Patent Publication Number: US-8992213-B2

Title: Sealing mechanism for a vacuum heat treating furnace

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/149,507 filed Feb. 3, 2009, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to vacuum heat treating furnaces, and in particular, to a sealing mechanism for a cooling fan drive shaft that penetrates the wall of a vacuum heat treating furnace. 
     2. Description of the Related Art 
     Many of the known vacuum heat treating furnaces have an internal gas quenching system. The gas quenching system includes an internal fan for circulating an inert cooling gas over the heated metal parts and through an internal heat exchanger. Commercially available embodiments of such furnaces also have an internally mounted electric motor for driving the gas circulation fan. An example of such a furnace is that sold under the registered trademark “TURBO TREATER” by Ipsen Inc., the assignee of the present application. 
     The interior of a vacuum heat treating furnace is subject to extreme temperature and pressure conditions. Depending on the type of material being heat treated, the interior of the furnace can reach a temperature of up to 3000° F. (1650° C.), be evacuated to a vacuum of down to about 10 −5  torr, and be backfilled with inert gas up to a pressure of up to about 12 bar (1.2 MPa). Under such operating conditions, the useful life of most electric motors is severely curtailed resulting in costly maintenance, repair, or replacement, and furnace downtime. Although the construction of the electric motors used in the known vacuum heat treating furnaces has been modified in various ways to overcome the problems associated with the extreme conditions encountered in such furnaces, none of the modifications have proven entirely satisfactory. The design modifications that work best are also the most expensive to implement. Lower cost modifications have not provided a reliable solution to the problem. 
     A desirable alternative to locating the fan drive motor inside the furnace vessel is to locate the motor outside the furnace where it is not subject to the temperature and pressure extremes encountered inside the furnace vessel. However, in order to locate the fan drive motor outside the furnace vessel, it is necessary to provide a seal where the drive shaft penetrates the furnace wall. The problem is to effectively provide a vacuum-tight seal for a vacuum as low as about 10 −5  torr, as well as to provide a gas-tight seal that is capable of sealing against a fluid pressure of up to 12 bar (1.2 MPa) or higher. 
     One solution to the foregoing problem is described in U.S. Pat. No. 5,709,544, the entire disclosure of which is incorporated herein by reference. The &#39;544 patent describes a dual seal arrangement that includes an inflatable seal and a lip seal that surround the fan drive shaft where the shaft passes through the furnace wall. The inflatable seal provides a vacuum-tight seal around the drive shaft when inflated. The lip seal provides a gas-tight seal around the drive shaft when the vacuum furnace is pressurized with a cooling fluid and the fan is being rotated. The dual-seal described in the &#39;544 patent has proved effective. However, the lip-type gas seal is a contacting seal and thus, is subject to wear when the drive shaft rotates in operation. In order to avoid premature wearing of the lip seal, some users have limited the rotational speed of the drive shaft. Although the shaft speed reduction benefits the service life of the lip seal, it adversely affects the cooling efficiency of the fan. Another drawback of the lip seal is that the higher the cooling gas pressure used, the greater the force on the lip seal against the drive shaft. The higher sealing force increases the wear rate of the lip seal. Therefore, it has also been necessary to limit the pressure of the cooling gas in order to avoid premature wearing of the lip seal. Although the use of reduced gas pressure benefits the service life of the lip seal, it adversely affects the efficiency of cooling a work load in the furnace. 
     In addition to the foregoing drawbacks, the dual seal described in the &#39;544 patent includes numerous components which are installed and assembled in place. Maintenance of the seals required disassembling and then re-assembling the seals and the hardware that supports them in the vacuum furnace. Consequently, when it is necessary to perform maintenance on the seals, the furnace has to be shut down for an extended period of time. Extended shut-down periods are highly undesirable in production manufacturing facilities. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided a vacuum heat treating furnace that includes a pressure vessel having a wall that defines a chamber, a fan disposed inside the chamber for circulating a cooling gas therein, a motor disposed externally to the pressure vessel, and a drive shaft operatively connected to the fan and the motor through an opening in the wall of the pressure vessel. The vacuum furnace of the present invention further includes a dual seal mechanism disposed around the drive shaft adjacent the opening in the pressure vessel wall. The dual seal mechanism includes an inflatable first seal surrounding the drive shaft for providing a vacuum-tight seal around said drive shaft when inflated. The dual seal mechanism also includes a second seal surrounding the drive shaft adjacent to the inflatable first seal. The second seal has an inside diameter that is dimensioned such that a gap is present between the second seal and the drive shaft. The dual seal mechanism further includes a channel disposed adjacent to the second seal for conducting a purging fluid to the gap between the drive shaft and the second seal. A means for injecting the purging fluid into the gap is operably connected to the channel. 
