Compressors are broadly classified into two basic categories: Rotodynamic compressors and Positive Displacement compressors. These two broad classes can be further split as:
Positive Displacement compressors
- Rotary compressors
- Reciprocating compressors
These compressors have rotating equipment parts imparting momentum to the gas particles which is later converted to pressure. For the rotodynamic type of compressors the flow is continuous. These compressors are often smaller in size and produce much less vibration than the positive displacement compressors.
These compressors operate with the same principle as that of the centrifugal pumps. The gases come on from an axial direction to the rotating compressor impeller, which then imparts a radial velocity to the gas particles. These particles then hit the diffuser where the velocity is converted to pressure head. The impellers usually operate with high rotational velocities, typically in the range of 9000-15000 RPM for compressors used in chemical industry. The compressors can have either a single casing with multiple stages or multiple casings with intercoolers between them to reduce the power required to drive the compressors. The compressors are typically driven by gas / steam turbines or electric motors.
The stable operating region for a centrifugal compressor occurs between the ‘surge point’ and the ‘choke point’. Surge point corresponds to minimum flow of stable operation. Surge point is characterized by the reversal of main flow in the compressor, excessive vibration and sound coming from the compressor. Choke point of a compressor at a given operating speed occurs at the maximum flow limit.
Following figure gives an idea of the structure of a centrifugal compressor.
Figure 1 - Structure of a centrifugal type compressor
Axial Flow compressors
These compressors are primarily used for applications involving large gas flowrates and relatively low outlet pressures, as compared to centrifugal compressors. Axial flow compressors are usually more efficient than centrifugal compressors. An axial flow compressor consists of a large number of blades attached to a rotating blade with stationary adjustable blades fixed to the compressor casing. This arrangement of blades creates multiple stages resulting in high efficiency and pressure ratio per casing. The operation of an axial flow compressor is governed by the rotational speed of the blades. The stable operating range for an axial flow compressor is however narrow, compared to the centrifugal compressors. Typical revolution speeds for axial compressors are in the range 1000-3000 RPM.
Figure 2 - Stationary and rotating blades in an axial flow compressor
Positive displacement compressors
These types of compressors operate with fixed volumetric gas flows but they can achieve a range of differential pressures. They can be further classified between ‘rotary compressors’ and ‘reciprocating compressors’.
There are many variants of rotary type compressors depending on the details of the mechanism employed. But all of them have two things in common – fixed volumetric flow and one or more rotating shafts. Following schematics are very explanatory of the various mechanisms of rotary type positive displacement compressors.
Figure 3 - Schematic indicates the Screw type rotary positive displacement compressor
Figure 4 - Mechanism of a liquid-piston type rotary positive displacement compressor
Figure 5 - Schematic showing the mechanism of a sliding vane type
Figure 6 - rotary positive displacement compressor
Reciprocating type of compressors is typically used for applications involving low gas flows and high discharge pressures. These compressors can give develop compression ratios as high as 10 per stage depending on the allowable discharge gas temperature. Following pictorial is very explanatory of the mechanism of a reciprocating type compressor.
Figure 7 - Double acting two-stage reciprocating compressor