The Use of Mineral Oils in Refrigeration Systems
For many years, refrigeration systems have been using mineral oils. The results have shown adequate features with both HCFC and CFC refrigerants. Refrigeration systems use lubricating oils to lubricate the compressors. They should normally be confined to the compressor crankcase.
However, small amounts of oil are always entrained in the refrigerant in the circuit (even with oil separators or special piping configuration) and migrate through the condenser, expansion valve and evaporator.
This oil migration through the different components of the circuit creates a problem, as oil occurs in heat exchangers. This creates a film on the tube wall. The presence of oil impairs the effectiveness of the heat transfer mechanism.
- An ideal lubricating oil for a refrigeration system would have the following properties:
- Good lubricating properties
- Good low-temperature miscibility and solubility
- Moisture Content is low
- Good thermal and chemical stability (especially at high discharge temperature).
With the development of chlorine-free refrigerants, conventional mineral oils appeared to be no longer adequate. This is mainly because of their immiscibility with these new refrigerants.
Synthetic oils were then considered for use with the new alternative refrigerants. A polyol ester oil appeared as the most suitable lubricant. Especially for its miscibility with hydrofluorocarbon (HFC) and the low temperature operating range.
There are limits to the available data regarding the effects of using refrigerant/oil mixtures on two-phase flow. Thus, it is difficult to realize the impact. Therefore, the primary objective of the study is to understand the impact of oil on heat transfer.
Flow Regimes
The different flow regimes identified by researchers during evaporation inside horizontal tubes include:
- Bubble flow
- Plug flow
- Slug flow
- Wavy flow,
- Annular flow
- Mist flow
Slightly different terminology sometimes characterizes similar flow patterns. Figure 1 from Collier et al. (1994) describes the flow patterns observed during evaporation inside a horizontal tube, including cross-sectional views of the flow.
Refrigerant Mixtures
Kattan et al. (1998) experimented with ternary mixtures such as R123, R402A and R404A, whereas Shin et al. (1997) investigated binary mixtures of refrigerants and hydrocarbons. Murata et al. (1993) obtained local heat transfer coefficients for non-azeotropic refrigerant mixtures (NARMs) with R1231R134a in smooth and grooved tubes.
Empirical correlations were developed for pure R123 and a mixture of R1231R134a. Torikoshi et al. (1993) investigated experimentally the heat transfer of HFC refrigerant mixtures with R32 and R134a. Heat transfer coefficients for mixtures are generally lower than the heat transfer coefficient for the pure components. There is a striking contrast between the study findings and the results of the above discussed experimental research.
Several alternatives are available for HCFC and CFC refrigerants. However, these alternatives comprise of two or more elements. These alternative refrigerants blend present problems in their use. The issues relate to classical theory.
The classical theory focuses on heat transfer among the pure substances. The issues with classical theory highlighted the need for new research that would focus on refrigerant mixtures.
Jung et al. (1989) worked on 4 refrigerants (R22, R114, R12, R152a). As well as refrigerant mixtures and refrigerant/oil mixtures. The experimental results, obtained for annular flow (quality above 5%) with the mass flux between 230 and 720 kg/m2s and heat fluxes ranging from 10 to 45 kW/m2.
They correlate with two-phase multipliers and compared with existing correlations. Jung found that the Martinelli-Nelson correlation over-predicted their experimental data for pure and mixed refrigerants by 20%. And proposed their own correlation to correlate their results in a superior way.
In their correlation, Jung et al. (1989) defined a new two-phase multiplier to take into account the heat transfer during flow boiling in a more convenient way than the Lockhart-Martinelli which is based on an adiabatic flow. Jung et al. (1989) showed that the acceleration pressure gradient accounted for less than 10% of the measured pressure drop. They decided that there was no need for this acceleration term in the total pressure drop.
Oil Mineral Results
The use of oil minerals resulted in unsatisfactory results with the development of HFC refrigerants. It also led to the development of new synthetic oils.
A few studies, listed in Schlager (1988a), started to investigate these new synthetics oils in the early ‘90s. Most of them studied the effects of oil on R134a. Hambraeus (1993) produced extensive work on the effects of three oils on the evaporative heat transfer and pressure drop.
