WO2003064949A1

WO2003064949A1 - Counter-rotating tunnel furnace

Info

Publication number: WO2003064949A1
Application number: PCT/US2003/002121
Authority: WO
Inventors: Oleg Nikolayevich Chunyayev; Lidiya Omovna Chunyayeva; Leonid Leonidovich Tovagnyanskiy; Om Iosifovich Zayets
Original assignee: Jamar Venture Corporation
Priority date: 2002-01-31
Filing date: 2003-01-22
Publication date: 2003-08-07

Abstract

The present invention pertains to a counter-rotating tunnel furnace (10) useful for diffusion surface alloying, diffusion carbide surface alloying of metal items, pipes, and construction beams in powder packs, and for firing of ceramic and fire-resistant materials. The invention utilizes a counter-rotating principle which involves transport of workpieces (35) inside the furnace via conveying devices (36, 37) situated side-by-side and moving in an alternate direction so that the furnace can reuse, or exchange, heat between the cooling outgoing workpieces and the incoming workpieces.

Description

COUNTER-ROTATING TUNNEL FURNACE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The benefit of prior U.S. Provisional Application No. 60/353,626 filed January 31, 2002 is hereby claimed.

FIELD OF THE INVENTION [0002] The present invention relates generally to tunnel furnaces, and more particularly, to a counter-rotating tunnel furnace.

BACKGROUND OF THE INVENTION [0003] Tunnel furnaces are well known and have been the subj ect of numerous improvements over the years. They are useful for diffusion surface alloying and diffusion carbide surface alloying of metal items including pipes and construction beams in powder packs, and for firing of ceramics and fire-resistant materials such as bricks and cement blocks, and further are widely used due to high productivity and recuperation of heat by means of forced convection.

[0004] With respect to diffusion alloying, the method and composition for diffusion alloying of ferrous metals as described in U.S. Patent No. 6,197,436 to Zayets et al. and the method and composition for diffusion treatment of ceramic materials as described in U.S. Patent No. 5,939,144 are hereby incorporated herein. Notably, diffusion coating of workpieces of metal, ceramics and the like having a preferred coating composition forms a protective surface layer. As a result of chemical and thermal processing of the base metal, the surface acquires high wear and corrosion resistance characteristics to satisfy the requirements for long-term performance in various environments.

[0005] With today's tunnel furnaces, diffusion surface alloying and diffusion carbide alloying is limited by intense erosion of the pack material (i.e. , Nichrome and Inconel). Additionally, the large dimensions of continuous tunnel furnaces increase investment cost while its use of fuel and electric with forced convection heat exchange tends to be inefficient when dealing with massive loads having small relative surface area.

[0006] The counter-rotating principle of the present invention involves transport of the workpieces inside the furnace via conveying devices situated side-by- side and moving therethrough in alternating directions so that the furnace can reuse, or exchange, most of the heat of cooling outgoing workpieces to incoming workpieces entering the furnace thereby decreasing furnace dimensions and the time needed for heating the incoming workpieces. The counter-rotating principle utilizes radiant heat exchange which can be several times more effective than forced convection heat exchange at temperatures of 300°C to 600°C, and which can be more than ten times effective at temperatures above 600°C. As such, alternate movement of workpieces in the furnace via conveying devices provides an effective heat exchange even with massive loads of small relative surface area.

SUMMARY OF THE INVENTION [0007] The present invention pertains to a counter-rotating tunnel furnace useful for diffusion surface alloying and diffusion carbide surface alloying of metal items including pipes and construction beams in powder packs, and for firing of ceramics and fire-resistant materials such as bricks and cement blocks. [0008] The counter-rotating tunnel furnace has a chamber including opposing first and second ends and a plurality of zones extending laterally thereacross to define a heat exchange area. Each of the opposing ends is provided with a movable door which preferably extends laterally across the chamber to define opposing sides for the heat exchange area. The plurality of zones includes at least one isothermal temperature zone located intermediate the chamber as well as radiant heat exchange zones (300°C and above), and convection heat exchange zones or convection-radiant (mixed) heat exchange zones (below 300°C). The isothermal temperature zone is heated to a desired maximum temperature by a heat source. [0009] There is further provided at least one pair of conveying devices for transporting the workpieces (i.e., metal items, ceramics, bricks, etc.) through the plurality of zones in the chamber wherein each conveying device is situated in a side- by-side relationship in the chamber and transports the workpieces in alternate directions at equivalent speeds to maximize heat exchange of the workpieces within the heat exchange area. The at least one pair of conveying devices further cooperate to form a continuous production loop.

