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718 CHAPTER 32 1954 Guidei or, using the vaiuds from Example 5: Tb + 2rb. = 0.26 - 0.04 - 0.12 = 0.10 in. ra + rib = 0.26 - 0.04 - 0.02 - 0.12 = 0.08 in. The loss in the elbows will be assumed to be ^ = 12 or 10 additional equivalent feet, the friction loss of head per 100 equivalent ft is then for duct (1) 0.10 = 0.286 in. 0.15 + 2X0.10 for duct (2) 0.08 = 0.4 in. 0.10 + 0.10 Using Friction Chart Fig. 1, the duct diameter of Section B, to carry 750 cfm witha. loss of 0.286 in. per 100 ft, is found as 10 in., and the duct diameter of Section D, to carry 750 cfm with a loss of 0.4 in. per 100 ft is 9.8 in. Equivalent rectangular ducts are 10 X 8 in., the velocity in both ducts is 1350 fpm. The actual loss in ducts (1) and (2) is: 0.04 + 20 X + 15 X + 0.12 = 0.26 0.06 + 10 X 0 4 + 10 X ^0 4 + 0.12 = 0.26 For final survey of ducts selected see Table 6 (Tabulation of Results). 3. Static Regain Method When this method is used, the velocity is reduced at each branch or take-off so that the recovery in static pressure due to this reduction will offset the friction in the succeeding section. This method is based on the convertibility of static, pressure and velocity pressure, as discussed in a preceding section on Pressure Changes. If no friction or dynamic losses occurred, the change in velocity head would be completely converted into a regain in static pressure, which for standard air would be: \4005/ (ZlY \4005/ (18) where H, ~ theoretical head recovered (static regain), inches of water. Vi = initial velocity of standard air, feet per minute. Vi = velocity of standard air after reduction, feet per minute. ' , Under ideal conditions, 0.7 to 0.8 of the velocity head is actually recovered, blit for practical design an average recovery of 0.5 is assumed. The actual velocity head recovered Hr, then becomes J= [_V4005/ \4005/ (19) The advantage of the static regain method is that it provides a con venient means of designing a long run of duct (or an entire system) so that essentially the same static pressure will be obtained at each outlet. This simplifies outlet selection and system balancing. On large systems or very long runs, where it may not be feasible or economically desirable to design for zero static pressure loss between outlets, the method may be used to size Air Duct Design - cum 719 ducts for a uniform predetermined, loss. This loss or gain is net, that is, it is the friction loss compensated by any static pressure gain made available by a change in velocity. (The latter effect is commonly neglected in the Equal Friction Method.) Charts for the practical application of the principles of static regain to duct design, are presented in Figs. 12 and 13. These charts are based on Equation 19, as applied to rectangular ducts of average construction with dimension ratios of 3 to 1 or less. Note that the gain or loss indicated on me charts is the net gain or loss in the duct section considered (normally me distance between two outlets); it should not be confused with static Pressure loss per 100 ft, or total pressure loss in the duct. The total loss or gain in the outlet run is the summation of the losses or gains in the suc cessive sections figured. (Losses in outlets, coils, or similar items are