It gives information about Advanced Vapor Compression Cycles: Trans-critical cycle and their types, Ejector refrigeration cycle and their types. Presentation of cycle on P-h and T-s chart.

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Mechanical Engineering
Subject:- Heating, Ventilation and Air
Conditioning (ME 411)
B.Tech.Mechanical
Purushottam W. Ingle
Assistant Professor

2. • Advanced Vapor Compression Cycles: Trans-critical cycle and their
types, Ejector refrigeration cycle and their types. Presentation of cycle
on P-h and T-s chart.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

3. Transcritical Cycle
• Due to harmful effects of the synthetic refrigerants on the
environment, CO2 has been revived as a potential refrigerant.
• Due to the low critical temperature of CO2, the gas cooler is operated
above the critical pressure and the evaporator is operated below that;
hence the cycle is called transcritical cycle.
• Along with eco-friendliness, CO2 systems have various advantages
over conventional systems such as, compatibility with normal
lubricants and common machine construction materials, non-
flammability and non-toxicity, greatly reduced compression ratio, easy
availability, high volumetric refrigerant capacity, and excellent heat
transfer properties.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

4. • The major disadvantage of CO2 cycle is lower COP due to huge
expansion loss compared to conventional refrigerants and hence the
cycle needs modifications.
• There are several reasons for modifying the basic single-stage
transcritical cycle, including improvement of COP, capacity
enhancement for a given system and component size, adaptation of
the heat rejection temperature profile to given requirements and
keeping the pressure ratio and discharge temperature of the
compressor within limit.
• Cycle modifications such as use of internal heat exchanger, expansion
turbine, multi-staging, two-phase ejector, vortex ntube and parallel
compression economization.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

5. TRANSCRITICAL CO2 CYCLE AND OPTIMUM DISCHARGE PRESSURE
• In the transcritical
refrigeration cycle (Fig. 1),
the vapor from evaporator is
compressed to supercritical
(gas cooler) pressure (1-2),
rejected heat at gliding
temperature (2-3) and
expanded (2-3) from
supercritical pressure to
subcritical (evaporator)
pressure to give cooling (4-
1).
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

6. • For the conventional cycle, condensing temperature is chosen based
on coolant temperature in the condenser and corresponding
saturated pressure is taken as the condensing pressure.
• But for supercritical heat rejection no saturation point exists, so the
gas cooler pressure is independent of the refrigerant temperature at
gas cooler exit (point 3 in Fig. 1).
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

7. • 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.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

8. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

9. 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.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

10. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

11. Simple vapour compression cycle
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

12. Transcritical cycle process
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

13. Types of Transcritical cycle
• Transcritical cycle with throttle valve
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

14. Transcritical refrigeration system with internal heat exchanger (suction
line heat exchanger)
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

15. • Enhancement on cycle efficiency can be a substantial 25%.
• The internal heat exchanger can increase the cooling capacity and
COP up to 11.9% and 9.1%, respectively.
• COP may improve by 10% by using internal heat exchanger in
residential application. Through the experimental study on CO2
residential air-conditioning system with internal heat exchanger.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

16. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
Expansion with work recovery

17. • Another option to reduce the expansion losses is the use of a work
producing device (expander), which has potential for COP
improvement. Enthalpy decreases in expansion process (process 3-4
in Fig. 4) and hence the net work requirement reduces and cooling
effect increases, which will improve the COP of the cycle.
• Work recovery turbine with isentropic efficiency of 60%, may reduce
contribution to total irreversibility by about 35% and cause an
average increase of COP by 25%.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

18. MULTISTAGE CYCLE
• Performance of the CO2 transcritical cycle can be improved by using
multistage compression with inter-cooling.
• By using ‘subcooling’, the COP could be improved while the capacity
increased and the necessary high pressure reduced.
• The subsequent theoretical and experimental investigations on the
multistage transcritical CO2 cycles for refrigeration/heat pump and air
conditioning showed the significant performance improvement over
the basic single stage cycle.
• Several types of configurations such as flash gas removal, flash gas
inter-cooling, compression inter-cooling for a multistage transcritical
CO2 cycle can be adopted to improve the system performance
depending on the requirement.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

19. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

20. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

21. Two stage cycle with Intercooler
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

22. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

23. Ejector refrigeration cycle
• In transcritical CO2 cycle, regenerating expansion energy and
increasing refrigerant pressure by means of an ejector is an effective
way to improving the COP. In addition, the ejector simplifies the
process of controlling the gas cooling pressure in the CO2 cycle.
• The gas cooling pressure of the CO2 cycle could be controlled by
changing the throat area of ejector nozzle.
• Ejector refrigeration is similar to normal vapour compression system,
only compressor is replaced by liquid circulation pump, boiler and
ejector.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

24. • Due to heat added in the boiler evaporated refrigerant goes in high
temperature and pressure by using low grade heat energy.
• The high pressure refrigerant expand through the primary nozzle in
the ejector and create very low pressure region at the primary nozzle
exit plane.
• Low pressure allows a liquid refrigerant in the evaporator to vaporize
at low temperature to create refrigerating effect.
• Evaporated fluid (secondary fluid) will mix with primary fluid in mixing
chamber of ejector, the mix stream is discharged through diffuser to
condenser where vapour is condensed.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

25. • Liquid refrigerant which accumulated in condenser is return back to
the boiler by pump and other remaining fluid expanded through
throttling valve to the evaporator to complete the cycle.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

26. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

27. 9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

28. • In ejector expansion refrigeration cycle (Fig. 6), the primary flow from
the gas cooler (state 3) and the secondary flow from the evaporator
(state 8) are going through primary and secondary nozzles,
respectively.
• Constant pressure/area mixing chamber (p-h diagram of CO2 cycle
with constant pressure mixing ejector is shown Fig. 7) and diffuser
(10-5) of the ejector and then separated in forms of vapour (state 1)
and liquid (state 6) so that this ratio should matched with the inlet
ratio of primary and secondary flows.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

29. • Then the liquid
circulates through
expansion valve (6-
7) and evaporator
(7- 8), whereas the
vapour circulated
through compressor
(1-2) and gas cooler
(2-3).
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

30. • cooling capacity and COP simultaneously improve by up to 8% and
7%, respectively compared to conventional valve expansion system,
and ejector is able to recover up to 14.5% of the throttling losses.
• Experiments confirmed that the high-side pressure control integrated
into the ejector could be used to maximize the system performance.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

31. Single Ejector Refrigeration System with precooler and pre-heater
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

32. • The liquid refrigerant returning to generator is pre-heated by using
hot refrigerant arriving from ejector exhaust.
• The liquid refrigerant cooled by pre-cooler using cold vapour
refrigerants leaving the evaporator before reaching the evaporator.
• Refrigerants arriving from condenser cooled and heated before
passing through boiler.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

33. SERS combined with power cycle
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

34. • If we required multiple output from one system then it will be
congeneration and generation.
• This system widely applied for solving present challenges at small
scale.
• Primary flow of ejector is turbine discharge and secondary flow is
evaporated fluid from evaporator mixed each other at mixing
chamber of ejector.
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon