Which is the most practical fluid for the Rankine cycle?
The Rankine cycle is the most commonly used vapor power cycle.
In most practical systems, water is the working fluid for this cycle, and the produced vapor is steam.
There are a few systems that utilize an organic fluid, typically a hydrocarbon or ammonia, which produce the vapor of the organic fluid.
In such cases, the cycle is referred to as the organic Rankine cycle.
Water-steam Rankine cycles and their variations are the principally used cycles of all coal and nuclear power plants.
The processes contained in a Rankine cycle are:
A) (1-2) Isentropic compression
B) (2-3) Constant pressure heat addition
C) (3-4) Isentropic expansion
D) (4-1) Constant pressure heat rejection
In the above processes, the working fluid will undergo phase change during the heat addition and heat rejection process and the most practical fluid for the Rankine cycle is water because air cannot be used in the Rankine cycle and air-fuel, fuel is not the working fluids.
Improvement of Rankine cycle
The thermal efficiency of the Rankine cycle can be improved by increase the average temperature at which heat is transferred to the working fluid in the heating process, or decrease the average temperature at which heat is transferred to the surroundings from the working fluid in the cooling process.
Increasing the average temperature during the heat addition process increases the boiler pressure.
The maximum boiler pressure is limited by the tube metallurgical material problem in the boiler.
Increasing the boiler pressure increases the moisture content of the steam at the turbine exit which is not desirable.
Increasing the average temperature during the heat addition process without increasing boiler pressure can be done by superheating the steam to high temperature with a superheater.
Superheating the steam to a higher temperature also decreases the moisture content of the steam at the turbine exit which is very desirable.
Increasing the average temperature during the heat addition process can be accomplished with a superheater.
The moisture content of steam at the turbine exhaust can be decreased by reheating the steam between the stages of a multi-stage turbine.