Patent Publication Number: US-6903306-B2

Title: Directional cooling system for vacuum heat treating furnace

Description:
RELATED APPLICATIONS 
   This continuation-in-part application claims priority under 35 U.S.C. §120 to U.S. application Ser. No. 10/154,457, filed May 23, 2002, which claims priority to U.S. application Ser. No. 09/597,496, filed Jun. 20, 2000, both of which are incorporated herein by reference in entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to vacuum heat treating furnaces, and more specifically to a vacuum heat treating furnace having a precision-controlled, directional cooling system that provides uniform cooling of a workpiece load. 
   BACKGROUND 
   Known vacuum heat treating furnaces employ cooling gas injection systems to rapidly cool workpieces from the heat treating temperature. The workpieces are heated in a hot zone which is enclosed by a hot zone wall that retains heat inside the hot zone. After heat treatment, cooling gas is injected into the hot zone to cool the workpieces. The cooling gas flows across the hot zone to cool the workpieces and exits through one or more exit ports in the hot zone wall. The exit ports are typically small to minimize the escape of heat from the hot zone during heat treatment. 
   One problem with known vacuum treating furnaces occurs when the workpiece is not cooled uniformly. In many furnaces, the stream of cooling gas contacts one part of the workpiece load more than other parts, resulting in areas that receive too little or too much cooling. When workpieces are not cooled uniformly, the finished workpiece may not exhibit the desired properties, such as hardness and ductility. Non-uniform cooling is a common problem in systems that draw cooling gas to exit ports located at only one end of the hot zone. Non-uniform cooling is also a problem in furnaces where the flow of cooling gas is fixed in one configuration that cannot be adjusted or adapted to cool workpieces having different sizes and geometries. 
   Directional cooling systems have been developed to improve cooling by controlling the flow of cooling gas that enters the hot zone. In directional cooling systems, injection of cooling gas can be concentrated in different sections of the hot zone to cool specific areas of the workpiece. Although directional cooling systems provide better control of cooling gas entering the hot zone, the cooling gas stream is typically discharged from one end of the hot zone. As a result, the cooling gas stream is drawn to one section of the hot zone, which still results in uneven cooling along the length of the workpiece. 
   Another problem with known directional cooling systems is the placement of actuators, dampers, and other moving components in the hot zone. When moving components are routinely exposed to high temperatures in the hot zone, the components become damaged over time, increasing maintenance and equipment downtime. As a result, the known vacuum heat treating furnaces and cooling systems fall short of the needs of furnace users who desire uniform cooling of workpieces and reduced maintenance of their vacuum furnaces. 
   SUMMARY OF THE INVENTION 
   The above-described problems associated with the known vacuum heat treating furnaces are overcome to a large degree by the vacuum heat treating furnace in accordance with the present invention. According to a first aspect of the present invention, there is provided a heat treating furnace for providing directional cooling of a workpiece load. The heat treating furnace includes a hot zone enclosure defining a hot zone therein. The hot zone enclosure has a side wall, a first end wall, and a second end wall. The side wall has one or more slots formed therethrough and along the length thereof. The heat treating furnace also includes means for injecting a cooling gas into the hot zone through the hot zone enclosure. The heat treating furnace further includes means for directing the cooling gas to exit the hot zone enclosure through one or more of the slots. 
   In accordance with a second aspect of the present invention, there is provided a hot zone enclosure for a heat treating furnace. The hot zone enclosure includes a side wall and first and second end walls. The side wall has one or more slots formed therethrough and along the length thereof. The slots are covered to limit the escape of heat from the hot zone during heat treatment. In one embodiment of the invention, the slots are covered by actuated bungs. In another embodiment, the slots are aligned with stationary baffles spaced inwardly or outwardly from the slots. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary as well as the following detailed description will be better understood when read in conjunction with the drawings in which: 
       FIG. 1  is a top plan view in partial section of a vacuum heat treatment furnace in accordance with the present invention. 
