Document gNoBakvZLv9x2Z3mpnVRdDdJ

HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 45 or 90 deg. In some cases it may be advisable to take the flow connection off the top of the main and the return connection from the side. With forced circulation and high velocities, it is advisable to let the water enter at the top of the radiator and leave at the bottom of the opposite end. With gravity circulation the flow connection may be either at the top or at the bottom of the radiator. With short radiators both flow and return may be at same end, but top and bottom. Unless used as heating surface, all piping, both flow and return, should be insulated. EXAMPLES OF PIPING DESIGN The following graded series of examples of the design of hot water piping systems will illustrate the fundamental principles and methods. The differences between reversed return 'and direct return systems are shown, and the methods of balancing the several radiators or circuits are illustrated: A simple gravity system is shown in Fig. 7 and an elementary forced circulation system is diagrammed in Fig. 8. Elementary Gravity System Example 1. A simple gravity-circulation system is illustrated in Fig. 7 with one radiator that is giving off heat at the rate of 20,000 Btu per hour or 20 Mbh. The boiler CHAPTER 16. HOT WATER HEATING SYSTEMS AND PIPING ence greater than 20 F, and if the circuit is of 1)4 in. pipe, the circulation willTake place with a temperature difference smaller than 20 F. To find, for example,' the temperature difference at which a circuit of l.in. pipe would transmit the required 20 Mbh, assume the difference to be 40 F. The ph (pressure head) would be (Fig. 4, from 200 to 160) 175 milinches per foot, or 1750 for the system. The fh for the system may be found from Fig- 3; the chart of this figure is based on a temperature difference of 20 F; if the tempera ture difference were 40 F, the heat conveyed would be twice that shown in the chart. Hence, find 10 Mbh on the lower scale, proceed vertically upward to the intersection with the 1 in. line, and from there to the left scale and read 13 milinches per foot. Note that the velocity would then be only about 5 in. per second. The total/A would then be 45 x 13 or 585 milinches. Since the ph would be 1750, circulation would take place with a temperature difference less than 40 F. The required temperature difference may be determined by constructing the diagram of Fig. 9, from which it appears that the temperature difference with which the 1 in. pipe circuit would function is about 30 F. Hence, if the flow riser temperature is 200, the return riser temperature will be 170, and the average water temperature in the radiator, about 185 F. Elementary Forced Circulation System Example 2. Design a system for the piping arrangement shown in Fig. 8, according to one of the outlined procedures. The ph developed by the circulating pump and the Fig. 7. Gravity System Fig. 8. Forced Circulation System imparts heat to the water at the same rate, and the water circulates at a uniform velocity. The thermal or gravity pressure head which produces the circulation is equal to the friction head which resists the circulation. The circuit consists of I boiler, 1 radiator, ' 2 ells, 1 radiator valve and a total of 24 feet of pipe. Solution. W*th the average water temperatures of 200 and 180 F in the supply and return risers, respectively, the pressure head will be 90 milinches per foot of water column. This pressure head may be found from Fig. 4. Since the center of the radiator is 10 ft above the center of the boiler, the total pressure head of the circuit is 10 x 90, or-900 milinches, or 0.9 inches of 190 F water. The friction head of the circuit must then also be 900 milinches. The friction head of 1 ft of 1 in. pipe is found from Fig. 3 to be about 46 milinches at 20 Mbh, and the corresponding velocity 9 in:-per Second. (Note that all values in Fig. 3 are based on a temperature difference of 20 F.) Similarly, if a 1)4 in. pipe were to be used, the friction head would be about 12 milinches per foot and the corresponding velocity about 5 in. per second, from Fig. 3. To find the friction head in the elbows, boiler, radiator and valve, Table 3 is used, and the entire circuit is found to be equal to 10 elbow-equivalents plus 24 ft of pipe. Each elbow-equivalent is equal to a pipe length of 25 times the nominal diameter. Then, the equivalent lengths of straight pipe are 45 ft of 1 in..pipe or 50 ft of 1)4 in. pipe. Hence, if 1 in. pipe is used, the fh (friction head) of the circuit will be 45 x 46, or 2070 milinches, and if 1)4 in. pipe is used, the fh will be 50 x 12, or 600 milinches. A 1 in. pipe would, therefore, be too small and a 1)4 in. pipe too large to permit the'desired circulation with a flow-return temperature difference of 20 F. If the circuit is of 1 in. pipe, the circulation will take place with a-temperature differ- 320 Fig. 9. Determination of Required Temperature Difference pipe size may be assumed and the flow-return temperature difference found; or, the ph developed by the pump and the flow-return temperature difference may be assumed and the pipe size found; or the pipe size and the flow-return temperature difference may. be assumed and the ph found which the circulating pump must develop. Solution. Assume that the circulating pump will develop a ph of 2 ft or. 24,000 milinches and that a 1 in. pipe is to be used. The equivalent length of the circuit will then be 45 ft, as in Fig. 7, and the available ph will be 24,000/45, or 533 milinches per foot. In Fig. 3,.find 533 on the left scale, move horizontally to the intersection with the 1 in: pipe line, and read about 77 Mbh delivered by the pipe (with a velocity of about 35 in. per second) for a temperature difference of-20 F. Since the circuit is to deliver only 20 Mbh, the temperature difference-will be 20 divided by 77 and multiplied by 20, or 5.2 F. Hence, if the flow riser temperature is 200, the return riser temperature will be . about 195, and the average water temperature in the radiator, about 197.5 F. If a )4 in. pipe were used instead of a 1 in., the equivalent length of circuit would be ' 35 ft instead of 45; the unit ph, 686 milinches instead of 533; the velocity, 27 in. per second instead of 35; the temperature difference, 19.5 instead of 5.2; and the average water temperature in the radiator, about 190.5 instead of 197.5 F. If the 1 in. pipe is used for the circuit, the gravity ph will be 22 milinches per foot, or 220 for the circuit (Fig. 4, 200 to 195). Since this is only 1 per cent of the pump ph (24,000 milinches), it may be neglected in the calculation, as was done previously. However, there are cases in which the gravity ph is so large compared with the pump ph, that it should be included in the calculation. 321