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E9H5SWS0 712 CHAPTER 52 '.959 Guide Procmu Table 9 .... Resistance to Corrosion of Materials Used for Hoods and Ducts48 0 j "O 1 t3 | | ts 1 |3 ,e | | 1 o E | 8 S J 4X z 1 81 v _ i5e ? Ca 5 | g 3* iz 1 2-5 il 5*8 20 |l I Iot |3 75 S V 8 X *o a IJ |1 ij 3 E 11 *j j? si -ox Sy || |3 if <>> Asphalt PfienoCc Resin Acid, Chromic (Plating and Anodising).............. EP EP s ES SP S S P SF P EF E Acid, Hydrochloric........... EP P E S F E E4 E F E E EF S S F EP F E4 E Acid, Hydrofluoric........... P S S S EF Acid, Nitric........................ P EE E FP F F SE Acid, Oxalic (Anodizing). ES E S E Acid, 8ulfuric.................... P P S5 E6 E E7 p E S EP S F F ES E E5 EP E Acid Dipping and Pick- ling (Horiz Ducts)........ P FE E Alkaline Cleaner (Caustic 140-180 F)....................... EP EP E SE ES S SE E Alkaline Cleaning Solu- tions............. ................... 2 E E E EE E Alkaline Plating (Cu, Cd, Ag)................................... EP EP E S ES 8 S S S E Plating and Anodizing (Horizontal Ducts)....... P SE E Plating, Bright Nickel__ SP SP SE S SE Platts? and Diorime (Ver- tical Ducts).................... P F EE Tanning, Dressing, Dye- ing. Bleaching Sol......... S8 S9 E Water, Boiling and Steam. EP SP E E E ES E ES E Decarbonising Ageots___ 3 E E Degreasing Compounds: Organic A Mineral Sol.. E E E Enamels and Lacquers (Coating A DiddIm). .. ES Fabric Impregnation........ S Salt, Supersaturated So- lution 300-400 F............. 3 S E Wire Insulation............. S Approximate rVTt* E--Excellent............................. Over 5 Tears 8--Satisfactory......................... 1 to 6 yean F--Fkir...................................... H to 3 yean P--Poor..................................... 6 months Cocnbiaatioas (EP)..................Indefinite rating Notes: 1. Ratinfs ere based upon collated Said service reports. Indefinite rati&ts an given where reports are inconsistent due to varying operating conditions (such as dilution, exposure to weather, eta.). Blank spaces <tenote insufficient information, not poor service. 3. Mild soaps, detergents, water soluble oils. 140-180 P. S. Creosote oil, kemiae, ethylene dichloride. 4. To 3D percent: E; 60-ft) percent, L50 F: 8. 6. To 60 percent, plating: E; 30 percent anodizing, agitated 80 F: 8. 6..To SO percent plating: 8; over SO percent pickling, 30 percent anodizing: E. 7-5-23 percent, 135-215 F pickling: S; plating, pickling, anodising: E. &. Except arid. 9. Arid. 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 properly, 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 713 Table 10____Resistance to Corrosion of Materials Used for Exhaust Fans48 u1 a e Jj 1 6 4 T 1 5 j X o | o J i < T5 2J J % $2 >iX 13 X 3 | 4. z8 *8 5 | 2| 8 *8 | < i < Acid Dipping, Pickling, and StripEP Alkaline Cleaning........................... EF s s E E E E SP EP E s E ES E E F s ES EEF p E Anodizing......................................... SF s E EE EE E Coating--Enamel and Lacquer... E E E E Decarbonizing................................. ES EP EE Degreasing and Metal Cleaning.. ES ES E Hardening and Quenching............ ES ES Plating.............................................. EP S E ES S ES E E E Washing and Rinsing..................... EP E E SP SE E F Cigcificqfton Approximate Service life E-Ewelleat Over 6 yon 8--Satisfactory S to S jwn p ...... s mantM k> 3 years p Poor___ 6 montlm Combinatione...................Variable performance Note: Eatings ere faeeedapon collated field service reports. Forward curved fen are not recommended fcr opeaventilation eervke. Cleanout door* and bottom drains ere recommended os ell fans o open tank operatkma Tba depoaita which build up on fen wheels end casings should be removed by regnler cleaning Repaint where any eoeting bee been broken and removed. heavy dust loads or moist air are encountered. Unless MATERIALS REQUIRED FOR CORROSION the hood design is altered or there are accumulations in RESISTANCE the hood or branch pipe between hood and point of hood suction reading, the air volume exhausted from a hood Id many cases, exhaust systems including hoods, ducts, cannot change without a change in hood suction reading. air flow producing equipment, and air cleaning equipment The Pitot tube can be used for routine check purposes will require protective coatings if carbon- steel is used, or instead of the static pressure method but requires care in other materials of construction will be required. Need for reading velocity pressures in an exact position with the such materials will be determined by (a) the corrosion tube paralleling the flow of. air. The hood suction method " rates on interior or exterior duct surfaces,, (6) protection of checking can, however, be more readily delegated to against product contamination, and (c) explosion hazards. an assistant having no technical- training. U-gages have Construction for corrosion protection is the most dif been standard plant equipment long enough to eliminate ficult due to the complex factors that influence rate of any feeling of uncertainty in their use. corrosion. It is-seldom possible to predict the concentra Since pressure readings vary as the square of the ve tion, composition, and dry-bulb and dew-point tempera locity or volume of flow, a slight change in flow is mag tures that will exist in actual operations. A . guide for nified by a comparison of gage readings. Normally, a re selecting corrosive resistant materials based on a survey duction of volume or velocity of 10 to 15 percent will of actual experiences has been reproduced in Tables- 9 not be sufficient to reduce the effectiveness of the exhaust and 10.4* system. This range is equivalent to a reduction in static pressure readings of 19 to 30 percent. REFERENCES 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 casing 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 main ducts due to insufficient conveying velocities, condensation of oil or water vapors on duct walls, adhesive characteristics of material exhausted. The material in this chapter is based largely on the recom mendations of the American Conference of Governmental In dustrial Hygienists as published in the Industrial Ventilation Manual, 1958 Edition. 1J. J. Bloomfield and J. M. DallaValie: The Determination and Control of Industrial Dust (U. S. Public Health Service Bulletin 217,1935). * J. M. DallaValle: Exhaust Hoods (Industrial Press, New York, 1952). 4 T. Hatch: Design of exhaust hoods for dust control systems (Journal of Industrial Hygiene and Toxicology, Vol. 18, 1938, p. 595). 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. Loses 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 lack of maintenance, improper operation, wear, etc. * W. C. L. Hemean: Air dilution in industrial ventilation (Heating and Ventilating, February 1941). *B. F. Postman: Practical application of industrial exhaust ventilation for the control of occupation exposures (American Journal of Public Health, Yol. 30, 1940, p. 149). * W. N. Witheridge: Principles of Industrial Process Ventila tion (University of Michigan, Inservice Training Course, Oc tober 1945).