| 摘要 |
<p>A porous preform vitrifying device which comprises a furnace core pipe receiving a porous preform, and a means for forcibly feeding nitrogen or the like into an exhaust gas pipe extending from the furnace core pipe in order to equilibrate the pressure between the furnace core pipe and the furnace body surrounding the same and to minimize pressure variation in the furnace core pipe, wherein on the basis of a signal indicating the difference between the pressure in the furnace core pipe and the pressure in the furnace body, the amount of gas to be fed into the furnace core pipe and furnace body and the amount of gas to be discharged therefrom are controlled and so is the amount of nitrogen to be fed from the furnace core pipe into the exhaust gas pipe.</p> |
