## 2 Current Methodology (ASHRAE 90.1-2019 Standard)  
A data center is different from ordinary commercial facilities, as it has a high sensible heat rate. Rack coolers are better if designed only for a sensible rate (without wet coils), and the coils can even be above the dew temperature [7]. Figure 1: Growth of global data center instances [2]. Figure 2: Distribution of electricity consumption in a typical DC with power usage effectiveness (PUE) = 2.1 [6]. Table 1 shows the minimum efficiency requirements and the Net Sensible COP in ASHRAE 90.1-2019 Standard indicated for floor-mounted air conditioners and condensing units serving computer rooms [8] considering the rating conditions for dry-bulb (DBT) and dew-point (DPT) temperatures. \begin{tabular}{c c c c c c} \hline \hline
**Equipment Type** & **Standard Model** & \begin{tabular}{c} **Net Sensible Cooling** \\ **Capacity COP** \\ \end{tabular} & \begin{tabular}{c} **Minimum Net** \\ **Sensible COP** \\ **(kW/kW)** \\ \end{tabular} & \begin{tabular}{c} **Rating Conditions** \\ **Return Air** \\ **(DBT/DPT)** \\ \end{tabular} & **Test Procedure** \\ \hline \multirow{6}{*}{Air-cooled} & \multirow{2}{*}{Downflow} & \(<\)23 kW & 2.70 & \multirow{6}{*}{\begin{tabular}{c} \\ \(\geq\)23 and \(<\)86 kW \\ (Class 2) \\ \end{tabular} } \\ \cline{3-5}  & & \(\geq\)86 kW & 2.36 & \multirow{6}{*}{\begin{tabular}{c} \\ (Class 2) \\ \end{tabular} } \\ \cline{3-5}  & \multirow{2}{*}{Upflow-duct} & \(<\)23 kW & 2.67 & \multirow{6}{*}{\begin{tabular}{c} \\ \(\geq\)23 and \(<\)86 kW \\ (DBT/DPT) \\ \end{tabular} } \\ \cline{3-5}  & & \(\geq\)86 kW & 2.33 & & \\ \cline{3-5}  & \multirow{2}{*}{Upflow-nonduct} & \(<\)19 kW & 2.16 & \multirow{2}{*}{\
\begin{tabular}{c} \\ \(\geq\)19 and \(<\)70 kW \\ (Class 1) \\ \end{tabular} } \\ \cline{3-5}  & & \(\geq\)70 kW & 1.89 & & \\ \cline{3-5}  & \multirow{2}{*}{Horizontal} & \(<\)19 kW & 2.65 & \multirow{2}{*}{
\begin{tabular}{c} \\ \(\geq\)19 and \(<\)70 kW \\ (Class 3) \\ \end{tabular} } \\ \cline{3-5}  & & \(\geq\)70 kW & 2.47 & & \\ \hline \hline \end{tabular}   
The values for the most used equipment, which is the downflow, are based on a fixed characteristic of return temperature and dew point based on Class 2. According to TC 9.9, classes are divided according to the types of equipment/needs of a data center, as shown in Table 2[9]. \begin{tabular}{c c c c c} \hline \hline
**2008 Classes** & **2011 Classes** & **Applications** & **Information Technology Equipment** & **Environmental Control** \\ \hline
1 & **A1** & &
\begin{tabular}{c} Enterprise servers, \\ storage products \\ \end{tabular} & Tightly controlled \\ \hline
2 & **A2** & & &
\begin{tabular}{c} Some control \\ \end{tabular} \\ \hline
3 & \begin{tabular}{c} **A3** \\ \end{tabular} & Data center & \
\begin{tabular}{c} Volume servers, \\ storage products, \\ personal computers, \\ workstations, \\ laptop, \\ printers \\ \end{tabular} &
\begin{tabular}{c} Some control, \\ use of free cooling \\ techniques when \\ allowable \\ \end{tabular} \\ \hline
3 & \begin{tabular}{c} **A4** \\ \end{tabular} & & \
\begin{tabular}{c} Office, \\ home, \\ transportable \\ environment, etc. \\ \end{tabular} &
\begin{tabular}{c} Personal computers, \\ workstations, \\ laptops, \\ printers \\ \end{tabular} & Minimal control \\ \hline
4 & C & \
\begin{tabular}{c} Point of sale, \\ industrial, \\ factory, etc. \\ \end{tabular} &
\begin{tabular}{c} Point of sale equipment, \\ ruggedized controllers or computers, \\ PDAs \\ \end{tabular} & No control \\ \hline \hline \end{tabular}   
A1--A data center environment with strict control of the psychometric parameters: dry-bulb temperature (DBT), dew-point temperature (DPT), relative humidity (RH), and in a mission-critical operation. Generally developed for large companies with a large number of racks.