[0010] Each conveying device of the at least one pair of conveying devices preferably includes a pair of rail lines extending through the plurality of zones and at least one platform, preferably a plurality of platforms, for transporting the workpieces along the pair of rails. Notably, conveyor belts, or the like, may be a suitable substitute for the rail lines and platforms.

[0011] The conveying devices transport the workpieces through each zone at a desired speed, stopping in zones as needed for desired periods of time to properly diffusion surface alloy or diffusion carbide surface alloy metal items including pipes and construction beams in powder packs, or to fire ceramics or fire-resistant materials (i.e., bricks and cement blocks). Notably, after leaving the isothermal temperature zone, the workpieces begin cooling down and transferring, or exchanging, heat throughout any remaining zones. The alternate directions of the conveying devices utilize the exchange of heat from cooling outgoing workpieces by warming up the incoming workpieces thereby allowing the tunnel furnace to have decreased dimensions. Most notably, the exchange of heat from the outgoing workpieces to the incoming workpieces decreases the length of time needed for heating the incoming workpieces in the isothermal temperature zone thereby increasing output and reducing the amount of electricity used per workpiece.

[0012] In one embodiment, the plurality of zones include opposing convection- radiant heat exchange zones, at least one pair of opposing radiant heat exchange zones intermediate the convection-radiant heat exchange zones, and an isothermal temperature zone located intermediate the at least one pair of radiant heat exchange zones. Each zone is separated by a movable divider which preferably extends laterally across the chamber and enables the zones to be temperature regulated. The movable dividers cooperates with the at least one pair of conveying devices to allow transport of the workpieces through the chamber. Each opposing convection-radiant heat exchange zone further includes at least one air flow device for circulating air. [0013] In another embodiment, the plurality of zones include opposing convection heat exchange zones, at least one pair of opposing radiant heat exchange zones intermediate the convection heat exchange zones, and an isothermal temperature zone intermediate the at least one pair of radiant heat exchange zones. A heat source is provided for heating the isothermal temperature zone to a desired maximum temperature while opposing shields and ah flow devices are positioned in the chamber, preferably within the convection heat exchange zone, so that the air devices circulate and the shields are positioned to direct air through the heat exchange area. [0014] Accordingly , it is an obj ect of the invention to utilize a counter-rotating principle wherein transport of workpieces within a tunnel furnace is performed via conveying devices situated side-by-side and moving in alternating directions so that the furnace can reuse, or exchange, most of the heat of cooling outgoing workpieces. [0015] It is another object of the invention to utilize heat exchange among workpieces inside a tunnel furnace in order to decrease tunnel furnace dimensions and reduce the cost thereof.

[0016] Lastly, it is another object of the invention to utilize heat exchange among workpieces inside a tunnel furnace during diffusion surface alloying, diffusion carbide surface alloying of metal items, pipes, and construction beams in powder packs, and during firing of ceramic and fire-resistant materials to increase the output of the workpieces and reduce the amount of electricity used per workpiece. [0017] The invention will be further described in conjunction with the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] Fig. 1 is a top plan view of one embodiment of the counter-rotating tunnel furnace wherein the top of the furnace has been removed therefrom; [0019] Fig. 2 is a top plan view showing a variation of the invention of Fig. 1 wherein the counter-rotating tunnel furnace is provided with two pairs of conveying devices;

[0020] Fig. 3 is a top plan view of another embodiment of the present invention wherein the top of the furnace has been removed therefrom; and

[0021] Fig. 4 is a graph showing the increase and decrease in temperature within the counter-rotating tunnel furnace of Fig. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0022] The present invention pertains to a counter-rotating tunnel furnace useful for diffusion surface alloying and diffusion carbide surface alloying of metal items including pipes and construction beams in powder packs, and for firing of ceramics and fire-resistant materials such as bricks and cement blocks. [0023] Fig. 1 shows one embodiment of the present invention which concerns a counter-rotating tunnel furnace 10 having a chamber 12 including opposing first and second ends 14 and 16 provided with a movable door 18 and a plurality of zones 20 extending laterally across the chamber 12 to define a heat exchange area 22. The movable doors 18, likewise, extend laterally across the chamber 12 to define opposing sides 24 for the heat exchange area 22.

