# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 2 Experimental Procedure  
Experimental measurements were made using a self-developed water-based physical model of the FDU, illustrated in Figure 1, which includes a description of the individual components. The dimensions of the physical model of the aluminium refining ladle are shown in Figure 2. The model was constructed in the scale 1:1 while observing dynamic and kinematic similarity as described in detail in [42]. The actual system of aluminium, hydrogen and nitrogen as the refining gas was replaced with water, oxygen and argon. These model media achieve similar values at 20 \({}^{\circ}\)C as the actual media at the operating temperature [9]. A comparison of the properties of the media is provided in Table 1. The basic principle of physical modelling of dissolved hydrogen removal from the aluminium melt was thus represented by dissolved oxygen removal from water using argon. In-Pro6860i/12/120/mA Ex optical probes by METLER-TOLEDO (Greifensee, Switzerland), including their components allowing the measurement of dissolved oxygen content up to 26 mg\(\cdot\)L\({}^{-1}\) (26 ppm), were chosen for continuous measurement of the oxygen content. In order to maintain measurement accuracy, we regularly calibrated the probes with the special iLink-RS485/VP8 device by METLER-TOLEDO (Greifensee, Switzerland), which contains ambient pressure and relative humidity sensors. The measurement accuracy of the probes is \(\pm 1\%\). A system had to be developed for the probes, which would comply with the requirements for oxygen concentration measurement including communication via a PC, data archiving and data evaluation. A more detailed description of the model design and the measurement methodology is available in [42]. The measurement methodology consisted of the following steps:  
* Establish the required position of the vessel using a lifting platform in order to ensure the adequate working height of the rotor;
* Fill the model with water; the water level reaches a height of 720 mm from the upper edge of the vessel;
* Set the required gas flow according to the requirements of the given experiment variant while keeping a gas pressure of 0.4 bar;
* Establish saturation with oxygen to the value of 18 ppm with the rotor rotational speed of 350 rpm and gas pressure of 0.4 bar. This is followed by a delay of 120 s; during this time, the oxygen saturation runs down, the oxygen concentration becomes homogenized in the water volume and stabilized at the required value of 22 ppm. The rotor speed is off during this delay period;
* Set the required rotor speed for the given experiment variant;
* Start the measurement after the delay period (start argon blowing, rotor speed and recording of data from the optical probes at the same time);
* Stop the measurement (switch off the rotor and stop the gas feed) as soon as the mean oxygen concentration from the measuring probes reaches 0.5 ppm. Figure 1: Description of physical model workspace. Figure 2: Dimensional diagram of the vessel. This study evaluated 6 different types of rotors. Each rotor is composed of a shaft and an impeller as illustrated in Figure 3a. The studied impellers are shown in Figure 3b. The rotor is connected to the shaft by a threaded connection, or the rotor and shaft are made as a monoblock, as is the case with rotors D and E. Impellers A, B and E are circular with diameters of 190 mm. The height of rotor A is 115 mm and its upper part is indented. The design of impeller E is very similar; this impeller differs in its height (81 mm) and shaft coupling. Impeller B with a height of 105 mm has circumferential protrusions. Impeller C is star-shaped with a diameter of 192 mm and height of 90 mm. The most complex shape is shown by impeller D with an external diameter of 192 mm and height of 80 mm. A square impeller was also tested--impeller F with a side length of 134 mm and height of 61 mm. The foundry used the manufacturer's recommended working height for the rotors. To closely simulate operating conditions, physical modelling also adhered to these recommendations.