Document dYwbX0kVrDXJL4JqBER11Qd7R

814 CHAPTER 41 1949 Guide, area actually occupied by the air flow, for convenience they are divided into two general classes--those caused by changes in direction of the duct and those caused by changes in cross-sectional area of the duct. Conduit transitions are representative of changes in cross-sectional area and bends (elbows) are representative of changes in direction of the duct. . Shock losses vary substantially as the square of the velocity of the air flow and are therefore conveniently expressed as a fraction of the' velocity head. Air Duct Design 815 where Hi := total shock pressure loss, inches of water: y,, = velocity of air stream, feet per minute. C = an experimentally determined constant (shock loss coefficient) Fig. 3 which shows diagrammatically the relation of velocity pressure to velocity for standard air (Vm = 4005 VhI) can be conveniently used to find the total shock pressure loss for any duct element with known shock loss coefficient C. Shock losses are independent of the roughness of the duct walls and. therefore cannot be computed correctly as friction losses. It is neverthe- ! Fig. 3. .Relation Between Velocity and Velocity Head fob Standabd. Aib the velocity pressure corresponding to the mean velocity of flow, inches of water. . ah' empirical factor based on experiments and termed shock factor or loss coefficient:. It can be seen from Equation A that the shock factor is independent of both density and the unit used and that it represents the number of velocity heads lost at the conduit transition or bend. , Values of the shock factor , . for.various .duct elements are sometimes tabulated6'7. It .should be kept ' in mind, however, that absolutely reliable: shock factors have not yet;,been - fully established for all duct elements and that difficulties have been en countered in correlating experimental data of different investigators. A , comprehensive study of the loss factors of duct elements is being conducted ; by the A.S.H.V.E. Research Laboratory for ihe purpose of obtaining ; ..exact, data. ' For standard air Equation 4 changes to: Fig. 4. Loss of Pressure in Elbows less customary to express the dynamic and friction losses in duct sections or elements in equivalent length of duct in feet or in diameters to facilitate their computation as. friction losses. Formulas have been developed; for expressing these relations6. I PRESSURE LOSSES IN ELBOWS It is customary to express the dynamic and friction losses in elbows as equal to a number of diameters of round pipe, or a number of widths-of rectangular pipe, or equivalent length of duct9. The curves in Fig. 4 give the number of diameters or widths of pipe which have a frictional resistance equivalent to the pressure drop in the elbows. Curves B and (7 are based .on tests of round and square elbows10 of .ordinary good sheet metal- con struction. ' Values obtained from Curve A should be used when there is any doubt ;as to quality of duct construction. ' It is suggested that this curve be used for rectangular elbows and five-piece elbows as it will thus allow an.additional factor of safety without seriously affecting the design. \ As indicated in Fig. 4, long radius elbows will' offer much less resistance, to the flow of air than" short radius elbows. Experience has shown that