Document pe9o9vdybOQZX5JV02xdzN8q7

748 CHAPTER 34 1953' Guide 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. .Most dry; types, however, employ a throw-away type of medium which; is held in. permanent metal frames so that the difficulty of cleaning the medium is avoided. Some dry filters are 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 impingement type. Viscous impingement unit filters represent the general type of air cleaner now in use. They have approached standards in size, and their overall: dimensions are small when compared with their, ratings. Throw-away units are often installed in series so that the one in' front, which usually becomes plugged with lint, can be discarded, after which the downstream unit is moved to the front and 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 upkeep are generally lower, whether of the cleanable or the throw-away type. They require more careful attention than the automatic oil type if the resistance is to be maintained within reasonable limits. FILTER INSTALLATION Many air cleaners are available in units of convenient size for handling when installing, cleaning, of replacing. Such units are usually designated as filters or unit filters. A typical unit filter may be 20 in. square and from one to several inches thick, depending on the manufacture and proposed use. In large systems, a number of such units are installed adjacent to each other and collectively called a bank of filters. Air cleaners are commonly installed in the outdoor air intake ducts of buildings, and generally in the recirculating and by-pass air ducts as well. Cleaners are logically placed ahead of heating or cooling coils and other air conditioning equipment in the system to protect them from dust. The character of the dpt arrested by the filters in an air intake duct is likely to be mostly particulate matter of a greasy nature, while lint may pre dominate in dust from within the bulding. The published performance data for all air filters are based on straight through unrestricted air flow. Filters should be installed so that the face 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. The 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 depreciation as'well as expense and convenience of maintenance. The following recommendations apply , to filters (also washers) installed with central fan systems: ,Air Cleaning 749 , 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. Air washers should, whenever possible, be installed between the tempering and. heating coils to protect them from extreme cold in winter. : 7. 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. 8. Filters 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 itself 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 shell charcoals, are sometimes called activated charcoals or activated carbon. These materials can adsorb approximately 50 percent of: their own weight 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 whichdepends 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