Document dnQGEx87V1yxrOVkab0oy85RR

American Society of Heating and Ventilating Engineers Guide, 1929 Table 1. Heat Required to Raise 1000 Cu. Ft. of Air from various Outside Entering Wet Bulb Temperatures to various Dew Point Temperatures xrn TorreSPONDING RELATIVE HUMIDITIES AT 70 DeG. FaHR. See Marks' Engineers Handbook. Wet Bulb Temperature op Entering Air. Deg. Fahr. -10 0 10 20 30 40 50 60 Relative Humidity. Per Cent at 70 Deo. Fahr. (and Dew Point. Deg. Fahr.) 30% (37.25) 1194 984 750 510 300 40% <44.5) 1452 1246 1025. 779 496 178 50% (50.5) 1653 1447 1228 983 700 384 60% (55.3) I860 1663 1445 1200 920 603 220 70% (59.6) 2044 1840 1621 1377 1097 783 394 80%. (63.5) 2245 2039 1822 1581 1300 987 619 181 raise the temperature of 1000 cu. ft. of air pier minute between the limits specified. The heat required for heating the air is given by the formula H =1000 it " it where H = B.t.u. required to raise >000 cu. ft. of air from a temperature of /, to a temperature of t2 deg. fahr. AIR FITTERS Air filters are distinguished from air washers in that they clean the air without the use of water and without the addition of a water spray. They are. usually of the following types: 1. The .viscous filter, depending upon the dirt impinging on a surface covered with a viscous fluid or oil. 2. The dry filter, which removes the dirt from the air by straining or filtering it through cloth, felt or paper screens, the openings in which are too small to allow the passage of dirt, or which removes the dirt by passing the air through tortuous passages: VISCOUS AIR FILTERS All of the so-called viscous type , air filters operate on The principle of adhesive impingement. Investigation has shown that the dirt and dust in air, including, the soot and carbons--the great destructive elements in modern ventilation -work--are trapped and retained by successive impingements on oil coated surfaces. This is not a new idea--for years roads and floors, have been oiled to keep the dust from flying about. Considerable ingenuity, however, has been shown in the various arrange ments of metallic surfaces which are formed into filter units and made available for air cleaning work. While. the: arrangement of filtering media and the kind of materials used are almost unlimited, there are certain rather definite requirements 436 Chapter XXIX--Air Cleaners for a practical commercial filter, and these have limited the possible constructions. To fulfill the essential requirements of clean air an air filter must: 1. Be efficient in dirt removal. 2. Interpose low resistance to air flow. 3. Have a large dust holding capacity. 4. Be easy to clean and to handle. There are other factors of importance, of course, among them weight, strength and permanency. In order to secure maximum efficiency, it is necessary to divide the air into innumerable fine streams; the more intimately and frequently the air is brought into contact with the viscous-coated media the better the Fig. 1. Resistance to Air-Flow of a Typical Unit Air Filter cleaning will be. Theoretically seven impingements are sufficient; more will give better service. As the dirt and dust are leached out of the air and collected on the adhesive coated surfaces, additional supplies of binding liquid are required in order to bind further layers of dirt. For this reason filters are usually designed to. hold somewhere within the unit additional supplies of the binding liquid, capable of distribution by capillary attraction, or by some mechanical means. Investigations in this country and abroad demonstrate that the first impingement of dust-laden air on a viscous coated surface removes 60 per cent of the dust, the next impingement takes 60 per cent of what then remains--that is, 24 per cent--the next impingement removes 9.6 per cent, etc. The principle of progressively packed media is embodied to a greater or less extent in all unit viscous filters. This arrangement gives relatively large spaces for the collection of dirt in the front part of the filter, where the bulk of the solids are taken out, without undue increase in resistance. At the back of the filter the openings are smaller and finer dust particles are removed. The charging'liquid used with these filters should be odorless, fire-proof, and germicidal in its action. 437