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Centrifugal Compressor Efficiency
Centrifugal compressor mainly contains three major parts, namely Nozzle, rotor or impeller and involute casing. The impeller is of three types based on the type of vanes, which are given as follows: 1. Radial vanes: Vanes are straight. 2. Forward curved vanes: Exterior tip of the vane is inclined in a clockwise direction if the impeller is rotating in a clockwise direction. 3. Backward curved vanes: Exterior tip of the vane is inclined in anticlockwise direction if the impeller is rotating in a clockwise direction. Basically, the centrifugal compressor compresses air by taking it from the atmosphere to make it useful for a purpose. It uses an external power source to compress air. Now to maximize the efficiency of the compressor, it is required to compress the maximum volume of air using minimum power from the external power source. So, the ways to increase the efficiency of a centrifugal compressor would be discussed in the article.

First, have a look at how the centrifugal compressor works: First air enters into the nozzle where the velocity of air increases and pressure reduces. After that, it passes through the rotating impeller where energy is imparted to the air. Then air moves centrifugally outward through the vanes of the impeller. This path is a diverging path and acts like a diffuser in which 50% of the total pressure rise takes place. After passing through the impeller, air passes through an empty space which is a diffuser where rest of the pressure rise occurs, then air gets collected inside casing where it reaches the highest pressure, and finally, the compressed air is sent for use.

There is a number of compressors, some of which use inlet guide vanes which are stationary and some do not use them. Compressors in which inlet guide vanes are not used, there are chances that eddies may form due to the rotating motion of airflow. Due to this, there will be power losses which reduce the efficiency of the compressor. The compressor in which inlet guide vanes are used, the airflow is more like laminar. So, there will be no formation of eddies here which saves power relative to the previous case and also efficiency is more. This implies that to compress a certain volume of air, there will be less power requirement in case where inlet guide vanes are used with respect to the other one. Another way to increase the performance of the compressor will be to use variable diffuser geometry. It helps rise in the pressure of air to a larger extent. Performance of compressor can also be enhanced by using water or liquid washing systems.

Efficiency; The efficiency of a compressor is defined as the ratio of the desired output effect to the input. The kinetic energy of air increases at nozzle due to a drop in enthalpy(h) which results in pressure drop. Entropy(s) vs enthalpy(h) plot: Entropy(s) remains constant in ideal conditions so, dwideal = hy – hx dwactual  = h’y – h’x eff =  (hy – hx )/( h’y – h’x)  = Cp (Ty – Tx ) / cp( T’y – T’x) eff    = (Ty – Tx ) / ( T’y – T’x)