Abstract:
A combustion tube comprises a generally cylindrical body with an outwardly extending annular tube stop spaced from one end of the combustion tube for engaging a combustion tube mounting assembly and fixing the tube in a precise position. Near the opposite end of the tube is an enlarged opening for receiving an upper seal assembly of a combustion furnace with the outer annular shoulder of the upper end of the tube having a rolled edge to facilitate the insertion of the tube through the seal in the upper seal assembly of the furnace. The combustion tube is made of quartz glass to withstand the temperatures encountered in the furnace. The combustion tube is specifically designed and adapted to be precisely positioned in an induction furnace with an easy tube removal system for the furnace.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) and the benefit of U.S. Provisional Application No. 61/448,400 entitled C OMBUSTION  T UBE , filed on Mar. 2, 2011, by Gordon C. Ford. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a combustion tube for an induction furnace employed in an analytical instrument. 
     Quartz combustion tubes have been used in connection with combustion furnaces for many years for enclosing a sample for the analysis of elements, such as carbon or sulfur. One commercially available analyzer is Model No. CS600, available from Leco Corporation of St. Joseph, Mich. Such an analyzer uses a quartz combustion tube which is typically used to maintain a pressurized oxygen-rich environment for the combustion of a sample in a ceramic crucible held within an induction coil surrounding the combustion tube. U.S. Pat. Nos. 2,809,100, 3,923,464, and 4,234,541 are examples of systems employing such quartz combustion tubes. The disclosures of these patents are incorporated herein by reference. 
     During the combustion process, byproducts of combustion frequently cause deposits on the combustion tube. The quartz tube must be cleaned and eventually replaced to maintain the accuracy of sample results. The mounting of a combustion tube in existing furnaces equipped with an auto cleaner is both time consuming and cumbersome, requiring that fluid fittings and electrical connections be removed and the auto-cleaner device removed from the combustion tube area of the furnace. The combustion tube is removed from the top of the furnace housing once the disassembly has been completed. Once a new combustion tube has been installed, the furnace has to be reassembled, frequently including the connection of fluid couplings which can lead to leaks in the system if not properly accomplished. Thus, the maintenance, removal and replacement of combustion tubes in existing furnaces is difficult, time consuming, and leads to downtime for the operation of an analyzer. 
     Accordingly, there exists a need for an improved combustion tube for use in an analytical combustion furnace which can be readily accessed without disconnecting fluid fittings, auto-cleaners, or the like from the furnace assembly. 
     SUMMARY OF THE INVENTION 
     The system of the present invention accomplishes this goal by providing an improved combustion tube which cooperates with a mounting system for the combustion tube. The combustion tube is removably installed through an aperture in the floor of a furnace housing and can be manually or automatically unlocked from the floor of the furnace housing by a cam-locking mechanism for positioning the combustion tube in an open area of the furnace below the furnace housing for easy removal and replacement. 
     In a preferred embodiment, a combustion tube includes a generally cylindrical body with an annular outwardly extending ring for releasably engaging a mounting assembly. The mounting assembly includes a base seal assembly which is placed on a lower seal assembly of a combustion furnace and is raised to automatically align with the upper furnace seal. Cams in the floor of the furnace housing engage a cam associated with the base seal assembly to lock the combustion tube in place as it is raised into the furnace housing. As a result of this construction, a downwardly removable combustion tube is provided, and the steps of disassembling the furnace components, including the self-cleaning mechanism located above the combustion tube, is eliminated as is the necessity for skilled technicians to replace the combustion tube. An operator can readily unlock the combustion tube from the furnace and lower it for access and replacement. 
