Document 2jgapDaDnmvM0Bk30D6JgqGbp

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 Further, the velocity contours are identical for similar hood shapes when the hoods are reduced to the same basis of comparison. These facts are applicable to all hood problems so that when the velocity contour distribution is known, the air flow required can be determined. Fig. 1 shows the contour distribution in two axial planes perpendicular to the sides of a rectangular hood with a side ratio of one-half. The distribu tion shown is identical for all openings with a similar side ratio provided the mapping is as shown in the figure. The contours, of course, are expressed as percentages of the velocity at the opening. Fig. 1. . Velocity Contours for a Rectangular Opening with a Side Ratio of One-Half. Contours are Expressed as Percentages of the Velocity at the Opening Air Flow from Static Readings The volume of air flow through any hood may be determined from the following equation: where Q = 4005/ A y/HT (2) Q = volume of air flow, cubic feet per minute. A = area of connecting duct, square feet. kt = static suction at throat of hood, inches of water. = orifice or restriction coefficient which varies from 0.6 to 0.9 depending on the shape of the hood. 732 CHAPTER 40. INDUSTRIAL EXHAUST SYSTEMS Aii average value of / is 0.71, although for a well-shaped opening a value of 0.8 may be used. The factor / is determined from the equation: where h, is the velocity head in the connecting duct. The term static suction is not a good measure of the effectiveness of a hood unless the area of the opening and the location of the operation with respect to the hood are known. This is clearly indicated by Equation 1 which shows that the velocity at any point along the axis varies inversely as the area of the opening and the square of the distance. However, this formula coupled with Equation 2 should serve to indicate the velocity conditions to be expected when operations are conducted external to the hood opening. Large Open Hoods Large hoods, such as are used for electroplating and pickling tanks, should be sub-divided so the area of the connecting duct is not less than one-fifteenth of the open area of the hood. Frequently, it will be found necessary to branch the main duct in order to obtain a uniform distri bution of flow. Canopy hoods should extend 6 in. laterally from the tank for every 12 in. elevation, and wherever possible they should have side and rear aprons so as to prevent short circuiting of air from spaces not directly over the vats or tanks.- In most cases, hoods of this type take advantage of the natural tendency of the vapors to rise, and air velocities may be kept low. Cross drafts from open doors or windows disturb the rise of the vapors and therefore provision must be made for them. The air velocities required also depend upon the character of the vapors given off, cyanide fumes, for example, requiring an air velocity of approxi mately 75 fpm on the surface of the tank and acid and steam vapors requiring velocities as low as 25 to 50 fpm. The total volume of air flow necessary to obtain these velocities may be approximately determined from the following simple formula: Q = 1.4PDV (4) where Q = total volume of air handled by hood, cubic feet per minute. P = perimeter of the tank, feet. D - distance between tank and hood opening, feet. V = air velocity desired along edges and surface of tank, feet per minute. Lateral Exhaust Systems The lateral exhaust method, as developed for chromium plating, is applicable in many instances in preference to the canopy type hoods. The method makes use of drawing air and fumes laterally across the top of vats or tanks into slotted ducts at the top and extending fully along one or more sides of the tanks. The slots are 2 in. wide and for effective `Health Hazards in Chromium Plating, by J. J. Bloomfield and Wm. Blum (U. S. Public Health Report, Vol. 43; No. 26. September 7, 1928). 733