Cooling System For Vacuum Furnaces

Abstract

A vacuum furnace construction of a type particularly suitable for brazing, heat treating, sintering, annealing and other thermal processing of metal components involving a heating thereof to temperatures ranging up to about 2,300.degree.F. The furnace incorporates a cooling system to effect a rapid cooling of the workpieces at the conclusion of the thermal process without exposing the components to oxidation attack at the high temperatures prevalent in the vacuum furnace during the cooling cycle. The invention further contemplates an improved heat exchanger construction through which an inert cooling gas is adapted to be circulated for reuse in the cooling system of the vacuum furnace.

Description

BACKGROUND OF THE INVENTION

The use of vacuum-type furnaces for the thermal processing of workpieces is receiving widespread acceptance because of the protection from oxidation attack afforded to such workpieces at the elevated temperatures employed. While the vacuum furnace of the present invention is particularly applicable for use in brazing, significant advantages can also be obtained in the utilization of such furnaces in various types of heat treating, sintering, annealing and similar thermal processes frequently requiring temperatures that are as high as about 2,300.degree.F at which metal components are extremely susceptible to chemical reaction and oxidation attack.

A continuing problem associated with high temperature vacuum furnaces has been the long length of time required to cool the workpieces at the completion of a thermal processing cycle in order to enable the furnace to be opened and the workpieces to be removed therefrom. The elaborate radiant heat insulation required in furnaces of this type in addition to the construction necessary to enable the attainment of high vacuums has at least in part been responsible for rendering cooling systems as heretofore proposed either unsuccessful from an operational standpoint or impractical from a commercial standpoint. A further complication is the necessity of protecting the metal work components from oxidation attack during the cooling cycle at least until reasonably moderate temperature levels are attained at which they are less prone to oxidation attack. Among the various solutions heretofore proposed are the circulation of an inert gas through the furnace which has been found inadequate to provide the requisite cooling capacity as well as the direct introduction of liquid nitrogen into the work zone. The latter technique, although effective insofar as cooling is concerned, is very expensive and can only be employed on parts that are to be quenched at an oil quench rate or faster.

The cooling system in accordance with the vacuum furnace construction of the present invention provides for a simple and economical solution to the cooling problem of thermally processed parts whereby a significant reduction in the process cycle is achieved thereby substantially increasing the efficient use of the furnace and the productive output thereof.

SUMMARY OF THE INVENTION

The benefits and advantages of the present invention are achieved by a vacuum furnace which consists of a structural enclosure defining an internal three-dimensional chamber within which a thermal insulating liner is placed and is spaced inwardly of the inner surfaces of the enclosure defining an outer zone and an inner work zone in which the workpieces are adapted to be disposed during a thermal processing cycle. The work zone further incorporates a support on which the workpieces are adapted to be placed and the inner surface of the insulating liner is provided with a plurality of heating elements for heating the work zone as well as the workpieces therein. The outer zone is divided by means of a baffle into an inlet zone connected to an inlet port through which a cooling gas such as argon, for example, is adapted to enter and an outlet zone formed with an outlet port for discharging the cooling gas from the furnace.

The insulating liner is formed with an annular nozzle arrangement disposed in communication with the inlet zone for receiving and discharging the cooling gas in the form of a high velocity stream and in impinging and cooling relationship with respect to the workpieces in the work zone to effect a relatively rapid cooling thereof at the completion of a thermal processing cycle. The cooling gas preferably travels in a toroidal flow pattern within the work zone and is withdrawn through a discharge opening in the insulating lining positioned at a point remote from the nozzle. The discharge opening is positioned in communication with the outlet zone from which the cool gas is withdrawn and is passed through a heat exchanger for a cooling thereof.

The vacuum furnace further includes a removable closure in a wall of the furnace for gaining access to the interior of the furnace and a removable section in the heat insulating liner for gaining access to the work zone and work positioned on the work support therein. Preferably, the nozzle arrangement is embodied in the removable section of the insulating lining and is disposed adjacent to the removable closure in the furnace to facilitate a loading and unloading of workpieces from the work zone. The vacuum furnace also includes a system including control means incorporating sensing means therein for sensing the temperature and pressure existing within the vacuum furnace and for controlling the quantity and rate of circulation of the inner cooling fluid through the furnace.

