Document 10V421y6K2Ljxq98NR1gp48va

340 CHAPTER 24 1960 Guide From a hygienic viewpoint where air is to be recirculated, only the contaminant concentration in the recirculated air stream, rather than the collection efficiency, should be con sidered. Correlation between the various efficiencies is poor due to the wide variation in particle size, size distribution, and shape. Resistance or pressure drop through any given collector is related to the operating principle, and varies about as follows: low resistance collectors--up to 2 in. water; me dium resistance collectors--2 to 8 in. water; high resistance collectors--8 to 25 in. water. In centrifugal collectors of the wet or dry type (power driven impellers or rotors) resistance alone is not of significance since these devices are also air movers and therefore the factor of paramount importance is the overall power consumption of the unit. This is also true of certain types of wet collectors which require auxiliary pumping power. The ability of any unit to maintain specified air or gas flow during its operating cycle is also a function of its operating principle. For example, inertial collectors of the simple and centrifugal type arc able to maintain constant resistance regardless of concentration and particle size. How ever, adhesive materials may collect at the entry, through out the unit, and at the exit, and thus cause a reduction in air flow and serious maintenance problems. With the ex ception of packed towers and wet filters, wet collectors operate at constant resistance throughout their operating cycles. However they may also be adversely affected by adhesive or corrosive materials. Wet filters or packed towers will eventually become clogged because of retention of inert dusts. Units employing fabric or other filter media will have resistance influenced by the concentration, particle size, shape, and parking characteristics of the aerosol. Their overall resistance during the operating cycle may increase gradually over a known or determined period. Auxiliary cleaning devices such as separate compartments, alternately shaken, reverse air flow, or reverse jet air flow may main tain constant or nearly constant resistance or insure only small fluctuations in air or gas flow. Electrostatic pre cipitators have the outstanding characteristic of maintain ing a constant air flow with an accompanying low resist ance. Adsorbers behave similarly to packed towers in that they provide constant resistance until the voids become seriously plugged with inert dusts. This generally requires a matter of several months on normal applications. It has been found that inertial separators require the least maintenance. Simple wet collectors rank next in this respect. Complex wet collectors and fabric arrestors require maintenance on a definitely scheduled basis. Electrostatic precipitators require the usual maintenance connected with high-voltage equipment and may vary widely. The main tenance of adsorber type units is minima] and is based primarily on the amount of inert material encountered. INERTIAL SEPARATORS There are many types in this group. These range from simple settling chambers utilizing gravitational forces, to complex centrifugal devices using abrupt directional changes or motor driven rotors. They all have the desirable features of constant resistance during their operating cycle and do not require any special construction for temperatures under 750 F. Usually they require less maintenance than other types of collectors. Table 1.... inertial Separator Characteristics KM Efficiency Portid* Size Rem- fang* In Micron* lone* (Pm- >100 100-10 <o Drop! Oeteripfron and Kamatkt Settling Meand baffle cham bers Low Low Low A large chamber to re duce velocity to permit settling. Auxiliary baf fles are sometimes used to improve perform ance. Requires large space. Large ter cy clones High Me Low dium to low Low Considered as single body over 3 ft in diam eter. Gases and material must be dry. Air-tight dust bins or continuous re moval of material must be provided. Space re quirements moderate. Refined design cen trifu gals High High Me Me dium dium to low Available as multiple small-diameter tubes, scroll-shaped bodies, and multiple louvers. Gases and material must be dry. Air-tight dust bins or continuous re moval of material must be provided. Space re quirements moderate. Integral High rotor type High Me See re Rotor acts as air mover dium marks and separating element. to Usually limited to dry low granular materials and dry gases. Low space requirements. No external pressure loss involved but power requirements are higher than usual exhauster. Slotted scroll fan High Me Low dium to low See re Separation takes place in marks in fan scroll. No external loss involved but power requirements are higher than usual exhauster. On the basis of their important characteristics inertial separators may be classified according to Table 1. SCRUBBERS AND OTHER WET COLLECTORS In these collectors many methods are used to wet the contaminant and remove it from the air stream. A number 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 Air Cleaning 341 - Air washers Wet filters Packed towers Spray towers Centrifugal or inertial scrubbers Venturi scrubber Table 2 .... Characteristics of Wet Collectors Efficiency Fail'd* awe range rn micron* >10 10-1 <i High Medium Low In. Water Wafer Rate h Qdlsns Par 1000 Co Ft Gat Mair.taaanca Oetcripffon and Ramarkt Low (less than 1) 5-15 Medium Composed of one or more banks of low-pressure sprays and flooded eliminator plates. Space requirements are large. Water is usually recirculated. High High Medium Low (less than 1) 3-8 Medium Composed of one or more banIts of low-pressure sprays flooding packed cells of glass or other fibers. Space requirements are large. Limited to low loadings of solid particles. High High Medium High 5-10 High Composed of granular or ceramic packing. Space requirements large. Generally limited to gas absorption. High Medium Low Low 5-10 Low Usually a vertical chamber equipped with lowpressure water sprays at the top or along the walla. Baffle plates may be provided for contacting. Space requirements medium. Usually applied for hot gases. High High Medium to high Medium (see re marks) K-w See re marks This classification includes most of the com mercial wet collectors. They may utilize singly or in combination, low- or high-pres sure sprays, wetted plates, rotating mem bers, liquid gas contact by induction or cen trifugal action. Some of these units (disintegrators) have high overall power requirements for driving the rotor while others have high power con sumption for raising water pressures for spray nozzles. Maintenance varies from low to high depend ing on component parts. Simple gas liquid contact units would be classified as low. Units having rotating members may vary from medium to high depending on the type of service. High High High High (12-25) 5-10 Medium Consists of a simple venturi with low-pressure sprays at throat followed by a cyclonic type separator. Space requirements are low. Unit functions independent of grain loadings. High velocity at venturi throat may cause erosion problems. 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 eliminates 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. FILTERS Filters for industrial air and gas cleaning consist pri marily of cloth dust collectors and the ultra or absolute type of filter. Cloth dust collectors are generally constructed in the form of housings with cloth envelopes drawn over wire screen frames or with vertical cloth bags ot tubes. Cloth filters depend to a great extent on the accumulated dust