Patent Publication Number: US-9891000-B2

Title: Center heating element for a vacuum heat treating furnace

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/866,178, filed Aug. 15, 2013, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to vacuum heat treating furnaces and in particular to a heating element arrangement for a vacuum heat treating furnace. 
     Description of the Related Art 
     Prior to the present invention, a center heating element arrangement was used in a vacuum aluminum brazing furnace manufactured by Ipsen Inc., the assignee of the present application. A center heating element is a heating element bank that is positioned between two separate workloads inside a vacuum furnace. This arrangement allows for faster and more uniform heating of such loads because when the load is split into two sections, the center heating element radiates heat toward the inside-facing surfaces of the workloads while the peripheral heating elements radiate heat toward the outside-facing surfaces of the workloads. This has only been accomplished in the past by providing the center element bank with its own dedicated power terminals, variable reactance transformer power supply, and a thermocouple for effecting temperature control. 
     The known center heating element arrangement leaves something to be desired with respect to design and operational flexibility. The requirement for a separate power supply complicates the power supply requirements for the vacuum furnace and results in a more costly design. That design also requires more penetrations in the furnace wall. Also, the fixed-in-place nature of the known arrangement prevents the vacuum furnace from being used for larger-size workloads. 
     In view of the foregoing, it would be desirable to have a vacuum heat treating furnace that avoids the undesirable aspects of the known arrangement for a center heating element, while still providing the benefits of a center heating element. Moreover, it would also be desirable to have a vacuum heat treating furnace having a center heating element that does not have its own connections outside the vacuum furnace, no separate power transformer, and no separate control thermocouple. It is further desirable to provide a vacuum heat treating furnace having a center heating element that is readily removable so that work loads of different sizes can be heat treated in the furnace. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a vacuum heat treating furnace for the heat treatment of metal parts that includes a pressure/vacuum vessel, a hot zone enclosure positioned inside the pressure vessel to define a hot zone within the heat treating furnace, a heating element array positioned inside said hot zone enclosure, and a source of electric power for energizing the heating element array. In the vacuum heat treating furnace of this invention, the heating element array includes a first heating element, a second heating element, and a third or center heating element. The first heating element is suspended on an inner wall of the hot zone enclosure in a first region of the hot zone. The first heating element has a first end and a second end and the first end is connected to the source of electric power. The second heating element is suspended on the inner wall of the hot zone enclosure in a second region of the hot zone opposite to the first region. The second heating element has a first end and a second end and the first end is connected to the source of electric power. 
     The center heating element is suspended from the inner wall of the hot zone enclosure along a vertical chord or diameter of the hot zone. The center heating element has first and second ends. First and second connection terminals are provided at the first end of the center heating element. The first connection terminal is connected to the second end of the first heating element and the second connection terminal is connected to the second end of the second heating element. In this manner, the center heating element is connected to the source of electric energy through said first and second heating elements. The heating element array further includes a support member having a first end attached to the hot zone enclosure and a second end connected to the second end of the center heating element for supporting the center heating element in the furnace hot zone. 
     In accordance with another aspect of the present invention, the heating element array is provided with a first removable/reusable fastener that attaches the first terminal connection to the second end of the first heating element and a second removable/reusable fastener that attaches the second terminal connection to the second end of the second heating element. 
     In accordance with a further aspect of the present invention the heating element array is provided with a third removable/reusable fastener that attaches a first end of the first sub-element to the second end of the first heating element and a fourth removable/reusable fastener that attaches the first end of a second sub-element to the second end of the second heating element. 
     In accordance with a still further aspect of the present invention, the vacuum heat treating furnace of this invention includes two or more heating element arrays as described above that are arranged in spaced coaxial relation along the length of the furnace hot zone. 
