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Thermodynamics

Intercooled Gas Turbine Simulator

Visualise an ideal Brayton cycle whose compression is split into two stages with an intercooler between them. Adjust the pressure ratio, turbine inlet temperature, specific heat ratio and intercooler effectiveness to see the work saved, turbine work and net work update in real time, with a T-s diagram animation and work-versus-pressure-ratio charts.

Parameters
Overall pressure ratio r_p
Total pressure ratio from compressor inlet to outlet
Compressor inlet temperature T₁
K
Ambient air temperature; also the intercooling target
Turbine inlet temperature T₃
K
Peak combustor-exit temperature, capped by material limits
Specific heat ratio γ
Air's c_p/c_v — about 1.40 for room-temperature air
Intercooler effectiveness ε
1.0 cools fully to inlet temperature, 0 means no cooling
Results
—
Per-stage pressure ratio
—
Compressor work (intercooled) (kJ/kg)
—
Compressor work (single-stage) (kJ/kg)
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Compressor work saved (kJ/kg)
—
Turbine work (kJ/kg)
—
Net work (kJ/kg)
—
Layout and T-s diagram — cycle animation

Flow path: LP compressor → intercooler → HP compressor → combustor → turbine. On the T-s diagram the "intercooling notch" between the two compression stages shows the rejected heat and the resulting work saving.

Compressor work vs pressure ratio
Net work vs pressure ratio
Theory & Key Formulas

$$w_{c,IC}=c_p\big[(T_{2}-T_1)+(T_{2}'-T_{i})\big],\qquad r_{stage}=\sqrt{r_p}$$

Compressor work with intercooling, w_{c,IC}. T₂ is the stage-1 exit temperature, T_i the temperature after intercooling, T₂' the stage-2 exit temperature. The work saving is greatest when each stage takes the square root of the overall pressure ratio r_p and the intercooler returns the air to near the inlet temperature.

$$T_2=T_1\,r_p^{(\gamma-1)/2\gamma},\qquad T_i=T_2-\varepsilon\,(T_2-T_1)$$

Stage-1 exit temperature T₂ and the temperature after intercooling T_i. Each stage's isentropic temperature factor uses the square-root exponent; the higher the effectiveness ε, the closer T_i is to the inlet temperature T₁.

$$w_t=c_p\,T_3\Big(1-r_p^{-(\gamma-1)/\gamma}\Big),\qquad w_{net}=w_t-w_{c,IC}$$

Turbine work w_t and net work w_net. T₃ is the turbine inlet temperature and c_p the specific heat at constant pressure. Every bit of compressor work cut adds directly to the net work.

What is the Intercooled Gas Turbine?

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I saw the term "intercooling" for gas turbines. What does that device actually do?
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In short, it is a way to "cool the air partway through compression". A normal gas turbine pushes the intake air up to high pressure in one go with its compressor. An intercooled machine splits that compression into two stages and slips a heat exchanger between the first and second stages. That is the "intercooler". It cools the air heated by the first stage before it enters the second stage.
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You deliberately cool it? After going to the trouble of raising its temperature by compressing it — isn't that wasteful?
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Good question. The key is that "the work needed for compression is proportional to the temperature at which you compress". Hot air is "stiff" — it takes extra effort to squeeze the same pressure ratio. Cold air is "soft". So cooling the air before it enters the second stage sharply reduces the work the second-stage compressor consumes. The "Compressor work saved" card on the left is showing exactly that saving.
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I see! So the work you saved turns into extra engine output?
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Exactly. A gas turbine's net output is the difference between "the work the turbine extracts from the combustion gases" and "the work the compressor consumes to pressurise the air". In fact the compressor can swallow more than half the turbine's gross output. So any compressor work you save translates almost entirely into extra net output. With the default settings, the compressor work that costs about 299 kJ/kg single-stage drops to about 247 kJ/kg with intercooling — a saving of around 52 kJ/kg.
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If it's that worthwhile, why isn't every gas turbine fitted with an intercooler?
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Here is the subtle part. On its own, intercooling actually lowers the thermal efficiency slightly. Because the air enters the combustor cooler, more fuel must be burned to heat it to the same turbine inlet temperature. The power (specific power) goes up, but the efficiency — the fuel economy — drops. So real machines almost always pair it with a "regenerator", which uses exhaust heat to preheat the air before combustion. Intercooling plus regeneration raises both the power and the efficiency.
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How you split the pressure ratio between the two stages must matter. Is there a rule?
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There is. The total compressor work is minimised when each stage handles the same pressure ratio — the square root of the overall ratio. For an overall pressure ratio of 12, each stage takes the square root of 12, about 3.46. If the ratio is biased toward one stage, that stage's temperature rise grows and the total work increases. This tool assumes that optimum split. In practice, combining that optimum split with a high-effectiveness intercooler is how marine propulsion and large industrial gas turbines reach high specific power.

