Abstract:
A convection heating system for vacuum furnaces is described. The convection heating system including a hot zone enclosure defining a hot zone, a plurality of gas injection nozzles for injecting a cooling gas into the heat treatment zone of furnace, an exit port for permitting discharge of the cooling gas from the hot zone, and means for circulating a process gas within the hot zone to provide convective heating and cooling. Each gas injection nozzle has a flap disposed pivotally supported therein for substantially preventing the escape of heat from the hot zone during a heating cycle, but for permitting the injection of the cooling gas into the furnace hot zone during a cooling cycle. The gas exit port includes a flap pivotally mounted therein for impeding the unforced outward flow of a gas from the heat treatment zone during a heating cycle

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of application Ser. No. 09/597,496 filed on Jun. 20, 2000, the disclosure of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to vacuum heat treating furnaces, and in particular, to a convection heating system for vacuum furnaces having a unique combination of features that provides significantly improved heat retention and heat transfer during heating and cooling cycles, respectively.  
         BACKGROUND OF THE INVENTION  
         [0003]    Known vacuum heat treating furnaces available hitherto incorporate cooling gas injection systems to provide cooling of metal parts from the elevated heat treatment temperature. Among the components of the cooling gas injection system used in such furnaces are a plurality of nozzles for conducting the cooling gas into the furnace hot zone. The gas injection nozzles used in the known systems are generally tubular or cylindrical in shape and have an unobstructed central opening that extends along the length of the nozzle.  
           [0004]    A problem arises when using such nozzles in a vacuum heat treating furnace. Because the known nozzles have unobstructed openings therethrough, heat can be lost from the hot zone during the heating cycle. Such heat loss occurs when the heated atmosphere in the furnace hot zone escapes the hot zone through the cooling gas nozzles and is cooled in the plenum or, in a plenumless furnace, in the space between the hot zone and the furnace wall. The heated gas is cooled as it traverses the plenum, or the annular space between the hot zone and the water-cooled furnace wall in a plenumless furnace, and reenters the hot zone at a lower temperature. This problem occurs in vacuum furnaces that utilize convection heating.  
           [0005]    In addition, in the known vacuum heat treating furnaces with forced gas cooling, a return path is provided so that the cooling gas can be recirculated and cooled. This return path usually includes an opening in the hot zone enclosure so that the cooling gas can exit the hot zone. This opening in the hot zone wall also permits heat to escape from the hot zone during heating.  
           [0006]    The above-described heat loss results in a non-uniform heating of the metal parts and higher energy use. When the metal parts do not uniformly attain the desired heat treating temperature, the properties desired from the parts are not achieved. Consequently, a need has arisen for a heat treating furnace having a forced gas cooling function which substantially prevents the heat in the hot zone from exiting the hot zone during a convection or other heating cycle. It would be highly desirable to have a simple device for injecting cooling gas into a vacuum heat treating furnace which substantially inhibits the escape of heated gas therethrough without the need for actuators and the mechanical linkage systems needed to operate such actuators.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with the present invention, a heat treatment furnace having forced gas cooling or quenching capability is provided. The heat treatment furnace according to this invention includes an outer furnace wall inside of which a heat shielded enclosure is provided. The heat shielded enclosure contains an interior space, or hot zone, in which a work piece may be placed/positioned for heat treatment. The enclosure is designed with substantial thermal insulation to impede the outward flow of heat from the hot zone. The enclosure includes a plurality of orifices disposed in a selected area or areas of the enclosure wall. A plurality of nozzles are provided in communication with the orifices so that a cooling gas may be injected into the hot zone through the nozzles during a cooling cycle. The nozzles include a flow control means that is adapted for allowing an inward flow of the cooling gas during a cooling cycle, but which impedes the outward flow of heat from the hot zone during a heating cycle. In a first embodiment of the flow control means, each nozzle includes a flap disposed in a channel formed through the nozzles. The flap is pivotally supported in the channel in such a manner so as to impede the outward flow of heat from the hot zone, but to permit the inward flow of the cooling gas. The furnace further includes a gas exit port disposed in a wall of the heat shielded enclosure. The gas exit port provides a passageway through which the