## 3 Results and Discussion
Figure 9 also shows that with the increase of the charge, the LIFT experimental value is almost unchanged at 600 RT and the LIFTs of the system are reduced by 0.11 \({}^{\circ}\)C and 0.18 \({}^{\circ}\)C at 700 and 800 RT, respectively. Figure 8: Liquid entrainment factor as a function of charge at each cooling capacity. The influence of the variation of the cooling capacity, evaporation temperature and condensation temperature on COP can be considered by the calculation based on the compressor efficiency map, which is obtained at a certain suction superheat, so it can be considered that there is no liquid carryover effect. Therefore, the difference between the calculated and test COP can reflect the effect of the liquid entrainment factor. Figure 10 shows the efficiency factor contours of the test compressor, where \(\Phi\) is the percent of the rated volumetric flow rate and \(\Psi\) is the percent of the rated polytropic work, as described by Equations (24) and (26), respectively. \[\Phi=\frac{V_{s}}{V_{n}} \tag{24}\]  
where \(V_{n}\) is the rated volumetric flow rate, \(V_{n}\) = 1.283 m\({}^{3}\cdot\)s\({}^{-1}\). \(V_{s}\) is the refrigerant volumetric flow rate at evaporator state, calculated by Equation (25). \[V_{s}=m_{t}v_{e} \tag{25}\]  
where \(v_{e}\) is the specific volume of refrigerant vapor at evaporator state, determined by the evaporation pressure and the suction enthalpy. \(m_{t}\) is the refrigerant flow rate calculated by Equation (1) assuming no liquid carryover. \[\Psi=f\ln(p_{c}/p_{e})\frac{p_{c}v_{c}-p_{e}v_{e}}{\ln(p_{c}v_{c}/p_{e}v_{e})}/ w_{n} \tag{26}\]  
where \(p_{c}\), \(p_{e}\) are condensation and evaporation pressure respectively, \(v_{c}\) is the specific volume of refrigerant vapor at condenser state, determined by the condensation pressure and discharge enthalpy; \(f\) correction factor is introduced to correct the error of calculating the polytropic compression work by using the approximate polytropic process equation [22]. \(f\) is about 1.005 for all test points. \(w_{n}\) is the rated specific polytropic work, \(w_{n}\) =21.4 kJ\(\cdot\)kg\({}^{-1}\). Once \(\Phi\) and \(\Psi\) are known, the efficiency factor \(\eta\) of the compressor is obtained from the compressor map, then the polytropic efficiency \(\eta_{c}\) is calculated by Equation (27). \[K=\frac{Q_{e}}{A_{e}\Delta t} \tag{22}\]0  
where \(\eta_{n}\) is the rated compressor efficiency, \(\eta_{n}\) = 0.802, provided by the chiller manufacturer. By adding the polytropic compression power, transmission loss and motor loss, the input power of the chiller can be obtained, thereby obtaining the calculated COP. \[K=\frac{Q_{e}}{A_{e}\Delta t} \tag{22}\]1 \(t_{e}\), LIFT as a function of charge at each cooling capacity. Figure 10 also shows each test point. It can be seen from the figure that the test points coincide under each cooling capacity. The variation of \(\Phi\) and \(\Psi\) under each cooling capacity is less than 1.0% and the variation of efficiency factor is less than 0.5%. It can also be seen from the figure that if LIFT (\(\Psi\)) remains unchanged, the compressor efficiency factor increases with the increase of cooling capacity under part load. Figure 11 shows the variation of test and calculated COP with refrigerant charge at each cooling capacity. The calculated COP values of each curve in Figure 11 were corrected by a constant (0.06, \(-\)0.03, \(-\)0.05 for 600, 700, 800 RT, respectively) so that the calculated COP at the first point of each curve coincides with the experimental value, to eliminate the influence of the calculation model error and clearly reflect the trend of the influence of the liquid entrainment on COP. As can be seen from Figure 11, the variation of the calculated COP is small (less than 0.5%) for each curve, mainly because the temperature LIFT is almost constant for each curve (0-0.18 \({}^{\circ}\)C, see Figure 9) and the variations of the cooling capacity are also small for each curve (1.1-1.4%).