Document MK0zmKGyY7MZRNq6ZNa6Vk5V

HEATING VENTILATING AIR CONDITIONING GUIDE 1944 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 B, 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 total fh 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 ph 344 + 32, or about 11 milinches per foot. With this ph, a % in. pipe will convey about 5 Mbh (Fig. 3), or 0.5 gpm. Hence, only 5 -4- 60, or about 8 per 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 and result in an unsatisfactory installation. To secure a larger flow of water through the radiator it is necessary to increase the/A 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, or by reducing the 1)4 in. main between the points A and B to the next smaller size, t.e. 1 in. 200 F 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 1 gpm and a % in. pipe is 40 milinches per foot, or 32 x 40, or 1280 for the radiator circuit. The main circuit consists of 4 ft of 1 in. pipe and two reducing tees. The two reducing tees may be placed equal to 0.8 elbow equivalents (Table 3), and the equivalent length of the main circuit equal to 5.7 ft. 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 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 l 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 320 HOT WATER HEATING SYSTEMS AND PIPING CHAPTER 16. 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 throBuygdhetchrearsaindgiatthoer ims ainicnrefraosmed15M0 ptoer1ceinn.t.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 totalfh 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. ReIvneraserdevaenrdseDdirreecttuRrnetusyrnstSemysttehmesradiators 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 the water flowing through the radiator farthest from the boiler, as illus- -1000'--------------------------------------------- ---------------------------------- r1iI I^ I 1-------1Pipe s5i"zes IQ--I __~52"___I--Q.-'ij. o J-5---D----_- 31SM0b_h Oforle3a_c2h7p0ip0e CQ 2250 (XI1} ~ rrr ~ " fr'll \ 9~ ~~ 450 io 900 ll~~~i350~12 * -- -- ll- ~ilL - - 8 3600 Mbh f* ) Is* -, 8" pipe Fig. 14. Two-Pipe Reversed Return System trated in Fig. 14. In a direct return system the radiators are connected so that all water returns to the boiler along the most direct path after it has passed through its radiator, as illustrated in Fig. 15. ExampleJ. In Fig. 14, sixteen air conditioning units, each demanding 450 Mbh, are to be supplied with water from a central plant. The system is divided into two equal parts as shown. Each part supplies eight units and has, therefore, eight circuits. The total length of each of the eight circuits is about 1170 ft. Circuit 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-16 17-18 Load Mbh 3600 3150 2700 2250 1800 1350 900 450 450 1 3600 1 7200 Table 7. Tabulated Data for Example 7 I Elbows -- I NO. 65 130 130 130 130 130 130 130 FT 142 130 130 130 Unit Friction Milinches per Ft 100 180 150 100 210 125 113 190 Total Friction, Milinches 7800 23.900 19.500 13.000 29.800' 16,200 14.700 24.700 20.000 65 130 1.8 0 100 Total 7.800 11 fV7ft