Fig. 374.
As the pressures increase, however, the machines become more and more complicated, owing not only to the greater power required, but also to the heating of the air during compression. The use of water-jackets for cooling the air in the compression cylinders is general, but this does not effect thorough cooling, as only a small portion of the air in the cylinder comes in contact with the jacketed parts. This difficulty has led to the use of compound machines, in which case inter-coolers are generally used between the different stages of compression, which cause the air to shrink in volume between the stages.
Briefly summed up, the chief advantages of multi-stage over single-stage compression are:
1. Lower average temperature, resulting in lower average pressure, and permitting the compression of the same volume of air with less expenditure of energy.
2. Increased safety and ease of lubrication. When high final temperatures prevail, part of the lubricating oil vaporizes, and wear on the piston and cylinder becomes rapid. Under exceptional circumstances the combination of air and oil vapor may reach the proportions of an explosive mixture, and if the compression temperature passes its flash point damage may result. Such accidents are, however, very rare even in single-stage work; in multi-stage compression, with proper intercooling, they are impossible.
3. Greater effective capacity in free air. The final pressure in the low pressure cylinder is much lower than in a single-stage machine, and the air confined in the clearance spaces when expanded down to atmospheric pressure occupies comparatively little space. Consequently the inflow of air through the suction valves begins at an earlier point in the stroke.
4. The air delivered by a two-stage or multi-stage compressor is dryer than that furnished by a single cylinder. Under constant pressure the power of air to hold watery vapor decreases with its temperature, and during its passage through the inter-cooler much of the original moisture in the air is precipitated. Consequently less trouble is experienced from condensation in the discharge pipe.
A properly designed inter-cooler should reduce the air in the cylinders to the temperature of the outside air. The economy of compressing in several stages—or, in other words, compound compressors—is shown from the fact that in compressing air up to 100 lbs. the heat loss reaches about 30 per cent. By compressing in two stages, this loss is cut down to less than half; and in four stages, it is reduced to four or five per cent. It is evident, therefore, that the higher the pressure required the more essential is the use of compound machines.