THE CARNOT VAPOUR CYCLE
The Carnot vapour cycle is used as an ideal power cycle for steam power plants.
Consider an ideal Carnot cycle as shown is the Figure.
- Here the working fluid is water, in the cyclic process 1-2, water is heated reversibly and isothermally in a boiler, in the process 2-3 it is expanded isentropically in a turbine, in the process 3-4, then expanded water is condensed reversibly and isothermally in a condenser and finally in the process 4-1, the fluid is compressed isentropically by a compressor to the initial state.
The four Carnot cycle processes are :
• Isothermal heat addition from process 1-2,
• Isentropic expansion of steam in an expander from process 2-3,
• Isothermal heat rejection in the condenser from process 3-4, and
• Isentropic compression of a mixture of vapour and liquid from process 4-1.
By considering 1 kg of water as working fluid for a vapour power cyclic process
Heat added = Q1, which is shown on T-S diagram as area (1-2-2′-1′-1) = T1 ΔS,
ΔS = change in entropy.
Heat added = Q2, area (3-2′-1′-4-3) on T-S diagram = T2 ΔS
Net Work done = W = Q1 – Q2 = T1 ΔS – T2 ΔS = (T1 – T2) ΔS