[0024] The plurality of zones 20 include opposing convection-radiant heat exchange zones 26, at least one pair of opposing radiant heat exchange zones 28 intermediate the convection-radiant heat exchange zones 26, and an isothermal temperature zone 30 located intermediate the at least one pair of opposing radiant heat exchange zones 28. Three sets of opposing radiant heat exchange zones 28a, 28b and 28c are shown in Fig. 1. Each opposing convection-radiant heat exchange zone 26 includes at least one air flow device 32, preferably a fan, for circulating air, as indicated by arrows, within the zone 26.

[0025] Noticeably , the plurality of zones 20 are separated by movable dividers

34 which preferably extend laterally across the chamber 12 enabling the zones 20 to be temperature regulated. The dividers 34 and doors 18 preferably are composed of Nichrome or Intonel to provide adequate insulation. [0026] A heat source 33, preferably at least one electric heater, heats the isothermal temperature zone 30 to a desired maximum temperature. Accordingly, the isothermal temperature zone 30 is monitored to keep the desired temperature constant. [0027] At least one pair of conveying devices 36 is provided for transporting workpieces 35 (i.e., metal items, ceramics, bricks, etc.) through the zones 20 of the chamber 12. A single pair of conveying devices 36 is shown in Fig. 1. Each conveying device 37 is situated in a side-by-side relationship in the chamber 12 and transports the workpieces 35 in alternate directions (indicated by arrows) at equivalent speeds therethrough to maximize heat exchange within the heat exchange area 22. Each conveying device 37 of the pair of conveying devices 36 includes a pair of rail lines 38 extending through the plurality of zones 20 and at least one platform 40, preferably a plurality of platforms, for transporting the workpieces 35 along the pair of rails 38. Notably, conveyor belts (not shown), or the like, may be a suitable substitute for the rail lines 38 and platforms 40. The rail lines 38 preferably are composed of steel and the platforms 40 insulated and preferably the same, or similar in size.

[0028] The platforms 40 on each pair of rail lines 38 cooperate with one another to form a train as they move along the rail lines 38 through the furnace 10. The platforms 40 may be coupled together much like boxcars and pulled through the furnace 10, or may move via hydraulics. Each platform 40 separates from the train upon exiting the furnace 10 while the workpieces 35 on each platform 40 are removed therefrom.

[0029] Notably, the pair of conveying devices 36 in Fig. 1 cooperate to form a continuous production loop for continual transport of workpieces though the furnace. More specifically, each pair of rail lines 38 for each conveying device 37 of the pair of conveying devices 36 directs platforms 40 exiting the furnace 10 onto the other corresponding pair of rail lines 38, as indicated by arrows. Once on the other corresponding pair of rail lines 38, workpieces 35 are loaded onto the platform wherein the platform 40 can begin another trip through the furnace 10 in an alternate direction thereby keeping production continuous. [0030] Accordingly, the movable doors 18 and dividers 34 cooperate with the pair of conveying devices 36 to allow transport of the goods 35 through the furnace 10. Preferably, the doors 18 and dividers 34 are automated and programmed to move in a vertical direction, or swing in a direction away or towards incoming and/or outgoing workpieces 35, thereby allowing the workpieces 35 to pass through the furnace 10. As such, the doors 18 and dividers 34 help regulate the temperature in the zones 20 by keeping heat contained therein.

[0031] Due to the alternate movement of the workpieces 35 through the furnace 10, it is estimated that a zone tunnel furnace 9 as shown in Fig. 1 will produce a temperature in opposing radiant heat exchange zones 28a that is about 80% of the temperature in the isothermal temperature zone 30 while the temperature in radiant heat exchange zones 28b will be about 60% of the temperature in the isothermal temperature zone 30, the temperature in radiant heat exchange zones 28c will be 40% of the temperature in the isothermal temperature zone 30, and the temperature in convection-radiant heat exchange zones 26 will be 20% of the temperature in the isothermal temperature zone 30. The artisan will appreciate that the percentages will change depending upon the number and size of the zones 20. Additionally, the number of pairs of conveying devices 36, and number and size of the zones 20 can be modified to meet necessary productivity and investment-to- effectiveness ratios. The more zones 20, the greater the heat efficiency. [0032] Fig. 2 illustrates a variation of the invention of Fig. 1 wherein the counter-rotating tunnel furnace 10 includes two pairs of conveying devices 36a and 36b situated in a side-by-side relationship in the chamber 12. Similarly, each conveymg device 37 of the two pairs of conveying devices 36a, 36b transports the workpieces 35 in alternate directions through the chamber with each pair of the two pairs of conveying devices 36a, 36b cooperating to form a continuous loop. [0033] Fig. 3 shows another embodiment of the counter-rotating tunnel furnace

