Basics of Air Pressure: Displacement Compression and Dynamic Compression

In the two graphs below, you'll find the pressure-volume relationship for a theoretical compressor and a realistic diagram for a piston compressor illustrated (respectively).

The stroke volume is the cylinder volume that the piston travels during the suction stage. The clearance volume is the volume underneath the inlet and outlet valves and above the piston. It must remain at the piston's top turning point for mechanical reasons.

Differences between the stroke volume and suction volume are due to the expansion of air remaining in the clearance volume before the suction starts. The practical design of a compressor, e.g. a piston compressor, results in a difference between the theoretical p/V diagram and the actual diagram.

The valves are never completely sealed and there is always a degree of leakage between the piston skirt and the cylinder wall. In addition, the valves can not fully open and close without a minimal delay. This results in a pressure drop when gas flows through the channels. The gas is also heated when flowing into the cylinder as a consequence of this design.

In a dynamic compressor, the pressure increase takes place while the gas flows. The flowing gas accelerates to a high velocity by means of the rotating blades on an impeller. The velocity of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser.

Depending on the main direction of the gas flow used, these compressors are called radial or axial compressors. Compared to displacement compressors, a small change in the working pressure of dynamic compressors results in a large change in the flow rate.

Each impeller speed has an upper and lower flow rate limit. The upper limit means that the gas flow velocity reaches sonic velocity. The lower limit means that the counter pressure becomes greater than the compressor's pressure build-up, which means return flow inside the compressor. This, in turn, results in pulsation, noise and the risk for mechanical damage.

In theory, air or gas may be compressed isentropically (at constant entropy) or isothermally (at constant temperature). Either process may be part of a theoretically reversible cycle. If the compressed gas could be used immediately at its final temperature after compression, the isentropic compression process would have certain advantages.

In reality, the air or gas is rarely used directly after compression, and is usually cooled to ambient temperature before use. Consequently, the isothermal compression process is preferred, as it requires less work. A common, practical approach to executing this isothermal compression process involves cooling the gas during compression. At an effective working pressure of 7 bar, isentropic compression theoretically requires 37% higher energy than isothermal compression.

A practical method to reduce the heating of gas is to divide the compression into several stages. The gas is cooled after each stage before being compressed further to the final pressure. This also increases the energy efficiency, with the best result being obtained when each compression stage has the same pressure ratio. By increasing the number of compression stages, the entire process approaches isothermal compression. However, there is an economic limit for the number of stages the design of a real installation can use.

At constant rotational speed, the pressure/flow curve for a turbocompressor differs significantly from an equivalent curve for a positive displacement compressor. The turbocompressor is a machine with a variable flow rate and variable pressure characteristic. On the other hand, a displacement compressor is a machine with a constant flow rate and a variable pressure. A displacement compressor provides a higher pressure ratio even at a low speed. Turbocompressors are designed for large air flow rates.

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