Document jB43KJk0Da0vbZ3aQXVd3ZymQ

336 CHAPTER 24 1960 Guide on chains, and are alternately dipped in oil and exposed to the air stream. Charged-Media Electronic Air Cleaners The charged-media type air cleaner consists of a dielectric filtering medium, usually arranged in pleats as in typical dry filters. No ionization means are employed. The dielectric fil tering medium may consist of glass fiber mat, cellulose mat, Effective face area is further defined as that area of the filter face inride the enclosing frame, flanges, or seals, but not corrected for any area occupied by any bracing, and re taining or strengthening grid members, located within the en closing frame. The rated capacity of general ventilation filters is often based on face velocities in the range of 250 to 520 fpm. Re sistance to air flow, measured in inches of water column, at rated capacity, is generally available from the manufacturer (see Catalog Data Section). A suitable allowance must be made in the system design for increase in resistance as the filter becomes loaded with dust. Tig. 3 indicates the range of resistance to air flow found on tests of four unit filters. Type A is a dense pack used in bacterium control; Type B is a medium pack used for general ventilation work; Type C is a low-resistance unit, for use where low resistance is the im portant factor and maximum cleaning efficiencies are not es sential; and Type D is a high-velocity viscous-impingement filter. The effectiveness of air cleaners in removing a given dust from the air is called efficiency or arrestance. If the efficiency is denoted by B, then algebraically E is given by: Medio Type Electronic Air Cleaner or other similar material, and is supported on or in contact with a gridwork consisting of alternately grounded and charged members, the latter usually being held at a potential of 12,000 volts d-c. An intense and non-uniform electrostatic field is thus created through the dielectric medium. Airborne particles approaching the field are polarized and drawn to ward filaments or fibers of the media. The general arrange ment illustrating this type of filter is shown in Fig. 2. The precipitator of this type offers resistance to air flow, when clean, on the order of 0.10 in. of water at 250 fpm veloc ity and, unlike the ionizing type, the resistance of the chargedmedia type electronic cleaner rises as dust is accumulated on the media. Because of these characteristics the filter tends to equalize the air distribution over the face of the filter. lake typical replaceable-media mechanical filters, the charged-me dia precipitator is serviced by replacing the filtering medium. The dielectric properties of the media are impaired when the relative humidity exceeds 70 percent. AIR CLEANER PERFORMANCE AND TESTING Atmospheric air cleaners are generally rated in terms of the three more critical factors in performance; namely, (1) re sistance vs air flow, (2) efficiency or arrestance vs dust load at design air flow, and, (3) resistance vs dust load at design air flow. Air flow is expressed in cubic feet per minute and is there fore a function of face velocity and filter size. Face velocity is defined as the average velocity of the air entering the effec tive face area of the cleaner and may be expressed by the equation: V = Q/A (1) where V - face velocity, feet per minute. Q air flow, cubic feet per minute. A ~ effective face area, square feet. where Di = amount of dust per unit volume in uncleaned air. Dt = amount of dust per unit volume in cleaned air. A number of procedures for evaluating air cleaner effi ciency have been proposed and are in use. These procedures differ principally in the type of test dust that is used and in the method of measuring the dust content of the entering and leaving air. The numerical value of the arrestance or efficiency of an air cleaner may be different lor different meth ods of test even when the same test dust is used, and is gen erally different for different test dusts using the same test method. Caution to be exercised in interpreting published arrestance data, since the test efficiency may be different from that obtained in an actual installation, or in the space being supplied with air. The two most common methods for evaluating efficiency are: (1) the weight method, in which the weight concentra tion of dust in the air entering and leaving the filter is meas ured by some suitable means, and (2) the dust spot test in which the dust content of the entering and leaving air is measured in terms of its ability to stain filter paper. The particle count method is not used for efficiency evaluation, except in investigation of filter performance on specific par- Air Cleaning 337 tides such as pollen, or on certain industrial dusts harmful to health. Dust particles can be captured on microscope slides by means of one of the various kinds of impingement devices. The process is useful if inspection and analysis of dust are de sired, but particle counting is not sufficiently precise for eval uating the efficiency of a cleaner operating on a heterogene ous dust. Among the weight methods that have been used are the Air Filter Institute