       FIG. 2  is an end view in partial section of the vacuum heat treatment furnace in  FIG. 1  as viewed along line  2 — 2  in FIG.  1 . 
       FIG. 3  is an end view in partial section of the vacuum heat treatment furnace in  FIG. 1  as viewed along line  3 — 3  in FIG.  1 . 
       FIG. 4  is a perspective view of a cooling gas nozzle used with the vacuum heat treatment furnace in FIG.  1 . 
       FIG. 5  is a partial sectional view of the cooling gas nozzle of  FIG. 4  taken through line  5 — 5  in FIG.  4 . 
       FIG. 6  is a perspective view of a pin that may be used with the cooling gas nozzle in FIG.  4 . 
       FIG. 7  is a rear elevation view of the cooling gas nozzle of FIG.  4 . 
       FIG. 8  is a side sectional view of the vacuum heat treatment furnace of  FIG. 1  as viewed along line  8 — 8  in FIG.  1 . 
       FIG. 9  is a side sectional view of the vacuum heat treatment furnace of  FIG. 1  as viewed along line  9 — 9  in FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, a heat treating furnace in accordance with the present invention is shown and designated generally as  20 . The heat treating furnace  20  has a hot zone  32  that includes a side wall  30 , a first end wall  30 ′ and a second end wall  30 ″. Cooling gas can be injected into the hot zone  32  and onto a workpiece from several angles relative to the workpiece. The cooling gas is injected through a plurality of nozzles  50  installed through the side wall  30 . The side wall  30  has one or more elongated slots  36 . In this manner the cooling gas is caused to flow uniformly over the length of the workpiece to provide efficient removal of heat and improve front to back cooling uniformity. 
   A damper assembly  80  is provided to control the direction and flow rate of the cooling gas stream through the hot zone  32 . The damper assembly  80  has two or more dampers  82  that connect the hot zone  32  to a blower unit  60 . Each damper  82  is located in proximity to one of the slots  36  and is adjustable to draw gas flow into the slot in closest proximity to the damper. The dampers  82  are operable individually or in combination to create a cooling gas stream with a desired magnitude and flow direction through the hot zone. The dampers  82  are controlled by actuators  86  that are thermally isolated from the hot zone  32 , to prevent damage to the actuators from heat generated in the hot zone. 
   Referring now to  FIGS. 1 and 2 , the furnace  20  will be described in greater detail. The furnace  20  may be constructed with a variety of exterior configurations and orientations. In  FIG. 1 , the furnace  20  is shown as a generally horizontal cylindrical vessel. The hollow interior of the furnace  20  is enclosed by a double outer wall  22  and a domed, double wall door  24 . The double outer wall  22  has an open end  26  that is sealed by the door  24 . The door  24  is preferably attached to the pressure vessel  22  by hinges and is movable to expose the open end  26  and provide access to the interior of the furnace  20 . 
   The hot zone  32  has an array of heating elements  33  mounted inside the hot zone  32  for applying heat to a workpiece placed in the furnace. The heating elements  33  extend around the hot zone  32  and are arranged along the length of the hot zone  32  to distribute heat uniformly throughout the hot zone. The hot zone walls  30 ,  30 ′, and  30 ″ are configured to retain heat in the hot zone and minimize transfer of heat from the workpiece during heating. A variety of heat retention mechanisms may be used to retain heat in the hot, zone. As shown in  FIG. 2 , the hot zone  32  is surrounded by a thermal insulation layer  31  connected to the hot zone walls  30 ,  30 ′, and  30 ″. 
   Referring again to  FIG. 1 , a convection fan  52  is mounted inside the hot zone  32  and has a plurality of flat blades. The convection fan  52  is mounted on a shaft  56  driven by a motor  54  mounted in the door  24  outside the hot zone  32  between the end wall  30 ′ and the outer wall of door  24 . The motor  54  is operable to rotate the shaft  56  and fan  52  to provide convective heating in the hot zone during a heat treating cycle. 
