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Two Basic Compression Principles: Displacement Compression and Dynamic Compression

Compressor Types Compressors Basic Theory Compressed Air Wiki Compressed Air

Before you can learn about the different compressors and compression methods, we first have to introduce you to the two basic principles for the compression of gas. After that, we will compare the two and look into the different compressors in these categories.

What are the two basic principles of compression?

displacement and dynamic compression
There are two generic principles for the compression of air (or gas): Positive displacement compression and dynamic compression. The first one includes, for example, reciprocating (piston) compressors, orbital (scroll) compressors and different types of rotary compressors (screw, tooth, vane). In positive displacement compression, the air is drawn into one or more compression chambers, which are then closed from the inlet. Gradually the volume of each chamber decreases and the air is compressed internally. When the pressure has reached the designed build-in pressure ratio, a port or valve is opened and the air is discharged into the outlet system due to continued reduction of the compression chamber's volume. In dynamic compression, air is drawn between the blades on a rapidly rotating compression impeller and accelerates to a high velocity. The gas is then discharged through a diffuser, where the kinetic energy is transformed into static pressure. Most dynamic compression are turbocompressors with an axial or radial flow pattern.

What are positive displacement compressors?

piston compressor
A bicycle pump is the simplest form of a positive displacement compression, where air is drawn into a cylinder and is compressed by a moving piston. The piston compressor has the same operating principle and uses a piston whose forward and backward movement is accomplished by a connecting rod and a rotating crankshaft. If only one side of the piston is used for compression this is called a single-acting compressor. If both the piston's top and undersides are used, the compressor is double acting. The pressure ratio is the relationship between absolute pressure on the inlet and outlet sides. Accordingly, a machine that draws in air at atmospheric pressure (1 bar(a) and compresses it to 7 bar overpressure works at a pressure ratio of (7 + 1)/1 = 8).

The compressor diagram for positive displacement compressors

The two graphs below illustrate (respectively) the pressure-volume relationship for a theoretical compressor and a more realistic compressor diagram for a piston compressor. The stroke volume is the cylinder volume that the piston travels during the suction stage. The clearance volume is the volume just underneath the inlet and outlet valves and above the piston, which must remain at the piston's top turning point for mechanical reasons.

The difference between the stroke volume and the suction volume is due to the expansion of the air remaining in the clearance volume before suction can start. The difference between the theoretical p/V diagram and the actual diagram is due to the practical design of a compressor, e.g. a piston compressor. 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, which results in a pressure drop when the gas flows through the channels. The gas is also heated when flowing into the cylinder as a consequence of this design.

Formula isothermal compression

Compression work with isothermal compression:

Formula isentropic compression

Compression work with isentropic compression:

These relations show that more work is required for isentropic compression than for isothermal compression.

What are dynamic Compressors?

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. As compared to displacement compressors, dynamic compressors have a characteristic whereby a small change in the working pressure 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.

Compression in several stages

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 the 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.

What is the difference between a turbocompressor and a positive displacement compressor?

At constant rotational speed, the pressure/flow curve for a turbocompressors differs significantly from an equivalent curve for a positive displacement compressor. The turbocompressors 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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