# 1911 Encyclopædia Britannica/Power Transmission/Pneumatic

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III.—Pneumatic
Every wind that blows is an instance of the pneumatic transmission of power, and every windmill or sail that catches the breeze is a demonstration of it. The modern or technical use of the term, however, is confined to the compression of air at one point and its transmission to another point where it is used in motors to do work. The first recorded instance of this being done was by Denis Papin (b. 1647), who compressed air with power derived from a water-wheel and transmitted it through tubes to a distance. About 1800 George Medhurst (1759–1827) took out patents in England for compressing air. He compressed and transmitted air which worked motors, and he built a pneumatic automobile. William Mann in 1829 took out a patent in England for a compound air compressor. In his application he states: “The condensing pumps used in compressing I make of different capacities, according to the densities of the fluid to be compressed, those used to compress the higher densities being proportionately smaller than those previously used to compress it to the first or lower densities,” &c. This is a very exact description of the best methods of compressing air to-day, omitting the very important inter-cooling. Baron Van Rathen in 1849 proposed to compress air in stages and to use inter-coolers between each stage to get 750 ℔ pressure for use in locomotives. For the next forty years inventors tried without success all manner of devices for cooling air during compression by water, either injected into the cylinder or circulated around it, and finally, with few exceptions, settled down to direct compression with no cooling worthy of mention. Only in the last ten years of the 19th century were the fundamental principles of economical air compression put into general practice, though all of them are contained in the patent of William Mann and the suggestion of Van Rathen. The first successful application of compressed air to the transmission of power, as we know it, was at the Mont Cenis Tunnel in 1861. The form of compressor used was a system of Water rams-several of them in succession-in which water was the piston, compressing the air upwards in the cylinder and forcing it out. Although the air came in Contact with the water, it was not cooled, except slightly at the surface of the water and around the Walls of the cylinders. The compressors were located near the tunnel, and the compressed air was transmitted through pipes to drilling machines working at the faces in the tunnel. Rotary drills were tried first, but were soon replaced by percussion drills adapted from drawings in the United States Patent Office, copied by a French and Italian commission from the patent of J. W. Fowle of Philadelphia. H. S. Drinker (Tunneling, Explosive Compounds and Rock Drills, New York, 1893) states positively that the first percussion drill ever made to work successfully was patented by J. J. Couch of Philadelphia in 1849. Shortly afterwards Fowle patented his drills, in which the direct stroke and self-rotating principle was used as We use it now. The first successful drill in the Hoosac Tunnel was patented in 1866 by W. Brooks, S. F. Gates and C. Burleigh, but after a few months was replaced by one made by Burleigh, who had bought Fowle’s patent and improved it. Burleigh made a compressor, cooling the air during compression by an injected spray of water in the cylinders. The successful work in the Mont Cenis and Hoosac Tunnels with the percussion drilling machines caused the use of compressed air to spread rapidly, and it was soon found there were many other purposes for which it could be employed with advantage.
The larger tunnels and metal mines were naturally the earliest to adopt pneumatic transmission, often using it for pumping and hoisting as well as drilling. In Paris and Nantes, in Berne and in Birmingham (England), street tramways have been operated by pneumatic power, the transmission in these, however, being in tanks rather than, pipes. Tanks on the cars are filled at the central loading stations with air at very high pressure, which is used in driving the motors, enough being taken to enable the car to make a trip and return to the loading station. Several attempts in pneumatic street traction were made in America, but failed owing to financial troubles and the successful introduction of electric traction. It is used Very successfully, however, both in Europe and in America., in underground mine haulage, being especially adapted to coal mines, where electricity would be dangerous from its sparks. The copper smelting Works at Anaconda, Montana., U.S.A., uses twelve large pneumatic ​locomotives for charging the furnaces, removing slag, &c. Many stone quarries have a central plant for compressing air, which is transmitted through pipes extending to all working points, and operates derricks, hoists, drills, stone cutters, &c., by means of motors. Every considerable ironworks, railroad shop or foundry has its pneumatic transmission plant. Also in the erection of the larger steel bridges or buildings a pneumatic transmission system is part of the contractor's outfit, and many railways have a portable compressing plant on a car ready to be moved to any point as needed.
Dr Julius G. Pohle, of Arizona, patented in 1886, and introduced extensively, the use of compressed air for lifting water directly, by admitting it into the water column. His plan is largely adopted in artesian wells that do not flow, or do notflow as much as desired, and is so arranged that the air supply has a back pressure of water equal to at least half the lift. If it is desired to lift the water 30 ft. the air is admitted to the water column at least 30 ft. below the standing water surface. The air admitted being so much lighter than the water it displaces, the column 60 ft. high becomes lighter than the column 30 ft. high and is constantly released and flows out at the top. The efficiency of this method is only 20 to 4O<%), depending on the lift, but its adaptation to artesian wells renders it valuable in many localities.