10 which has a chamber 12 including opposing first and second ends 14, 16 provided with a movable door 18 and a plurality of zones 20 extending laterally thereacross to define a heat exchange area 22. Similarly, the movable doors 18 extend laterally across the chamber 12 to define opposing sides 24 for the heat exchange area 22 and preferably are composed of Nichrome or Intonel to provide adequate insulation. [0034] The plurality of zones 20 include opposing convection heat exchange zones 44, opposing radiant heat exchange zones 46 intermediate the convection heat exchange zones 44, and an isothermal temperature zone 30 intermediate the opposing radiant heat exchange zones 46. A heat source 33, preferably at least one electric heater, heats the isothermal temperature zone 30 to a desired maximum temperature. The isothermal temperature zone 30 is monitored to keep the desired maximum temperature constant.

[0035] Similar to Fig. 1 , at least one pair of conveying devices 36 is provided for transporting workpieces 35 through the zones 20 of the chamber 12. A single pair of conveying devices 36 is shown in Fig. 3. Each conveying device 37 is situated in a side-by-side relationship in the chamber 12 and transports the workpieces 35 in alternate directions, as indicated by arrows, at equivalent speeds therethrough to maximize heat exchange within the heat exchange area 22. Each conveymg device 37 of the pair of conveying devices 36 includes a pair of rail lines 38 extending through the plurality of zones 20 and at least one platform 40, preferably a plurality of platforms, for transporting the workpieces 35 along the pair of rails 38. Notably, conveyor belts (not shown), or the like, may be a suitable substitute for the rail lines 38. The rail lines 38 preferably are steel and the platforms 40 insulated and preferably the same, or similar in size.

[0036] Additionally, the platforms 40 on each pair of rail lines 38 cooperate with one another to form a train as they move along the rail lines 38 through the furnace 10. The platforms 40 may be coupled together much like boxcars and pulled through the furnace 10, or may move via hydraulics. Each platform 40 separates from the train 42 upon exiting the furnace 10 while the workpieces 35 on each platform 40 are removed therefrom.

[0037] Similar to Fig. 1, the pair of conveying devices 36 in Fig. 3 further cooperates to form a continuous production loop. More specifically, each pair of rail lines 38 for each conveying device 37 of each pair of conveying devices 36 directs the platforms 40 exiting the furnace 10 onto the other corresponding pair of rail lines 38, as indicated by arrows. Once on the other corresponding pair of rail lines 38, workpieces 35 are loaded onto the platform 40 wherein the platform 40 can begin another trip through the furnace 10 in an alternate direction thereby keeping production continuous. Accordingly, the movable doors 18 cooperate with the pair of conveying devices 36 to allow transport of the workpieces 35 through the chamber 12. Preferably, the doors 18 are automated and programmed to move in a vertical direction, or swing in a direction away or towards incoming and/or outgoing workpieces 35, thereby allowing the workpieces 35 to pass through the furnace 10. [0038] Additionally, opposing shields 48 and air flow devices 50, preferably fans, are positioned in the chamber 12 so that the air flow devices 50 circulate and the shields 48 are positioned to direct air through the heat exchange area 22. More specifically, each opposing shield 48 extends longitudinally along the chamber 12 substantially intermediate each conveying device 37 of the at least one pair of conveying devices 36 within the convection heat exchange zone 44 and is spaced apart from the movable door 18 to define an air flow path 52 between the movable door 18 and each shield 48. Each shield 48 should extend about 15-25% the length of the tunnel furnace 10. Each opposing air flow device 50 is positioned to circulate air through the air flow path 52, as indicated by arrows, and located within the convection heat exchange zone 44. Notably, the shields 48 and air flow devices 50 help control the temperature within the furnace 10 by circulating the air while the doors 18 help regulate the temperature in the furnace 10 by keeping the heat contained therein.

