Classifications of Compressors

Most commonly used compressors can be divided into two groups. One is the positive displacement compressor and the other dynamic compressor.

Positive Displacement Compressors

In positive displacement compressors, the compression is accomplished by movement of a displacing element. By physically reducing the volume of the gas, pressure is increased. The two main types of positive compressors are the reciprocating compressor and rotary compressor.

1. Reciprocating Compressors

The reciprocating compressor is likely the most well known and the most widely used of all compressors. In the reciprocating compressor, the compressing and displacing element is a piston moving linearly within a cylinder. A reciprocating compressor has many moving parts and, therefore, lower efficiency. Each cylinder assembly includes piston, cylinders, cylinder heads, suction and discharge valves, and the parts necessary to convert rotary motions to reciprocating motion.

The main advantage of reciprocating compressors are: the ability to use them as non-lubricating; high-pressure ratios through multi-staging; and variable capacity. The disadvantages are: high maintenance, pulsation, and unbalanced forces created by motion.

2. Rotary Compressors

In rotating compressors, the positive displacement is accomplished by the positive action of rotating elements. These compressors perform compression in an intermittent mode and do not use inlet and discharge valves. Several configurations of a rotary compressor are possible.

The vane type of rotating compressor does not produce pulsating flow, and requires less space compared to a reciprocating compressor. However, it does require 5% to 20% more horsepower than a reciprocating compressor and uses 10 times the oil of a reciprocating compressor.

Helical lobe compressors have two inter-meshing rotors, each with a helical form, and they compress and displace gas.

Centrifugal Compressors

In centrifugal compressors, the rotating impellers impart a velocity head to the fluid which is converted to pressure head by expansion. These compressors have few moving parts since only the impeller and shaft rotate. Centrifugal compressors provide continuous delivery without cyclic variations. These compressors also have low maintenance and oil consumption costs.

The main advantage of centrifugal compressors are: high capacity, virtually oil-free operations, high pressure capability, and relatively low maintenance. The disadvantage are: they are relatively expensive compared to reciprocating and screw compressors, inflexible in change in pressure ratios and capacities, and extremely sensitive to vibrations.

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