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698 CHAPTER 33 1950 Guide Engineebs and incorporated in a code.4 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 dusl-spot or blackness test for cleaner efficiency was developed at the National Bureau of Standards.6 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 and of the volumes of the air samples are then indicators of the filter effectiveness. A special photometer 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 I '' Air Cleaning 699 airi cleaiier 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-impinge ment type and the other a dry filter with a cellulose 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, p.c. . . . . / SELECTION AND MAINTENANCE For removing fine dust or liquid particles which show no gravitational settling tendency, electrical precipitators are highly effective in air cleaning Fro. 2. Resistance to Air Flow op Typical Unit Air Filters tests of electrostatic air cleaners, no dust is 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 in dust spots of equal area the downstream spot shall not be any. dirtier than the upstream spot when 10 times as much air is drawn ; through it as is drawn 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 atmospheric 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.4 " Curves are obtained during such tests to show the relation between dust load and resistance. '' ' Fig. 2 indicates the general range of. resistance !to air flow through unit' air filters. Type A is a dense pack used in bacterium control; Type B is' a 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 applications. The electrostatic equipment is comparatively expensive to install and.maintain. However, in many applications where the cleanest air possible is needed,'this expense is justified by the results obtained with properly installed and operated electrostatic air filters. . The advantage, of the automatic impingement type filter consists in the :smali'amount5ofAttention which it- requires. Such devices are therefore to be recommended where-labor, is scarce, or where reliable and frequent . attention to filters cannot be assumed. This type of equipment is not any ; better in dust.arrestance than some unit filters, and it ranks next to precipi tators in first cost. ..' ' -Unit filters constitute the majority of air cleaners now in use,?and some choice is possible between the types available. Where lint ini an eminently dry state predominates, a dry filter obviously may be preferable to other : types because of its .lint-holding capacity. If the . lint is greasy or if oil. vapor exists in the air, the dry filter, if it is of the cleanable type, may;be troublesome, since grease tends to make it difficult to clean. The cleaning difficulty, is avoided, if a throw-away Type of medium is used- Some'dry filters are capable of high efficiencies, compared to other unit filters on fine