Abstract:
An analytical combustion furnace has an input port for receiving a holder for an analytical sample. A purge block is coupled to the input port of the combustion furnace and has an input end for receiving the holder and a gas inlet and a gas outlet. A door selectively closes the input end of the purge block, and a pressurized seal is coupled between the purge block and the door. A source of purging gas is coupled to the gas inlet of the purge block for pressurizing the seal. The gas outlet of the purge block communicates with the seal to allow purging gas to escape the area of the seal. In one embodiment, a pressurized seal surrounds a push rod, which seal is pressurized by a purging gas to continuously purge the entry of the push rod into the purge block, thereby eliminating atmospheric contamination.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to combustion furnaces and particularly to improved seals for sealing the entry to such furnaces. 
         [0002]    Analyzers for elements, such as carbon, sulfur, and nitrogen, require the combustion of specimens, typically solids or liquids, in a holder, such as a crucible or combustion boat. In many instruments, the holder for the combustion sample is a ceramic holder. Horizontal furnaces, such as described in U.S. Pat. No. 5,064,617 issued Nov. 12, 1991, to O&#39;Brien et al., the disclosure of which is incorporated herein by reference, utilize a ceramic combustion boat which is extended within an electrically heated furnace for combusting samples within a ceramic combustion chamber to temperatures of from about 1000° C. to about 1500° C. U.S. Pat. No. 5,314,662, issued May 24, 1994, to Heinz et al., discloses one system for automatically sequentially loading specimens into such a furnace and includes a mechanism for the admission and withdrawal of a combustion boat for an analysis. The disclosure of the U.S. Pat. No. 5,314,662 is also incorporated herein by reference. 
         [0003]    With the increasing interest in the accurate determination of low concentration levels of elements, such as nitrogen, by various industries, the sealing of the furnace&#39;s entry way against atmospheric contaminants is becoming increasingly important. Although O-ring face-type seals have been employed in the past as has the use of purging gases in a purge block holding the combustion sample prior to entry into the furnace, such seals still can allow the admission of some atmospheric nitrogen, particularly where a push-rod is employed which extends through the purge block to position a sample holder, such as a combustion boat, into the combustion furnace itself. Also, the combustion furnace must be opened for the removal of a spent combustion boat and the admission of a new specimen in a new combustion boat. This process also allows the admission of atmospheric nitrogen and other atmospheric contaminants which must then be purged from the system. 
         [0004]    Although existing furnace seals and purge blocks provide acceptable results, with the increasing industry demand for the measurement of low concentrations of analytes such as nitrogen, they are often limited by how the atmospheric nitrogen content impacts the blank measurement. As an example, if the blank nitrogen level was 50 ppm as when UHP oxygen (99.995%) is used as a carrier gas during an analysis, nitrogen levels in the surrounding atmosphere is more than 15,000 times the nitrogen level found in the blank. Thus, in order to achieve low level blanks, all atmospheric contamination must be avoided or eliminated. Thus, not only must the atmosphere be purged for the sample compartment but seals used to isolate the sample compartment from the surrounding environment must be highly effective. This is a major challenge where the seals must be cycled open during each sample to introduce new samples into the purge block sample-holding compartment. 
         [0005]    In addition, sliding seals employed with push-pull rods for the introduction and withdrawal of samples from a furnace also present a challenge for maintaining effective isolation of the combustion chamber from the surrounding atmospheric environment. Small imperfections or scratches in the sliding rod can allow environmental contamination into the chamber and any other leaks likewise contaminate the specimen to be combusted and analyzed. 
       SUMMARY OF THE INVENTION 
       [0006]    The system of the present invention solves these problems by providing an opening to an analytical combustion furnace having an input port for receiving a holder for an analytical sample for the combustion of the sample for the analysis of the byproducts of combustion. A purge block is coupled to the input port of the combustion furnace and has an input end for receiving the holder and a gas inlet and a gas outlet. A door selectively closes the input end of the purge block, and a pressurized seal is coupled between the purge block and the door. A source of purging gas is coupled to the gas inlet of the purge block for pressurizing the seal. The gas outlet of the purge block communicates with the seal to allow purging gas to escape the area of the seal in a controlled manner. 
         [0007]    Further, the invention contemplates including a pressurized seal surrounding a push rod, which seal is pressurized by a purging gas to continuously purge the entry of the push rod into the purge block, thereby eliminating atmospheric contamination. 
