Axial and centrifugal-type compressors typically fall into the category of rotodynamic compressors.
Term 'rotodynamic compressor' typically refers to a continuous-flow compressor, which uses rotating impellers in order to add velocity and pressure to the gas undergoing compression. Compared to positive displacement type compressors, dynamic compressors are typically smaller in size and create less vibration.
Centrifugal compressors typically consist of one or more stages. Each stage typically consists of an impeller (rotating element) and a diffuser (stationary element).
In a centrifugal compressor, the gas typically enters the impeller axially and is discharged radially. The gas hurled out radially by the rotating impeller blades. When the gas The gas then flows through the circular chamber (diffuser), following a spiral path where the velocity head of the gas gets converted to pressure head as per Bernoulli’s principle. To put it simply, the impeller imparts kinetic energy to the gas fluid, whereas the diffuser converts part of that kinetic energy into pressure head.
The extent of pressure rise in a centrifugal compressor mainly depends on the rotational speed of the impeller and the impeller size. However, the maximum allowable speed is limited by the strength of the structural material of impeller blades and sonic velocity of the gas. As a result, limitations on maximum achievable pressure rise exist. These limitations are overcome by the use of multistage centrifugal compressors, which are able to compress the gas to desirable pressure using multiple stages of centrifugal compressors operating in series.
Figure 1 - Typical impeller and diffuser at centrifugal compressor
High efficiency of the centrifugal compressors and the fact that they can be offered as packaged items at small sizes is offset by their high initial cost and the sophisticated vibration monitoring system that is usually required because of the high rotational speeds at which centrifugal compressors operate. Among others, centrifugal compressors are used in natural gas processing plants, process air applications etc.
Axial flow compressors are typically used at applications with low differential pressure (head) requirements and high flow rates. A typical axial compressor consists of a drum, to which blades of specific geometry are attached.
Contrary to centrifugal compressors, axial flow compressors do not change the direction of the gas: the gas typically enters and exits the compressor in an axial direction (parallel to the axis of rotation). First, the gas is accelerated and then diffused so that its pressure is increased.
Typical applications of big-size axial compressors are those used to compress the air intake of gas turbines. These are typically multistage axial compressors, consisting of several stages. Each stage typically consists of a rotor and a stator. A rotor is a set or rotating blades, whose role is to accelerate the gas flow. A stator is a set of stationary blades whose role is to convert the velocity energy gained in the rotor to pressure energy, similar to the impeller – diffuser combination used in the centrifugal compressors.
Axial compressors have relatively high peak efficiency. On the other hand, their efficiency is good over a narrow rotational speed range. Their big weight, combined with their high starting power requirements are some of their drawbacks.
Figure 2 - Axial compressor schematic