About the viscosity reduction of oils by diluted ammonia at high temperatures.

Abstract

Ammonia refrigeration plants count as one of the most important, if not the most important, in the industry as they continue to be at the major choice for many industry-cooling applications. Nowadays, R717 is also used in high temperature heat pumps, with high pressures and high discharge temperatures. Up until now, the compressors have been lubricated by reliable mineral oils. However, due to rising requirements, e.g. thermal stability and viscosity behavior, mineral oils are not always the best solution. Higher discharge temperatures and pressures leads to problems with these oils, which can occur especially for high temperature applications. One of these problems is that R717 is soluble in oil at high temperatures, reducing the viscosity of the lubricant. Thus, this phenomenon could result in lubricating problems, ending up in wear and mechanical problems. This article deals with measurements of solubility, vapor pressure, and viscosity of four different oils mixed with R717 up to 50 bar and 140 °C, two hydrotreated mineral oils, a polyalphaolefine oil and a mixed synthetic hydrocarbon. For the pure oils, the density was determined based on the bending vibration principle; the pressure of at least three refrigerant-oil mixtures per oil was determined using a static measurement method. After the pressure measurement, the miscibility gaps were calculated and the concentrations of the mixtures fulfilling the working point conditions 50 bar and 140 °C were defined. Because these mixtures segregate at room temperature, a new method for measuring the viscosity was developed to prevent the segregation. The viscosity of the pure oils and of the predefined mixtures were measured and the viscosity reduction was calculated. Finally, the results were compared graphically and arithmetically in the paper.