Document V3Rkz61N28mXvQDmD9EmN0Qvo

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 Where the dust load is heavy, a filter bank may be installed ahead of the precipitator. At the present time electrostatic equipment is comparatively expensive and bulky and is therefore only used when the expense is justified by the need for the cleanest air possible. . The'advantage of the automatic impingement type filter consists in the small amount of attention 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 precipitators in first cost. Unit filters constitute .the majority of, air cleaners now is use, and some choice is possible between the types available. Where lint in an eminently dry state predominates, a dry filter obviously may-be preferable to other types on account of its lint-holding capacity. If the lint is greasy or if a *' a X a CE . CHAPTER 29. AIR CLEAN INC DEVICES upkeep are generally lower, whether of the cleanable or the throw-away type. The viscous impingement unit filter requires more careful attention than the automatic oil type if the resistance is- to be maintained- within reasonable limits. Safety Requirements An investigation of safety ordinances should be made by the engineer when the installation of an air cleaner of any considerable size is con.templated. It is possible that a combustible filtering media may not be permitted in accordance with some existing local regulations. Combus tion of dust and lint on a filtering medium is possible, though the medium itself may not burn. , , ,Fig. 3. Dust and Lint Holding Opacity of.Two Filters ; oil -vapor exists in'the air, the dry filter may be' troublesome,-since grease tendsito plug such-filters and makes: them difficult to clean, if--they are of . the cleanable type. The cleaning difficulty is avoided if a throw-away type ofi.medium is used. - Some dry filtersfare: capable of high efficiencies, compared to other unit filters on fine, particles; but their dust-holding capacity for such, dust may be inferior to that of the viscous impinge ment.type.. ' ... ' Viscous impingement unit filters represent the general 'type'of air cleaner now in use: They have approached standards in size and their over-all dimensions are small when compared with their ratings'.'-' : Throw-away units are often installed in seriqs so .that the one in front, which usually becomes plugged with lint; can be discarded. The one located downstream is .then moved to the front and is replaced by a new unit. -...... Viscous.impingement unit filters do not have efficiencies as high as can be expected with some other types of unit filters, but their first cost and ,572 Fig. 4. Resistance Curves for 6-14 Mesh Activated Carbon ODOR ADSORPTION Activated carbon is sometimes used to remove malodorous substances from air in connection with air conditioning.. Cocoanut shell carbon is usually employed for the purpose. This is a hard, granular and very porous substance with the power of adsorbing vapors and minute droplets from air. It will under favorable conditions adsorb some substances to the extent of 60- per cent of its own weight. The carbon is spread from % to % in. thick in. trays through which air is circulated at a rate of from 30 to 50 fpm. The resistance of the bed varies with the velocity as shown in Fig. 4. Eventually, the carbon approaches saturation and must be reactivated or revivified by heating to drive out the captured impurities. For. this 573