The ester-based oils, tested with R134a and R152a, had different viscosities, and the oil content was increased up to 4% (mass). The experimental heat transfer coefficients were compared with the correlations of Pierre and Shah, and a good agreement was found for an oil-free refrigerant. The heat transfer coefficient for the refrigerant/oil mixture was generally lower than that for the pure refrigerant.
There were special cases, which showed an increase in heat transfer with increasing oil content. The local effect of oil on the heat transfer coefficient was an increase at low vapour qualities and a reduction for high qualities. Good agreement was found with the findings of Wors0e-Schmidt (1960) and Mathur (1976).
The oil viscosity was considered the significant property influencing the heat transfer coefficient. The total pressure drop results agreed well with correlations from Hambraeus (1995) published a good summary of investigations on the effects of oil on evaporative heat transfer, showing that oil could either decrease or increase the heat transfer coefficient depending on the test conditions.
Experimental results were presented for R134a with three different ester-based oils. They investigated the effects of thermodynamic properties on heat transfer.
From the experiments, the decrease in heat transfer coefficient seems to depend on the oil viscosity. There is a greater reduction resulting from a high-viscosity oil. The decrease in heat transfer coefficient was fairly well estimated by including the mixture viscosity in correlations for pure refrigerants.
A study of pressure drop is presented here to complete the effects of oil on evaporation. Souza et al. (1993) tested several oils including Polyalkylene Glycol (PAG) and ester oils with R134a with a flow regime predominantly annular. The pressure drop was found to increase as the oil concentration increases. A correlation was developed to predict the two-phase pressure drop of refrigerant/oil mixtures. This correlated results with R134a and R12 with 5 different oils with a mean deviation of 3.3%.
Effects of Oil on Evaporative Heat Transfer
The effects of oil on evaporative heat transfer have been studied extensively by Hambraeus, 1995). The influence of oil on the physical and thermal properties of refrigerant mixtures was investigated in detail. On the basis of experimental tests on three synthetic oils with R134a. No correlations were developed due to the large variations in test conditions. But a survey of studies taking into account the oil effects was presented.
A few studies propose correction factors to conventional heat transfer correlations, to predict the effects of oil, but these corrections are often limited to particular conditions and they may not be generalized. Correction factors have been developed by Tichy et al. (1986) for R12.
Effects of Naphthenic-Based Oil on Pressure Drop
Tichy et al., (1986) also investigated the effects of a naphthenic-based oil on the pressure drop of R12 during evaporation. The data were compared with the frictional pressure drop and void fraction relationship.
The best correlation for oil-free refrigerant pressure drop was used to correlate oil/refrigerant mixtures.
For evaporation, a Duckler II correlation was modified to account for the effects of oil. The addition of oil dramatically increased the pressure drop and increased the pressure drop by 63 to 86% for evaporation. Tichy et al. observed that the presence of oil promoted the formation of annular flow increasing heat transfer.
In the dry-out region of evaporation (high vapour quality), the heat transfer coefficient was reduced. This is because of the formation of an oil-rich layer. Relatively limited work has been undertaken on the effects of oil on condensation compared to evaporation.
A general finding of Schlager et al. (1988a) is that the condensing heat transfer coefficient is always degraded with the presence of oil in the refrigerant.
Effects of Local Vapor Quality on Heat Transfer
Results from Tichy et al. (1985) revealed that the local vapor quality has little impact in reducing the heat transfer. The experimental tests with R12 and a miscible oil showed that 2% and 5% oil concentrations resulted in a 10% and 23% decrease in heat transfer coefficient. This is much lower than that for evaporation.
The comparison of observed flow patterns was consistent with the Breber et al. (1980) map, but unsatisfactory for the maps from Soliman (1971). Their results were correlated by an extension of the Shah (1979) correlation and predicted 82% of the data within ±20%.
This correlation is valid for the entire range of the experimental data (94< G <944 kg/m2s, 3.5< q’ <690 kW/m2, 4.8< Pabs <9.3 bar, 0.2< x <0.8), regardless of the flow pattern. But the authors reported that it may not be as accurate as one developed for specific flow regimes.
The reduction in local heat transfer coefficient with oil is probably due to alteration of the transport properties in the liquid layer.
References
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