Porous glass base material manufacturing apparatus, method for manufacturing porous glass base material, and method for manufacturing glass base material for optical fiber

A method for manufacturing porous glass base material comprising:

• supplying a liquid raw material containing an organic siloxane raw material in a liquid state supplied from a raw material tank to a vaporizer;
• mixing the liquid raw material of the organic siloxane with a carrier gas in a vaporizer and vaporizing the liquid raw material to make the raw material mixed gas in which the raw material gas and carrier gas are mixed;
• supplying the raw material mixed gas to a burner via a raw material gas pipe;
• and
• forming soot of porous glass particles on the surface of a starting base material by releasing the raw material mixed gas into the flame of the burner that moves relative to the starting base material along the longitudinal direction of the starting base material rotating around a rotation axis along the longitudinal direction,
• wherein the raw material gas pipe is insulated and kept warm by double insulation, which is a combination of an inner layer heat-insulation material provided on the outside of the raw material gas pipe and an outer layer heat-insulation material provided on the outside of the inner layer heat-insulation material.