Document 449jdLRy0rVYMMMOp0gQOzr3a

630 CHAPTER 37 1965 Guide And Data Book of wet collectors are similar in design to dry collectors with the addition of sprays or liquid contactors. The mech anism of separation in wet collectors depends on inertial *forces such as impaction and impingement, and in some. ', cases, diffusion. Wet collectors do not require any special construction for high temperatures. They are suitable for use on mixed, contaminants such as gases and solid or liquid particles. The collection of dust in a wetted form eiiminatas a sec ondary dust problem in disposal of collected material. Protection against corrosion and freezing may be necessary and disposal of waste liquids may become a problem. With the exception of the packed towers and wetted filters they ' all have a constant resistance during their operation. On the basis of their important characteristics, wet col lectors may be classified according to Table 2. Table Z .... Characteristics of Fabric Collectors Kind Efficiency Partidt axa range in >10 10 to 1 <i Reorf- Entering Ve/oerfy Mainte nance Cloth collector High High Mod Me- 1-6 Medium . (shaken or erate dium to High rapped)* to (peri high odic) Cloth -collec High tors (reverse air or jet cleaning)* High Mod Me erate dium to to high high 10-25 Medium to low Fabric Collectors Filters for industrial air and gas cleaning consist primarily * Sp*ca requirement* Urge. Requires scheduled deeping. "Spec* requiremeate medium. Cleaning cccataat during operating cycle. Added horaepower tued for rerene jet action. of fabric collectors. These are frequently referred to as doth dust collectors and are generally constructed in the form of housings with doth envelopes drawn over wire screen frames or with vertical cloth bags or tubes. Fabric collectors depend tiecs. Low-voltage cleaners for industrial loadings to date do to a great extent on the accumulated dust layer for their not have sufficient inherent dust-holding capacity. One ex effectiveness. Their basic or initial efficiency depends upon ception in the field of exhaust systems is that of the oil-mist the type of fabric employed. Felted media such as wool gives - _ collector, which functions satisfactorily on a liquid aerosol higherinitial removal oq very fine particles toah woven fabrics. - . .. with the conventional low-voltage type (2 stage) electrostatic The increase in resistance to air flow, resulting from the * precipitator. accumulated dust layer, is related to rate of flow, con Industrial precipitators employ an assembly of parallel centration, and characteristics of the contaminant. Re collector electrodes of various constructions, including cor moval of the collected material is accomplished through, rugated plate, rod curtains, or perforated plate. Air fiow periodic shaking by single mechanisms or combinations of is usually horizontal, although special construction may per them employing rapping, or through reversal of air flow, mit vertical air fiow. High-voltage collectors were made or by means Of high-velocity reverse air jets. With shaking in the form of vertical pipes, and are used for high oper or rapping, air flow is not constant and filtering velocities ating pressures,and for wet-collector designs where water con are usually held under 6 fpm. With reverse jets or reverse tinuously flows downward inside the pipe walls of the col air flow, substantially constant air fiow is maintained and lector electrode. The negative discharge electrodes or rods filtering velocities are generally higher (up to 35 fpm). are accurately centered between, the usual 9 in. collector Filtering velocities in all fabric collectors are highly de electrode spacing, the latter being of positive charge. Pre pendent upon the aerosol, fume, or dust being Collected, j cipitation occurs in a single stage wherein. ionization and The ultra or absolute type of filter may consist of pleated collection art carried oq simultaneously throughout the cellulose asbestos paper, deep sand beds, composite glass unit, and depend on high potentials of 25,000 to 75,000 wool layers, compressed glass fiber in the form of paper, volts. The high-voltage direct current is supplied by either or batte and resin wool. The use of this type of filter is mechanical or electronic rectifiers. limited to Concentrations in the range of outdoor air where-' High efficiencies are obtained by allowing time for contact very high efficiencies, greater than 09.95 percent are required. in the collector zone, and by proper ratio of air flow velocity Fabric collectors are not suitable for high temperatures, to transverse velocity of negatively charged particles toward above 300 F, acid mists or vapors, or mixed contaminants such positively charged collector plates. as wet corrosive gases and solid or liquid particles. Cotton Air velocities vary from 240 fpm to 480 fpm with a fabrics are limited to ISO F, whereas wool is limited to constant pressure drop of less than } in. water. Since the temperatures under 200 F A wide variety of natural and _ maximum efficiency .is obtained with the finer particles at synthetic filter media are available for various temperature low concentrations, and the precipitation is not effective for and corrosion conditions but caution must be exercised large particles, mechanical collectors are frequently used in relative to rupture, high temperatures, and corrosive gaya series with these units. When property'selected, efficiencies are.often high enough Attention should be directed to the . prohibition of air to permit recirculation' of air except when handling toxic recirculation to occupied spaces. High-voltage cleaning contaminants. On the basis of their important characteristics, equipment produces ozone and nitrogen oxides in excessive fabric collectors may be classified according to Table 3. quantities. The major-Bource' of maintenance usually will be in conjunction, with the high-voltage rectifying equip Electrostatic Precipitators .. ment rather than the collector. Although usually used for elevated temperature - work, unlaws the gases are precon Electrostatic precipitators for industrial air and gas dean* ditioned, temperatures in excess' of 700 F should be avoided. ing differ materially from the low-voltage design* described Collector electrode cleaning is accomplished by a rapping in Chapter 36, although the principles of operation are sim device, either electrical or pneumatic, without interrupting ilar. For industrial concentrations, it is obvious that more operation. The dry plate surfaces release the dust into severe demands are made upon methods o! clehniig' the hoppers below, the plates. Some wet Cleaning methods are collector, disposal of collected material, and servicing' prac- also used. Industrial Ait and Gas Cleaners 631: 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 Ooe of the important problems in industrial air and gas cI.ning is the removal of soluble, insoluble, or combustible eases 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- 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 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 ample 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 oo as & 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. 1. 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 nnyales 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. 2. Adsorbers. These units consist of a chamber filled with a granular adsorbent The adsorbent may be activated char coal, 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 and some- in organic gases. They may be reactivated by the use of steam or heat. Resistance depends upon the absorbent mesh rise, 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 precleanere is necessaiy 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. 3. Combustion Devices. This group of industrial air cleaners entails the use of high temperatures or catalysis combined with some elevation in temperature to destroy or decompose organic and some inorganic gases which create obnoxious odors. The devices are divided into those in which combustion is obtained by use of liquid or gaseous fuels and the secondary air for combustion is provided by the contaminated air. In essence, the amplest form involves passing the contaminated air through the combustion chamber of a boiler or fire box. Special fire boxes or brick checkerworb 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 decon taminated is passed. The contaminants break down and are reduced to elemental gases of an innocuous nature. Catalysts reduce the amount of heat necessary but may become con taminated by sulfur or other dements. The performance of a combustion device is dependent upon the retention period of the contaminant in the high tempera ture zone. Removal or destruction is thus dependent on velocity and surface area of the heat-transfer device. If prop erly 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 organio 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 re quired by ordinary fuel-burning equipment Catalytic units become contaminated slowly and will require removal for reactivation at certain intervals depending upon the applica tion. 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 combustion. Catalytic units require temperatures of at least 500 F and consequently it may be necessary to preheat the contaminated air to this value. BIBLIOGRAPHY C. E. MQler: Pointers on selecting equipment for industrial gas cWning (Chemical and Metallurgical Engineering, March 1938, p. 132). ;. . W. A. Schmidt and E. Anderson: Electrical precipitation (Electrical Engineering, August 1938, p- 332). A. W. Simon and L C. Kron: Electrical precipitation (Elec trical Engineering, February 1932, p. 93). I