# A Bench Evaluation Test for Refrigeration Oils in a Refrigeration System Using a Screw Compressor
## 2 The Theoretical Basis of an Accelerated Degradation Bench Test  
Synthetic fluids often lose their structural integrity via thermal, oxidative, and chemical degradation [1]. Increases in these reactions would result in an increase in degradation speed, resulting in low service life of the refrigeration oil. The degradation speed is related to chemical substances, temperature, and pressure. For most lubricants, the temperature is the main influential factor. The relationship between temperature and useful life time of many synthetic lubricants has been tested, as shown in references [16, 17]. It is found that the correlation of the synthetic lubricants between the working temperature, \(T\), and the service lifetime, \(H\), can be described by the following equation:  
\[\log H=aT+b \tag{1}\]  
where \(a\) and \(b\) are coefficients as shown in Table 1.  
It can be seen that the coefficient \(a\) for these oils is similar, between \(-0.0295\) and \(-0.03\). If the average value \(-0.0296\) was taken as the coefficient \(a\) for all of those oils, the lifetime of the lubricant oil under working temperature \(T_{1}\) and \(T_{2}\), representing the real operation temperature and the accelerated test temperature, respectively, would be calculated in the following equation:  
\[\log\frac{H_{1}}{H_{2}}=-0.0296(T_{1}-T_{2}) \tag{2}\]  
Therefore, the service time of the accelerated test can be obtained by Equation (2) with the combination of the lifetime under the real operation and the two temperature values. According to this equation, if the working temperature was increased by \(10\ ^{\circ}\)C, the lifetime would be reduced by \(50\%\). This provides the theoretical base for the accelerated degradation bench test. In this paper, in order to evaluate the lifetime of the refrigeration oil, the discharge temperature of the compressor is controlled to be much higher than normal working temperature in the actual refrigeration system. The refrigerant R134a is used as the working refrigerant in the experimental study, and the standard operating conditions are listed in Table 2.  
To accelerate the degradation process of the refrigeration oil, the discharge temperature of the studied compressor is increased to \(94\ ^{\circ}\)C. The testing condition is shown in Table 3.  
According to Equation (2), \(100\) h of service time of the refrigeration oil in the test bench system is equivalent to about \(1668\) h of service time in an actual system under the standard operating condition. Thus, \(500\) h of operating time under the experimental working conditions are set to evaluate the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Oils** & \(a\) & \(b\) \\ \hline Mineral Oils & \(-0.0295\) & \(5.8984\) \\ Alkyl Phosphates & \(-0.0296\) & \(6.3664\) \\ Alkyl Silicates & \(-0.0296\) & \(7.5068\) \\ Polyglycols & \(-0.0296\) & \(8.185\) \\ Synthetic Hydrocarbons and Diesters & \(-0.0296\) & \(8.8652\) \\ Silicones & \(-0.0296\) & \(9.5505\) \\ Polyol Esters & \(-0.0296\) & \(10.006\) \\ Polyphenyl Ethers & \(-0.03\) & \(13.152\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Coefficients \(a\) and \(b\) for Equation (1). \begin{table}
\begin{tabular}{c c} \hline \hline
**Item/Unit** & **Value** \\ \hline Evaporating temperature/\({}^{\circ}\)C & \(-15\) \\ Evaporating pressure/bar & \(1.64\) \\ Condensing temperature/\({}^{\circ}\)C & \(45\) \\ Inlet temperature of the compressor/\({}^{\circ}\)C & \(25\) \\ Discharge pressure of the compressor/bar & \(11.60\) \\ Discharge temperature of the compressor/\({}^{\circ}\)C & \(52\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Standard operating conditions of the refrigeration oil in an actual refrigeration system.