Document 2J4e3MNwKvpkYZBxnmErKzN9R

EXHAUST SYSTEMS, GENERAL 6 COMFORT VENTILATION interferes with the effectiveness of the ex haust hood, nor should he be allowed to make adjustments in the exhaust system He should tell his supervisor when he no tices a decline in exhaust control Hoods must be fixed wherever possible, as it is false security to depend on em ployees to move them around as needed Hood classification Hoods have been classified by Brandt* according to their shape as follows 1. Enclosing hoods Spray paint booths Laboratory hoods Luminous dial painting hoods Abrasive blasting cabinets Some gnnder hoods 2 Rectangular or round hoods Welding Stone cutting Down-draft table tops for degreasing, cleaning, and cementing Drilling Masonry saws (not portable) Material transfer 3 Slot hoods Lateral exhausts at tanks or vessels such as used for degreasing, pick ling, and plating At table tops or on flat surfaces where the contaminant is released over a large area 4 Canopy hoods Above tanks, furnaces, melting pots, tables, and the like Hood summary The points to remember in good hood design are 1. Do not expect an exhaust hood to "reach out" very far and suck in the contami nant. With a pressure hose, a stream of air can be blown for many feet from the hose, but the process does not work m reverse 2. Enclose the process as much as possible 3. Use flanges, side shields, and baffles to the fullest 4. Use flared connection at the end of the duct 5. Be sure the air velocity at the point of contaminant dispersal, die capture veloci ty, is high enough Achieve this by em phasis on mass air movement into the hood face, rather than hood face velocity. 6 Do not draw the contaminant past the operator's face 7 Take advantage (when positioning the hood) of air movement caused by heat currents and trajectory, if heavy particles are thrown off 8 Keep the hood horn interfering with the operator's work. Ducts After contaminated air has been drawn into a hood, ducts serve the purpose of guiding the air to an air denner or to the outdoors When air passes through any duct or pipe, friction must be overcome, that is, energy must be expended The amount of this friction loss must be calcu lated before the system is installed, so the proper size fan and motor can be pur chased However, this is a task for a plant engineer, not for the safety director Sev eral excellent references have appeared in the post few years which remove the guesswork from duct design (See "Refer ences" at the end of this chapter, especial ly Brandt, DallaValle, and Hemeon, and the publications of American Foundrymen's So ciety and the American Conference of Gov ernmental Industrial Hygienists ) Some general comments on sizing of ducts will be helpful to the safety man in appraising the merits of a given system Starting pout in designing a local ex haust system is determining how many cu bic feet of air per minute (cfm) must be handled by each hood to control the con taminant released in the workroom (see the topic "Hoods" in this chajpter). Based on such data, careful duct design accomplishes these objectives mamtains proper transport velocity so the contaminant, if it is a dust or fume, will not settle out and plug the pipe, holds power consumption to a mini mum; "keeps the system "balanced" at all times Balance means that duct branches near the fan, where suction is high, must not rob air horn branches at the far end For 0Brandt, Allen D "A Summary of De sign Data for Exhaust Systems " Heat ing and Ventilating (May 1945), 28-13