When air is compressed into a smaller volume, if the temperature remains constant, the pressure increases directly in proportion to the decrease in volume; that is, if the volume is reduced one-half the pressure will be doubled; if reduced to one-third the pressure will be trebled, and so on for any decrease in volume. There is, however, another and most important factor in the problem which must he considered in all cases, particularly where high pressures are concerned, viz., the increase in temperature and consequent increase in volume due to the heat developed during compression. When air is compressed, part of the work done during compression is converted into heat, which must be taken up by the air compressed, the result being to very materially raise its temperature and increase its volume, thus adding largely to the work required to be done.
The single stage compressor consists primarily of an air-cylinder, in which the air drawn from the atmosphere is compressed, a steam cylinder, located above the air-cylinder, the two being connected by a suitable centre piece, steam and air pistons mounted on a com mon piston rod, and a valve motion controlling steam admission and exhaust. The compressor is double acting, steam being admitted alter nately on either side of the steam piston which, being directly connected with the air piston, causes both to move up and down. On the upward stroke of the air piston the air above it is compressed and discharged into the main reservoir, while the space below is filled with air drawn from the atmosphere. On the down ward stroke this operation is reversed. The steam exhaust is piped to the smokestack or to the exhaust cavity of the saddle.
The great increase in compressor capacity called for in modern service has resulted in a much more careful consideration of the matter of steam consumption than formerly, and as a result the Westinghouse 8/2-inch cross com pound compressor was developed for the specific purpose of combining maximum capacity and highest efficiency, by compounding both the steam supplied and the air compressed to the extent that, while this compressor has a capacity over three times greater than the well-known 9w-inch single stage compressor, the steam con sumption per 100 cubic feet of air compressed is but one-third.
While it is advantageous to keep the air as cool as possible during compression, heating it is an advantage, as soon as it leaves the re ceiver, for the reason that by this heating its volume or its pressure is increased. So im portant is this advantage, theoretically, that de vices called reheaters are frequently employed to heat the air just before it passes to the motor or tool which it operates. Reheaters are made in many forms, the usual one being a kind of stove or oven through which the air passes by means of a spiral pipe or some other arrange ment which allows it to be quickly heated.
The air reheater shown in the sectional drawing consists of a series of hollow annular rings bound together by four tie rods, the upper and lower rings having flanged openings projecting through the shell for pipe connec tions. These rings are surrounded by a sheet steel shell, the space between being filled with asbestos packing, which prevents loss of heat by radiation. Air enters at the bottom ring and leaves at the top, the openings connecting the rings being so arranged that the air is forced to travel through the heater in a circuitous passage. These heaters should be placed as near as possible to the work and the outlet pipe should be of large diameter, short and care fully covered, to prevent losses by radiation.
See AIR-PUMP; COMPRESSED AIR; CONDENSER.