cooling gas introduced into the hot zone via the nozzles may exit the hot zone for recirculation and cooling. The gas exit port is also configured to impede the outward flow of heat from the hot zone during a heating cycle of the furnace. In a preferred embodiment of the gas exit port, the exit port includes a pivotally mounted panel in the passageway for impeding the unforced outward flow of heat from the hot zone. The exit port panel also functions to prevent the unforced introduction of cooler gas into the hot zone. A gas circulation means is also provided within the heat shielded enclosure for providing stirring circulation of the heated atmosphere within the hot zone to convectively heat or cool a work piece that is being heat treated in the furnace. The circulation means may conveniently be provided as a fan. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The foregoing summary, as well as the following detailed description of a preferred embodiment of the present invention, will be better understood when read in conjunction with the drawings, in which:  
         [0009]    [0009]FIG. 1 is a schematic view partially in section of a vacuum heat treating furnace in accordance with the present invention;  
         [0010]    [0010]FIG. 1A is a detail view of an alternative arrangement for the end wall structure of the vacuum heat treating furnace shown in FIG. 1;  
         [0011]    [0011]FIG. 2 is a sectional view taken along line  2 -- 2  of FIG. 1 showing the end wall of the heat shielded enclosure;  
         [0012]    [0012]FIG. 3 is a perspective view of a cooling gas nozzle in accordance with the present invention;  
         [0013]    [0013]FIG. 4 is a cross-sectional side elevation view of the cooling gas nozzle of FIG. 3 as viewed along line  4 -- 4  therein;  
         [0014]    [0014]FIG. 5 is a front elevation view of the cooling gas nozzle of FIG. 3;  
         [0015]    [0015]FIG. 6 is a rear elevation view of the cooling gas nozzle of FIG. 3;  
         [0016]    [0016]FIG. 7 is a perspective view of a pin for attaching the cooling gas nozzle of FIG. 3 to a furnace hot zone wall; and  
         [0017]    [0017]FIG. 8 is a cross-sectional side elevation view of a gas exit port in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring now to the drawings wherein like reference numerals refer to the same or similar elements across the several views, and in particular to FIG. 1, there is shown a heat treating furnace generally designated  10  which includes a pressure vessel having a double outer wall  12 , preferably of generally cylindrical shape, and a domed double end wall  14 . The space between the double walls can be insulating space to impede the flow of heat or can be liquid filled and used as a cooling jacket, if desired. End wall  14  includes a cylindrical motor housing and support  16  which has a flanged outer edge  16   a  which mates with a flanged edge  18   a  of an end closure  18  for the motor housing. End closure  18  is removable for servicing the motor  20 . Although not shown here, the flanges are provided with suitable fastening means (e.g., bolts) and sealing means (e.g., gasket seal). A motor  20  is supported within the housing  16  and is provided with electrical connections which pass through motor housing wall  16  in a sealed manner.  
         [0019]    The opposite end of the vacuum furnace  10  is provided with a double-wall end closure  24  having a sealing flange  24   a  which cooperates with a sealing flange  12   a  on the cylindrical double wall structure  12 . A furnace of the present invention may vary in size, but is typically quite large, having a diameter of perhaps six feet or more. In such large structures the end closure  24  is supported in a way not material to the present invention, but which enables it to be conveniently moved away from the end of the structure to allow the introduction into the furnace hot zone of work pieces to be heat treated, typically supported on refractory pallets. Although not shown the furnace requires heating elements  25  or other means of heating. One such heating element arrangement is shown in FIG. 2.  
         [0020]    As shown in FIG. 1, a heat shielded enclosure, or hot zone wall, generally designated  26 , conforming to the shape of the outer wall  12  is suitably supported in the pressure vessel by structure not shown, but well known in the art. In a cylindrical furnace, such as that shown in the drawings, a cylindrical hot zone wall  28  is preferably generally arranged coaxially with the longitudinal axis of the pressure vessel. The hot zone wall  28  is spaced inwardly a uniform spacing distance from the outer furnace wall  12 . In the embodiment shown in FIG. 1, the hot zone enclosure  26  is substantially cylindrical. However, the enclosure  26  and hot zone wall  28  may have other cross-sectional shapes such as square, rectangular, or polygonal, as needed for a particular application. The hot zone enclosure  26  is lined internally with a refractory material to resist the intense processing heat. The hot zone enclosure  26  is designed to retain the heat within the enclosure and impede its flow outwardly and to provide a hot zone  40  therein into which work pieces to be heat treated are positioned.  
