Attaining High Energy Efficiency with Less Materials Using
Smaller-Diameter, Inner-Grooved Copper Tubes
Vol. 14, Issue 1A


Laboratory Experiments Advance Knowledge of Copper Tubes, Coils, Refrigerants and System Performance

The First International Congress of Refrigeration (ICR) was organized in Paris in 1908, a few months after the creation of the nonprofit Association Française du Froid (AFF). The 26th ICR convened again in Paris in 2023. While there have been many Congresses in the intervening 115 years, perhaps none have been more consequential than this latest Congress.




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The ICR remains the premier international conference for sharing research on all aspects of the vapor compression cycle, including residential, commercial and industrial applications of Refrigeration, AC and Heat Pumps (RACHP), and their effects on comfort, the global food chain and industrial production.


Furthermore, at this Congress, another issue was at top-of-mind, namely, the climate crisis and the environment.

For these reasons and more, our review of select papers (which began in the previous issue the MicroGroove Update newsletter) continues here!

Five Sections, Ten Commissions

L'Institut International du Froid / International Institute of Refrigeration (IIF/IIR) is organized into five sections (A, B, C, D and E), which are further split into ten commissions. Table 1 includes links to website landing pages for select Commissions.

Table 1 — Select Commissions (Sections B & E) and Sub-Commissions (B1, B2, E1 and E2) in this Article

Section A

Cryogenics and liquefied gases

A1 Cryophysics & Cryoengineering

A2 Liquefaction & Separation Of Gases

Section B

Thermodynamics, equipment and systems

B1 Thermodynamics and Transfer Processes

B2 Refrigerating Equipment

Section C

Biology and food technology

C1 Cryobiology, Cryomedicine & Health Products

C2 Food Science & Engineering

Section D

Storage and transport

D1 Refrigerated Storage

D2 Refrigerated Transport

Section E

Air conditioning, heat pumps and energy recovery

E1 Air Conditioning

E2 Heat Pumps and Energy Recovery

The previous issue of the MicroGroove Update newsletter (Volume 13, Issue 2) reviewed select research papers from Commission E2 on Heat Pumps and Energy Recovery. This article reviews select papers from Commissions B1, B2 and E1 as well as a few more papers from E2.

Each of the 99 Technical Sessions typically included five individual presentations on the topic of the TS. Additionally, posters lined the hallways of the Congress. Eighty five Works in Progress and Innovations were gathered into a Book of Abstracts.

Table 2 Select Technical Sessions from Commission B1

on Thermodynamics & Transfer Properties

Technical Sessions

Keynote ‎


Components, Systems


Condensation | PCM & Slurries ‎


Thermophysical Properties

28, 53, 73

Heat Exchangers


Two phase flows and phenomena


Heat Exchangers | Heat Pumps ‎




Commission B1 on Thermodynamics and Transfer Processes

B1 is perhaps the most consequential of the commissions, since the research can apply to several RACHP product categories. Table 2 lists select Technical Sessions from Commission B1.


The keynote was delivered by Laura Fedele from the National Research Council, Construction Technologies Institute, Padova, Italy. Her paper on Knowing properties of fluids to understand their potentialities in RACHP sector (Paper 1149) provides a high-level meta-analysis of trends and literature in the development of primary and secondary fluids with emphasis on low GWP refrigerants and mixtures as well as phase change materials and nanofluids.


Environmental Impacts of HFO Blends


Besides the keynote, a review paper by Laura Fedele and four coauthors was presented on the first day of the Congress in Technical Session 11 on Components, Systems (Paper 0442).


In phasing down HFCs, the environmental impacts of widespread adoption of HFO blends should not be downplayed. One measure of the environmental impacts is the Total Equivalent Warming Impact (TEWI). TEWI includes not only the GWP of the refrigerant molecules in the atmosphere but also is a measure of the greenhouse gas (GHG) emissions associated with the use and disposal of a particular piece of refrigeration equipment. It includes the environmental effects of the manufacture and disposal of the HFOs.


