A steam turbine is a mechanical device that converts thermal energy from steam into mechanical energy. The steam generated from the boiler is heated to a very high temperature and pressure before it passes over the turbines at high speeds. The steam strikes the turbine blades at high speeds causing it to rotate on a shaft. The rotation of the turbine drives a generator which produces electricity. Most electricity generated is by steam turbines which act as the prime mover. They are used for power generation in power plants and for ship propulsion where it generates electricity onboard to power the ship.
Types of steam turbines.
1. Impulse Turbines
2. Reaction Turbines.
Impulse turbines consist of fixed nozzles that cause the steam flowing through them to exit at high speeds. These jets of steam contain significant amounts of kinetic energy which act on the turbine blades shaped like buckets. As the steam flows through the nozzles its pressure falls from inlet pressure to exit pressure. Due to the high ratio of expansion of steam the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle.
In reaction Turbines the rotor blades are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam passes through nozzles formed by the rotor. Steam is directed to the rotor by fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both stator and rotor, with steam accelerating through stator and decelerating through rotor, with velocity remaining constant across the stage, but pressure and temperature decrease, reflecting the work performed in driving the rotor.