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Turbine Engines: Steam Turbines and Gas Turbines

A turbine engine turns a fast-moving gas into spinning motion. A steam turbine gets hot steam from a boiler. A gas turbine compresses air, burns fuel in it, and sends the hot gas through a turbine. The spinning shaft drives a generator, a ship propeller or a jet. Efficiency = useful work out ÷ heat in.

🎬 Step-by-step story

  1. A turbine is a wheel with blades. A fast jet of gas or steam hits the blades and pushes them. The wheel spins.
  2. Steam turbine: a boiler heats water into hot steam. The steam rushes through the turbine and spins it. The shaft turns a generator and the bulb lights.
  3. Steam gets bigger as it loses pressure. So the blades get bigger from the first stage to the last. Many stages take out more energy.
  4. Gas turbine: the compressor squeezes air. The combustor burns fuel in it and makes very hot gas. The gas spins the turbine, which also runs the compressor.
  5. The exhaust gas is still hot. A heat-recovery boiler uses it to make steam for a second, steam turbine. This is a combined plant and wastes less heat.
  6. Free play: slide the fuel. More energy in makes the shaft spin faster and gives more power. Go back to compare steam and gas.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does a jet of gas spin a wheel?

The gas has fast-moving mass. When it hits and is turned by the blades it pushes on them, and a push on blades at the rim makes the wheel turn. Watch the red marker go round.

Where does the steam come from?

From the boiler. Fuel burns under the red box, the water inside turns to steam and flows along the pipe to the turbine.

Why not one big blade ring? Why many stages?

One ring cannot take out all the energy smoothly. Many stages take a little at a time, so the turbine is more efficient and the blades are not damaged. Look at the rings getting bigger.

Why does the gas turbine need its own compressor?

Air squeezed to high pressure burns fuel with a much bigger push. The turbine on the same shaft runs the compressor. See the blue air turn orange after the combustor.

Why is the exhaust still hot, and can we use it?

The gas cannot give all its heat to one turbine. The leftover heat can boil water for a second turbine. That is the combined cycle.

What happens if we give more fuel?

More energy goes in, so the hot gas pushes harder and the shaft spins faster and gives more power, until the metal limit. Try the slider.

What is a turbine engine?

A turbine is a wheel (rotor) with many curved blades. A fast stream of steam or gas hits the blades and pushes them round. So the energy of the moving fluid becomes spinning motion of a shaft.

Compared with a piston engine, a turbine has only one kind of motion: spinning. It runs smoothly, with little shaking, and a small turbine can give a lot of power. That is why turbines drive power-station generators, ships and aircraft.

Fixed blades (nozzles or guide vanes) between the moving blades steer the gas onto the next row.

Steam turbines

A steam power plant works in a loop (the Rankine cycle):

  1. Boiler: fuel heat turns water into high-pressure hot steam.
  2. Turbine: steam expands through the blades, pressure and temperature fall, the shaft spins.
  3. Condenser: the used steam is cooled and turns back to water.
  4. Feed pump: pushes the water back into the boiler.

The steam grows in volume as it expands, so the blades are bigger in each stage. Big plants have several stages (high, medium and low pressure).

Steam turbines are used where there is a steady heat source: coal, gas, nuclear or solar thermal power stations, and large ships.

Gas turbines

A gas turbine uses hot gas from burning fuel, not steam. It has three main parts on one shaft (the Brayton cycle):

  1. Compressor: sucks in air and squeezes it to high pressure. The blades get smaller as the air gets squeezed.
  2. Combustor: fuel is sprayed in and burns. The gas becomes very hot, at nearly the same pressure.
  3. Turbine: hot gas expands through the blades and spins them.

The turbine makes more power than the compressor needs. Net work = turbine work − compressor work. The extra goes to a generator, a propeller, or (in a jet engine) the gas leaving at the back pushes the plane forward.

Gas turbines start fast (minutes), are light and compact, and are used in aircraft, quick-start power plants and fast ships. Their exhaust is very hot, so it can be reused: the combined cycle adds a steam turbine run on the exhaust heat and reaches about 55 to 60% efficiency.

Efficiency and losses

Efficiency η = work out ÷ heat in. A modern steam plant is about 35 to 45%. A single gas turbine is about 30 to 40%. The rest is waste heat: the condenser water and the hot exhaust. Raising the temperature and pressure of the gas gives better efficiency, but the blades must survive the heat.

Shaft power: P = torque × angular speed = T × 2πN/60, with N in revolutions per minute.

Try it: a pinwheel

Make a paper pinwheel. Blow gently, then hard. Next, hold it above a hot cup of tea (careful) and watch rising steam turn it. Predict: which blows faster, the pinwheel or the 3D wheel at 20% fuel? Check with the slider: more energy in gives faster spin.

Key formulas and definitions

Worked examples

1. A steam plant takes in 100 MW of heat and gives 38 MW of electric power. Find its efficiency and the heat wasted.

η = 38 / 100 = 0.38 = 38%. Waste heat = 100 − 38 = 62 MW (mostly in the condenser).

2. In a gas turbine the turbine produces 60 MW and the compressor needs 25 MW. Find the net power.

Net = 60 − 25 = 35 MW. Notice that 25 MW of the turbine output only runs the compressor.

3. A turbine shaft turns at 3000 rpm with a torque of 100 N·m. Find the power.

ω = 2π × 3000 / 60 = 314.16 rad/s. P = T × ω = 100 × 314.16 ≈ 31 416 W ≈ 31.4 kW.

4. A gas turbine has efficiency 0.35. Its exhaust heat runs a steam cycle of efficiency 0.30. Find the combined-cycle efficiency.

η = 0.35 + 0.30 − 0.35 × 0.30 = 0.65 − 0.105 = 0.545, so about 54.5%.

Common mistakes

Practice quiz

1. In a steam plant the part that turns used steam back into water is the:
2. The correct order in a gas turbine is:
3. Why do steam turbine blades get bigger along the shaft?
4. Net work of a gas turbine is:
5. Which is best for quick start and light weight?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is the difference between a steam turbine and a gas turbine?

A steam turbine uses steam made in a separate boiler. A gas turbine uses hot gas made inside itself by burning fuel in compressed air.

How does a jet engine relate to a gas turbine?

A jet engine is a gas turbine. The turbine takes just enough power to run the compressor, and the fast hot gas leaving at the back pushes the aircraft forward.

What is a combined cycle plant?

It uses a gas turbine first, then uses its hot exhaust to boil water for a steam turbine. Two turbines from one fuel give about 55 to 60% efficiency.

Where this is taught

Japan高校（専門学科）1〜3年Prime Movers

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