Initial The capacity and effi ciency of a compressor is also affected by the initial heating of the air. Under ideal condi tions the cylinder would, on the suction stroke, be filled with air at full atmospheric pressure and at a temperature no higher than that of the outside air, but such ideal conditions are im possible of attainment. Even with an unob structed inlet passage air will not flow into the cylinder without some indifference in pressure to force it in, and when, as in many compres sors, the inlet valves are of the spring weighted poppet type, this difference as to its effect upon capacity and efficiency becomi.s a serious mat ter. Then again, the entering air comes in con tact with the cylinder walls, cylinder head, pis ton and clearance surfaces which have become highly heated during the preceding compression stroke, and is heated thereby to a temperature above that of the surrounding air. This not only reduces the volume of free air at the out side temperature which can be handled, but also raises the terminal temperature of compression.
It is essential to economy in air compression: (1) That the percentage of clearance be kept as small as possible. (2) That the inlet and out let areas should be large in order to reduce friction losses and heating and to ensure the complete filling of the cylinder at all speeds. (3) That the entering air should pass through short ports, in a solid stream, and over as small an extent of heated surface as possible. (4) That the cylinders and cylinder heads should be water-jacketed in order to talce away as much of the heat of compression as possible. To accomplish this as well as to facilitate cleaning, all ,water spaces and pipes should be large and an ample supply of cooling water used, as cold as can be obtained. Suitable hand holes or openings for cleaning out water spaces should be provided and made use of frequently, and the water should be as clear and free from sediment as possible. (5) That multi-stage compression should be used for anything be yond very moderate pressures.
Hydraulic Compression of The method of compressing air by means of falling water forms a most interesting topic in the subject of air compression. One of the old forms of compressing air is by means of a trompe or water bellows. Many improvements have been made on this early apparatus and distinct types developed from it, prominent among which are the Frizell, Baloche, Krahnass, Taylor and Arthur devices. There are several phenomena in connection with hydraulic air compression that at first sight seem paradoxi cal. In compressing air by hydraulic means, the
air becomes drier during the compression, but whatever may be its initial condition as to hu midity at the end of compression it will be saturated with moisture. Again isothermal compression is secured and, generally speaking, at uniform temperature a given volume of air implies a capacity for a certain weight of water whether the air is at a pressure of one or 100 atmospheres, but if the air is compressed through a range from one to 100 atmospheres, its volume will be reduced, if the compression is isothermal, to 1/100 part of the original vol ume, and in consequence 99/100 of the weight of moisture it originally held will be precipi tated. In connection with this type of corn-, pressor it has been found that the compressor air contains less oxygen than the free air of the atmosphere and in consequence its use in mines is not as beneficial as air from other types of compressors. The losses inherent in hydraluic compression are: (1) The heat expended in impregnating the water with air; (2) a loss which may be called the slip due to the velodty with which the bubbles tend to rise; (3) a loss due to the increasing solution of the atr in the water with the increasing pressure as the water and air descend.
Multi-Stage Compression.— It frequently happens that high pressures are demanded for commercial purposes, and in order to satisfy this demand, avoid the danger of explosion due to high temperatures and reduce the losses due to adiabatic compression, engineers have adopted a multi-stage system of compression- compressing the air partly in one cylinder, pass ing it through an intercooler where its tempera ture and volume are reduced, then compressing it still further in a second cylinder, and, if the pressures required are high, this compressed air is passed to a second intercooler, thence to a third cylinder and in some cases a third inter cooler and a fourth cylinder, and required to secure the desired compression pressure eco nomically. The advantages of this system of compression more than offset the extra expense in constructing the compressor. The larger the volume of the intercooler, the more time is required for the compressed air to cool; for this reason receiver intercoolers, as they are termed, are more efficient than those of small volumetric capacity. See Ant COMPRESSORS; Ant PUMP; CONDENSER; POWER TRANshuSsION.
Hiscox, 'Compressed Air> (New York 1901); Peek, 'Compressed Air Plant' (ib. 1913) ; Unwin, :Compressed Air' (ib. 1903); Simons, 'Compressed (ib. 1914).