Aside from the effects on the atmosphere, Fedele et al. examined how HFOs in the atmosphere break down into toxic chemicals (PFAS) that can contaminate drinking water (Paper 0442). As the industry contemplates the adoption of HFOs, it is essential to first investigate their potential environmental effects.


Zeotrope Mixtures Inside Microfin Tubes


The term azeotropic refers to a mixture that boils at one temperature. In other words, the liquid and the vapor have the same composition. It comes from the Greek word zein, meaning to boil, hence azeotropic means no change when boiled. Conversely, the components of a non-azeotropic or zeotropic mixture have different boiling points, that is, the components don’t evaporate or condense at the same temperature or behave as one substance.


For refrigerants, a zeotropic mixture exhibits a temperature glide, which means that the phase change occurs within a temperature range of about four to seven degrees Celsius rather than at a single temperature. The names and compositions of many new zeotropic mixtures are given in a UNEP update on ASHRAE 34.


Refrigerant mixtures and temperature glide were the subject of much research presented in the B1 Commission.


TS 25 on Condensation | PCM and Slurries included several investigations of temperature glide in smaller diameter copper tubes. Such research is essential for developing the tube correlations for heat exchanger simulations. Tube correlations provide empirically measured Heat Transfer Coefficients (HTCs) for different conditions of temperature, pressure and quality. Typically, this research is performed on smaller diameter copper tubes with and without microfins, i.e., internal enhancements, or inner-grooves.


For example, Mainil et al. studied the two-phase flow, condensation heat transfer of R454C (a zeotropic mixture) inside 3.5 mm OD copper tubes with microfins (Paper 0747). They reported on the effects of mass velocity and vapor quality. HTCs were measured for mass velocities from 50 to 200 kg /m2s, vapor qualities from 0.1 to 0.9, and saturation temperatures of around 20 °C.


Similarly, Liu et al. studied condensation heat transfer of two mixtures (R450A and R454B) inside 5.0 mm OD copper tubes with microfins (Paper 0931). R450A is a zeotropic mixture made of R1234ze(E) and R134a (0.58/0.42 by mass), whereas R454B is a zeotropic mixture made of R32 and R1234yf (0.689/0.311 by mass). The comparison between the two mixtures revealed that compared to R454B the HTCs for R450A were about 15 percent higher at high vapor qualities. However, R450A shows higher frictional pressure gradient.


Hirose et al. studied zeoropic refrigerant mixtures inside 4 mm tubes (Paper 0418). They proposed a new heat transfer correlation for refrigerant mixtures at condensation. They studied four compositions of R32/R152a and two compositions of R32/R1234ze inside 4 mm OD microfin tubes. An average saturation temperature of 35 °C was used for the experiment. The proposed correlation is for a pure refrigerant that is corrected by a coefficient F, which represents the effects of mass, velocity and quality.


Also, in TS 25, Longo et al. studied the condensation heat transfer of hydrocarbons (Paper 0708). New experimental data was collected during propylene condensation inside a 4.2 mm ID copper with internal microfins. (Note that propylene or propene differs from propane in that it contains one double bond.)


Modeling Mixtures


Refrigerant properties need to be well understood and quantified to simulate and build efficient RACHP products. In recent years, there have been dozens of refrigerant mixtures introduced as candidates for replacing HFCs in a multitude of RACHP applications.


Commission B1 Technical Sessions 28, 53 and 73 were dedicated to the theoretical modeling of refrigerant properties, including mixtures.


Refrigerant blends seek tradeoffs between properties such as flammability, temperature glide and GWP. One theme of the papers in Commission B1 is to theoretically model these tradeoffs according to the composition of the mixtures. For example, R1234yf is a hydrofluoroolefin (HFO) with an ultralow GWP. R32 is a hydrofluorocarbon with less flammability but higher GWP. Propane has high flammability (A3) and ultralow GWP. An R290-HFO mixture would have an ultralow GWP and be less flammable than R290.


Akasaka modeled mixtures of R1234yf with R290 (Paper 0662). The model predicts the bubble point pressure and single-phase density within the range of experimental uncertainties.


