# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 3 Results
### Analysis of Energy Consumption and Comfort in Different Ventilation Systems  
The energy consumption of a ventilation system consists of the energy required for air transportation and the energy consumed for air conditioning. The energy consumption for air transportation refers to the energy required for the fan to drive airflow through the ducts, which can be calculated using Equation (13). \[P_{F}=kQ^{3} \tag{13}\]  
In the equation, PF represents the energy consumption for air transportation (kW) and k is a coefficient determined by the fan type. The energy consumption for air conditioning refers to the energy required to handle the load inside the subway carriages. The energy consumption for air conditioning can be calculated using Equations (14) and (15). \[P_{C}=\frac{Q_{L}}{COP} \tag{14}\]  
\[Q_{L}=c_{p}Q\Delta t \tag{15}\]  
In the equation, PC represents the energy consumption for air conditioning (kW), QL is the HVAC system load (kW), COP is the coefficient of performance of the air conditioning system, cp is the specific heat capacity of air at constant pressure (kJ/(kg\(\cdot\)K)), \(\rho\) is the air density (kg/m\({}^{3}\)), Q is the fresh air volume (m\({}^{3}\)/s), and \(\Delta\)t is the temperature difference between the outlet of the total heat exchanger in the HVAC system and the return air inlet of the subway carriages. In this study, when calculating ventilation energy consumption, COP was set to 4.2, cp was set to 1.013 kJ/(kg\(\cdot\)C) (corresponding to dry air at 25 \({}^{\circ}\)C), and air density was dynamically calculated using the incompressible ideal gas state equation. The outdoor design temperature was set to 34.8 \({}^{\circ}\)C (according to the relevant standard GB 50736-2012) [39]. The coefficient k was set to 0.8. It was assumed that the HVAC system included a total heat exchanger, with a heat exchange temperature difference of 2 \({}^{\circ}\)C. Details can be found in Table 4. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Parameter** & **Value** & **Unit** & **Explanation** \\ \hline COP & 4.2 & & According to the relevant standard China Academy of Building Science (2015) [40] \\ \hline C\({}_{\text{P}}\) & 1.013 & kJ/(kg\(\cdot\)\({}^{\circ}\)C) & Corresponding to dry air at 25 \({}^{\circ}\)C \\ \hline \(\rho\) & 1.169 & kg/m\({}^{3}\) & Corresponding to dry air at 25 \({}^{\circ}\)C \\ \hline T\({}_{\text{i}}\) & 25 & \({}^{\circ}\)C & According to the relevant standard China Academy of Building Science (2012) [39] \\ \hline T\({}_{\text{o}}\) & 34.8 & \({}^{\circ}\)C & According to the relevant standard China Academy of Building Science (2012) \\ \hline k & 0.8 & kJ-s\({}^{2}\)/m\({}^{9}\) & According to the relevant literature [PERSON] et al. (2021) [41] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Setting of parameters related to energy consumption calculation. Since the temperature remains constant, the primary factor affecting energy consumption is the air supply velocity. As shown in Table 5, when both ventilation systems operate at the same air supply velocity, the SFRC system demonstrates a higher overall energy-saving rate than the MV system. Based on the boundary conditions of the supply and return air outlets of both ventilation systems, the SFRC system exhibits lower energy consumption during the air-conditioning process. This may be attributed to differences in the area of the supply and return air outlets. The table indicates that at the maximum air velocity, the SFRC system achieves energy savings of 9.59%, while at a moderate air velocity, the energy savings reach 34.91%. For the MV system, apart from the low-air velocity condition where energy consumption in the air-conditioning process remains the same, no significant energy-saving advantage is observed. Compared to previous indicators such as CR and PRP, the SFRC system demonstrates greater efficiency and safety. As a common mode of daily transportation, the airflow distribution inside subway cars plays a significant role in passenger comfort.