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960 CHAPTER 45 1952 Guide I^ Design Based on Total. Air Flow '' Where the foregoing factors Ire not known, the usual method of de signing an exhaust system is to base the air flow through the system on rates of flow (through each hood) which have been found by expeiehce to provide adequate control. " For woodworking systems the sizes of branch connections in common use are given in Table 2. Similar data for grinding and buffing wheels are given in Table 3. Velocity Contours - It is possible, by use of a specially constructed Pitot tube,12 to map con tours of equal velocity in any axial plane located in the field of influence. It has been found that the positions of these contours for any hood can be Table 3.- Branch Pipe Sizes fob Gkinding and Buffing Hoods Based on a Pipe Velocity of 4500 fpm: Type of Wheel Wheel Size Diameter, In. Min. Max. Maximum Width In. Area Sq In. Branch Pipe Minimum Diameter, In. Grinding 9 9 18 18 24 24 30 30 36 Disc Grinding 20 20 30 8 Buffing, Polishing and Scratch 8 16 Brushing 16 24 24 30 i 30 3 3 175 . 4 4 300 5 5 500 6 6 700 7. .300 4 5 2 50 ' 3^ 3 150 4 4 300 5 6 600 6 expressed as percentages of the velocity at the hood opening, and are purely functions of the shape of the hood.1* , Further, the velocity contours are identical for similar hood shapes when the hoods are reduced to the same basis of comparison. These facte 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 having a side ratio of one-half. The distribu tion shown is idential for all openings with a similar, side ratio, provided the mapping is as shown in the figure. The contours are expressed as per centages of the velocity at the opening. Low Velocity Systems On multiple installations of the same operation, it is often possible to institute a great saving in power cost by designing an exhaust system using low velocities in the main ducts. Such a system, for use in grinding and shaping porcelain, has been described.14 In these operations, the separate machines are grouped around a central plenum chamber, and exhausted by means of a low pressure fan connected to the plenum. In Industrial Exhaust Systems 961 one such case, a power saving of over 90 percent was obtained.. A similar design technique15 has been described for use in ventilating plating tanks. Canopy Hoods Canopy hoods are being replaced by other types of hoods, such as siotted hoods at tank operations. : Where canopy hoods are used, they should ex tend 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- Fig. 1. Velocity Contours fob Rectangular Opening with a Side Ratio of One-Half. Contours are Expressed as Percentages of the . Velocity at the Opening 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. Gross drafts'from open doors or windows disturb the rise of the vapors, and therefore 'considera tion must be given them. The air velocities required also depend upon the character of the vapors given off. The recommended minimum- cap ture velocity is 100 fpm. The quantity of air which must be exhausted to obtain any given cap ture velocity is expressed by the following equation: Q = 1.4 PDV where , Q = quantity of air exhausted by hood, cubic feet per minute. P = perimeter of the tank, feet. D = distance between tank and hood opening, feet,V =' capture velocity, feet per minute. ' (5)