Open cycle gas turbine with intercooling

A simple open-cycle constant-pressure gas turbine draws atmospheric air, compresses it, burns fuel in it at constant pressure, expands the hot gas in a turbine and exhausts it to the atmosphere (Brayton/Joule cycle). With intercooling, compression is split into LP and HP stages with an air cooler between them.

Sketch of the plant

             water in/out
                 ||
 Air->[LPC]-2->[ IC ]-3->[HPC]-4->[CC]-5->[ T ]-6->
  1     ||                 ||      ^       ||  exh
        ++======shaft======++======|=======++==>Gen
                                  fuel

LPC = low-pressure compressor, IC = intercooler, HPC = high-pressure compressor, CC = combustion chamber, T = turbine.

T–s diagram

 T
 |                   5
 |                  /|
 |                 / |
 |         4     /   |
 |         |---'     |
 |         |   2     6
 |         |  /|    /
 |         | / |   /
 |         3   1--'
 +----------------------- s
 1-2 LPC, 2-3 intercooling at p_i,
 3-4 HPC, 4-5 combustion at p_2,
 5-6 turbine, 6-1 exhaust (p_1)

How intercooling improves the plant

• Compressor work per kg is $W = c_p T_{in}\left[r^{(\gamma-1)/\gamma} - 1\right]$, i.e. proportional to inlet temperature. Cooling the air from $T_2$ back to $T_3 \approx T_1$ at the intermediate pressure makes the HP stage work much smaller.
• Total compressor work is minimum when intercooling is perfect ($T_3 = T_1$) and the stage pressure ratios are equal: $p_i = \sqrt{p_1 p_2}$.
• Turbine work is unchanged, so net work and work ratio increase; a smaller plant gives the same output.
• Because air now leaves the HPC colder ($T_4$ lower), more fuel is needed in 4–5, so intercooling alone gives only a small change in efficiency. When combined with a regenerator, the larger gap between exhaust temperature $T_6$ and $T_4$ allows more heat recovery, and thermal efficiency rises noticeably.
• Other ways to raise efficiency: regeneration, reheating, higher turbine inlet temperature, optimum pressure ratio and better component efficiencies.