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

An 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 ±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 ⅜ 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⁻² s⁻¹, respectively, and measured coefficients were in the range of 1400 to 3900 W m⁻² K⁻¹. 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.