Document GKDzLKZmkj3yEBqRYORY5Rwx

572 CHAPTER 31 1965 Guide And Data Book OUTLET NO. 2 9 SEC-E 10 FT FAN SEC-A 20 FT SEC-B 10 FT SEC-C 15 FT SEC-0 >0 FT . 5 FT 1 15 FT i Rg. 12.... Dud Layout for Example 6 Tile principal limitation of the equal-friction method - is that it does not differentiate between runs having several transitions, elbows, etc., and runs having noire. Only the actual length of duct is considered; this 'and the flow- rate fix the duct size. Moreover, when computing the system re sistance, the designer must be siire that the pressure losses of transitions, elbows,' etc.,' are included and added to the straight pipe losses. If the pressure available for the ductwork is known, as it is for packaged equipment, this pressure can be divided' by the total'equivalent length of the run apparently having the highest resistance, to obtain a design friction loss'value per foot,- for use with fig. 2 or 3. Hence, for these applications/ it is not necessary to select an initial velocity. However, the weakness of this method is that the resistances of fitting? must be expressed in terms of equivalent length. Since transitions,1 elbows, etc., have predominantly dynamic , losses,."the equivalent length of a particular-fitting varies considerably with its actual size. Note that, for example, the values for el bows in Fig. 7, Chapter 7 of the 1964 Guide And Data Book are related to duct size, and note also that the elbow losses in Fig. 9 of this chapter are given in terms of the number of diameters. Hence, when the available pressure is known, the method requires that the duct size be estimated in advance. The calculated duct size should therefore.be compared with the initial estimate, and if there is a considerable difference, a recalculation should be made n*ing the calculated size. - Less dampering is required if the method is modified-so that only the main duct is by the equal-friction method. The fan is selected for tius total duct resistance in the manner described for ' the velocity-reduction - method.- The pressure available at each branch is divided by its equivalent length in hundredsof feet, to obtain a design friction loss value,or use with Fig.- 2 or Fig.-3, in conjunction;with the branch flow rate. The branch ducts are sized as nearly as possible to dissi pate all of the available pressure. v .' f<': When this method is;ufied- care should be exercised to prevent velocities in short branches from becoming excessive, to avoid noiseIprobleras..This U-easily prevented during,the design process, because the velocity can be read directly,from the friction chart. If velocity,is excessive, read horizontallyto the left and select a diameter -which yields a lower velocity; The : damper for this,,run will have to dissipate the excess pressure. Since ductwork attenuates noise,to some extent,.the damper should'be located as close, to the main as-.possibie. Sound treatment for this branch should also be considered. An alternative solution may be .to .revise the duct layout to increase- the resistance .of the: run,, e.g.,-by,,i relocating the branch take-off so that the total duct length is increased. Table 7------Tabulation of Results (Example 6) Section A B C D B Bow loti Cfcn 2500 1750 1000 750 750 FricSoa per too ft hLthO. 0.2 0.2 0.2 0.2 0.2 Docf Ota. fa. 17.0 14.8 12.0 10.7 10.7 Vabdtr Fpm 1600 1480 1290 1190 1190 Dart 20 X 13 15 X 12 15 X 8 12 X 8 12 X 8 Example 6: (Equal-Friction Method)- A duct layout is shown in Fig. 12. Outlets Noe. 1 and 2 deliver 750 cfm each and outlet No. 3 delivers 1000 cfm. Having selected a velocity of 1600 cfm in Section A, size the duct system and determine its static-pressure requirement. Solution: The total cfm to be handled is 2500 cfm. From Fig. 3, with 2500 cfm and 1600 fpm velocity, read a diameter of 17 in. and a friction loss of 0.2 in. of water per 100 ft. By subtraction, the flow rate in Section B is 1750 cfm. Along the 0.2 friction line in Fig. 3, all of the ducts can be sized immediately because the flow rates are known. Results are presented in Table 7. The rectangular equivalents were selected from Table 2 with the objective of having the same duct depth for ail branch runs. The duct run to outlet No. 3 has the highest apparent re sistance. It is decided to fabricate the elbow in Section C with a radius ratio of 1.2; hence, from Fig. 8 with H/W " 1.9, LfW " 8. Since W ~ 1.25 ft (15 in.), the additional equivalent length due to the elbow L is 10 ft .The total equivalent length of the run is therefore (20 + 10 + 15 + 10 + 15) = 70 ft. .There fore, at 0.2 per 100 ft, the duct resistance is 0.2 X 0.70 = 0.14 in. of water. Including the outlet pressure of 0.12 in., the staticpressure requirement of the duct system is 0.26 in. of water. The design is now complete, and dampers will be relied upon for adjusting the