DEVELOPMENT OF HOUSE HEATING 



141 



by the post, F. This valve is usually closed. When steam 

 begins to generate in the boiler, it begins to exert pressure. 

 This pressure is transmitted to the radiator and hence to 

 the valve. The upper half of the valve consists of a metallic, 

 bellows-like capsule open below. The upper plate, I, of this 

 capsule carries the seat of the needle valve, E. Now as long 

 as the valve is cool the pressure of a few ounces within the 

 valve is sufficient to spring the bellows-like capsule up, raising 

 the plate, /, and therefore the 

 valve seat, E, off from the 

 needle. This opens the valve. 

 If any air is in the radiator, 

 it is at once forced out. The 

 lower half of the valve con- 

 sists of a closed bellows-like 

 capsule, A. The upper plate 

 of this capsule carries the post, 

 F. This lower capsule con- 

 tains a liquid which is very 

 volatile, i.e., it turns to a 

 vapor at relatively low tem- 

 peratures. Its vapor pressure 

 increases with increased tem- 

 perature exactly as in the case of water, only much more 

 rapidly. Now as soon as all of the air has been forced out of 

 the radiator, the hot steam enters the valve. This heats the 

 capsule, A, and greatly increases the vapor pressure of the 

 liquid which it contains. This increased pressure expands the 

 bellows-like capsule and forces the post, F, upward. The 

 needle is thereby forced into the vent, closing the valve. This 

 prevents the escape of steam into the room. After the air 

 which was in the radiator and the pipes when the system was 

 first set up has once been expelled, all the vacuum valves are 

 supposed to remain closed to prevent any steam from escaping 

 or air from entering the system. The entire system is sup- 

 posed to contain a perfect vacuum except for the water vapor. 



FIG. 103. The vacuum valve. 