         [0021]    An end wall  30  of construction similar to the hot zone wall  28 , is attached at one end thereof. A movable end wall  32  is disposed at the opposite end of the heat shielded enclosure  26 , and is of similar construction thereto. End wall  32  is dimensioned to substantially close the open end of the enclosure  26 . The movable wall  32  which completes the heat shielded enclosure  26  is affixed to and moves with the furnace end closure  24 . End closure  24  includes a cylindrical motor housing  65  and support  66 . The motor housing  65  is generally cylindrical in shape and has a central longitudinal axis substantially aligned with the central longitudinal axis of the enclosure  26  when the movable end wall  32  is engaged to close the open end of the enclosure  26 . A convection motor  70  is supported within the housing  65  on support structure  67 . The convection motor  70  is provided with electrical connections  68  which pass through and are sealed at motor housing wall. The convection motor  70  is also provided with optional water cooling by means of inlet water tubing  64   a  and outlet water tubing  64   b  which pass through and are sealed at the motor housing wall.  
         [0022]    A convection fan  60  is attached to a hub  60   b , which is mounted to the shaft  62  of the convection motor  70 . The hub  60   b  extends through an aperture in the movable end wall  32  so that the fan  60  is located inside the hot zone when the end closure  24  and end wall  32  are in the fully closed position. The convection fan  60  in the embodiment shown in FIGS. 1 and 1A has flat blades  60   a  attached to the hub  60   b  on the shaft  62 . Because the blades  60   a , hub  60   b , and shaft  62  are disposed within the hot zone  40  during the heating cycle of the furnace  10 , those components are preferably made of a refractory material capable of withstanding the very high temperatures attained within the hot zone  40 . One such suitable material is carbon reinforced carbon (CFC) manufactured by C-CAT, Inc. of Fort Worth, Tex., USA. In operation, the convection fan  60  circulates or stirs the gas within the hot zone  40  during a convection heating cycle to provide more rapid and uniform heating of work pieces present within the hot zone  40 . In addition, during a cooling cycle the convection fan  60  may be used to assist circulation of the cooling gas within the hot zone  40  to provide more rapid and uniform cooling of the work pieces.  
         [0023]    The hot zone wall  28  of the heat shielded enclosure  26  is perforated with a plurality of orifices  36 . Optionally, a plurality of orifices  38  perforate the end wall  30  also. The orifices  36 ,  38  are so distributed over the wall areas as to permit the flow of cooling or heat treating gas in several directions in the hot zone  40 , toward the work pieces being treated. The orifices  36 ,  38  may have any shape and pattern of distribution at the enclosure wall  28  and end wall  30  that is suited to provide the desired flow of gas into the hot zone  40 . For example, the orifices  36 ,  38  may comprise a series of holes in the walls  28 ,  30 . Alternatively, the orifices  36 ,  38  may comprise one or more longitudinal slots.  
         [0024]    A plurality of gas injection nozzles  39  are disposed in communication with the orifices  36 ,  38  to provide a means for injecting a cooling gas into the hot zone  40  during a forced gas cooling cycle of the heat treating furnace when the work pieces are rapidly cooled from the heat treating temperature. The gas injection nozzles  39  include a means for substantially preventing the egress of heat from the hot zone  40  during the heating cycle of the furnace  10 . The gas injection nozzles  39  may comprise any structure that permits the forced flow of gas therethrough, but which also impedes the flow of heat that would otherwise be induced by natural convection therethrough. For example, the nozzles  39  may comprise a baffle structure in gaseous communication with the orifices  36 ,  38 . In a preferred embodiment, the nozzles  39  have a flap valve which is described more fully hereinbelow.  
         [0025]    The gas injection nozzles  39  are fastened to the hot zone wall  28  by any appropriate means. This arrangement can be seen more easily in FIG. 6. Suitable fastening means include pins, bolts, wires, threads, twist-lock tabs, or retaining clips. The means for attaching the nozzle  39  to the hot zone wall  28  preferably provides for easy installation and removal of the nozzle  39  to facilitate assembly and maintenance of the heat treating furnace  10  and/or its heat shielded enclosure  26 . A preferred means for attaching the nozzle  39  to the hot zone wall  28  is described more fully below.  
