They say that, admitting that the cooling surfaces are equal, we
have in one case _more time_ to absorb the heat than in the other. This
is true, and here we come to an important class division in air
compressing machinery--_high speed and short stroke_ as against _slow
speed and long stroke_. Hydraulic piston compressors are subject to the
laws that govern piston pumps, and are, therefore, limited to a piston
speed of about 100 feet per minute. It is quite out of the question to
run them at much higher speed than this without shock to the engine and
fluctuations of air pressure due to agitation of the water piston. The
quantity of heat produced, that is, the degree of temperature reached,
depends entirely upon the conditions in the air itself, as to density,
temperature and moisture, and is entirely independent of speed. We have
seen that it is possible to lose 21.3 per cent. of work when compressing
air to five atmospheres without any cooling arrangements. With the best
compressors of the dry system one-half of this loss is saved by water
jacket absorption, so that we are left with about 11 per cent.
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