Surface Machinery and Methods for Oil-Well Pumping

42 SURFACE MACHINERY AND METHODS FOR OIL-WELL PUMPING.
on both strokes of the piston. For air compression to less than 100
pounds to the square inch, single-stage compressors are satisfactory
and are most common; for pressures up to 500 pounds to the square
inch, two-stage compressors are used. For oil-field practice single
and two stage compressors meet requirements.
Two-stage compressors have two cylinders of different sizes that
compress the air by stages. The free air enters the larger cylinder,
is partly compressed, and passes through a cooling device called an
intercooler to the smaller cylinder where compression is completed.
Because of the high temperatures caused by compression in a
single stage to pressures above 100 pounds to the square inch, lubri-
cation of the cylinder and valves is difficult. The oil used tends to
deposit a coke-like substance in the ports and pipes, decreasing
the air discharge area and adding to the compressor load. A two-
stage compressor eliminates the overheating of cylinders and the re-
sultant operating troubles, as compression is distributed between the
two cylinders.
COMPRESSOR EFFICIENCY.
Even the most efficient compressor loses much power. The ratio
of the power available in the compressed air to the power required
to compress the air seldom exceeds 70 per cent, and is often much less.
Due to the expansion of air by heat, a less weight of air is com-
pressed at each stroke when intake temperatures are higher. The
lower the initial temperature the lower will be the final temperature,
the greater the final weight of compressed air, and the greater the
efficiency of compression.
COOLING DEVICES.
Use of an intercooler between the high and low pressure cylinders is
the most important factor in two-stage compression. The intercooler
is a closed cylindrical steel tank containing a water coil; circulating
water and air ports lead to the two cylinders. The location of the in-
tercooler differs with the size of the compressor. It is generally
placed crosswise--in the smaller compressors beneath the air cylin-
ders and in the larger compressors above the cylinders. By cooling
the partly compressed air entering the high-pressure cylinder, the
intercooler greatly increases compressor efficiency.
The cylinder barrels and heads are water-jacketed to keep the
cylinder walls cool enough for good lubrication and to prevent
cumulative heating.
AIR RECEIVERS.
An air receiver is a closed steel tank that receives and stores air
from the compressor. It is especially valuable for intermittent air
consumption, as pressures can be built up in the receiver, and much

George, H. C. Surface Machinery and Methods for Oil-Well Pumping. Washington D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc12407/. Accessed July 31, 2014.