TUNNELS THROUGH HARD ROCK 83 



to extract the heat from the air before it leaves the com- 

 pressor cylinder. Air compressors may tirst be divided into 

 two classes, according to the means employed for cooling the 

 air, as follows: (1) Wet compressors, and (_') dry compress- 

 ors. A wet compressor is one which introduces water directly 

 into the cylinder during compression, and a dry compressor is 

 one which admits no water to the air during compression. 

 Wet compressors may be subdivided into two classes: (1) 

 Those which inject water in the form of spray into the cylinder 

 during compression, and (2) those which use a water piston 

 for forcing the air into confinement. 



The following brief discussion of these various types of 

 compressors is based on the concise practical discussion of 

 Mr. W. L. Saunders, M. Am. Soc. C. E M in "Compressed Air 

 Production." The highest isothermal results are obtained by 

 tin- injection of water into the cylinders, since it is plain that 

 the injection of cold water, in the shape of a finely divided 

 spray, directly into the air during compression will lower the 

 teni{>erature to a greater degree than simply to surround the 

 cylinder and parts by water jackets winch is the means of cool- 

 ing adopted with dry compressors. A serious obstacle to water 

 injfctioii, and that which condemns this type of 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 rebating. 

 Tin- limitation to the speed of a compressor is also an important 

 objection. The chief claim for the water piston compressor is 

 that its piston is also it cooling device, and that the heat of 

 compression is absorlwd by the water. Water is so poor a 

 conductor ot heat, however, that without the addition of sprays 

 it H safe to say that this compressor has scarcely any cooling 

 advantages at all so far as the cooling of the air during com- 

 -ion is concerned. The water piston compressor operates 

 at slow speed and is expensive. Its only advantage is that it 

 has no dead spaces. In the dry compressor a sacrifice is made 



