Rankine cycle is explained along with T-s, P-v, diagrams, reheat, etc. All the formulas and examples are well captured to have a basic idea.
Let’s learn the learn Rankine cycle with basic, definition, history.
The Rankine cycle is one of the popular cycles widely used by power plants. It is used to convert the pressure energy coming from the steam to mechanical energy with the help of steam turbines.
The Rankine cycle is named after the William John Macquorn Rankine professor at Glasgow university.
It is defined as a cycle, where heat energy is supplied to the system via the boiler where the fluid typically water is converted to high pressurized steam, and the steam is passed over the turbine and the work is generated.
The friction losses and others are neglected in the ideal Rankine cycle to for simplification of the calculation.
So, we will see a typical ideal Rankine cycle its processes, and some of its variations. Also, what changes are there when we compare the ideal with the real cycle?
Let’s see the diagram of the Rankine cycle and its main components.
There are four main components,
In the ideal Rankine cycle, there are four processes running, we will check each of them and what will happen in each process.
It can be understood with the help of the T-S diagram better.
In this process, the working fluid will be pumped from low pressure to high pressure. The fluid is in the liquid form at the start of this process.
The water is heated to the saturation temperature, keeping the entropy constant.
In case of this process the high-pressure liquid will enter the boiler. The fluid will be heated at the constant pressure with the help of an external heat source and it will become a dry saturated vapour.
The required energy needed for heating up can be calculated with the help of enthalpy entropy chart or steam tables.
The dry saturated vapour at the end of the process 2-3 will be expanded through the turbine in this process. It will generate the power.
Due to expansion the temperature and the pressure of the vapor will be decreased. Same as the above the output can be easily calculated.
At this process the low temperature and low pressure vapour will be condensed in the condenser. It will be converted into saturated liquid.
So, these are the four process in the ideal Rankine cycle.
Let’s see the P-v diagram,
The irreversibility’s due to the fluid friction, heat loss to the surroundings the Rankine cycle differs from the ideal one.
Losses like fluid friction drop the pressure in the boiler, condenser, and the piping between the components, and hence the network output is reduced.
Thus, more heat addition will be required for maintaining the network output and hence more fuel or charge.
The general efficiency equation of the simple Rankine cycle,
Thermal efficiency = WTurbine – WPump / QBoiler
Let’s consider the followings,
Then we can write,
The ideal Rankine cycle has no losses and its considered as a ideal power plant cycle. But in actual obviously there are losses and reduced work output.
As the water droplets will condense water droplets will heat the turbine at a very high speed and the turbine will get eroded, the blades and efficiency of the turbine will be reduced.
The solutions like superheating of steam are employed to overcome these problems.
So, to overcome the losses and get more net power output some variations are adopted in the Rankine cycle.
Some variations are discussed below, like
The reheating cycle work is to remove the moisture carried by the steam at the final stages of the expansion process.
The reheating prevents the vapor from condensing during the expansion, thus reduced the damage in the turbine blades and improves the efficiency of the cycle because there will be more heat flow into the cycle at higher temperatures.
Regenerative is named because the fluid emerging from the condenser usually a subcooled liquid, is heating by the steam tapped from the hot portion of the cycle.
As you can see in the image above.
In the thermal power plant, the Rankine cycle is used to produce electricity.
Here, water is pressurized by the pump, takes up the heat and produces vapor in the boiler, and later on, expands in the turbine to generate electricity.
The details of the thermal power plant are already captured in a separate article.
Hence, we have learned the basics of the Rankine cycle along with T-S, P-V, diagrams, reheat, regeneration, etc. Any questions, please let us know.