         [0008]    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 
         [0009]      FIG. 1  is a perspective view of a combustion furnace utilizing the pressurized gas seals of the present invention; 
           [0010]      FIG. 2  is an enlarged perspective view of the input end of the purge block and its door in an open position and showing a combustion boat and push-pull rod for manipulating the combustion boat into the combustion furnace; 
           [0011]      FIG. 3  is a perspective view of the purge block and push-pull rod assembly, shown in a closed position, for the combustion and analysis of a specimen; 
           [0012]      FIG. 4  is a perspective view of the furnace purge block and push-pull rod assembly shown in an open position; 
           [0013]      FIG. 5  is a left side elevational view of the combustion chamber, the purge block, and the push-pull rod assembly; 
           [0014]      FIG. 6  is a fragmentary vertical cross-sectional view of the structure shown in  FIG. 5 ; 
           [0015]      FIG. 7  is an enlarged vertical cross-sectional view of the purge block and door; 
           [0016]      FIG. 8  is an enlarged cross-sectional view of the door for the purge block, the housing for the push-pull rod, and an interface plate; 
           [0017]      FIG. 9  is a cross-sectional view of the housing for the push-pull rod, taken along section line IX-IX in  FIG. 8 ; 
           [0018]      FIG. 10  is an enlarged fragmentary vertical cross-sectional view (reversed as viewed in  FIG. 8 ) of the purge block and rod housing assembly; and 
           [0019]      FIG. 11  is a greatly enlarged cross-sectional view of the circled area XI in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Referring initially to  FIG. 1 , there is shown an analyzer  10  embodying the combustion furnace and pressurized gas purged seals of the present invention. Analyzer  10  can be a TruMAC® nitrogen analyzer, recently commercially available from the Assignee of the present invention, Leco Corporation of St. Joseph, Mich. Analyzer  10  includes several components including a furnace  12 , a carousel  14  for holding ceramic combustion boats with specimens to be analyzed which are introduced by an elevator  16  into the inlet of purge block  20  of furnace  12 , as illustrated in  FIG. 2 . The analyzer  10  is a nitrogen analyzer but, depending on the detection system employed, can analyze other elements, such as carbon, sulfur, and the like. Typically, with a nitrogen sample, a thermo-conductivity cell is employed in a detection system such as described in U.S. Pat. No. 5,233,308 issued Aug. 3, 1993, to Willis, the disclosure of which is incorporated herein by reference. Samples placed in a sample holder  18 , such as a ceramic combustion boat ( FIGS. 2 ,  6 , and  10 ), will typically be solid 1 gram samples, although they can take other forms as well. The samples are introduced into the furnace  12  through a purge block  20  then into a combustion tube  100 , as best seen in  FIG. 6 . The combustion system contained within furnace  12  can generally be of the type described in the above-identified U.S. Pat. No. 5,064,617. 
         [0021]    The inner combustion tube  100  is a generally cylindrical tube (made of mullite) which positions a combustion boat  18  adjacent boat stop  102  and is enclosed within an outer closed-end cylindrical mullite combustion tube  104  ( FIGS. 5 and 6 ). An oxygen lance  106  is supplied with a flow of oxygen from a source of purified oxygen  108 , shown schematically in  FIG. 6 . Source  108  is UHP oxygen, which is 99.995% pure and is also coupled to the purge block gas inlet  34  ( FIG. 7 ) and push-pull rod  80  housing  90  at oxygen inlet  70  ( FIG. 10 ). Source  108  is coupled to the oxygen lance  106  through a fitting  109  ( FIG. 6 ). The connection of the oxygen source  108  to the various fittings is shown schematically in  FIG. 6  by the dashed line connectors  107 , it being understood that the connections  107  are typically tubes with suitable gas-tight fittings, such as fitting  109  for transferring the flow of oxygen to the furnace components. 
         [0022]    The purge block  20  is employed for admitting sample-holding combustion boats  18  into combustion tube  100  and withdrawing them subsequent to the combustion of a sample. Block  20  has an inlet end  22  leading to an internal annular cavity  24  for holding a sample-holding combustion boat prior to an analysis during a period of time sufficient for purging the atmospheric contamination from the combustion boat and sample once the door  40  for the purge block  20  is moved from an open position, seen in  FIG. 4 , to a closed position, shown in  FIG. 3 . Cavity  24  ( FIG. 4 ) of purge block  20  includes a generally axially aligned, U-shaped support structure having a floor  23  for supporting a combustion boat  18 , as seen in  FIG. 6 , once admitted to the inlet end  24  of the purge block. Purge block  20  is fixedly and sealably coupled to the input port  110  ( FIG. 6 ) of the combustion system including combustion tube  100  by a generally circular flange  26 , including O-ring seals  27  ( FIG. 7 ) and suitable fasteners  29  which, as seen in  FIGS. 1-4 , sealably secure the outlet end of purge block  20  to the inlet  110  of furnace  12  and combustion tube  100 . Thus, once assembled, purge block  20  is sealably secured to furnace  12  and the combustion tube components contained therein. 