     These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary front perspective view of a combustion furnace embodying the present invention; 
         FIG. 2  is a front perspective view of the furnace housing with the cover removed showing the combustion tube and locking mechanism for the tube, shown in an unlocked position; 
         FIG. 3  is a front perspective view of the furnace shown in  FIG. 2 , showing the lowering and raising of the combustion tube through an aperture in the furnace housing floor; 
         FIG. 4  is a front perspective view of the furnace shown in  FIG. 3 , showing the combustion tube lowered from the furnace housing in a position for removal/replacement; 
         FIG. 5  is a perspective view of the improved combustion tube; 
         FIG. 6  is a front elevational view of the combustion tube; 
         FIG. 7  is a top plan view of the combustion tube; 
         FIG. 8  is a bottom plan view of the combustion tube; 
         FIG. 9  is an enlarged fragmentary view of the circled area IX of  FIG. 6 ; 
         FIG. 10  is an enlarged fragmentary cross-sectional view of the circle area X of  FIG. 6 ; 
         FIG. 11  is an enlarged perspective view of the combustion tube, shown mounted within the lower seal assembly; 
         FIG. 12  is an exploded perspective view of the combustion tube and lower seal assembly of the furnace; 
         FIG. 13  is an enlarged fragmentary cross-sectional view of the interface between the combustion tube and the lower seal assembly; 
         FIG. 14  is an exploded perspective view of the lower seal assembly; and 
         FIG. 15  is an enlarged cross-sectional view of the lower seal assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , there is shown an analyzer  10  which includes an induction furnace assembly  20 , including the auto cleaning mechanism  30  described in detail in U.S. patent application Ser. No. 13/198,746, filed Aug. 5, 2011, entitled C OMBUSTION  F URNACE  A UTO  C LEANER , the disclosure of which is incorporated herein by reference. The analyzer components themselves can be similar to those employed in a carbon sulfur analyzer, Model No. CS600, available from Leco Corporation of St. Joseph, Mich. The analyzer  10  is shown in  FIG. 1  with the cover shrouds removed to expose the components of the induction furnace, which are also shown in  FIGS. 2-4 . The detachable auto cleaner assembly  30  is removably mounted by a bayonet connection to a heated filter assembly  40 , which is sealably secured at  65  to the top wall  51  of combustion housing  50 . Housing  50  additionally includes side walls  53  and  55 , an interface back wall  57 , and floor  58 . The housing  50  is mounted to structural base  17  of the analyzer  10 , which includes housing  50 . The front of furnace housing  50  is enclosed by a quickly removable door  52 , which, when removed as seen in  FIG. 2 , exposes the combustion tube  60  of the present invention ( FIGS. 5-10 ). An induction coil (not shown) surrounds the combustion tube to heat an analytical specimen held in a ceramic crucible  14  ( FIG. 1 ) when introduced into the hot zone of combustion tube  60  by vertically movable pedestal  12 . Tube  60  is sealably coupled to the lower end of filter assembly  40  by an upper seal assembly  65 . The pedestal  12  ( FIG. 1 ) for holding a sample-holding crucible  14  is positioned on a cup-shaped lower seal assembly  16  and is raised and lowered into combustion tube  60  by means of a pneumatic cylinder  18  and cylinder rod  22  ( FIG. 3 ) coupled to assembly  16 . In the position shown in  FIG. 1 , the cylinder rod  22  is in a lowered retracted position within cylinder  18 . 
     A combustion tube base assembly  80  ( FIGS. 3 ,  4 ,  11 , and  12 ) sealably couples the lower end  63  of tube  60  to the pedestal&#39;s lower seal assembly  16  such that, during combustion of a sample, oxygen flows upwardly through a gas inlet in assembly  16  to sweep byproducts of combustion into gas outlet  13  ( FIG. 1 ) for analysis. Oxygen is also supplied to the upper end of combustion tube  60  by an oxygen inlet  15  and suitable passageways to an inlet lance to direct oxygen into crucible  14  during combustion. Housing  50  includes cam-actuated locking assembly  100  ( FIG. 2 ) which cooperates with base assembly  80  to allow the combustion tube  60  to be easily withdrawn from underneath the floor of combustion housing  50 . Assembly  100  includes arms  122 ,  124  on pivoted cams  106 ,  108  mounted to housing  50  by a mounting plate  114 , as described in detail in U.S. patent application Ser. No. 12/889,628 filed on Sep. 24, 2010, entitled E ASILY  R EMOVABLE  C OMBUSTION  T UBE , the disclosure of which is incorporated herein by reference. 
     Referring to  FIGS. 5-11 , there is shown a combustion tube  60 , which has a generally cylindrical body  62 , including a first or lower end  63  and a second or upper end  64 . Spaced from the lower end a predetermined distance “d” (shown in  FIG. 6 ) is a tube stop or stop collar  66  comprising an annular ring or shoulder formed in and integral with the cylindrical body  62  of the combustion tube  60 . The opposite end  64  of the tube is enlarged to provide a somewhat larger diameter “D”, as seen in  FIGS. 6 and 7 , such that the open mouth  67  of the upper end  64  of tube  60  mates with an internal O-ring in the upper seal assembly  65  ( FIGS. 2-4 ) of the combustion furnace  20 . Surrounding the annular upper end  64  of the combustion tube  60  is a rolled edge  68  ( FIG. 9 ) which provides a rounded interface between the upper end of tube  60  as it engages the seal of the upper seal assembly  65 , as illustrated in  FIG. 2 . 
