SECT, in.] CONDENSATION OF STEAM. 147 



This is the cause of the failure of every method of slow condensation. It cannot be 

 too prompt, unless a sacrifice of power is made in some other way to gain that 

 promptness, and to which the effect gained by condensation is not equivalent. 



281. Water has been found the most effective cold body for condensation ; 

 it has great specific heat, perhaps greater than any other body; it is a rapid 

 conductor of heat, and in a jet applies an immense proportion of cooling surface 

 to the steam. 



Now since water is frequently difficult to be procured of a low temperature, and 

 sometimes not in sufficient quantity, it becomes important to inquire what effect is 

 produced by given proportions at given temperatures. 



282. The weight of the water, W, required for condensation, multiplied by 

 the quantity x t its temperature is raised, gives the heat it absorbs ; and in the 

 steam engine, where the operation is repeated in the same vessels and at the 

 same temperatures, the excess of the temperature T x of the steam above that to 

 which the condensing water is raised, added to 1000, and the sum multiplied by 

 the weight w of the steam, must be equal to the heat absorbed by the condensing 

 water. That is 



W (* - = (1000 + T - x) i 

 or W w ( 10 + T x) , _ w (1000 + T) + W t 



X t W + W 



283. When the temperature of the condensed water is equal to the tem- 

 perature of the steam, the quantity of water would be equal to that which simply 

 reduces the steam to water without change of temperature ; or 



w _1000tt> 

 T-t 



But in this case no effect would be obtained. Any greater quantity of cold water 

 reduces the elastic force, but it must be so far reduced as to render the accession of 

 power more than equivalent to that required to work an air pump, and cover the 

 expense of a supply of water, and the extra cost of the engine. 



284. In low pressure steam T = 220, and t may be taken at 52 the mean 

 temperature, and if the temperature of the condenser be 100, then 



w _ w (1000 + T x) _ w (1000 + 220 100) _ , 

 ,-* 100-52 



That is, 23 i- times the quantity of the water required for steam, will be the 



1 To make this equation general, let s be the specific heat of the condensing body, and C the heat 

 of conversion, and s' the specific heat of the body in vapour, then W s (x t) = w s (C + T x). 



