Document YGXK5DwmOg1NY1p9Xx3KGKMw8

464 CHAPTER 24 1946 Guide Many large heating systems contain some one-pipe and some two-pipe sections. The piping system shown in Fig. 13' functions with a flow-return temperature difference.of 40 deg. ' Solution. Since the four radiators are each to deliver 15 Mbh, and since the water is to leave the boiler at 200 F and return at 160 F, Radiator 1 will receive 200 F water, Radiator 2, 190 F water, Radiator 3, 180 F water, and Radiator 4, 170 F water. Since the system is to supply 60 Mbh with a temperature difference of 40 deg and since 1 lb of water liberates 1 Btu when cooled 1 deg, it is necessary that water circulate through this system at the rate of 60,000 '+ 40,* or 1,500 lb per hour, or 25 lb per minute. As suming one gallon of water to weigh 8.33 lb, water must circulate in the system at the rate of 3 gpm. If the temperature difference were 20 deg instead of 40, the circulation would be at the rate of 6 gpm. It is well to remember that, with a temperature difference of 20 deg, water circulating at the rate of 1 gpm will convey heat at the rate of 10 Mbh. The chart of Fig. 4 shows the rate of circulation in gpm on the upper scale and the corresponding heat conveyance on the lower scale. The system may be divided into 5 separate systems. Each of the four radiators, with Circuit 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-16 16-17 17-18 Table 7. Tabulated Data for Example 7 Load Mbh 3600 3150 2700 2250 1800 1350 900 450 450 3600 7200 Pipe Length Ft 65 130 130 130 130 130 130 130 Elbows No. 1.8 0 0 0 2 0 0 0 Equivalent Length Ft 78 130 130 130 142 130 130 130 Size In. 6 5 5 5 4 4 3M 2M Unit Friction Milinches per Ft Total Friction. Milinches 100 / 180 150 100 7,800 23.900 19,500 , 13.000 210 29.800 125 16.200 113 14.700 190 24,700 Estimated Friction 20,000 65 1.8 130 0 78 130 - 6 100 8 90 Total 7.800 11,670 188,570 its flow and return lines constitutes an elementary heating system (similar to Example 1), and the flow main with its two risers is also a complete elementary system. . If the center of the boiler is 4 ft below the center of the flow main, and if the flow, riser contains 200 F water and the return riser 160 F water, the head for the main circuit is (Fig. 5) 4 x 175, or 700 milinches. The circuit consists of 110 ft of pipe and 10 elbow equivalents; its equivalent length is about 150 ft if a 2 in. pipe is used as the ma!m. The average head will be 700 -* 150, or 4.7 milinches. According to Fig. 4 or Table 1 for a 4.7 milinch friction, a 2 in. pipe will convey about 70 Mbh. Since the pipe is to convey only 60 Mbh, it is slightly too large but should be used. The friction will,-then, be 3.5 milinches instead of the permissible 4.7. The water wilhcirculate with a temperature difference slightly less than 40 deg, and the three last radiators would receive water slightly warmer than indicated in Fig. 13. / As the water flows in the main and arrives at one of the four points marked A, the flow will be divided and a portion of the water will take the short path in the.main to the px)int B, and the remainder will take the long p>ath through the radiator to the p>oint B. Since the two paths together offer less resistance to the flow than the one path alone, the unit friction will be less than 3.5 milinches between .the points A and B. If the distance from A to is 4 ft, and if, for example, the flow in the short path is 2 gpm, the head forcing the water along the two paths is 4 x 1.6, or about 6-railinches. Since the friction of the long path is much greater than the friction of the short path, only a com paratively small portion .of the water would take the long path. However,,the gravity head of the radiator supplies, an additional head for the long path'. If the center of the 'radiator is 4 ft above the main and if the radiator circuit is designed for a temperature difference of 30 deg, the radiator head will be about 4 x 120, or 480 milinches.. In that case, the head is 6 milinches for the short path and 486 milinches for the long path. The radiator circuit consists of 11 ft of pipe and about 14 elbow equivalents. If the circuit "is of 1 in. pip>e, its equivalent length is about 40 ft and the unit friction should be 486 4- 40, or 12 milinches. For this friction and 30 deg temperature difference, a 1 in. Hot Water. Heating Systems and Piping 1 465 Fig. 13. One-Pipe Gravity Circulation System pipe conveys about 15 Mbh (Fig. 4). A 1 in. pipe is, therefore, the correct size, and the water would flow through the radiator with a temperature difference of 30 deg. Since each of the four radiators delivers one fourth of the total heat, and since the total temperature difference is to be 40 deg, the water would cool 10'deg in every radiator if all the water pjassed through every radiator. Since the water cools about 30 deg in flowing through the. radiator only 3% of the water flows through the radiator and % flow through the main from p>oint_A to p>oint B. It follows from these calculations that, if the main is of 2 in. pipe arid the radiator 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 deg and that, at the radiator con nections, water will circulate through the radiators at the rate of 1 gpm and that, conse quently, between radiator branch connections the friction is less since only 2 gpm flow through the main between those promts. One-Pipe Forced Circulation System Example 6. The system shown in Fig. 13 as a gravity circulation system may be changed to a forced circulation system by inserting a circulating pump as shown in Fig. 14. 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% in. pipe may be selected for the main and % in. pipe for thge radiator branches and risers, and 20 deg as the flow-return temperature difference. Since the system is to deliver 60 Mbh with a temperature difference of 20 deg, the pump must circulate 60 -r 10, or 6 gpm. For this load, the unit friction for a 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 1)4 in. pipe. -The total friction 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, Fig. 14.. One-Pipe Forged Circulation System