Техническое чтение для энергетиков. Бухарова Г.П. - 73 стр.

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75
The torque requirements of boiler-feed pumps and most other pumps are
satisfied by motors with low starting current and normal starting torque. Most boiler-
feed-pump motors are rated for a temperature rise of 40 degrees С above ambient and
have class A insulation. Where the ambient temperature is above 40 degrees, class В
insulation is used.
Although drip-proof construction is usual, special enclosures are sometimes
used to reduce the noise level of the motor or to protect the motor from flyash and
other unfavorable atmospheric conditions. Noise can be reduced by using pipe- or
base-ventilated motors in which the inlet and exhaust for cooling air are at a remote
location. In particularly dirty locations enclosed motors are used. Air for such motors
is cooled by either an air-to-air or an air-to-water heat exchanger. Since outside air is
never drawn into the motor, the windings are protected from contamination.
In addition to boiler-feed pumps, numerous other pumps are associated directly
with the water cycle of the plant or perform auxiliary functions. These include pumps
for handling circulating water, condensate, drain water, raw water, water-purification
chemicals, ash, flood water, water for fire protection, sump water, lubricating oil, and
station water supply. Usually the largest of these are the circulating-water pumps. In a
typical station, there are two circulating-water pumps per turbine with a total
horsepower slightly less than one percent of the turbine rating. The remaining pumps
range in size from a fraction of a horsepower for small chemical-feed pumps to 100
to 300 horsepower for some raw-water, ash, and fire pumps. The size of the driving
motor in a particular application is determined by the head and capacity requirements,
which are influenced by the nature of the water source.
The location, and the speed and torque requirements* of these pumps usually allow
the use of standard drip-proof, squirrel-cage induction motors with low starting
current and normal starting torque. Most pump motors have a synchronous speed of
1,200, 1,800 or 3,600 rpm; however, some motors, such as the circulating-water
pumps, may require speeds as low as 277 rpm. Vertical motors are frequently used
for pumping service because they require much less floor space.
THE CHILDHOOD OF THE STEAM-ENGINE
Before the Middle Ages nearly all the energy available was that of human and
animal muscles, later to be supplemented by some wind and water power in the more
advanced civilizations. But there were enough poor people everywhere who could be
made to work for the rich and mighty, providing for their needs.
We must try to visualize these circumstances if we want to understand why
science and engineering did not develop very much during the Middle Ages – why,
for instance, the steam-engine was not invented, and electricity discovered, a
thousand years earlier. Certainly many creative minds turned to the problem of
harnessing the forces of Nature for the benefit of Man in those 'dark' centuries; but
there seemed to be no need for such devices, and even if they had been invented no
one would have dared to suggest how they might have been used. Life on earth,
except for the chosen few, was to be a valley of tears, toil, and sweat, and any attempt

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