Document LGaOy94zanXv89OVvGrjvrdq

American Society of Heating and Ventilating Engineers Guide, 1932 Solution. The friction heads in the boiler, radiator, valve, and tee may be expressed in terms of the friction head in one elbow according to the values given in Table 1. Having done this, each of the five circuits consists of 155 ft of pipe and about 24 elbow equivalents. The friction head of one elbow is approximately equivalent to that in a pipe having a length equal to 25 diameters. Assuming that the average pipe size in this case will be about 1in., one elbow equivalent may be placed equal to about 3 ft of pipe and the total length of the circuit equivalent to about 227 ft of pipe. Having determined the equivalent pipe length, assume the rate at which the water is to be forced through the system. This rate may vary widely. The water may flow through the radiator so that it will cool 10 deg or 20 deg or any other reasonable number of degrees. In this case, assume a 10-deg drop. Since the system is to dissipate 66,000 Btu per hour, the pump must circulate 6600 lb of water per hour or 13.2 gpm based on the nominal density of water of 8.336 lb per gallon at 62 F. (One gallon of water per min ute at this density will deliver 10,000 Btu per hour with a temperature drop of 20 deg). The next step in the design is to assume the velocity at which the water is to circulate through the system. This also may vary materially. As the velocity is increased, the sizes of the pipes and the cost of the system are decreased, but the cost of operating the circulating pump is increased. The designing engineer should make a careful study to determine the velocity which will produce the most economical installation for the particular case in hand. In this case, assume a velocity of about 1 Yi fps for a 1^4-in. pipe. Reference to Fig. 4 shows that for a lM-in. pipe and a velocity of 18 in. per second, the friction head is about 100 milinches per foot, or about 2 ft for a circuit of 227 ft, if the pipe sizes for that circuit are chosen so that the average friction head is about 100 milinches per foot of pipe. The pipe sizes may now be selected from Fig. 4 by making allowance for the fact that Fig. 4 is based on a temperature drop of 20 deg and that the system to be designed is to Table 1. l 90-deg elbow._'.... .............. 1 45-deg elbow..................... 1 90-deg long turn elbow... 1 open return bend :............ 1 open gate valve................ 1 open globe valve............... 1 angle radiator valve........ 1 radiator--............................ 1 heater.................................. 1 tee...... ................................. Elbow Equivalents3 1.0 ...... 0.7 ...... 0.5 ..... 1.0 ...... 0.5 ..... 12.0 ..... 2.0 ...... 3.0 ...... 3.0 (Note*5) *The loss of head in one elbow can be expressed in terms of the velocity head by the formula: where h aL 2g (1) h *= the loss of head in feet, v the velocity of approach in feet per second, and 2g = 64.4 ft per second per second. bTbe loss of head in tees when water is diverted at right angles through a branch of the tee varies with ' the per cent diverted. When the water diverted is less than 60 per cent of that approaching the tee, the loss of head, in elbow equivalents, may be expressed as follows: *e II* p2t (2> where \= the loss of head in elbow equivalents, vi = the velocity of approach, vt = the velocity of water diverted at right angles. Values in elbow equivalents for the most common percentages of water diverted in a lxlxl-in. tee are as follows: 25% 16.0 33% 9.0 50% 4.0 1.8 For other percentages the approximate values may be secured by*interpolation. When the water is diverted from the tee.into a smaller size branch, as in a lxlx%-in. tee, approximate values may be secured by means of Formula 2. 114 rictionF Head in iuncncj per Foot o r PipeN Chapter 8--Hot Water Heating Systems and Pipe Sizes Fig. 4. Friction Heads in Pipes for a 20 deg. Temperature Difference of the Water in the Flow and Return Lines 115