Document gKyDN2nwNRjBL9mJ2vaOkora

802 CHAPTER 31 1958 Guide Air Duct Design 803 Fig. 7. Relation Between Velocity and Velocity Head fob Standard Aib where the aspect ratio, H/W = 1/4, then if the bend of the same radius ratio had been made in the plane of the narrow dimension giving an aspect ratio, H/W = 4. ; Data now available for losses in compound bends,7 8 where two or more elbows are close together, do not warrant refinement of design calculations beyond use of the sum' of the losses for the individual elbows. Actually, the losses-are somewhat less than for two bends when they are placed to form a U bend, and somewhat more when they form a reversed, or S bend. Where angles of other than 90-deg bend are encountered, the ;loss may be considered as directly proportional to the angle of bend. Losses8 for Fig. 9. Loss in 90-Deg Elbows of Round Cross Section elbows discharging air directly into a large space are higher than those given for elbows within duct systems. Turning vanes may be advantageously employed in elbows, both to re duce the pressure loss and to provide a more uniform velocity distribution downstream from the bend9,10. Vanes and concentric splitters are par ticularly recommended where miter elbows are used, because even the simplest vane forms will produce a substantial saving in pressure loss. PRESSURE LOSSES IN DIVIDED-FLOW FITTINGS Analysis11 of available data for losses at branch take-offs indicates that the loss in the straight-through section is about 35 percent of that for abrupt enlargement (see next section, Losses Due to Area Changes) in volving the same ratios of velocities; that the loss in the diverted-flow sec tion depends on the ratio of the velocity of diverted flow to total flow and on the angle the take-off makes with the main, and is at a minimum for a natural relation that exists between these two variables. That is, there is a natural angle of efflux corresponding to the velocity ratio. Some repre sentative branch losses are given in Table 5. Fig. 8. Loss in 90-Deg Elbows of Rectangular Cross Section Fig. 10. Portion of Duct System for Example B