132 THE PRODUCTION AND USE OF HEAT 



152. Equipment for Steam Heating. As in the case of hot 

 water heating, the equipment consists of a HEATER, generally 

 called BOILER, of connecting pipes, and radiators (Fig. 96). 

 In fact, a plant for hot water heating may be, and some- 

 times is, used for steam heating with but slight modification. 

 This is so, notwithstanding the fact that the two systems are 

 very different in principle. 



153. Hot Water and Steam Systems Contrasted. The two 

 systems may be contrasted as follows: 



1. In the hot water system, the radiator is heated by re- 

 ceiving the SENSIBLE HEAT given off by the water as it is 

 cooled in passing through the radiator. 



In the steam system, water is turned into steam in the 

 boiler. The steam then passes up through the pipes to the 

 radiator and is there condensed, giving off its HEAT OF VAPORI- 

 ZATION. This is the source of the heat which keeps the radiator 

 hot. The hot water, condensed steam, returns to the boiler, 

 often at 100C. 



2. In the hot water system, the water is kept in circulation 

 by its own unequal density. The hot water, being lighter, is 

 pushed upward into the radiator while the colder, heavier 

 water sinks into the heater, crowding the hot water upward. 



In the steam system, the steam is forced upward into the 

 radiators by the unequal pressure upon it. In the boiler, 

 more water is constantly being changed into steam, tending 

 thereby to increase the pressure. In the radiator, the steam is 

 constantly condensing into water, tending thereby to decrease 

 the pressure. This means that there is always a lower pressure 

 in the radiator than in the boiler and, consequently, steam is 

 constantly being forced from the boiler into the radiator. 



3. In the hot water system, the heater, the pipes and the 

 radiators are always completely filled with water. 



In the steam system, the boiler is but partlly filled with 

 water. 



4. The ordinary hot water system is never closed but always 

 open at the expansion tank. 



