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FIG. 35.—HOW THE VENA CONTRACTA RAISES WATER

A very ingenious apparatus for compressing air was invented in the earliest years of the iron age to furnish a continuous blast of air for the Catalan forges. This compressor, known as a “trompe,” can hardly be termed a machine because it contains no moving parts except water, which is the motive power, and the air which it traps and compresses. To understand its operation we must look into a peculiar property of water flowing out of a reservoir into a pipe or nozzle. There is a converging motion that tends to contract the jet of water just after it leaves the pipe. This is known as the vena contracta. It produces a partial vacuum in the pipe. If air ports are opened into the pipe at this point, air will be sucked in to fill the vacuum and will be carried out of the pipe by the friction of the water. In Figure 35 a pipe is shown running from the vena contracta to a water tank below. The rise of water in this pipe indicates the degree of vacuum produced by the jet.

FIG. 36.—THE “TROMPE” BY WHICH COMPRESSED AIR WAS FURNISHED FOR CATALAN FORGES

This principle is used in the hydraulic-air compressor or “trompe” as it is called. Water flows out of one reservoir through a pipe into another reservoir lower down. (See Figure 36.) Air enters the pipe through ports at the point where the vein of water contracts and is carried down into the second reservoir. This reservoir is sealed so that the air is trapped in it. The water passes out through a pipe which is carried high enough to keep a certain pressure of air in the reservoir and prevent it from blowing out.

FIG. 37.—DIAGRAMMATIC SECTION OF A LARGE HYDRAULIC AIR COMPRESSOR IN MICHIGAN

There is a compressor of this type, constructed on an enormous scale, in the northern part of Michigan. A sketch of the compressor is given in Figure 37. The air reservoir in this case is a huge underground rock-walled chamber nearly 350 feet below the surface, 8 feet wide, 26 feet high, and about 280 feet long. There are three intake pipes, 5 feet in diameter, each filled with an annular funnel-shaped head, which sucks air into the water and carries it down into the chamber. At the bottom of each intake pipe there is a concrete block with a conical top projecting up into the pipe. The column of water flowing down the pipe is spread out into an annular stream by the conical block and the bubbles of air escape into the chamber. The water outlet of the chamber is an inclined shaft which leads up about 270 feet to the surface of the ground where it discharges into the tail race. The water is forced up this inclined shaft by the pressure of the air trapped in the chamber. The mouth of the shaft is, of course, below the level of the water in the chamber so that there is no chance for the air to escape unless the pressure becomes excessively high, when it will force the water level below the mouth of the shaft and blow out. The discharge sometimes forms a geyser 700 feet high. The air will continue to blow until the pressure is reduced enough for the water level to rise and cut off access of the air to the mouth of the shaft. Under normal conditions there is a fall of 343 feet from the water level at the top of the intake pipe to the water level in the chamber, and a vertical rise of 271 feet from the water level in the chamber to the tail-water level. The difference, or 72 feet, represents the working head. With all three intakes operating, a total of 5,000 horsepower is developed. Each intake delivers 11,930 cubic feet of air per minute at a pressure of 128 pounds. Air enters the intake heads through tubes ⅜ inch in diameter and there are 1,800 of them to each head. The air is employed to operate machinery and tools in an adjacent mine.

One advantage of this type of air compressor is that it cools the air while compressing it. This was hardly an advantage in the Catalan forges, but when the air is used to drive machinery it is important that it be precooled. When air is compressed by mechanical means a great deal of heat is generated and the machines must be water jacketed to extract this heat, but in the hydraulic compressor the air bubbles are compressed as they pass down with the water to the reservoir and the water absorbs the heat, delivering cool compressed air at the bottom of the intake pipe.