Vacuum degassing method and its apparatus

A vacuum degassing method for degassing molten glass, the method comprising the steps of:

• providing a vacuum degassing vessel in a vacuum housing;
• sucking a molten substance into the vacuum degassing vessel through a first uprising pipe;
• controlling a first flow quantity of the molten substance rising in the first uprising pipe by rotation of a first flow quantity control means which is positioned in a first downfalling pipe, said first downfalling pipe being provided upstream of said first uprising pipe and downstream of a storage area for said molten substance with respect to a flow direction of said molten substance and having a bottom which is connected to a bottom of the first uprising pipe;
• degassing the molten substance in the vacuum degassing vessel such that the degassed molten substance flows down through a second downfalling pipe;
• and
• controlling a second flow quantity of the degassed molten substance flowing in the second downfalling pipe by rotation of a second flow quantity control means positioned in a second uprising pipe, said second uprising pipe being provided downstream of the second downfalling pipe with respect to a flow direction of said degassed molten substance and having a bottom which is connected to the bottom of the second downfalling pipe;
• wherein top portions of the first and second flow quantity control means are positioned outside of the vacuum housing, and the first and second flow quantity control means maintain a first surface of the molten substance int he vacuum degassing vessel at a level which is substantially the same as a level of a second surface of the molten substance which is in said storage area before said degassing step.