# A Bench Evaluation Test for Refrigeration Oils in a Refrigeration System Using a Screw Compressor
## 4 Results and Discussion
### Viscosity Evolution  
The viscosity of the oil and the refrigerant-oil mixture is important to ensure the compressor and the refrigeration system operate safely and reliably [3; 18; 19]. Figure 4 shows the viscosity evolution of the two studied oils. The viscosity at the first point of \(-100\) h represents the viscosity of the fresh oils before they are injected into the refrigeration system. The viscosity at the point of \(0\) h indicates the initial viscosity of the oils, after they are loaded into the refrigeration system with a few minutes of operation, to ensure all the oils are well mixed in the system.  
The viscosities of both of the two oils decreased after being injected into the system during the initial running for the first few minutes, which indicated that the refrigerant had effects on the oil viscosity. The oil characteristics are different between the fresh oil and refrigerant-oil mixture. Therefore, this implies that the bench test for the oil is important and meaningful.  
As shown in Figure 4, as the refrigeration system continues to operate, the viscosity of the oil A increased slowly and then reached a stable condition after the system operated for about \(400\) h.  
Figure 4: Oil viscosity evolution (Data at \(-100\) h represents the fresh oil before loading into the system; data at \(0\) h represents the initial oil after loading into the system; the vertical lines refer to the error bars).  
Figure 3: Variation of the compressor efficiency, \(\eta_{\mathrm{c}}\), during the testing.  
However, oil B shows a different trend. The viscosity of oil B continuously decreased during the first 300 h and then gradually stabilized. According to the standard (DIN 51503-2), a viscosity variation of \(\pm\)15% would trigger the oil drain. However, it was found that the viscosity of oil A decreased from 147.55 mm\({}^{2}\)/s to 105.95 mm\({}^{2}\)/s after being loaded into the system. This was a decrease of approximately 28.2%, which was higher than the 15% specified by the standard used. After 73 h of operation, its viscosity increased to 127.55 mm\({}^{2}\)/s, which was 13.6% lower than the fresh oil viscosity. After 500 h of operation, the viscosity increased to 139.22 mm\({}^{2}\)/s, which was 5.6% lower than the fresh oil viscosity. Since oil A is a mature oil that has been successfully applied in refrigeration systems, it could be concluded that a rapid initial decrease in the oil viscosity should not trigger the oil drain. These results indicated that the effectiveness of a given refrigeration oil should not be determined by the drop of the viscosity in the initial stage. The justification for the quality of the oil viscosity should depend on whether the system has reached stable operation. The decision of whether or not the oil should be replaced should be made only when the variation of the viscosity of the refrigeration oil in the system is larger than 15% during the stable operation. This further demonstrated that the bench test evaluation is important for justifying the efficacy of the refrigeration oil.  
The viscosity of the oil B dropped from 144 mm\({}^{2}\)/s to 128 mm\({}^{2}\)/s in the initial stage, representing a decrease of approximately 11%. The viscosity continued to drop to 100 mm\({}^{2}\)/s during subsequent testing, or approximately a 22% drop from the operating hours 50 to 275. Based on the aforementioned standard, this oil B does not meet the requirement and should be replaced. However, the viscosity soon stabilized between 100 mm\({}^{2}\)/s and 114 mm\({}^{2}\)/s for the rest of the experimental running period. This variation is less than 15%. Meanwhile, the compressor efficiency and discharge temperature, and the refrigeration capacity, did not show a significant change. Also, no rotor abrasion was found. These observations indicated that there was no need to drain the refrigeration oil. Thus, the current standard for the oil drain seems too strict and should consider the variation of the refrigeration oil during the long term operation.