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724 CHAPTER 52 1960 Guide Table 3 .... A Guide to Ventilation Rates for Typical Industrial Equipment (Continued} State or Local Segalatiorts Should bo CoaaJted and followed Whore Highor VeatSelioo Cote* or* SpociSod Operation Type of Hood V<p?rfsfetfes Air Row Utuot Transport Velocity Fpm Remarks and References Pouring hoods Foundry Side hood 200 to 300 cfm per linear ft of hood with slot velocities of 1500 fpm. Exhaust take-off every 8 to 10 ft 3500 Ref. 20 Rock drilling Dry drilling (rock) Special trap (see refer- 60 cfm--vertical (downward) erences) work 200 cfm--horizontal work 3500 Ref. 26,27. May vary with size and speed of drill. Wet drilling offers alternate con trol methods Rubber calender rolls Canopy--side panels 75-100 fpm indraft 3500 Ref. 24, 31 Quarts fusing Booth on beneh 150-200 fpm at face Ref. 34, 35 8creens Vibrating Flat deck Enclosure 150-200 fpm indraft through hood openings but not less than 25-60 cfm per sq ft of screen area 3500 Ref. 15, 19 Shakeouts Foundry Enclosure 200 fpm through all openings in enclosure, but not less than 200 cfm per sq ft of grate area 3500 Ref. 20, 22 Spray coating Booth--operator inside 100-200 fpm at booth cross-sec tion Booth--operator out 150-200 fpm at booth cross-sec side tion Booth--downdraft 100-200 fpm downdraft 1500-2000 1500-2000 1500-2000 Ref. 15 Use higher ventilation rate for small booths 4 sq ft or less Tanks, open surface See Table 6 Ref. 36, 37, 38, 39 Tumbling mill* Hollow trunnion type Exhaust connection by manufacturer Use branch diameter same size as exhaust outlet. For round mills branch dia should be X dia of mill; for square mills branch dia should be 1 in. plus X side dimension of mill 3500-5000 Ref. 14 Tumbling mills, drums, Enclosure cages, barrels 400 fpm through openings but not less than 75 cfm per sq ft plan area 3500 Where equipment is enclosed and dust tight during rota tion, enclosure may not be needed if feed and discharge operations can be otherwise controlled Welding Local hood with flange Downdraft bench Booth 6 in. from are--150 cfm 6-9 in. from arc--275 cfm 8-10 in. from arc--425 cfm 10-12 in. from arc--600 cfm 150-250 cfm per sq ft grille area 100 fpm at booth face 2000-4000 2000 2000 Ref. 40 Woodworking See Table 7 Ref. 22, 41 Miscellaneous Complete enclosure Packaging, machines, granulators, enclosed oust producing units Packaging, weighing Booth container filling, in spection Downdraft 100-400 fpm indraft through in spection or working openings, but not less than 25 cfm per sq ft of enclosed plan area 50-150 cfm per sq ft of open face area 75-150 cfm per sq ft of dust pro ducing plan area 3000 3000 3500 Ref. 19 Industrial Exhaust Systems 725 For high canopy hoods. Equation 5 may be used: q. - 7.4 i. B,w (5) tokere l, = effective height, feet. The effective height l. may be taken as the actual vertical distance from hood to hot surface plus twice the width of the hot surface. Where it is necessary to have openings at the top of a hood that is filled with heated air, leakage of the hood contents through these openings may be prevented by u<dng sufficient ventilation to obtain a face velocity V as calculated from the following equation: v-c (6) where = height of the air column, feet. A) area of the (sharp-edged) openings, square feet. H, = sensible heat released to air stream, Btu per minute. C = a coefficient depending on the excess of temperature inside the hood above room temperature with values as follows: Temperature excess 0-200 200-400 400-600 600-800 (F deg) Value of C 20 18 16 14 The American Society of Heating and Air-Condi tioning Engineers has recognized the need for design criteria for determining exhaust ventilation requirements for hot processes in industry. Research studies on this subject have been in