AMERICAN STEAM ENGINEERING 287 



the continental practice being about in volumes 30 to 1 as 

 compared to the American of 25 to 1. 



Examining the types of engines in use. \Vhat is econ- 

 omy in a power and traction station? Engine economy 

 guarantee at rated load is not the real measure, as loads arc 

 very variable. To ascertain what the real economy is we 

 must know what is the variation in large plants and what is 

 the average loads on engines in these circumstances. Cross 

 compound condensing units give good results. The varia- 

 tion of temperature in the low pressure cylinder is very small 

 because of the reheater receiver. Such an engine is prac- 

 tically two single engines with each cylinder acted upon by 

 a governor; it consists of two engines with smaller ranges of 

 temperature variation. This is American practice. Engines 

 for this class of work necessitate ample receiver capacity. 



It has been suggested to adapt triple expansion to 

 traction work and variable loads by using a throttle gov- 

 ernor on the high pressure, an automatic on the intermedi- 

 ate, and fixed cut-off on .the low pressure. A sudden over- 

 load would quickly raise the initial steam pressure and let 

 out the cut-off on the high pressure; the intermediate auto- 

 matic cut-off would provide the balance of power and deliver 

 a fairly constant volume of steam at a slightly varjdng pres- 

 sure; the cut-off being fixed and the vacuum constant, the 

 range of temperature would be fairly constant. During the 

 overloads such an engine would not be economical, but it 

 would be so for the average load for which it was designed 

 and to which it had been regulated. 



German and continental engineers generally try to 

 secure absolutely steady load by the use of large storage 

 batteries, in which case triple expansion condensing units 

 are to be recommended, and also the use of superheated 

 steam; but although this reduces the cost of the power gen- 

 erated, we must consider the extra cost of the power supplied, 

 due to loss in accumulators and to interest, maintenance, 

 and sinking fund involved by the installation battery. "Wliere 

 money is to be got at a low rate of interest, expensive fuel 

 and water may indicate the use of batteries. 



