An overview of the effect of lubricant on the heat transfer performance on conventional refrigerants and natural refrigerant R-744
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)
- et al.
Nucleate pool boiling of R-114 and R-114-oil mixtures from smooth and enhanced surfaces-I. Single tubes
International Journal of Heat and Mass Transfer
(1995) - et al.
Nucleate pool boiling of R-114 and R-114-oil mixtures from smooth and enhanced surfaces-II. Tube bundles
International Journal of Heat and Mass Transfer
(1995) - et al.
Boiling of ammonia/lubricant mixture on a horizontal enhanced tube in a flooded evaporator with inlet vapor quality
International Journal of Refrigeration
(2008) Boiling of new refrigerants: a state-of-the-art review
International Journal of Refrigeration
(1996)- et al.
The influence of a low viscosity oil on the pool boiling heat transfer of the refrigerant R507
International Journal of Refrigeration
(2001) Enhancement of R123 pool boiling by the addition of Hydrocarbons
International Journal of Refrigeration
(2000)The effect of lubricant concentration, miscibility, and viscosity on R134a pool boiling
International Journal of Refrigeration
(2001)Effect of bulk lubricant concentration on the excess surface density during R123 pool boiling
International Journal of Refrigeration
(2002)Effect of Refrigerant oil Additive on R134a and R123 boiling heat transfer performance
International Journal of Refrigeration
(2007)- et al.
Influence of oil on nucleate pool boiling heat transfer of refrigerant on metal foam covers
International Journal of Refrigeration
(2011)
Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend
International Journal of Heat and Mass Transfer
Heat transfer of oil-contaminated HFC134a in a horizontal evaporator
International Journal of Refrigeration
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
Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map
International Journal of Heat and Mass Transfer
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)
Heat transfer and pressure drop of HFC134a-oil mixtures in a horizontal condensing tube
International Journal of Refrigeration
In-tube condensation of R134a and ester oil: empirical correlations
International Journal of Refrigeration
Heat transfer during forced convection condensation inside horizontal tube
International Journal of Refrigeration
Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes
International Journal of Refrigeration
Flow condensation pressure drop characteristics of R410A oil mixture inside small diameter horizontal microfin tubes
International Journal of Refrigeration
Condensation heat transfer characteristics of R410A–oil mixture in 5 and 4 mm outside diameter horizontal microfin tubes
Experimental Thermal and Fluid Science
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
Two-phase flow pattern in small diameter tubes with the presence of horizontal return bend
International Journal of Heat and Mass Transfer
Condensation heat-transfer and pressure drop characteristics of refrigerant R-290/R-600a-oil mixtures in serpentine small-diameter U-tubes
Applied Thermal Engineering
Influence of Oil on R-410A Two-phase pressure drop in a small U-type wavy tube
International Communications in Heat and Mass Transfer
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
Critical review of flow boiling heat transfer of CO2–lubricant mixtures
International Journal of Heat and Mass Transfer
Fundamental process and system design issues in CO2 vapor compression systems
Progress in Energy and Combustion Science
Flow boiling heat transfer of carbon dioxide in horizontal mini-tubes
International Journal of Heat and Mass Transfer
In-tube cooling heat transfer of supercritical carbon dioxide Part 1: experimental measurement
International Journal of Refrigeration
In-tube cooling heat transfer of supercritical carbon dioxide Part 2: comparison of numerical calculation with different turbulence models
International Journal of Refrigeration
Effect of lubricating oil on cooling heat transfer of supercritical carbon dioxide
International Journal of Refrigeration
Study on two-phase flow pattern of supercritical carbon dioxide with entrained PAG type lubricating oil in a gas cooler
International Journal of Refrigeration
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
Models of thermodynamic and transport properties of POE VG68 and R410A/POE VG68 mixture
Frontiers of Energy and Power Engineering in China
Flow boiling characteristics and flow pattern visaulzation of refrigerant/lubricant mixtures
International Journal of Refrigeration
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
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
11(3)
Nucleate boiling performance of refrigerants and refrigerants–oil mixtures
Journal of Heat Transfer
Nucleate pool boiling of refrigerant/oil mixtures
Experimental Heat Transfer
Pool boiling of R-11 and R-123 oil–refrigerant mixtures on plain and enhanced tube geometries
ASHRAE Transactions
The effect of oil contamination on the nucleate pool boiling performance of R-114 from a porous coated surface
ASHRAE Transactions
Some observations of the foaming characteristics in the nucleate boiling performance of refrigerant–oil mixtures
ASHRAE Transactions
Berechnung des Wärmeü bergangs verdampfender binärer Flüssigkeitsgemische
Chemie Ingenieur Technik
The enhancement of nucleate boiling by foam
Heat Transfer
Nucleate pool boiling heat transfer of R134a and R134a-PVE lubricant mixtures on smooth and five enhanced tubes
Journal of Heat Transfer
Cited by (32)
A novel constant pressure control method for nucleation boiling of CO<inf>2</inf>/lubricant in molecular dynamics simulation
2024, International Journal of RefrigerationRecent progress in pool boiling heat transfer of low GWP refrigerants with the effect of POE lubricant oil
2023, Thermal Science and Engineering ProgressExperimental investigation of condensation heat transfer of zeotropic refrigerant/oil mixtures in plate heat exchanger
2023, International Journal of Thermal SciencesBoiling heat transfer of CO<inf>2</inf>/lubricant on structured surfaces using molecular dynamics simulations
2023, Applied Thermal EngineeringStratification and heat transfer characteristics of partially miscible lubricating oil-refrigerant mixtures during pool boiling
2022, Applied Thermal EngineeringExperimental investigations on cooling heat transfer of CO<inf>2</inf>-lubricant mixtures in horizontal tubes at supercritical pressure: A review
2022, International Journal of RefrigerationCitation 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.