Document Z4a9Epxe8Qrmgb4rvmJ0gerBY

566 Chapter 31 1945 Guide where = absolute viscosity, pounds per foot second (see Fig. 3) p = density, pounds per cubic foot. The absolute viscosity of dry air at various temperatures is given in Fig. 3. It is assumed that the viscosity is not appreciably affected by the moisture content. For temperatures ordinarily used in heating, ventilating and air con ditioning work, the correction for viscosity'may be neglected, without. serious error. The correction equation would then be simplified to: . Ha = X 5 (6) Example 1. Assume that it is desired to circulate 10,000 cfm of air through 75 ft of 24 in. diameter pipe. Find 10,000 cfm on the left scale of Fig. 2 and move horizontally right to the diagonal line marked 24 in. The other intersecting diagonal shows that the velocity in the pipe is 3200 fpm. Directly below the intersection it is* found that the friction per 100 ft is 0.41 in.; then for 75 ft the friction will be 0.75 X 0.41 = ,0.31 in. In a like manner any two variables may be determined by the intersection of the lines representing the other two variables. . Circular Equivalents oi Rectangular Ducts Where rectangular ducts are used it is frequently desirable to know the equivalent diameter of round pipe to carry the same capacity and have the same friction per foot of length. Table 1 gives directly the circular equivalents of rectangular ducts for equal friction and capacity;- which are based on values determined from Equation. 7: 4d = 1.265 (a b)' a+b (7) . where a = one side of rectangular pipe, feet or"inches. ' 6 = other side of rectangular pipe, feet or inches. " d = equivalent diameter of round pipe for equal friction per foot of length to carry . the same capacity, feet or inches. Rectangular equivalents of round ducts are also given in the curves of Fig. 4 which are plotted from data based on Equation 7. To use the chart, locate the diagonal curve giving the diameter of the round duct. The width and height of an equivalent rectangular or square duct may then be read as the abscissa and the ordinate,of any point of the curve. Multiplying or. dividing the length of each.side of a pipe by a constant is the same as multiplying or dividing the equivalent, round size by-the same constant. Thus, if the circular equivalent of an 80 x 24-in. duct is Table 1.' Circular Equivalents of Rectangular Ducts for Equal Friction ---- 8n>s RiCTANQUDAB Duct 3 3.5 4 4.5 5 5.5 8 8.5 9 9.5 10 10.5 5.2 5.4 5.5 5.7 5.7 5.9 6.0 6.2 6.1 6.3 6.5 6.7 6.5 6.7 6.9 7.1 6.9 7.1 7.3 7.5 7.3 7.5 7.7 7.8 5.8 6.3 6.8 7.2 7:7 8.1 5.9 6.5 7.0 7.4 7.8 8.3 11 11.5 6.0 6.2 6.6 6.7 7.1 7.2 7.6 7.7 8.0 8.2 8.5 8.6 12 12.5 13 13.5 14 14.5 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0 7.1 7.3 7.4 7.5 7.4 7.5 .7.7 7.8 7.9 8.1 7.9 8.0 8.2 8.4 8.5 8.6 8.3 8.5 8.7 8.8. 8.9 9.1 8.8 9.0 9.2 9.4 9.5 9.6 15 15.5 6.9 7.0 7.6 7.7 8.2 8.3 8.7 8.9 9.2 9.4 9.8 9.9 16 7.1 7.8 8.4 9.0 9.5 10.1 Table 1. 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