Experimental investigations of the influence of dissolved ammonia on the low temperature-cold-flow properties of refrigeration oils.

Ammonia (R717) is a well-established refrigerant in compression cooling technology. Refrigeration oils, such as mineral oils (MO), polyalphaolefins (PAO) or alkyl benzenes (AB), are used for these applications, as they are nonpolar and nearly immiscible with ammonia. However, it was found that at temperatures above 100 °C up to ten weight percentage ammonia is soluble in refrigeration oils. Even at 0 °C up to one weight percentage ammonia can dissolve. This solubility behavior has an influence on the flow properties of the refrigeration oil (viscosity) and on the properties of the refrigerant (e.g. saturation vapor). Furthermore, at very low application temperatures, a decrease of the pour point of the refrigeration oils was observed. There is great interest regarding the relationship between the amount of dissolved ammonia and the reduction of the viscosity as well as the reduced pour point of the refrigeration oil at low temperatures. Therefore, this paper includes results of measurements of the dynamic viscosity of pure refrigerant oils and ammonia-refrigeration oil-mixtures with ≤0.5 weight percentage ammonia, the calculation of the kinematic viscosity and the percentage viscosity reduction by dissolved ammonia. The measurements of viscosity took place in a cooled
oscillating piston viscometer from room temperature to -40 °C and maximum 20,000 mPa·s, respectively. The temperature-dependence of the viscosity of the pure oils was compared with the mixtures. A calculation of the pour point of the mixtures is possible by using the viscosity value of the pure oils at the pour point.