Effect of lubricating oil contamination on evaporation in refrigerants R12 and R22

Abstract

SUMMARYAn investigation has been made into the effect of oil concentration on evaporation heat transfer coefficients in refrigerant-oil mixtures flowing in a horizontal tube. A new correlation is presented for heat transfer coefficients in convective evaporation of refrigerant-oil mixtures that predicts the results of the present study within approximately f 20%. The paper reports measurements of evaporation heat transfer coefficients in refrigerants R12 and R22, both oil-free and with two concentrations of Shell Clavus 32 oil. A 1.8 m long 2 in O/D copper tube (8.05 mm I/D) was used, at evaporation temperatures of -5"C, 0°C and + 5°C. Heat flux and mixture mass velocity were kept constant at 2500 W m-2 and 155 kg m-' s -l , respectively, and measured coefficients were in the range of 1400 to 3900 W m-* K-I. The results showed that, for a complete evaporator, 2% oil may be expected to increase the heat transfer coefficient by 12%, but 10% oil returns the coefficient to oil-free values.

Measurement and Correlation of Flow-Boiling Heat Transfer of a R-410A/Oil Mixture Inside a 4.18 mm Straight Smooth Tube

“…The second kind of correlations, such as the Cawte et al (1996) correlation and the Tichy et al (1986) correlation, are also verified with the present data. Figure 6a depicts comparisons of the predicted values of these correlations with the present experimental data.…”

“…Figure 6a depicts comparisons of the predicted values of these correlations with the present experimental data. It shows that the deviations of the Cawte et al (1996) correlation and the Tichy et al (1986) correlation are within -80% to -50% and -50%-30%, and neither of them can predict the experimental data of R-410A/oil mixture satisfactorily. The poor predictability is due to the fact that these correlations do not consider the influence of mixture properties, and they only owe the influence of oil presence on heat transfer to nominal oil concentration.…”

“…The second category is developed in terms of a two-phase multiplier to correct the single-phase heat transfer coefficient of an oil-free refrigerant (Cawte et al 1996;Tichy et al 1986). A two-phase multiplier for heat transfer is defined as the ratio of the heat transfer coefficient of a two-phase refrigerant-oil mixture to that of a liquid-phase oil-free refrigerant, as presented in Equation 8:…”

“…The available experimental research shows that the flow mechanism in small-diameter tubes is different from that in conventionally-sized tubes (Wei et al 2007a;Coleman and Garimella 1999). The research on the heat transfer performance of different refrigerant-oil mixtures, such as the R-12/oil mixture (Cawte et al 1996;Tichy et al 1986;Hughes et al 1984;Worsoe-Schmidt 1960), R-22/oil mixture (Wei et al 2007a(Wei et al , 2007bCawte et al 1996;Suguru et al 1991;Schlager et al 1989aSchlager et al , 1989bSchlager et al , 1990, R-134a/oil mixture (Eckels et al 1998a(Eckels et al , 1998b(Eckels et al , 1994Hambraeus 1995;Nidegger et al 1997;Zurcher et al 1997;Fukushima and Kudou 1990), and R-407C/oil mixture (Zurcher et al 1998a(Zurcher et al , 1998b, etc., show that different refrigerant-oil mixtures in different diameter tubes have different heat transfer characteristics. However, there are no paper reports documenting the heat transfer performance of a R-410A/oil mixture flow boiling in small tubes, and the existing research results of refrigerant-oil mixture flow boiling in the normally-sized tubes may not be extended to a R-410A/oil mixture in small tubes.…”