Document jBZK53ZJ3bN9Vdmdk4OYZBxBN

706 CHAPTER 52 1959 Guide Table 3 .... A Guide to Ventilation Rates for Typical Industrial Equipment (Continued) State or Local Kegelatiota SboaU be Comalted and Followed Where Higher Veatfafran Rate* or* Specified Operation Tjrpt of Hood VeaiBatioe Air Row Uwaf Tnatspof Velocity Fpm Resort* aod References Pouring hoods Foundry Side hood 200 to 300 cfm per linear ft o: hood with slot velocities ol 1500 fpm. Exhaust take-of every 8 to 10 ft 3500 Ref. 20 Rock drilling Dry drilling (rock) Special trap (see refer- 60 cfm--vertical (downward] erenees) work 200 cfm--horizontal work 3500 . Ref. 28,27. 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 bench 150-200 fpm at faee Ref. 34, 35 Screens Vibrating Flat deck Enclosure 150-200 fpm indraft through hood openings but not less than 25-50 cfm per sq ft of screen area 3500 Ref. 15, 19 8hakeouts 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. 38, 37, 38, 39 Tumbling mills 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 l 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 arc--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 200Q Ref. 40 Woodworking See Table 7 Ref. 22, 41 Miscellaneous Complete enclosure Packaging, machines, granulators, enclosed dust 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 faee area 75-150 cfm per sq ft of dust pro ducing plan area 3000 3000 3500 Ref. 19 Industrial Exhaust Systems 707 For high canopy hoods, Equation 5 may be used: q, m 7.4 (5) where lt -- 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 ncing sufficient ventilation to obtain a face velocity V as calculated from the following equation: where lm -- height of the air column, feet. Af -> 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.*1 Induced Air How 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.0 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 failing 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 falling material is much less if the space through which the material falls is en closed effectively than if not enclosed, and can be re- Table 4 .... Indueed-Air-Row Equivalent of the Energy in Falling, Unenclosed Streams of Partides (Foe a tofid* flow rate R of 1 U> per tee aod a tpeerfie groWfy ( I referred to water) Faffing Dtdaace,* Area, tq ft Air Row Eqe/ratecf,' Cfm, to be nettipBed by Partkte Size, MiDbaefem 1 2 5 10 20 50 100 3 ft . 6 ft 12 ft 20 ft 30 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 X 350 300 250 200 150 600 500 350 300 200 2 900 700 600 400 350 4 1500 1200 900 700 500 8 2300 1800 1500 1000 900 15 3500 2800 2200 1500 1300 25 5000 4000 3200 2300 1800 50 8000 6000 5000 3700 3000 X 560 450 350 270 220 1 900 700 600 420 350 2 1500 1100 900 700 550 4 2300 1800 1500 1000 850 8 3600 2900 2300 1700 1300 15 5300 4200 3500 2600 2100 25 7800 6000 5000 3600 2900 50 -- 10000 8000 6000 4700 X 800 650 500 370 300 1 1300 1000 800 600 500 2 2000 1600 1300 960 760 4 3300 2600 2000 1500 1200 8 5000 4100 3200 2400 1900 15 8000 6400 5000 3700 2900 25 -- 9000 7000 5000 4000 X 1000 850 650 500 400 1 1700 1400 1100 800 600 2 2700 2100 1700 1300 1000 4 4400 3500 2700 2000 1600 8 6800 5400 4200 3200 2500 15 10000 8200 6600 4800 4000 25 -- 9000 6800 5400 From Plantand Aium VtntHatian by W. C- L. Hemoon, Tbo Industrial Press New York <MS5). 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 min 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