Document 2REqnM1egMrODLXG0RpBDwEjR

American Society of Heating and Ventilating Engineers Guide, 1937 divided by 231 (total equivalent length of short branch) = 306 milinches per foot which is 66 milinches per foot more than is available in the longer circuit. Therefore approxi mately 12 per cent more capacity is available in the pipes which will change the pipe size only a slight amount. If it is necessary to correct this variation, a stop cock may be placed in the return line of the short branch and adjusted after installation. However, if the variation be of sufficient magnitude, the pipes in the shorter branch should be sized accordingly. In this case the pipe size should be selected according to a frictional loss of 240 milinches per foot. Due to the fact that Section BJ requires a capacity of 57 Mbh, which is slightly over 1 in., use a 1 in. pipe in BJ and QR. Section Table 2. Capacities of Pipes in Mbh (1000 Btu per Hour) and Velocities of Water In Pipes in Inches per Second for Forced Circulation Systems with a Total Friction Head of 2 ft and for a Maximum Temperature Drop of 10 F 1 Pm Sob . (Inches) 2 Equivalent Length op Pipe (Feet*>) 83 4 5 | 6 1 ' ' Equivalent Total Length op Pipe in Feet in Longest Circuit 9 100 150 200 250 300 350 400 Unit Friction Head, in Milinches 240 160 120 96 80 69 60 i 6.2 4.8 4-1 s.4 2.9 2.6 2.4 15 12 10 9- 8 7.5 7 H2 13.2 10.3 8.6 7.3 6.2 18 14 12 11 10 6.0 5.5 9 8.5 i 2.3 25.0 19.2 16.3 14-4 12.5 12.0 n.t 22 17 15 13 12 11 10.5 1M 3.0 52.8 40.8 34-8 31.2 27.8 26.4 24.0 27 21 18 16 15 14 13 m 3.5 79.2 60.7 51.2 45.6 40.8 40.0 36.0 30 23 20 18 16 15 14 . 2 4.0 153.8 120.0 104-0 93.5 86.4 81.5 73.8 36 28 24 22 20 18 17 VA 6.0 250.0 192.0 164.6 149.0 139.2 135.8 122.5 41 32 28 25 22 21 19 3 6.5 444-0 348.0 294-0 270.0 254.0 240.0 223.0 48 37 32 29 26 24 22 For other temperature drops the capacities of pipes are to be changed correspondingly. For example, for a temperature drop of 30 F, the capacities shown in this table are to be multiplied by 3. The velocities remain unchanged. ^Approximate length of pipe in feet equivalent to one elbow in friction head. This value varies with the velocity. JK has a load of 47 Mbh which is approximately 1 in. Therefore use 1 in. in JK and PQ. KL carries 35 Mbh which is approximately halfway between % and 1 in. Use in. in KL and 1 in. in OP. Section LM requires a capacity of 20 Mbh, therefore use in. in LM and NO. Size radiator branches as previously described. Many times a A in. pipe will prove to be too large, but at the present time it seems to be general practice to avoid pipes smaller than this size, especially for hot water installations. If a number of heating systems are to be designed for similar conditions, i.e., for a total friction head of 6 ft and a temperature drop through the radiators of 20 F when the maximum quantity of heat is being delivered 608 ii at i! Chapter 33--Hot Water Heating Systems and Piping to the building, a table such as Table 3 may be prepared from the data of pig. 4. Having this table, the pipe sizes for the system of Example 1 can be e&sily selected. For example, for Sections AB and RI, each supplying 85 Mbh, the equivalent pipe length of the system is 299 ft. In the table the length shown nearest to this length is 300 ft. In the 300-ft column, a 1-in. pipe is too small and a pipe is too large. The lf^-in. pipe will therefore be selected. For other systems, it will be economical to operate with different friction heads, and tables may be prepared similar Table 3. Capacities of Pipes in Mbh (1000 Btu per Hour) and Velocities of - Water in Pipes in Inches per Second for Forced Circulation Systems with a Total Friction Head of 6 ft and for a Maximum Temperature Drop of 10 Fa Pits Sms (Inches) Equivalent LENGTH or Pm (Frarb) or CmcuiyEquivalent Total Length Pips in Feet nr Longest 400 600 1000 Unit Feiction Head, in Milinches iso 120 90 72 14 7-4 18 6.0 15 5.0 13 3.8 10 s.4 3.1 9 7.5 15.8 22 12.7 18 10.8 16 8.4 12 7.7 11 6.7 9 2.5 so.o 27 24.0 22 20.4 19 15.8 15 13.9 12.5 13 11 lK 3.3 64.8 33 52.5 26 44-4 23 33.6 18 SO.O 26.8 16 14 IH 4.0 96.0 37 76.8 31 64-8 26 50.1 20 44.7 40.8 18 15 5.0 192.0 44 153.0 36 130.0 30 100.1 24 90.0 78.0 21 18 2H 6.0 300.0 50 244-0 41 206.0 35 161.0 26 144-0 130.0 24 21 7.5 550.0 58 436.0 48 368.0 42 287.0 32 249.0 228.0 27 24 For other temperature drops the capacities of pipes are to be changed correspondingly, rur for a temperature drop of 30 F. the capacities shown in this table are to be multiplied by 3. The velocities rem^aAinppurnocxhiamnagteedl.ength of pipe in feet equivalent to one elbow in friction head. This value varies with he velocity. to Tables 2 and 4, which are based on total friction heads of 2 and 18 ft, respectively. Example 2. Design a direct return two-pipe forced circulation system for the layout shown in Fig. 5 assuming a temperature drop of 10 F through the radiation. For this system the length of the pipe line from the boiler to the highest radiator on the farthest nser and back to the boiler is about 250 ft. There are about 16 elbow equivalents hav ing an equivalent pipe length of about 50 ft, so the total equivalent pipe length is about 300 ft. Solution. The same pipe size tables may be used as those developed for the reversed return system of Fig. 3, Table 3 which provides for a friction head of 6 ft.