How steam turbines work?

No thermal power plant works without steam turbines, designed specifically to energy generation. Are this subject’s post.

Blades

The turbine’s blades are in shape of airfoil. To when the steam reaches the turbine, one side will have bigger pressure and the blade moves.

A steam turbine has many alternated stages of fixed and mobile blades.

Those stages are necessary to extract the maximum energy from the steam. The static blades (stator) increase the steam’s speed, because they act like nozzles. The temperature and pressure always fall even with increase of speed due to conservation of energy.

The mobile blades (rotor) can be two types: impulse or reaction. The impulse blades stay in the first stages where the pressure is bigger, while the reaction blades stay in last stages.

These are different types of blades used in steam turbines. Are made in a form that stay in the best angle to receive the steam.

Why the blades in the last stages are bigger? Because when the steam pressure falls, the volume expands proportionally. If all the stages were in the same size, the steam flux would be too fast and the blades could be damaged.

Turbines of different pressures

Many steam turbines have two symmetrical sides and the steam reaches the middle and are distributed equally to both sides. What is the finality to have two symmetrical sides? The steam flux creates an axial motion and this force needs to be balanced with an opposing direction force to the rotor stays in the right position.

It is common a thermal power plant have a low power turbine (LP), a intermediary turbine (IP) and another of high power (HP), all in the same shaft.

Why it is necessary? According to Carnot theorem, a thermal machine has bigger efficiency when bigger is the difference between the input and output temperatures of the machine.

$\eta =1-\frac{Tc}{Th}$

Where $\eta$ is the yield, $Tc$ is the lower temperature in output and $Th$ is the higher temperature in the input. However, blades have a maximum temperature limit of operation. After the first stage, the steam goes to a boiler to be reheated and return to the turbines.

Turbine control

To keep the steam turbine constant are used control valves in ducts. An electronic governor is used to control valves and consequently the speed turbine.