compressor capacity can be extended to certain amount exceeding designed capacity at some cutting down of discharge pressure and some increase of driving power.
in centrifugal compressor, discharge pressure attainable by impeller varies by inlet air density. When inlet air temperature lowers, discharge pressure rises. When inlet air temperature rises or inlet valve is throttled, discharge pressure lowers. That is, the attainable discharge pressure in centrifugal compressor is in proportionate relation to the density of inlet air.