   A heat shielded enclosure  40  is mounted inside the furnace  20  in the annular space between the double outer wall  22  and the hot zone wall  30 . The enclosure  40  is connected to the interior surface of the double outer wall  22  by a welded flange or other means of support. An annular space or plenum  42  is formed between the side wall  30 , the end wall  30 ″, and the heat shielded enclosure  40 . The enclosure  40  surrounds a portion of the side wall  30  and terminates near the end wall  30 ′. An end wall  41  connects the terminal end of the enclosure  40  to the side wall  30  such that the plenum  42  is substantially enclosed between the hot zone wall and enclosure, as shown in FIG.  1 . An annular duct  43  is formed between the double outer wall  22  and the enclosure  40 . 
   Cooling gas is injected into the hot zone  32  and removed from the hot zone in a closed loop system. As shown in  FIG. 2 , the duct  43  is operatively connected to the hot zone  32  by a plurality of conduits  53 . The conduits  53  are arranged around the hot zone  32  so that the cooling gas can be introduced into the hot zone from several angles around the workpiece. The side wall  30  has a plurality of orifices  34  that are coaxially aligned with a plurality of orifices  44  extending through the enclosure  40 . The conduits  53  extend between the orifices  34 ,  44  and through the plenum  42  to form a direct passage from the annular duct  43  to the hot zone  32 . A plurality of gas injection nozzles  50  are mounted on the side wall  30  in communication with the conduits  53 . 
   Referring now to  FIGS. 4-7 , the gas injection nozzles  50  (hereinafter “nozzles”) will be described in greater detail. The nozzles  50  provide a means for injecting a cooling gas into the hot zone  32  during a forced gas cooling or quenching process. The nozzles  50  are also constructed to substantially prevent the egress of heat from the hot zone  32  during a heat treating cycle. A variety of structures may be used for the nozzles  50  to permit forced flow of cooling gas into the hot zone while impeding the convection of heat from the hot zone. In the preferred embodiment, the nozzles  50  have a flap valve  51 . The nozzles  50  extend through the thermal insulation layer  31  and are attached to the side wall  30 . A variety of fasteners may be used to secure the nozzles  50  to the side wall  30 , including pins, bolts, wires, threads, twist-lock tabs, or retaining clips. The means for attaching the nozzle  50  to the side wall  30  preferably provides for easy installation and removal of the nozzle to facilitate assembly and maintenance of the heat treating furnace  20  and/or its hot zone  32 . 
   Referring now to  FIGS. 4 and 5 , there are shown the details of a preferred arrangement for a gas injection nozzle  50 . The gas injection nozzle  50  is formed of a forward portion  121  which is exposed in the hot zone  32  and a rear portion  125  which extends through the insulation layer  31  and is attached to the side wall  30 . A first central opening  123  is formed through the length of the forward portion  121  and a second central opening  127  is formed through the length of the rear portion  125 . The first central opening  123  and the second central opening  127  are aligned to form a continuous channel through the nozzle  50 . The rear portion  125  has an annular recess  129  formed at the end thereof. The annular recess  129  is formed to accommodate a rounded flange or collar  101  that extends inwardly from the side wall  30  at an orifice  34 . 
   A pair of boreholes  128   a  and  128   b  are formed or machined in the forward portion  121  of nozzle  50  for receiving the fasteners that attach the nozzle  50  to the side wall  30 . A preferred construction for the fastener is shown in  FIG. 6. A  pin  140  has a first end on which a plurality of screw threads  142  are formed to permit the pin  140  to be threaded into a threaded hole in the hot zone wall. It will be appreciated that instead of the screw threads  142 , the first end of pin  140  can be provided with twist-lock tabs, or a transverse hole for accommodating a retaining clip. The other end of the attachment pin  140  has a transverse hole  144  formed therethrough for receiving a retaining clip to hold the nozzle  50  in place. 