The present invention further contemplates an improved heat exchanger construction including a pair of spaced headers formed with an inlet port and an outlet port, respectively, for receiving the inert cooling gas to be cooled. The headers are interconnected by a plurality of conduits and a corresponding plurality of heat exchanger tubes extend through the conduits and through the headers at the ends thereof and project outwardly of the headers such that the ends of the tubes can be connected to a supply of a heat exchange fluid. In this manner, no connections are present along the lengths of the heat exchange tubes which are disposed within the evacuated interior of the heat exchanger so that any leaks that may occur therein can be located and repaired without removing the heat exchange fluid and without contaminating the atmosphere within the vacuum furnace by heat exchange fluid.

Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, partly schematic, illustrating a vacuum furnace system and ancillary equipment and controls arranged in accordance with a preferred practice of the present invention;

FIG. 2 is a magnified side elevational view, partly in section, of a vacuum furnace and the cooling system incorporated therein;

FIG. 3 is a magnified fragmentary sectional view of the furnace closure seated against the upper wall of the vacuum furnace as encompassed by the dotted circle indicated at 3 in FIG. 2;

FIG. 4 is a schematic side elevational view of the relative disposition of the controls, heat exchanger and vacuum pump of a vacuum furnace system as shown in FIG. 1;

FIG. 5 is a plan view of a heat exchanger;

FIG. 6 is an end elevational view of the heat exchanger shown in FIG. 5; and

FIG. 7 is a magnified fragmentary sectional view of the header portion and heat transfer tubes disposed in the transverse conduits of the heat exchanger shown in FIG. 6 as viewed substantially along the line 7--7 thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings and as may be best seen in FIG. 1 a vacuum furnace system of a general type to which the present invention is applicable includes a vacuum furnace 10 connected by means of conduits 12 and 14 to a pumping and cooling system 16 which is controlled by a control system housed in a control panel 18. The control system incorporates appropriate temperature and pressure sensing devices (not shown) for monitoring the conditions prevalent in the interior of the vacuum furnace. The vacuum furnace 10 as best seen in FIGS. 2 and 3 consists of a generally circular cylindrical receptacle or tank 20 having a dish shaped bottom 22 to which a series of upright legs 24 are securely affixed. The upper portion of the tank 20 is provided with a generally flat top panel 26 which is securely fastened by means of screws to an annular flange 28 encircling the periphery of the upper edge of the tank. The top panel 26 is formed with a circular opening 30 in the center thereof over which a dome shaped closure member 32 is removably positioned in sealing engagement thereagainst.

As best seen in FIG. 3, the lower edge of the closure member 32 is provided with an annular flange 34 which is provided with a groove 36 in the lower face thereof in which a resilient O-ring type seal 38 of a heat-resistant elastomeric material, such as a silicone rubber, is removably disposed. The face of the O-ring seal 38 is adapted to be seated in sealing engagement against the upper surface of the top panel 26 adjacent to the opening 30.

In a manner similar to that shown in FIG. 3, an O-ring seal 40 is adapted to be received in the face of the groove of the annular flange 28 for sealing the peripheral portion of the top panel 26 to the tank 20.

A radiant heat insulating liner 42, including a generally circular cylindrical sheet metal side wall 44 connected at the upper and lower edges thereof to annular radially inwardly extending end walls 46, is mounted in spaced substantially concentric relationship with respect to the inner surface of the wall of the tank and is supported in spaced relationship from a bottom thereof by means of supporting legs 48. The lower wall of the insulating liner is formed with an outlet assembly comprising a generally dish-shaped member 50 formed with an axially downwardly extending discharge tube 52 and an inverted conically-shaped deflection plate 54 supported on spacers 56 which is positioned in spaced central alignment above the discharge tube 52.

