90 per cent or better on atmos pheric. air. Dust-holding capacity is defined as the amount of dust which a filter can retain and have a resistance less than some arbitrary value. The term applies only to non-automatic air cleaners. Determination. of dust holding capacity is an. objective of each test under the A.S.H.V.E. Standard Code*. Curves are obtained during such-tests to show the relation between dust load and resistance. Typical curves are shown in Fig. 2. Type A is a dense pack used in bacterium control; Type B is a Fig. 2. Resistance to Air Flow of Typical Unit Air Filters medium pack used for general ventilation work; and Type C is a low resistance unit) for use where low resistance is the important factor and maximum cleaning efficiencies are not essential. At the National Bureau of Standards two injectors are provided on the air cleaner testing apparatus. One injector is used to contaminate the air stream'with Cottrell precipitate,-' previously described. This dust is used to make - both efficiency determination and dust-holding capacity tests. The other injector contaminates the air stream with cotton linters with which lint-holding capacity-tests are made. The curves in Fig. 3 illustrate the difference in the characteristics of two. filters, one a viscous- impingement type and the other a dry filter with a celulose fiber medium. The two injectors, can be operated either separately or simultaneously. A total dust deposit of 4 per cent cotton linters and 96 per cent Cottrell precipitate gives a deposit on a filter closely resembling those that occur in Washington, D. C. SELECTION AND MAINTENANCE If effectiveness in arresting dust were always the primary consideration, an electrostatic cleaner might be used for all air cleaner Installations'. 8Loc. Cit. Note 4. 571