, which
the slow moving compressor seeks to erase. We are quite safe in saying
that the element of _time alone_ in the stroke of an air compressor
could not possibly effect a saving of more than half of this, or 51/2 per
cent. Now, in order to get this 51/2 per cent. saving, we reduce the speed
of an air-compressing engine from 350 feet per minute to 100 feet per
minute. We must, therefore, in one case have a piston area _three and
one-half_ times that of the other in order to get the _same capacity of
air_, and in doing this we build an engine of enormous proportions with
heavy moving parts. We load it down with a large mass of water, which it
must move back and forth during its work, and thus we produce a
percentage of friction loss alone equal to twice or even three times the
51/2 per cent. heat loss which is responsible for all this expense in
first cost and in maintenance, but which really is not saved after all
unless water injection in the form of spray also forms a part of the
system.
It is obvious that cost of construction and maintenance have much to do
with the commercial value of an air compressor.
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