Similarly, Imai et al. use molecular simulations to demonstrate sufficient prediction accuracy for R1132a and R1123/propane (Paper 0877). They also newly investigated the vapor-liquid equilibrium for R1132a/CO2, a candidate refrigerant blend for ultra-‎low temperature application.


Kawahara et al. combined measurements of surface tension with molecular simulations (Paper 0880). Their focus was on R1132a (1,1-difluoroethylene, CH2=CF2) and a binary mixture of R1123 (trifluoroethylene, CHF=CF2) with propane. R1132 is a low GWP candidate for low-temperature applications; and R1123/R290 for the air conditioning temperature range. The simulation data overlaps with the surface tension data obtained by the experiment (using a differential capillary rise method).


Besides refrigerant blends, mixtures include refrigerant oil mixtures. The International Copper Association China collaborated with the Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University in a study of the refrigerant-oil mixtures inside smaller diameter tubes (Paper 0063). This paper was presented in TS87 on Boiling. The corresponding author for this paper is Professor Guoliang Ding.

Table 3 Select Technical Sessions from Commission B2 on Refrigerating Equipment

Technical ‎Sessions


‎07, 30‎

Refrigerants and Refrigerant Mixtures

35, 78‎

Energy Efficiency ‎



14, 51

Environmental and Societal Footprint


Commission B2 on Refrigerating Equipment


The B Section on Thermodynamics, equipment and systems encompasses the B1 Commission on Thermodynamics and Transfer Properties as well as the B2 Commission on Refrigerating Equipment. Not surprisingly there is considerable overlap between B1 and B2. Table 3 lists select Technical Sessions from the B2 Commission.


Advances in CO2 Technology


Refrigerating equipment using CO2 as a refrigerant remains at the frontier of research. Two B2 Technical Sessions were dedicated to CO2. That is in addition to the three Technical Sessions on CO2 that were part of Commission E2 on Heat Pumps and Energy Recovery as covered the a previous issue of the MicroGroove Update.


Researchers from SINTEF and the Norwegian University of Science and Technology (NUST) were already mentioned for their research papers on CO2 presented in the E2 Commission. As previously noted, Armin Hafner has figured prominently in cutting-edge research on CO2. For the B2 commission he appears as a coauthor on Papers 1538, 1546 and 1625.


The B2 Commission included many papers on refrigerants and refrigerant blends, including new research on R744 mixtures as covered in Technical Sessions on Refrigerants and Refrigerant Mixtures. See Papers 1864, 1973, 1746, and 2028.


It can be seen therefore that new research on CO2 figured prominently at the ICR 2023. CO2 holds great promise for low-GWP refrigerating systems and that message was loud and clear at ICR 2023.

Commission E1 on Air Conditioning


The air conditioning sector is facing demands for improvements in efficiency and the adoption of low-GWP refrigerants. The keynote presentation by Professor Xianting Li and Chenjiyu Liang from Tsinghua University in Beijing says it all: Can we further improve the energy efficiency of air conditioning system significantly? According to these authors, a comparison of the existing air-conditioning system with an ideal air conditioning system shows that the energy consumption of an existing air conditioning system can be decreased by 59 percent after eliminating the efficiency losses.

Table 4 lists select Technical Sessions from Commission E1 on Air Conditioning. Besides providing guidance on energy-efficient air conditioning systems, Commission E1 includes design studies of air conditioners that use smaller diameter copper tubes.


In TS 24, for example, Utage et al. present a performance simulation of an inverter-operated split air conditioner using HFC-161 as an alternative refrigerant to HC-290 and HFC-32 (Paper 0327). The parameters were optimized to maintain the original cooling capacity: evaporation temperature 7 °C, condensation temperature 45 °C, and subcooling temperature 8.3 K. The total superheat was 11.1 K and the compressor displacement was 21 cm3. Copper tube diameters were 5 mm and 9 mm for the condenser and evaporator, respectively, with a capillary tube diameter of 1.65 mm. Tube length was 650 mm. The system gave a cooling capacity of 5.105 kW with a power consumption of 1.450 kW, exhibiting the EERIS of 4.78. The charge was optimized at 450 g and the discharge temperature was lowered in the range of 6 °C to 15 °C compared to HC-290 and HFC-32.


