compressors are designed with some form of de-
vice for keeping the air cool during compression.
Two systems are used, by which It is attempted
to l»:eep the air cool during compression, and
tliese systems divide air compressors into two
classes, as follows: ( 1 ) Wet compressors which
introduce water directly into the cylinder during
compression, (a) in the form of a spray, and
(b) by the use of a water piston; (2) Dry com-
pressors, which admit no water directly into
the cylinder, but have the cylinder surrounded
by a jacket, into the space between which and
the cylinder walls water is forced and kept in
circulation. The water piston compressor is now
seldom used. Cooling by the water spray injec-
tion gives the most efficient results as far as cool-
ing the air is concerned, but it has so many
objections that it has been found to be the best
practice to use the less efficient but vastly more
simple water-jacket system, and endure the loss
of iieat which miglit be saved by using water
spray injection. Accordingly, we find that most
air compressors are nowadays provided with a
water-jacketed air cylinder for cooling the air
during compression. Vertical air compressors
have the steam cylinders placed vertically above
the air cylinders: horizontal compressors Iiave
the steam and air cylinders placed horizontally
one ahead of the other. Direct air compressors
have the steam and air piston on the same piston
rod, so that the thrust of the steam piston gives
a direct thrust on the air piston; indirect acting
compressors transfer the thrust of the steam pis-
ton by means of cranks and gearing to the air
piston rod. A simple acting compressor is one
which compresses air on the forward stroke of
the air piston only, the back stroke doing no
useful work; a double acting compressor com-
presses air on both the forward and back strokes
of the air piston. A two-stage compressor part-
ly compresses the air in one cylinder, from
which it is passed to a second cylinder, where it
is furtlier compressed. Generally, the air in
passing from the first to the second cylinder
passes through an inter-cooler, where it is cooled
by water. Three-stage and four-stage compres-
sors are sometimes employed. A duplex air com-
pressor consists of a riglit-hand steam and air
cylinder and a left-hand steam and air cylinder,
each side being capable of being run separately.
or the two sides can be run together. A duplex
compressor may liave either the air cylinders or
the steam cylinders, or both air and steam cylin-
ders compounded. Air compressors may have
the steam cylinders replaced by a pulley, so that
they may be operated by a belt, or by a water
wheel obtaining power from a head of water.
Whatever the form of compressor which is used, the mechanical action in compressing the air is that of a piston working in a cylinder, exactly as in the case of a bicycle pump. As each cylinderful of air is compressed, it is forced into a sheet-iron or sheot-steel tank called a receiver. This receiver is cylindrical in form, and serves as a reservoir of compressed air for supplying the machine which is operated by air pressure. The receiver is often provided with an arrangement for cooling the contained air by water. While it is advantageous for the reason given aliove to keep the air as cool as possible during compression and while it remains in the receiver, as soon as it leaves the receiver heating it is an advantage, for the reason that by this heating its volume or its pressure is increased. So important is this advantage, theoretically, that devices called reheaters are often employed to heat the air just before it passes to the motor or the 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 arrangement which allows it to be quickly heated. Some of the many methods of utilizing compressed air in engineering and the arts are given in the following list of uses, compiled by a prominent American manufacturer of air compressors: Rock drills, coal cutters, pumps, hoisting engines, and other machinery in mines and tunnels, air brakes on railroad and street cars, switches and signals, engines, hoists, cranes, stone carving and boiler calking tools, chipping tools, polishing machines, riveters, punches, hammers, tapping, screwing and drilling machines, stay-bolt cutters, angle iron shears, paint machines, sand blast apparatus, molding machines, wood bundling machines and shop tools of e'ery description, oil fires under ovens, furnaces, and boilers, and in fifty other applications, such as welding, annealing, tempering, oil illuminating lights, pneumatic transmission tubes, street railway motors and mine locomotives, passenger and freight elevators, sheep shearing machines and cloth cutters, railway crossing gates and jacking up cars, and steering gear of vessels, charging; pneumatic dynamite guns and projectiles, and automatic sprinkler systems for fire protection, tunnel driving by the pneumatic process: sinking caissons for structural foundations; pumping wells by air lift pump method: conveying and elevating acids, chemicals, and other liquids; racking otT beer in breweries: aerating water supplies of cities, towns, and villages: agitating fluids, such as asphalt, molasses, and chemical solutions: mixing nitro-glycerinc: removing hose from mandrels in rubber factories: inflating tires: testing tinware. pipe, hose, and other manufactured products required to stand pressure; increasing and maintaining pressure on hydraulic elevators; sprays of all descriptions, including physicians', hospitals", sanitariums', and baths: spraying solution in the manufacture of silk ribbon: moving and elevating grain, culm, and other material; cleaning carpets, car cushions, etc.; unloading dump cars: raising sunken vessels: supplying divers in submarine operations: refrigerating, ventilating, and cold storage; manufacture of various gases: disposition of sewage: and for a large number of other duties in railroad shops, chemical works, and in connection with a wide variety of experiments and patented processes. For the great majority of these uses an air pressure below 75 pounds per square inch is ample, but for charging the tanks of compressed air locomotives, for liquefying gases, etc., much higher pressures are required. The highest known pressure to which air has been compressed is 400<l atmospheres (about 60.000 pounds) per square inch, but this was a hxboratory experiment. The safe limit of pressure for u.se in the arts to-day is largely determined by the strength of the retaining vessel or reservoir, and has reached its limit at about 3000 pounds per square inch. To obtain these great pressures specially designed air compressors have to be constructed.
For a concise and reailable history of air compressors and of the use of compressed air, consult: Saunders, Compressed Air Production (Nevr
