Document 7MjLzR4EM961a7gXv3Ld3QGQo

HEATINC VENTILATING AIR CONDITIONING CUIDE 1943 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 3 which shows that the velocity at any point along the axis varies approxi mately inversely as the square of. the distance. However, this formula coupled with Equation 1 should serve to indicate the velocity conditions to be expected when operations are conducted external to the hood opening. Table 2. Branch Pipe Size for Woodworking Machine Hoods Type of Machine Self feed table saw Other single saws Saws with Dado Head Band saws Disc sanders Triple drum sanders Size, In. Mill. Max. 18 18 2 23 3 6. 18 18 28 26 32 32 . 38 38 .48 30 30 36 36 42 42 48 No. OF Branches Minimum Diameter, In. . Bottom Branch Top Branch Others 254 14 15 15 2: 4 - : 4 254 255 14 15 244 254 3 5 .4 4 17 18 19 1 10 Single drum sanders: (area in sq in.) Horizontal belt sanders Vertical belt sanders Jointers Single planers. Tenoner 350 700 . 1400 350 700 1400 2800 9 9 14 6 69 9 14 .8 8' 20 20 20 26 26 36 1 ' 4a 5 6 7 2 - 5. V6 14 15 16 14 1 5 15 1 6 1 .7 25 4 4 5 aNot over 10 in. diameter. 734 CHAPTER 40. INDUSTRIAL EXHAUST SYSTEMS Table 3. Rates of Flow Through Branch Pipes Grinding and Buffing Wheels Pipe Diameter, In. 3 4 5 6 .7 Air Volume, Cfm 225 400 600 900 1200 Design Based on Total Air Flow Where the foregoing, factors are not known, the usual method- of designing an exhaust system is to base the air flow through the system on rates of flow through each hood which have been found by- experience to provide adequate control. For woodworking systems the rates of flow given in Table 1, calculated on the basis, of a branch velocity of 4000 fpm, are adequate'for control. Using these air flow rates, Table 2 gives the size of pipe connections to be used with the more common wood working machines. Properly designed grinding and buffing wheel , hoods have been found to be adequately controlled when the rates of air flow given in Table 3, calculated on the basis of a branch velocity of 4500 fpm, are used. Table 4 gives the minimum branch pipe sizes to be used on the more common sizes of grinding aiid buffing wheels. In some states grinding, polishing and buffing wheels are subject to regulation by codes. (See Standards Chapter 48.) The static suction requirements, which range from 1)4 to 5 in. water displacement in a U-tube, must be followed in such states although in several instances they may appear to be excessive. Frequently, in these operations, a large part of the wheel must be exposed and the dust-laden air within the hood is thrown outward by the centrifugal action of the wheel, thus counter acting useful inward draft. This tendency may be diminished by locating the connecting duct so as to create an air flow of not less than 200 fpm past the lower edge of the wheel. Table 4. Branch Pipe Sizes for Grinding and Buffing Hoods Type of Wheel Grinding Disc Grinding Buffing, Polishing and Scratch Brushing Wheel Size Diameter, In. Min. Max. 9 18 24 30 - 9 . 18 24 30 36 20 20 .30 8 8 16 16 24 24 30 Maximum Width In. i 3 4 5 6 ._ 2 3 4 6 Area Sq In. 30 175 300 500 700 300 50 150 300 600 Branch Pipe . Minimum Diameter, 3 4 5 6 7 4 5 3J4 4 5 6 73d