Also in TS24, Phadake et al. replace a condenser that has horizontally orientated copper tubes with a falling film condenser that has vertically oriented inner-grooved copper tubes. (Paper 370). In both cases, the condenser tubes are 5 mm in diameter. The goal is to achieve the same cooling capacity and EER with the same boundary conditions. A 12 percent reduction in R290 refrigerant charge was achieved with the falling film design. The tubes are inner-grooved copper tubing with a 5 mm outside diameter. The condenser uses 52 tubes copper tubes in two rows with aluminum fins.


As could be expected, Commission E1 on Air Conditioning included a broad range of papers on diverse topics, including papers on low-GWP refrigerants, Energy Efficiency and Optimal Design and Control. The papers are mainly on systems and applications. In many cases, smaller diameter copper tubes are part of the solution.

Additional Papers from Commission E2 on Heat Pumps and Thermal Recovery


The ICR 2023 Part One article titled Heat Pumps Take Center Stage in Paris focused on select papers from Commission E2 on Heat Pumps. (See MicroGroove Update, Volume 13, Issue 2.)

Here are a couple more papers from Commission E2 that specifically mention smaller diameter copper tubes in heat pump applications.


In E2 TS 96 on Refrigerant charge assessment or reduction, Marcel van Beek and Thijs van Gorp described the design process and final design of a demonstration prototype air-to-water heat pump using a low refrigerant charge of R-290 (Paper 831). The prototype was built using commercially available components. Evaporator dimensions are 800 mm x 360 mm x 52 mm (Height x Width x Depth) using 5 mm OD copper tubes three rows deep and with 40 tubes per rows in height. The coil has 10 parallel circuits. On the airside, the fin pitch is 1.8 mm and there are two five-bladed, electronically commutated (EC) fans with a diameter of 350 mm. This prototype design has a measured nominal heating capacity of 3.4 kW and a measured system COP of 4.5 at A7/W35.


In the Poster Session 4, Vasu et al. optimized the circuitry of an indoor unit of a heat pump (Paper 0528). The design used 5 mm OD copper tubes with R466A. The paper gives the geometrical specifications of an optimized fin-tube heat exchanger that could be used as the indoor unit of a mini-VRF heat pump. A performance simulation was carried out according to the circuitry of fin-tube heat exchangers. Compared to the reference circuitry, the best circuitry for the indoor unit of the heat pump has an increase in heat capacity of 10.5 percent. The performance simulation program was developed based on tube-by-tube method.

Table 4 Select Papers from Commission E1 on Air Conditioning

Technical Sessions

Refrigerants. Equipment ‎

76, 24

Energy Efficiency (Keynote)


Energy Efficiency | Systems and applications (Includes PCM)


Optimal design & control | Systems and applications


Advanced Heat Exchangers from the LU-VE Group


Among the many excellent papers in the E2 Commission on Heat Pumps & Energy Recovery was the paper titled Geometry miniaturization in fin-and-tube heat exchangers for refrigerant charge reduction presented by Stefano Filippini from the LU-VE Group [7].

The paper was presented on the last day of ICR 2023 in TS 96 on Refrigerant Charge Assessment and Reduction. It provides a close look at the new heat exchanger geometry using very small diameter tubes (i.e., 4 mm diameter copper tubes). Experiments set up to validate the theoretical analysis show good agreement between theory and experiment regarding heat transfer performance as well as air pressure loss. This paper will be featured in an “In the Spotlight” column on the LU-VE Group in a future issue of the MicroGroove Update newsletter.



That brings us to the end of this sampling of papers from the quadrennial International Congress of Refrigeration. Credit is due to the Association Française du Froid (AFF) for bringing together hundreds of researchers from academia and industry to share their latest accomplishments in RACHP. It is truly amazing to see the technology advance, step-by-step, year-after-year. This newsletter can only provide a glimpse of the countless advances. This latest ICR has succeeded in providing a window into the voluminous research from laboratories around the world.

