AIR COMPRESSOR, a device used for compressing air, or, in other words, which makes the same weight of air occupy a less volume or a greater weight occupy the same volume. Any device which reduces the volume of air with a pressure increase is an air com pressor. When released from the containing reservoir the air tends to resume its volume and this tendency is utilized by making the air perform mechanical work as it expands. Al though compressed air has been used for work ing engines in confined situations, it is not at all likely that it will ever come into extensive use, owing to the great waste of power attend ing it. This waste arises from two causes: (1) The friction due to forcing the compressed air along a great length of pipe, and (2) the loss from the dissipation of the great heat which results from its compression. The greater the original compression of the air the higher its temperature will rise; and as this caloric, which cannot be kept from escaping, is practically a part of the bulk of the air, it fol lows that the loss of power from this cause will increase with the pressure or tension of steam engines there is always a ,mall quantity of water in the cylinders and slide-valves, aris ing from the condensation of a portion of the steam, and this suffices to lubricate the piston and valves. It is well known that when steam is superheated so highly as to prevent a slight condensation in the cylinder and slide-valves, they are very rapidly destroyed. Air rises in the air. Even were it possible to prevent the escape of the heat by covering the vessels and pipes with some non-conducting substance, it would not be practicable to use the hot air in the same way as steam is used, because the lubricating material necessary to keep the pis ton and slide-valves from °tearing') would be decomposed by the high temperature. In temperature when very much compressed, and we cannot use it until its temperature falls; and as this involves a great waste of power, it follows that where economy is of great conse quence, air cannot be used as a mode of trans mitting mechanical power. Indeed, no fluid can be economically used for transmitting power for any considerable distance.
In its early history compressed air as a motive power for vehicles was not successful due to the fact that the storage tanks in use would admit of only small pressures. With its continued use and the development of stor age tanks permitting the air to be compressed to an initial pressure of about 5,000 pounds per square inch it has made that class of motive power available for trucks used in manufactur ing plants and for other vehicles designed to carry heavy weights for short distances. The
extensive plants necessary for compressing the air and the fact that the storage tanks must be refilled frequently, as well as the inconvenience of transporting a large number of charged cylinders, make it impracticable as a motive power for vehicles used for commercial or military purposes.
The air compressor consists essentially of a cylinder in which atmospheric air is compressed by a piston, the power for driving which may be derived from a steam engine, water-wheel or electric motor. The air cylinder is generally double-acting and is provided with inlet and discharge valves in each cylinder head. The usual types of compressors are and duplex. Direct-connected compressors are driven by direct-current induction or synchron ous motors, the rotors of which are of large diameter to produce a proper relation between the peripheral and rotative speeds.
The work performed by a compressor is, broadly speaking, that of increasing the pres sure of air (or other gas) by reducing its i volume or compressing it into a smaller space. Usually the lower or initial pressure is the 'atmospheric pressures at the point of location of the compressor, while the higher or terminal pressure is fixed by the requirements of the particular case, and may be anywhere from 10 to 30 pounds (gauge pressure) per square inch as in blowing engine practice, up to 80 to 125 pounds per square inch for rock drills, pneu matic tools, etc., and up to 1,500 to 2,000 pounds i per square or even higher, for special pur poses. Atmospheric pressure (or zero gauge pressure) equals 14.7 pounds absolute pressure per square inch at sea-level (equivalent to 30 inches barometer) and becomes less as the altitude above sea-level increases, the decrease being approximately one-half pound, or one inch in mercury column, for each 1,000 feet in crease in altitude. As the work of compression depends upon the initial and terminal absolute pressures (absolute pressure being equivalent to gauge pressure plus atmospheric pressure) the altitude at which the compressor is to work is of great importance and must always be taken into consideration.