## 3 Start-Up Characteristics  
### Start-Up Characteristics with Rated Refrigerant Charge  
The dynamic characteristics during the start-up and shutdown of the air conditioner can be described based on the refrigerant movement inside the system [18]. When the compressor starts, the refrigerant is drawn from the evaporator and suction line and discharged into the condenser by the compressor. In the early stage of the compressor operation, the amount of refrigerant that condenses in the condenser is insufficient, and the refrigerant exits the condenser with the gas mixture; thus, the refrigerant entering the evaporator is insufficient for replacing the refrigerant that has been sucked out of the evaporator by the compressor. Consequently, the mass of the refrigerant in the evaporator decreases continuously, and the pressure and temperature change rapidly. The compressor continuously draws the refrigerant from the evaporator outlet and discharges it into the condenser; thus, the mass of the refrigerant in the condenser increases, and the refrigerant vapor gradually condenses. As the refrigerant at the outlet of the condenser becomes subcooled, the fluctuations of the pressure and temperature slow down.  
The results in this study are very similar to those of the study described above [18]. Figures 3 and 4 present the dynamic characteristics of the main components of the system at a rated refrigerant charge and outdoor and initial indoor temperatures of 35 \({}^{\circ}\)C. Figure 3 shows the change in the discharge and suction pressures of the compressor according to the elapsed time during the initial operation of the air conditioner. The discharge pressure rapidly increased until 70 s owing to the operation of the compressor; subsequently, its  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Outdoor** & **Initial Indoor** & **Refrigerant Charge** & **Charge** & \multirow{2}{*}{**Type**} \\  & **Temperature** & **Temperature** & **Amount (g)** & **Uncertainty (\%)** & \\ \hline
23 & 40 & 28 & 750 & 1.33 & Training \\
24 & 45 & 28 & 750 & 1.33 & Training \\
25 & 35 & 35 & 750 & 1.33 & Training \\
26 & 40 & 35 & 750 & 1.33 & Training \\
27 & 45 & 35 & 750 & 1.33 & Training \\
28 & 35 & 35 & 800 & 1.25 & Test \\
29 & 40 & 35 &slope decreased, and the discharge pressure converged to the steady-state value. The suction pressure greatly decreased in the first 80 s and then increased again until 100 s. Afterward, the pressure increased gradually and tended to converge to a steady-state value.  
The temperature distributions in the flow direction of the refrigerant in the condenser and evaporator at several time points after the start-up are shown in Figure 4. Before operation, the internal temperature of the condenser was uniform; it can be inferred that most of the condenser was occupied by superheated vapor. As the compressor started to run, the overall temperature of the condenser increased, and the refrigerant condensed and became subcooled at the outlet. After 230 s, three distinct phases appeared in the condenser; the occurrence of superheated refrigerant at the inlet and subcooled refrigerant at the exit can be clearly confirmed.  
In addition, the temperature of the evaporator was uniform before the operation of the air conditioner. As shown in Figure 3, the evaporation pressure decreased sharply owing to the operation of the compressor; thus, the temperature of the refrigerant at the inlet of the evaporator decreased at first. Subsequently, the superheating point shifted to the exit, and the saturated refrigerant section gradually increased. The evaporation temperature decreased rapidly at the beginning and then gradually increased over time.