It will be interesting to know what advances will be made between 26th ICR held in Paris in 2023 and 27th ICR to be held in South Korea in 2027!


Noteworthy Papers from B1 Commission on Thermodynamics and Transfer Processes





Dr. Laura Fedele, Knowing properties of fluids to understand their potentialities in RACHP sector.

Components, Systems



Dr. Laura Fedele, Dr. Silvia Trini Castelli, Dr. Piera Ielpo, Prof. Claudio Zilio, Dr. Sergio Bobbo, The environmental impact of HFOs from TEWI to PFAS. A review.


Condensation | PCM and Slurries



Afdhal Kurniawan Mainil, Hakimatul Ubudiyah, I Wayan Sugita, Keishi Kariya, Akio Miyara,Condensation of R450A and R454B inside a compact microfin tube.



Yuce Liu, Luisa Rossetto, Andrea Diani, Development of the correlation condensation heat transfer for nonazeotropic refrigerant mixtures inside 4mm OD small-diameter microfin tubes



Masataka Hirose, Daisuke Jige, Norihiro Inoue, Condensation Heat Transfer of Hydrocarbons Inside a Compact Horizontal Microfin Tube



Giovanni A. Longo, Simone Mancin, Giulia Righetti, Claudio Zilio, Investigation on Condensation Heat Transfer of R454C inside Small Diameter Microfin Tube.


Thermophysical Properties



Ryo Akasaka, A Thermodynamic Property Model for R1234yf and Propane Mixtures.



Tomoaki Imai, Takehiro Miura, Masato Hashimoto, Tetsuya Okumura, Chieko Kondou, Molecular simulation for property prediction of low GWP refrigerant mixtures.



Takemasa Kawahara, Ryutaro Nonaka, Tetsuya Okumura And Chieko Kondo, Surface tension measurement with assistance of a molecular simulation for low GWP refrigerant mixtures.





Mr. Guang Li, Dr. Dawei Zhuang, Ms. Liyi Xie, Dr Prof. Guoliang Ding, Mr. Yifeng Gao, Mr. Ji Song, Effect of miscibility on flow boiling heat transfer characteristic of refrigerant-oil mixture inside small diameter ‎tube.


Noteworthy Papers from B2 Commission on Refrigerating Equipment





Pierre Barroca, Armin Hafner, Kacper Kuczynski, Pierre Hanf, Bart Verlaat

Performance and operation limits of a Remote Ultra-Low Temperature R744 transcritical chiller



Håkon Selvnes, Sigmund Jenssen, Alexis Sevault, Jan Bengsch, Kristina Norne Widell, Marcel Ulrich Ahrens, Shuai Ren, Armin Hafner, Integrated CO2 refrigeration and heat pump system for a dairy plant: Energy analysis and potential for cold thermal energy storage.



Stefanie Blust, Pierre Barroca, Pierre Hanf, Paolo Petagna, Bart Verlaat, Armin Hafner, R744 cooling technology at ultra-low temperature.


Refrigerants and Refrigerant Mixtures



Rafael Larrondo, Francisco Vidan-Falomir, Michal Haida, Daniel Sánchez García, Jacek Smolka, Ramon Cabello López, Experimental evaluation of the novel R744/R1270 blend in a transcritical refrigeration plant



Francisco Vidan-Falomir, Rafael Larrondo, Daniel Sánchez García, Manel Martínez-Angeles, Daniel Calleja-Anta, Laura NebotAndres, Rodrigo Llopis Doménech, Ramon Cabello López, Experimental evaluation of alternative CO2-based blends for transcritical refrigeration systems.



Emanuele Sicco, Manel Martínez-Angeles, Gabriele Toffoletti, Laura Nebot-Andres, Daniel Sánchez García, Giovanni Cortella, Rodrigo Llopis Doménech,

Experimental evaluation of different refrigeration system configurations using CO2-based blends as refrigerants.



Morgan Leehey, Stephen Kujak, Chemical Stability of HFO and HCFO Olefin Refrigerants and Their Potential Mechanistic Breakdown Pathways.


