Air shield for combustor firetube stack

Abstract

A combustor having an outer stack placed around a fire tube of the combustor to direct cooler air upwards to envelop an exhaust plume from the fire tube as an air shield or curtain barrier to prevent fugitive gases coming into contact with the exhaust plume and auto-igniting.

What is claimed is: 
     
         1 . A method for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, the firetube comprising an inlet disposed on top of the combustor box wherein the inlet is in communication with an interior of the combustor box, a burner assembly disposed in the inlet, one or more flame arrestors disposed on the combustor box configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the method comprising:
 drawing in shielding air into the combustor box;   conveying the shielding air from the combustor box along an exterior of the firetube up to the outlet of the firetube; and   enveloping the exhaust plume with the shielding air to form an air shield therearound, whereby the air shield forms a barrier between the exhaust plume and the fugitive gases to prevent the fugitive gases from auto-igniting.   
     
     
         2 . The method as set forth in  claim 1 , further comprising drawing in a portion of the shielding air into the firetube at one or more locations disposed along the firetube. 
     
     
         3 . The method as set forth  claim 1 , wherein the conveying of the shielding air comprises natural draft or convection. 
     
     
         4 . An apparatus for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the apparatus comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air. 
     
     
         5 . The apparatus as set forth in  claim 4 , wherein a top end of the outer stack is disposed below the outlet of the firetube. 
     
     
         6 . The apparatus as set forth in  claim 4 , wherein the firetube comprises one or more breather openings configured to allow the shielding air to be drawn into the firetube. 
     
     
         7 . An improved combustor, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the improvement comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air. 
     
     
         8 . The improved combustor as set forth in  claim 7 , wherein a top end of the outer stack is disposed below the outlet of the firetube. 
     
     
         9 . The improved combustor as set forth in  claim 8 , wherein the firetube comprises one or more breather openings configured to allow the shielding air to be drawn into the firetube.