# A Bench Evaluation Test for Refrigeration Oils in a Refrigeration System Using a Screw Compressor
## 1 Introduction  
Refrigeration oil is very important for lubricating, sealing, cooling and clearing the system of unwanted particles in the flow [1] in vapor compression refrigeration systems. In addition, the oil can decrease the vibration of the compressor [2]. This topic has been widely researched, and several standards have been formulated to select refrigeration oils for different types of compressors. These standards suggested different physical and chemical indexes for compressors under varying operating conditions. Thus, most research focused on the characteristics of fresh refrigeration oil. However, the physical and chemical indexes of fresh refrigeration oils sometimes failed to reflect the actual operating characteristics [3]. Performance tests of the refrigeration oil in refrigeration systems were often recommended for evaluating refrigeration oils. When the fresh refrigeration oil is injected into the compressor, chemical reactions, such as oxidation and decomposition during the compression process, result in degradation of the refrigeration oil [4]. These changes then alter the physical and chemical indexes, leading to failure of the fluid to provide proper lubrication. Reaction principles of lubricant oils for engines have also been well researched. The degradation was mainly caused by chemical reactions, while the reaction speed was related to the working temperature [5]. In addition to these studies, chemical models of the effects of the oil on different components within the system were related to the temperature [6]. For refrigerant oil, the chemical situation is much clearer, due to no oxidizing agent being present in the system. Thus,considering degradation, synthetic oil often performs better than mineral oil, due to its high chemical stability. It is reported that biodegradable lubricants could be used in the refrigeration system, but have not yet been widely applied in compressors [7, 8] due to the problem of degradation. Compared to the fresh refrigeration oil, physical and chemical indexes of the used refrigeration oil are more important to predict the oil life. Thus, research works have been carried out to determine the physical and chemical indexes for the oil drain. Standards, such as DIN 51503-2, suggest \(\pm\)15% of viscosity and acid value for the oil drain. For this reason, a bench evaluation test is often recommended to investigate the newly developed fresh oil. Since the degradation of the oil is long, the accelerated life test method is adopted. According to the literature [9], the rate of oil aging (degradation) is doubled if its working temperature is increased by 10 \({}^{\circ}\)C. Thus, a bench evaluation test of the fresh oil for air compressors can be carried out easily, because the oxidation of the fresh oil in an air compressor is accelerated. Such bench evaluation test results of lubricating oils for the reciprocating air compressors and screw air compressors have been applied successfully [10]. Another method to evaluate the oil drain is to monitor the status of the lubricating oil during operation by continuously testing the metal content in the lubricating oil [11]. However, such a test system is quite complicated. This issue becomes even more complicated when refrigeration oil is considered. Apart from the standard physical and chemical indexes of the refrigeration oil, the concentration of the refrigeration oil in the refrigerant is also an important factor in the refrigeration system [12] that adds another dimension of complexity to the analysis. Many research efforts have focused on both the oil concentration, as well as physical and chemical indexes of the refrigeration oil [13]. [PERSON] et al. [3] investigated the effects of refrigeration oil on the performance of an R600a refrigerator with a reciprocating compressor, using bench evaluation tests. However, very little literature was found to focus on bench evaluation tests for the life of the refrigerator oil.