     In accordance with a second aspect of the present invention, there is provided an apparatus for sealing a fan drive shaft in a heat treating furnace. The sealing apparatus includes a housing having an annular body and a central opening. An inflatable first seal surrounds the central opening of the annular body. A second seal surrounds the central opening and is adjacent to the inflatable first seal. The sealing apparatus also includes a channel formed in the annular body adjacent to the second seal for conducting a purging fluid into the central opening. 
     In accordance with a further aspect of the present invention, there is provided a fan drive system for a vacuum heat treating furnace. The fan drive system according to this aspect of the invention includes an electric motor adapted to be disposed externally to the vacuum heat treating furnace and a drive shaft adapted to be connected to a fan inside the vacuum furnace and to the motor through an opening in the wall of vacuum furnace. The fan drive system also includes a dual seal mechanism disposed around the drive shaft adjacent to an opening in the pressure vessel wall. The dual seal mechanism includes an inflatable first seal surrounding the drive shaft for providing a vacuum-tight seal around the drive shaft when inflated. The dual seal mechanism also includes a second seal surrounding the drive shaft adjacent to the inflatable first seal. The second seal has an inside diameter that is dimensioned such that a gap is present between the second seal and the drive shaft. The dual seal mechanism further includes a channel disposed adjacent to the second seal for conducting a purging fluid between the drive shaft and the second seal and means connected to the channel for injecting the purging fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description of a preferred embodiment of the present invention will be better understood when read with reference to the accompanying drawings, of which: 
         FIG. 1  is a partial side elevation view in partial section of a vacuum heat treating furnace in accordance with the present invention; 
         FIG. 2  is a detail elevation view in partial section of a dual seal arrangement used in a fan drive system in the vacuum heat treating furnace shown in  FIG. 1 ; 
         FIG. 3  is a side perspective view of the fan drive system shown in  FIG. 2 ; 
         FIG. 4  is a front perspective view of a seal cartridge in accordance with the present invention; 
         FIG. 5  is a front elevation view of the seal cartridge shown in  FIG. 4 ; 
         FIG. 6  is a side elevation view in partial section of the seal cartridge of  FIG. 4  as viewed along line A-A of  FIG. 5 ; 
         FIG. 7  is a second side elevation view in partial section of the seal cartridge of  FIG. 4  as viewed along line C-C of  FIG. 5 ; 
         FIG. 8  is front perspective view of a second embodiment of a seal cartridge in accordance with the present invention; 
         FIG. 9  is a front elevation view of the seal cartridge shown in  FIG. 8 ; 
         FIG. 10  is a side elevation view in partial section of the seal cartridge of  FIG. 8  as viewed along line A-A of  FIG. 9 ; and 
         FIG. 11  is a schematic diagram of a pneumatic system for use with the dual seal arrangement according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, and in particular to  FIGS. 1 and 2 , there is shown a vacuum heat treating furnace  10  in accordance with the present invention. The vacuum heat treating furnace  10  includes a pressure vessel  12  which encloses a chamber  13  wherein metal parts are heat treated. Pressure vessel  12  has a receptacle  14  formed through pressure vessel end wall  15 . The receptacle  14  is generally cylindrical in shape. 
     A forced gas cooling system is provided in the vacuum furnace  10  for directing a cooling gas over metallic work pieces after they are heat treated in the furnace. The cooling gas is an inert gas such as nitrogen, argon, helium, hydrogen or a mixture of at least two of those gases. The gas cooling system includes a gas circulating fan  18  and a fan drive motor  20  which is connected to the fan  18  by a drive shaft  22 . A heat exchanger is positioned in the chamber  13  to remove heat from the cooling gas as it is circulated by the fan. The fan drive motor  20  is mounted and supported outside the pressure vessel  12 . In a vacuum heat treating furnace that operates at very high temperatures, e.g., 2000-3000° F. (1093-1650° C.), the fan dirve motor  20  is preferably mounted at a distance from the pressure vessel  12 . In such an embodiment the fan drive motor  20  is coupled to the drive shaft  22  by means of a mechanical linkage such as a drive belt and sheave arrangement, a chain and sprocket arrangement, or a gear drive arrangement. 
