Document r3pOg1bMzVr6g4482orL8bjV

708 CHAPTER 32 1954 Guide forms will produce a substantial saving in pressure loss. The addition of vanes or splitters divides an elbow into parallel channels, each having more favorable radius and aspect ratios than the original elbow. Values of addi tional equivalent length L, for elbows of square cross-section having various' vane forms and combinations, may be found from Table 3-in which values of L/W are shown. LOSSES DUE TO AREA CHANGES . Area changes in ducts, generally unavoidable, are necessitated frequently.;: by the building construction or changes in the volume of air carried. Ex perimental investigations14'16 of pressure changes, and pressure losses at'j changes of the area of duct cross-sections, indicate that the excess pres- - sure loss over the normal friction loss is a dynamic loss due to a faster.; stream expanding into a slower stream, as determined by the actual areas., occupied by the flow.rather than the areas of the duct. No perceptible;; dynamic loss is due to the converging of the air stream itself where the flow;, is contracted, but the air stream continues to converge beyond the edge oh the contraction and reaches a minimum at the vena c&ntracta. This con- ,* traction of the air stream is shown in Fig. 8. For contraction, therefore,1, the dynamic loss is-caused by expansion from the vena contr'acta to the full'.; area following the contraction. Enlargement in area may be considered? as a special condition of general expansion following contraction. Fig,1? illustrates (a) abrupt enlargement and (b) abrupt contraction. -' ' 'l' : For a sudden symmetrical enlargement, a theoretical expression for the loss is , /, ' A' (p, - Pi)* 2g 2g .kl VENA ___________ '"<31 I CONTRACTA ---- ---- -.TIFig. 8. Aib Flow at Abbupt Enlabgement ob Contbaction op Air StbeaM Air Duct Design or for standard air; .70? where ! .4. \1 / T7 /tT Tr \. (7) K = pressure loss due to sudden enlargement, feet of fluid flowing. Ih = pressure loss due to sudden enlargement, based on standard air, inches of water. ---- ------- in Additional Equivalent Duct Length*. >> Additional Equivalent Length L = Duct Width W, in Feet, Multiplied by L/W Values Shown ' -----wviuj IU uuc uue.ii_ u--u--cWcVJ XUVV ^IV/1 OCUUUU 's = velocity in the outlet duct, feet per second. Ft = velocity of standard air in the inlet duct, feet per minute. F* = vejpcity of standard air in the outlet duct, feet per minute. = area of the inlet duct, square feet. ^t = area of the outlet duct, square feet. * The loss for a sudden symmetrical contraction, he can similarly be ex pressed as A. (l _iYW> \ Ai) 2g (8)