The upper end wall of the insulating liner is provided with a nozzle assembly including a lower dish-shaped member 58 of a generally circular conical configuration which is formed with a discharge opening 60 in the center thereof. A conical deflector plate 62 is supported by spacers 64 in spaced substantially parallel relationship above the upper surface of the dish-shaped member 58 and defines an annular downwardly convergent nozzle opening 66 which communicates with the discharge opening 60.

As best seen in FIG. 2, the insulating liner, including the nozzle assembly and the discharge assembly, divide the interior of the vacuum furnace into an outer zone and an inner work zone. The inner work zone is provided with a table or support 68 projecting upwardly through the discharge assembly on which workpieces which are to be thermally processed are adapted to be stacked. A plurality of electrical resistance heating elements 70, which preferably comprise graphite tape elements, are supported between upper and lower terminals 72, 74 which extend through insulators 76 mounted in correspondingly-shaped apertures formed in the side wall 44 of the insulating liner. The heating elements 70 are arranged in vertically spaced relationship around the inner surface of the insulating liner and around the workpieces adapted to be placed on the upper surface of the work support 68. The ends of the terminals 72 are connected to a common annular bus bar 78, while the ends of the terminals 74 are connected in common to an annular bus bar 80. Each bus bar is in turn connected to a feed-through terminal extending in sealed relationship through the wall of the tank such as the feed-through device indicated at 82 in FIG. 2, which is connected to the bus bar 80.

The interior of the insulating liner is covered with a suitable heat-resistant insulating layer 84 which may be comprised of any one of a variety of suitable commercially available materials including carbon felt and ceramic fiber blankets. The inner surfaces of the nozzle assembly and a discharge assembly of the insulating liner are similarly covered with the insulating layer.

In accordance with the arrangement shown in FIG. 2, the angularity of the openings defined by the nozzle opening 66 and the discharge opening in the discharge assembly are oriented in relationship to the disposition of the insulating liner and insulating layer thereon so as to intercept any heat rays radiating or reflecting from the surfaces within the interior of the work zone including the radiant heating elements 70. Accordingly, the outer zone is maintained at a substantially lower temperature than the work zone and this is further controlled by means of cooling channels 86 which are brazed or welded to the exterior surfaces of the vacuum furnace including the side walls of the tank 20, the dome-shaped closure 32, the top panel 26 and the dish-shaped bottom 22. A suitable cooling fluid, such as water for example, is circulated through the channels via a suitable header and flow control valves (not shown) connected thereto to facilitate a cooling of the vacuum furnace at the completion of a thermal processing cycle and to maintain the shell of the tank within an acceptable temperature range throughout the cycle.

The outer zone between the insulating liner and the inner wall of the tank is divided by an annular baffle 88, as best seen in FIG. 2, into an upper or inlet zone and a lower or outlet zone. The upper zone is provided with a flanged inlet port 90, to which the conduit 12 is connected for supplying a cooling fluid such as substantially dry argon to the upper portion of the furnace for discharge in cooling relationship through the nozzle opening 66. The lower zone is connected to a flanged outlet port 92, to which the conduit 14 is connected for removing the inert heat-exchanged gas from the lower zone as it is discharged from the work zone through the discharge assembly. The conduit 14 is also effective for removing air and other gaseous products from the furnace interior in order to attain the desired vacuum during the thermal processing cycle.

Referring now to FIG. 4, the conduit 14, as schematically shown, is connected to the inlet side of a blower, such as a Rootes blower 94, which functions both to evacuate the interior of the vacuum furnace 10, as well as to circulate the inert cooling gas through the furnace and heat exchange equipment. During a vacuum thermal processing cycle, the blower 94 is supplemented by a vacuum pump 96 which is connected to the outlet side of the blower 94 through remotely-actuated valve 98 which is controlled by the control system within the control panel 18.

At the conclusion of a thermal processing cycle, and at the initiation of a cooling cycle, the valve 98 is closed and inert gas is admitted into the interior of the furnace from a supply tank indicated at 100 through a remotely-actuated valve 102, which is connected by means of a supply line 104 to the lower outer zone of the furnace. The inert gas is withdrawn from the interior of the furnace through the conduit 14 by the blower 94 and is circulated through a heat exchanger 106 to effect a cooling thereof, whereafter it passes through a remotely-actuatable control valve 108 back into the upper outer zone of the furnace for discharge through the nozzle assembly against the workpieces disposed within the work zone.

In order to provide a uniform rate of cooling of the workpieces and to avoid an overload of the heat exchanger, the control system through the flow control valve 102 effects a controlled addition of inert gas to the vacuum furnace in response to the temperature of the inert gas discharged from the outlet side of the blower 94 as sensed by a temperature sensing device 110 connected to the control panel 118. For example, at the conclusion of a vacuum brazing operation of workpieces at a temperature of about 1,400.degree.F, electric power to the heating elements is discontinued and the interior of the furnace immediately commences to cool through normal radiation and convection approaching a level of about 1,000.degree.F, at which time the inert cooling gas cooling cycle is initiated. The inert or other nonoxidizing gas is introduced through the supply line 104 into the furnace until a pressure of about 0.2 atmospheres absolute is attained. As the workpieces within the furnace progressively cool, the quantity of heat removed therefrom by the cooling gas decreases, which is reflected in a reduction in the temperature of the gas as sensed by the sensing device 110. As the temperature of the gas decreases, the control system causes an actuation of the flow control valve 102, causing additional gas to be admitted into the interior of the furnace, causing an increase in the pressure thereof so as to increase the density of the cooling gas and the quantity of heat extracted thereby. Accordingly, by monitoring the temperature and pressure of the cooling gas being circulated through the vacuum furnace, a substantially uniform rate of heat removal is provided, which in turn imposes a substantially uniform load on the heat exchanger 106.

It will be understood that alternative satisfactory arrangements and controls for the ancillary equipment can be employed in combination with the vacuum furnace construction as shown in FIG. 2 in order to obtain the increased cooling rate of the workpieces at the completion of a thermal processing cycle. It will also be understood that the chemical reactivity of the cooling gas as established by its composition and nonoxidizing properties can be varied consistent with the nature of the parts being processed and their susceptibility to chemical attack and deterioration at the elevated temperatures encountered during the course of the cooling cycle.

Referring now to FIGS. 5-7, a gas-to-liquid heat exchanger is shown which is constructed in accordance with another embodiment of the present invention to provide for a rapid cooling of the cooling gas, while at the same time minimizing the likelihood of a contamination of the vacuum furnace atmosphere as a result of leakage of the heat exchange liquid utilized for cooling the gas. As shown, the heat exchanger comprises a first header 112 having an inlet port 114 at one end thereof and being closed by a cap 116 at the opposite end thereof. A second header 118 is disposed in spaced substantially parallel relationship with respect to the first header and is formed with an outlet port 120 at an end opposite to the inlet port in the first header and is closed at its opposite end by means of a cap 122. A plurality of conduits 124 extend in spaced transverse relationship relative to the first and second headers and the end portions thereof extend through the header walls and project into the interior of the headers. The end portions of the conduits are secured in gas-tight sealed relationship in the headers such as by means of brazing or welding.

A heat transfer tube 126 extends axially through each conduit 122 and the end portions thereof project through the outer walls of the headers and are sealingly fastened therein such as by means of welding. The central portion of the heat transfer tubes 126 is formed with a plurality of radially extending elements or bristles 128 secured in heat exchange contact at their inner ends to the outer surface of the tube. The bristles 128 serve to substantially increase the effective heat transfer area of the heat transfer tubes and are adapted to be disposed in heat transfer relationship with the gas passing through the transverse conduits from the first header to the second header. An extraction of heat from the bristles through the wall of the transfer tubes is accomplished by a cooling liquid such as water which enters an inlet elbow 130 at the closed end of the first header 112 as shown in FIG. 5 and is discharged at the opposite end of the second header 118 through a discharge elbow 132. The cooling liquid travels in a serpentine manner from the inlet elbow to the discharge elbow by means of the U-shaped connectors 134 secured to the projecting end portions of the heat transfer tubes connecting adjacent pairs to each other. In accordance with this arrangement the integral nature of the heat transfer tubes disposed within the headers and transverse conduits minimizes any possibility of a leakage of liquid into the evacuated interior of the heat exchanger and a contamination of the atmosphere of the vacuum furnace. Any leakage that may occur most likely will occur at the connections between the ends of the tubes and the U-shaped connectors 134 which is exterior of the heat exchanger and can be readily observed and repaired as may be required from time to time.

