Document 73mYVN4vdRR0qBpxepqozmY6

602 CHAPTER 33 1948> Guide Unit filters constitute the majority of air cleaners now in 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 tKe lint is greasy or if oil 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. The cleaning difficulty is avoided if a throw-away type of medium is used. 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 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. METHODS OF INSTALLATION Many air cleaners are available in units of convenient size for handling when installing, cleaning or 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, often, in the recirculating 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 Air Cleaning Devices 603 dust arrested by the filters in an air intake duct is likely to be mostly particulate matter of a greasy nature, while lint may predominate in dust from within the building. 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 uniformly over the entire filter surface, using baffles or diffusers if neces sary. - 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 operating 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 per cent 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 clean liness 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 and washers 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 connections 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 re sistance 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 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 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