Document qmdyNDkNepZBaRBo8eQXmzj4G

930 CHAPTER 45 1950 Guide Every pipe should be kept open and unobstructed throughout its entire length, and no fixed screen should be placed in it, although, the use of a trap at the junction of the hood and branch pipe is permissible, provided it is not allowed to fill up completely. The passing of pipes through fire walls should be avoided wherever possible, and floor sweep connections should be so arranged that foreign material cannot be easily introduced into them. At the point of entrance of a branch pipe with- the main duct, there should be an increase in the latter equal to their sum. Some state codes specify that the combined area be increased by 25 per cent. While this is not always good practice, and is frequently done at the expense of a reduced air velocity, it is often done where future expansion of the exhaust system is contemplated. Duct Resistance The resistance to flow in round galvanized duct riveted and soldered at the joints may be obtained from Figs. 1 or 2, Chapter 31. The pressure drop through elbows depends upon the radius of the bend. For elbows .Table 6 Loss Thbocqh 90-Deo Elbows Ratio or Exmow Csntbb Lins Radius to PlPB DZAAIETHB OB DEPTH AfPBOXXMATB LOBS IV PSB ChNT ' op Velocity Hbad 1 80 u 31 2 22 2i 19 whose centerline radii vary from 100 to 250 per cent of pipe diameter, the loss may be estimated from Table 6. RESISTANCE OF SYSTEM The resistance of the exhaust system is composed of three factors: (1) ioss through the hoods, (2) collector drop, and (3) friction drop in the duct, system. ; -The loss through the hoods is usually assumed to be equal to one-half the suction at the hoods. Where possible the resistance of the particular collector to be used should be obtained, from the manufacturer. -Friction drop in the pipes must be computed for each section where there is a change in area or in velocity. The Velocities should be found 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 in this chapter and Figs. 1 or 2, Chapter 31. . 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. 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 orthreshold limits18. Too much emphasis cannot be placed on the necessity of testing exhaust ^sterns' frequently by determining the concentration 'of atmospheric contamination at the1 worker's breathing level19. Commonly accepted Industrial Exhaust Systems 931 values of threshold limits for usual atmospheric contaminants will be found in Tables 3,4 and 5, Chapter 8. AIR FLOW PRODUCING EOUEPMENT In any type of exhaust system some form of air flow producing equip-ment is required to create the pressure necessary to cause the air to flow through the system to the discharge stack. The principal types of air moving equipment are centrifugal exhaust fans, disc or propeller fans, axial flow fans and venturi ejectors. Table 7. Cobbosion Resisting Matbbials fob Exhaust Systems* MATERIAL Metals ACIDb Acetic Chromic Htdrockloric Htdmk . FLUORIC Nitric Phor* paoaic Sul phurous Sul* . psuric Dil.jCooo. DiL Cone. DU. Cone. DiL |Coce. DiL Cone. DU jCone. DiL |Cone. Dil | Cone. Aluminum-------------- Good Fair Poor No Data Poor Good Poor Magnesium awl AUoyn__ No Data Good j Poor No Data Poorj Good No Data No Data Poor No Data Poor No Data Lead and Lead-Coated__ Poor Good Poor Poor Poor Poor Good Good Poor Mo!y Alloy (60 Ni--2QMo -20 ft)_ Good No Data Fair No Data Poor Poor No Data Good ^Mond Metal ' Fur Poor Fair Poor Goodd Fair Poor Fair Poor Goode Poor Bronx*__ Poor SBam Iron-__ Fair jGood No Data Stainless Steele (18 Cr8 Ni) cameled Rt*el_- _____ Miscellaneous Asbestos Comp Good No Data Good No Data Foir Poor Good Good Poor Good No Data Poor Good Good . Poor Poor Good except against strong adds and alkalis. Good No Data Good ; Poorjcoode Good No Data Good > Wood_________________ Bobber Some woods are decomposed or soflesed Easier I Poor j other*. | Poor la general plmtiw resist weak adds and are decomposed by concentrated acid. * Standard Prutloe Sheet No. 115 (Divisksi of Industrial Hygiene, New York State Labor Department). , Acid mists in air are mare corrosive than as liquid in storage tank. Galvanized iron not resistant to acid. Stainless steel of (24 Cr--10 Nt) fairly resistant at low temperature far ECl and HtPO*. Under most conditions. * At room temperatures. PROTECTION AGAINST CORROSION AND ABRASION Manufacturers generally provide special fans for the handling of various industrial wastes. When corrosive or abrasive materials are conveyed, the fan blades and interior of the fan housing should be protected from wear. This may be accomplished by placing the collector on the suction, side of the fan. Excellent protection against many corrosive acids may be obtained by lining the interior surfaces of the ducts and fans, including wheels, with rubber. The removal of gases and fumes in many chemical plants requires that metals used in the construction of the exhaust system be resistant to chem-