outlets to the design now rates.,, If refinement is necessary, the modified method can be applied to Sections D and E. First, the static pressures available at the junctions with the main of the Section D and E branch ducts are obtained. For Section A it is the system pressure of 0.26 minus the frictioo presure-loss in Section A. Tne latter is 0.20 X (20/100) * 0.04; hence, the pressure at the entrance of Section B is 0.22 in. of water. The pressure available for the duct; work is 0.22-0.12, or'OilO. Assume the equivalent lengths of the branch take-off and the elbow to be 10 ft The total equiva lent length of Section D is then (10 + 10 -f 10 -f 5) - 35, and the friction loss per 100 ft required to dissipate 0.10 in. of water is 0.10 X (100/35) " 0.29. With this'unit friction loss and a flow rate of 750 cfm, fig. 2 yields a diameter of 10.0 in.' and a velocity of 1380 fpm. .' `' Section E is sized in a jprailar manner. The nresure available is 0-26 minus the friction loss in Sections Aana B, or 0.20. With the outlet pressure loss of 0.12 deducted,.the.available ductwork pressure loss is 0.08 in. Assuming that the branch take-off loos is equivalent to 10 ft. ,of duct, the total equivalent length is 20 ft. Tne required friction loss is 0.08 X (100/20) " 0.40. With this unit friction-loss and a flow rate of.750 cfm, Fig. 2 yields a diame ter of 9.4 in,-and a velocity of 1580 fpm. An equivalent rectangular size is 9 X 8 in. A comparison of these, results with those-in Table 7, shows that the modified method has reduced the size of Section D somewhat and that of Section B considerably. The reduced sizes accomplish more economically what would otherwise have to be done with dampers. . Static-Regain Method . - Consider a straight rim* of-duct with several branch take offs attached. The flow rate of air along , the run is progres sively reduced by the amount diverted into each successive take-off. If, for example, the size of the run were the'same throughout its length, the.velocity would become progres sively less in accordance with Equation 3. When velocities are reduced, a conversion of velocity pressure into-static pressure occurs '{as weD as a loss in total pressure): -The principle of the statk^regain method is :to size a duct run so Air;Duct Design 573 thnt the increase in static pressure (regain) at each take-off junction just offsets tbn pressure loss of the succeeding sec tion of the run. . . . . 1 ; ,-:b The method provides a convenient means.of designing a long fun of duct having several take-offs so, that essentially the sarnie static pressure,exists at-the entrance.to,each branch, outlet, or terminal.takeoff. As a consequence, outlet or tennis nal selection and.system balancing -is sifnplified..The method b particularly,`suited.to large' installations.-haying.several long runs of .duct! with"each run having many take-offs of Supply outlets attached!'For'this type of application, little or no dampening b ordinarily required to balance the system. If the1.distance between branch take-offs is either'very snail oryefy'great! it may.not be feasible or economically desirable to design for.the aww static pressure at each junc tion. In such cases, the method rj,ri be used to size the main for either, a,progressively lower static presure (net. static- pressure loss) or.a progressively higher pressure.(net static- pressure gain); ; n':Z' . V -f. i ; The duet length of any section- should' include the equiv- alentiength of-any elbows or transitions within the section. The charts apply to constructions -where regain takes place unaccompanied by., radical change in direction, namely, to etrdightrthroughiseciitms of divided-flow-fittingB. - If no friction-or 'dynamic losses occurred at the junction;' there*would be no loss in total' pressure^. and the change in velocity pressure 'would be completely converted into a re gain (rise) in static pressure/which; for standard air, would'. tchere' ^ j' '. ^ , .- ; P, = theoretical static-pressure regain, inches of water. 1 < Vi -- velocity in.main upstream of branch, feet per minute. Pi -- velocity in main downstream of branch, feet per min ute. 4005 -- .conversion factor of velocity'equivalent to 1 in...water " ' It has been found by test that the regain' coefficient'across a takeoff or an outlet will equal about 0.90 for well-designed and constructed ducts with no reducing section immediately after the outlet. However, for practical design, an average recovery of 0.75`is assumed. Hence the actual regain Pr b: p- - 0 75[(jss)' - (^)'l <I3) !l In any duct section through which'air is fiowing-thefe is a'continuous loss of pressure due to friction.'This pressure toss may be expressed as: " ' - .V - - 00270 (^.)(i'^) . : (14)' where .: H/ " pressure toss, inches of water. L -- duct length, feet.. - -d " duct diameter, inches. ,, ( ,