By far the most serious obstacle to water injection, and that which condemns the wet compressor, is the influence of the injected water upon the air cylinder and parts.  Even when pure water is used, the cylinders wear to such an extent as to produce leakage and to require reboring.  The limitation to the speed of a compressor is also an important objection.  The claim made by some that the injected water does not fill the clearance spaces, but is aerated, does not hold good, except with an inefficient injection system.  The writer has increased the speed of an air compressor (cylinders 12 in. and 12 in. by 18 in., injection air cylinder) ten revolutions per minute by placing his fingers over the orifice of the suction pipe of the water pump.  The boiler pressure remained the same, the cut-off was not changed and the air pressure was uniform, hence this increase of speed arose from the fact that the water was restricted and the clearance spaces were filled with compressed air, which served as a cushion or spring.  While the volume of compressed air furnished by this compressor would be somewhat reduced by the restriction of the water, yet the increase in speed which was obtained without any increase of power fully compensated for the clearance loss.

Mr. John Darlington, of England, gives the following particulars of a modern air compressor of European type: 

“Engine, two vertical cylinders, steam jacketed, with Meyer’s expansion gear.  Cylinders, 16.9 inches diameter, stroke 39.4 inches; compressor, two cylinders, diameter of piston, 23.0 inches; stroke 39.4 inches; revolutions per minute, 30 to 40; piston speed 39 to 52 inches per second, capacity of cylinder per revolution, 20 cubic feet:  diameter of valves, viz., four inlet and four outlet, 51/2 inches; weight of each inlet valve, 8 lb.; outlet, 10 lb.; pressure of air, 4 to 5 atmospheres.  The diagrams taken of the engine and compressor show that the work expended in compressing one cubic meter of air to 4.21 effective atmospheres was 38,128 lb.  According to Boyle and Mariotte’s law it would be 37,534 lb., the difference being 594 lb., or a loss of 1.6 per cent.  Or if compressed without abstraction of heat, the work expended would in that case have been 48,158.  The volume of air compressed per revolution was 0.5654 cubic meter.  For obtaining this measure of compressed air, the work expended was 21,557 pounds.  The work done in the steam cylinders, from indicator diagrams, is shown to have been 25,205 pounds, the useful effect being 851/2 per cent. of the power expended.  The temperature of air on entering the cylinder was 50 degrees Fah., on leaving 62 degrees Fah., or an increase of 12 degrees Fah.  Without the water jacket and water injection for cooling the temperature it would have been 302 degrees Fah.  The water injected into the cylinders per revolution was 0.81 gallon.”

We have in the foregoing a remarkable isothermal result.  The heat of compression is so thoroughly absorbed that the thermal loss is only 1.6 per cent.; but the loss by friction of the engine is 14.5 per cent., and the net economy of the whole system is no greater than that of the best American dry compressor, which loses about one-half the theoretical loss due to heat of compression, but which makes up the difference by a low friction loss.