Document 2q8dK7pmo6DrM5KGpB7w9xNQ6

; 654 CHAPTER 33 1949 Guide Air Cleaning Devices 655 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 over-all 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. 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 time. 7. Filters installed close to air inlet should be protected from the weather by suit able 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. FILTER INSTALLATION Safety Requirements Many air cleaners are available in units of convenient size for handling An investigation of safety ordinances should be made by the engineer when installing, cleaning or replacing. Such units are usually designated when the installation of an air cleaner of any considerable size is contem as filters or unit filters. A typical unit filter may be 20 in. square and from plated. It is possible that combustible filtering media may not be per one to several inches thick, depending on the manufacture and proposed mitted in accordance with some existing local regulations. Combustion of use. In large systems, a number of such units are installed adjacent to dust and lint on a filtering medium is possible, though the medium itself each other and collectively called a bank of filters. may not bum. Air cleaners are commonly installed in the outdoor air intake ducts of buildings and, often, in the recirculating air ducts as well. Cleaners are ADSORPTION OF VAPORS OTHER THAN WATER logically placed ahead of heating or cooling coils and other air conditioning Many of the foreign gases in the atmosphere are selectively adsorbed by equipment in the system to protect them from dust. The character of the charcoal. Included are many of the organic gases, such as those emanating dust arrested by the filters in an air intake duct is likely to be mostly from animals and people, some of the gaseous constituents of combustion, particulate matter of a greasy nature, while lint may predominate in dust alcohols, ketones, esters, and gaseous products of putrefaction. from within the building. Charcoals differ widely in their adsorptive capacity. Those which have The published performance data for all air filters are based on straight ' marked adsorption characteristics, such as properly prepared coconut shell through unrestricted air flow. Filters should be installed so that the face charcoals, are sometimes called activated charcoals or activated carbon. These area is at right angles to the air flow whenever possible. Eddy currents materials can adsorb approximately ,50 per cent of their own weight of and dead air spaces should be avoided and air should be distributed uni many organic gases at 70 F. The charcoal may be used for a long time by formly over the entire filter surface, using baffles or diffusers if necessary reactivation at high temperatures, under which condition it gives up the 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: 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 1. The filter must be of ample`size for the amount of air it is expected to handle. metal containers which are grouped in frames and set in the air stream. An overload of 10 to 15 per cent is regarded as the maximum allowable. When air The percentage removal of an organic gas, such as carbon tetrachloride, is volume is subject to increase, a larger filter should be installed. 95 per cent or above when placed in intimate contact with the carbon at 2. The filter must be suited to the operating conditions, such as degree of air.clean 70 F. In commercial apparatus there may be a by-pass effect which de liness required, amount of dust in the entering air, type of duty, allowable pressure drop, operating temperatures, and maintenance facilities. pends on the physical arrangement of the charcoal containers. This by passing reduces the percentage removed in the total gas passing through . 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 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 . The following recommendations apply to filters and washers installed, the requirements for contaminant-free air, minus the outdoor air, divided with central fan systems: by the fraction denoting the percentage removal of the gas in question in ' the adsorber bank which is to be used. ! 1. Duct connections to and from the filter should change size or shape gradually to i insure even air distribution over the entire filter area. Adsorbers may be applied to reduce objectionable gases entering through i