# A Review on Two-Phase Volumetric Expanders and Their Applications
## 8 Discussion
### Comparison of the Two-Phase Devices  
The literature overview allowed for the comparison of the performance of devices in the presence of two-phase flows, as employed in heat pumps, cooling applications, and power production, as summarized in Table 5. Nonetheless, most of the data came from studies about heat pumps and cooling cycles, since they appeared to be a debated topic to employ two-phase expanders. As a result, CO\({}_{2}\) seemed to be the working fluid primarily involved in assessing the performance of the devices (Figure 20). However, the literature indicated that other fluids were also employed in these applications, such as R134a, R245 fa, and R22. Conversely, water steam was the fluid mostly investigated for power production. Figure 19: Timeline of the research about two-phase volumetric expanders. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multicolumn{2}{c}{ \begin{tabular}{c} **Tolerance to** \\ **Wet** **Expansion** \\ \end{tabular}  } & \multirow{2}{*}{\(\mathbf{P}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{T}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{N}\)} & \multirow{2}{*}{\(\hat{m}\)} & \multirow{2}{*}{\(\mathbf{P}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} & \multirow{2}{*}{\(\mathbf{\eta}_{\text{in}}\)} \\  & & & & & & & & & & & & & & \\ \hline \multirow{2}{*}{Scroll} & \multirow{2}{*}{Medium} & \multirow{2}{*}{8–131} & \multirow{2}{*}{20–145} & \multirow{2}{*}{4000–104} & \(1.8\times\) & \multirow{2}{*}{0.4–12} & 0.70–0.80 & & 0.66–0.65 & 0.60–0.60 & 0.80–0.60 & Medium \\  & & & & & & & & & & & & \\ \multirow{2}{*}{Screw} & \multirow{2}{*}{High} & \multirow{2}{*}{3–100} & \multirow{2}{*}{40–180} & \multirow{2}{*}{400–180} & \(1500\)–0.02–24.80 & \(6\)–2300 & 0.10–0.20–0.0 & & 0.27–0.50–0.60 & Medium \\  & & & & & & & & & & & & \\ \multirow{2}{*}{Vanes} & \multirow{2}{*}{Medium} & \multirow{2}{*}{21–91} & \multirow{2}{*}{32–54} & \multirow{2}{*}{500–3000} & \(0.092\)–0.200 & 0.138–0.90 & \(0.15-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.20-0.900-0.500-0.78 & Medium \\ \multirow{2}{*}{Piston} & \multirow{2}{*}{Medium–low} & \multirow{2}{*}{12–100} & \multirow{2}{*}{33–120} & \multirow{2}{*}{500–3000} & \(0.091\)–0.0–0.0 & \(0.20-0.20-0.20-0.50-0.10-0.50-0.10-0.50-0.10-0.50-0.10-0.50-0.10-0.50-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10-0.10.10-0 The survey revealed how scroll machines exhibited the highest isentropic efficiency in a restricted range between 0.70-0.80, while the other devices showed values belonging to a broader interval between 0.10-0.70 depending on the working conditions. Nonetheless, the aim to provide a complete assessment of the devices required a comparison in terms of overall efficiency, since it is also indicative of the mechanical efficiency. As a result, the literature revealed that twin screw machines appeared to be the most promising to handle two-phase fluids with a high liquid fraction in a wide range of operating conditions, as revealed by the highest values of the overall efficiency of between 0.27-0.56 [8].