It has been stated above, in referring to the bicycle pomp, that air is heated by compression. As heat causes air to expand, a cubic foot of hot air, at, say, 75 pounds pressure, will decrease in volume when cooled, and thus bring about a reduction in pres:are to something he than 75 pounds. Evidently, therefore, a loss of work done in eompression results from the beating of the air. The amount of the loss is estimated! at 21.3 per cent. of the total work clone in compress ing air to 75 pounds pressure. To save this loss, 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 keep the air cool during compression, and these 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 va:tly more simple \rater-jacket system, and endure the loss of heat which might 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 have 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 further 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 right-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 roll together. A duplex compressor may have 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 whi h 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 sheet-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 fur the reason given above 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 in creased. So important is this advantage, theo
retically, that devices called reheaters are often employed to heat the air just before it passes to the motor or the tool which it operates. Re heaters 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 util izing 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, ham mers, tapping, screwing and drilling machines. stay-bolt cutters, angle iron shears, paint ma chines, sand blast apparatus, molding machines, wood bundling machines and shop tools of every 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 rail way 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; sink ing caissons for structural foundations; pump ing wells by air lift pump method: conveying and elevating acids, chemicals, and other liquids; racking off beer in breweries: aerating water supplies of cities, towns, and villagbs: agitating fluids, such as asphalt, molasses, and chemical solutions: mixing nitro-glycerine; removing hose from mandrels in rubber factories: inflating tires: testing tinware, pipe. hose. and other manu factured products required to stand pressure; increasing and maintaining pressure on hydrau lic elevators; sprays of all descriptions, includ ing physicians', hospitals'. sanitariums', and baths; spraying solution in the manufacture of silk ribbon; moving and elevating grain, cubit, and other material ; cleaning carpet's, car cush ions, etc.: unloading dump ears; raising sunken vessels: supplying divers in submarine opera tions; refrigerating. ventilating, and cold stor age: manufacture of various gases; disposition of sewage: and for a large number of other duties in railroad shops. chemical works, and in con nection 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 4000 atmospheres (about 60.000 pounds) per square inch, hut this was a laboratory experiment. The safe limit of pres sure for use in the arts to-day is largely deter mined 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 compres sors have to be constructed.
For a concise and readable history of air com pressors and of the use of compressed air, con sult: Saunders, Compressed dir Production (New York, 1902): for a somewhat more technical discussion of the production and use of com pressed air, F. Richard:, Air (New York, 18115). The most comprehensive descrip tive treatise on compressed air in English is Ilisenx, Compressed Air and Its Appliention.s. INew York, 1901).