Nuclear and coal-based thermal power plants together produce almost half of the world's power. Steam turbines lie at the heart of these power plants. They convert thermal energy in the steam to mechanical energy. This video will explain the inner workings of the steam turbines and why they are constructed in the manner they are in a step-by-step, logical manner. To understand its basic workings, let's first observe one of their blades.
You can see that the blade of a steam turbine has an airfoil shape. When the high energy fluid passes over it, this airfoil shape will create a pressure difference. This will subsequently create lift force.
The lift force will rotate the turbine. In short, the energy in the fluid transfers to the mechanical energy of the rotor. To further understand steam turbine operation, let's understand fluid energy in greater depth.
A fluid has three forms of energy due to its speed, pressure, and temperature. As the blades absorb energy from the fluid, all three forms of energy come down. The low velocity jet is of no use to produce effective lift force. To increase velocity, the fluid is passed through a stator section. The stator set is stationary and attached to the turbine casing.
You can see that flow area decreases along the stator and the speed thus increases. In short, the stator acts like a nozzle. As the speed of the jet increases in the stator, kinetic energy increases.
As there is no net energy transfer in the fluid and stator section, the pressure and temperature of the jet should decrease to keep the total energy constant. Now, the next row of rotors is added. The stator also makes sure that the flow coming out of it will be at an optimum angle of attack to the next rotor set.
After that, another nozzle set is added. Many such sets are used in a steam turbine. There is an important term while designing steam turbines, namely degree of reaction. This term is calculated by dividing pressure and temperature energy by the total energy change in the rotor.
Pressure and temperature energy together is called enthalpy. The degree of reaction decides what type of steam turbine it is. As the pressure of the steam undergoes a drastic reduction during steam turbine operation, its volume increases proportionally. To accommodate such an expanded steam, we have to increase the flow area.
Otherwise, the flow speed will become too high. This is the reason why the steam turbine blades are too long towards the outlet. You can see how long the last stage turbine blades are compared to the first stage blades.
The tips of such long blades will have very high velocity compared to the root. A twist is given to it so that all blade cross sections will remain at an optimum angle of attack. This kind of large turbine uses two such symmetrical units.
You can see how the steam is equally divided between these units. High capacity power plants use different stages of steam turbines such as high pressure turbine, intermediate pressure turbine, and low pressure turbines. All these units are attached to a single rotating shaft. The shaft in turn is connected to a generator.
The reason for such different stages is quite interesting. With greater steam temperature comes greater power plant efficiency. This is according to the second law of thermodynamics.
But we cannot have temperature greater than 600 degrees Celsius, since the turbine blade material will not withstand temperature more than that. Temperature of the steam decreases as it flows along the rows of the blade. Consequently, a great way to increase power plant efficiency is to add more heat after the first stage.
So after the first stage, the steam is bypassed to the boiler and more heat is added. This is known as reheating. This will increase the steam temperature again, leading to higher power plant efficiency and output. One challenging problem in power plant operation is to keep the speed of the steam turbine constant. This is important since frequency of the electricity produced is directly proportional to the generator speed.
However, depending on the load or power demand, the steam turbine speed will vary. To keep the steam turbine speed constant, a steam flow governing mechanism is used. If the steam turbine rotates at a higher speed, the control valve will automatically reduce the steam flow rate to the turbine until the speed becomes normal.
If the turbine rotates at a low speed, the inverse will be done. In this way, the balance of power demand and power supply will be perfectly synchronized. To learn more about Degree of Reaction and its implications, please check the next video.
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