Simple vapour compression cycle and transcritical cycle are same but only Difference in Heat rejection Process. In the transcritical cycle process, the heat rejection takes place at pressures and temperatures above the critical point – that is, in the fluid region. A condition in the fluid region is often referred to as a gas condition. For the transcritical cycle process, the heat rejection is therefore called gas cooling and subsequently the heat exchanger used is called a gas cooler.

1. Prof.R.S.KOLHE
Prof. R.S. KOLHE
ME (HEAT POWER)
BE (MECHANICAL)
Transcritical
Refrigeration Systems

2. Properties of carbon dioxide
• CO2 : Natural substance
• Designation : R744
• Used as dry ice for
transport cooling
• Triple point : - 56.60 c
and 5.2 bar
• Critical point : 310 c 73.8
bar

3. Critical properties of selected
refrigerants
Refrigerant Critical pressure
(bar)
Critical temperature
(0 c )
R22 49.9 96.1
R134a 40.6 101.1
R404A 37.3 72
R410A 49 71.4
R600a 36.4 134.7
R717 (Ammonia) 113.3 132.3
R744 (Carbon dioxide 73.8 31

4. Tcr=101.1 0 c
Condensation
T atm =30 0 c (assume)
R 134a
Tcr=31 0 c
Condensation
T atm =30 0 c
R 744 (CO2)
Condensation process can not be
used
T supercritical =50 0 c
Gas cooler
T atm =30 0 c
R 744 (CO2)
Gas cooler can be used instead of
condensor
Condensation process can be used

5. Transcritical Refrigeration Systems
• Simple vapour compression cycle and transcritical
cycle are same but only Difference in Heat rejection
Process.
• In the transcritical cycle process, the heat rejection
takes place at pressures and temperatures above the
critical point – that is, in the fluid region.
• A condition in the fluid region is often referred to as
a gas condition.
• For the transcritical cycle process, the heat rejection
is therefore called gas cooling and subsequently the
heat exchanger used is called a gas cooler.

6. Transcritical cycle process
QE

7. Transcritical cycle process
• Process 1-2 :The transcritical cycle process begins with a one-stage
compression from state point 1 to 2. During this process, the temperature
rises significantly – and, for carbon dioxide, can reach a level of 130°C.
• Process 2-3:The heat rejection process from state point 2 to 3 occurs at
constant pressure above the critical point. The temperature during this
process varies continuously from the inlet temperature (at state point 2) to
the outlet temperature (at state point 3).
• Process 3 - 4: The expansion process from state point 3 to 4 occurs at
constant specific enthalpy. The inlet condition is supercritical (above the
critical point) and the outlet is two-phase (mixture of liquid and vapour).
• Process 4 - 1: The heat absorption process (evaporation) from state point 4
to 1 occurs at constant pressure, and the evaporation part also at constant
temperature. The outlet condition (compressor inlet condition) is slightly
superheated.

9. Simple vapour compression cycle

10. Transcritical cycle process
QE

11. Subcritical and Transcritical
Refrigeration cycle processes
Critical Point
Critical Point

12. Types of Transcritical cycle
• P h diagram
Transcritical cycle with throttle valve

13. Transcritical refrigeration system with internal heat
exchanger (suction line heat exchanger)

14. Transcritical refrigeration system with internal heat
exchanger and an automatic valve as expansion device

15. Transcritical refrigeration system with internal heat exchanger
and a thermostatic valve as expansion device

16. Two stage compression with an
intercooler
1
2
3
4
5
6

17. Two stage compression with flash gas
bypass

18. References
[1] “Theory and ”, M.M. Abbott & H.C. van Ness, Schaum’s outline series,
McGraw-Hill, Second edition, 1989.
[2] “Control of high-pressure in transcritical vapour compression cycle”, EP 0
604 417 B1
[3] “Method of operating a vapour compression cycle under trans- or
supercritical conditions”, EP 0 424 472 B2.
[4] “Refrigerating system with pressure control valve”, WO 5890370

19. Have a Good Day
19