It will also be appreciated that alternative satisfactory heat transfer tubes can be employed in lieu of the bristle-type tube 126 shown in FIG. 7. In the embodiment illustrated, the bristles or heat transfer elements are integrally formed on the surface of the tubing by means of a mechanical cutting action leaving the cut chips attached at their inner ends to the tube surface, assuring excellent heat transfer between the bristles and tube walls. Alternative satisfactory heat transfer tubes are commercially available which can be satisfactorily employed to achieve benefits similar to those obtained by the tube 126.

While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages as hereinabove set forth, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

Claims

What is claimed is:

1. A vacuum furnace comprising a structural enclosure defining a three dimensional chamber, an insulating liner in said enclosure and disposed in spaced relationship from the inner surfaces thereof dividing said chamber into an outer zone and an inner work zone, a support in said work zone for supporting work to be thermally processed, heating means for heating said work zone, removable closure means sealingly disposed on said enclosure for gaining access to said chamber, an inlet port in said enclosure for introducing a cooling fluid into said outer zone, an outlet port in said enclosure for withdrawing the cooling fluid from said outer zone, a baffle disposed between the interior surface of said enclosure and the exterior surface of said insulating liner for dividing said outer zone into an inlet zone disposed in communication with said inlet port and an outlet zone disposed in communication with said outlet port, said insulating liner including a removable section for gaining access to said work zone, nozzle means in said liner for directing cooling fluid from said inlet zone into said work zone and in cooling relationship with respect to any work on said support therein, discharge means in said liner for discharging cooling fluid from said work zone into said outlet zone, cooling means interposed between said inlet port and said outlet port exteriorly of said enclosure for cooling said cooling fluid withdrawn from said work zone, and including means for circulating said cooling fluid through said inlet zone, through said work zone, through said outlet zone and through said cooling means.

2. The vacuum furnace as defined in claim 1 further including cooling means disposed in heat transfer relationship with the exterior surface of said enclosure.

3. The vacuum furnace as defined in claim 1 in which said nozzle means in said liner is disposed in a position remote from said discharge means therein.

4. The vacuum furnace as defined in claim 1 wherein said insulating liner comprises sheet steel having a graphite heat insulating wool over the inner surfaces thereof.

5. The vacuum furnace as defined in claim 1 in which said nozzle means comprises a dish shaped member formed with an aperture in the center thereof disposed in communication with said work zone and a conically shaped member disposed in spaced relationship from said dish shaped member and defining therebetween an annular nozzle passageway disposed in communication with said outer zone and the cooling fluid therein, said dish shaped member and said conical member oriented so as to intercept any radiant heat waves reflected through the aperture of said nozzle means.

6. The vacuum furnace as defined in claim 1 in which said nozzle means are oriented so as to introduce the cooling fluid into said work zone in the form of a high velocity stream directed toward any work on said support therein and to cause a toroidal flow pattern of cooling fluid within said work zone.

7. The vacuum furnace as defined in claim 1 in which said work zone is of a generally cylindrical configuration formed with said nozzle means in one end thereof and said discharge means in the opposite end thereof and wherein said heating means extend longitudinally around the inner side of said work zone encircling said support and any work thereon.

8. The vacuum furnace as defined in claim 1 in which said removable section of said liner incorporates said nozzle means therein and is disposed adjacent to said removable closure means in said enclosure.