Document byB57LNan0vMjGpDD3gbBMw1

526 Chapter 28 ____________ .__________' 1945 Guide VISCOUS IMPINGEMENT TYPE CLEANERS The medium in a viscous impingement type filter is usually a fiber pack for non-automatic types or a series of metal plates for automatic self cleaning types. In either case, the medium is treated with a viscous substance, often an oiL .or grease, called the adhesive or the satufant, intended to retain dust particles which come in contact with it.; Also, in either case, the arrangement is such that the air stream is broken iip into many small air streams and these are caused to abruptly change direction a number of times in order to throw the dust particles, by momentum, against the adhesive. Several desirable characteristics of an adhesive for air cleaners of this type are: 1. Its surface tension should be such as to produce a homogeneous film or coating on the filter medium. 2. The viscosity should vary only slightly with, normal changes of temperature. 3. it should prevent the development of mold spores and bacteria on the filter medium. 4. The liquid should have high capillarity, or ability to wet and retain the dust at all operating temperatures. 5.. Evaporation should be slight. . 6. It should be fire resistant. 7. It should be odorless. Various fibrous materials have been used-as 'filtering media in unit filters of the viscous impingement type. This includes glass fiber, steel wool, similar wool of non-ferrous metals, wire screen, animal hair, hemp fibers, and other materials. In. such filters, the medium is often packed more densely on the discharge than on the approach-side in order to increase the dust holding capacity. This results in a selective arrestance of dust with the larger particles nearer the approach face. The arrange-' ment also permits some penetration of lint into (but not through) the filter, so. that the amount of lint which can be tolerated on the filter is also increased. Due to plane surface area the viscous impingement type filter, however, may be inferior to some dry types where the air carries a high percentage of lint. The resistance pf air filters obviously increases with the air flow through them. Face velocities of about 300 fpm and resistances in the range from 0.1 to 0.2 in. water, when the device is new and clean, are usual for venti lation system filters. ' Special filters with low resistances are available for use with gravity warm air furnaces and for other, uses where only low pressure is available. The resistance of these.filters increases with dust or lin.t loading and it is the resistance due to this cause which ordinarily necessitates servicing. The rate of loading obviously depends upon the amount as well as the kind of dust in the air and for this reason, periods between servicing cannot be predicted. Manometers are often installed to indicate the pressure drop across filter banks and they serve to indicate when the filter requires cleaning.- The pressure drop tolerated differs between operators and system designs. The resistance of a filter bank can be kept desirably low by periodically servicing some but not all of the units in the ' bank at one time, providing the difference in resistance between the clean and dirty filters is small.' The method of cleaning viscous impingement unit filters differs for different types of filters and kinds of dust. Much dry dust or lint can often be removed by rapping the filter. Air Cleaning Devices... ~ .- - . .. - .527 Throw-away filters are constructed of inexpensive materials and are designed to. be discarded after one use. The frame is frequently a com bination of cardboard and wire. Cleanable types usually have metal frames. Various cleaning methods - have been recommended'including: air jet, water jet, steam jet, washing in kerosene; and dipping in an oil. The latter may serve both to clean the filter and add the necessary adhesive. Automatic Viscous Filters In an automatic air filter, means are provided to remove the dust from the medium mechanically. Automatic filters with moving cloth media have been constructed. The media is supported on rollers and moves slowly and continuously across the air stream and then through some .cleaning mechanism, such as a beater, a vacuum cleaner, or a brushing arrangement. Such filters, however, are not now in wide use, possibly because of mechanical difficulties and the rapid and sometimes permanent increase of resistance when oily matter is present in the air. ; The medium in a typical automatic filter at present consists of a series of specially formed metal plates mounted on a pair of chains. The chains . are mounted on sprockets located at the top and bottom of the filter housing, so that the filtering medium, can be moved as a continuous curtain up one side and down the other side of the sprockets. The arrangement is such that, at the bottom, the medium passes through a bath of special oil which both serves to remove the dirt from the plates and acts as an adhesive when the cleaned plates next pass through the air stream. The plates forming the filtering medium or curtain usually overlap each other and due to their special shape many small air passages are formed between them. These air passages turn abruptly one or more times in order to give the impingement effect. An electrically driven rotating device is usually supplied with an auto matic filter. The device may be set to move the curtain periodically or a special switch, actuated by pressure drop, may be used to govern its motion. Such a switch will cause the gear to move the curtain when the resistance of the filter to air flow becomes excessive and will stop it when the resistance becomes sufficiently low. In operation, the resistance of an automatic filter will remain approxi mately constant as long as proper operation is obtained. A resistance of in- water at a face velocity of 500 fpm is typical of this class. DRY AIR FILTERS As the name implies, adhesives are not used on dry air filters. The media in such filters are usually fabrics or fabric-like materials. Media of: wi felt, cotton batting (both glazed and unglazed), celulose fiber and other materials have been used commercially. . The medium in a filter of this class is usually supported by a wire frame in the form of pockets or V-shaped pleats in order to increase the area exposed to the passage of air. A 2 ft square unit may contain from 15 to 30 sq ft of medium. Dry air filters are likely to have a comparatively high lint-holding capacity on account of the large area of medium used. Wool felt media are troublesome to clean when impregnated with greasy dust and they are too expensive to discard frequently. Both vacuum cleaning and.dry cleaning have been used for reconditioning wool felt filters.