[0039] The desired maximum temperature of the isothermal temperature zone

30 in Fig. 3 remains constant while decreasing in temperature in a direction therefrom towards the opposing convection heat exchange zones 44. The graph of Fig. 4 illustrates this decrease in temperature. Accordingly, the number of pairs of conveying devices 36 and the length of the zones 20 can be modified to meet necessary productivity and investment-to-effectiveness ratios. The greater the length of the zones 20, the greater the heat efficiency. [0040] With respect to the transport of workpieces 35 in Figs. 1-3, the workpieces 35 are loaded onto each conveying device 37, more specifically, onto the platforms 40 thereof for transport through the furnace 10 and are removed upon exiting.

[0041] Specifically, with respect to surface alloying, the workpieces 35 preferably are loaded into containers (not shown) situated on the platforms 40 of each conveying device 37. Each container (not shown) is packed with an alloying mixture in a powdered form which has been weighed and mixed and placed in the container (not shown). The workpieces 35 preferably are cleaned (i.e., degreased), for example in a weak acid solution, prior to being placed in the container (not shown). . The containers (not shown) are hermetically sealed then transported through the furnace 10 on the platforms 40. After cooling and exiting the furnace 10, the containers are unsealed and the workpieces 35 removed therefrom properly coated. [0042] Accordingly, the pair of conveying devices 36 transports the workpieces 35 through the zones 20 at a desired speed, stopping in zones 20 as needed for desired periods of time to properly diffusion surface alloy or diffusion carbide surface alloy the workpieces 35 such as metal items including pipes and construction beams in powder packs, or to fire ceramics or fire-resistant materials (i.e., bricks and cement blocks). The workpieces 35 should be of the same type but may be different as long as the temperatures needed for diffusion surface alloying or diffusion carbide surface alloying of metal items including pipes and construction beams in powder packs, or for firing of ceramics or fire-resistant materials are the same, or similar.

[0043] The workpieces 35 enter a maximum temperature within the isothermal temperature zone 30 such that, after leaving the isothermal temperature zone 30, the workpieces 35 begin cooling down and transferring, or exchanging, heat throughout any remaining zones 20. The alternating directions, as indicated by arrows, of each conveying device 37 allows the outgoing workpieces 35 to warm-up the incoming workpieces 35 thereby allowing the tunnel 10 furnace to have decreased dimensions. Most notably, the exchange of heat from the outgoing workpieces 35 to the incoming workpieces 35 decreases the length of time needed for heating the incoming workpieces 35 in the isothermal temperature zone 30 thereby increasing output and reducing the amount of electricity used per workpiece 35.

[0044] While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims. [0045] What is claimed is:

Claims

CLAIMS 1. A counter-rotating tunnel furnace comprising: a chamber including opposing first and second ends and having a plurality of temperature regulated zones extending laterally thereacross the chamber to define a heat exchange area, said plurality of zones including at least one isothermal temperature zone located intermediate the first and second ends of said chamber; a heat source for heating said at least one isothermal temperature zone to a desired maximum temperature; and at least one pair of conveying devices for transporting workpieces through said plurality of zones of said chamber wherein each conveying device of said at least one pair of conveying devices transports said workpieces in alternate directions through said chamber to maximize heat exchange within said heat exchange area.

2. The tunnel furnace as recited in claim 1 wherein said plurality of zones further include at least one pair of opposing convection-radiant heat exchange zones and at least one pair of opposing radiant heat exchange zones intermediate said convection-radiant heat exchange zones, said isothermal temperature zone intermediate said at least one pair of radiant heat exchange zones.

3. The tunnel furnace as recited in claim 1 wherein said plurality of zones are separated by insulated movable dividers and each said opposing first and second ends is provided with an insulated movable door, each movable divider and movable door cooperating with said at least one pair of conveying devices allowing transport of said workpieces through said chamber.

4. The tunnel furnace as recited in claim 3 wherein each said movable door extends laterally across said chamber to define opposing sides for said heat exchange area, and each said movable divider extends laterally across said chamber.

5. The tunnel furnace as recited in claim 1 wherein said each conveying device of said at least one pair of conveying devices, situated in a side-by-side relationship in said chamber, cooperate to form a continuous loop through which the workpieces travel.

6. The tunnel furnace as recited in claim 1 wherein said each conveying device of said at least one pair of conveying devices move in alternate directions through said chamber at equivalent speeds.

7. The tunnel furnace as recited in claim 5 wherein said each conveying device of said pair of conveying devices further comprise a pair of rail lines extending through said plurality of zones and at least one platform for transporting said workpieces.