     In accordance with a further aspect of the present invention there is provided a method for configuring a vacuum heat treating furnace for holding different size workloads. The method is implemented with a vacuum heat treating furnace that has a pressure/vacuum vessel, a hot zone enclosure positioned inside said pressure vessel to define a hot zone within the heat treating furnace, a heating element array positioned inside said hot zone enclosure, and a source of electric energy. The heating element array includes a first heating element suspended on an inner wall of the hot zone enclosure in a first region of the hot zone. The first heating element has a first end and a second end and the first end is connected to the source of electric energy. The heating element array also includes a second heating element suspended on the inner wall of the hot zone enclosure in a second region of the hot zone opposite to the first region. The second heating element has a first end and a second end and the first end is connected to the source of electric energy. The heating element array further includes a center heating element suspended from the inner wall of the hot zone enclosure along a vertical chord of the hot zone. The center heating element has first and second connection terminals at a first end thereof and a second end. The first connection terminal is connected to the second end of the first heating element and the second connection terminal is connected to the second end of the second heating element. In this manner the center heating element is connected to the source of electric energy through the first and second heating elements. The heating element array further includes a hanger assembly having a first end attached to the hot zone enclosure and a second end connected to the second end of the center heating element. The method includes the steps of disconnecting the center heating element from the first and second heating elements, removing the center heating element from the vacuum heat treating furnace, and then connecting a jumper heating element between the second end of the first heating element and the second end of the second heating element, whereby an electrical conduction path is established between the first and second heating elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary as well as the following detailed description will be better understood by referring to the drawings wherein: 
         FIG. 1  is an end elevation view of a vacuum heat treating furnace according to the present invention; 
         FIG. 2  is a side elevation view in partial cross section of the vacuum heat treating furnace shown in  FIG. 1  as viewed along line  2 - 2  thereof; 
         FIG. 3  is a detail view of a hanger assembly for a center heating element used in the vacuum heat treating furnace of  FIG. 1  as viewed along line  3 - 3  thereof; 
         FIG. 4  is an end elevation view of a vacuum heat treating furnace according to the present invention with the center heating element removed; 
         FIG. 5  is a side elevation view in partial cross section of the vacuum heat treating furnace of  FIG. 4  as viewed along line  5 - 5  thereof; 
         FIG. 6  is a schematic view of an alternate arrangement of a center heating element array according to the present invention; 
         FIG. 7  is a top plan view of the center heating element array of  FIG. 6 ; 
         FIG. 8  an end elevation view of the center heating element array of  FIG. 6 ; 
         FIG. 9  is a perspective view of the center heating element array of  FIG. 6 ; 
         FIG. 10  is a perspective view of an alternate embodiment of the center heating element hanger assembly shown in  FIG. 3 ; 
         FIG. 11  is an edge view of the hanger assembly shown in  FIG. 10 ; 
         FIG. 12  is a side elevation view of the hanger assembly shown in  FIG. 10 ; 
         FIG. 13  is a bottom plan view of the hanger assembly shown in  FIG. 10 ; and 
         FIG. 14  is a side elevation view in partial cross section of the hanger assembly shown in  FIG. 12  as viewed along line  14 - 14  thereof. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings and in particular to  FIGS. 1 and 2 , there is shown an embodiment of a vacuum heat treating furnace (vacuum furnace)  10  according to the present invention. The vacuum furnace  10  includes a pressure/vacuum vessel  12  that is designed in the known manner to withstand levels of superatmospheric pressure and subatmospheric pressure that are used for a typical vacuum heat treating cycle. A hot zone enclosure  14  is mounted inside the pressure/vacuum vessel  12 . Hot zone enclosure  14  is formed of a heat resisting and/or a heat reflecting material that has sufficient rigidity for the enclosure to maintain its shape. The hot zone enclosure is attached to the interior wall of the pressure/vacuum vessel  12  in any known manner. The hot zone enclosure  14  may have any desired cross-sectional geometry including circular, as shown in  FIG. 1 , and polygonal geometries. Hot zone enclosure  14  defines a hot zone  16  in the vacuum furnace  10 . Hot zone  16  is dimensioned to accommodate therein one or more workloads  20   a,    20   b.  The workloads  20   a,    20   b  are supported on rails  19  that are each supported by a plurality of rail supports  18  that extend from the bottom of the pressure/vacuum vessel  12 , through the hot zone enclosure  14 , and into the hot zone  16 . 