     An upper seal assembly  65  includes an internal O-ring seal on a collar that engages the inner cylindrical surface of the mouth  67  at the upper end  64  of combustion tube  60 . The furnace  20  also includes a base assembly  80 , shown in detail in  FIGS. 12-15 , which receives the lower end  63  of combustion tube  60  with tube stop  66  indexing the position of the tube with respect to the lower base assembly  80 . 
     The base assembly  80  has a generally cylindrical collar  82  with a central aperture  81  therein for receiving the combustion tube  60 , as seen in  FIG. 11 . Near the upper edge of collar  82  is an annular recess  83  for receiving an O-ring seal  84 . A locking cap  85  is threaded onto collar  82  and compresses the O-ring seal once the combustion tube has been inserted therein for sealably holding the combustion tube within the collar  82 . Cap  85  engages stop collar  66  of combustion tube  60  compressing seal  84  ( FIG. 13 ) and vertically aligns the combustion tube  60  with respect to the base assembly  80  and ultimately the furnace  20 , as seen in  FIGS. 2-4  and  13 . Collar  82  includes a lower cylindrical sleeve  86  having internal annular recess  87  for receiving a second O-ring  88  for sealing the base assembly  80  onto the lower seal assembly  16  of the furnace. Further, the assembly  80  includes a metal RFI shield  89  and a third sealing O-ring  78  ( FIGS. 13 and 15 ) positioned within an annular recess  91  of collar  82 . 
     Collar  82  includes a cam  90  which has an outer annular vertical surface  92 , an upper chamfered surface  94  chamfered upwardly and inwardly at an angle of about 45° from the vertically extending annular surface  92  and a lowered chamfered surface  96  inwardly and downwardly beveled at an angle of about 45° from surface  92 . The two surfaces thus form an angle of about 90° between them to fit within a 90° slot  107  (not shown) in the cams  106  and  108 . Collar  82  also includes an annular flange  93  having a diameter greater than aperture  59  in furnace housing floor  58  to engage the undersurface of floor  58  when the combustion tube is in an installed locked position as seen in  FIG. 3 . The cam  90  so-defined interengages with cam pins of cam-actuated assembly  100  for locking and unlocking the combined combustion tube  60  and base assembly  80  as described in greater detail the above identified &#39;628 application. 
     The combustion tube  60  is made of quartz glass to withstand the temperatures of combustion, which can reach 3000° C. The combustion tube  60  has an overall length (i.e., height) of about 6.12 inches, with the inner diameter D near the top having a diameter of about 1.28 inches. The outer diameter of the cylindrical body  62  of the tube is about 1.48 inches, while the outer stop collar  66  has an outer diameter of about 1.58 inches and a minimum radius of curvature at its intersection with the outer surface  62  of tube  60 . The radius of curvature of the collar itself of about 0.010 inches. The vertical height of collar  66  was about 0.157 inches. Thus, the collar  66  projects outwardly from the outer surface  62  of combustion tube  60  about 0.065 inches, a distance sufficient to interface with the base assembly  80  and cap  85  to index and mount the combustion tube to the base assembly  80 . The distance to collar  66  from the bottom edge  63  of tube  60  was about 0.83 inches to provide a distance sufficient for the sealed engagement of the lower end  63  of the combustion tube with seal  84 , as illustrated in  FIG. 13 . The wall thickness of tube  60  was, in one preferred embodiment, about 2 mm. Although these dimensions are illustrative of the preferred embodiment of the invention, other sized furnaces may incorporate combustion tubes with proportionally larger or smaller dimensions, as long as the unique mounting and sealing arrangement is provided for the combustion tube. The important feature is that a stop collar is provided in spaced relationship to the lower edge of the combustion tube to allow its indexable and sealable mounting to a base assembly for easy removal, particularly in a furnace having an easily removable combustion tube design. 
       FIG. 3  illustrates the assembly of the combustion tube  60  to the lower seal assembly  16 , which includes an upper ring  65  threadably mounted to the collar  82  of the base assembly. As seen in  FIGS. 3 ,  11  and  13 , when combustion tube  60  is inserted into the base assembly  80 , the stop collar  66  engages the cylindrical bottom edge of upper ring  85  to precisely position the combustion tube with respect to the base assembly  80 . Engaging the bottom of upper ring  85  assures positive removal of the combined assembly  80  and tube  60  when disengaged from furnace  20 . This allows assembly  80  to cooperate with the raising and lowering mechanism, including the piston rod  22 , lower seal assembly  16 , and pedestal  12  ( FIGS. 1 and 3 ) to precisely position the combustion tube  60  within the analytical furnace  20 . Thus, with the improved combustion tube of the present invention, an indexing stop is provided to precisely mount and position the combustion tube in the furnace, particularly when used in connection with an easily removable combustion tube mechanism. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.