8. The tunnel furnace as recited in claim 1 wherein each of said at least one platform of said each conveying device of said pair of conveying devices is insulated and transports workpieces through said chamber.

9. The tunnel furnace as recited in claim 2 wherein each said opposing convection-radiant heat exchange zone includes at least one air flow device for circulating air.

10. The tunnel furnace as recited in claim 1 further including opposing shields and opposing air flow devices positioned respectively to circulate and direct air through said heat exchange area.

11. The tunnel furnace as recited in claim 1 wherein said each said opposing shield extends longitudinally along said chamber substantially intermediate said each said conveying device of said at least one pair of conveying devices and further spaced apart from said isothermal temperature zone and from said movable door to define an air flow path between said movable door and said each said shield, each said air flow device positioned to circulate air through said air flow path.

12. A counter-rotating tunnel furnace comprising: a chamber mcludmg opposing first and second ends each provided with an insulated movable door, and a plurality of temperature regulated zones extending laterally across said chamber to define a heat exchange area, said plurality of zones including opposing convection-radiant heat exchange zones, at least one pair of opposing radiant heat exchange zones intermediate said convection-radiant heat exchange zones, and an isothermal temperature zone located intermediate said at least one pair of radiant heat exchange zones, each said plurality of zones separated by insulated movable dividers; a heat source for heating said isothermal temperature zone to a desired maximum temperature; and at least one pair of conveying devices for transporting workpieces through said zones of said chamber, each conveying device of said at least one pair of conveying devices situated in a side-by-side relationship in said chamber and transporting said workpieces in alternate directions through said chamber to maximize heat exchange within said heat exchange area.

13. The tunnel furnace as recited in claim 12 wherein said at least one pair of conveying devices cooperate to form a continuous loop.

14. The tunnel furnace as recited in claim 12 wherein each said movable divider extends laterally across said chamber, and each said movable door extends laterally across said chamber to define opposing sides for said heat exchange area, the dividers and doors cooperating with the at least one pair of conveying devices to allow transport of the workpieces through the chamber.

15. The tunnel furnace as recited in claim 12 wherein said each conveying device of said at least one pair of conveying devices transports said workpieces in alternate directions through said chamber at equivalent speeds.

16. The tunnel furnace as recited in claim 12 wherein said each conveying device of said pair of conveying devices includes a pair of rail lines extending through said zones and at least one insulated platform for transporting said workpieces along said pair of rail lines.

17. The tunnel furnace as recited in claim 12 wherein each said opposing convection-radiant heat exchange zone includes at least one air flow device for circulating air.

18. A counter-rotating tunnel furnace comprising: a chamber including opposing first and second ends provided with an insulated movable door and a plurality of zones extending laterally thereacross the chamber to define a heat- exchange area, said plurality of zones including opposing convection heat exchange zones, opposing radiant heat exchange zones intermediate said convection heat exchange zones, and an isothermal temperature zone intermediate said opposing radiant heat exchange zones; a heat source for heating said isothermal temperature zone to a desired maximum temperature; opposing shields and air flow devices, each said air flow device positioned to circulate and each said shield positioned to direct air through said heat exchange area; and at least one pair of conveying devices for transporting workpieces through said zones of said chamber, each conveymg device of said at least one pair of conveying devices situated in a side-by-side relationship in said chamber and further transports said workpieces in alternate directions at equivalent speeds through said chamber to maximize heat exchange within said heat exchange area.

19. The tunnel furnace as recited in claim 18 wherein said at least one pair of conveying devices further cooperating to form a continuous loop.

20. The tunnel furnace as recited in claim 18 wherein each said opposing shield extends longitudinally along said chamber substantially intermediate each said conveying device of said at least one pair of conveying devices and is further spaced apart from said isothermal temperature zone and from said movable door to define an air flow path between said movable door and each said shield, each said air flow device positioned to circulate air through said air flow path.

21. The tunnel furnace as recited in claim 18 wherein each said insulated movable door extends laterally across said first and second ends of said chamber to define opposing sides for said heat exchange area for cooperating with said at least one pair of conveying devices allowing transportation of said workpieces through said chamber.

22. The tunnel furnace as recited in claim 18 wherein said each conveying device of said pair of conveying devices includes a pair of rail lines extending through said plurality of zones and at least one insulated platform for transporting said workpieces along said pair of rails.

23. The tunnel furnace as recited in claim 18 wherein each said opposing shield is located within said convection heat exchange zone.