Document jm934J6bDwMq19wmjK021KgqR

American Society of Heating and Ventilating Engineers Guide, 1930 Fig. 12. Two-Pipe, Direct-Return, Forced Circulation System for Six Buildings shown in Fig. 1.1, is $3,193. Assuming the life of the main to be 20 years and the interest rate 6 per cent, the cost of the main is $278 per year. The power required to circulate the water through the main is 150,000 X 15.9, or 2,385,000 ft. pounds per hour, or 1.2 hp. Assuming that the power required, to circulate the water through the central plant is 15 per,cent of the amount stated, that the efficiency of the pump is 75 per cent,-and that the cost of current is 2l/2 cents per kilowatt-hour* the cost of circulating the water is 3.44 cents per hour, or $144 per year if the pump is to be operated 20 X 30 X 7, or 4,200 hours a year. 'After completing two additional calculations similar to the one just Fig. 13. Influence of the Circulating Pump upon the Pressure in the Flow Main 384 Chapter 21--Piping for Hot Water Heating Systems described, for the system in which the largest pipes in the main are 4 in. and 6 in. respectively, the following results are obtained: Largest Main, Inches Velocity in Same Total. Friction Inches per Second In Feet 4 93 34.8 5 58 15.9 6 40 6.47 Annual Cost 8221 278 340 Annual Cost op Circulation 8316 144 59 Combined Cost 8537 422 399 This comparison assumes that the cost of the pump is practically the same in the three cases and that the cost of forcing the water through the six buildings remains constant. - . Optimum Velocity of Wafer in Forced Circulation Systems Based upon these assumptions, the system having the 6-in. main with a velocity of 40 in. per second in that main is the most economical to install. For this installation the cost of circulating the water would be about 2 per cent of the cost of the heat, if the latter is estimated at 50 cents per 1.000. 000 B.t.u., and if it is further assumed that the average heat demand will be one-half of the maximum, i.e., that the average will be 1,500,000 for the plant which was designed for a maximum demand of 3.000. 000 B.t.u. per hour. In a manner similar to that described before, every forced circulation system should' be analyzed before definite pipe sizes are adopted. In large installations it may be found advantageous to transport the heat from the central plant to the central point of a group of buildings by means of high-pressure steam and to install at this central point a heat exchanger and a circulating pump. , Effect of Location of Expansion Tank on Pressure in Forced Systern In most central heating systems it is necessary to use an expansion tank. Such a tank is generally installed near the circulating pump and on its suction side; the pump being located in the return line near the heater., With such an arrangement, the pressure in the main remains constant at the suction side of the pump; it rises sharply in the pump and then decreases gradually along the main, as. indicated in Fig. 13. In this figure, the line 1, 2, 3, 4, is a base line; the line 5, 6, 7, 8, is a line parallel to the base line drawn so that the distance 1 to 5 represents the pressure in the main when the system is not in operation. This pressure is deter mined by the elevation of the expansion tank and must be such that sufficient pressure is maintained in the highest radiator to prevent boiling when the water is heated to the maximum temperature for which the system was designed. The line 9, 10, 11, 12, 5, is drawn so that the vertical distances from its points to the corresponding points of the base line show the pressure in the respective parts of the main when the water is being forced through the main by the circulating pump, neglecting-the slight reduction in pres sure head caused .by the velocity head. An inspection of Fig. 13 shows that, with the expansion tank located at the suction end of the pump, 385