     Vacuum furnace  10  also has one or more heating element arrays or banks to provide radiant heat within the hot zone  16 . As shown in  FIG. 1  a heating element array according to the present invention includes a first lateral heating element  24 , a second lateral heating element  26 , and a third or center heating element  28 . First lateral heating element  24  is positioned in a lateral region of the hot zone  16  and second lateral heating element  26  is positioned in a second lateral region of the hot zone  16  that is diametrically opposite to the first lateral region. The first lateral heating element  24  and the second lateral heating element  26  are shaped to conform generally to the shape of the hot zone enclosure  14 . The center heating element  28  is positioned along a vertical chord of the hot zone  16  that is centrally located between the first and second lateral heating elements. Preferably, the center heating element is positioned along the vertical diameter of the hot zone. For some applications it may be desirable to have the center heating element located off the diameter of the hot zone, for example, to accommodate two different size workloads. 
     First lateral heating element  24  includes heating element segments  38   a,    38   b,    38   c,  and  38   d.  The heating element segments  38   a  and  38   b  are connected together at a first heating element support  39   a.  Heating element segments  38   b  and  38   c  are connected together at a second heating element support  39   b  and heating element segments  38   c  and  38   d  are connected together at a third heating element support  39   c.  The heating element supports  39   a,    39   b,  and  39   c  extend from and are attached to the hot zone enclosure  14  at spaced intervals as shown. Second lateral heating element  26  includes heating element segments  40   a,    40   b,    40   c,  and  40   d.  The heating element segments  40   a  and  40   b  are connected together at a first heating element support  41   a . Heating element segments  40   b  and  40   c  are connected together at a second heating element support  41   b  and heating element segments  40   c  and  40   d  are connected together at a third heating element support  41   c.  The heating element supports  41   a,    41   b,  and  41   c  extend from and are attached to the hot zone enclosure  14  at spaced intervals as shown. 
     First lateral heating element  24  is connected at one end to a first terminal connector  32  that extends through the hot zone enclosure and through the wall of the pressure/vacuum vessel  12 . Likewise, second lateral heating element  26  is connected at one end to a second terminal connector  34  that also extends through the hot zone enclosure and through the pressure/vacuum vessel wall. The other ends of the first and second terminal connectors  32 ,  34  are adapted to be connected to a source of electric power  36 . Arrangements for such power sources and suitable connections therefor are known to persons skilled in the art and no special design is needed for the arrangement according to the present invention. However, it is an advantage of the present invention that fewer power sources are needed for the heating element arrangement according to this invention than would be needed for the known heating element arrangements that include a center heating element. 
     The center heating element  28  is formed of a first heating element segment  42   a  and a second heating element segment  42   b.  First heating element segment  42   a  has a connection terminal  45   a  at a first end thereof whereby the first heating element segment  42   a  is connected to the first lateral heating element  24 . Second heating element segment  42   b  has a connection terminal  45   b  at a first end thereof whereby the second heating element segment  42   b  is connected to the second lateral heating element  26 . The first and second heating element segments  42   a  and  42   b  are connected to each other at second ends thereof as described in more detail below. In this manner the center heating element  28  provides a continuous conductive path between the first and second lateral heating elements  24  and  26 . With the foregoing arrangement, the center heating element  28  is connected to the electric power source  36  through the first and second heating elements  24 ,  26  and forms a complete electrical circuit therewith. 
     In the embodiment shown in  FIG. 2 , the center heating element  28  preferably includes a second pair of heating element segments  43   a,    43   b  of which heating element segment  43   b  is one member. The second ends of the heating element segments  42   a  and  42   b  of the center heating element  28  are connected together by means of a support connector  48 . Heating element segments  43   a  and  43   b  are also connected to the support connector  48  in spaced relation to the heating element segments  42   a,    42   b.  The support connector  48  is connected to one end of a heating element hanger assembly  46  that extends from and is attached to the hot zone enclosure  14 . The support arrangement for the center heating element  28  is shown in more detail in  FIG. 3 . 
