SUMMARY.-In the construction of air-compressors, the present
tendency is to use metal throughout. For piston-packing, rings
or segments In east-iron or bronze are employed; for stuffing
boxes, soft alloys, and for valves in machines for distributing
power, plates of steel, resting in bronze seats, either with or
without springs, are recommended. With regard to dimensions,
starting with the volume of compressed air to be furnished per
minute under a given pressure, the useful volume to be given to
the compressor is first to be calculated, keeping in view the
fact that the compressing piston, if on the hydraulic system,
should not travel faster than 15 revolutions per minute; or, if
direct-acting not more than 60 revolutions. The useful volume
determined, the stroke of the piston is fixed so that the velocity
per second shall not exceed 29.25 inches for hydraulic piston-compressors,
or 58 inches for direct-acting compressors. If the diameter is
too large, two cylinders are employed.