Reversed Carnot Cycle
Sadi carnot in 1824 proposed a heat engine cycle as a measure of maximum possible conversion of heat into work.
All reversible heat engines working between the same two temperatures have the same efficiency irrespective of the amounts of heat that they draw from the source.
According to carnot’s theorem, no heat engine can work more efficiently than a carnot’s engine working between the same limits.
carnot cycle comprising of
1. Two reversible iso-thermal processes and
2. Two reversible adiabatic processes
A cyclic heat engine operating on the carnot cycle is called carnot heat engine. A reversible heat engine cycle is an engine operating in the reverse way (ie) receiving heat from a low temperature region, discharging to a unser such conditions, the cycle is called heat pump cycle or a refrigeration cycle.
A reversed carnot cycle can be employed as a refrigeration cycle which wouid be measure of maximum possible refrigeration cycle operating between two temperature limits.
The working substance assumed as air. The cycle consists of two reversible isothermals and two reversible adiabatics (isentropics) and its PV and TS diagrams as shown .
Process 1 – 2:
Air is compressed isentropically, where S1=S2 and T1 raises to T2
Process 2 – 3:
Air rejects heat isothermally to the hot reservoir at T2, where T2=T3.
Process 3 – 4:
Air is expanded Isentropically, where S3=S4 and T3 drops to T4.
Process 4 – 1:
Air absorbs heat isothermally from the cold reservoir at T1, where T1=T4.
From TS diagram
Heat absorbed from the cold reservoir
Q1= Area 1456 = T1 . ds
Q1 = T1 (S1– S4)
Heat rejected to hot reservoir
Q2 = Area 2356 = T2 . ds
Q2 = T2 (S2 – S3)
Therefore, the net work done
W = Q2– Q1 = (T2 – T1) ds
W = (T2– T1) (S1– S4)
W = Area 1234
The performance of a refrigerator working on a reversible carnot cycle is maximum. However, there is no refrigerating machine working on this cycle, because the adiabatic stroke needs high speed while the isothermal stroke needs extremely low speed, which are not practicable.