     As shown in  FIG. 3 , the hanger assembly  46  is preferably formed of two bar segments  46   a  and  46   b  which are arranged in spaced parallel relation to each other. First ends of the bar segments  46   a  and  46   b  are attached to the hot zone enclosure  14  by any suitable means. Second ends of the bar segments  46   a  and  46   b  are attached to the support connector  48 . In the preferred arrangement shown, a pin  54  is inserted through aligned holes in the ends of bar segments  46   a  and  46   b  and in the support connector  48 . In order to provide electrical isolation between the support connector  48  and the hanger assembly  46 , a ceramic bushing or sleeve  52  is positioned around a central portion of pin  54  that extends through the pin hole of support connector  48 . In addition, ceramic ring spacers or collars  53   a  and  53   b  are disposed around the pin  54  on either side of the support connector  48 . Preferably, spacer  53   a  is located between one side of support connector  48  and bar segment  46   a.  Spacer  53   b  is located between the other side of support connector  48  and bar segment  46   b.  The heating element segments of the center heating element  28  are attached to the support connector  48 . In the embodiment shown in  FIG. 3 , a threaded graphite stud  56  extends through aligned holes in the ends of the heating element segments  43   a  and  43   b  and in the support connector  48 . The threaded stud  56  is held in position by means of graphite nuts  58   a  and  58   b  threaded to each end of the stud  56 . Stud  56  and support connector  48  are formed of a conductive material, preferably graphite, to provide a conductive path between the heating element segments  43   a  and  43   b.  Although not shown in  FIG. 3 , the heating element segments  42   a  and  42   b  are connected to the support connector  48  in a similar manner. 
     Referring back to  FIG. 1 , connection terminal  45   a  is attached to heating element segment  38   d  by means of a removable/reusable fastener  62   a.  Likewise, connection terminal  45   b  is attached to heating element segment  40   d  by means of a removable/reusable fastener  62   b . Connection terminal  45   a  is attached to heating element segment  42   a  by means of a removable/reusable fastener  64   a  and connection terminal  45   b  is attached to heating element segment  42   b  by means of a removable/reusable fastener  64   b.  The removable/reusable fasteners  62   a,    62   b,    64   a,  and  64   b  are preferably embodied as a combination of a threaded graphite stud and corresponding graphite nuts as described above. In this arrangement the threaded studs extend through aligned holes in the connection terminals and the corresponding heating element segments. The nuts are threaded onto the opposing ends of the threaded stud and tightened to complete the connection. Other types of removable/reusable fasteners known to those skilled in the art can also be used. In an alternative embodiment, the connection terminals  45   a,    45   b  can be formed integrally with the heating element segments  42   a  and  42   b,  respectively, such that the removable fasteners  64   a  and  64   b  would not be needed. 
     Referring now to  FIGS. 6-9 , there is shown an alternate arrangement for the center heating element array according to the present invention. In the arrangement shown in FIGS.  6 - 9 , the center heating element array includes a plurality of heating elements  128   a - 128   h  arranged in side-by-side relation along the length of the vacuum furnace hot zone. Each heating element  128   a - 128   h  is attached to a hanger assembly  110   a - 110   h,  respectively. Heating element  128   a , which is typical, includes first and second heating element segments  142   a  and  142   b.  In the same manner as described above, heating element segments  142   a  and  142   b  are connected at respective first ends thereof. Heating element segment  142   a  has a connecting segment  138   d  and heating element segment  142   b  has a connecting segment  140   d.  In the embodiment shown in  FIGS. 6-9 , a first end of connecting segment  138   d  is attached to a second end of heating element segment  142   a  by means of a removable/reusable fastener as described above. A second end of connecting segment  138   d  is adapted to be connected to the first heating element  24  by means of a removable/reusable fastener. A first end of connecting segment  140   d  is attached to a second end of heating element segment  142   b  by means of a removable/reusable fastener. A second end of connecting segment  140   d  is adapted to be connected to the second heating element  26  by means of a removable/reusable fastener. 
