Document XOE52oqJ1DdpMZmeqbQKNNkkw

342 CHAPTER 24 1960 Guide Table 3 ... .Characteristics of Fabric Filters Kind fflo*ocy Portiefe ciza rang* in Accu tane* ^Storing Vt&dty Maintenance >!0 10 fa I <1 Clothcollector High (shaken or rapped)* High Mod erate to high Me- 1-6 Medium to high (peri odic) Cloth collect High ors (reverse air or jet cleaning)6 High Mod erate to high Me dium to high 10-35 Medium to low Absolute type* High High High Me 3-14 None (see dium remarks)* * Bp*ee requirements lure- Requires eebednled efekninsb requirements ""lil'11TM Cleeaiai eaustsat during operating pyda Addedborsepcrtrtr used far werae jet Action. 8iBe0 requirements ----h Requires replacement of urut wbea prednernuaearesietaaee b attained. Ordinary leaning ant possible. layer for their effectiveness. Their basic or initial efficiency depends upon the type of fabric employed; felted media such as wool gives higher initial removal than woven fabrics. The increase in resistance to air flow, resulting from the q/M'-Hmnlftterf dust layer, is related to rate of flow, con centration, and characteristics of the contaminant. Re moval of the collected material is accomplished through periodic chairing by single mechanisms or combinations of them employing rapping, or through reversal of air flow, or by means of high-velocity reverse air jets. With shaking dr rapping, air flow is not constant and filtering velocities are usually held under 6 fpm. With reverse jets or reverse air flow, substantially constant air flow is maintained and filtering velocities are generally higher (up to 35 fpm). Filtering velocities in all fabric collectors are highly de pendent upon the aerosol, fume, or dust being collected. The ultra or absolute type of filter" may consist of pleated relliitose asbestos paper, deep sand beds, composite glass wool layers, compressed glass fiber in the form of paper, or batts and resin wool. The use of this type of filter is limited to concentrations in the range of outdoor air where very high efficiencies, greater than 99.95 percent are required. Fabric filters are not suitable for high temperatures, above 300 F, acid mists or vapors, or mixed contaminants such as wet corrosive gases and solid or liquid particles. Cotton fabrics are limited to 180 F, whereas wool is limited to temperatures under 200 F. A wide variety of natural and synthetic filter media are available for various temperature and corrosion conditions but caution must be exercised relative to rupture, high temperatures, and corrosive gases. When properly selected, efficiencies are often high enough to permit recirculation of air except, possibly, when han dling toxic contaminants. On the basis of their important characteristics, fabric filters may be classified according to Table 3. ELECTROSTATIC PRECIPITATORS Electrostatic precipitators for industrial air and gas clean ing differ materially from the low-voltage designs described in Part I of this chapter, although the principles of opera tion are similar. For industrial concentrations, it is obvious that more severe demands are made upon methods of cleaning the collector, disposal of collected material, and servicing practices. Low-voltage cleaners for industrial loadings to date do not have sufficient inherent dust-holding capacity. One exception in the field of exhaust systems is that, of the oil-mist collector, which functions satisfactorily on a liquid aerosol with the conventional low-voltage type (2 stage) electrostatic precipitator. Industrial precipitators employ an assembly of parallel collector electrodes of various constructions, including cor rugated plate, rod curtains, or perforated plate. Air flow is usually horizontal, although special construction may permit vertical air flow. High-voltage collectors were made in the form of vertical pipes, and are used for high oper ating pressures and for wt-coUector designs where water continuously flows downward inside the pipe walls of the collector electrode. The negative discharge electrodes or rods are accurately centered between the usual 9 in. col lector electrode spacing, the latter being of positive charge. Precipitation occurs in a single stage wherein ionization and collection are carried on simultaneously throughout the unit, and depend on high potentials of 25,000 to 75,000 volts. The high-voltage direct current is supplied by either mechanical or electronic rectifiers. High efficiencies are obtained by allowing suitable time for contact in the collector zone, and by proper ratio of air flow velocity to that of transverse velocity of the negatively charged particles toward the positively charged collector plates. Air velocities vary from 240 fpm to 480 fpm with a constant pressure drop of less than Vi in. water. Since the mmcimnm efficiency is obtained with the finer particles at low concentrations, and the precipitation is not effective for large particles, inertial separators are frequently used in series with these units. Attention should be directed to the prohibition of air recirculation to occupied spaces. High-voltage cleaning equipment produces ozone and nitrogen oxides in excessive quantities. The major source of maintenance usually will be in conjunction with the high-voltage rectifying equip ment rather than the collector. Although usually used for elevated temperature work, unless the gases are precon ditioned, temperatures in excess of 700 F should be avoided. Collector electrode cleaning is accomplished by a rapping device, either electrical or pneumatic, without interrupting operation. The dry plate surfaces release the dust into hoppers below the plates. Some wet cleaning methods are also used. Electronic precipitators require large spaces, maintenance may vary widely, and initial costs are high. Efficiency is a function of the aerosol encountered and usually ranges above 90 percent. ABSORBERS, ADSORBERS, AND COMBUSTION DEVICES One of the important problems in industrial air and gas cleaning is the removal of soluble, insoluble, or combustible gases and vapors from fluid streams before their discharge to the atmosphere. These contaminants may create prob lems which are distinct from the particulate removal dis cussed previously. It has been pointed out that scrubbers and other wet collectors may remove gases and particulate matter from contaminated air streams. However, the units Air Cleaning 343 described are intended primarily for particulate removal with the exception of the spray or packed towers or wet cell