     A support plate  24  is disposed within the receptacle  14  to provide a wall or bulkhead between chamber  13  and the ambient environment outside pressure vessel  12 . Fan drive motor  20  is attached to the support plate  24  by any suitable means. The support plate  24  has an opening  28  through which the drive shaft  22  extends. A dual seal mechanism  30  is disposed in opening  28  where it is affixed to and supported by the support plate  24  around the drive shaft  22  to provide a vacuum-tight seal and a substantially gas-tight seal. As shown in  FIG. 3  a coil of metal tubing  74  is wrapped around the dual seal mechanism  30  on the inboard side of the support plate  24  for conducting a cooling medium such as water. The metal tubing  74  penetrates the support plate  24  through to the outboard side thereof. Vacuum seals  78   a  and  78   b  are provided around the metal tubing  74  where the tubing penetrates through the support plate  24  to provide substantially vacuum-tight seals around the tubing. Connectors  76   a ,  76   b  are affixed to the tubing ends for connection to a source of the coolant. It will be appreciated by those skilled in the art that arrangements of cooling coils other than that shown in  FIG. 3  can be utilized. 
     Referring now to  FIGS. 4 to 7 , the dual seal mechanism  30  is illustrated in greater detail. The dual seal mechanism is preferably constructed as a cartridge containing an inflatable seal and a non-contacting seal. A housing  32  that is attached to support plate  24  by suitable fasteners, has a central opening. A first circumferential recess  36  is formed in housing  32  around the central opening. The recess  36  is dimensioned for receiving an inflatable seal  34 . The inflatable seal  34  is a generally ring-shaped tube preferably formed of fabric reinforced silicone or another gas-impermeable, flexible material which can be inflated. The tube can have any suitable cross section, but is preferably rectangular or oval in cross section. The cross section of the inflatable seal  34  is dimensioned to fit within recess  36  and be clear of the drive shaft  22  when the inflatable seal is deflated. When the inflatable seal  34  is inflated, it expands beyond the limits of recess  36  to press on the circumference of the drive shaft  22  to form a vacuum-tight seal between the drive shaft and the housing  32 . A radial channel  37  is formed in the housing  32  to provide a communication port between the inflatable seal  34  and a source of pressurized gas or other fluid for inflating the inflatable seal  34 . A gas-tight tube  38  (shown in  FIGS. 2 and 3 ) is connected to the radial channel  37  and extends through the support plate  24  to the pressurized gas source. The gas-tight tube  38  also permits the inflatable seal  34  to be connected to a vacuum, if desired, so that it can be deflated sufficiently to be clear of the drive shaft. A suitable type of inflatable seal is one sold under the registered trademark “PNEUMA-SEAL” by the Engineered Products Division of Pawling Corporation, Pawling, N.Y. If desired, a thermal heat shield can be installed over the gas-tight tube  38  on the inboard side of support plate  24 . 
     The dual seal mechanism  30  has a non-contacting seal  40  adjacent to the inflatable seal  34 . The non-contacting seal provides a controlled clearance or gap around the drive shaft  22 . The controlled gap is dimensioned so that the shaft can rotate substantially freely at any angular velocity and with any furnace pressure without causing significant wear of the seal material. There is a small amount of gas leakage from the furnace chamber through the gap to the atmosphere. However, the gas leakage rate is held to an acceptable level by proper selection of the gap distance which is preferably about 0.002-0.005 inch (0.05-0.125 mm). In a preferred embodiment, a packing material that can “wear in” is included around the shaft to narrow or eliminate the gap. The packing material is applied between the shaft surface and the seal cartridge body to provide a smaller gap after some of the packing material is worn away. Preferred materials for such a design include graphite rope packing, GRAPHFOIL rings, TEFLON rings, ceramic fiber rings, or other suitable material. 
     As shown in  FIG. 6 , the non-contacting seal  40  includes a bushing  42 . The bushing  42  is press fit into a second recess in the housing  32  adjacent to recess  36 . The bushing  42  is preferably formed of material that is generally softer than the drive shaft  22 . In a preferred embodiment the bushing  42  is machined from graphite-metal alloy. A commercial form of such a material is sold under the registered trademark GRAPHALLOY. The bushing  42  has a circumferential groove  44  formed around the internal circumference. A plurality of small bore holes  46  are formed in the bushing  42  between the outer surface and terminating in the circumferential groove  44 . The circumferential groove  44  and bore holes  46  are situated on the bushing  42  such they align with a channel  39  that is formed around the inside circumference of the housing  32 . A second radial channel  41  is formed in the housing  32  to provide a communication port between the channel  39  and the purging gas supply tube  45 . With this arrangement, a purging gas can be injected into the gap between bushing  42  and the drive shaft  22  to prevent outside air from being drawn into the furnace chamber when the furnace is transitioned from a subatmospheric pressure to a superatmospheric pressure. 
