140 THE PRODUCTION AND USE OF HEAT 



the absolute pressure is 74.7 Ib. and that the temperature of 

 the steam is between 150 and 160C., about 153 or 154C. 

 If the gauge indicated 80-lb. pressure, what would be the ab- 

 solute pressure ? What would be the temperature of the steam ? 

 Can you suggest why it is that, when a high-pressure steam 

 boiler bursts, the steam scalds those near worse than does 

 ordinary steam? 



162. Reducing the Pressure in a Steam Heating Plant 

 When high pressures are carried on the boilers of steam heating 

 plants, it is necessary to reduce the pressure before the steam 

 reaches the radiators. Radiators are not usually constructed 

 so as safely to withstand such high pressures. When the 

 steam reaches the building to be heated, the pressure is usually 

 reduced to about 5 Ib. gauge pressure by a device called a 



REDUCING VALVE (Fig. 97). 



163. The Vacuum System of Steam Heating. One objec- 

 tion to the ordinary PRESSURE SYSTEM for residence heating is 

 the fact that no steam is generated until the temperature of 

 the water in the boiler has been raised to a point above 100C., 

 or 212F. This means, not only that considerable fuel is 

 consumed before any steam is generated, but it also means 

 that in mild weather, when a little heat is required, the fire 

 often gets such a start in order to produce any steam at all that 

 the house becomes overheated. To overcome this defect in 

 steam heating of residences the VACUUM SYSTEM is often used 

 in place of the pressure system. The ordinary pressure 

 system can usually easily be changed into a vacuum system. 

 All that is necessary is that the system be made air- and steam- 

 tight and VACUUM VALVES be placed upon all radiators. 



164. The Vacuum Valve. The purpose of the VACUUM 

 VALVE is threefold: (1) To permit any air which may be in 

 the radiator to escape; (2) to prevent any steam from escaping; 

 (3) to prevent any air from re-entering the radiator. One 

 type of vacuum valve is shown in Fig. 103. The valve is 

 screwed into the radiator near its top by means of the threaded 

 nipple, C. At E is a needle valve, the needle being supported 



