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624 CHAPTER 36 1965 Guide And Data Book than, filters which have higher resstaaces. In most systems, resistance is deliberately added in the form of a perforated plate, pre-filters, or after-filters for .the purpose of obtaining a uniform distribution of air..The resistance, including the baffles or after-filters, is constant and generally ranges from 0.15 to 0.25 in. of water at the usual velocities of 300 to 400 {pm. Such plates or filters, however, cannot compensate for defects in the duct layout ahead of the cleaner. Suitable screens of no coarser than 16 mesh should be installed across ail outdoor air entrances to prevent insects, leaves, bits of paper, and similar material from entering the cleaner. Where lint is present inappreciable quantities, devices for lint,re moval should be installed ahead of the cleaner. Electronic cur cleaners of the ionising type are .efficient, low-pressure-drop devices for removing fine dust and smoke particles. They are available in fixed collector-plate and moving collector-plate types. The fixed collector-plates are often coated with a special oil as.an adhesive. .Cleaning-is generally accomplished by washing the cells in place with hot water from a water hose or by means of fixed or moving nozzle systems. The bottom of the equipment is made water tight and provided with a drain. In one moving-plate type, the grounded elements on which the dirt collects are mounted so as to form a travelling curtain which intermittently-passes through a reservoir containing a fireproof chemical adhesive. The unit is equipped with wipers which remove the collected dirt from the plates. It then settles as sludge in the bottom of the adhesive reservoir from which it is removed periodically. Electronic Air Cleaners of the stationary plate type are sometimes used without any adhesive treatment on the plates. Under such conditions the precipitator becomes an agglomer&tor which is followed downstream by a secondary filter, and the dry agglomerates produced in the precipitator are allowed to blow off and be caught by the downstream.fiiter. .The use of an automatic replaceable-media.filter for catching these agglomerates results in an overall combination which provides a high degree of cleaning efficiency and also the freedom from maintenance associated with an automatic . filter of this design. When handling certain types of atmos pheric dust, the ionising wires and struts may require periodic cleaning. Electrical forces drive all particles to the collecting surface but cannot hold them there. In fact, after a particle touches the collecting surface, the electric force reverses and tends'to pull it off, and the dust is held only by adhesion. It is, there fore, very important to be sure that either the dust is naturally adherent or that the plates are always covered'with adhesive. - The exception is given in the preceding paragraph describing the role of dry plates or agglomerators. - Space Charge r . Dust which passes through an ionizer and is charged but not ' removed carries the electrical charge into the space. If'con tinued on a large scaled this will build up a space c/iarye in ` 'the space which tends to drive this charged dust to the.walls.14 ' Thus, an electronic air cleaner which, for any reason, charges but does not remove the dust, can blacken walla faster than if no cleaning device were used. SBEOION AND MAINTENANCE To evaluate filters and air cleaners properly for a particular application, two factors should be carefully weighed: (1) the degree of air cleanliness required and (2) disposal of ihe dirt ' after it is removed from the air. These' factors affect initial costs, operating costs, and the extent of maintenance that will be required. Savings from reduction in housekeeping expenses, protection of valuable property and equipment, ability-to carry on dust-free manufacturing processes, unproved work ing conditions and even health benefits, should be credited against the cost of installing and operating an adequate system. The capacity and physical sue of the unit required may emphasize the need for low maintenance cost. Operating costs, predicted life, and efficiency are more important thyj first cost, because air cleaning is a continuing process. While electronic air cleaners have a higher first cost, they exhibit very high efficiencies in cleaning atmospheric air, due largely to their ability to remove fine dust which shows no gravitational effect They are equally effective on coolant oil mists from high-speed cutting and grinding machines. Sys tem resistance remains unchanged as dirt is being collected, and the resulting residue is disposed of directly to prepare the equipment for further duty. The charged-media type elec tronic air cleaners are less effective than the ionising type when compared on an individual test basis. However, they incorporate mechanical -filtration with electrostatic precipi tation and thus provide the effectiveness of a dry type air filter should an electrical outage occur. Recent developments in extended surface dry type filters using very fine fiber glass wool webs or other materials, have brought the efficiency of these filters well into the range of the electronic or electrostatic types. First costs of these filters are lower than for electronic types, but higher than for panel types. Operating and maintenance costs are higher than for either panel types or electronic air cleaners. Pressure drop is greater and slowly increases during its useful Hfe- Choice should be based on considerations of both first costs and operating costs as well as the degree of cleaning efficiency. < An advantage of the automatic filters of.both the movingcurtain and replenishable-media type is the small amount of attention which. they require. Such devices are therefore recommended for consideration where large volumes of air are being handled, where the installation is remote, or where reliable and frequent attention to filters cannot be assumed. ' The relatively constant pressure drop of this type of filter is also an advantage. The first cost is substantially higher than that of unit filters, but the saving in maintenance cost tends to offset thia disadvantage. Viscous-impingement unit filters do not have efficiencies as high as can be expected from dry types of unit filters, but their first cost and upkeep are generally low. They require more careful attention than the moving-curtain