Temperatures in "cold" section of turbine?
 

I would like to know roughly how hot the air becomes at the last stage of compression (before it reaches the combustion chambers) in a modern turbine.

Reason I'm asking is because I've heard differing things - one is that the compression stage section is relatively cold and could therefore be made out of non heat resistant materials such as composites etc. The other is that compressing air creates lots of heat and hence the opposite. Which one is true?

I don't have an exact answer, but on the B737, the bleeds trip off if the temp exceeds 490F, so it can get at least that hot.

The more compression the more heat. Each compressor stage adds. So yes it's cold enough in the front stages to use composites although the dirt in the air will wear them a lot faster than steel or Titanium. In the high performance engine the compression ratios can drive the temperature at the compressor exit well above 1000 deg F.

For turbines, the more stages the lower the exit temperature, but then there is the efficiency and weight trades and the need to decide whether you want jet pipe thrust or Fan thrust etc. etc.

Somehow I think that the free turbines used in ships (cue Barit1) would have the lowest turbine exit temperature

The compressor delivery temperature will be mainly a function of the entry temperature and overall pressure ratio.
Taking 30:1 as a typical axial flow compressor ratio and ISA sea level take-off conditions, the temperature would be around 470 deg C assuming adiabatic compression.

I have a graph for an RB211 showing about 250C at the point of entry into the combustion chamber.

Not that the RB211 is particularly modern :p

Cheers.
NSEU

Compressor Discharge Temperature
 

For comparison, P&WC PW150A (on Bombardier Q400) CDT is around 450 deg C (840 def F) at 250 PSIA. That's pretty hot for a 3-Stage Axial plus Single-stage Centrifugal compressor arrangement.

Slightly off-subject, I believe that if ceramic engine parts could be made at the same cost - or cheaper than the existing alloy parts, engine durability and longevity will benefit. Until then, the cost of manufacturing these parts will ensure the technology will remain stuck where it is. Several manufacturers have experimented with ceramic turbine blades with great success, but the cost made this option impossible.

Plastics / composites in turbine engine compressors? I doubt it, but I'm happy to stand corrected.

An engine I was very familiar with (14 stage axial compressor) used:

Inconel first few stages for birdstrike resistance
Titanium in mid stages for light weight and corrosion resistance
Inconel in the last stages because of high temperature

Often - usually even - it's cycle temp. that dictates the material.