Furnaces having dual gas screens and methods for operating the same

Abstract

Furnace assemblies are provided including a furnace defining an internal process chamber extending therethrough. A first gas screen is coupled to the furnace. The first gas screen is configured to introduce a first gas into the internal process chamber at a first end of the furnace. A second gas screen is positioned adjacent to the first gas screen at an opposite end of the first gas screen from the furnace. The second gas screen is configured to introduce a second gas to provide a seal for the first end of the furnace. The furnace may be a draw furnace and the process chamber may be an internal draw chamber.

1 - 18 . (canceled).  
   
   
       19 . A method for providing a desired gas flow in a draw furnace for manufacturing optical fiber, the method comprising the steps of: 
 providing a first gas screen positioned adjacent a first end of the draw furnace;    providing a second gas screen positioned adjacent an end of the first gas screen opposite from the draw furnace;    injecting a process gas into the draw furnace through the first gas screen at a process gas flow rate;    injecting a sealing gas through the second gas screen at a sealing gas flow rate; and    selecting the process gas flow rate and the sealing gas flow rate to provide a desired flow rate of the process gas from the downstream end to an upstream end of the draw furnace and to provide a desired flow rate of the sealing gas from the second gas screen downstream from the downstream end of the draw furnace so as to reduce introduction of contaminant gases into the draw furnace while a preform positioned in the draw furnace is heated.    
   
   
       20 . The method of  claim 19  wherein the process gas includes an inert gas.  
   
   
       21 . The method of  claim 20  wherein the process gas is selected from a group consisting of helium, nitrogen, argon and a mixture of helium, nitrogen or argon and wherein the sealing gas is selected from a group consisting of nitrogen, argon and a mixture of nitrogen and argon.  
   
   
       22 . The method of  claim 20  wherein the sealing gas is a heavier gas than the process gas.  
   
   
       23 . The method of  claim 19  wherein the process gas comprises helium and wherein the sealing gas comprises argon.  
   
   
       24 . The method of  claim 19  wherein the step of selecting the process gas flow rate and the sealing gas flow rate further comprises the step of selecting the process gas flow rate and the sealing gas flow rate to provide at least one of a carbon monoxide concentration in the draw furnace of less than about 50 parts per million (ppm) while the preform is heated and an oxygen concentration of less than about 25 ppm while the perform is heated.