Document 6wd1MQZadVmgZK1DKmRG15Qk1

576. _____ Chapter 31. _____ 1945 Guide (6) 'Duct resistance: The longest run is----- -------------------------_-------------------------------- = 150 ft - Two, 58 x 30 in. elbows (150% ratio) ^ ^ ^ ^------------- = ^ Two, 30 x 15 in. elbows (-150% ratio) ^ ^ ^ ^ >------- -- 65 ft i O v> 06 v IK Three, 15 x 30 in. elbows (75% ratio)------- ^-------- :------------- ~ 131 ft Total equivalent run.;________:;----- 472 ft 472 ft at 0.028 in. per 100 fts______________ ________ _____________ __ 0.132 in. (7) Allowance for damper adjustment, 25% of 0.132......... ..........;------------------ 0.033 in. (8) Supply grille resistance (from manufacturer's tables)0.036 in. Total static pressure loss of system__ ____________ -0.952 in. The fan is selected from the manufacturer's ratings to deliver 22,935 cfip at a static pressure of 0.952 in. as outlined in Chapter 29. Example S. If the rooms and offices of the hotel building of' Example 2 are to be Table 4. Pipe Sizes for Example 2s Volume of Air (cfm) 22,935 12,510 10,425 8,340 6,255 4,170 2,085 Diameter of Pipe (Inches) . 56 - 45 . 42 39 35 29.5 23 Equivalent Size of Rec tangular Duct (Inches) 60x44 58x30 50x30 42x30 42x24 30 x 24 30 x 15 . ^Velocity through grilles (not shown) to be approximately 300 fpm. served from a manufactured unit with a capacity of 22,935 cfm against an external resistance of 0.35 in., the known resistances are calculated as: (1) Outdoor air inlet______________________ _____ :--------------------------------------- 0.094 in. (2) Allowance for damper adjustment.:. L 1--------------------------------- -------------- .0.033 in. (3) Supply grille resistance (from manufacturer's tables)...---------------------------0.036 in. Total known resistance.----------------------------------------------------------------- 0.163 in. Subtracting, this from the total available resistance: 0.35 in. -- 0.163 in. = 0.187 in. available for duct resistance. Known length of run 150 ft The duct width is then estimated for the following elbow'calculations: Four 150% ratio'elbows, 4 x 13 x 3.5 ft.---------------------- ------- i82 ft Three 75% ratio elbows, 3 x 35 x 1.5 ft------------------------------ 158 ft Total estimated length-------------------------------------------------- 490 ft The duct friction per 100 ft is then 0.187 4.90 = 0.0382 in. and the mains and branches are sized from the 0.038 in. friction line in Fig. 2. If it is desired to size each branch for equal resistance, the total resistance back to the point of juncture is calculated and the branch is then sized in a manner similar to that outlined in Example 3. Air Duct Design 577 ~~ ~DUCTCONSTRUCTIONDETAIES Straight sections of round duct are usually formed by rolling the sheets to the proper radius and grooving the longitudinal seam. Rectangular ducts are generally constructed by breaking the corners, and grooving the longitudinal seam, although some fabricators still use the standing seam due to lack of equipment. Elbows and transformation sections'are gener ally formed with Pittsburgh corner seams because this [seam [is easier to Grooved seam Standing seam S slip Drive slip End slip Ooubte seam Pittsburgh seam Bar slip Reinforced bar slip Pocket slip Angle connection Fig. 7. Sheet Metal Duct and Arrangement Details lock in place than the double seam, but complicated fittings such as double compounded elbows are usually constructed with double seam corners. The construction of these various seams as well as the types of girth connections are shown in Fig. 7. The application of the various slips and connections is outlined in Table 5. The end slip may be used wherever S slips are recommended. Where drive slips are used the end slip may be applied on the-narrow side of the duct and only the drive slips on the