Document 37kJ5JQmjG84o56vqODLLvVBa

1128 CHAPTER 45 1958 Guide y Table 6. Ventilation Rates fob Open Surface Tanks**-87 Note I--Column A refers to tamk with hood alone one side or two parallel sides when one hood is against a wall or a baffle running length of tank and as high as tank is wide; also to tanks with exhaust manifold along center Htib with ^ becoming tank'width in Wfl* Ratio. fyjiiTnTi B refers to free standing tank with hood along one side or two parallel sides. Note 2--Complete control-of the vapors and mist from degreasing operations requires the same ventila tion rates as recommended for pickling. However, the solvents employed in degreasing operations are rela tively volatile, and the solvent loss caused by evaporation increases rapidly as the exhaust rate at the tana increases. For this reason perfect control is usually sacrificed in favor of lower solvent evaporation rates. Where solvent loss does not present an important cost.or operating problem, the exhaust rates gJ*e" fox pickling should be adhered to, but where solvent loss must be kept at a minimum, an exhaust rate (m 50 tarn per sq ft of area is commonly employed. Where this rate is used, control will be adequatei onjy if the is located in an area free of drafts and if the degreasing operations are carried out in accordance with a rigid schedule.0 and dilution have taken place may be approximated for low canopy hoods above horizontal surfaces and where no heat from steam is involved, as q, = 5.4 A.* h (AJ)` (3) where A% -- surface area of hot body, square feet* 1 Af * the temperature difference, hot body to room air, Fahrenheit degrees. Similarly, where the heat is furnished by steam from a tank of hot water, q, = 290 A. -V^TC where G = the rate of steam formation, pounds per (square foot'water surface) (minute)- Industrial Exhaust Systems 1129 For high canopy hoods, Equation 5 may be.used: q, = 7.4 He'/> (5) where U = effective height, feet. The effective height k 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 using sufficient ventilation to obtain a face velocity V as calculated from the following equation: - 'V (6) where h = height of the air column, feet. A t = 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 (F deg) 0-200 200-400 400-600 600-800 Value of C 20 -18 - 16 14 The American Society of Heating and Air-Conditioning Engineers has recognized the need for design criteria for determining exhaust ventila tion requirements for hot processes in industry. Research studies on this subject have been in progress since 1952, and the results to date have re cently been published.42 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 handlinjg 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 jailing heights has been computed by Hemeon13 and the theoretical equivaIcnt induced air flow resulting from unenclosed air streams is summarized in Table 4. Where falling streams occur largely within an enclosure such as a storage j f1'ib should be recognized that much of the air set in motion is recircufatecf within the bin. Exhaust volumes from the enclosure need only the extra air induced before the falling material enters the enclosure although the recirculating induced air can cause localized positive pressures na outward leakage if the enclosure is not of air tight construction. ,, The amount of air induced by falling material is much less if the space urough which the material falls is enclosed effectively than if not enclosed,