Wet compressors may be subdivided into two classes.
(1) Those which inject water in the form of a spray into the cylinder during compression.
(2) Those which use a water piston for forcing the air into confinement.
The injection of water into the cylinder is usually known as the Colladon idea. Compressors built on this system have shown the highest isothermal results, that is, by means of a finely divided spray of cold water the heat of compression has been absorbed to a point where the compressed air has been discharged at a temperature nearly equal to that at which it was admitted to the cylinder. The advantages of water injection during compression are as follows:
(1) Low temperature of air during compression.
(2) Increased volume of air per stroke, due to filling of clearance spaces with water and to a cold air cylinder.
(3) Low temperature of air immediately after compression, thus condensing moisture in the air receiver.
(4) Low temperature of cylinder and valves, thus maintaining packing, etc.
(5) Economical results, due to compression of moist air (see table 3).
TABLE 3.—SHOWING THE RELATIVE QUANTITY OF WORK REQUIRED TO COMPRESS A GIVEN VOLUME AND WEIGHT OF AIR, BOTH DRY AND MOIST—ALSO RELATIVE VOLUMES WITH AND WITHOUT INCREASE OF TEMPERATURE FROM COMPRESSION.