Document V3Y0mzqx0YeXRwwVQMKGy7K24

696 Chapter 40 1945 Guide Table 1. Rates of- Flow Through Branch Pipes .Woodworking Machines Pipe Diameter, In. 3 4 5 . Air Volume, Cfm 200 350 550 Pipe Diameter, In. 6 7 8 Air Volume, Cfm 800 1100 1400 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. Min. Max. No. OF Branches 2 Minimum Diameter, In. Bottom Branch 5 Top Branch 4 Others 18 1 18 1 4. 5 15 22 4 4 2 32 5 4 3 62 5 5 18 1 4 18 28 1 . 5 26 32 2 4 4 32 38 2 5 4 38. 48 3 5 4 4 30 1 . 7 30 36 1 8 36 42 . 1 9 42 48 1 10. Single drum sanders: (area in sq in.) Horizontal belt sanders Vertical belt sanders Jointers Single planers* Tenoner 350 700 1400 9 6 9 8 20 26 350 700 1400 2800 9 14 6 9 14 8 20 20 26 36 1 2 2 1 1 1 1 1 1 1 1 2 4a 5 6 7 5 4. 6- 4 4 5 6 4 5 5 6 7 55 aNot over 10 in. diameter. Table 3. Rates of Flow Through Branch Pipes Grinding and Buffing Wheels Pipe Diameter, In. 3 4 5 Air Volume, Cfm 225 400 600 Pipe Diameter, In. 6 7 Air Volume, Cfm 900 1200 Industrial Exhaust Systems 697 hood so that the volume of air necessary to produce results is. as small as possible. This will reduce the size of equipment,, the power required by the system, and also the heating load requirements in winter. Air Flow from Static Readings 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: where Q = 4005 / A (1) Q = volume of air flow, cubic feet per minute. A = area of connecting duct, square feet. ht = static suction measured 3 diameters from throat of hood, inches of water. / = orifice or restriction coefficient which varies from 0.6 to 0.9 depending on the shape of the hood. An average value of/is 0.71, although for a well-shaped opening a value of 0.8 may be used. The factor / is determined from the equation: where hv is the velocity head in the connecting duct. The static 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 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. 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 4500fpm, are used. Table 4 gives the minimum branch pipe sizes to be used on the more common sizes of grinding and 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 lj^> to 5 in. water displacement in a [/-tube, must be followed in such states although in several instances they 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.