Document reznym6oeB17pMmB8BoYpJ0wa

680 CHAPTER 31 1952 Guide Fig. 7. Portion op Duct System-fror Example 3 forms will prodiice a substantial saving in pressure loss. The addition of vanes or splitters, divides an elbow into parallel channels, each having more favorable radiud 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 frqm Table 3 in which values of L/W are shown. . ; . ;f : LOSSES DUE TO AREA CHANGES Area changes' in ducts,' generahyuriavoidable, are necessitated frequently by the building construction or changes in the volume of air carried. Ex perimental investigations1416 of pressure changes, and pressure losses at changes of the area of duct cross-sections, indicate that the excess pres-, sure loss over the; normal friction loss is a dynamic lbss 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.r; 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 of the contraction and reaches a minimum at the vena contracta: This con traction of the air stream is shown in Fig. 8. For contraction, therefore, the dynamic loss is caused by expansion from the vena, contracta to the'full area following the contraction. Enlargement in area may be considered as a special condition of general expansion following contraction. Fig: 8 'illustrates'(a) abrupt enlargement and (b) abrupt contraction. . For a sudden symmetrical enlargement, a theoretical expression for the' loss is. . . _ . . A-.. Fig. 8. Air Flow at Abrupt Enlargement or Contraction of Air Stream Air Duct Design or for standard air; 681 where \ At/ \4005/ V 4005 / (7) -- pressure loss due to sudden enlargement, feet of fluid flowing. ffo = pressure loss due to sudden enlargement, based on standard air, inches of water. Table 3. Pressure Loss in Vaned Elbows op Square Cross-section Expressed in Additional Equivalent Duct Length*. *> Additional Equivalent Length L = Duct Width W, in Feet, Multiplied by L/W Values Shown 1 b Varies: A " a large number of small arc vanes; B = a small number of large are vanes; C hollow vanes Having different outside and inside curvature: D = four vanes with radius of 0.1 W; E = single splitter with mdius of 0.5 W; F - no vanes or splitteis. p Pi -- velocity in the inlet duct, feet per second. vt = velocity in the outlet duct, feet per second. Vl = velocity of standard air in the inlet duct, feet per minute. Ft = velocity of standard air in the outlet duct, feet per minute. Ai = area of. the inlet duct, square feet. At = area of the outlet duct, square feet. The loss for a sudden symmetrical contraction, he can similarly be ex pressed as