Document 15Zez6n7vjQrnnYbGNJokadgm

American Society of Heating and Ventilating Engineers Guide Duct Resistance The resistance to flow in any galvanized duct riveted and soldered! the joints may be obtained from Fig. 3, Chapter 20. The pressure through elbows depends upon the radius of the.bend. For elbows centerline radii vary from 50 to 300 per cent of pipe diameter, the loss be estimated from Table 6. It is sometimes convenient to expressrtH resistance of an elbow in terms of an equivalent length of duct of the diameter. Thus with a throat radius equal to the pipe diametertU resistance is equivalent to a section of straight pipe approximately diameters long, while with a throat diameter radius 1 Yi times the dSI meter, the resistance is about the same as that of seven diameters^ straight pipe. COLLECTORS The most common method of separating the dust and other matemU from the air is to pass the mixture through a centrifugal or cycl<j%jI collector. In this type of collector the mixture of the air and material! is introduced on a tangent, near the cylindrical top of the collector, '^1 the whirling motion sets up a centrifugal action causing the compel tively heavy materials suspended in the air to be thrown against the stda of the separator, from which position they spiral down to the tail piece! while the air escapes through the stack at the: center of the collector's The, diameter of the cyclone should be at least 3 times the diamefeS of the fan discharge duct. When two or more separate ducts ent^a cyclone; gates should be provided to prevent any back draft throughjg system Which may not be operating. Cyclones working in conjunctiot| with two or more fans should be designed to operate efficiently at thirds capacity rating. The following formula is useful in computing! loss through a cyclone when the velocity of the air in the fan discharft| duct is known: ^ -013 (mo)2 where hc = the pressure drop through the cyclone, inches of water. V = the air velocity in the fan discharge duct, feet per minute. If a cyclone is used to collect light dusts such as buffing wheel dusSl feathers and lint, the exhaust vent should be large enough to permit;^ air velocity of 200 to 500 fpm. This will, of course, require a cyclones la rger dimensions than given for the foregoing, general case. -9 When a high collection efficiency is desired, or the material is very fat multicyclones may be used. These are merely small cyclones arranged^ parallel which utilize the principle of high centrifugal velocity to attora separation. The capacities and characteristics of this type of separatajj should be obtained from the manufacturers. Cloth Filters Filter cloths are used when the material collected by an exhaust sysjl|| is valuable or cannot be separated from the air with an ordinary cyclop 354 Chapter. 21--Industrial Exhaust Systems 'Iso employed when it is desirable to recirculate the air drawn Mg** by the exhaust system, which otherwise might entail con^"Tlloss in heat. Bag filters which are properly housed may be gasabie u1 nde_r, suction.; Bag houses used in the manufacture of zinc oxide ither chemical products are operated on the positive side of the fan. hjji! cotton anud adosbuecbstiuoos culuouthis acuree cuojimiiiumuoinulyy uusbecdu a<isb fnilitering *? '-.' wWhhen woolen cloths are employed, the filtering capacities vary -- to 10 cfm per square foot of filtering surface, depending on the ffre0jtlPv;?/2er 0{ the mater*ia-*l collected. ^The. .r.a..tes fror . cotton andtasbe.stos r5fi^are slightly lower. The type of filter cloth and the rates of filtration 0f course, on the material to be collected and the fan capacity, increase of resistance varies with the amount of material per- JJ&gTto build up on the surface of the filter and can be determined only iPfSeriment. The limits of the increase may be regulated by adjust*nt of the shaking or cleaning mechanism. These limits may be iHlated further according to the capacity of the.fan and the effective Sjsj'jbnance of the hoods and the duct system. RESISTANCE OF SYSTEM fe maintained resistance of the exhaust system is composed of three : (1) loss through the hoods, (2) collector drop, and (3) friction op;jri the pipes. Me loss through the hoods is usually assumed to be eqiial to the suction pSmtained at the hoods. The collector drop in inches of water is given approximately by Equation 6, but where possible the resistance of the " iiSilar collector to be used should be ascertained from the manu- Enction drop in the pipes must be computed for each section where ttiereis a change in area or in velocity. Find the velocities in each section Sfppe starting with the branch most remote from the fan. The friction 3rop for these sections can be determined by reference to Table 6. Total friltion loss in the piping system is the friction drop in the most remote Branch plus the drop in the various sections of the main, plus the drop intSie'discharge pipe. SELECTION OF FANS AND MOTORS Manufacturers generally provide special fans for the collection of ^arious industrial wastes. These are available for the collection of coal dS&wood shavings, wool, cotton and many other substances. For parfrcular features concerning special fans, consult tlie Catalog Data SfdttSj of The Guide and manufacturers' data. When substances _.J an abrasive character are conveyed, the fan blades and housing be protected from wear. This may be accomplished by placing a tojlector on the negative side of the fan or by lining the housing, and %o future expansion' of :an exhaust system is contemplated, the,Tan: should be chosen to provide the calculated air volumq./:>'SIi^nl{l; rever, the exhaust system be required to. handle moferair in ihe. e, the motor should be adequate for theimaximum load njfrtflsMftid.r v ' ' m Tot babasiT 355