Document vxBZYLpzpY8oxrqNn0qDZxxq

284 CHAPTER 21 1959 Guide Air Duct Design 285 CU FT OF AIR PER MINUTE (Bamd en. Stamford Air of 0.075 (b por cu ft doodiy flowing Arowgli overago, deca, roaod, gafvoatrod octal duds bovfog approximately 40 loath par. 100 ftj Caution; Do not extrapolate below cbort. fig. 2.... Friction of Air in Straight Ducts for Volumes of 10 to 2000 Cbn (Based ort Standard Air of 0.075 lb pot to ft dootity flowing through owerogo, deem, round, gtdvuiuzed metal duct* hortng approximately 40 joint* per 100 ffj fig. 3 .... Friction of Air in Straight Ducts for Volumes of 1000 to 100,000 Cfm The distinction between static and total pressure is im portant, because'the former is conventionally used as the basis for system design, but the latter determines the actual . mechanical energy that must be supplied to the system. Note in Fig. 1 that the static pressure decreases and then increases in the direction of flow. Moreover, it even becomes negative (below atmospheric). Therefore, in dealing with static pres sures, distinction must always be made between static pres sure loss (Section AB) and static pressure 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,