Document wgzbenY16xKxyoE4R4aQM8zX6

924 CHAPTER 45 1950 Guide Design Based on Total Air Flow ' Where the foregoing factors are not known, the usual method of de signing an exhaust system is to base the air flow through the system oh rates of flow (through each hood) which have been found by experience 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 tube1* to map con tours of equal velocity in any axial plane located in the field of influence. Ithas been found thatthe positions of these contours for any hood can be Table 3. Branch Pipe Sizes fob Grinding and Buffing Hoods Based on a Pipe Velocity of iSOO /pm. Type of Wheel Grinding . Disc Grinding Buffing, -Polishing and Scratch Brushing Wheel Sub Diameter. In. Min. Max. 9 9 18 18 . 24 24 30 ` 3 36 20 20 30 8 8 16 16 24 24 30 Maximum Width in. Area Sq In. Branch Pips Minimum Diameter, In. i 30 3 175 4 300 . 5 500 6 700 3 4 5 6 7 300 .... 4 5 2 50 3 150 4 300 6 600 3H 4 5 6 expressed as percentages of the velocity at the hood opening .and are purely functions of the.shape of the hood1*. 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 showB 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 ex pressed as percentages 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 described14. 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 925 one such case a power saving of over 90 per cent was obtained. A similar design technique1* has been described for use in ventilating plating tanks. Canopy Hoods Canopy hoods are being replaced by other types of hoods, such as slotted hoods at tank operations. Where canopy hoods are used, they should Fig. 1. Velocity Contours fob Rectangular Opening with a Side Ratio of One-Half. Contours abb Expressed; as Percentages of the Velocity at the Opening 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. The recommended minimum capture velocity is 100 fpm.. The quantity of air which must be exhausted to obtain any given capture velocity is expressed by the following equation: Q - 1.4 PDV where Q = quantity of air exhausted by hood, cubic feet per minute. P TM perimeter of the tank, feet. . D. = distance between tank and hood opening, feet. Y capture velocity,-feet per minute.; . (5)