     In accordance with an aspect of this invention, the center heating element  28  is removably connected to the first and second lateral heating elements  24  and  26  so that the center heating element  28  can be removed when needed in order to permit a single large workload to be heat treated in the vacuum furnace  10 . The use of removable/reusable fasteners to attach the heating element segments  42   a  and  42   b  to respective heating element segments  38   d  and  40   d  is preferred to facilitate the removal and reinstallation of the third heating element  28 . This arrangement provides greater flexibility for the user of the vacuum furnace according to the invention with respect to the sizes of workloads that can be accommodated in one vacuum furnace. 
     In the embodiment shown in  FIG. 4 , the center heating element is removed. The heating element segments  38   a,    38   b,  and  38   c  are connected to heating element segments  40   a,    40   b , and  40   c  to form a complete electrical circuit. A jumper segment  38   e  is connected between heating element segments  38   c  and  40   c,  preferably by use of removable/reusable fasteners as described above. 
     The heating element segments  38   a - 38   d,    40   a - 40   d,    42   a,  and  42   b  are preferably formed of graphite as known to those skilled in the art. The heating element segments can be formed of a refractory metal such as molybdenum. When the center heating element  28  is formed of a refractory metal, it is preferred that insulating spacers will be fixedly positioned between the heating element segments  42   a  and  42   b  in order to prevent contact between the segments when the center heating element is energized. The lateral heating elements used in the embodiments described and shown herein can be realized by use of curved graphite heating elements as described in U.S. Pat. No. 5,965,050, the entirety of which is incorporated herein by reference. 
     Referring to  FIG. 5 , there is shown an alternate arrangement of the first heating element  24  used in the heat treating furnace of  FIG. 4 . In the arrangement shown in  FIG. 5 , the first heating element  24  has two sub-elements  24   a  and  24   b  electrically connected and arranged in parallel. A connector  47  provides an electrical connection between sub-elements  24   a,    24   b  and the connector terminal  32 . In like manner, although not shown, the second heating ( 26  in  FIG. 1 ) has two sub-elements (not shown) that are connected in parallel. 
     Referring now to  FIGS. 10-14 , there is shown an alternate embodiment of the heating element hanger assembly of  FIG. 3 . The hanger assembly  110  includes a bar  111  and a bracket  112 . Bracket  112  is generally U-shaped in cross section. The hanger assembly  110  also includes a rod  113  that extends through holes in a bottom end of bracket  113 . Rod  113  is used for connecting the hanger assembly  110  to the center heating element according to the present invention. Retaining wires  114   a  and  114   b  are used to hold the rod  113  in place. The retaining wires are typically inserted in diametric through-holes at each end of rod  113  and are formed in a manner so as to prevent them from falling out. 
     Hanger assembly  110  also includes first and second pins  115   a  and  115   b  for attaching the bar  111  to the bracket  112 . The pins  115   a  and  115   b  extend through aligned holes in the bar  111  and bracket  112 . The pins  115   a  and  115   b  are retained in position with pairs of retaining wires  116   a  and  116   b,  respectively. The retaining wires are typically inserted in diametric through-holes formed in the ends of the pins  115   a  and  115   b  that extend beyond the sides of bracket  112 . The bar  111 , bracket  112 , rod  113 , pins  115   a  and  115   b,  the retaining wires  114   a,    114   b,    116   a,  and  116   b  are preferably formed of a refractory metal such as molybdenum. In order to electrically insulate the bar  111  from bracket  112 , ceramic sleeves  117   a  and  117   b  are positioned around the central portions of the pins  115   a  and  115   b  that extend through the holes in bar  111 . In addition, ceramic collars  118   a  and  118   b  are positioned around pin  115   a  so as to prevent contact between bar  111  and the sidewalls of bracket  112 . Additional ceramic collars  118   c  and  118   d  are positioned around pin  115   b  between bar  111  and the side walls of bracket  112 . 
