Document rpzE42YNv1aOm9X67yxwLrjBJ

620 CHAPTER 36 1965 Guide And Data Book are nevertheless valid and significant because'all were subject to identical standardised procedures. ' ;1 Test methods of the National Bureau of Standards have validity for similar reasons, but because the testing is per formed for government departments and on the bads of confi dential reporting, toe test value is largely restricted to govern* meat personnel and contractors doing government work. / Dioctyl-phtoalate (DOP) smoke, a homogeneous aerosol of 0-3-micron particle size, is .used to test a special class of extremely high efficiency filters, used principally to oontrol particulate matter of extreme toxicity, and for supply syf* terns to areas requiring exceptional cleanliness. Since thejtest aerosol and procedures are standardized, results .on this family of air filters can be compared in a valid manner. { Research on the operating characteristics of-air filters con tinues, and techniques are in use and' under development. However, thpsp are beyond the scope of this chapter. TYPES OF AIR CLEANERS Ail the common types of air cleaners fall into two broad categories: (1) filters proper, wherein toe accumulating dost load results in a pressure drop increase continuously to theend of an operating period determined by maximum'permisable pressure drop, generally 0.5 to l.O.in: of water;'at'the end of this period filters are either replaced or washed for re-use; and (2) the ionizing type electronic cleaner in which deposits are removed at the end of an operating.cycle, usually by washing, - automatically or manually, or by automatically moving, the plates through an oil bath. Pressure'drop is essentially, con^ stent throughout the operating cycle. ` ,,i; .' This classification of air cleaners',is set forth as follows: Air Fitter* > M. jv 1. Viscous Impingement (a) Frame, panel or unit types (b) Moving curtain type 2. Dry Type (Extended Surface, Deep Folded)-' s *' '' (a) Thick beds of fibers, porous sponge, etc. (b) Thin oonwoven mats .. *: 3- Charged Media, Electronic (Dryh .. . Electronic, Air Cleaner, Ioniser-Plate Type 1. Stationary plate type, washable 2. Rotating plate type. 'j Unil or Panel Type. A number of different materials are used as the filtering medium in unit filters of the viscous- impingement type. These include animal hair, vegetable fibers, metallic wools, fibers of glass, and various configura tions of metal screens and mesh material, crimped wire, screen, woven mesh, metal stampings or plates, or sometimes a com bination of these. The arrangement of the medium in this type of filter'actually involves two bade configurations*. 1. The kigh-ecttxtiy type in which the filtering medium is placed on edge to offer a low resistance to the.flow of air. This type of filter design generally does not have any recommended direction of air flow, filters in this category carry a nominal face velocity rating of bom 480 to 520 fpm. Due to the possibility of face loading, these high-velocity viscous-impingement filters are apt to be adversely affected if there are excessive amounts of lint in the air being handled. 2. The progressive pack or proaressvoe density design, in which the medium ispacked more densely on the leaving air side than on the entering air side. Such an arrangement permits the accumu lation of dirt throughout the depth of the media, with the larger particles bong removed near the air entering face of the filter, and selective removal taking place throughout the depth of medium, filters of this design generally are rated at a face ve locity of 300 to 350 fpm. The adhesive or oil used on a viscous-impingement type filter requires c&reful selection. In general, it is good practice to follow the filter manufacturer's recommendations. De sirable characteristics of a suitable adhesive are: (a) a low percentage of volatiles,to prevent excessive evaporation, (b) a viscosity that varies only slightly with normal changes of temperature, (e) ability to inhibit growth of Bacteria and mold spores, (d).high capillarity, or the ability.to wet and retain toe dust particles, .(e) a* high flash point and fire point, and- (0 freedom from odor.'; .Unit types of viscous-nnpingement filters are' usually pro1-' duced in.nominal thicknesses of^l-and 2. inland in several standard wi*** The initial resistance of typical filters of this category usually ranges from 0.08 to 0.15 in- of water. The resistance of any air filter increases with an increase in toe rate of air flow. ,In the case of unit types.of viscous filters,' the initial resistance at other than normal rating can be approxi mated from Equation 3: Ri = fliOTt/Yi)1-7 . . (3) Viscous Impingement Filters Viscous impingement filters are flat, shallow assemblies in which the approach velocity of toe air stream- equals.,the velocity through the filter media. Thus, the 'air':velocity through toe filter media is high (250 to 520 fpm),'The. filter mpri'A is commonly composed of relatively coarse fibers in a bed of high porosity and, in the case of thicker,beds, of-rela* tively high dust-holding