PVT-measurements for high-temp ammonia heat pump lubricants.

Ammonia (R717) is a commonly used refrigerant, often used in food, beverage and chemical industry for example. Recently, R717 is also used in high-temperature heat pumps. Typically, hydrocarbon oils like mineral oil (MO), polyalphaolefine (PAO) and alkyl benzene (AB) are applied for lubricating R717 compressors. In our previous experiments as well as the latest literature it was found that at high temperatures ammonia dissolves in these oils and thus affects the viscosity stronger than expected. The solubility of R717 increases from around 1 % at room temperature to more than 10 % at temperatures above 100 °C. For this reason, it is necessary to think about generating corresponding PVT diagrams (also known as Daniel Plots) for such systems. The paper deals with measurements of solubility, vapor pressure, density and viscosity of four different oils mixed with ammonia up to 50 bar and 140 °C. The density was determined based on the bending vibration principle; the pressure of three to five refrigerant-oil-mixtures (90 – 99 wt.% oil) per oil was determined with a static measurement method. With the results from the pressure measurement, the miscibility gap was calculated. To prevent segregation during the viscosity experiment a complex measurement setup was developed. After measuring the viscosity of the pure oils and the mixtures, the viscosity reduction was calculated and Daniel Plots were created. For the first time, high temperature behavior of several types and viscosity grades of hydrocarbon oils with ammonia can be compared by Daniel Plots to choose the best oil for high-temperature ammonia heat pump applications.