A remarkable pneumatic transmission system was installed in 1890 by Priestly in the Snake River Desert, Idaho, U.S.A. On the north side of the river is a cliff, nearly perpendicular, about 300 ft. high. One hundred and ninety feet above the river, for a considerable distance along the cliff, streams of water gush out from between the bottom of the great lava bed and the hardened clay of the old lake bottom. Priestly, without knowledge of Pohle's system, built a pipe line down the bluff and trained the water into it in such a way that it carried a very considerable quantity of air in the form of bubbles along with it down the pipe, compressing it on the way. The air was collected at the bottom in a covered reservoir, and taken up the cliff again to the lower part of an inverted siphon pipe, one side of which reached down from the water-supply about 60 ft. and the other side reached up and over the bluff. Allowing the water to fill both sides of the pipe to the level of the water-supply, he admitted his compressed air at about 75 lb pressure into the long side of the pipe near the bottom, and soon had water flowing upwards over the cliff and irrigating a large tract of rich lava land. He had made a power, a transmission and a motor plant without a moving part. A similar compressor was installed near Montreal, Canada, in 1896; another at Ainsworth, British Columbia, in 1898; and another at Norwich, Connecticut, U.S.A., in 1902. These are called hydraulic air compressors and show an efficiency of about 70%. They are particularly adapted to positions where there is a large flow of water with a slight fall or head.
The actual transmission of power by air from the compressor to the motor is simple and effective. The air admits of a velocity of 15 to 20 ft. per second through pipes, with very slight loss by friction, and consequently there is no necessity for an expensive pipe system in proportion to the power transmitted. It is found in practice that, allowing a velocity as given above, there is no noticeable difference in pressure between the compressor and the motor several miles away. Light butt-welded tubing is largely used for pi ing, and if properly put in there is very slight loss from leakage, which, moreover, can be easily detected and stopped. In practice, ei sponge with soap-suds passed around a joint furnishes a detective agency, the escaping air blowing soap bubbles. In good practice there need not be more than 1% loss through leakage and 1% possibly through friction in the pneumatic transmission of power. Air develops heat on compression and is cooled by expansion, and it expands with heat and contracts with cold. For the purpose of illustration suppose a cylinder 10 ft. long containing 10 cub. ft. of air at 60° F., with a frictionless piston at one end. If this piston be moved 7½ ft. into the cylinder, so that the air is compressed to one quarter of its volume, and none of the heat developed by compression be allowed to escape, the air will be under a pressure of 90 lb per square inch and at a temperature of 460° F. If this air be cooled down to 60° F. the pressure will be reduced to 45 lb per square inch, showing that the heat produced in the air itself during compression gives it an additional expansive force of 45 lb per square inch. 'The average force or pressure in compressing this air without loss of heat is 21 lb per square inch, whereas if all the heat developed during compression had been removed as rapidly as developed the average pressure on the piston would have been only II lb per square inch, showing that the heat developed in the air during compression, when not removed as fast as developed, caused in this case an extra force of 10 lb per square inch to be used on the piston. If this heated air could be transmitted and used without any loss of heat the extra force used in compressing it could be utilized; but in practice this is impossible, as the heat is lost in transmission. If the piston holding the 2½ cub. ft. of air at 45 lb per square inch and at 60° F. were released the air expanding without receiving any heat would move it back within 3½ ft. of the end only, and the temperature of the air would be lowered 170° F., or to IIO° F. below zero. If the air were then warmed to 60° F. again it would move the piston the remaining 3½ ft. to its starting point. It is seen that the ideal air-compressing machine is one which will take all the heat from the air as rapidly as it is developed during compression. Such “ isothermal compression ” is never reached in practice, the best work yet done lacking 10% of it. It follows that the most inefficient compressing machine is one which takes away no heat during compression-that is, works by “ adiabatic compression, ” which in practice has been much more nearly approached than the ideal. It also follows that the ideal motor for using compressed air is one which will supply heat to the air as required when it is expanding. Such “ isothermal " expansion is often attained, and sometimes exceeded, in practice by supplying heat artificially. Finally, the most inefficient motor for using compressed air is one which supplies no heat to the air during its expansion, or works by adiabatic expansion, which was long very closely approached by most air motors. In practice isothermal compression is approached by compressing the air slightly, then cooling it, compressing it slightly again, and again cooling it until the desired' compression is completed. This is called compression in stages or compound compression. Isothermal expansion is approximately accomplished by allowing the air to do part of its work (as expanding slightly in a. cylinder) and then warming it, then allowing it to do a little more and then warming it again, and so continuing until expansion is complete. It will be seen that the air is carefully cooled during compression to prevent the heat it develops from working against compression, and even more carefully heated during expansion to prevent loss from cold developed during expansion. More stages of compression of course give a higher efficiency, but the cost of machinery and friction losses have to be considered. The reheating of air is often a disadvantage, especially in mining, where there are great objections to having any kind of combustion underground; but where reheating is possible, as W. C. Unwin says, “for the amount of heat supplied the economy realized in the weight of air used is surprising. The reason for this is, the heat supplied to the air is used nearly five times as efficiently as an equal amount of heat employed in generating steam.” Practically there is a hotair engine, using a medium much more effective than common air, in addition to a compressed-air engine, making the efficiency of the whole system extremely high. (A. de W. F.)