200 STEAM ENGINES 



Calculation of Brake Horsepower. When the necessary 

 data, as previously noted, have been determined, the brake 

 horsepower may be calculated by the formula 

 2,r R WN 



Hb 



in which /?& = brake horsepower; 

 7T = 3.1416; 



R = length of brake arm, in feet; 

 W = net pull or pressure, in pounds; 

 N = number of revolutions per minute. 



EXAMPLE. A Prony brake with an arm 6 ft. long was placed 

 on the flywheel of an engine running at 200 R. P. M. What 

 brake horsepower was being developed when the net pressure 

 was 140 lb.? 



SOLUTION. Applying the formula, 



2X3.1416X6X140X200 



33,000 





CONDENSERS 



Types of Condensers. There are two types of condensers 

 in general use, namely, the surface condenser and the jet con- 

 denser. In the former, the exhaust steam comes in contact 

 with a large area of metallic surface that is kept cool by con- 

 tact with cold water. In the latter, the exhaust steam, on 

 entering the condenser, comes in contact with a jet of cold water. 

 In either case, the entering steam is condensed to water, and 

 in consequence a partial vacuum is formed. If enough cold 

 water were used, the steam on entering would instantly con- 

 dense and a practically perfect vacuum would be obtained 

 were it not for the fact that the feedwater of the boiler always 

 contains a small quantity of air, which passes with the exhaust 

 steam into the condenser and therefore partly destroys the 

 vacuum. To get rid of this air, the condenser is fitted with an 

 air pump, which pumps out both the air and the water formed 

 by condensation. 



Surface Condensers. In the surface condenser, the exhaust 

 steam and the injection water are kept separate throughout 



