Document Lp5eXEdYXaOerdRD4MgLjX63g

760 CHAPTER 41 1948 Guide - the round pipe sizes are obtained and then, referring to Table 2, the equivalent rectan gular sizes are selected as shown in Table 5. The pressure at the outlets nearest the fan will be greater than at the pipes farther along the run so that the former will tend to deliver more than the calculated amount of ' air. To remedy this condition, volume regulating dampers should be located at the base of each riser, or in each branch duct, and adjusted for proper distribution. At points where branches leave the main it may be advisable, depending upon the nature of the installation, to install adjustable splitters similar to those shown in Fig. 7 where the main duct divides into the 58 x 30 in. and 50 x 30 in. branches. Resistance Losses for the System (1) Outdoor air intake, 1000 fpm velocity (1.5 heads X 0.0625)1.'__ ___ 0.094 in. (2) Filters (from manufacturer's tables)____ __ .0.250 in. (3) Tempering coil loss (from manufacturer's tables):_______________________ 0.074 in. (4) Air washer loss (from manufacturer's tables).0.250 in. (5) Reheating coil loss (from manufacturer's tables)_____________ _____ _____0.083 in. (6) Duct resistance: The longest run isJ = 150 ft Two, 58 x 30 in. elbows (150% ratio) 2 X X 58____ = 126 ft - Two, 30 x 15 in. elbows (150% ratio) 2 X X30 = 65 ft Three, 15 x 30 in. elbows (75% ratio) 3X ^X 15 = 131 ft Total equivalent run:;________________________ 472 ft. 472 ft at 0.033 in. per 100 ft____________________________ ______-0.156 in." (7) Allowance for damper adjustment, 25% of 0.156'______________ _____ ;__ 0.039 in. (8) Supply grille resistance (from manufacturer's tables)...______ ...__________0.036 in. Total static pressure loss of system0.982 in.' The fan is selected from the manufacturer's ratings to deliver 22,935 cfm at a static pressure of 0.982 in. as outlined in Chapter 32. Example 4. If the rooms and offices of the hotel building of Example 3 are to be 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 inlet0.094 in. (2) Supply grille resistance (from manufacturer's tables)0.036 in. Total known resistance.;'0.130 in. Subtracting this from the total available resistance: 0.35 in. -- 0.130 in. = 0.220 in. available for duct resistance and damper adjustment. If 25 per cent is allowed for damper adjustment, the total allowable duct friction = 0.220-H 1.25 = 0.176 in. 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. __________________ Three 75% ratio elbows, 3 x 35 x 1.5 It___________________ 182 ft 158 ft Total estimated length 490 ft The duct friction per 100 ft is then 0.176 4- 4.90 = 0.036 in. and the mains and branches are sized from the 0.036 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 4. i4ir Duct Design 761 Table 5. Pipe Sizes foe Example 3* 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 30x24 30 x 15 ^Velocity through grilles (not shown) to be approximately 300 fpm. DUCT CONSTRUCTION DETAILS Straight sections of round duct tire usually formed by rolling the sheets to the proper radius arid 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 comer seams because this seam is easier to 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 showninFig. 8- The application of the various slips and connections is outlined in Table 6. The end slip may be used wherever 5 slips are recommended. Where drive slips are used the end slip may be applied on the narrow side of the duct and the drive slips on only the maximum side. Ducts 25 to 30 in. in size should be reinforced between the joints, but not necessarily at the joint. Ducts 31 in. and up should be reinforced at the joint and between the joints; if drive slips are used the