Reverse Carnot Cycle

Reverse Carnot Cycle refrigerationgyaan.com/reverse-carnot-cycle October 10, 2019

Published by skhattar36@gmail.com on Reverse Carnot Cycle is a reversible cycle, it is used as an example of a refrigeration cycle operating between a constant temperature heat source and sink. On this basis, various refrigeration cycles are being compared.

Similarly, a refrigeration working on reverse Carnot cycle, has a maximum COP. It is not possible to make the refrigerating machines working on the principle of reversed Carnot cycle.

Watch Video At: https://youtu.be/PBVK3NZd5bA

A reversed Carnot cycle uses air as a working medium as shown in Fig. 1 and Fig 2 as a Reverse Carnot Cycle working and T-S diagram.

Fig 1 Reverse Carnot cycle

Process of Reverse Carnot Cycle Four processes of the cycle are as follows.

Process 1-2: Isentropic Compression.

Process 2-3: Isothermal Compression.

Process 3-4: Isentropic Expansion.

Process 4-1: Isothermal Expansion.

1/3

Fig. 2 T-S diagram of Reverse Carnot Cycle

Let p1, v1, and T1 be the pressure, volume, and temperature of the air.

1. Isentropic Compression: The air is compressed isentropically as shown by the curve1-2 in the T-S diagram. During this process, the pressure increases from p1 to p2 and the specific volume decreases from v1 to v2 and the temperature increases from T1 to T2. During this process Entropy ‘s’ remains constant (s1=s2). No heat is observed or rejected by the air.

2. Isothermal Compression: Air is compressed isothermally at a constant temp T2=T3 as shown by the curve 2-3 in the T-s diagram. During this process, the pressure of the air increases from p2 to p3, specific volume decreases from 2 to v3 and the temperature remains constant T2=T3.

Heat rejected by the air: qr = T3(S2-S3) = T2(S2-S3) where qr denotes the heat rejection.

3. Isentropic Expansion: Air is expanded isentropically as shown by the curve 3-4 in the T-s diagram. Pressure of the air decreases from p3 to p4, specific volume increases from v3 to v4 and the temperature decreases from T3 to T4. No heat is rejected or absorbed by the air.

4. Isothermal Expansion: Air is compressed isothermally at a constant temp T4=T1 as shown by the curve 4-1 in the T-s diagram. The pressure of the air decreases from p4 to p1, specific volume increases from v4 to v1.

Heat absorbed by the air: qa = T4(S1-S4) = T1(S2-S3) where qa denotes the heat absorption.

Work Done Work done during the cycle per kg of air 2/3

work done = Heat rejected – Heat absorbed = qr – qa = T2(s2-s3) – T1(s2-s3) = (T2 – T1)(s2 – s3)

C.O.P Coefficient of performance of the system working on the Reverse Carnot cycle.

C.O.P = Heat absorbed/ work done = qa/qa – qr = T1( S1 – S3 )/ ( T2 – T1 ) (S1 – S3 ) = T1/ T2 – T1 Reverse Carnot cycle is most efficient between the fixed temperature limits yet no refrigerator could be made using this cycle. This is due to the fact that the isentropic process requires high speed and isothermal process requires extremely low speed

3/3