The wet compressor of the second class is the water piston compressor, Fig. 18.

[Illustration:  FIG. 18.—­HYDRAULIC AIR COMPRESSOR.]

The illustration shows the general type of this compressor, though it has been subject to much modification in different places.  In America, a plunger is used instead of a piston, and as it always moves in water the result is more satisfactory.  The piston, or plunger, moves horizontally in the lower part of a U shaped cylinder.  Water at all times surrounds the piston, and fills alternately the upper chambers.  The free air is admitted through a valve on the side of each column and is discharged through the top.  The movement of the piston causes the water to rise on one side and fall on the other.  As the water falls the space is occupied by free air, which is compressed when the motion of the piston is reversed, and the water column raised.  The discharge valve is so proportioned that some of the water is carried out after the air has been discharged.  Hence there are no clearance losses.

This hydraulic compressor seems to have a certain charm about it, which has resulted in its adoption in Germany, France and Belgium, and by one of the largest mines in the United States.  Its advantages are purely theoretical, and without certain adjuncts which have been in some cases applied to it, even the theory is a very bad one.

The chief claim for this water piston compressor is that its piston is also its cooling device, and that the heat of compression is absorbed by the water.  So much confidence seems to be placed in the isothermal features of this machine that usually no water jacket or spray pump is applied.  Mr. Darlington, who is one of the stanch defenders of this class of compressors, has found it necessary to introduce “spray jets of water immediately under the outlet valves,” the object of which is to absorb a larger amount of heat than would otherwise be effected by the simple contact of the air with the water-compressing column.  Without such spray connections, it is safe to say that this compressor has scarcely any cooling advantages at all, so far as air cooling is concerned.  Water is not a good conductor of heat.  In this case only one side of a large body of air is exposed to a water surface, and as water is a bad conductor, the result is that a thin film of water gets hot in the early stage of the stroke and little or no cooling takes place thereafter.  The compressed air is doubtless cooled before it gets even as far as the receiver, because so much water is tumbled over into the pipes with it, but to produce economical results the cooling should take place during compression.

Water and cast iron have about the same relative capacity for heat at equal volumes.  In this water piston compressor we have only one cooling surface, which soon gets hot, while with a dry compressor, with water jacketed cylinders and heads, there are several cold metallic surfaces exposed on one side to the heat of compression, and on the other to a moving body of cold water.