progress since 1952, and the results to date have been published." Induced Air Flow Where quantities of individual particles are projected through an air space by gravity or by process forces, volumes of room air in proportion to the momentum of the particles are set in motion with resulting intermixing and flow in the same direction as the particles. Exhausted volumes from hoods or enclosures must be sufficient to include this induced air flow if control is to be effective. Induced air flow should be evaluated from high speed rotating machines including pulverizers, from material handling systems employing high speed belts or involving large tonnages of falling granular material, and from escaping compressed air jets from pneumatic tools. The energy represented by material of various particle sizes and falling heights has been computed by Hemeon.** The theoretical equivalent induced air flow resulting from un enclosed air streams is summarized in Table 4. Where falling streams occur largely within an enclosure such as a storage bin, it should be recognized that much of the air set in motion is recirculated within the bin. Exhaust volumes from the enclosure need only include the extra air induced before the falling material enters the enclosure although the recirculating induced air can cause localized positive pressures and outward leakage if the enclosure is not of airtight construction. ' The amount of air induced by failing material is much /less if the space through which the material falls is en closed effectively than if not enclosed, and can be re- Toble 4 ..., Induced-Air-How Equivalent of the Energy in Falling, Unendosed Streams of Particles (for a solidt Bow rota k of t lb per tee and o specific gravity i I referred to water) FoDing Distance, i Shwoza Area, tq ft Air Flow Eqvivaler.1, Cftn, f be tmdtipGod by (ft/z)ln Partide She, MBEnetcns ? 2 5 10 20 50 100 3 ft X 450 350 220 180 150 80 65 1 750 550 350 300 250 . 150 100 2 1200 850 550 450 350 200 170 4 1900 1400 850 700 600 350 250 8 3000 2200 1400 1100 900 500 400 15 4600 3300 2000 1700 1400 800 650 25 6400 4600 3000 2500 2000 1000 900 50 10000 7500 4700 4000 3200 1800 1400 6 ft X l 2 4 350 300 250 200 150 600 500 350 300 200 900 700 600 400 350 1500 1200 900 700 500 12 ft 8 15 25 50 X l 2 4 2300 3500 5000 8000 560 900 1500 2300 1800 2800 4000 6000 450 700 1100 1800 1500 2200 3200 5000 350 600 900 1500 1000 1500 2300 3700 270 420 700 1000 900 1300 1800 3000 220 350 550 850 20 ft 8 15 25 50 X 1 2 4 3600 2900 5300 4200 7800 6000 10000 800 1300 2000 3300 650 1000 1600 2600 2300 3500 5000 8000 500 800 1300 2000 1700 2600 3600 6000 370 600 960 1500 1300 2100 2900 4700 300 500 760 1200 30 ft 8 15 25 X 2 4 5000 8000 -- 1000 1700 2700 4400 4100 6400 9000 850 1400 2100 3500 3200 5000 7000 650 1100 1700 2700 2400 3700 5000 500 800 1300 2000 1900 2900 4000 400 600 1000 1600 8 6800 5400 4200 3200 2500 --15 10000 8200 6600 4800 4000 25 -- 9000 6800 5400 From Flantaad Promts VntQotion by W. C- L. Hemeon, The Industrial Press New York (1SSS). duced to a minimum by enclosing as well as possible the openings at the feed point of the material. DUCT SYSTEM DESIGN The duct system will consist of branch ducts connected to a main duct that will convey the air from the hoods to the exhaust fan and the air cleaning equipment, if used. Round ducts should be used wherever possible. Their gage size and construction differ from air supply practice due to the rougher usage encountered, and in the case of dusts, to the abrasive effect. (See later section on Construction Specifications for Local Exhaust Sys tems.) Usual conveying velocities are shown in Table 8. Where .solid contaminants are handled, recommended ve locities must be maintained throughout the system to prevent material from settling in the ducts and obstructing