26 BULLETIN 141, U. S. DEPARTMENT OF AGRICULTURE. 



' BELT-DRIVEN PUMPS. 



The ordinary steam-driven pump requires from 100 to 200 pounds 

 of steam per horsepower hour ; therefore it is more economical to use 

 a belt-driven pump which can be driven directly from the line shaft. 

 By this means the power for pumping is furnished by the engine, 

 which will develop a horsepower hour on from 40 to 70 pounds of 

 steam, depending on the type of engine, steam pressure, the condi- 

 tions of its valves, etc* One disadvantage of the belt-driven pump 

 is that it becomes necessary to run. the engine when it is desired to 

 pump water into the boiler, but that is not a serious objection, as 

 the engine is usually in operation, during the time steam is kept on 

 the boiler and can be easily run for the purpose. The belt-driven 

 pump can be easily adjusted to feed the boiler continuously at just 

 about the rate required. It is usually provided with a tight and 

 loose pulley or some form of clutch so that the pump can be started 

 and stopped at will when the machinery is in operation. 



It may 'be well to state here that in order to pump hot water 

 satisfactorily the pump should be placed always below the source of 

 supply. If this is not done the pump will either fail entirely or 

 operate very unsatisfactorily, depending on the temperature of the 

 water. In pumping cold water there is an atmospheric pressure of 

 14.7 pounds per square inch on the surface of the water, which will 

 support a column of water approximately 30 feet high. Allowing 

 for frictional resistance in the suction pipe and valves, if the pump is 

 within 20 feet above the supply it will operate satisfactorily, pro- 

 vided the piston speed of the pump is not too great to allow the 

 water to follow the piston. The vapor pressure of water at 50° F. 

 is only about one-quarter of a pound per square inch, and therefore 

 is negligible as compared with atmospheric pressure. With water 

 at 212° F. the vapor pressure is just 14.7 pounds per square inch, 

 or just equal to the atmospheric pressure, and if we try to lift water 

 at that temperature by suction the hodj of water will not rise at 

 all. but the steam vapor will rise from the surface of the water and 

 follow the piston. With water at 200° F. there will be about 3.3 

 pounds 1 pressure by the atmosphere in excess of the vapor pressure, 

 which is sufficient to raise the water approximately 7.9 feet, but 

 with no excess pressure to' overcome the frictional resistance of the 

 pipe and to lift the valves, to say nothing of giving velocity to the 

 water. The pump, therefore, should be placed at least 3 feet below 

 the supply when pumping hot water in order to have head enough 

 to force it through the supply pipe at the required velocity and to 

 lift the valves in the pump chamber. The "pump should be placed 

 near the water supply, and the supply pipe should be as straight and 

 free from bends as possible. The springs of the pump valves should 

 be made as light as practicable in order to insure proper operation. 