     The hanger assembly  110  further includes a disc  121  positioned on bar  111  and retained in position with a retaining wire  122  that extends through a small hole in bar  111 . A small notch  123  is formed in bar  111  near the end thereof that is distal from the bracket  112 . The disc  121  is positioned at a distance along bar  111  from the notch  123  so that the bar  111  can extend through the hot zone insulation a distance sufficient to allow it to engage with a catch on the hot zone enclosure by twisting the hanger assembly  110 . When the hanger assembly  110  is thus installed, the disc  121  abuts the hot zone insulation. The twist-lock feature is described in U.S. Pat. No. 4,321,415, the entirety of which is incorporated herein by reference. 
     The foregoing text describes the features of a single heating element array in accordance with the present invention. However, as shown in  FIGS. 2 and 5 , a vacuum furnace according to the present invention may also include two or more heating element arrays. Referring to  FIGS. 2 and 5 , a further embodiment of the vacuum furnace  10  will have additional heating element arrays positioned within the hot zone  16  and positioned at intervals along the longitudinal axis of the pressure vessel  12 . A second heating element array includes a first lateral heating element  124 , a second lateral heating element (not shown) and a center heating element  128 . A third heating element array includes a first lateral heating element  224 , a second lateral heating element (not shown) and a center heating element  228 . In addition, a fourth heating array includes a first lateral heating element  324 , a second lateral heating element (not shown) and a center heating element  328 . This concept can be extended to any desired and effective number of heating element arrays. 
     In accordance with the present invention it is contemplated that in the heating element arrays, the heating element segments that make up the respective heating element arrays can be formed to provide different electrical resistances or watt densities at different locations in the heating element arrays. This arrangement allows for placement of heating elements having an electrical resistance and/or watt density selected to provide more or less heat as needed in the furnace hot zone to provide better temperature uniformity in the workload. The electrical resistances and watt densities of the heating element arrays are varied by using a first heating element having a geometry in one segment of a heating element array and a second heating element having a different geometry from that of the first heating element in another section of the heating element array. For example, in one embodiment the heating element segments located in an upper region of the hot zone will have a geometry that provides a first watt density and the heating element segments in a lower region of the hot zone will have a different geometry to provide a second watt density having a different magnitude than the first watt density. It is also contemplated that, when more than one heating element array is present in the vacuum furnace of this invention, all of the heating element segments in one heating element array will have the same geometry and all of the heating element segments in another heating element array will have a different geometry. In this manner the radiant heat output from one heating element array will be different from the radiant heat output of another heating element array, whereby the heat applied to the workload will be different in different zones of the vacuum furnace. Thus, for example, the heating element segments in the heating element array nearest the door of the vacuum furnace will have a geometry that provides a watt density sufficient to provide greater heat output than that of an inboard heating element array(s) in the hot zone. This aspect of the invention is described in copending U.S. patent application Ser. No. 13/728,122, the entirety of which is incorporated herein by reference. 
     It is further contemplated that when multiple heating element arrays are present in a vacuum furnace according to this invention, the heat output of each heating element array can be adjusted or trimmed at the electric power source. This is conventionally realized by use of a variable reactance transformer associated with the electric power source and connected to each one of the heating element arrays in the manner known to those skilled in the art. This aspect of the invention is described in U.S. Patent Application Publication No. 2013/0175256, the entirety of which is incorporated herein by reference. 
     In view of the foregoing description of preferred embodiments of a vacuum furnace according to the present invention, some of the advantages of the inventive concepts will be apparent to those skilled in the art. For example, the vacuum heat treating furnace according to the present invention includes a center heating element that is operatively connected to an electric power source through two lateral heating elements so that the center heating element is energized without the need for a separate power connection to the center heating element. Also, the center heating element used in the vacuum furnace of this invention is removably connected to the lateral heating elements and to the hot zone enclosure so that the center heating element can be easily removed, thereby permitting the vacuum furnace to be reconfigured for a single large workload to be processed in the vacuum furnace. 
     The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features or steps shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. For example, the heating element arrangement including a removable center heating element according to the present invention could be adapted for use in a vertical vacuum furnace. Accordingly, the invention incorporates variations that fall within the scope of the invention as described.