# Safety and Energy Implications of Setback Control in Operating Rooms during Unoccupied Periods
## 2 Materials and Methods
### Data Analysis
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Standard**} & \multirow{2}{*}{**OR Type/Ventilation Mode**} & \multirow{2}{*}{\begin{tabular}{c} **Minimum Total Air** \\ **(ach) 1** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Pressure** \\ **Relationship to** \\ **Adjacent Areas** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Minimum** \\ **Pressure** \\ **Differential** \\ \end{tabular} } \\ \hline \multirow{3}{*}{
\begin{tabular}{c} UNE 171340:2020 \\ (Spain) [21] \\ \end{tabular} } & High risk ORs/ & \multirow{3}{*}{20} & \multirow{3}{*}{15} \\  & unidirectional airflows & & & \\ \cline{1-1} \cline{4-5}  & High risk ORs/ & & & \\ \cline{1-1} \cline{4-5}  & mixing ventilation & & & \\ \cline{1-1} \cline{4-5}  & General & & & \\ \cline{1-1} \cline{4-5} ANSI/ASHRAE/ASHE & All ORs/Unidirectional & & & \\ \cline{1-1} Standard 170-2021 & airflow & & & \\ \cline{1-1} \cline{4-5} (USA) [9] & & & & \\ \hline \hline \end{tabular}  
* ach: air changes per hour.  
\end{table} 
Table 4: Ventilation requirements in operating rooms (ORs). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Test** & **Duration (hours)** & **Dates** \\ \hline
1 & 78.7 & 7-10 December \\
2 & 83.3 & 17-21 January \\
3 & 13.6 & 7-8 April \\
4 & 65.5 & 26-29 July \\
5 & 83 & 15-19 August \\ \hline \hline \end{tabular}  
ORs must be always be maintained at a positive pressure with respect to the adjoining spaces. Table 4 gathers the minimum air changes per hour and the pressure relationships to adjacent areas specified in the Spanish Standard [21] and the ANSI/ASHRAE/ASHE Standard 170-2021 [9].  
\end{table} 
Table 3: Tests dates and duration. Since it operates with chilled water, the electric energy savings derived from the effect of the setback in the cooling coil demand can be calculated by multiplying the result from Equation (2) with the European Seasonal Energy Efficiency Ratio (ESEER) of the chiller. The air-cooled chiller ESEER is 4.83, thanks to the integration of the adiabatic cooling pads at the air inlet of the condenser [22]. Similarly, the energy savings in the heating coil are due to the lower ventilation thermal loads:  
\[Heating\ coil\ savings=\left(\dot{V}\cdot\frac{1}{v}\cdot\Delta h_{\mathit{sens\ hc}}-\dot{V}_{\mathit{setback}}\cdot\frac{1}{v}\cdot \Delta h_{\mathit{sens\ hc\ setback}}\right)\cdot\Delta t \tag{3}\]  
In this case, the enthalpy variation considered \(\Delta h_{\mathit{sens\ hc}}\) is only sensible heat and corresponds to the heating of the outdoor conditions up to the set supply temperature during the regular operations or during the setback (Table 2). In this case, energy savings in the heating coil involve natural gas savings in the boiler, whose performance, \(\eta\), is 97%. The energy savings associated with natural gas consumption can be calculated by dividing the results from Equation (3) from the boiler performance. Finally, the energy savings due to less vapor injection will depend on the humidity ratio variation achieved in the treated air, \(\Delta w\), and the water vapor latent heat, \(C_{L}\):  
\[Vapor\ injection\ energy\ savings=\left(\dot{V}-\dot{V}_{\mathit{sup}}\right) \cdot\frac{1}{v}\cdot\Delta w\cdot C_{L}\cdot\Delta t \tag{4}\]  
All the psychometric variables required are calculated from the measured ones (Table 1) through the known psychrometric expressions [23]. The CO\({}_{2}\) emissions corresponding to the natural gas and to the electricity consumption are obtained through the conversion factors for Spain [24]:  
\[Carbon\ emission\ conversion\ factor\ for\ natural\ gas=0.252\ \mathrm{kg_{CO_{2}}/kWh} \tag{5}\]  
\[Carbon\ emission\ conversion\ factor\ for\ electricity=0.331\ \mathrm{kg_{CO_{2}}/kWh} \tag{6}\]