Greenhouse Heating 



pounds, and also some other power, such as an auxiliary boiler at higher 

 pressure, or electricity, to drive the pump which returns the water of con- 

 densation. In some cases the water of condensation is conveyed to an open 

 well, or reservoir, and from there into the boiler. Another plan, and a 

 more common one, is to carry the return water directly into a receiver at 

 the suction end of the pump. If a steam pump is used, it is of a special 

 construction for this purpose, and works automatically, forcing a constant 

 stream of return water into the boiler, the volume of which will vary with 

 the weather. This water is frequently made to return into the receiver of 

 the pump relieved of pressure ; that is, after the water reaches the receiver, 

 any steam vapor which collects in the receiver is allowed to escape. This 

 plan makes a return water temperature of more than 212 degrees Fahr. 

 impossible, and the usual temperature is much below this. For reasons 

 which will be given further on, any plan which reduces the pressure on 

 return water, between the radiation pipes and the boiler, is not favored by 

 the writer. 



A higher boiler pressure, viz., from twenty-five to fifty pounds, is more 

 commonly employed in operating the positive return system. The boiler 

 pressure must be sufficient to properly work the steam pump. If a lower 

 pressure is desired, the steam cylinder of the pump must be made larger. 



With this increased pressure, an auxiliary boiler is not required. The 

 return pump is connected direct to the boiler. It is not desirable to carry 

 this high pressure into the radiation pipe. A reducing valve is a neces- 

 sary adjunct to the boiler in this system. This valve is entirely automatic, 

 and can be adjusted to any pressure from one to ten pounds, or more, at 

 the will of the operator, to suit the requirements of the weather. 



This method of positive return, as applied to steam heating, is adapted 

 to large plants only. It is not contended that it is the equal in economy to 

 the so-called gravity system. There are inherent in the system certain 

 features which, in spite of the best efforts of the engineer to prevent it, 

 absorb a considerable number of heat units, which the gravity system is 

 not guilty of. To start with, there is a slight friction loss in the reducing 

 valve ; then, after the return water reaches the pump, there is still another 

 loss, for power is required to drive this pump. Notwithstanding this slight 

 loss, which cannot be entirely eliminated, t'he positive system is quite popu- 

 lar with florists. It is not within the writer's knowledge that any florist 

 who has used it has condemned it on account of poor economy. To attain 

 the best results from this method the return water should be conveyed direct 

 to the receiver of the pump without any diminution of pressure; that is, 



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