Document NNk76kKaRQyMZV5XVv9zyBv2V

HEATING VENTILATING `AIR CONDITIONING GUIDE 1943 branches are of 1 in. pipe, water will circulate through the system at the rate of 3 gpm with a temperature difference somewhat less than 40 F and that, at the radiator connec tions, water will circulate through the radiators at the rate of 1 gpm and that, conse quently, between radiator branch connections the fh is at a lower rate since only 2 gpm flow through in the main between those points. One-Pipe Forced Circulation System Example 6. The system shown in Fig. 12 as a gravity circulation system may be changed to a forced circulation system by inserting a circulating pump as shown in Fig. 13. The location of the expansion tank should then be changed as indicated. Solution. To design this system the pipe sizes and the temperature difference may be assumed; for example, 1)4 in. pipe may. be selected for the main and % in. pipe for the radiator branches and risers, and 20 F as the flow-return'temperature difference. Since the system !is to deliver 60 Mbh with a jteraperature difference of 20 F, the pump must circulate 60 -i-. 10, or 6 gpm. For this load, the unitfh fora 1)4 in. pipe is 86 milinches. The main circuit consists of 110 ft of pipe and 10 elbow equivalents and.may be placed equal to 136 ft of pipe. CHAPTER 16. HOT WATER HEATINC SYSTEMS AND PIPING The fh for 5 gpm and a 1 in. pipe is 240 milinches per foot, or 5.7 x 240, or 1370 milinches for the main circuit. Since this is only slightly more than the calculated fh for the radiator circuit, it is evident that the flow through the radiator will be slightly more than 1 gpm, and it is not necessary to make a second trial calculation. The quantity of water flowing through the radiator can be varied by varying the distance between the points A and B, where the radiator branches join the main. In order to deliver 15 Mbh to the radiator with a temperature difference of 20, it is necessary that 1.5 gpm flow through the radiator; since, in this case, the flow through the radiator is only 1 gpm, the temperature difference must be 30 F. If the water enters the radiator at 190. F, the average water temperature will be 175 F.. The quantity of water circulating through the radiator may be varied con 150* The total fh for the main circuit is 136 x 86, or .11,696 milinches, or practically 1 ft for a flow of 6 gpm. At the points A, a portion of the water will be diverted through the radiator circuit, and as a result less than 6 gpm will flow in the main between the points A and,, and the fh will be slightly less than 86 milinches per foot between these; points. But the difference will be so small that it may be neglected and the totaljh from A to B assumed to Be 4 x 86, or 344 milinches. The pH forcing the water through the radiator.circuit will then be 344 milinches. The radiator circuit consists of 11 ft of pipe and about 14 elbow equivalents and may be placed equal to 32 ft of pipe and the available ph344 -=- 32, or about 11 milinches per foot. With this ph, a % in. pipe will convey about 5 Mbh (Fig. 3), or 0.5 gprm Hence, only 5 4- 60, or about 8 peV cent of the water, would flow through the radiator, if the. radiator's gravity head is not'considered. The water, would, then, have to cool 60 F in order to deliver-15 Mbh, and the average radiator temperature would be 170 if the water entered at'200. This would require a large radiator arid result in an unsatisfactory installation. To secure a larger flow of water through the radiator it is necessary to increase the fh of the short path A-B in. the main. This may be done by inserting special resistance tees.at points A and.B, or by inserting an orifice resistor between points A and B,,ot by reducing the 1)4 in. main between the points .4 .and B to. the next smaller size,' i.e. 1 in. The relative quantity of water flowing through the radiator may then be found by . trial calculations. Assume, first, that 1 gpm will flow through the radiator and 5 gpm through the main. The ph for l.gpm and a % in. pipe is 40 milinches per foot,or32 x.40, or 1280 for the radiator circuit^ . .. 4 ' The main circuit consists.of 4 ft of 1 in. pipe and two reducing tees. THetwo reducing tees may be placed equal to 0.8 elbow equivalents (Table 3), and the equivalent length of the main circuit equal tp 5.7 ft. . 326 siderably without an appreciable effect on the quantity of heat dissipated by the radiator. This is evident from the following calculation. Assume that Radiator 2 has been.designed so that it will dissipate 15 Mbh when its flow of water is at the rate of 1 gpm, and when its average temperature is 175 F. Assume that the flow of water is increased 50 per cent--from 1 gpm to 1.5 gpm. The water will then flow through the radiator in two-thirds the time and will cool two-thirds as much; i.e., it will cool 20 F instead of 30 F, and the average radiator temperature will be 180 F instead of 175 F. If the surrounding temperature is 70 F, the temperature differences, radiator and surroundings, will be 110 and 115 F, respectively. Consequently, the heat dissipation will be increased only about 6 per cent when the quantity of water flowing through the radiator is increased 50 per cent. ' 4" cT1 4" 1 4-1 1 1 d 5" 5" (3 3rd . Pipe sizes d. 5- q 5" 21" 2i" d 450 Mbh 10 . 900 . 11 9[-J-| 450 Mbh |t] 900 tl] Mbh for each pipe ___ . 3600 ___ . 5" UJ 6" 0 \ tXI 3150 1X1 2700 1350 .12 135 g] r 1800-* lo 12 2250 GQ 1 5" 6" ]T7 r 16 3150 15 2700 14 2250 1*3 IS 3600 Mbh C---S- J| CfsM. 8" pipe .' Fig. 15. Two-Pipe Direct Return System By decreasing the main from to 1 in. between radiator branches while the flow is decreased from 6 to 5 gpm, the fh in that section of the main is increased from 344 to 1370 milinches; or 926. milinches. Hence, for the four radiator sections the increase is 3704 milinches, and the total/A for the circuit will be.11,696 plus 3704, or 15.4 in. instead of 11.7 in. as first calculated. ' The pump must, therefore, circulate 6 gpm against a head of 1.3 ft, Reversed and Direct. Return Systems In a reversed return system the radiators are connected, so that'all circuits are practically of equal length and so that the water flowing through the radiator nearest the boiler must travel practically as far as 327