ICR 2019: an overview of current refrigeration research (part I)

Presentation of the main scientific and technical topics covered in the papers of the 25th IIR International Congress of Refrigeration and summary of some of the 14 keynotes.

PART I - See Part II

The topics addressed in the papers and keynotes presented during the International Congress of Refrigeration (ICR 2019) in Montreal, Canada, on August 24-30, 2019, illustrate very well the current research areas in the refrigeration sector.

The main topics covered were:

  • Low-GWP refrigerants: alternative refrigerants (HFOs, HFO-HFC blends, CO2, ammonia, hydrocarbons, water) per application, related safety issues, refrigerant charge reduction
  • Thermodynamics: thermodynamic properties of refrigerants, heat and mass transfer issues, nanofluids and nanolubricants, ionic liquids
  • Phase-change materials and slurries and energy storage: applications in cryogenics, food refrigeration, air conditioning, etc.
  • Alternative refrigeration technologies to vapour compression: absorption-adsorption, caloric cooling, solar cooling
  • Compressors: control, lubrication, advances in linear compressors
  • Heat exchangers: micro- and mini-channels, two-phase flow, frosting and defrosting issues
  • Cold chain: monitoring, thermal properties of food, advances in refrigerated storage and transport and in household refrigerators
  • Commercial refrigeration: advances in supermarket refrigeration, CO2/sub> systems, ejector technology, display cabinets
  • Air conditioning: dehumidification, liquid desiccants, energy efficiency, alternative refrigerants, safety issues related to the use of hydrocarbons, high-performance buildings
  • Heat pumps: sorption heat pumps, high-temperature heat pumps, ground-source heat pumps
  • Cryogenics (cryocoolers, cryopreservation, cryotherapy) and liquefied gases (LNG).

The 14 keynotes alone represent a good synthesis of the areas that generate the most research. They also synthesize the current state of knowledge and challenges in these fields. In this issue of the Newsletter as well as in the next, we will offer a summary of these keynotes, theme by theme.

The central theme of alternative low-GWP refrigerants and refrigerant charge reduction was the subject of 4 keynotes:

In his keynote[1] titled “Thermodynamics of the new refrigerants”, Mark Mc Linden stresses that the number of acceptable refrigerants is decreasing due to environmental constraints but, at the same time, new options are becoming available. These “new” refrigerants include new molecules, such as the HFOs (hydrofluoroolefins) containing a carbon-carbon double bond, as well as some “old” fluids, such as ammonia, carbon dioxide and hydrocarbons. The HFC (hydrofluorocarbon) refrigerants, although being phased down, remain in use—especially as blend components. The author reviews the thermodynamic characteristics that underpin the choice of a refrigerant. Refrigerants have different thermodynamic characteristics, and these determine their relative performance in a refrigeration cycle. He concludes that the design engineer must balance the tradeoffs presented by different refrigerants and design the system to best accommodate the properties of the chosen fluid.

In another keynote [2], Yongchan Kim and Dongchan Lee emphasise that heat transfer coefficient and pressure drop of a refrigerant are essential parameters to optimally design heat exchangers in a refrigeration system. They investigated evaporation heat transfer characteristics of several low-GWP HFO refrigerants (R1234yf, R1234ze(E), and R1233zd(E)) in different types of plate heat exchangers, which are now widely adopted in the refrigeration industry thanks to their high compactness and efficiency. They conclude that the brazed plate heat exchanger shows higher heat transfer coefficient, whereas the shell and plate heat exchanger exhibits a lower pressure drop.

Refrigerant mixtures have become commonly used for air-conditioning and refrigeration applications. Most mixtures are zeotropic ones and therefore exhibit a temperature glide during evaporation and condensation. In their keynote, Jiazhen Ling, Yunho Hwang and Reinhardt Radermacher [3] stress that the temperature glide may cause capacity losses due to a reduced approach temperature – defined as the temperature difference between two fluids – in heat exchangers. Without a proper design, a heat exchanger may lose up to 27% of its capacity when using a zeotropic mixture having 5.2 K temperature glide. However, by simply changing the refrigerant path to be counter flow, such capacity loss can be minimized. Refrigerant mixture has the benefit of control system capacity by altering its composition circulating in the cycle. The paper studies a R1234yfa/R32 mixture and reports a 4 % cooling capacity reduction for every 10% reduction in R32 concentration.

Reduction of refrigerant charge is attractive, particularly because a low charge may expand the possibilities for some very good refrigerants (high cycle efficiency, high heat transfer/pressure drop performance, etc.) in locations where these fluids are either totally restricted or only allowed in limited quantities due to flammability or toxicity issues. This is particularly true for fluids like ammonia and hydrocarbons, which in some applications and locations are already only accepted below certain levels (typically 150 kg for NH3 and 150 or 50 g for HCs). In his paper [4], Pega Hrnjak presents several strategies for charge reduction:

  • Introduction of a secondary fluid
  • In the compressor: reducing the internal volume, reducing the quantity in lubricant, reducing solubility to reduce refrigerant absorption
  • In vessels by reducing volume and liquid levels
  • In pipes by reducing internal volume (diameter and possibly even length)
  • In heat exchangers by reducing tube diameter and length.

Because it is the most attractive, the last strategy is elaborated in detail. Obviously, to reduce the charge, internal volume needs to be reduced, but one must be careful to balance the adverse effects regarding either increased pressure drop or reduced heat transferred.

[1] Mark Mc Linden, Thermodynamics of the new refrigerants, https://bit.ly/2n7vNo5

[2] Yongchan Kim, DongChan Lee, Evaporation heat transfer characteristics of low GWP refrigerants in plate heat exchangers, https://bit.ly/2lDiOdq

[3] Jiazhen Ling, Yunho Hwang, Reinhard Radermacher, Vapor compression heat pumps with refrigerant mixtures, https://bit.ly/2nDpoB5

[4] Pega Hrnjak, Rational approach to refrigerant charge reduction, https://bit.ly/2n7zGcF

See part II