352 APPLIED SCIENCE 



tern, the air escapes ahead of the steam and finally when 

 steam reaches the air valve, the heat of the steam expands a 

 plug in the valve, which thus closes automatically. As the 

 air valves often get out of order, it is a great convenience to 

 run small J4 ia. pipes from each radiator to the boiler-room. 

 The engineer can then open each air pipe until steam appears. 

 When this happens he can be certain that the coils are 

 working properly. 



397. Steam Productions from Water. The weight of 

 water required to make 1 cu. ft. of steam at any pressure is 

 the same as the weight of 1 cu. ft. of steam. 



Therefore, the weight of water is obtained by multiplying 

 the number of cubic feet of steam required by the weight of 

 one cubic foot. 



EXAMPLE. How much water will it take to make 300 cu. ft. of 

 steam at 100 Ibs. absolute pressure? 



One cubic foot of steam at 100 Ibs. pressure is given as weighing 

 .2307 Ibs. Therefore, 300 cu. ft. will weigh 300 X .2307, or 69.21 

 Ibs. of water. 



One cubic foot of water may, for any practical purpose, be 

 reckoned to weigh 62 J/ Ibs., and the weight of one gallon of 

 water may be taken as 8-^- Ibs. Therefore, 69.21 Ibs. divided by 

 62.5 gives 1.1 cu. ft., or 69.21 divided by 8.3 gives 8.34 gals. 



At atmospheric pressure one cubic foot of steam has nearly 

 the weight of one cubic inch of water, and the weight increases 

 very nearly as the pressure; therefore, the rule: Multiply 

 the number of cubic feet of steam by the absolute pressure in 

 atmosphere and the product is the number of cubic inches of 

 water required to give the steam. 



In all such calculations, for practical purposes, a liberal 

 allowance must be made for loss and leakage. 



