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Thermal Power Plant
How Does a Steam Turbine Work?
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. These notes will explain the inner workings of the steam turbines, and why they are constructed in the manner the are in a step-by-step logical manner.
To understand the basic working principles of steam turbines we should firstly observe one of their blades.
We can see that the blades of a steam turbine have 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 we should understand fluent energy and greater death.
A fluid has three forms of energy:
Speed Pressure 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 stationery and attached to the turbine casing, so the flow area decreases along the stator and the speed thus increases.
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 blades. The tips of such long blades will have very velocity compared to the root. A twist is given to it so that all blades cross- sections will remain at an optimum angle of attack.
This kind of large turbine uses to such symmetrical units, you can see how mush the steam is equally divided between these units.
High capacity power plant uses different
stages of steam turbines such as:
High-Pressure Turbines
Intermediates Pressure Turbine
Low-Pressure Turbine.
All these units are attached to single rotating shaft, and the shaft in turn is connected to a generator.
The reason for such different staged is quire interesting:
With greater steam temperature comes greater power plant efficiency.
According to the second law of thermodynamics: CARNOT'S THEOREM
= −
But we cannot have temperature greater than 600° since the turbine blade material will not withstand temperature more than.
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.
And this is known as "Re-Heating".
"Re-Heating". 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.
