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898 CHAPTER 46 1949 Guide Table 2. Branch Pipe Size for Woodworking Machine Hoods Based on a Pipe Velocity of 4000 fpm. Type or Machine Self feed table saw Other single saws. . Saws with Dado Head Band saws' Disc sanders Triple drum sanders Size. In. Min. Max. No. op Branches 18 2 3 18 26 32 38 30 . 36 42 18 2 3 6 18 ' 28 32 38 48 30 36 . 42 48 2 1 1 T 2 2 2 1 1 2 2 3 1 1 1 1 Minimum Diameter, In. Bottom Branch 5 4 5 5 4 5 `4 5 5 7 8 9 10 Top Branch 4 14 5 6 4 4 5 Others 4 4 4 .. 4 Single drum sanders; ' (area in sq in.) Horizontal belt sanders Vertical belt sanders Jointers hi ' ! ' ' \ Single planers Tenorier 350 700 1400 350 700 1400 2800 9 9 14 6 69 9 .... 14 8 8 20 20 20 26 26. 36 1, 2 2 1 1 1 1 1 1 1 1X 2 4a 5 6 7 5 6 4 5 6 4' 5 5: ,6 7 .. '5 4 4' '- ,,,, 5 . \> ,, &Not over 10 In. diameter. - 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 Industrial Exhaust Systems .. 899: assistance in keeping the ducts clear. The air quantity, required to capture dust which is thrown or projected in a. direction.away from the hood.at considerable velocity, 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. . Air Flow from Static Readings State Codes for local exhaust systems at certain operations list minimum static suction requirements which may range from l-J in. to 5 in. water column. Frequently, in grinding, buffing and polishing 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. The static suction at the throat of a hood is frequently used in practice as a measure of the effectiveness of control. Where the hood coefficient is known the volume of air flow through any hood may be determined from the equation: Q = 4005 JAy/h, (2) where Q = quantity of air exhausted, cubic feet per minute. A = area of connecting duct, square feet. h> = static suction measured at approximately 3 diameters from throat of hood, inches of water. / = orifice or restriction coefficient which varies from 0.6 to 0.95 depending on the shape of the hood* Ah average value of / is 0,8, although for a well-shaped opening a value of 0.85 to 0.9 may be used. The factor f is determined from the equation: where hv is the velocity head in the connecting duct. The sialic 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 4' which shows that the velocity at any point along the axis varies approximately inversely as the square of the distance. : This formula coupled with Equa tion 2 should serve to indicate' the velocity conditions to be expected when operations are conducted externally to the hood opening. Axial Velocity Formula for Hoods : When the normal flow of .air into a hood is unobstructed, Equation 4 may be used to determine the air velocity at any point along' the axis11: V = ----0--.-1- -Q-- i* + 0.1 A (-0 where V = velocity at point, feet per minute. Q =. quantity of air exhausted, cubic feet per minutex = distance, along axis, feet. A = area of opening, square feet. 'X