   Referring to  FIGS. 5 and 7 , a flap  131  is disposed in the first central opening  123  and is pivotally supported by a pin  133  which traverses holes in sidewalls  135   a  and  135   b  of forward portion  121 . The flap  131  is positioned and dimensioned so as to close the central opening  123  when it is in a first position, thereby preventing, or at least substantially limiting, the transfer of heat out of the hot zone  32  and the unforced introduction of cooling gas into the hot zone through the central channel of the nozzle. In a second position of the flap  131 , as shown in phantom in  FIG. 5 , the central opening  123  is open to permit the forced flow of cooling gas through the nozzle  50  and into the hot zone  32  during a cooling or quenching cycle. The position of the flap  131  relative to the central channel may be influenced by gravity, depending on the position and orientation of the nozzle  50  on the side wall  30 . In some sections on the side wall  30 , the flap  131  is maintained in the first or closed position by the force of gravity. In other areas of the side wall  30 , the flap  131  may be pivoted toward the second or open position under the force of gravity, leaving the nozzles open. For this latter set of nozzles, biasing means, such as a counterweight or a spring, can be used to maintain the flaps  131  in the closed position. The biasing means should provide a biasing force strong enough to maintain the flaps  131  in the normally closed position against the force of gravity, but less than the force of the cooling gas on the flap when cooling gas is being injected. In this way, the flap  131  can be maintained in the closed position during heat treatment and be readily pivoted to the open position when cooling gas is injected through the nozzle  50 . 
   The nozzle  50  and the flap  131  are preferably formed from a refractory material such as molybdenum or graphite. They may also be formed of a ceramic material if desired. In the embodiment shown, the forward portion  121  is rectangular in cross section and the rear portion  125  is circular in cross section. However, the shapes of the forward and rear portions of nozzle  50  are not critical. Preferably, the forward portion  121  has a larger cross-sectional area than the rear portion  123  so that the forward portion  121  will press against the thermal insulation  31  to help keep it in place during operation of the heat treating furnace. Similarly, the shapes of the first and second central openings  123  and  127  are not critical. The first central opening  123  is preferably square or rectangular for ease of fabrication and the second central opening  127  is preferably circular for ease of adaptation with the opening in the side wall  30 . 
   The side wall  30  has a structure that allows uniform application and removal of cooling gas along the length of the workpiece. The cross section of the side wall  30  may have any of a variety of shapes, including circular, square, rectangular, polygonal, or other cross sectional shape. In the preferred embodiment, the side wall  30  is cylindrical, as shown in FIG.  2 . The nozzles  50  are arranged around the cylindrical wall to inject cooling gas radially inwardly onto the workpiece from a plurality of locations around the workpiece. One or more slots  36  extend along the side wall  30  and connect the hot zone  32  to the plenum  42 . The slots  36  may have any shape and dimension to provide a passage for removing heat uniformly along the length of the hot zone  32  and workpiece. In addition, the side wall  30  may have several slots formed therein. As shown in  FIGS. 1 and 2 , the side wall  30  has four linear slots  36  offset from each other at about 90° intervals around the circumference of the wall. The slots  36  extend substantially the length of the side wall  30  so that injected cooling gas can form a gas stream that exits through the slots along the length of the hot zone  32  in a uniform manner. 
   The slots  36  cooperate with means for limiting the escape of heat from the hot zone during a heating cycle. The slots  36  may be covered by actuated bungs that are operable in an open condition to allow cooling gas to discharge from the hot zone during a cooling cycle, and in a closed position to minimize the escape of heat from the hot zone by convection during a heating cycle. In the preferred embodiment, the slots are covered by a plurality of baffles  38  that are radially aligned with the longitudinal slots  36  and spaced therefrom. The baffles  38  are formed of a thermal insulating material and dimensioned to substantially cover the slots  36 . In this way, the baffles  38  minimize the escape of heat from the hot zone  32  by convection during a heating cycle. The baffles  38  are stationary with no actuated components or moving parts. As a result, the baffles are less susceptible to the types of damage and wear that occur when actuated parts are repeatedly exposed to heat from the hot zone. 