     In an alternative embodiment, the bushing  42  is made from bronze, another metal, or a metal alloy suitable for use as a bushing material. In the alternative embodiment which is shown in  FIG. 6 , a plurality of additional grooves  48  are formed around the inside circumference of the bushing. The grooves  48  are preferably filled with a packing material such as the graphite rope packing described above. 
     A further embodiment of a seal cartridge in accordance with the invention is shown in  FIGS. 8 to 10 . A seal cartridge  130  includes a housing that is formed from a plurality of rings  132   a ,  132   b ,  132   c ,  132   d , and  132   e . A recess  136  is formed around the inside circumference of ring  132   d . An inflatable seal  134 , as described above, is positioned in the recess  136 . A first radial channel  137  is formed in ring  132   d  to permit the inflatable seal to be connected to the gas-tight tube  38  for inflating and deflating the inflatable seal  134 . A second recess or groove  139  is formed around the inside circumference of ring  132   b  at a location that is displaced longitudinally from the recess  136 . A second radial channel  141  is formed in ring  132   b  to provide a communication port between the channel groove  139  and the purging gas supply tube  45 . With this arrangement, a purging gas can be injected into the gap between the seal  130  and a sleeve  142  that is attached to the fan drive shaft. Additional grooves  148  are formed around the inside surfaces of rings  132   a ,  132   b , and  132   c . Sealing rings  144  are positioned in each of grooves to provide sealing. The sealing rings are preferably made of carbon graphite. 
     The sleeve  142  is fitted over the portion of the drive shaft  22  disposed within the seal cartridge  130 . The sleeve  142  has a set screw hole formed therein to permit a setscrew (not shown) to couple the sleeve  142  onto drive shaft  22  whereby sleeve  142  is caused to rotate with drive shaft  22 . First and second grooves  146   a  and  146   b  are formed in the inside surface of sleeve  142  to permit sealing rings (not shown) to be inserted between the sleeve  142  and drive shaft  22 . The sleeve  142  has a very hard outer surface which is highly finished, preferably to about 8 RMS. The outer surface of sleeve  142  is preferably hardened with a thin coating of a material such as hard chromium plating or chromium III oxide (Cr 2 O 3 ), to provide a very hard surface on the sleeve. The coating is preferably applied by electrodeposition or by a thermal spray deposition technique such as plasma spraying. The combination of hardness and smoothness of the sleeve surface provides an excellent contact surface for the inflatable seal  34  and the seal rings  144 . The hard smooth surface of sleeve  142  also provides very good wear resistance for long life. It will be appreciated that the sealing surface sleeve  142  is easily replaceable and prevents scoring and wearing of the drive shaft  22  itself. 
     Referring now to  FIG. 11  there is shown a gas subsystem  100  for inflating and deflating the inflatable seal  34  and providing a purging gas to the non-contacting seal  40 . The subsystem  100  includes a source  110  of pressurized gas. The pressurized gas is preferably an inert gas such as nitrogen, argon, helium, or a combination thereof. A check valve  112  is connected to the outlet of the pressure source  110 . The outlet side of check valve  112  is connected to a T-connector  113  to bifurcate the gas supply line. All of the gas supply lines described herein are preferably formed of metal tubing using appropriate gas-tight fittings and connectors. Manual shut-off valve  114   a  is disposed in gas supply line  115   a  to the inflatable seal and a second shut-off valve  114   b  is disposed in the gas supply line  115   b  to the non-contacting seal. A pressure regulator  116  is connected in the supply line  115   a  downstream from the shut-off valve  114   a  for controlling the pressure in the supply to a preset value. A first solenoid valve  102  and a second solenoid valve  103  are connected to the supply line  115   a  and to the gas-tight  38  to the inflatable seal, downstream from the pressure regulator  116 . The gas-tight tub  38  connects to the inflatable seal from the outlets of the first and second solenoid valves  102 ,  103 . A pressure switch  106  is tapped into the gas supply line between the solenoid valves  102 ,  103  and the inflatable seal. 
     A second pressure regulator  118  is connected in the other gas supply line  115   b  downstream from the shut-off valve  114   b . A third solenoid valve  104  and a fourth solenoid valve  105  are connected to the supply line  115   b  and to the supply line  45  to the non-contacting seal, downstream from the pressure regulator  118 . The supply line  45  connects to the seal cartridge from the outlets of the third and fourth solenoid valves  104 ,  105 . 
