Question 8 (10%) Small gas turbines, say less than 500kW, generally use one or t

Question

Question 8 (10%) Small gas turbines, say less than 500kW, generally use one or two stages of centrifugal gas turbines use axial flow compressors exclusively. compressor rather than axial flow compressors, whereas large mass flow rate, high pressure ratio What are the advantages offered by centrifugal compressors over axial flow compressors (2%) a) and why are they preferred in small gas turbines? b) Why do axial flow compressors become necessary as flow rate and pressure ratio increase? (396) What are the three most common methods of increasing the operating speed range of the (5%) c) gas turbine which also assist in the prevention of compressor stall?

Explanation / Answer

A. High efficiency of the centrifugal compressors and lowers 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.

B.The value 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. So as are result Axial flow compressors are necessary to obtain higher flow rates and pressure ratios because of large mutlistaging possible.

C.Material Selection: Material Selected should be such that it should allow turbine to operate at much higher spees of rotation.

Bleed valves: Compressors are unstable machines. If the mass flow is too low the aerofoils can stall, this creates a phenomenon called rotating stall. The bleed valves avoid these dire consequences by blowing air out and increasing the air flow through the compressor. The performance effect is very negative, SFC increases and a lot of noise is generated.

Aerofoil Design: Aerofoil must be design such that it must avoid stall phenomena even operating at very high speeds.

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