Single and Multiple Stage Compressors
As noted in the compressor descriptions, most compressor designs are available in single or multiple stage designs. When air is compressed, its temperature increases. Compressing in multiple stages allows cooling to occur between the stages, which saves work in the compression process.
Air compressor performance typically approximates isentropic compression, allowing an increase in the air temperature over the course of the compression process. This is approximated in the curve below. As we reduce volume from V1 to V2, pressure increases from P1 to P2. The blue shaded area represents the amount of work done on the air to change from P1/V1 to P2/V2.
However, in a two-stage compressor, air is partially compressed using isentropic compression, and then cooled in a heat exchanger, called an intercooler. This brings the compression process closer to isothermal (constant temperature) compression, which is more efficient. The blue shaded area below represents the amount of work done to change the air from P1/V1 to P2/V2 in two stages, with an intercooler. The green shaded area represents the savings achieved by two stage compression versus single stage.
We can see that there is a distinct efficiency advantage to multiple stages when intercoolers are used. Without intercooling, however, the savings would not be realized and the efficiency would be no different than a single stage machine.
Multi-Stage Oil Free and Lubricated Reciprocating Compressors
Oil free compressors and lubricated reciprocating compressors realize significant savings from multiple stages of compression, with two to three stages of compression for typical plant pressures of 90 to 125 PSIG. Reciprocating compressors may go to 5 or more stages to reach thousands of PSIG operating pressures.
Two Stage Oil Flooded Screw Compressors
Oil flooded rotary screws are offered in two-stage arrangements by some manufacturers. However, intercooling is very limited in these machines due to oil flooded nature of the machine and the risk of moisture condensation as the air is cooled. Every compressor brings in some moisture with the intake air. Higher ambient temperature and relative humidity means more water vapor that is brought into the compressor.
Air can hold less moisture as pressure increases, but more moisture as temperature increases. For this reason, oil flooded compressors are thermostatically controlled to maintain operating temperatures high enough so that water does not condense in the compressed air. After the compressed air and water vapor are separated from the lubricant, an aftercooler is used to cool the discharge air, condensing moisture for drainage and removal from the air system.
If true intercooling (back to ambient temperature) was used in an oil flooded compressor, water would condense in the compressor and mix with the oil, providing reliability problems. Therefore, partial “intercooling” is performed by introducing a spray of slightly cooled oil between the stages. The cooling effect is minimal to prevent condensation. Since the cooling is minimal, the advantages of the two stage compressor are minimal as well. Efficiency is improved only marginally, and the expense and complexity of the compressor are increased by nearly doubling the number of moving parts.