Document 3JVGXozVObYRbb3b1KBxZeZex

American Society of Heating and Ventilating Engineers Guide, 1930 With the system designed as a gravity flow system, the total drop in temperature through the system was 25 deg.; the total heat output was 54,000 B.t.u., the quantity of water circulating through the system was 54 000 ---, or 2,160 lb. per hour. .The heater pressure head was 0.4 in. The ,. 2,160 X 0.4 ,,,, . corresponding power was------------- or 72 ft- pounds per hour; or 27,500 hp. Similarly, the power developed by the six-radiator pressure heads may be found to be about 200,000 hp. After having changed the system from gravity to forced circulation, much more power will be supplied by the pump than was supplied by the gravity pressure heads. Consequently, more water can be circulated through the system, thereby reducing the temperature drop through the system and as a result, smaller radiators and pipes may be used, thereby further increasing the velocity of the water and the power necessary to circulate it. !i ' ' ' i Chapter 21--Piping for Hot Water Heating Systems side of safety. Neglecting the radiator pressure head, the flow of water from A to B will be so that the friction head in the two possible paths will be equal to each other and equal to the pressure heacf from A to B. One path consists of a 1J4 in- pipe, 4 ft. long; the other, of a. 1 in. pipe, 7 ft. long and 16 elbow equivalents. By a few trial calculations it is found that the friction head from A to B is about 85 per cent of that calculated before, and that about 900 lb. per hour flow through the . , . , 9,000 in J radiator. The cooling in the radiator must, therefore, be ~gQg~, or 10 deg. j. The average temperature of the water will be 200 deg. in the first radiator, and 196 deg. in the sixth. The sixth radiator will be 4 per cent larger than the first instead of 25 per cent as calculated for the gravity system. cza^it o= . Fig. 9. Radiator Connections for a One-Pipe System Illustrating the Influence of the Pressure Drop in the Main upon the Circulation through the Radiator Let it be assumed that the total temperature drop is to be 5 deg. instead of 25 deg., and that the main shall be 1}4 in. instead of 3 in. In this case the quantity of water circulating through the system will be --11:--, or 10,800 lb. per hour, and the friction head will be 440 mil-inches per foot of pipe and about 1,850 mil-inches per elbow. The total fription head will be about 135 X 440, plus 10 X 1,850, or 6.49 ft. The velocity of the water in the main will be about 42 in. per second. The power required to produce the circulation will be about ^ hp. Z<j Assume that the radiator connections are to be reduced from l'J in. to 1 in., and that the distance along the main between radiator connections is 4 ft. as shown in Fig. 9. The pressure head between the points A and B must be such that it will force 10,800 lb. of water per hour from A to B. A part of this water moves along the straight pipe from A to B and the remainder flows through the radiator. The flow through the radiatorjis caused partly by the pressure head from A to B and partly by the pressure head produced in the radiator risers; the latter is very small compared with the former and may be neglected, the resulting errors being on the 380 < Fig. 10. One-Pipe Forced Circulation System for Six Buildings Relative Economy of Gravity and Forced Circulation Comparing the economy of the gravity with the forced circulation system it may be concluded that it is more economical to use gravity circulation except when the increased cost produced by installing, the pump and by operating, maintaining, and replacing it, when that becomes necessary, is less than the saving effected by using the smaller pipes and smaller radiators, made possible by the use of the pump. There are some cases where gravity circulation cannot be used. For example: If in Fig. 11 the six rectangles represent six buildings instead of six radiators, each building requiring 500,000 B.t.u. to be supplied from the central plant, it will almost always be necessary to adopt forced cir culation for such an installation. The heating main can be arranged as it could be to supply six radiators in a building, as a one-pipe system, Fig. 10; as a two-pipe reversed-returned system, Fig. 11, or as a two-pipe direct-return system, Fig. 12. 381