Rankine cycle or vapor and liquid cycle is a cyclic process on which steam power plant works. The main principle of Rankine cycle is to produce high pressure and high temperature steam which strikes the blades of turbine and produces maximum power at the output. Components used in the Rankin cycle process are turbine, condenser, pump and boiler.
Steam losses the pressure inside the turbine due to which turbine rotates with high speed. Turbine is coupled with generator which converts the mechanical energy of steam turbine into electrical energy.
The exhaust steam is sent to the condenser where steam is completely converted into water at constant pressure.
Pump is used to raise the pressure of water equal to the boiler pressure.
In boiler, high energy steam is generated by heating the water at constant pressure.
The block diagram of Rankine cycle process is as follows:
The ideal Rankine cycle operates in the four steps namely
Isentropic Compression 1-2 :
Isentropic compression is done from point (1-2) through the pump. When all of the steam becomes a saturated liquid (after passing through the condenser), pressure of the water is raised up to the boiler pressure by using the feedwater pump. This process is said to be isentropic.
Isobaric Heat Addition 2-3 :
In boiler, the fuel(coal) is burn to generate heat. Heat is added in the boiler in order to convert saturated liquid into steam. Addition of heat takes place at constant pressure.
Isentropic Expansion 3-4:
The dry saturated steam expands in the steam turbine and losses pressure inside the turbine. Turbine is coupled with alternator which converts the mechanical energy of steam turbine into electrical energy and generates power.
Isobaric Heat Rejection 4-1:
In this process, wet steam comes out of the turbine and enters in condenser. The exit steam rejects heat in the condenser at a constant pressure to become a saturated liquid. Saturated liquid fed back into feedwater pump.
Now, we will see that how to draw the Rankine cycle t-s diagram.
During the stage of 1-2, the pressure of liquid is raised by using pump. Hence pump performs a work in order to compress the water.
Heat is added to convert saturated liquid to saturated vapor at constant pressure. 2-2` curve shows the work of economizer which ensures that water is not cold so that the operation of boiler is no affected.
Steam losses pressure inside the turbine thereby allowing the turbine to do some useful work.
Heat rejection occurs in a condenser at constant pressure. Rejection of heat is represented by “Qout” in the above ts diagram.
Rankine cycle is a model that describes the performance of thermal based power plants. From the above discussion, we can say that working fluid (water) in a Rankine cycle follows a close loop. The operating fluid is continuously evaporated & condensed and the cycle continues in the same manner.