         [0023]    In order to admit specimens in holders, such as combustion boat  18 , the door  40  of purge block  20  must be opened ( FIGS. 2 and 4 ) and closed ( FIGS. 1 and 3 ), and securely sealed against environmental contamination once closed and purged by the flow of oxygen from source  108 . The push rod  80  is employed for pushing the combustion boat from elevator  16  into the furnace and moves with respect to the door  40  through a sealed housing  90  ( FIGS. 8-11 ). Thus, there are two areas that must be positively sealed for maintaining a controlled environment during an analysis. First, the purge block holding the sample and combustion boat therein in the cavity  24  in the area between the door  40  and inlet end  22  of the purge block. Secondly, the interface between push rod  80  and housing  90  is sealed by pressurized seals which are constantly purged by the flow of purified oxygen from source  108  as described below. 
         [0024]    The cup-shaped door  40  is coupled to housing  90  by a mounting plate  86  ( FIGS. 8 and 10 ) to move in unison on a carriage  110  slideably mounted to the stationary frame  114  of analyzer  10  in a conventional manner using roller bearings and suitable guides. Carriage  110  is driven by a first motor  112  (FIGS.  1  and  3 - 5 ) also secured to stationary frame member  114  of the analyzer  10 . This motor and a screw jack drive  116  extending between motor  112  and carriage  110  serve to move the door  40  and housing  90  between the open and closed positions seen in  FIGS. 2 and 3 , respectively. A second motor  118  ( FIG. 5 ) and screw jack drive  120  are coupled to carriage  110  and are employed for advancing and retracting push-pull rod  80  with respect to housing  90  with motor  118  traveling with carriage  110  as the door  40  moves between open and closed positions. An actuator  122  is coupled to rod  80  and is employed for rotating the rod, such that an L-shaped pushing element  82  ( FIGS. 2 and 10 ) at the end of the rod can be in a position as shown in  FIG. 2  for engaging the trailing edge of combustion boat  18  for pushing the boat as rod  80  is advanced into the combustion tube  100 . Subsequently, upon completion of combustion, actuator  122  rotates the rod  80  90°, such that rod  80  can be advanced to position element  82  on the leading edge of the boat  18 . Actuator  122  then rotates rod  80  90° for engaging the leading edge of boat  18  withdrawing the boat from the combustion chamber in a manner described in U.S. Pat. No. 5,314,662 (noted above). Housing  90  and mounting plate  86  are sealably secured together by O-rings  92  ( FIGS. 8 and 10 ) and fasteners  85  extending through outer cover plate  88  ( FIGS. 3 ,  4 ,  8 , and  10 ). The improved sealing mechanism for the internal cavity  24  of purge block  20  and the combustion tube  100  is now described in connection with  FIGS. 4 ,  7 ,  8 , and  10 . 
         [0025]    The inlet end  22  of purge block  20  is generally annular, as seen in  FIG. 4 , and includes an annular outer recess  28  ( FIGS. 7 and 10 ) for receiving a pair of opposed facing spaced-apart annular cup-shaped seals  30  and  32  ( FIG. 7 ). Seals  30 ,  32  are preferably made of nitrile/Viton® having a durometer hardness of about 60 or 70. They are sized to fit in close relationship with wall  28  of block  20  and surface  43  of door  40 , as best seen in  FIG. 7 . Seals  30  and  32  are positioned in spaced relationship on opposite sides of an oxygen inlet  34  and an oxygen outlet  36 , which includes a flow restrictor  37 . Oxygen is supplied between seals  30  and  32  at a pressure of about 10 psi and a flow rate of approximately 100 cc/min to pressurize the seals  30 ,  32  and urge their inner and outer cylindrical surfaces  31 ,  33  tightly against the inner cylindrical wall  28  of block  20  and the outer surface  43  of the annular flange  42  of cup-shaped door  40 . As best seen in  FIGS. 2 ,  7  and  10 , the annular flange  42  of door  40  extends within the annular space between wall  28  of block  20  and surrounds the floor  23  of the purge block. The outer surface  43  ( FIGS. 2 ,  7  and  8 ) of annular flange  42  of door  40  engages surfaces  31  of seals  30  and  32 . Upon actuation of motor  112  to position the door  40  in a closed position, as seen in  FIGS. 1 and 3 , purging gas from source  108  is supplied through inlet  34  into the annular volume  35  between cup seals  30  and  32  to pressurize the seals to provide a tight seal between door  40  and purge block  20 . The continuous flow of oxygen through restricted outlet  36  initially for approximately two minutes together with oxygen through lance  106  is sufficient to purge the cavity  24  and combustion chamber  100 , as well as pressurize seals  30  and  32  to a tightly sealed position. The oxygen continues to flow during the subsequent heating of the furnace and combustion of a sample, which typically takes from four to five minutes. The pressurized volume  35  between seals  30  and  32  prevents the admission of atmospheric contaminants during the combustion of a specimen. 
