Thermodynamic properties of ammonia-oil-mixtures that influence the efficiency of an evaporator.

Typically, hydrocarbon oils like mineral oil (MO), polyalphaolefine (PAO) and alkyl benzene (AB) are used for lubricating R717 compressors. It is known and confirmed by our previous experiments as well as the latest literature that ammonia dissolves in these oils not only at high temperatures but also at low temperatures. This partial solubility affects the viscosity and the pour point of the oil. There is a huge interest to understand the cold-flow properties of oil in the evaporator. This also includes the mass flow of liquid on the inner surface of the evaporator, that influences the heat transfer coefficient significantly. However, the heat transfer in evaporators is not influenced by the mass flow alone. There are three phases in the R717 evaporator, the liquid refrigerant with a low load of oil, the oil-rich phase and the gaseous ammonia phase. To develope refrigeration cycles with low energy demand, the knowledge of the interaction between the three phases and their influence on the efficiency of the plant becomes more important.
The presentation deals with measurements of low temperature solubility and thermal conductivity of different oils mixed with ammonia. The goal of our presentation is to show how difficult it is and what have to be done in future to predict which oil is the best and most effective regarding heat transfer in an ammonia evaporator.