Document rp81YMNNJzbJ67G94bX494OMq

254 CHAPTER 21 1962 Guide And Data Book Table 3 .... A Guide to Ventilation Rates for Typical Industrial Equipment (Continued) State or local Sogtrfofioa* SkoaU bo Coantterf aad FoOowmd Wtm HiglMr VonfSafiaa Catos aro Spaa'lted Opervtioa Type of Hood VontSofMa AkFtew l/toef Tramped VaJocffy Fpa baaii and titenam Pouring hoods Foundry Side hood 200 to 300 efm 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 efm--vertical (downward) enenees) work 200 efm--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 bench 150-200 fpm at face Ref. 34, 35 Screens Vibrating Flat deck Enclosure 150-200 fpm indraft through hood openings but not less than 25-50 efm 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 efm per sq ft of grate area 3500 Ref. 20, 22 Spray coating Booth--operator inside Booth--operator out side Booth--downdraft 100-200 fpm at booth cross-sec tion 150-200 fpm at booth cross-sec tion 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 n)ill Hollow trunnion type Exhaust connection by manufacturer Use branch diameter same sise as exhaust outlet. For round mills branch dia should be ) dia of mill; for square mills branch dia should be 1 in. plus ^ side dimension of mill 3500-5000 Ref. 14 Tumbling mills, drums, Enclosure cages, barrels 400 fpm through openings but not less than 75 efm 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 efm 6-9 in. from arc--275 efm 8-10 in. from arc--425 efm 10-12 in. from arc--600 efm 150-2S0 efm per sq ft grille area 100 fpm at booth face 2000-4000 2000 2000 Ref. 40 Woodworking See Table 7 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 efm per sc - ft of enclosed plan area 50-150 efm per sq ft of open face area 75-150 efm per sq ft of dust pro ducing plan area 3000 3000 3500 Ref. 22, 41 Ref. 19 Industrial Exhaust Systems For high canopy hoods. Equation 5 may be used: q, - 7.4 L* H(5) tjfcerc , l, = effective height, feet. The effective height L may be taken aa the actual ygrtical distance from hood to hot surface plus twice the vidth 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 sufficient ventilation to obtain a face velocity V as calculated from the following equation: 255 Table 4 .... Induced-Air-Flow Equivalent of the Energy in Fading, Unenclosed Streams of Particles r- trhen l height of the air column, feet. Af - area of the (sharp-edged) openings, square feet. B, -- sensible heat released to air stream, Btu per minute. *> 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 Societt op Heating, Refrigerating anb Ais-CoNnmoNZNG Engineers has recognized the need for design criteria for determining exhaust ventilation require ments for hot process in industry. Research studies on this subject have been in progress since 1952, and the results to date have been published.0 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 compresed air jets from pneumatic tools. The energy represented by material of various particle axes and falling heights has been computed by Hemeon.a 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 ttti* 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 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- New Ytrk (1855). 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 uspd Round ducts should be used wherever possible. Thar gage size and construction differ from air supply practice due to the rougher usage encountered, and in the <agp 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