         [0026]    The concept of providing a pressurized flow of purging gas, such as oxygen, to both provide pressurized sealing to the purge block and purge contaminants is likewise applied to the interface between push-pull rod  80  and its housing  90 . Housing mounting plate  86  is sealably secured to the door  40  through the O-rings  94  ( FIG. 8 ) over an annular door coupling  44 , as best seen in  FIG. 7 . Housing  90 , in turn, is sealably coupled to plate  86  by O-rings  92  and fasteners  85 . Housing mounting plate  86  includes an annular opening  96  ( FIG. 8 ) which receives door coupling  44  to sealably couple housing  90  to door  40 . Suitable fasteners, such as fasteners  87 , secure plate  86  to door  40 . 
         [0027]    As best seen in  FIGS. 8-11 , housing  90  includes an axially extending cylindrical port  93  which receives a ported bearing  50  for rotatably and axially supporting the motion of push rod  80  within housing  90 . Ported bearing  50  can be made of a suitable bearing material, such as phosphorous bronze or the like, and includes radially extending ports  52  which communicate with an annular recess  98  ( FIG. 11 ) formed in housing  90 , such that oxygen from an oxygen inlet  70  ( FIGS. 8 ,  10 , and  11 ) coupled to source  108  can flow in the area surrounding rod  80  through the ported bearing  50 . On opposite sides of ported bearing  50  are opposed facing cup seals  60  and  62 . Seals  60 ,  62  are made of Turcite® (PTFE with carbon graphite) having a durometer hardness of about 70 and are sized to closely surround the outer surface  84  of rod  80  and engage port  96  of housing  90 , as best seen in  FIG. 11 . Oxygen from supply  108  is admitted through oxygen inlet  70  and exits the ported area in annular recess  98  through restricted exhaust port  72  ( FIG. 9 ) also communicating with the annular recess  98 . The oxygen is supplied at inlet  70  at approximately 10 psi with a flow rate of 5 cc/min to pressurize the surfaces  61  of cup seals  60  and  62  against the outer cylindrical surface  84  of rod  80  and the outer surfaces  63  of seals  60 ,  62  against port  96 . This assures a secure seal between rod  80  and housing  90  as the rod is moved axially and rotated during the admission of a sample-holding combustion boat  18  and its subsequent withdrawal by operation of the rod  80 . The continuous supply of pressurized purging purified oxygen through recess  98  in the area between seals  60  and  62  purges any contaminants that may exist on rod  80  and also prevents admission of any atmospheric contamination in the area of the rod interface with housing  90 . Housing  90  includes an outer cover plate  88  ( FIGS. 8 and 10 ) and an interface mounting plate  86  between the housing  90  and door  40 . Plate  86  is sealably secured to housing  90  by fasteners  85  and O-rings  92  and subsequently to door  40  by fasteners  87 . In some embodiments, the housing may be of unitary construction and coupled directly to the door or part of the door itself. 
         [0028]    Thus, in the two critical areas of sealing the combustion furnace associated with analyzer  10 , namely, the purge block  20  and the housing  90  for push-pull rod  80 , employ pressurized seals and continuous pressurized flow of purified oxygen to prevent the admission of contaminants from the atmosphere during the loading and unloading of a sample as well as purging of the purge block and rod areas before and during an analysis of a specimen. This allows extremely low levels of analytes to be measured without contamination from the surrounding environment. The analyzer itself can be of the type commercially available from Leco Corporation of St. Joseph, Mich. 
         [0029]    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.