Frequently Asked Questions

Intercooling means splitting the compression of a gas turbine into two stages instead of one, and placing a heat exchanger called an intercooler between them to cool the air. Compressing a gas heats it up, and hotter gas takes more work to squeeze a given pressure ratio out of. By cooling the air heated by the first stage before it enters the second stage, the second stage needs less work. This tool calculates that saving as you vary the pressure ratio and the intercooler effectiveness.
The work needed for compression is roughly proportional to the temperature at which the compression happens. Cold air is "softer" and needs less work for the same pressure ratio. In single-stage compression the heat of the first stage raises the inlet temperature of the second stage, so the second stage starts hot and consumes extra work. Intercooling dumps that heat to the surroundings (atmosphere or cooling water) so the second stage starts cold. In theory the saving is largest when the compression is split so each stage handles the square root of the overall pressure ratio, and the intercooler is effective enough to return the air close to the inlet temperature.
No — on its own, intercooling actually lowers the thermal efficiency slightly. Cooling the air drops the temperature of the air entering the combustor, so more fuel is needed to heat it to the same turbine inlet temperature. The specific power (net work) definitely increases, but the efficiency falls. For this reason real machines almost always pair intercooling with a regenerator (recuperator) that recovers exhaust heat to preheat the air before combustion. The combination of intercooling and regeneration raises both the power and the efficiency.
When the overall pressure ratio r_p is shared between two stages, the total compressor work is minimised when each stage handles the same pressure ratio, the square root of r_p. This follows from differentiating the sum of the two stage works and setting it to zero, under the ideal assumption that the intercooler returns the air to the same inlet temperature each time. For example, with an overall pressure ratio of 12, each stage handles the square root of 12, about 3.46. If the pressure ratio is biased toward one stage, that stage's temperature rise grows and the total work increases.

Real-World Applications

Marine gas turbine propulsion: Intercooling pays off most in naval propulsion. High output (high specific power) is demanded from a limited hull volume, and seawater offers an excellent, readily available cooling source — exactly the conditions intercooling thrives on. Marine gas turbines that combine intercooling and regeneration, such as the WR-21, deliver compact, high power while keeping good fuel economy across a wide range from low to high load.

Large industrial and power-generation gas turbines: Stationary industrial gas turbines also adopt intercooling where output must be maximised. Air is bled partway through the compressor, cooled in an external heat exchanger and returned. In hot climates, where warm intake air increases compressor work, the specific-power boost from intercooling is especially valuable. It is often designed into combined cycles together with regenerators or steam injection.

Aeroderivative engines: Some aeroderivative gas turbines — aircraft engines repurposed for ground power and mechanical drive — also incorporate intercooling. Layering the specific-power gain of intercooling onto the high pressure ratio and light weight inherited from aircraft engines pushes the output-per-unit-weight even higher. This suits applications such as peaking power and emergency standby that demand fast starts and high power density.

Thermodynamics teaching and cycle comparison: Intercooling is an ideal topic for studying "what each modification to the Brayton cycle buys you", alongside regeneration and reheat. It helps to organise them: intercooling raises specific power but lowers efficiency, regeneration raises efficiency, reheat raises specific power. By varying the pressure ratio and effectiveness in this tool and watching the gap from single-stage compression, you grasp the compressor work saving intercooling brings intuitively.

Common Misconceptions and Pitfalls

The most common misconception is "fitting an intercooler also raises the thermal efficiency". Intercooling does reduce the compressor work and raise the specific power (net work), but in itself it does not raise the thermal efficiency. On its own it lowers it slightly, because the cooler air entering the combustor needs extra fuel to reach the same turbine inlet temperature. "More output" does not equal "better fuel economy". If you want efficiency too, pairing it with a regenerator — which this tool does not model — is a prerequisite. Understand intercooling as "a modification that goes after output" and regeneration as "a modification that goes after efficiency".

Next, assuming "the ideal-cycle figures are the real machine's performance". The compressor work and net work this tool computes are ideal values, treating air as an ideal gas, the compression as perfectly adiabatic (isentropic) and the specific heat as constant. Real compressors and turbines have isentropic efficiencies; losses increase the compressor work and reduce the turbine work. The intercooler also has a pressure drop, and recovering the lost second-stage inlet pressure costs work. A real machine's specific power is usually lower than this tool's value, so use the tool to read "the direction and magnitude of the intercooling effect".

Finally, the simplification that "setting the intercooler effectiveness to 1 is always best". The higher the effectiveness — the closer you cool toward the inlet temperature — the more compressor work you save, but that requires a large heat-transfer area and cooling source, making the unit heavy and bulky and increasing piping pressure loss. If water is used for cooling, securing the water and protecting against freezing become issues. A real design balances the specific-power gain from higher effectiveness against the constraints of weight, volume, pressure loss and cooling-source availability. Treat the effectiveness slider as an ideal upper bound, and evaluate a real machine at a "realistically achievable effectiveness".

How to Use

  1. Set overall pressure ratio (typically 8–16 for industrial gas turbines) using the pressureRatioNum slider. The simulator automatically splits this equally between two compressor stages.
  2. Enter inlet air temperature (ambient, usually 288–310 K) and turbine inlet temperature (1200–1500 K for modern engines) using tempInletNum and tempTurbineInletNum respectively.
  3. Adjust gamma (specific heat ratio; 1.4 for air) if modeling different working fluids, then observe real-time calculations for per-stage pressure ratio, compressor work, turbine work, and net cycle output in kJ/kg.

Worked Example

Consider a power plant cycle: overall pressure ratio = 12, inlet temperature = 300 K, turbine inlet temperature = 1400 K, gamma = 1.4. Each compressor stage sees pressure ratio of √12 ≈ 3.46. Single-stage compression would require approximately 520 kJ/kg work. With intercooling between stages, cooling air back to 300 K reduces the second stage work; total intercooled compressor work drops to ~380 kJ/kg, saving 140 kJ/kg. Turbine work output reaches ~920 kJ/kg, yielding net work of ~540 kJ/kg—a 26% efficiency gain versus non-intercooled operation.

Practical Notes

  1. Intercooling effectiveness is most valuable at pressure ratios above 10:1; below 8:1, the weight penalty of the cooler often outweighs efficiency gains in aero applications.
  2. Real intercoolers reduce outlet temperature to 85–95% of inlet, not perfect 100%; enter slightly higher tempInletNum to reflect this 15% pinch-point loss.
  3. Marine and stationary power plants exploit intercooling readily; military turbofans rarely adopt it due to nacelle volume constraints and compressor blade count limits.