   The baffles  38  may be positioned radially inwardly from the slots  36  into the hot zone  32 , as shown in FIG.  2 . Alternatively, the baffles  38  may be installed radially outwardly from the slots  36  in the plenum  42 . In either case, the baffles  38  form gaps  41  between the edges of the baffles and the hot zone side wall. The gaps  41  provide passages between the hot zone  32  and plenum  42  to permit cooling gas to exit the hot zone during cooling gas injection. Any of a variety of connectors may be used to support the baffles  38  in the hot zone or plenum. In  FIG. 2 , the baffles  38  are supported by a pair of rods  39  mounted to the inside of the side wall  30 . The rods  39  are preferably formed of a high strength, high temperature material, such as carbon/carbon or molybdenum. During a cooling cycle, the cooling gas flows around the baffles  38  and rods  39  and exits through the slots  36  into the plenum  42 . 
   Referring back to  FIG. 1 , the cooling gas injection system will be described in more detail. Cooling gas is conveyed in a closed loop system that supplies forced cooling gas into the hot zone  32  and removes heated gas from the hot zone. The cooling gas is recirculated through the annular duct  43 , hot zone  32 , and plenum  42  by a blower unit  60  mounted between the double outer wall  22  and the heat shielded enclosure  40 . The blower unit  60  has a housing  62  that adjoins one end of the heat shielded enclosure  40 . A blower fan  66  is mounted in the blower unit  60  and has a suction end  72  and a discharge end  73 . The blower fan  66  has a plurality of fan blades mounted on a drive shaft  68 . The drive shaft  68  is connected to and driven by a motor  67 . In the preferred embodiment, the motor  67  is mounted outside the double outer wall  22  of the furnace, and the shaft  68  extends through the double outer wall. In this way, the motor  67  is readily accessible for repairs on the outside of the furnace  20 . In addition, the motor  67  is not subjected to the extreme heat generated inside the hot zone  32 . The blower fan  66  is operable to force cooling gas through the duct  43  and into the nozzles  50  with sufficient pressure to inject the gas past the flaps  131  and into the hot zone  32 . The direction of cooling gas flowing through the duct  43  is shown by the arrows “A” in FIG.  1 . The gas enters the hot zone through the cylindrical hot zone wall  30  and contacts the workpiece from about 360° around the workpiece. In this way, the cooling gas contacts the workpiece evenly on all sides. Cooling gas flows across the surface of the workpiece and absorbs heat from the workpiece. 
   The blower unit  60  is connected in communication with the plenum  42  and is operable to draw the heated gas from the hot zone  32  and into the plenum  42 . The direction of cooling gas flowing through the plenum  42  is shown by arrows marked “B” in FIG.  1 . The plenum  42  and housing  62  of the blower unit  60  are connected by exit ports or openings  46  in an end wall of the heat shielded enclosure  40 . When the blower fan  66  operates, it creates a suction draft in the housing  62  and plenum  42 . The suction in the plenum  42  draws heated cooling gas out of the hot zone  32  and through the longitudinal slots  36 . 
   Referring now to  FIGS. 1 and 3 , the heat shielded enclosure  40  preferably has four exit ports  46 . For clarity, only two exit ports  46  are shown in FIG.  1 . The exit ports  46  are generally positioned in axial alignment with the four longitudinal slots  36  on the hot zone wall  30 . Each exit port  46  forms a passage that permits heated cooling gas to be drawn from the plenum  42  into the blower housing  62 . Cooling gas that enters the blower housing  62  is drawn toward the suction end  72  of the blower fan  66 . 
