26 BULLETIN 747, IT. 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 bod5 7 °f 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' 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. 
