Document 2JyVj2vzNbnpQ0y5zpvN7J836

;854 CHAPTER 33 1958 Guide area is at right angles to the air flow whenever possible. Eddy currents and dead air spaces should be avoided, and air should be distributed uni formly over the entire filter surface, using baffles or diffusers if necessary. Failure of air filter installations to give satisfactory results can, in most cases, be traced to faulty installation or improper maintenance or both. The most important requirements of a satisfactory and efficiently oper ating air filter installation are: 1. Tlie filter must be of ample size for the amount of air it is expected to handle. An overload of 10 to 15 percent is regarded as the maximum allowable. When air volume is subject to increase, a larger filter should be installed. 2. The filter-must be suited to the operating conditions, such as degree of air cleanliness required,. amount of dust in the entering air, type of duty, allowable pressure drop, operating temperatures, and maintenance facilities. 3. The filter type should be the most economical for the specific application. The first cost of the installation should be balanced against efficiency and depreciation as well as expense and convenience of maintenance. ' The following recommendations apply to filters installed with central fan systems: , 1. Duct connections to and from the filter should change size or shape gradually to insure even air distribution over the entire filter area. 2. Sufficient space should be provided in front as well as behind the filter to make it accessible for inspection and service. A distance of two feet may be regarded as the minimum. 3. Access doors of convenient size should be provided in the sheet metal connec tions leading to and from the filters. 4. All doors on the clean air side should be lined with felt to prevent infiltration of unclean air. All connections and seams of the sheet metal ducts on the clean air side should be as air-tight as possible. 5. Electric lights should be installed in the chamber in front of and behind the air filter. 6. Filters installed close to an air inlet should be protected from the weather by suitable louvers, in front of which a large mesh wire screen should be provided. 7. Filters; other than electronic air cleaners, should have permanent indicators to give a warning when the filter resistance reaches too high a value. 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 contem plated. It is possible that combustible filtering media may not be per mitted in accordance with some existing local regulations. Combustion of dust and lint on a filtering medium is possible, though the medium itsell may not bum. ADSORPTION OF VAPORS OTHER THAN WATER Many of the foreign gases in the atmosphere are selectively adsorbed by charcoal. Included are many of the organic gases, such as those emanating from animals and people, some of the gaseous constituents of combustion, alcohols, ketones, esters, and gaseous products of putrefaction. Charcoals differ widely in their adsorptive capacity. Those which have marked adsorption characteristics, such as properly prepared coconut she charcoals, are sometimes called activated, charcoals or activated carb These materials can adsorb approximately 50 percent of their own weig Air Cleaning 855 of many organic gases at 70 F. The, charcoal may be used for a.long time by reactivation at high temperatures, under which condition it gives up the adsorbed gases. Temperatures of approximately 1000 F are desirable for reactivation. Charcoals for use in air handling systems should be able to stand physical handling, including reactivation, without excessive loss by breakage or dusting. As applied in air handling systems, the charcoal is placed in perforated metal containers which are grouped in frames and set in the air stream. The percentage removal of an organic gas, such as carbon tetrachloride, is 95 percent or above, when placed, in intimate contact with the carbon at 70 F. In commercial apparatus there may be a by-pass effect which depends on the physical arrangement of the charcoal containers. This by-passing reduces the percentage removed in the total gas passing through the adsorber. Resistance to air flow is usually selected within the general range of resistance of impingement filters. The required quantity of recirculated air to be treated is determined by dividing the requirements, for contaminant-free air, minus the outdoor air, by the fraction denoting the percentage removal of the gas in question in the adsorber bank which is to be used. Adsorbers may be applied to reduce objectionable gases entering through the outdoor air inlet. They may also be used to reduce the odors caused by exhausts from processing. Adsorber beds, in all cases, should be pro tected from dust, free oil and grease. PART H--INDUSTRIAL AIR AND GAS CLEANERS Industrial development and growth of industrial areas have had a cumulative effect upon the problem of controlling contaminants. Not only has the atmosphere in many cities become more polluted, but the intensity of pollution at the points of control has increased. Accompanying this increase in pollution there has been a growing consciousness of the need for more effective air cleaning among housewives, store managers, industrialists and legal inspectors, and this has resulted in increasing severity of regulations pertaining to collection of dust and contaminants. Air cleaning for the supply system is usually accomplished by means of some type of air filter. To prevent escape of industrial dust into the at mosphere, some type of collector is required. An industrial air cleanmg installation is designed to perform one or more of the following 6 functions: 1. Prevent property. a nuisance or physical damage to an individual, a plant, or adjacent 2. Prevent re-entry of contaminants to working spaces. 3. Reclaim usable material. 4. Reduce fire, explosion, or other hazards. 5. Permit recirculation of cleaned air to working spaces. 6. Allow utilization of cleaned gases for processes. DEGREE OF AIR CLEANING REQUIRED The amount of material which can be discharged is established by local * sTMte regulations prepared by pollution control, labor or health departTVi8 Standards are established to prevent injury to persons or property. e nee(f for control within the plant is often of prime interest to the