Document 82X4EM1MpO7q9GMaZD2gB3jry

American Society of Heating and Ventilating Engineers Guide, 1936 boiler is the only one which receives water at approximately the tem perature at which it leaves the boiler. All other radiators receive cooler water and must be proportionally increased in size, so the total heating surface in the system is considerably larger than that in a corresponding two-pipe system. The pipe sizes in gravity circulation systems may be varied. As the pipe sizes are decreased, the temperature drop through the radiators, which produces circulation, is increased and it becomes necessary to increase the temperature of the water leaving the boiler so that the mean temperature in the radiator remains constant. For example, Fig. 8 shows diagrammatically an elementary heating system which will function with either lj^-in. or 1-in. pipe. The radiator is required to deliver 27 Mbh, and the circuit consists of 30 ft of pipe and .20 elbow equivalents. If 1 }4-in. pipe is used, the system will operate correctly if the water temperatures in the flow and return risers are 200 F and 180 F, respectively. The mean water temperature in the radiators will then be 190 F and, if the radiator is located in air having a temperature of 70 F, the size of the radiator must be sufficient to deliver 27. Mbh under these conditions. Fig. 8. An Elementary System If 1-in. pipe is used, the system will function correctly with water tem peratures in the flow and return risers of 210 F and 170 F, or of 200 F and 160 F. In the. first case, the mean water temperature is again 190 F and the same, size radiator may be used as with the 1 pipe, but the temperature of the water leaving the boiler must be raised from 200 F to 210-F. In the second case, the temperature of the water leaving the boiler is the same as for the 134-in- pipe, but the mean water temperature in the radiator is lowered from 190 F to 180 F, and theoretically the size of the radiator should be increased about 1234 Per cent to deliver the required 27 Mbh (See Table 3, Chapter\6, 1933 Guide). This indicates the extent to which pipe sizes arid radiator sizes may be decreased by increasing the temperatures of the water in the boiler, as is possible in closed systems and in open systems in which the open expansion tank is located sufficiently high to secure a pressure in the boiler equal to that existing in the boiler of the closed system. Example 8. Design a one-pipe gravity circulation system for the layout shown in Fig. 6. Assume that the main circuit consists of 150 ft of pipe, 7 elbows, and one boiler. Solution. Replace the boiler by 3 elbow equivalents and assume that the size of the main will be about 2 in. According to Table 6, Column 2, a 2-in. elbow is equivalent to 4 ft of pipe, and the total equivalent length of the main will be about 150 plus 40, or 190 ft. Assuming that the center of the boiler will be about 4 ft lower than the horizontal portion of the main and that the temperature drop in the system is to be 35 F, Table 6 may be used to determine the size of the mains. Note from Column 8, for a 200-ft i 598 Chapter 33--Hot Water Heating Systems and Piping Table 6. Capacities of Mains in Mbh, for One-Pipe and for Two-Pipe Direct ' Return Gravity Circulation Systems with a Total Friction Head of 0.6 In., a Temperature Drop of 35 F, when the Mains are 4 Ft Above the Center of the Boiler 1 1 2 l3 Pipe Size (Inches) Equivalent Length op Pipe (Feeto) 75 1 81 5 6 7 19 10 Equivalent Total Length op Pipe in Feet in Longest Circuit | 100 125 150 175 200 | 250 : 300 Unit Friction Head, in Miunchee n 350 zo8.0 6.0 4.8 4.0 3.4 3.0 2.4 1.7 136 3.0 43.0 87.6 33.0 80.0 27.0 25.0 22.2 20.S 18.7 2 4.0 8S.0 72.0 68.0 67.0 61.0 .48.0 42.0 88.0 85.0 236 4.5 140.0 116.0 100.0 90.0 81.6 76.4 67.S 61.0 56.0 3 5.0 284-0 204.0 176.6 160.0 143.0 183.0 110.0 107.5 100.0 334 5.5 347.0 800.0 260.0 286.0 214.0 200.0 177.0 160.0 146.0 4 6.0 490.0 '422.0 370.0 884.0 297.0 278.0 248.0 223.0 205.0 Approximate length of pipe in feet equivalent to one elbow in friction head. This value varies with the velocity. length, that a 2-in. mam w01 supply 48 Mbh and a 236-in. main, 75.4 Mbh. Since the system to be designed is to supply 66 Mbh, a 2-in. pipe is too small and a 23'6-in. pipe too large. The solution is to use some 2-in. and some 236-in. pipe. Since the 236-in is nearer the correct size than the 2-in., select 2-in. pipe for the first 50 or 60 ft out of the boiler and 2>4-in. for the remaining pipe back to tne boiler. Tables 7 and 8 may be used to design the radiator risers and connections. According to Table 7, for 12 Mbh the flow riser should be J6 in. and the return riser 1 in., and the riser branches should be 1 in. and 134 in., respectively. Note that according to Table 8, both radiator tappings should be 1 in. To simplify the construction, select 1-in. flow Table 7. Maximum Capacities of Risers3 in Mbh, and Velocities of Water in Pipes in Inches Per Second for One-Pipe and for Two-Pipe Direct Return Gravity Circulation Systems with a Drop of 35 F Through Each Radiator Pipe Sizb (Inches) Flow Return 34 36 34 34 34 34 34 1 11 1 134 134 1)4 D6 134 134 Equivalent Length ' or Pipe (Fhctc) 1.0 . 1st Floors Vel (In. per Sec.)d Mbh Flow Return 1.5 9 2.3 2.3 12 3.2 2.0 2.0 18 2.5 2.5 21 3.0 2.0 3.0 26 3.0 3.0 84 4.0 2.5 3.5 48 3.0 3.0 2nd Floor Mbh 6 6-4 10.1 12.8 20 25.2 43 3rd and 4th Floors Mbh 6.8 8.0 14.0 17.1 26.0 84 55 third ,pressur* hea?? ?! 45?' I800; 3150. and 4500. respectively, for the first, second. nKtiiM ?,nd ?n heads of 200 miUnches for the first floor radiators and con- 700 millnches for ail other radiators and their connections risers nSer branches-the pipin which connects the risers to the mains, are to be one size larger than the the wbdtj?mate length of pipes in feet equivalent to one elbow in friction head. This value varies with dVelocities apply to the riser branches. 599