Document LoJ3dEJ6Q63X5K4mKmKmpDpzX

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 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 29, Air Cleaning Devices. 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 duct system. The loss through.the hoods is usually assumed to be equal to the suction maintained at the hoods. Where possible ,the resistance of the particular collector to be used should be ascertained from the manufacturer. 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 and Fig. 2, Chapter. 32. Total friction loss in the piping system is the friction Table 7. Accepted Standards for Toxic Concentration of Fumes, Dusts and Mists1 Fumes Cadmium oxide.______ _______ ___ Chlorodiphenyl:............. ;................... Lead or oxides of lead........................ Mercury or mercury compounds.___ Pentachloronaphthalene................. ... Trichloronaphthalene....... ................ Zinc oxide........................................... Dusts Asbestos__ :_______________ ______ _______:_ Cement-.............. ........... '............... .................... Gypsum::............................... _.;11................. Lead or compounds of lead........ '................. Manganese__ .;___ _____ 1____________ ______ Marble._______________________ ___ ;.. ' Silica (more than 70 per cent free silicon-dioxide .Silica (more than 10 per cent free silicon-dioxide Silica (less than 10 per cent free silicon-dioxide).. Mists ' Chromic acid_____ _ Sulphuric acid.____ !. Milligrams per Cubic Meter 0.1 1.0 0.15 .0.1 0.5 5.0 15.0 Million Particles per - Cubic FooTb 5 100 100 0.15c 6C 100 . 5 10 100 - Milligrams per Cubic Meter - 0.1 5.0 RAdapted from Safe Concentrations of Certain Common Toxic Substances Used- in Industry, by M. Bowditch, C. K. Drinker. P. Drinker, H. A. Haggard, and H. Hamilton (Journal oj Industrial Hygiene and Toxicology. Vol, 22, No. 6, June, 1940). Industrial Code Bulletin Nol 35 (New York State Labor Depart ment). Study of Asbestosis in Asbestos Textile Industry (U. S. Public Health Bulletin No. 241, 1938). Chronic Manganese Poisoning in an Ore Crushing Mill (Cl. S. Public Health Bulletin No. 247, 1940). bDetermined by light field- or equivalent technique. - '1 cMilligrams per cubic meter. 744 CHAPTER 40: INDUSTRIAL EXHAUST SYSTEMS \ < Table 8. Corrosion Resi$ting'-Materials for Exhaust Systems1 MATERIAL : Metals. ACIDb - Acetic Chromic Hydro- ; ;Htdro- CHLORIC FLUORIC Nitric Pbos- . Sul PHOBIC PBU&ous phuric ` .DiLjConc. DiLjConc. DiljConc. DiP j Cone. DillCone. DiLjConci DiL Cone: Dil| Cone. Aluminum Good Fair Poor No Data Poor Good Poor Poor . Poor Magnesium and Alloys__ No Data- Good j Poor No Data Poorj Good No Data No Data No Data No Data Lead and Lead-Coated__ Poor : ` Good Poor Poor' Poor Poor Good Good] Poor Moly Alloy (60 Ni--20Mo -20 Ft) Monel Meta]--' ' Good-. No Data Fair No Data' Poor.. Fair Poor Fair) Poor Goodd Fair| Poor Poor Fur 'No Data Good l>oor GoodejPoor Bronze^____ Poor Good Silicon Iron..:_:.' Fur jGood No Data - Fair Poor : Good Good ' No Data Good Stainless Steele (is CrSNi) EnamelpH Rfjel Miscellaneous - Asbestos Comp Good No Data - Good No Data Poor Good No Data Poor Good 1 Good Poor Poor Good except against strong adds and alkalies Good Poor Goode No Data Good Wood________ RubberPlastics. Some woods are decomposed or softened faster than others. 1 1 1 1 PoOT 1. 1 Poor In general plastics resist weak acids and are decomposed by concentrated acid. ^Standard Practice Sheet No. 115 (Division of Industrial Hygiene, New York State Labor Department). bAcid mists in air are more corrosive than as liquid in storage tank. Galvanized iron not resistant to acidr ^Stainless steel of (24 Cr--10 Ni) fairly resistant at low temperature for HCl and Hi PO*. ' dUnder most conditions. At room temperatures. 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 or threshold limits10. 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 usual atmospheric contaminants, such as fumes, dusts and mists are given in Table 7. Similar data covering gases and vapor will be found in Chapter 28. `Criteria for Industrial Exhaust Systems, by J. J. Bloomfield (A.S.H.V.E. Transactions, Vol. 40. 1934. p. 353).; 745