An overview of the effect of lubricant on the heat transfer performance on conventional refrigerants and natural refrigerant R-744

https://doi.org/10.1016/j.rser.2012.04.054Get rights and content

Abstract

This review provides an overview of the lubricant on the heat transfer performance, including nucleate boiling, convective boiling, shell side condensation, forced convective condensation, and gas cooling, for conventional refrigerants and natural refrigerant R-744. Various parameters affecting the heat transfer coefficient subject to lubricant, such as oil concentration, heat flux, mass flux, vapor quality, geometric configuration, saturation temperature, thermodynamic and transport properties are discussed in this overview. It appears that the effect of individual parameter on theProd. Type: FTP heat transfer coefficient may be different from studies to studies. This is associated with the complex nature of lubricant and some compound effect accompanying with the heat transport process. In this review, the authors try to summarize the general trend of the lubricant on the heat transfer coefficient, and to elaborate discrepancies of some inconsistent studies. The lubricant can, increase or impair the heat transfer performance depending on the oil concentration, surface tension, surface geometry, and the like. For the condensation, it is more well accepted that the presence of lubricant normally will impair the heat transfer performance due to deposited oil film. However, the deterioration is comparatively smaller than that in nucleate/convective boiling. For the effect of lubricant on R-744 with convective evaporation, the general behavior is in line with the convectional refrigerant. For gas cooling, the lubricant cast significant effect on heat transfer coefficient especially for a higher mass flux or at a smaller diameter tube.

Introduction

A critical component in all refrigeration and air-conditioning systems is the compressor for compressing the circulating refrigerant vapor. In many applications the lubricant, which is essential for lubrication of the moving parts, is charged during the manufacture of the compressor and is expected to work reliably for the entire life of the unit. The main function of lubricant oil is to lubricate the internal moving parts of the compressor. On the other hand, the lubricant provides a seal between the moving parts enabling efficient vapor compression. Gibb et al. [1] had shown that the benefits of the introducing more energy efficient refrigeration lubricants can lead to a reduction in energy consumption as high as 15% and indirect reductions in emissions of the greenhouse gas CO2. With properly designed lubricants, Gibb et al. [1] estimated that up to 80% of the industrial refrigeration and air-conditioning systems replaced in the next 20 years in the USA could result in annual energy saving up to 200,000 GW h corresponding to 11 million metric tons of carbon in reduced CO2 emissions. Despite its crucial role in increasing the energy efficiency of compressor, in typical operation of an air-conditioning or refrigeration system, a small amount of lubricant oil may migrate from the compressor and into another part of the system, such as the evaporator, condenser, expansion device, and connecting piping, thereby inevitably altering the heat transfer and frictional characteristics of the refrigerant. As a consequence, it is imperative to realize its role on the heat transfer performance as far as system efficiency is concerned.

The presence of lubricant alters the physical properties of refrigerant mixtures. Among the difference in thermophysical properties, the influence of viscosity is especially imperative since the viscosity of lubricant oil is about two to three orders higher than that of refrigerant. On the other hand, the corresponding surface tension of lubricant is approximately one order higher than that refrigerant. Hence the presence of lubricant oil would considerably affect the thermodynamic and transport properties of refrigerant, casting a significant impact on the heat transfer characteristics. Fig. 1 is a schematic of the associated properties of R-410A/POE VG68 mixture based on the calculated results of Wei et al. [2] with oil concentration ranging from 0% to 30%. Normally, only a very slightly drop of mixture density subject to the rise of lubricant concentration is seen, followed by a moderate decrease of specific enthalpy. It should be mentioned that the density of the lubricant oil can be lower, equal, or higher than that of refrigerant. In the meantime, a detectable rise of surface tension and a sharp rise of viscosity is encountered.

There had been many reviews concerning the influences of lubricant oils on the heat transfer characteristics of refrigerant, for instance [3], [4], [5], some general behaviors of the lubricants were reported and some controversies still exists. Hence the major objective of this review is to summarize the important findings and clarify some possible causes of the inconsistency. In addition, the second objective is to review the effect of lubricant on the heat transfer performance of the environmentally friendly natural refrigerant—R-744.