   As shown in  FIG. 1 , the blower unit  60  includes one or more heat exchangers  64  located in proximity to the suction end  72  of the blower fan  66 . The heat exchangers  64  each contain a heat transfer surface, such as tubing coils, that contacts the stream of heated cooling gas as the gas is pulled toward the suction end  72  of the blower fan  66 . The heat transfer surface removes heat from the cooling gas to lower the temperature of the gas. After the temperature of the cooling gas is lowered, the blower unit  60  recycles the cooling gas back to the hot zone  32 . Any of a variety of liquid coolants or refrigerants can be circulated through the tubing coils to act as a heat sink. The blower unit  60  has a manifold  63  with two or more inlets adapted to receive the heated cooling gas. For clarity, the manifold  63  in  FIG. 1  is shown with two inlets. The manifold  63  has an outlet in proximity to the suction end  72  of the blower fan  66 . As such, the suction end  72  of blower fan  66  is operable to draw the cooling gas from the blower housing  62  into the inlets of manifold  63 , as shown by the arrows marked “C”, and through the heat exchanger  64 . The cooled gas is then drawn out of the manifold  63  and into the suction end  72  of blower fan  66 . The blower fan  66  discharges the cooling gas through the discharge end  73  of the fan. The discharge end  73  of the blower fan  66  is positioned in the duct  43  such that cooling gas is forced out of the fan and into the duct, as shown by the arrows marked “A”. The blower fan provides a back pressure or draft in the duct  43  to force cooling gas through the duct and into the nozzles  50 . The back pressure is sufficient to open the flaps  131  in the nozzles  50  so that the gas can be injected into the hot zone  32 . 
   As stated earlier, the duct  43  conveys forced cooling gas to the hot zone  32 , and the plenum  42  directs heated cooling gas from the hot zone to the suction side the blower unit  60 . In addition, the duct  43  is preferably sealed from the plenum  42  and blower housing  62  to prevent leaking of forced cooling gas from the duct into the return flow. The wall of the blower housing  62  has a flared edge  65  that fits around the wall of the heat shielded enclosure  40 . The edge of housing  62  and the edge of enclosure  40  form an annular recess that is filled by a ring shaped seal  74  to prevent cooling gas from leaking from the duct  43  into the housing  62 . The seal  74  is preferably formed of a heat resistant material, such as aluminum oxide or other technical ceramic material. 
   The furnace  20  has a directional cooling feature that permits the cooling gas stream to be manipulated in a variety of flow patterns to cool a workpiece in a selected manner. The flow pattern of the cooling gas in the hot zone is manipulated by controlling the amount of suction present at each longitudinal slot  36 . By controlling the amount of suction at each longitudinal slot  36 , the cooling gas stream is directed toward some of the slots and converges toward specific areas of the workpiece in the hot zone  32 . The exit ports  46  are configured to be fully opened, fully closed, or partially open. Allocation of the suction is regulated by controlling the extent to which each exit port is open or closed. By closing an exit port completely, the suction generated by the blower fan  66  through that exit port is cut off. This provides more suction at the slots located in proximity to other ports that are open. 
   The exit ports  46  may be operated with any of a variety of mechanisms in a wide range of configurations. As shown in  FIGS. 1 and 3 , each exit port  46  is circular and has an associated damper assembly  80 . Each damper assembly  80  has a circular frame  81  that is aligned with an exit port  46 . The frames  81  extend from the wall of the blower housing  62 , and into the housing. A disk shaped damper  82  is rotatably mounted inside each frame  81  and has a diameter generally equal to the diameter of the frame  81 . The dampers  82  are mounted on shafts  83  that extend through the side of the frames  81 . The shafts  83  are rotatable to pivot the dampers  82  inside the frames  81 . As shown in  FIG. 3 , the rotation of each damper disk  82  is illustrated by the arrows marked “D”. Each damper  82  is pivotable to a fully open position, a fully closed position, and an infinite number of positions in between the fully open and fully closed positions. In the fully open position, the circumference of the damper  82  is oriented in a plane essentially parallel to the longitudinal axis of the frame  81 . As such, the exit port  46  is virtually unobstructed by the damper  82 , allowing a maximum flow of cooling gas through the exit port  46 . In the fully closed position, the circumference of the damper  82  is oriented in a plane essentially normal to the longitudinal axis of the frame  81 . In this position, the exit port  46  is substantially closed to gas flow by the damper  82 . 
