COMPRESSED AIR. Compressed air has a wide application in the various branches of mechanical engineering and in the arts and manufactures. As compared with steam, com pressed air transmission of power is valuable and convenient because (a) its loss in trans mission through pipes is relatively small, (b) the question of the disposal of exhaust steam under ground is avoided and (c) the exhausted air is a help in ventilation. Although electricity has become a rival of compressed air in many branc.hes of work, their spheres of usefulness are not identical. The first cost of an electric plant is lower than that of an equivalent com pressed air plant; but a point is soon reached where compressed air transmission becomes the cheaper. In recent years the principles of air compression have become better understood and a substantial improvement has taken place not only in the design of the compressors them selves, but also in the installation of pipe lines and in the operation of the machines. In the production, transmission and employment of compressed air a greater total efficiency is now realized than was formerly thought possible.
Cooling During Compression.— If air at atmosphenc pressure and 60° F. could be com pressed to 100 pounds gauge pressure and all the heat due to the work of compression taken away as fast as generated, so that the tem perature during compression would remain con stant, the mean effective pressure during one stroke of the air piston would be 30.2 pounds. If, on the other hand, none of the heat due to the work of compression was talcen away the mean effective pressure during the stroke would be 41.6 pounds and the terminal temperature would reach 485° F. As the power required for compression is directly proportional to the mean effective pressure, it will be seen that the additional power required in the latter case is 3754 per cent. In practice neither extreme is reached, for it is impossible to completely cool the air during compression, and, on the other hand, some of the heat of compression will be radiated; but the lower extreme is the ideal, and the nearer it can be approached the more eco nomical the compressor will do its work.
Heat of Various plans for talcing away the heat generated in compression, such as injecting a spray of water into the cyl inder, circulating cooling water through the piston and around the heads and cylinder barrel, etc., have been tried. The use of the cooling
spray, or so-called gwet compression,0 has long since been abandoned, as has also the plan of circulating water through the piston, for the disadvantages more than offset the advantages. Cylinder heads and barrels are still water jacketed, not so much on account of the heat that can be taken from the air as to keep the cylinder cool enough for proper lubrication. The most effective means for talcing away the heat of compression and reducing the amount of power required consists of dividing the com pression into two or more stages, depending upon the terminal pressure, and cooling the air as much as possible between stages by means of suitable cooling apparatus; the water-jacketing of the cylinders being retained for the reason above stated. Where the work of compression is done in two or more cylinders, it is cus tomary to so fix the ratio of cylinder volumes as to divide the work equally between the cylinders. By using two-stage compression and cooling the air between the stages to its initial temperature (60° F.), without considering the cooling by water-jacketing, it is possible to reduce the mean effective pressure to 35.5 pounds as compared to 41.6 pounds, which is equivalent to a saving of 15 per cent. At the same time the terminal temperature will be only 245° F. instead of 485° F.
Clearance.— Another factor in compressor design is the clearance in the compressor cyl inders. It is not possible to run a compressor without some space between the piston and cylinder head at the end of the stroke, and in addition to this space there is the volume of the inlet and discharge passages between the valves and cylinder bore. The aim of all compressor builders should be to make this clearance space as small as possible in propor tion to the volume swept through by the piston; for at the end of the stroke the clear ance space is filled with air at the terminal pressure, which must expand back to the initial pressure before the inlet valve is opened. This as particularly important in single stage com pression, as at discharge pressures ordinarily used the expanding of the compressed air in the clearance space back into the cylinder seri ously affects the volumetric efficiency of the compressor.