Section snippets

Effect of lubricant on pool boiling

For pool boiling, the effect of lubricant on heat transfer is rather small at a low weight concentration, i.e., ω<3%, and for 2%<ω<4%, some types of lubricant can enhance the pool boiling, and with a higher concentration (ω>5%), almost all types of lubricant oil reduce pool boiling heat transfer. In practical application, the refrigerants used in flooded evaporator are usually miscible in lubricant oil. Miscibility is very important since an immiscible oil may form a film on the evaporator

Effect of lubricant on the thermofluids characteristics of R-744

When compared with the convectional refrigerants, literatures concerning the effect of lubricant on the heat transfer performance of R-744 (CO2) is comparatively few. A variety of lubricants can be used in CO2 refrigeration systems. In certain systems, synthetic hydrocarbons such as alkylbenzenes (ABs) and polyalphaolefins (PAOs) can still be used even though they have poor solubility with CO2 [78]. The poor solubility of the synthetic hydrocarbons is compensated by their excellent low

Conclusions

The present review provides an overview of the lubricant on the heat transfer performance, including nucleate boiling, convective boiling, shell side condensation, forced convective condensation, and gas cooling, for conventional refrigerants and for natural refrigerant R-744. There are various parameters affecting the heat transfer coefficient subject to the presence of lubricant, such as oil concentration, heat flux, mass flux, vapor quality, geometric configuration, saturation temperature,

Acknowledgments

The authors are indebted to the financial support from the Bureau of Energy, the Ministry of Economic Affairs, Taiwan and supporting funding from the National Science Council of Taiwan (100-ET-E-009-004-ET & 101-ET-E-009-003-ET).

References (101)

  • CC Wang et al.

    Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

    International Journal of Heat and Mass Transfer

    (2003)
  • K Hambraeus

    Heat transfer of oil-contaminated HFC134a in a horizontal evaporator

    International Journal of Refrigeration

    (1995)
  • B Dawidowicz et al.

    Heat transfer and pressure drop during flow boiling of pure refrigerants and refrigerant/oil mixtures in tube with porous coating

    International Journal of Heat Mass and Transfer

    (2012)
  • L Wojtan et al.

    Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map

    International Journal of Heat and Mass Transfer

    (2005)
  • L Wojtan et al.

    Investigation of flow boiling in horizontal tubes: Part II—Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes. International Journal of Heat and Mass Transfer

    48(14)

    (2005)
  • DW Shao et al.

    Heat transfer and pressure drop of HFC134a-oil mixtures in a horizontal condensing tube

    International Journal of Refrigeration

    (1995)
  • R Bassi et al.

    In-tube condensation of R134a and ester oil: empirical correlations

    International Journal of Refrigeration

    (2003)
  • TN Tandon et al.

    Heat transfer during forced convection condensation inside horizontal tube

    International Journal of Refrigeration

    (1995)
  • A Cavallini et al.

    Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes

    International Journal of Refrigeration

    (2000)
  • XC Huang et al.

    Flow condensation pressure drop characteristics of R410A oil mixture inside small diameter horizontal microfin tubes

    International Journal of Refrigeration

    (2010)
  • XC Huang et al.

    Condensation heat transfer characteristics of R410A–oil mixture in 5 and 4 mm outside diameter horizontal microfin tubes

    Experimental Thermal and Fluid Science

    (2010)
  • K Cho et al.

    Condensation heat transfer for R-22 and R-407C refrigerant–oil mixtures in a microfin tube with a U-bend

    International Journal Heat and Mass Transfer

    (2001)
  • CC Wang et al.

    Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend

    International Journal of Heat and Mass Transfer

    (2003)
  • MY Wen et al.

    Condensation heat-transfer and pressure drop characteristics of refrigerant R-290/R-600a-oil mixtures in serpentine small-diameter U-tubes

    Applied Thermal Engineering

    (2009)
  • IY Chen et al.

    Influence of Oil on R-410A Two-phase pressure drop in a small U-type wavy tube

    International Communications in Heat and Mass Transfer

    (2005)
  • IY Chen et al.

    Two-phase frictional pressure drop of R-134a and R-410A refrigerant–oil mixtures in straight tubes and U-type wavy tubes

    Experimental Thermal and Fluid Science

    (2007)
  • X Zhao et al.

    Critical review of flow boiling heat transfer of CO2–lubricant mixtures

    International Journal of Heat and Mass Transfer

    (2009)
  • M-H Kim et al.

    Fundamental process and system design issues in CO2 vapor compression systems

    Progress in Energy and Combustion Science

    (2004)
  • R Yun et al.

