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790 CHAPTER 31 1958 Guide the direction of the air flow. Static and velocity pressure are mutually convertible and either can increase or decrease in the direction of flow. Typical pressure changes in a .duct system are illustrated in Fig. 1. Be tween Sections A and B, the static and total pressures decrease uniformly because of friction between air molecules along the sides of the duct. Pressure losses in straight pipes are termed friction losses and are charac terized by dependence upon Reynolds number. (See Chapter 4.) Section BC is a converging section. The duct area is reduced, and con sequently the flow is accelerated. This is a stable and efficient process and the loss in total pressure is seen to be small. The marked decrease in static pressure is caused by conversion of static to velocity pressure in accordance with the equations above. Section CD presents the friction loss in the section of smaller duct. Since friction losses increase with nearly the square of the velocity, the pressure line falls much more rapidly than in Section AB. An abrupt expansion occurs at DE. The duct area is abruptly increased and the flow decelerated. The process is an inefficient one and the loss in total pressure is therefore large. The velocity pressure is decreased at B in accordance with Equations 2 and 3, and the static pressure rises. Tms and the pressure loss at BC are termed dynamic losses, and are characterized by being essentially independent of Reynolds number. Dynamic losses occur because of changes in the direction or velocity of the.air, and hence occur at duct transitions, bends (elbows) and obstructions such as dampersAs will be shown later, dynamic losses can be specified in terms of a con stant times the velocity pressure at a reference cross section. Beginning at Section E the velocity pressure is constant, and the static and total pressures again decrease uniformly due to wall friction. D>e static pressure at F is zero, as referred to atmospheric. The total pressure is the velocity pressure and is a measure of the kinetic energy of the stream as it discharges from the duct. The distinction between static and total pressure is important, because the former is conventionally used as the basis for system design, but tne latter dictates the actual mechanical energy that must be supplied to tn system. Note in Fig. 1 that the static pressure decreases and then increases in the direction of flow. Moreover, it even becomes negative (below a mospheric). Therefore, in dealing with static pressures, distinction mu Air Duct Design 791 always be made between static pressure loss (Section AB) and static pres sure change as a result of conversion of velocity pressure (Section BC). FRICTION LOSSES Pressure drop in a straight duct is caused by surface friction, and this friction loss is most readily calculated by means of the Air Friction Charts, Figs. 2 and 3, covering volume ranges of 10 to 2000 cfm, and 1000 to 100,000 cfm, respectively. These charts were developed by the ASHAE Re search Laboratory.1 The charts, Figs. 2 and 3, were constructed from the basic flow equation for the pressure loss in circular ducts (see Chapter 4): where V Ht = head loss due to friction, inches of water. / = a non-dimensional friction coefficient, which for air conditioning work de pends upon Reynolds number and the relative roughness of the conduit. Approximate values of / were taken from the work of Moody* where e -- 0.0005 ft. (See Chapter 4, Fig. 4, Relation Between Friction' Factor and Reynolds number.) It is numerically equal to the reciprocal of the num ber of duct diameters required to cause a pressure loss equivalent to one velocity pressure. I = length of conduit, feet. D -- inside diameter of conduit, feet. Ht - velocity pressure of mean velocity, inches of water. The air friction chart is based on standard air with a density of 0.075 lb per cu ft, flowing through average, clean, round, galvanized metal ducts having approximately 40 joints per 100 ft. Fig. 2 should not be used to obtain values below the charts by extrapolation, because critical flow would occur in this region and values so obtained would be unreliable. Variations in air temperature of the order of 20 deg from 70 F affect duct friction very little. Figs. 2 and 3 can therefore be used for all air systems with temperatures from 50 F to 90 F.1 For systems carrying air at much higher temperatures, however, the values found in the charts must be corrected.3 To determine the friction loss in such systems, the actual ow rate or velocity existing at the non-standard conditions must be used. ,, ^-'hanges in humidity, or the usual changes in barometric pressure or in e pressures required to circulate the air through the systems, have little jn uence on duct friction and can be disregarded in calculating friction osses m ordinary air-flow systems.1 For unusual conditions, the friction a ues obtained from Figs. 2 and 3 must be corrected.3 For ordinary applications, an excellent approximation for non-standard temperatures is where A,, = K K (5) friction loss under actual operating conditions, inches of water. 0 riction loss obtained from Figs. 2 and 3 using the actual flow rate or velocity g ___ exiswng under the non-standard conditions, inches of water. - friction loss correction factor, obtained from Fig. 4, dimensionless. For ducts of other than standard sheet-metal construction, correction