# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 4 Results and Discussion
The lowest values of parameter \(a\) were calculated for rotor C, where a low rate of gas bubble dispersion was observed, as well as a non-uniform distribution of the bubbles in the liquid volume. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Parameter**} & \multicolumn{5}{c}{\(a\)\(\cdot\)10\({}^{3}\)} \\ \hline \multicolumn{2}{c}{**Gas Flow Rate**} & \multicolumn{1}{c}{**(NI\(\cdot\)min\({}^{-1}\))**} & **13** & **15** & **17** & **19** & **21** \\ \hline \multirow{5}{*}{Rotor type} & A & (1) & \(-51.6\) & \(-48.2\) & \(-44.4\) & \(-42.3\) & \(-41.0\) \\  & B & (1) & \(-57.3\) & \(-54.0\) & \(-49.0\) & \(-46.0\) & \(-45.5\) \\ \cline{1-1}  & C & (1) & \(-70.4\) & \(-64.2\) & \(-60.1\) & \(-56.7\) & \(-52.1\) \\ \cline{1-1}  & D & (1) & \(-38.7\) & \(-36.2\) & \(-34.6\) & \(-33.3\) & \(-32.3\) \\ \cline{1-1}  & E & (1) & \(-55.8\) & \(-52.2\) & \(-49.3\) & \(-46.5\) & \(-43.3\) \\ \cline{1-1}  & F & (1) & \(-43.9\) & \(-41.5\) & \(-38.2\) & \(-37.4\) & \(-34.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Values of performance parameters \(a\) depending on refining conditions. Figure 6: Demonstration of the concentration curves obtained by physical modelling; gas flow rate: 17 NI\(\cdot\)min\({}^{-1}\), rotor speed: 350 rpm; (**a**) impeller A; (**b**) impeller B, (**c**) impeller C, (**d**) impeller D, (**e**) impeller E, (**f**) impeller F.  
Comparing the rotors according to their efficiency, their ranking (starting from the most efficient) is: D, F, A, followed by E and B, and finally C with the lowest rated power. As illustrated by Figure 7a, the gas flow provides a higher effect in rotors with a worse rating than those with a better rating. Furthermore, the differences between the rotors are higher at low gas flow rates than at higher flow rates. With higher gas flow rates, the bubbles become larger and they coalesce, thus decreasing the phase interface area [33; 37]. This combined effect probably caused the slowing down of the degassing effect of the rotors with increasing inert gas flow rates. As mentioned in the rotor descriptions, three propeller-type rotors (C, D, F) and three pump-type rotors (A, B, E) were tested. The evaluation results show that the two best rotors, D and F, were propeller-type rotors. As seen in Figure 3, these two rotors were the only ones which were not circular and which were the most complex. These characteristics meant that they were apparently more capable of transmitting the momentum to the model liquid. The transmitted momentum is involved in the creation of turbulence and thus affects bubble fragmentation and the transfer of mass. In contrast, rotor C was the least complexed, its shape was very compact and included only a hint of blades along the circumference. This shape was shown to provide the least efficiency. However, it should be noted that under operational conditions, rotors with more complex shapes may be subject to faster wear. This also depends on the material used to make the rotors and its interaction with the liquid metal. The expected calculated degassing times according to Equation (3) for each rotor are shown in Table 4. Figure 7b shows a graphical representation of expected refining times. In order to compare the expected degassing times, the required final concentration and the working speed of the rotor must be selected; the final concentration of 50% with 350 rpm was taken as an example. As shown in Figure 7b, the shortest time to achieve the required degassing quality was needed by rotor D, namely 77 s with a gas flow rate of 13 Nl-min\({}^{-1}\) and a speed of 350 rpm. Rotor F needed 87 s. However, according to Figure 7b, if the gas flow rate for rotor F increases to 17 Nl-min\({}^{-1}\), its efficiency would be virtually identical to that of rotor D at 13 Nl-min\({}^{-1}\), while other rotors would not achieve this performance even with the highest studied gas flow rate of 21 Nl-min\({}^{-1}\).