346 APPLIED SCIENCE 



drum in the boiler-room. This requires the use of a reducing 

 valve to let the pressure down between 5 and 15 Ibs. for the 

 heating coils. A receiving tank is necessary for the return 

 water, and a pump must be installed to force it back to the 

 boilers. Some mills have spare boilers that are used only for 

 heating purposes. These may be run at a low pressure, 10 Ibs., 

 and the steam may be passed directly into the heating system 

 without the use of a reducing valve. When the return water 

 is piped directly to the feed pipe of the boilers we have what 

 is known as a gravity return system. Since there is the same 

 pressure in the heating system that there is in the boiler, the 

 water of condensation runs back into the boiler simply by its 

 own weight. This requires that all heating pipes be on a 

 higher level than the water line in the boiler. If any radiators 

 or coils were lower than the boiler, they would, of course, fill 

 with water, and a pump would be required to return the water 

 from the low coils to the boiler. The gravity return system 

 is used in many dwellings, office buildings, churches, and 

 stores 



388. Low-Pressure Steam Heating. When steam at 

 atmospheric pressure is condensed into water at a tempera- 

 ture of 212 F., each pound of steam gives up 966 B. T. U. of 

 heat ; but if steam of 100 Ibs. gauge pressure (115 Ibs. absolute) 

 is condensed into water at 212 F., each pound of steam must 

 give up 1004 B. T. U., which is only 88 heat units more than 

 are contained in steam of atmospheric pressure. It is evident 

 from this that for heating purposes there is no advantage in 

 using stearn of a high pressure. One pound of exhaust steam, 

 only a pound or two over atmospheric pressure, is almost as 

 valuable an agent for heating purposes, as live steam at 100 

 Ibs. pressure direct from the boiler, 