capacity. These filters are charac terized by relatively low efficiency oh dust, good'efficiency on lint, and low cost. -\1* Viscous-impingement types of filters derive their nam* from' the fact that toe medium is treated with a viscous substance frequently referred to as oil or as adhesive. In the operation of the filter, the air stream is broken up into many'small columns, and these'individual'columhs of'air'are made to change direction many times during the course of. passage, through the filter. At each change'of direction, momentum carries the larger diist particles in a straight until they' impinge against the elements or fibers of toe nwthjm Wnd are held by the adhesive surface.u-"-u The filter may be of the unit or panel type: (i) the.perinsr neht type which is manually cleaned and replaced, `(2)' the disposable type which is replaced after it has accumulated its dirt load, or (3) the automatic movihg^curtaw type';' Bi " resistance at normal rating, Inches water.... .; resistance at air velocity. Ft, inches water. Fi e> face velocity at normal rating, feet per minute. Ft * face velocity at other than normal rating, feet per minute. Unit filters generally fit into metal'frames which are riveted or bolted together to form a filter bank or section. The rate of loading of the'filter' obviously depends upon the type and concentration, of the dirt in toe air being handled and upon the operating cycle.of the system. For this reason, periods between maintenance cannot be predicted with certainty: Manometers or draft gages are often installed to measure the pressure drop'across the filter bank and thereby indicate when the filter requires servicing. The final allowable pressure drop may vary from one installation to another; but in gen eral, unit filters are serviced when their operating resistance reaches 0.5 in. of water. With certain types of media a decline in efficiency may be the limiting factor in' operating life rather than the increased resistance due to diet load. y The manner of servicing unit filters depends upon then* construction. Disposable types of filters' are constructed if inexpensive materials and are designed to be discarded aftek one period of iise.The cell sides of this design are usually a N combination of cardboard and metal stiffeners. The'perma- . Air Cleaners 621 bent type of unit filters are generally constructed of metal in' order to withstand repeated handlings. Various cleaning methods have been recommended for filtera of this design,' but the widely used procedure involves'washing 'the filter with steam or water (frequently using a detergent) arid dipping or spraying the filter with its recommended ad hesive. The excessive adhesive should be allowed to drain from the filter before it is'put back into service. In order to provide as uniform an operating resistance as posable; it is sometimra advisable to use the rotation system of filter mainte nance where large banks are involved. In this procedure, a portion of the filter bank (20 -- 25 percent) is reconditioned on an established schedule. Certain arrangements of unit types of viscous filtera are designed to have the filtera washed in pirn* Qt^ning is generally accomplished by means of hot water sprayed from a hose or by toe use of fixed nozzles. At toe completion of the washing cycle, the necessary adhesive is applied. Where filtera are washed in place, provision should be made to collect and drain the water so as to prevent leak agefrom the filter housing. Moving-Curtain Viscous Filters. Automatic moving-curtain viscous filtera are available in two main types. In one design the filter marlinm is iimfadlAd on a travelling curtain which intermittently passes through an adhesive reservoir where the gives up its dust load, and at toe same time takes on a new coating of adhesive. The medium used in this design generally consists of metal panels or sections made up of screen wire, stamped plates or baffles, or reinforced mesh which,are attached to a pair of chains, and these in turn are mounted ' on sprockets located in the top and bottom of the filter hous ing. The mpdiA thus forms a continuous curtain which' moves up one face and down the other face of the filter. Arrange ment of media is such that the air is caused to change direction many times in its passage through the curtain and thus pro vides the impingement effect for the collection of dust, particles. -r: Automatic filters of this design generally utilize a timer for periodically moving the filter curtain. This timer,is so set that the curtain makes one complete revolution in 24 to 48 hr. Periodically the precipitated dirt must be removed from the adhesive reservoir. This is generally done by scraping the dirt - into a tray which can conveniently be suspended from the reservoir lip.;The frequency of dirt removal is a variable; but in normal operation this type of filter will require attention approximately once every three months. Where it is desirable to eliminate the maintenance, associated with removing toe precipitated dirt from toe reservoir, the adhesive may.be pumped