type if the resistance is to be m*tnfainiH within reasonable limits. If higher efficiencies are required, the dry type filters or elec tronic air cleaners should be given consideration. Table 1 lists some applications of various types of filters classified according to their efficiencies (AF1 test methods). FILTER INSTALLATION Many air cleaners are available in units of convenient size for handling when indadling cleaning, or replacing. Such units are usually designated as unit filters. A typical unit filter may be 20 or 24 in. square and from one to several inches thick, and may be either of the dry or viscous-impinge ment type. In large systems, the frames in which these units 'are installed are bolted or riveted together to form a filter ; bank. Automatic filters are constructed in sections offering two or three choices of widths and generally range in height from 3tol5ftm4to6in. increments. Several sections may be bolted together to form a filter bank. Air cleaners are commonly installed in the outdoor-air in take ducts of buildings, and generally in the recirculating and bypass air ducts as well. Cleaners are logically placed ahead "of heating or.cooling coils and other air-conditioning equip* 625 Air ? 1 in the system to protect them1'from: dust.' The dust , mjetted by the filters in an air-intake duct is.likely to be - Table 1 i.. Typical Applications of Filtere . Classified By Efficiency mostly particulate matter of a greasy nature, while lint may. - efficiency " predominate in dust from within the building. Typical Applications and Umttciiwu The published performance data for all air filters are 1. Wl% Dw* Sptf%., . based ' onstraightAhrongh unrestricted air , flow. Filters (AH) . Atmospheric) &ould 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 necessary. , - Failure of air-filter installations to give satisfactory rer suhs caul'in tnost 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 to' expected to handle. An overload of 10 to 15 percent is regarded, as. the maximum-allowable. When air volume is subject'to in^: crease, a larger filter ahouldbe installed. ' 2,' The fflter must1 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 thespecific application. The first cost of the installation should be balanced against efficiency and depredation 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 ate or shape gradually to insure even air distribution over the. . 35-50 '<10 Window air conditioner*, protec- tion of heat exchanger from lint < accumulations; relatively in effective on smoke, settling dust,. and pollen. , 50-70 , l , -. ` 10:20; v: Window air conditioners, pack-. aged air conditioners, domestic, warm air heating; effective on lint; somewhat effective on com mon ragweed pollen (generally. ' under. 70 percent); relatively, ineffective on smoke and stain-., ing type particles. 70-85 20-25 Air conditioners, domestic heating,central systems; fairly effective on ragweed pollen (generally over 85 percent); effective as; . prefilter for final cleanup filtera for white room, pharmaceutical,. etc.; relatively ineffective on smoke and staining particles. . 85-95 - 25-40. i.. Same as immediately preceding but with greater degree of effec tiveness; recommended mini mum for make-up air for paintspray; somewhat effective in re moving smoke and staining type ' particulate. 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.' * distance of 2 to 3 feet will be required.depending.on the filter aosea. 3. Access doors of convenient size should' be provided, in the sheet-metal connections leading to and from the filters.- . '4:'A1] doors on the dean-air side should be lined with felt to prevent infiltration of unclean air. All connections`and seams of the sheet-metal ducts on the dean-air side should be as airtight as possible.,Any filter bank must be calked to prevent bypass of untiltered air. This is most important when high effidency filters are used. 5. Electric-lights should be installed in the chamber in-front of and behind the air filter. - '' 1 ` 6: Filters installed dose to-an dr inlet should be protected from the-weather by suitable louvers, in front of which a large mesh wire screen should be provided. >95 >95 .35-60 60-85 .. Effective on finer airborne dust and pollen; reduce smudge and. stain materially; slightly effec tive on fume and smoke; ineneo-* tive on tobacco smoke. Used in, KtjilHmg recirculated and fresh tf systems; some types used in domestic heating and air conditioning; used as prefilters to high efficiency types. Effective on all pollens, majority - of particles causing smudge apd, stain, fume, coal and oil smoke;, partially effective on tobacco ' fnTK,lr<> Used same as for 35-60. percent range but better pro- ; tection. Some types reasonably ' effective on bacteria. 7. Filters^' 'other than electronic .air cleaners should have permanent indicators to jjpve a warning when' the filter resist-^ 8. Electronic air deaners should .'have`mi indicator or alarm system to indicate when high voltage is off or shorted out. ^ Safety Requirements Safety ordinances should be investigated when'-the instal lation of an air cleaner of any.considerable size is contem plated. Combustible filtering media may cot be-permitted in, accordance with some existing local regulations Combub*' tioa'of dust and lint on a filtering medium is possible, though , the medium itself may not burn. ; . ADSORPTION OF VAPORS OTHER THAN WATER Foreign gases in the atmosphere selectively adsorbed by charcoal include many of the organic gases, some of the gaseous constituents of combustion, alcohols, ketones, and gaseous products of putrefaction. Charcoals differ widely in adsorptive capacity. Those which v TOO" (Approx.) 100 (Approx.) 99+ Very effective on particles cawing smudge and stain, coal and oil smoke and fume. Highly effeo* ' tive on bacteria. Uses the same;as for 60-85 percent group but' better performance. Suitable for hospital surgeries, pharmsoeuti' cal preparation areas, and con-y trolled areas.1* Quite effective on tobacco smoke. . This group includes, among others,'. the DOP rated filters and some . electronic types- Electronic, types'used on general ventila tion in special applicationsrated types provide the oem protection available again* bacteria, radioactive dusts, nox ious dusts, all smoke and fume; used as clean-up filter with one or two stages of prefilters; suit able for all classes of clean rooms.UJ* have marked adsorption characteristics, such as properly