PUMPING WATERS. gi 



60 



- = 75 revolutions per minute, which would be quite 



oO 



fast enough for a small three-throw ram pump with 

 ordinary mitre valves. 



Now the horse-power required will be as follows : 

 3000 gallons per hour= 50 gallons per minute, and a 

 gallon of water weighs 10 Ib. The head, 300 ft., is 

 assumed to include the actual vertical height of the 

 delivery above the suction, plus friction in the pipe and 

 friction due to bends, all calculated as already pointed 

 out. 



Now 50 x 10 x 3000 = 1 50,000 foot-pounds of energy 

 required. A horse-power is taken as equal to 33,000 

 foot-pounds, while the engine will probably give So per 

 cent efficiency, the gearing, etc., 90, and the pump, 

 as stated, 80. The combined efficiency of the whole 

 will then be '8 x -9 x -3 = '576, say 60 per cent over 

 all efficiency. Hence, as the water horse-power is 

 equal to 



1 50,000 

 -^ - = 4-54 

 33,ooo 



the actual horse-power required will be 



say 8 horse-power. If the engine was a gas engine 

 this horse-power would have to be that which was 

 developed by the engine during a Brake test. Of 

 course the testing of such engines is quite out of the 

 province of this book, but the prospective buyer of 

 plant must be very careful upon what the makers 

 mean by a horse-power. The term nominal horse- 

 power is misleading, and should be strenuously 

 avoided. With gas engines and oil engines only the 



