STEAM INSTALLATION 189 



pound of water from 32 to 212 degrees but 

 that 966 B. T. U. (usually considered as 1000) 

 are required to change a pound of water at 

 212 degrees into steam. When the steam is 

 condensed in the coils it gives off this heat. 

 This is known as the latent heat of steam. It 

 may be defined as the amount of heat ab- 

 sorbed in changing from a liquid to a vapor 

 or the amount given off in changing from a 

 vapor to a liquid state. 



The problem in steam heating is to supply 

 an amount of radiating surface sufficient to 

 condense enough steam to furnish the 

 amount of heat required. Under ordinary 

 greenhouse conditions a square foot of steam 

 radiating .surface may be counted on to con- 

 dense approximately one quarter pound of 

 steam per. hour. Each square foot of radi- 

 ating surface will, therefore, provide a fourth 

 of 960 or approximately 240 B. T. U. per 

 hour. 



The number of B. T. U. required per hour 

 to heat a given house (see page 172), divided 

 by 240 will give, therefore, the number of 

 square feet of steam radiation required, and 

 from the table on page 174 the number of 

 linear feet of pipe may be easily determined. 

 Assuming a steam pressure of five pounds 



