Refrigerants are classified as completely miscible, partially miscible or immiscible according to their mutual solubility relations with mineral oils. Local heat transfer coefficients were determined from measurements of electrically delivered heat flux and tube wall temperatures in a siz-pass horizontal serpentine coil during evaporation of refrigerants R-502 and ammonia.

The R-502 experiments were at evaporating temperatures near -14 F (-26°C) where mineral oil is partialy miscible. At 2% to 10% oil concentration the heat transfer coefficient is reduced by 30% to 50% below that of oil-free R-502. At 1% or less oil concentration the reduction is small and might be neglected for many applications.

The ammonia experiments were performed at evaporating temperatures in the range of -10F to -40 F (-23°C to -40°C). The mineral oil was immiscible and formed a tar-like sludge that collected along the bottom of the tube. The heat transfer coefficient is reduced by 50% to 90% below that of oil-free ammonia.

Units: Dual