Compressor pressure ratio is calculated by dividing the total pressure after the last stage of compression by the total inlet pressure.
If there was no velocity change throughout the compressor the same result could be obtained by just dividing the static pressures taken at the same points.
Each rotor stator compression stage gives about 1.2:1 pressure ratio. On a multi-spool engine, the overall compressor ratio is found by multiplying each compressor ratio.
Single stage fan pressure ratio approximately 1.5: 1.
7-stage intermediate compressor pressure ratio approximately 5: 1.
6-stage high pressure compressor pressure ratio approximately 4:1
Overall compressor pressure ratio = 1.5 x 5 x 4 = 30:1
The actual pressure ratio per rotor/stator stage will vary but it is the average across the compressor that gives the figure of approximately 1.2:1.
To find the average for an engine with 14 stages and an overall pressure ratio of 30:1 as shown in the example above the calculation is to take the fourteenth root of 30 equals 1.275.
Using this number and the example engine at an inlet pressure of 14.7psi, a table can be constructed as follows:
Fan: 14.7 X 1.275 =18.74 Px Ratio = 1.275
CYCLE PRESSURE RATIO
The compressor pressure ratio calculated when using inlet pressures in flight.
If the aircraft were flying at 400kts with an intake ram pressure of 16psi at a compressor pressure ratio of 30:1 then the final delivery pressure would be 480psi.
FAN PRESSURE RATIO
The fan pressure ratio varies along the blade radius.
The pressure ratio of the outer blade section passing air to bypass is about 1.5:1 whereas the inner span section passing air to the core engine is closer to 1.2:1.
The blade twist, or stagger angle, ensures this distribution.
The pressure ratio of a compressor varies with its RPM.
Intake temperature can affect the pressure ratio but this is where the multi-spool compressor comes into its own.
We have already known that a reduction in air density will cause the LP compressor to increase it speed and thus maintain the pressure ratio.
This occurs as aircraft altitude increases thus the compressor pressure ratio will remain constant for a fixed power lever setting.
The increase in LP compressor speed will continue with reducing inlet air density until the maximum governed speed of the compressor is reached.
If the compressor becomes contaminated with dirt or oil its pressure ratio will reduce. This can cause excessive exhaust gas temperatures and low RPM.
Compressors can be washed. The maintenance manual instructions should be consulted before attempting this.
Blade damage will also affect the pressure ratio. It is usual for the compressor pressure ratio to reduce with engine deterioration over its life and regular performance checks in accordance with the maintenance schedule will monitor this.