Document OEvrXvDka7xne7b8yVg1ExkBK

730 CHAPTER 52 1960 Guide Process Table 9 .... Resistance to Corrosion of Materials Used for Hoods and Ducts46 S | o ij jo i | 1 _ac j E i |a 5 1 o E| 1 5 8 1X z ?! o <| O 8 1 j!* 1 1 g a z o 1 co o%. IS 1 | |3 c *X Asphalt il Si 3 gIS P pu U J| G 3 5* 11 >. "8 "* 11 > Phenolic Kevin Acid, Chromic (Plating and Anodising).............. Acid, Hydrochloric........... Acid, Hydrofluoric........... EP EP s ES EP P E S F PS SP S S P E E4 E F E E EF S Ss SF P SF EF EP F E4 E E E F Acid, Nitric........................ P EE E FP F F SE Acid, Oxalic (Anodising). ES E S E Acid, Sulfuric..................... P P S5 E8 E E7 P E S EP S F F ES E E5 EP E Acid Dipping and Pick- ling (Horis Ducts)........ P FE E Alkaline Cleaner (Caustic 140-180 F)....................... EP EP E sE ES S SE E Alkaline Cleaning Soiu- tions................................. 2 E E E EE E Alkaline Plating (Cu, Cd, Ag)................................... EP EP E S ES s s S S E Plating and Anodising (Horisontal Ducts)....... Plating, Bright Nickel___ SP SP P SE SE s E sE Platins and DinDinc (Ver-I tieal Ducts).................... P F EE Tanning, Dressing, Dyeing, Bleaching Sol......... Water, Boiling and Steam. Decarbonizing Agents___ S8 EP SP 3E E E S9 E ES E Es E E E Degreasing Compounds: Organic A Mineral Sol.. E E E Enamels and Lacquers (Coating A Dipping)... E S Fabric Impregnation........ S 8alt, Supersaturated So- lution 300-400 F............. 8 S Wire Insulation................. S Approximate Cfattffleoflon Service life S--EueUest...........................Over 5 ywie fi--^tkfsetorr......................... 1 to 8 !n F--Fair...................................... H to J rears p--.....................................8 p.wwth. Combination* (EP)..................Indefinite ratio* Note*: 1. Batin** sic beaed upon oolleted field service reports. Indefinite rating ere fives where reports sre inconsistent doe to wrist operstint eonditions (such * dilution. exposure to weather, etc-). Blank space* do* urte insufiictB&t information, not poor service. J- tjfld soaps, detergent*, water eotabie ails, 140-180 F. 8. Cieoeote oil, ethylene dichloride. 4. To 9) percent: E; 50-70 percent, 180 F: S. 8. To 50 percent, plating: E; 10 percent anodising, agitated 80 F: S. 8. To GO percent plating: S; over 80 percent pieklinc, 30 percent anodiiint-' 7A-S percent, US-tlS F pickling: 8; plating, pickling, soodmog: E. 8. Except add. 8. Acid. from the outdoors must be heated somehow to maintain comfortable temperatures in the area. The cost will de pend on the ratio of exhaust air to that required for ven tilation of the space occupied. MAINTENANCE OF PERFORMANCE Periodic inspection and checking of exhaust systems are necessary if control is to be maintained at the effective level of the original installation. Continued effectiveness depends on maintained design air volume flowing through^ the exhaust hoods. A checking procedure, therefore, must include some data to indicate at least relative air flow through the hoods. The static pressure or hood suction measurement will prove useful for such checking if data are available on air volumes and pressures at the time the system was installed. Testing and recording of such data for each new installation are of the utmost im portance. While hood suction readings have rightfully been dis carded as a means of measuring air flow, they do offer a quick and accurate method of measuring relative air flow. If the hood suction is known while an exhaust system is functioning property, its continued effectiveness can be assured so long as the hood suction is not reduced from its original value. Difficulty from plugging may be encountered where industrial Exhaust Systems Table 10,, Resistance to Corrosion of Materials Used for Exhaust Fans46 731 J EI J "9e < <3 w <5 i 11 1i 1 1 1< > 2 X | ce g> 11 8 J 1 ? 1S jj Acid Dipping, Pickling, and Strip- ping............................................... EP s E Alkaline Cleaning........................... EF s E E Anodising........................................ SF s Coating--Enamel and Lacquer... E E E Decarbonising........... ..................... ES E Degreasing and Metal Cleaning.. ES ES E Hardening and Quenching.......... ' ES ES Plating.............................................. EP s E Washing and Rinsing..................... EP E E E SP EP EE P ES S SP E E S s E ES E E F s ES EEF EE E sE E EF ES E E E EE F E OoMi'fteaKow E--'Excellent-.. S--Setixfecicey. F--Fair............. F--Poor............ Combinations.. Approximate Service Ufa Over 5 yean J to 6 yean 8 month* to 3 yean 6 months Variable periomimao* Note: Rating* are based upon collated field service reports. Forward curved fans are net recommended for opetttask ventilation eervice- Cleanout doon and bottom drains are recommended on all fa on open tank operatmns. The deposits which build up on fan wheels and >r should be removed by regular cleaning. Bepaint where any costing baa been broken and removed. heavy dust loads or moist air are encountered. Unless the hood design is altered or there are accumulations in the hood or branch pipe between hood and point of hood suction reading, the air volume exhausted from a hood cannot change without a change in hood suction reading. The Pitot tube can be used for routine check purposes instead of the static pressure method but requires care in reading velocity pressures in an exact position with- the tube paralleling the flow of air. The hood suction method of checking can, however, be more readily delegated to an assistant having no training. U-gages have been standard plant equipment long enough to eliminate any feeling of uncertainty in their use. Since pressure readings vary as the square of the ve locity or volume of flow, a slight change in flow is mag nified by a comparison of gage readings. Normally, a re duction of volume or velocity of 10 to 15 percent will not be sufficient to reduce the effectiveness of the exhaust system. This range is equivalent to a reduction in static pressure readings of 19 to 30 percent. A marked reduction in hood suction can.often be traced to one or more of the following items: 1. Reduced performance by the exhaust fan caused by re duced speed due to belt slippage, wear on rotor or casing, or an accumulation of material in the rotor or raring obstructing the air flow. 2. Incorrect direction of exhauster rotation. 3. Reduced performance caused by defects in the exhaust piping, such as accumulations of material in branch or mein ducts due to insufficient conveying velocities, condensation of oil or water vapors on duct walls, adhesive characteristics of material exhausted. . .4. Leakage losses caused by loose cleanout doors, broken joints, holes worn in duct (most frequently in elbows), or poor connection to the exhauster inlet. 5. Losses; in suction due to exhaust openings added to the system or due to a change of setting of blast gates in branch lines. 6. Increased pressure loss through the dust collector due to laefc of maintenance, improper operation, wear, etc. MATERIALS REQUIRED FOR CORROSION RESISTANCE Io many cases, exhaust systems including hoods, ducts, air flow producing equipment, and air cleaning equipment will require protective coatings if carbon steel is used, or other materials of construction will be required. Need for such materials will be determined by (a) the corrosion rates on interior or exterior duct surfaces, (6) protection against product contamination, and (c) explosion hazards. Construction for corrosion protection is the most dif ficult due to the complex factors that influence rate of corrosion. It is seldom possible to predict the concentra tion, composition, and dry-bulb' and dew-point tempera tures that will exist in actual operations. A guide for selecting corrosive resistant materials based on a survey of actual experiences has been reproduced in Tables 9 and 10.4* REFERENCES The materia) in this chapter is baaed largely on the recom mendations of the American Conference of Governmental In dustrial Hygienists as published in the Industrial Ventilation Manual, 195S Edition. *J. J. Bloomfield and J. M. DallaValle: The Determination and Control of Industrial Dust (U. S. Public Health Service Bulletm 217, 1935). *J. M. DallaValle: Exhaust Hoods (Industrial Press, New York, 1952). * T. Hatch: Design of exhaust hoods for dust control systems {Journal of Industrial Hygiene and Toxicology, Vol. 18, 1936, p. 595). * W. C. L. Hemeon: Air dilution in industrial ventilation (.Heating and Ventilating, February 1941). *B. F. Postman: Practical application of industrial wwlumet ventilation for the control of occupation exposures {American Journal of Public Health, Vol. 30, 1940, p. 149). * WttN' Witheridge: Principles of Industrial Process Ventila tion (University of Michigan, Inservice Training Course, Oc tober 1945).