    Flow boiling heat transfer of carbon dioxide in horizontal mini-tubes

    International Journal of Heat and Mass Transfer

    (2005)
  • C Dang et al.

    In-tube cooling heat transfer of supercritical carbon dioxide Part 1: experimental measurement

    International Journal of Refrigeration

    (2004)
  • C Dang et al.

    In-tube cooling heat transfer of supercritical carbon dioxide Part 2: comparison of numerical calculation with different turbulence models

    International Journal of Refrigeration

    (2004)
  • C Dang et al.

    Effect of lubricating oil on cooling heat transfer of supercritical carbon dioxide

    International Journal of Refrigeration

    (2007)
  • C Dang et al.

    Study on two-phase flow pattern of supercritical carbon dioxide with entrained PAG type lubricating oil in a gas cooler

    International Journal of Refrigeration

    (2008)
  • C Dang et al.

    Effect of PAG-type lubricating oil on heat transfer characteristics of supercritical carbon dioxide cooled inside a small internally grooved tube

    International Journal of Refrigeration

    (2010)
  • Gibb P, Randles S, Millington M, Whittaker A Lubricants for sustainable cooling. In: Proceedings of the 2003...
  • W Wei et al.

    Models of thermodynamic and transport properties of POE VG68 and R410A/POE VG68 mixture

    Frontiers of Energy and Power Engineering in China

    (2008)
  • EPB Filho et al.

    Flow boiling characteristics and flow pattern visaulzation of refrigerant/lubricant mixtures

    International Journal of Refrigeration

    (2009)
  • B Shen et al.

    Critical review of the influence of lubricants on the heat transfer and pressure drop of refrigerants, Part I: Lubricant influence on pool and flow boiling

    International Journal of HVAC&R Research

    (2005)
  • B Shen et al.

    Critical review of the influence of lubricants on the heat transfer and pressure drop of refrigerants, Part II: lubricant influence on condensation and pressure drop

    International Journal of HVAC&R Research

    (2005)

    11(3)

    (2005)
  • S Chongrungreong et al.

    Nucleate boiling performance of refrigerants and refrigerants–oil mixtures

    Journal of Heat Transfer

    (1980)
  • KI Bell et al.

    Nucleate pool boiling of refrigerant/oil mixtures

    Experimental Heat Transfer

    (1987)
  • RL Webb et al.

    Pool boiling of R-11 and R-123 oil–refrigerant mixtures on plain and enhanced tube geometries

    ASHRAE Transactions

    (1993)
  • Stephan K 1963. Influence of oil on heat transfer of boiling Freon 12 (Refrigerant 12) and Freon 22 (Refrigerant 22)....
  • AS Wanniiarachchi et al.

    The effect of oil contamination on the nucleate pool boiling performance of R-114 from a porous coated surface

    ASHRAE Transactions

    (1986)
  • CC Wang et al.

    Some observations of the foaming characteristics in the nucleate boiling performance of refrigerant–oil mixtures

    ASHRAE Transactions

    (1999)
  • Wallner R, Dick HG Heat transfer to boiling refrigerant–oil mixtures. in: Proceedings of the 14th International...
  • K Stephan et al.

    Berechnung des Wärmeü bergangs verdampfender binärer Flüssigkeitsgemische

    Chemie Ingenieur Technik

    (1969)
  • Stephan K Influence of oil on heat transfer of boiling Freon 12 (Refrigerant 12) and Freon 22 (Refrigerant 22). in:...
  • A Udomboresuwan et al.

    The enhancement of nucleate boiling by foam

    Heat Transfer

    (1986)
  • WT Ji et al.

    Nucleate pool boiling heat transfer of R134a and R134a-PVE lubricant mixtures on smooth and five enhanced tubes

    Journal of Heat Transfer

    (2010)
  • Cited by (32)

    • Experimental investigations on cooling heat transfer of CO<inf>2</inf>-lubricant mixtures in horizontal tubes at supercritical pressure: A review

      2022, International Journal of Refrigeration
      Citation Excerpt :

      However, various factors can affect the heat transfer characteristics. In addition to the factors listed by Wang et al. (Wang et al., 2012), other factors need to be further addressed, such as oil concentration, oil type, operating pressure and heat flux. Recently, Xu et al. (Xu et al., 2022) comprehensively reviewed the effect of lubricating oil on the flow characteristics of sCO2 in gas coolers.

    View all citing articles on Scopus
    View full text