through ofl clarifiers or can be allowed to circulate through large capacity settling tanka. Automatic viscous-impingement filters are aim available in which toe media is furnished in roll form so that it is fed automatically across toe face of toe filter, while toe dirty media is being rewound into a roll at toe bottom. Most designs of this type havethe media automatically fed across the face of the filter by-means of a timer control or pressure switch control. The timer control appears to be preferable because the pftdia can be replenished in email increments, and this results in a more uniform operating resistance. Filters of this W?gn are generally constructed to befail safe by having a signal that indicates when the roll of madia is about to become exhauBtad- At the same time the drive"motor is de-energized so that the filter cannot run out of media..The normal service require ments involve insertion of a clean roll of media at the top of toe filter and disposing of the loaded dirty roiL Automatic filters of this design are not limited in application to the vertical posi- tjon. Horizontal arrangements are avaiiable'for use'in connec tion with make-up-air units, air conditioning units, etc. Most automatic types of viscous filters are equipped with a fractional horsepower motor operating the drive mechanism through a gear reducer. The operatingperiod is adjustable so that the media travel can be adjusted for changes in dust concentration. In operation, the resistance of an automatic filter will remain approximately constant as long as proper operation is obtained. A resistance of 0.4 to 0.5 in. of water at a face velocity of 500 fpm is typical of this Masa, Dry Type Air Filters The media used in dry types of air filters are usually fabric-, like or blanket-like materials of varying thicknesses. Media of cellulose fibers, bonded glass, wool felt, asbestos, synthetics and'other materials have been used commercially. The medium in filters of this class is frequently supported by. a wire frame in the form of pockets or V-shaped pleats. In 6ther designs the media may be self-supporting.. Pleating of.the media provides a high ratio of madia area to face area., , The efficiency of dry type air filtera is usually higher than that of viscous impingement type filters, and toe variety of media available makes it possible to furnish almost any degree' of eWnirg efficiency desired. Modern dry type filter media and configurations give dust-holding capacity equal to or better than many viscous impingement filtera and efficiencies that are much higher. The high efficiency dry type filtera do not generally need prefilters to obtain good service life.'Ex ceptions to this, however, are the filtera rated at 99.9 percent efficiency on the DOP test aerosol Where highly efficient filtration is required, toe DOP rated dry filter, the electrostatic precipitator, and a second group of dry filters cover the field. The-first group are rated by the 0.3 micron DOP smoke method only and show 99.9+ percent efficiency. The electrostatic' precipitator, usually rated by atmospheric dust spot method of NBS or AFI, show efficien cies from ,90 to 98 percent. The higher efficiency dry filters that are usually rated by either NBS or AFI atmospheric dust spot, show performance up to about 95 percent The DOP rated filtera are made in an extended surface con- figuration of deep, spaced folds of all glass fiber paper-like material, or one made of a glass-asbestos mii. The NBS or AFI rated higher efficiency filtera are usually made of a mat of fine glass fiber in thicknesses from about l to 1 in. The use of the higher efficiency dry filters is expanding. The choice between electrostatic precipitators and-the applicable type of-dry filter depends on toe technical'requirements and an economic study of maintenance and replacement costa - In some designs of dry type air filters, the filter media is replaceable and is held in position in permanent metal cell tides. In other designs the entire cell is disposed of after it has accumulated its dirt load. Mention should be made that the absence of an adhesive on the filter medium does not necessarily qualify it as a dry type air filter. The distinguishing characteristics are generally the high efficiency produced by the smaller size fibers and the fact that the fibers are spaced close together to obtain a dense mat or pad. A low resistance pad consisting of large fibers having no adhesive coating would be a dry filter but would be less effective than a typical viscous-impingement filter. Tim initial resistance of a dry type filter will vary with the choice of medium. A number of commercial designs have an initial resistance of approximately 0.1 in. of water. It is customary to replace the media when a final resistance of 0.5 in. of water is reached. Other types of dry media providing, higher orders of cleaning efficiency will offer a higher resist ance to air flow. The overall operating resistance must be considered in the system d***" tine* that is the resistance against which the fan will be called upon to operate.' - /