Dr. Julius G. Pohle, of Arizona, patented in i886, and intro duced 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 not flow 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 3o ft. the air is admitted to the water column at least 3o ft. below the standing water surface. The air admitted being so much lighter than the water it displaces, the column 6o ft. high becomes lighter than the column 3o 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 Snake River desert, Idaho. On the north side of the river is a cliff, nearly perpendicular, about 30o 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 6o 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 transmis sion and a motor plant without a moving part. A similar com pressor was installed near Montreal, Canada, in 1896; another at Ainsworth, British Columbia, in 1898; and another at Norwich, Conn., in 1902. These are called hydraulic air compressors and show an efficiency of about 70%. They are particularly adapted to positions with a large flow of water with a slight fall or head.
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 piping, and if properly put in there is very slight loss from leak age, which, moreover, can be easily detected and stopped. In practice, a 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 leak age and 1% possibly through friction.
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 io ft. long containing io cu.ft. of air at 60° F, with a frictionless piston at one end. If this piston be moved 71 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 46o° F. If this air be cooled down to 6o° 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 com pressing 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 I i 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 io 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 prac tice this is impossible, as the heat is lost in transmission. If the piston holding the cu.ft. of air at 45 lb. per square inch and at 6o° F were released, the air expanding without receiving any heat would move it back within 31 ft. of the end only, and the temperature of the air would be lowered 170° F, or to io° F below zero. If the air were then warmed to 6o° F again it would move the piston the remaining 31 ft. to its starting point.