devices. Absorption devices are intended primarily for the re moval of readily soluble gases which can be removed by simple absorbing agents such as water or alkali; for ex ample, the removal of hydrochloric acid gas by a caustic spray. Adsorption apparatus is intended primarily for removal of organic vapors in either high concentration (solvent re covery) or low concentration (odor removal). The first process is usually carried on as a matter of economic re covery. In the second process, the concentration of the contaminant must be low enough so that the adsorbent will have a reasonable life before replacement or reactivation. Combustion, either total or catalytic, may be used to destroy organic compounds, either gaseous or particulate, which create odor problems. The use of catalysts to reduce the temperature needed for combustion is comparatively recent in this field. In some instances the heat of combus tion may be recovered for utilization. The temperature necessary for destruction of compounds varies with the nature of the contaminant. However, partial combustion may alter the compound to render it innocuous. The tem perature required for complete destruction is usually above 1100 F. Catalysts may only require heating to 500 F or may operate without heating if the gas temperature is above 500 F. Absorbers These may consist of spray chambers, packed towers, or wet-cell washers through which an absorbing agent is recirculated. The gases collected may be converted to in soluble salts or usable acids and other compounds. The performance of these devices depends upon several factors' such as the solubility of the gas, its vapor pressure, its rate of reaction with the absorbent, the velocity through the collector, and area of the absorbing surface either as spray droplets or wetted media. These are generally de signed for the specific purpose intended. They usually are intended to collect over 95 percent of the contaminant. Resistance depends upon the particular design and ranges from 1 to 10 in. of water in most installations. Their resistance during their operating interval is constant unless serious plugging of packing occurs. Spray units are seldom affected by plugging. Packed absorption devices require a precleaner to remove particulates if long trouble-free serv ice is desired. Maintenance of spray nozzles is a function of the degree of atomization used and of conditions encoun tered, such as purity of spray liquids. Protection against corrosion and freezing may be neces sary and disposal of waste liquids may create secondary problems. Adsorbers These units consist of a chamber filled with a granular adsorbent. The adsorbent may be activated charcoal, silica gel, alumina, and other treated solids. Activated charcoal is the most common and has the highest retention per unit weight for organic solvents. Performance of adsorbent beds depends upon their thickness, velocity of the gas passing, mesh size (surface area) of adsorbent, and temperature of the vapor being removed. They can be designed to obtain almost complete removal of organic vapors some in organic gases. They may be reactivated by the use of steam or heat. Resistance depends upon the adsorbent mesh size, depth, and velocity. It may range from less than one inch of water to several inches of mercury. Adsorbent beds are usually limited, because of their cost, to applications where recovery may be economically fea sible. Removal of particulate matter by use of precleaners is necessary if significant loadings are involved as the ad sorbent voids are readily plugged. They are not suitable for High temperature conditions. Silica gel and alumina ex hibit a strong preference for water vapor whereas charcoal does not. Combustion Devices This group of industrial air cleaners entails the use of high temperatures or catalysis combined with some eleva tion in temperature to destory or decompose organic and some inorganic gases which create obnoxious odors. The devices are divided into those in which combustion is ob tained by use of liquid or gaseous fuels and the secondary air for combustion is provided by the contaminated air. In essence, the simplest form involves passing the con taminated air through the combustion chamber of a boiler or fire box. Special fire boxes or brick checkerwork may also be employed. Catalytic devices consist of noble metal packed into frames in the form of ribbons or screens or may be ceramic granules coated with noble metal catalysts. The frames or packing are placed in a housing over which the gas stream to be decontaminated is passed. The con taminants break down and are reduced to elemental gases of an innocuous nature. Catalysts reduce the amount of heat necessary but may become contaminated by sulfur or other elements. The performance of a combustion device is dependent upon the retention period of the contaminant in the high temperature zone. Removal or destruction is thus dependent on velocity and surface area of the heat-transfer device. If properly designed, obnoxious odors are removed. The resistance of the direct combustion unit is negligible since it is an integral part of the fuel-combustion system. Catalytic devices behave in a manner comparable to packed beds or filters except that they'are not plugged by organic particulates but may be affected by inorganic solids. Maintenance of these types of units is essentially de pendent upon the contaminants encountered. The directcombustion system requires little care other than that required by ordinary fuel-burning equipment. Catalytic units become contaminated slowly and will require removal for reactivation at certain intervals depending upon the application. Economic factors may limit the use of direct-combustion methods unless there is a demand for the heat generated. They may best be applied to processes employing com bustion. Catalytic units require temperatures of at least 500 F and consequently it may be necessary to preheat the contaminated air to this value. X REFERENCES 1 8. S. Leopold and C. S. Leopold: Bronchial asthma and allied allergic disorders {Journal of the American Medical Asso ciation, March 7, 1925, p. 731). 1 L. H. Criep, M.D., and M. A. Green, MJ).: Air cleaning as an aid in the treatment of hay fever and bronchial asthma (Journal of Allergy, January 1936, p. 120). *0- M. Lidwell: The bacterial filtration efficiency of an elec trostatic air cleaner (Journal of the Institution of Heating and Ventilating Engineers, London, June 1951, p. 139).