# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 5 Conclusions  
The paper evaluates the degassing performance of rotors designed for refining aluminium melt in an FDU. Extensive experiments were carried out for this purpose using a physical model, aimed at evaluating and comparing the efficiency of degassing using six types of refining rotors. Two types of results were obtained using the physical model: visualization of the flow in order to map the gas behaviour in the liquid, and curves demonstrating oxygen concentration loss from the liquid. The concentration curves were evaluated based on a previously published data-processing method using the measured performance parameters, and the evaluation was demonstrated by determining the expected degassing time for certain defined gas flow rates. Such a comparison makes it possible to compare operating parameters of various rotors and select the most suitable with respect to the degassing time or gas consumption or other operating and economic parameters. The six rotors were evaluated not only to determine their order according to performance but also to demonstrate how the performance differences can be compensated. Differences were shown in the performance of some rotors, which can be compensated by modifying the gas flow rate. Other differences were so substantial that the different capabilities of the rotors could not be compensated even by moving from the lowest to the highest flow rate. The research findings can be summarized as follows:  
* The gas distribution pattern in the liquid was similar for all rotors. The highest concentration of gas was found around the rotor, from where the gas bubbles spread radially and ascended to the surface, while being concentrated predominantly around the shaft. Rotors A, D and F were better in terms of distributing the gas bubbles all the way to the walls of the vessel and below the rotor line. Rotors D and E were better able to disperse the gas to fine bubbles. * The ranking of the rotors, evaluated based on the dimensionless parameter \(a\), from the most efficient to the least efficient was as follows: D, F, A, E, B and C. As can be seen from the results, by changing rotor C to rotor D, the degassing efficiency can be increased by up to 45%. * The efficiency of the rotors corresponded well with the assumptions determined based on literature knowledge regarding the distribution and behaviour of gas bubbles in the liquid. Rotors D, F and A with higher degassing efficiency produced smaller bubble sizes and had a better ability to distribute bubbles throughout the ladle volume. * The degassing performance increased with increasing gas flow rates for all rotors. In the rotors with higher performance, an increase in the inert gas flow rate had a smaller effect in terms of increased degassing intensity. When the flow rate increases from 13 NI\(\cdot\)min\({}^{-1}\) to 21 NI\(\cdot\)min\({}^{-1}\), for rotor D the parameter \(a\) drops by 19.8%, while for rotor C, it drops by 35.1%. * The two best-ranked rotors (E and F) were propeller-type rotors. These rotors exhibit the highest complexity of their shape, which resulted in better transmission of the momentum to the liquid and a higher efficiency of degassing. On the other hand, in industrial conditions, rotors with complex shapes may provide a substantially shorter service life due to faster wear. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multicolumn{2}{c}{**Parameter**} & \multicolumn{5}{c}{**Degassing Time**} \\ \hline
**Gas Flow Rate** & **(NI\(\cdot\)min\({}^{-1}\))** & **13** & **15** & **17** & **19** & **21** \\ \hline \multirow{6}{*}{Rotor type} & A & (s) & 102 & 95 & 88 & 84 & 81 \\  & B & (s) & 113 & 107 & 97 & 91 & 90 \\ \cline{1-1}  & C & (s) & 139 & 127 & 119 & 112 & 103 \\ \cline{1-1}  & D & (s) & 77 & 72 & 69 & 66 & 64 \\ \cline{1-1}  & E & (s) & 110 & 103 & 98 & 92 & 86 \\ \cline{1-1}  & F & (s) & 87 & 82 & 76 & 74 & 68 \\ \hline \end{tabular}
\end{table} 
Table 4: Prediction of degassing time depending on the conditions of refining (resulting standard concentration: 50%, rotor speed: 350 rpm).