Code* and modifications of the former ASHVE Code* By the latter test method, a known weight of a prepared dust is injected into air supplied to the filter, and the quantity of dust in the cleaned air is determined by extracting and weighing the dust from a known volume of cleaned air. Dust extraction from the air is accomplished by drawing the air through a porous crucible or thimble by means of a high vacuum. The test dust specified in the ASHVE Code is a mixture of coal ash and carbon black. The Air Filter Institute Code, Section I, specifies a test dust as follows: "The test dust mixture consists of 72 percent standardized air cleaner test dust, fine (Arizona road dust, fine); 25 per cent K-l carbon black; and 3 percent No. 7 cotton listers (percentages by weight)." As in the ASHVE Code, the entering concentration (DJ is determined by feeding a known weight of the prepared test dust. The concentration of dust in the air leaving the filter (Dt) Is determined by passing the entire air flow through a high-efficiency filtering blanket known as an absolute filter, the g'Q in weight of which is measured. Test data and results are presented on standardized forms made available by the Air Filter Institute. The dust-spot or discoloration method of test for aircleaner efficiency was developed at the National Bureau of Standards.* In effect, this method is based on a comparison of the volumes of air before and after cleaning by the filter that must be drawn through unit areas of suitable filter paper to yield dust spots of equal opacity to transmitted light as de termined by means of a photometer. For example, if equal opacities are obtained when the amount of air sampled, per unit area of dust spot, is four times as great for the down stream paper as for the upstream paper, the efficiency of the filter is considered to be 75 percent, tinoe the dustiness of the cleaned air, as measured by change in opacity, is one-fourth that of the uncleaned air. To.obtain equal opacities, samples may be drawn at equal rates for equal times through un equal spot areas', or at equal rates through equal areas for unequal times, as best suite the circumstances. Tests may be made with atmospheric air, or with an artificial dust dispersed in the air stream. Dust-holding capacity is defined as the amount of dust which a filter can retain and have a resistance less than some arbitrary value. The term applies only to fixed-type air cleaners. Under the AF1* and the former ASHVE7 procedures dust-holding capacity is defined as the product of the weight of dust reaching the filter and the average efficiency at a spec ified resistance and air flow. Dust loading and arrestance determinations are made simultaneously, arrestance and re sistance being determined for each increment of feed with the results plotted against weight of dust fed or against accumu lated load in the filter. Dirt-loading measurements are *ndft at the National Bu reau of Standards* by feeding p mixture of 4 percent cotton lintgrs and 96 percent Cottrell precipitate (by weight) to the test filter. The two ingredients are separately dispersed into the air stream, using two aspirating-type injectors. The meas urements show the relationship between the pressure drop and the weight of dirt mixture received at the filter. The curves in Fig. 4 illustrate the difference in the dirt-loading characteristics of two filters, one a viscous-impingement type and the other a dry filter with an extended-surface cellulosefiber medium, with dust and with lint. SaECTlON AND MAINTENANCE To evaluate filters and air cleaners properly for a particu lar application, two factors should be carefully weighed: (1) tire degree of air cleanliness required and (2) disposal of the dirt after it is removed from the air. These factors affect ini tial costs, operating costs, and the extent of maintenance that will be required. Savings that accrue through reduction in housekeeping expenses, protection of valuable property and equipment, ability to carry on dust-free manufacturing proc esses, improved working conditions, and even health benefits should be credited against the cost of installing and operating an adequate system. The capacity and physical size of the unit required may emphasize the need for self-cleaning fea tures. Operating costs, predicted life, and efficiency are more important than first cost, because air cleaning is a continu ing -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 cleaners are less effective than the ionizing type, but are appreciably better than mechanical filters in the matter of particle collection. The advantage of the moving-curtain impingement type filter consists in the wmaJl amount of attention which it requires. Such devices are therefore to be recommended where labor is scarce, or where reliable and frequent atten tion to filters cannot be assumed. The constant pressure drop-of this type of filter is an advantage. The first cost is substantially greater than that of unit filters, and the dust arrestance may not be any higher. Unit filters constitute the majority of air cleaners now in use, and some choice is possible between the types avail-