HOW DO STEAM TURBINES WORK?

 

Nuclear and coal based thermal power plants together produce almost half of the world's electric power and steam turbines are the heart of these power plants. Steam turbines convert thermal energy into mechanical energy which inturn rotates the generator shafts to produce electricity. Charles Parsons first invented the modern steam turbine back in 1884.

WORKING PRINCIPLE OF STEAM TURBINE:

A steam turbine consists of multiple blades in airfoil shape. When a high pressure steam passes over it, a pressure difference is created due to the airfoil shape of the blade. The pressure difference creates a lifting force which rotates the turbine. Thus conversion of energy takes place that is from heat energy to mechanical energy. This is the basic working principle of a steam turbine and the turbine manufacturers rely on this for the best design.

HOW DOES A STEAM TURBINE WORK?

As the name implies that it is powered by steam, a high energy steam is injected into the turbine which transforms the potential energy of the steam into kinetic energy which causes continuous rotation of the blades. Anything that can flow is a fluid and hence steam is a fluid. A fluid has three forms of energy that are kinetic energy due to speed, pressure and temperature. All these three forms of energy get reduced when the turbine blades absorb and rotate. The low velocity will be incapable of generating enough lifting force to rotate the shaft. To overcome this challenge the blades are configured in such a way that the fluids are passed through the stator section. The stators are stationary and attached to the turbine casing and their design is such that the flow area decreases along the stator, that is the area of the stator where steam enters is wide and the area of the stator where steam exits is narrow, this design creates the increase in the speed of the steam just working similar to a nozzle. Thus the kinetic energy of the steam is revived. The whole turbine is configured by this design of blades and stators combination arranged adjacently. The stator is responsible to create an optimal angle of fire to the next blade. The overall design of the turbine is done to meet the required degree of reaction.

Degree of reaction = (Pressure Energy + Temperature Energy) change in the rotor / (Total Energy change in the rotor)

The degree of reaction is used to decide the type of turbine that is an impulse turbine or reaction turbine.

Since the volume of the steam gets increased upon reduction in the pressure within the turbine casing the turbine structure is designed with increased flow area towards the outlet. We can see that the steam turbine blades are long at the outlet whereas blades are too small at the inlet. Two such symmetrical units are attached on either side such that uniform distribution of steam takes place.

Depending upon the application different stages of steam turbine can be integrated such as high pressure turbine, low pressure turbine and intermediate pressure turbine. All these units may be attached to a common rotor which is connected to the generator to produce electricity.

EFFICIENCY OF A STEAM TURBINE:

The efficiency of a steam turbine depends on several factors like its type, application, stages etc. The modern steam turbines have efficiency of 80 - 90 %.

To increase the efficiency of the turbine, thermal power plants and industries utilize the same steam more than once by adding more heat after stage one. Here the steam is again fed to the boiler, the process is called reheating. This leads to a higher power efficiency of the steam turbine.

Other governors and turbine controllers are also integrated with the steam turbine to maintain the synchronous speed of the rotor. The rotor speed directly affects the frequency of the electricity generated.

Chola Turbo Machinery is one of the leading industrial steam turbine manufacturers in Bangalore, India. Contact us for all your industrial steam turbine needs.