Document OEky7Q6Yj1oyeO0jDkXg44b9p

American Society of Heating and Ventilating Engineers Guide, 1937 feathers and lint, the exhaust vent should be large'enough to permit an air velocity of 200 to 500 fpm. This will, of course, require a cyclone of larger dimensions than given for the foregoing general case. When a high collection efficiency is desired, or the material is very fine multicyclones may be used. These are merely small cyclones arranged inparallel which utilize the principle of high centrifugal velocity to attain separation. The capacities and characteristics of this type of separator should be obtained from the manufacturers. Cloth Filters Filters are used when the material collected by ah exhaust system is valuable or cannot be separated efficiently from the air with an ordinary cyclone.' They are also employed when it is desirable to recirculate the. air drawn from a room by the exhaust system, which otherwise might entail considerable loss in heat. Bag filters which are properly housed may be operated under suction. Bag houses used in the manufacture of zinc oxide and other chemical products are operated on the positive side of the fan. Wool, cotton and asbestos cloths are commonly used as filtering mediums. When woolen cloths are employed, the filtering capacities vary from M to 10 cfm per square foot of filtering surface, depending on the character of the material collected. The rates for cotton and asbestos cloths are lower. The type of filter cloth and the rates of filtration depend, of course, on the material to be collected and the fan capacity. The time increase of resistance varies with the amount of material per mitted to build up on the surface of the filter and can be determined only by experiment. The limits of the increase may be regulated by adjust ment of the shaking or cleaning mechanism. These limits may be regulated further according to the capacity of the fan and the effective performance of the hoods and the duct system. For additional information on Dust and Cinders, see Chapter 15, Air Pollution, p. 301. RESISTANCE OF SYSTEM The maintained resistance of the exhaust system is composed of three factors: (1) loss through the hoods, (2) collector drop, and (3) friction drop in the pipes. The loss through the hoods is usually assumed to be equal tq the suction maintained at the hoods. The collector drop in inches of water is given approximately by Equation 6, but where possible the resistance of the particular collector to be used should be ascertained from the manu facturer. Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. Find the velocities in each section of pipe starting with the branch most remote from the fan. The friction drop for these sections can be determined by reference to Table 6. Total friction 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 in the discharge pipe. 394 Chapter 21--Industrial Exhaust Systems EFFICIENCY OF EXHAUST SYSTEMS The efficiency of an exhaust system depends upon its effectiveness in reducing the concentration of dusts, fumes, vapors and gases below the safe or threshold limits9. Too much emphasis cannot be placed on the necessity of testing exhaust systems frequently by determining the concentration of atmospheric con tamination at the worker's breathing level. Commonly accepted values of threshold limits for the usual gases and vapors are given in Table 7. SELECTION OF FANS AND MOTORS Manufacturers generally provide special fans for the collection of various industrial wastes. These are available for the collection of coal J--* shavines. wool, cotton and many other substances. For Table 7. Threshold Limits of Common Vapors and Gases3 Substance Spec. Graf, of Gas or Vapor (Air 1) Inflammable Limits <%) Physiological Action Maximum Allowable Concentration (ppm) Ozone........... ---**--"** Carbon monoxide-------------Hydrogen sulphide.......---- Carbon tetrachloride........... 2.486 5.5 1.2678 2.2638 0.9671 - 1.190 2.73 1.1 5.3 rion-inflamm. do do do 12.5-74 4.3-46 1.4-7.0 7.5-26.5 non-inflamm. . irritant do do do asphyxiant do anesthetic do do 0.35 0.80 10.0 10.0 . 100.0 85-130 100.0 100.0 100.0 , .The Prevention of Occupational Diseases, by R. R. Sayers and J. M. DallaValle (Mechanical Engtnccriitt, Vol. 57, No. 4, April, 1935). particular features concerning special fans, consult the Catalog Data Section of' The Guide and manufacturers' data. When substances having an abrasive character are conveyed, the fan blades and housing should be protected from wear. This may be accomplished by placing a collector on the negative side of the fan or by lining the housing and blades with rubber. If no future expansion of an exhaust system is contemplated, the fan motor should be chosen to provide the calculated air volume. Should, however, the exhaust system be required to handle more air in the future, the motor should be adequate for the maximum load anticipated. Further information regarding the choice of fans and motors is given in nvjntoee 1*7 onrl AO PROTECTION AGAINST CORROSION The removal of gases and fumes in many chemical plants requires that metals used in the construction of the exhaust system be resistant to for Industrial Exhaust Systems, by J. J. Bloomfield (A.S.H.V.E. Transactions. Vol. 40,