         [0026]    Referring now to FIGS.  3 - 7 , an embodiment of the gas injection nozzle  39  will be described in greater detail. The gas injection nozzle  39  is formed of a forward portion  21  which is exposed in the hot zone  40  and a rear portion  25  which is attached to the hot zone wall  28  and end wall  30  to communicate with orifices  36  and orifices  38 , respectively. A first central opening  23  is formed through the length of the forward portion  21  and a second central opening  27  is formed through the length of the rear portion  25 . The first central opening  23  and the second central opening  27  are aligned to form a continuous channel through the nozzle  39 . The rear portion  25  has an annular recess  29  formed at the end thereof. The annular recess  29  is formed to accommodate a boss on the hot zone wall  28  around the orifice  36  as shown in FIG. 4.  
         [0027]    A pair of boreholes  33   a  and  33   b  are formed or machined in the nozzle  39  for receiving metal attachment pins that attach the nozzle  39  to the hot zone wall  28 . A preferred construction for the attachment pins is shown in FIG. 7. A pin  41  has a first end on which a plurality of screw threads  43  are formed to permit the pin  41  to be threaded into a threaded hole (not shown) in the hot zone wall. It will be appreciated that instead of the screw threads  43 , the first end of pin  41  can be provided with twistlock tabs, or a transverse hole for accommodating a retaining clip. The other end of the attachment pin  41  has a transverse hole  45  formed therethrough for receiving a retaining clip (not shown) to hold the nozzle  39  in place.  
         [0028]    A flap  31  is disposed in the first central opening  23  and is pivotally supported therein by a pin  33  which traverses holes in the sidewalls  35   a ,  35   b  of forward portion  21 . The flap  31  is positioned and dimensioned so as to close the central opening  23  when it is in a first position, thereby preventing, or at least substantially limiting, the transfer of heat out of the hot zone  40  and the unforced introduction of cooler gas into the hot zone through the central channel of the nozzle  39 . In a second position of the flap, as shown in phantom in FIG. 4, the central opening  23  is open to permit the forced flow of cooling gas therethrough into the hot zone  40  during a cooling or quenching cycle. For simplicity, the flap  31  is maintained in the first or closed position by the force of gravity. In such an arrangement the nozzle  39  is preferably oriented such that the flap will be normally closed. In a horizontally oriented vacuum furnace, as shown in the embodiment of FIG. 1, some of the nozzles  39  in the upper half of the hot zone  40  will necessarily be open a small amount because of the orientation of the nozzles  39  and the effect of gravity on the flap  31 . When it is desired to maintain the flaps  31  of such nozzles  39  in the normally closed position, biasing means, such as a counterweight or a spring, can be used. The biasing means should provide sufficient biasing force to maintain the flap  31  in the normally closed position, but the biasing force of the biasing means should be less than the force of the cooling gas on the flap  31  when it is being injected so that the flap  31  can be readily moved to the open position by the flow of the cooling gas.  
         [0029]    The nozzle  39  and the flap  31  are preferably formed from a refractory material such as molybdenum, graphite, or CFC. They may also be formed of a ceramic material if desired. In the embodiment shown, the forward portion  21  is rectangular in cross section and the rear portion  25  is circular in cross section. However, the shapes of the forward and rear portions of nozzle  39  are not critical. Similarly, the shapes of the first and second central openings  23 ,  27  are not critical. The first central opening  23  is preferably square or rectangular for ease of fabrication and the second central opening  27  is preferably circular for ease of adaptation with the opening in the hot zone wall  28 .  
