The vacuum system must handle the complete gas-flow and the significant amount of dust, created by condensed outgassing of the melt. Typical suction speed demand at the degassing pressure (~0.67 mbar) is between 50,000 m³/h to 250,000 m³/h. Required evacuation time: approximately 4-6 min. Fast evacuation is important, as the melt is continuously cooling down during the treatment. To avoid slag- foaming, a controlled pump-down process is often necessary.