Simple Rankine cycle is an ideal vapour cycle. It plays a major role in steam power plants. This cycle is mainly based on the conversion of input heat energy into output power, using turbine. The working fluid at the inlet repeatedly undergoes change of phase and in successive operations, energy is transmitted and output power is produced. Simple Rankine cycle consists of the following processes:
 Process 12: Adiabatic expansion of steam (in turbine)
 Process 23: Constant pressure heat rejection (in condenser) and
 Process 34: Isentropic compression (in pump)
 Process 41: Heating at constant pressure (in boiler)
All the above processes are reversible in nature.
Simple Rankine cycle can be easily understood if we understand its hs Diagram. hs diagram and block diagram of simple Rankine cycle are provided below:
hs Diagram of Simple Rankine Cycle:
Block Diagram of Simple Rankine Cycle:
Processes in Simple Rankine Cycle:
Process 12 (turbine):
Here, dry saturated steam from the boiler is allowed to expand in a turbine isentropically (i.e., entropy remains constant).
Let h_{1} be the enthalpy of steam entering the turbine
Let h_{2} be the enthalpy of steam leaving the turbine
Work done by the turbine is given by
W_{T} = h_{1} − h_{2}
Process 23 (condenser):
Wet steam from the turbine is fed into a condenser, to perform condensation (wet steam is converted to water). As it is a heat rejection process , heat from the steam is rejected into atmosphere.
Let h_{2} be the enthalpy of steam entering the condenser.
Let h_{3} be the enthalpy of water leaving the condenser.
Heat rejected from condenser
Q_{R} = h_{2} − h_{3} W or
Q_{R} = h_{2} − h_{f2} W
Since, h_{f2} = h_{3} (the output from condenser is a fluid and graphically, the enthalpy at point 2 and point 3 are same.)
Process 34 (pump):
Water from the condenser is pumped into the boiler using an external pump. During this process, pressure increases P_{3} to P_{4} isentropically.
The enthalpy and temperature of water also increase due to pump work.
Let P_{3} ,h_{3} be the pressure and enthalpy at stage 3 respectively.
Let P_{4} and h_{4} be the corresponding values at stage 4.
Work done by the pump is given by
W_{p} = h_{4} − h_{3} = V_{f3} (P_{4} − P_{3}) W
where,
V_{f3} → Specific volume of saturated water at pressure P_{3} (condenser pressure)
Note :
All the values of pressure here are substituted in N/m^{2}. All values of enthalpy are substituted in J/kg.
Process 41 (boiler):
Here the saturated water from the pump is heated using a constant heat source (like furnace). The input saturated water is heated till it reaches superheated condition. The temperature and enthalpy raises to a great extent, but the pressure is kept constant. The change of phase from liquid to vapour occurs in boiler.
Let h_{4} be the enthalpy of saturated water entering the boiler.
Let h_{1} be the enthalpy of superheated steam coming out of the boiler.
The heat supplied is given by
Q_{S} = h_{1} − h_{4}
See hs diagram above for better understanding of all the above processes.
Efficiency of Simple Rankine Cycle:
As we know, efficiency is the ratio between output and input. Here the output is work done and input is heat energy.
Net work done = work done in turbine + work done in pump
Net heat transfer = heat produced in boiler + heat rejected in condenser
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