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840 CHAPTER 46 1946 Guide 200 fpm, depending' on the type of hood used, are recommended as sufficient for safe controlConsidering the character of the industry, air velocities of this order may be extended to similar dusty operations. The method for approximately determining these velocities in terms of the velocity at the hood opening is given in Equation 3. No set rule can be given regarding the shape of a hood for a particular operation, but it is well to remember that its essential function is to create an adequate velocity distribution. The fact that the zone of greatest effectiveness does not extend laterally from the edges of the opening may frequently be utilized in estimating the size, of hood required. Where complete enclosure of a dusty operation is contemplated, it is desirable to leave enough free space to equal the area of the connecting duct. Hoods for grinding, polishing and buffing should fit closely, but at the same time should provide an easy means for changing the wheels. ` It is advisable to design these hoods with a removable hopper at the base to capture the heavy dust and articles dropped by the operator. Such provisions are of assistance in keeping the ducts clear. Air volumes used to control many Table 5. Recommended,Air Velocities Through Openings in Hoods Enclosing Operations or Located Over Zones of Generation of Dusts, Fumes, Vapors and Gases Released in Certain Manufacturing Processes Condition of Generation of Contaminant Released without noticeable movement...... .................... Minimum Air FPMVelocity, 50-100 Released with low velocity 100-200 Active generation.............'..... Released with great force.... 200-500 500-2000 Process Evaporation of vapors, exhaust from pickling, washing, degreasing, plating, welding, etc. Paint spraying in booth; inspection, sorting, weighing, packaging, low speed conveyor transfer points, rotating mix tures, barrel filling. Foundry shakeout, high speed conveyor transfer points, crushers, screens. Grinding, tumbling mills, abrasive clean ing. dust discharges may often be reduced by effective baffling or partial enclosure of an operation. This procedure is strongly urged where dusts are directed'beyond the zone of influence of the hood. Axial Velocity Formula for Hoods - When the normal flow of air into a hood is unobstructed, Equation 3 may be used to determine the air velocity at any point along the axis': V= 0.1. Q + 0.1 A where V = velocity at point, feet per minute. Q = volume of air handled, cubic feet per minute. x = distance along axis, feet. A - area of opening, square feet. (3). Velocity Contours It is possible by use of a' specially constructed Pitot tube' to map contours of equal 'velocity in any axial plane located .'in the field of in fluence.' It has been found that-the positions of these contours for any hood tan be expressed, as percentages of the velocity at the hood opening and are purely functions of the shape of the hood 8i Exhausting and Conveying Systems 841 Further, the velocity coiitours 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. - Low Velocity Systems Oh 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 described9. 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 this case a. power saying of over 90 per cent was obtained. A similar design technique10 has been described for use in ventilating plating tanks. Large Open Hoods Large hoods, such as may be used for electroplating and pickling tanks, should be sub-divided so the area of the connecting duct is hot less than one-fifteenth of the open area of the hood.1 Frequently, it will be.found necessary, to. branch the main duct in order to obtain, a uniform distri-: X