Airflow control system principles:

For any given engine there is only one set of conditions, mass flow, pressure ratio and rpm, at which all the compressor components are operating at their optimum effect. Compressors are designed to be most efficient in the higher rpm range of operation. The point at which the compressor reaches its maximum efficiency is known as the DESIGN POINT. Under design conditions the compressor produces a given compression ratio (ie.Volume 2/Volume 1) and the axial velocity (average velocity) of the gas remains approximately constant from the front to the rear of the compressor.

The Angle of Attack of the airflow to the compressor aerofoil blades will be at its optimum. This is the design condition and the compressor is operating at its optimum performance. Although compression ratio varies with rpm it is not proportional to rpm. This fact emerges due to the fixed blade angles, which can only be correct at the design point. To illustrate this fact, refer to the diagram showing rpm and compression ratio. Consider a compressor running at 8,000 rpm and its compression ratio is 10:1. Let us say that the volume of air entering the compressor is 100cm³. The volume of the air passing through the fixed outlet annulus of the compressor will be 10cm3.

Compressor R.P.M = 8,000 Compressor R.P.M. = 4,000
Compression Ratio = 10:1 Compression Ration = 4:1
Volume of gas (V1) = 100cm³ Volume of gas (V1) = 50cm³
Volume of gas (V2) = 10cm³ Volume of gas (V2) = 12.5cm³