rolling bearings, refrigerant lubrication, friction
Oil-free systems gain increased popularity in the chiller industry. This technology is often related to reduced maintenance, higher system efficiency and smaller environmental footprint. In refrigeration compressors which use oil for the lubrication it is impossible to avoid mixing of refrigerant and oil in the system. The system efficiency is affected by the contamination of the refrigerant by oil. One possibility to operate a compressor of a chiller without oil is the pure refrigerant lubrication (PRL) bearing technology. With this technology the traditional oil lubrication system can be removed completely and the bearings are lubricated only with refrigerant. This removes the challenges related to mixtures of fluids - on the chiller performance side (efficiency reduction due to oil contamination of the working fluid) as well as on the lubrication side (lubricant diluted by refrigerant). The challenge which pure refrigerant lubrication poses to rolling bearings was overcome by a special bearing development. Energy efficiency and total cost of ownership are major drivers of the industry. Therefore, accurate predictions of bearing power losses became a fundamental component of the bearing performance rating. For bearings operating in refrigerant environment, friction is even more important, since the heat generation affects the thermodynamics of the refrigerant, and can lead to refrigerant evaporation and starvation. A special bearing test rig has been developed, which allows the investigation of the bearing performance under pure refrigerant lubrication conditions. Its main components are the hermetic bearing test head and the refrigerant supply system. The test rig was designed to withstand the conditions present in refrigerant environments and enables the investigation of refrigerant condition and injection properties of typical bearing operating conditions. The refrigerant is pumped, filtered and conditioned in the lubrication circuit before it is injected into the bearing to provide cooling and lubrication. The bearings inside the stainless steel housing can be subjected to various loads and rotational speeds. The refrigerant properties such as film building ability, density and the boundary friction coefficient as well as possible evaporation influence the bearing friction characteristics. With the test rig the influence of the thermodynamic behaviour can be analysed. The mapping of the friction behaviour under various operating conditions allows the optimisation of the injection properties. PRL bearings enable machine designs which offer excellent overall system efficiency.