More about compressors In the last page we talked about compressor stages and how we could have many stages in one compressor. The problem with adding extra stages is that all these stages have to turn at the same speed because they are all connected to the same turbine. This becomes less efficient as more stages are added because on a fourteen stage compressor like the Rolls Royce Avon, the optimal rotational speed for stage one will not be optimal for stage fourteen. One answer is to have two or even three smaller compressors, each with its own turbine to drive it. Engines like the Rolls Royce RB211 , Trent and RB199 have three compressors. These are referred to as "triple spool engines". It is not so easy to add extra stages to a centrifugal compressor but it is possible to have two centrifugal compressors on one engine, either driven by separate turbines or arranged "back to back", driven by the same turbine. Some engines even combine an axial flow compressor with a centrifugal compressor. The air goes through the axial flow compressor first before getting a final boost by going through the centrifugal compressor.
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