Document VJNnBa6G3ZV6pNk8v1Oper7Zj

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 CHAPTER 32. AIR DUCT DESIGN 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 = H3 X S (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 ini; 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. CU FT OF AIR PER MINUTE .03 04 .06 08.1 2 3 4 .6 3 1 23 FRICTION IN INCHES OF WATER PER 100 FT WITH JOINTS Fig. 2. Friction of Air in Pipes 608 8 10 rio + cwr k T + t/Uo M* poises Tsdeg abs cent C- Sutherlands constant = 120 for air Mo at 10=1709 x Iff6 t c0. Pounds mass per ft per sec poises x 0.0672 0 40 80 120 ISO 200 240 SIR TEMPERATURE OEG FAHR . Fig. 3. Temperature--Viscosity.Curve for Dry Air Circular Equivalents of 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: d = 1.265 - Ya + b (?) where a = one side of rectangular pipe, feet or inches. b = 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 found ducts are also given in. the curves of Fig. 4 which are plotted from data, based on Equation 7. .. To use the 609