         [0030]    Referring back now to FIG. 1, cooling gas is preferably supplied to the nozzles  39  through a plenum  47 . Accordingly, the orifices  36 ,  38  are provided over an area of the enclosure wall  28  and end wall  30  selected to provide passageways for gaseous communication between the hot zone  40  and the plenum  47 . The plenum  47  is disposed in the passage between the furnace wall  12  and the enclosure wall  28  and extends around the back thereof, over the orifices  36 ,  38 . The plenum  47  includes a plenum wall  42  connected to the heat shielded enclosure wall  28  by radially inwardly extending plenum end wall  44  located between the orifices  36  and the open end  37  of the enclosure  26  to provide an annular flow channel around the hot zone wall  28 . The plenum wall  42  extends beyond the end wall  30  of the heat shielded enclosure  26  and the plenum  47  is continued by a planar plenum end wall  46  extending radially inwardly to a cowling  48 . A blower fan  50  is attached at hub  50   b  to shaft  52  of motor  20 . In the embodiment shown in FIG. 1, a heat shield  55  is mounted between the fan  50  and hot zone enclosure  26  in order to protect the fan and motor from the intense heat generated in the hot zone  40  during operation of the furnace. The cowling  48  has a curved or flared entry throat to minimize turbulence and promote efficient flow of the cooling gas from the blower fan  50 . The fan in the embodiment shown in FIG. 1 preferably has curved blades. The outward flow of air from blower fan  50  is directed in a generally radial direction throughout 360° in the space defined by the plenum  47 . The plenum  47  itself is adapted to handle the pressure and to keep the gaseous atmosphere relatively confined so as to cause relatively even flow through the nozzles  39  into the not zone  40 . Heat exchange coils  54  are preferably disposed in the recirculation channel between walls  46  and  14  to cool the recirculated cooling gas. Whether the coils are wound in helical layers as suggested in FIG. 1 is a matter of choice. The actual configuration of coils is not critical and may be varied a great deal.  
         [0031]    During a cooling cycle, the cooling gas, after entering the hot zone  40 , flows out of the hot zone  40  and into a coolant recirculation channel through the gas exit ports  34  as shown by the arrows “A”. The gas exit ports  34  may be provided in one or more of the movable end wall  32 , enclosure wall  28 , and end wall  30 . In the embodiments shown in FIGS. 1 and 1A, the gas exit ports are provided in the movable end wall  32 . The recirculation channel is defined by the furnace wall  12  and the outer plenum wall  42  and by the walls  46  and  14 . The gas exit ports  34  may comprise any structure that permits the forced flow of gas therethrough and also prevents the flow of heated gas therethrough that is induced by natural convection.  
         [0032]    A preferred arrangement of the gas exit port  34  is shown in FIG. 8. The gas exit port  34  comprises an exit port panel or flap  61  similar in function to the flap  31  of a nozzle  39 . The exit port flap  61  is disposed in exit port opening  63  which is formed in the movable end wall  32 . The exit port flap  61  is pivotally supported within the exit port opening  63  by a pin  69  which is held within the movable end wall  32 . The exit port flap  61  is positioned and dimensioned so as to close the exit port opening  63  when the flap is in a first position, thereby preventing, or at least substantially limiting, the transfer of heat out of the hot zone  40  and preventing the unforced introduction of cooler gas into the hot zone  40  through the exit port opening  63 . To enhance this function, the flap  61  is lined with thermal insulation  61 ′. In a second position of the flap  61 , as shown in phantom, the exit port opening  63  is open to permit the forced flow of cooling gas therethrough from the hot zone  40  during a cooling or quenching cycle. For simplicity, the exit port flap  61  is maintained in the first or closed position by the force of gravity. In such an arrangement the exit port flap  61  is preferably oriented such that it will be normally closed. The exit port flap  61  is preferably formed from a refractory material such as molybdenum, graphite, or CFC. The exit port flap  61  may also be formed of a ceramic material if desired. The shapes of the exit port opening  63  and exit port flap  61  are not critical. The exit port opening  63  and exit port flap  61  are preferably square or rectangular for ease of fabrication.  
         [0033]    Referring back to FIG. 1, a vacuum pump, shown schematically as block  159 , is provided for evacuating the furnace chamber. A controlled pressure gas supply  160  is also provided to introduce the processing gas into the furnace chamber. The processing gas is typically introduced at pressures elevated substantially above atmospheric pressure. Separate fluid supply and circulating means may be provided to supply coolant fluid to the furnace jacket  12 ,  14  and the end enclosure  24  and to the heat exchanger coils  54 , as needed.  
         [0034]    It will be recognized by those skilled in the art that changes or modifications may be made to the above described embodiments without departing from the broad, inventive concepts of the invention. It is understood, therefore, that the invention is not limited to the particular embodiment(s) disclosed, but is intended to cover all modifications and changes which are within the scope and spirit of the invention as defined in the appended claims. For example, the convection heating system according to this invention can be used in a vacuum heat treating furnace in which the cooling fan and heat exchanger coils are external to the furnace vessel.