Document Z20Ge6vdw8gEJa6L14mQZnX7

838 CHAPTER 46 . 1948 Guide. 10 diameters long; while with a center line radius 1J4 times the diameter, the resistance is apparently the same as. that of seven diameters of straight pipe. . r AIR CLEANING EQUIPMENT ^Primary dust separators, depend for their air cleaning action on centri fuging out the . dust particles or on imparting to the transporting air stream a quick change in direction, accompanied by a sudden reduction in velocity. They may be applied to exhaust systems from polishing and buffing wheels and, in some cases, as a preliminary separator ahead of a 'final dust arrester. Dust particles which cannot.be caught in inertia type separators are usually removed by one of the following methods: 1. Filtration. Of the filter arresters the cloth type, either of the screen, envelope, or bag construction, is most generally used for. industrial ex haust systems. Cloth arresters properly engineered for this job and handling dust, for which they are suited operate at a good efficiency. 2. Wet Collectors. Wet collectors have been in use. for many years,' but only recently have been, introduced for collection of. industrial dust from exhaust systems. They are particularly applicable where moisture is present in the air being exhausted. The resistance is relatively low in wet collectors suitable for dust removal and remains constant for a given exhaust system. 3. High-tension Electrical Precipitation. High-tension electrical preci pitation has been applied to removal of dust, fumes, and mists from air and other gases. (See section on Electric Precipitators in Chapter 33.) 4. Dynamic Precipitators. This equipment effects removal of entrained dusts by means,of a specially designed fan in which the rotating element imparts a centrifugal force to the dust particles. Originally these preci pitators were intended for the collection of. dry. dust, but recently the construction has been modified to permit the introduction of water into the dirty air intake, thus increasing its application to more kinds of dust. Consideration must be given to the type of dust handled , . .For additional information on dust and cinders, see . Chapter 33, 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 losis 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 obtained from the manufacturer. Table 8. Loss Through 90-Deg Elbows Elbow Center Line Radius in Per Cent of Pipe Diameter or Depth 50 100 150 200-300 Approximate Loss in Per Cent of Velocity Head 75 26 17 14 Exhausting and'Conveying Systems 839 Table 9. Corrosion Resisting Materials for. Exhaust Systems MATERIAL MgTATJi . ACIDb Acetic Chromic Hydro* CHLORIC Hydro fluoric Nitric Phos phoric Sul phurous Sul phuric DiLjCone. DU. Cone. DU. Cone. DiL Cone. DiL Cone. DiljConc. DiL Cone. .DiL Done. Ahnninum _ - Good Fair Poor- No Data Poor Good Poor Poor .; Poor Magnedrim and Alloya_ No Data Good | 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 Fe)_ Good Mood MetaL Fair No Data Fair No Data Poor Poor Furl Poor Good* Fur Poor Poor Fair No Data . Good Poor Good Poor Bronze__ Poor. Good Bttieon Itoil. Fur |Good ' No Data Fur' Stainless Steele (18 Cr* 8 Nt) - Good Enameled Steel--__ No Data Miscellaneous GoodNo Data ' Poor Good Poor'- No Data Poor Good Good Good Good > No Data- Good Poor : Good Po6r|Good . Poor No Data Good Asbestos Comp Good except against strong adds and alkalies Wood Robber Some woods are decomposed or softened faster than others. | Poor. | - | . Poor In general plastics resist weak adds 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 add. ^Stainless steel of (24 Cr--10 Ni) fairly resistant at low temperature ior.HCl and /7 PO*. ^Under most conditions. ... . cAt room temperatures. 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 8 in this chapter and Fig. 2, Chapter 41. 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 or threshold limitsI4. 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 level16. Commonly accepted values of threshold limits for usual atmospheric contaminants will be found,in Tables 3, 4 and 5, Chapter 10. . AIR FLOW PRODUCING EQUIPMENT ,, . In any type of exhaust system some form of air flow producing equip ment should be required to create the pressure necessary to cause the air