Document 2JGegxry222KDXm2k0vZM0mXg

American Society of Heating and Ventilating Engineers Guide, /19V3'S^'S^ Changing a System from Gravity to Forced Circulation f4 For example, let Fig. 8 represent the one-pipe gravity circulatdp system as designed. To change the system from gravity to forced cir<c| lation, insert.a circulating pump, at any point in the main. l| With the system designed as a gravity flow system, the total drop S temperature through the system was 25 deg.; the total heat output v,.Jj 54,000 B.t.u., the quantity of water circulating through the system ^ --?2r5?~> or 2,160 lb. per.hour. The heater pressure head was 0.4 in. corresponding power or 72 ft. pounds Per hour off 27-^00 ^p' Sfmilar,y- t^le Power developed by the six-radiator pressurj heads may be found to be about hp. 200,000 After having changed the system from gravity to forced circulation^ much more power will be supplied by the pump than was supplied by thef Chapter 11--Hot Water Heating llfeof `this water moves along the straight pipe from A to B and the fSS^der flows through the radiator. The flow through the radiator is ^ Ssea.part!y by the pressure head from A to B and partly by the pressure -:-.fed!pw*'uoed ,n a radht0r nferi,theJaer is very small compared ' nth the former and may be neglected. the resulting errors being on the Mm safe^- fleeting the radiator pressure head, the flow of water UroiriA to B will be so that the friction head in the two possible paths ;??:"Ownilelb'peaethqucaolntsoistesaochf aot1h/e4r iann.dpiepqeu*a4l tfot. tlhoengp;rethsseuorethheera, doffraom1 iAn tpoipBe' ^:f(ty long and 16 elbow equivalents. By a few trial calculations it is >:.X` t=Q 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 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. 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 54 000 .--Lg--, 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 friction 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 i hp. Assume that the radiator connections: are to be reduced from 134 mto-l 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. 196 . Fig. 10. One-Pipe Forced Circulation System for Six Buildings 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 'radiator. The cooling in the radiator must, therefore, beor 10 deg.. :.,-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. 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. . ; ` 197