   Each shaft  83  is operatively connected to and rotatable by an actuator  86 . Any of a variety of actuators  86  may be used, including electric actuators or pneumatic actuators. The actuators  86  are located on the outside of the double outer wall  22 . In this way, the actuators  86  are not subjected to the intense heat generated by the heating elements in the furnace  20 . The actuators  86  are connected to their respective shafts  83  by linkages  88  that extend through the housing wall of the blower unit  60 . The linkages  88  are preferably formed of a flexible material that allows the linkages to deflect as the walls of the housing  62  shift under thermal expansion and contraction. The damper assemblies  80  are independently operable and controlled by a central processor (not shown). Each actuator  86  is controlled by a signal positioner  84  that responds to electrical signals from the processor. The signal positioners  84  and actuators  86  convert signals from the processor into mechanical rotation of the shaft  83  to adjust the position of the dampers  82 . The processor is operable to precisely control the angular position of the dampers  82  and adjust the dampers to create a desired flow pattern of cooling gas in the hot zone. 
   Operation of the directional cooling system in the furnace  20  will now be described in more detail. The dampers  82  are operable to adjust the direction of cooling gas flow in the hot zone, as stated earlier. For example, one damper  82  may be open while the other dampers are closed to concentrate the cooling gas stream at one side of the hot zone  32 . The dampers  82  are also operable through modulation to adjust the magnitude of flow through each exit slot  36  in the hot zone side wall  30 . For example, some dampers  82  may be pivoted to the fully open position while others are modulated at an angle between the fully open position and fully closed position to partially obstruct the flow of cooling gas through the corresponding exit port  46 . The furnace  20  may be operated with an infinite number of damper settings to provide an appropriate cooling gas stream for a particular workpiece shape. 
   Referring now to  FIGS. 3 and 8 , one of the operating modes of the directional cooling system will be described. The furnace  20  has four dampers,  82 A,  82 B,  82 C and  82 D, which are disposed adjacent to exit ports  46 A,  46 B,  46 C, and  46 D, respectively. The exit ports  46 A,  46 B,  46 C, and  46 D are generally aligned with longitudinal slots  36 A,  36 B,  36 C and  36 D, respectively. The flow pattern of the cooling gas is illustrated when damper  82 A is in an open position and dampers  82 B- 82 D are in their closed positions. In this operating mode, the suction generated by the blower fan  66  is concentrated through the exit port  46 A. Since longitudinal slot  36 A is located closest to that exit port the suction generated by blower  60  is concentrated substantially entirely at slot  36 A. Therefore, the heated cooling gas in hot zone  32  is drawn preferentially to slot  36 A. The cooling gas converges around the side of the workpiece nearest slot  36 A and exits through slot  36 A into the plenum  42 . It will be readily apparent that the cooling gas can be conducted to any of the slots  36 A,  36 B,  36 C, or  36 D in the hot zone side wall  30  by opening the corresponding damper that is nearest to that slot and keeping the other dampers closed. 
   Referring now to  FIG. 9 , there is shown a second operating mode of the directional cooling system. In this mode the diametrically opposite dampers  82 A and  82 C are open. With this configuration, the suction generated by the blower  60  is divided between the exit ports  46 A and  46 C. Since longitudinal slots  36 A,  36 C are generally aligned with exit ports  46 A and  46 C, respectively, the suction draft is concentrated at slots  36 A and  36 C. The resulting gas flow in the hot zone is illustrated in FIG.  9 . In this operating mode, the cooling gas is drawn preferentially around two sides of a workpiece to form a flow pattern that provides more uniform cooling around the geometry of the workpiece. 
   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 shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.