     The operation of a vacuum heat treating furnace in accordance with the present invention will now be described. When a work load of metallic parts has been loaded into the chamber of the vacuum furnace, the pressure vessel is closed and sealed. A typical heat treating cycle includes evacuating the furnace chamber to a desired subatmospheric pressure while heating the work load up to the heat treating temperature, maintaining the work load at the heat treating temperature for a selected amount of time, and then shutting off the heating system. The inflatable seal is deflated and then the cooling fan drive motor is started and brought up to full speed. The furnace chamber is then backfilled (pressurized) with the inert cooling gas. In an alternative operating sequence, the furnace chamber is pressurized with the cooling gas and when the pressure in the chamber reaches a preselected superatmospheric pressure, the fan motor is activated to drive the circulating fan to circulate the inert gas over the work load and through the heat exchanger. When a slower cooling rate is desired, the furnace chamber can be backfilled with a partial subatmospheric pressure of the inert gas. 
     The fan does not operate during the heating/evacuation step and the drive shaft is thus in a static condition during that period of the heat treating cycle. The pressure set point on pressure switch  106  is preferably reached within about 3 seconds after solenoid valve  103  is opened in order to start and/or continue a cycle requiring a vacuum. Once the cycle reaches the state where the inflatable seal is deflated, i.e., solenoid valve  103  is closed and solenoid valve  102  is opened, the signal from pressure switch  106  is thereafter ignored by the system. 
     If the pressure switch  106  set point is not reached within the preferred time interval after solenoid valve  103  is opened, or any time thereafter while solenoid valve  103  is opened, then an alarm sounds, the heating/evacuation cycle is aborted, and solenoid valve  104  is opened to inject purge gas into the gap between the non-contacting seal and the drive shaft. The purge gas is injected into the gap at a pressure that is sufficient to prevent ambient air from being drawn into the furnace. 
     When electrical power is turned on to the furnace after a shut down, solenoid valves  102 ,  103 ,  104 , and  105  remain in the states they were in just prior to the power being turned off. There are two possible start-up conditions. Either the inflatable seal is inflated and no purge gas is being injected or the inflatable seal is deflated and the purge gas is being supplied to the non-contacting seal gap. A preselected time after the power to the vacuum furnace is turned on, preferably about 5 minutes, solenoid valve  103  is opened and solenoid valve  105  is closed, thereby causing the inflatable seal to be inflated and the purge gas to be stopped. The delay period allows any residual motor/fan rotation to stop completely before the inflatable seal is inflated. 
     At the start of the heating cycle, there is a preset delay period, preferably about 5 minutes, as described above for the powering up of the furnace. When a heat treating cycle is initiated, solenoid valve  103  and solenoid valve  105  remain in their initial positions while the furnace vacuum pump evacuates the furnace chamber. The solenoid valves remain in that state until the forced cooling portion of the heat treating cycle is initiated. 
     When the forced cooling cycle is initiated, solenoid valves  103  and  105  are de-energized causing them to close. Simultaneously, solenoid valves  102  and  104  are opened. Preferably, the opening of solenoid valve  102  is delayed for a preselected time, preferably about 3 seconds, after the time when solenoid valve  104  is opened in order to prevent air from being drawn into the furnace. When solenoid valves  102  and  104  are in their open (energized) positions, the inflatable seal is deflated and purge gas flows into the non-contacting seal gap. 
     There is a further time delay of preferably about 5 seconds after solenoid  102  opens until the cooling motor starts to provide sufficient time for the inflatable seal to deflate and retract from the drive shaft or sleeve surface. As described above, the cooling fan drive motor is preferably turned on and up to full speed before the furnace chamber is backfilled with the cooling gas. 
     When the cooling cycle is completed and stopped, solenoid valves  102  and  103  remain open to keep the inflatable seal deflated and to continue the gas purge in the non-contacting seal gap. The delay period is preferably about 5 minutes. After the delay period has elapsed, solenoid valves  103  and  105  are energized again to inflate the inflatable seal and stop the gas purge. This delay allows the fan motor to stop rotating completely before the inflatable seal is inflated. For any other cooling functions (vacuum cool, static cool), solenoid valves  103  and  105  remain open so that the inflatable seal is inflated and no purge gas is injected into the non-contacting seal gap. 
     In view of the foregoing description, some of the advantages of the dual seal according to the present invention should now be apparent. For example, the dual seal according to this invention is assembled in a compact cartridge that can be readily replaced when either of the seals fails or wears out. In addition, a dual seal is provided having a second seal that is designed to be substantially non-contacting with the fan drive shaft in order to minimize wear on the seal. A very small gap is provided between the seal and the drive shaft. This gap is dimensioned to minimize gas leakage from the furnace chamber when the furnace is pressurized with a cooling gas. Further still, the dual seal arrangement according to the invention includes means for providing a purging gas in the gap between the second seal and the drive shaft so that outside air is not drawn into the furnace chamber when the furnace is being transitioned from a subatmospheric pressure to a superatmospheric pressure.