What Are Turbine Power Plant Compressors? How Compressors Work?

The high flow rates of turbines and relatively moderate pressure ratios necessitate the use of rotary compressors. The types of compressors, which are commonly used, are of two types, centrifugal and axial flow types.

The centrifugal compressor consists of an impeller (rotating component) and a diffuser (stationary component). The impeller imparts the high kinetic energy to the air and diffuser converts the kinetic energy into the pressure energy. The pressure ratio of 2 to 3 is possible with single stage compressor and it can be increased upto 20 with three-stage compressor.

The compressors may have single or double inlet. The single inlet compressors are designed to handle the air in the range of 15 to 300 m3/min and double inlets are preferred above 300 m3/min capacity.

The single inlet centrifugal compressor is shown in figure below.

The efficiency of centrifugal compressor lies between 80 to 90%. The efficiency of multistage compressor is lower than a single stage due to the losses. The axial flow compressor consists of a series of rotor and stator stages with decreasing diameters along the flow of air.

The blades are fixed on the rotor and rotors are fixed on the shaft. The stator blades are fixed on the stator casing. The stator blades guide the air flow to the next rotor stage coming from the previous rotor stage.

The air flows along the axis of the rotor. The kinetic energy is given to the air as it passes through the rotor and part of it is converted into pressure. The number of stages required for pressure ratio of 5 is as large as sixteen or more.

A satisfactory air filter is absolutely necessary for cleaning the air before it enters the compressor because it is essential to maintain the designed profile of the aerofoil blades. The deposition of dust particles on the blade surfaces reduces the efficiency rapidly.

The advantages of axial flow compressor over centrifugal compressor are high isentropic efficiency (90 95%), high flow rate and small weight for the same flow quantity.

The axial flow compressors are very sensitive to the changes in airflow and speed, which result in rapid drop in efficiency. In both types of compressors, it has been found that lowering of the inlet air temperature by 15 to 20°C gives almost 25% greater output with an increase of 5% efficiency.

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