Document 71LeMddX4erVOg9mpED70NB2B

294 CHAPTER 21 1960 Guide Air Duct Design 295 CU FT OF AIR PER MINUTE (Bed oft' Stanford Ah of 0.075 tt per cu ft dandf? flowing (hrovgti overape, clean, round, gaJramzW motel duet* (raving approximofefy 40 jocnft per 100 ftj CooSon: Oo oof exfrapotat* fcolow dioif. Rg. 2 .... Friction of Air m Straight Duds for Volumes of 10 to 2000 Cfm o Standard Ah ef 0>75 lb per cv ft density flowing fbraogh average, dean, round, galvanized motel duct* having approximately 40 foioti per 100 ftj . Rg. 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 Pig. 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,.