## 3 Start-Up Characteristics
### Start-Up Characteristics with Refrigerant Charge  
The start-up characteristics were investigated according to the refrigerant charge at outdoor and indoor temperatures of 35 \({}^{\circ}\)C. Figure 5 presents the change in the condensation temperature for five refrigerant charges at the beginning of operation. The condensation  
Figure 4: Temperature distribution in heat exchanger with rated refrigerant charge; (**a**) Condenser, (**b**) Evaporator.  
Figure 3: Variation in compressor suction and discharge pressure with rated refrigerant charge.  
temperature increased rapidly when the compressor started operating. After approximately 120 s, the condensation temperature varied depending on the refrigerant charge amount; in addition, the condensation temperature increased more rapidly with the increasing refrigerant charge amount. Under overcharging conditions, the dynamic characteristics were similar to the rated charge amount until 220 s and then showed small deviations. After 220 s, the condensation temperature approached the steady-state values for 850 and 750 g; in the 950 and 650 g cases, it increased until approximately 580 s, and that of 550 g kept increasing within the investigated time range (Figure 5).  
The degree of superheat remained approximately constant, although the refrigerant charge changed owing to the active control of the EEV opening. However, as the refrigerant charge increased owing to the accumulation of refrigerant on the condenser, the condensation pressure increased, which increased the degree of subcooling. Since the degree of subcooling is sensitive to the refrigerant charge amount, it is widely used as a diagnostic parameter to determine the refrigerant charge amount [15; 16; 17]. As shown in Figure 6, the degree of subcooling exhibited a large increase with increasing refrigerant charge amount after 120 s and slowly decreased after 600 s. The degree of subcooling was proportional to the refrigerant charge amount; in addition, the degree of subcooling of 550 g was zero, which indicates that subcooling did not occur in this case.  
As shown in Figure 7, the evaporation temperature rapidly decreased during the initial 60 s of operation. The lower the refrigerant charge, the less refrigerant was in the evaporator, and the more the evaporation temperature decreased. The evaporation temperature decreased at approximately 30 s until it reached the minimum; afterward, it  
Figure 5: Variation in condensation temperature over time with refrigerant charge amount.  
Figure 6: Variation in degree of subcooling over time with refrigerant charge amount.  
experienced repeated rebounds until 220 s and stabilized. However, distinguishing the differences in the evaporation temperatures with respect to the refrigerant charge amount was difficult.  
The compressor discharge temperature was affected by the compressor inlet and outlet pressures, which tended to increase as the condensation temperature increased and the evaporation temperature decreased. As shown in Figure 8, the discharge temperature of the compressor increased more rapidly with the decreasing refrigerant charge amount at the beginning of operation because the suction pressure was relatively low at the beginning. After 120 s, the smaller the refrigerant charge, the more moderate the discharge temperature increase because the condensation temperature or pressure was relatively low. The condensation and evaporation temperatures increased with increasing refrigerant charge according to Figures 5 and 7. These two temperature effects on the discharge temperature offset each other; the cases with charge levels of 750 g or higher exhibited similar dynamic characteristics.