Document JNxxMRpkaMv3pzDvyraX4bZ3O

-S=C~T American Society of Heating and Ventilating Engineers Guide7i935~ 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 lj^-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 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 lj^-in. 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 1 J^-in. pipe, but the mean water (temperature o o Fig. 8. An Elementary System in the radiator is lowered from 190 F to 180 F, and theoretically the size of the radiator should be increased about 12J/ per cent to deliver the required 27 Mbh (See Table 3, Chapter 6, 1933 Guide). * This indicates the extent to which pipe sizes and 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 length, that a 2-in. main will supply 48 Mbh and a 234-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 234-in. pipe too large. The solution is to use some 2-in. and some 23'2-in. pipe. Since the 2 3'2-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 23'2-in. for the remaining,pipe back to the 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 34 in. and the return riser 1 in., and the riser branches should be 1 in. and 1 34 in., respectively. Note that according to Table 8, both radiator tappings should be 1 in. To simplify the construction, select 1-in. flow risers with 1-in. riser branches and 1-in. radiator tappings. Also select 134-in. return risers with 134-in. riser branches, and 134-in. radiator tappings. Similarly, for 18 Mbh, select 134-in. flow and return risers and riser branches, and 134-in. radiator tappings. 570 Chapter 33--Hot Water Heating Systems and Piping 'A t--------------------- ---- ~ . i --------- "'-S' t rle 6 Capacities of Mains in Mbh, for One-Pipe and for Two-Pipe Direct TA 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 Pip* 8a* (Inches) Equivalent Length op Pipe (Febt*0 3.0 6 93 4 5 78 10 It Equivalent Total Length or Pipe in Feet in Longest Circuit 75 100 125 150 .75 200 250 500 550 Unit Friction Head. in Milinches 8.0 4S.0 6.0 37.5 4.8 ss.o 4.0 30.0 3.4 27.0 3.0 25.0 2.4 22.2 2.0 20.2 1.7 18.7 2 ~~~234 4.0 83.0 72.0 63.0 57.0 51.0 48.0 42.0 38.0 35.0 4.5 140.0 115.0 100.0 90.0 81.5 75.4 67.2 61.0 56.0 3 5.0 34.0 204.0 175.5 160.0 148.0 133.0 110.0 107J> 100.0 3J4 5.5 S47.0 300.0 260.0 236.0 214.0 200.0 177.0 160.0 146.0 4 6.0 490.0 422.0 370.0 334,0 297.0 278.0 248.0 223.0 205.0 aApproximate length of pipe in feet equivalent to one elbow in friction bead. This value varies with the velocity. To develop a rule for determining radiator sizes, assume a system similar to that of Fig. 6, in which the total temperature drop is to be 35 F and which is equipped with 7 radiators, all radiators dissipating equal quantities of heat. The mean temperature of the water in the radiators will be reduced 5 F for each successive radiator. If the mean tempera ture of the water in the first radiator is 200 F, the mean temperature of the 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 Size (Inches) Flow Return Equivalent Length op Pipe (Feetc) 1st Floors VeL (In. perSec.)d Mbh Flow Return 2nd Floor Mbh 3rd and 4th Floors Mbh 36 34 1.0 5 6.2 34 34 6.4 8.0 34 34 1.5 9 2.3 2.3 10.1 14.0 34 1 12 3.2 2.0 12.8 17.1 11 2.0 18 2.5 2.5 20 26.0 1 134 21 3.0 2.0 25.2 84 H4 134 3.0 26 3.0 3.0 43 55 134 134 84 4.0 2.5 134 134 3.5 48 3.0 3.0 aThis table is based on pressure heads of 450, 1800, 3150, and 4500, respectively, for the first, second, third, and fourth floor radiators, and on friction heads of 200 milinches for the first floor radiators and con* nections, and 700 milinches for all other radiators and their connections. bThe riser branches, the piping which connects the risers to the mains, are to be one size larger than the risers. ^Approximate length of pipes in feet equivalent to one elbow in friction head. This value varies with the velocity. - ^Velocities apply to the riser branches.