Ana Paez, Benedicte Ballot-Miguet, Pascal Tobaly, Flammability classification and advantages of CO2/propane over pure CO2 in cooling systems.

Noteworthy Papers from E1 Commission on Air Conditioning

TS31 Energy Efficiency ‎


1157 (Keynote)

Prof. Xianting Li, Mr. Chenjiyu Liang, Keynote lecture: Can we further improve the energy efficiency of air conditioning system significantly?



Kohei Terashima, Tamaki Ito, Nao Kuniyoshi, Mitsuo Kojima, Tatsuo Nagai, Haruki Sato, Possibility of Cooling Using PV/T Solar Panels with an Ejector Refrigeration Cycle.


Wentao Wang, Chenjiyu Liang, Xianting Li, Energy-saving effect of grading treatment for fan coil unit and dedicated outdoor air system.


Refrigerants, Equipment ‎



Ashish Utage, Heramb Phadake, Kundlik Mali, Performance Simulation of an Inverter Operated Split Air Conditioner using HFC-161 as an Alternative ‎Refrigerant to HC-290 and HFC-32.



Heramb Phadake, Ashish Utage, Kundlik Mali, Effect of Falling film condenser on Refrigerant Charge for domestic split air conditioner.



Michael Petersen, Stephen Kujak, Gurudath Nayak, Evaluation of R-410A alternatives with lower Global Warming Potential in Air Conditioning and Heat Pump ‎applications.



Harrison Skye, Piotr A. Domanski, ‎Mark O. McLinden, Valeri Babushok, Ian Bell, Tara Fortin, Michael Hegetschweiler, Marcia Huber, Mark ‎Kedzierski, Dennis Kim, Lingnan Lin, Greg Linteris, Stephanie Outcalt, Vance Payne, Richard Perkins, Aaron ‎Rowane, Lower-GWP Non-Flammable Refrigerant Blends to replace HFC-134a.



Bruno Yuji Kimura de Carvalho, Predrag Stojan Hrnjak, Investigation on the use of subcooling control in reversible residential air-conditioning systems.


Bruno Yuji Kimura de Carvalho, Ankit Sethi, Ryan Hulse, Low GWP Refrigerants for Residential Heat Pump Applications: Evaporator Optimization and Cycle ‎Improvements.


Energy Efficiency | Systems and applications



Paul Kohlenbach, Uli Jakob, Philipp ‎Munzinger, Anja Werntges, The Potential of Photovoltaic Green Cooling with Natural Refrigerants.


Anna Pacak, Maciej Chorowski, Challenges of air conditioning decarbonization.


Giulia Righetti, Claudio Zilio, Domenico Feo, Marco Auerbach, Martin Butters, Simone Mancin, Experimental assessment of a 18 kWh Latent Thermal Energy Storage for air conditioning and space cooling‎.


Francesca Martelletto, Luca Doretti, Marco Noro, Simone Mancin, Numerical analyses of building thermal systems containing Phase Change Materials.


Optimal design & control | Systems and applications



Christopher Laughman, Vedang Deshpande, Hongtao Qiao, Scott A. Bortoff, Ankush Chakrabarty, Digital Twins for Vapor Compression Cycles: Challenges & Opportunities.


Naofumi Takenaka, Shohei Ishimura, Kazuya Watanabe, Shinichi Wakamoto, Development of a Continuous Heating Technology for Air Source Heat Pumps.


Diego Marchi, Vitor Alves, Adriano Ronzoni, Alexsandro Silveira, Christian Hermes, Experimental mapping of heat pump and water-cooled condensing household tumble dryers.

Additional Papers from E2 Commission on Heat Pumps and Energy Recovery (See MicroGroove Update, Issue 13-2 for more reviews of E2 Papers)



Marcel van Beek and Thijs van Gorp, Low Refrigerant Charge Air-To-Water Domestic Heat Pump Using R-290 as the Refrigerant.



Amal Vasu, Hyunjin Lee, Young-Soo Chang, Optimal design of the fin tube heat exchanger for an indoor unit of mini VRF heat pump system.

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