US3414658A

US3414658A - High vacuum furnace having improved support structures for the door and heating elements

Info

Publication number: US3414658A
Application number: US596683A
Authority: US
Inventors: Mark O Uitz
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-11-23
Filing date: 1966-11-23
Publication date: 1968-12-03
Anticipated expiration: 1985-12-03

Description

Dec. 3, 1968 3,414,658

M. O. UITZ HIGH VAC FURNACE HAVING IMPROVED SUPPORT STRUCTURES 2 Sheets-Sheet l OR THE DOOR AND HEATING ELEMENTS Filed Nov. 23, 1966 5 g INVENTOR. 2 2 MARK 0. UlTZ BY &o-TW

ATTORNEY Dec. 3, 1968 M. o UITZ 3,414,658

HIGH VACUUM FURNACE HAVING IMPROVED SUPPORT STRUCTURES FOR THE DOOR AND HEATING ELEMENTS Filed NOV. 23, 1966 2 Sheets-Sheet 2 a lo '1') CD E Q 2 g Lo 33 2; ID

3 INVENTOR.

MARK 0. UITZ ATTORNEY United States Patent 3,414,658 HIGH VACUUM FURNACE HAVING IMPROVED SUPPORT STRUCTURES FOR THE DOOR AND HEATING ELEMENTS Mark O. Uitz, Mountain View, Calif, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Nov. 23, 1966, Ser. No. 596,683 14 Claims. (Cl. 13-20) ABSTRACT OF THE DISCLOSURE A door on one end of the furnace is supported on rods to permit sliding the door away from the end of the container and the door is hinged to the rods to permit rotation of the door away from the container. A semicylindrical heater element fixedly mounted within the container and a similar element mounted on the inside of the door together form a cylindrical hot zone when the door is closed. Each of the heater elements is mounted on a pair of water cooled support members which provide reliable electrical connection to the heater elements when the furnace is in operation.

The present invention relates in general to improvements in high temperature, high vacuum, electrically heated furnaces useful, for example, in the fields of heat treating, thermocouple calibration, sintering, and metallurgical testing and, experimentation.

In particular, the invention relates to improvements in the means for supporting, cooling, and conducting current to the heating element of a furnace and to an improved construction for a furnace of the front loading type.

Previously available furnaces have been subject to a variety of problems. In some, premature heating element failures result when the current-conducting supports for the heater element leads expand or soften in high temperature operation. The consequent reduction of interface pressure between the heater element leads and the current-conducting supports increases the resistance across this interface, resulting in local heating and more expansion causing a runaway condition to develop. The result is that the supports either overheat sufficiently to fuse or else the clearance between heating element lead and current-conducting support becomes large enough to permit the heating element to slip, frequently resulting in short circuiting the element.

Earlier furnaces of the type employing a swinging or hinged door for convenient access to the interior of the furnace container have problems in addition to those mentioned above. When some of the internal parts of the furnace, such as a cold wall, are mounted on the door, as is frequently convenient, the additional clearance required for pivotal motion about the hinges necessitates the use of an inconveniently large door and con- .tainer. Furthermore, the hinges, which are usually welded to the container wall, increase the likelihood of leaks and mechanical damage because of thermal expansion forces resulting from different rates of expansion of container, door, weld metal, and hinge.

Earlier furnaces have sometimes employed two or more heater elements that form an enclosure or hot zone for containing the items to be heated. In order to facilitate placing these parts in the hot zone, one of the heater elements was frequently designated to be removed or loosened and swing aside with the result that current connections to the element are not always fully tightened by the operator at the beginning of each test 7 3,414,658 Patented Dec. 3, 1968 times results in enough electrical resistance to produce a great deal of unwanted heat at the current connections causing fusion of the metal surfaces and damage to the current conductors.

Consequently, it is an object of the present invention to provide a support structure for high-temperature heating elements wherein mechanical support and electrical and thermal contact for the heating element are reliably maintained without operator attention despite frequent temperature cycling and long continued high temperature operation.

Another object of the present invention is the provision of a furnace having a door mounted upon a support structure which permits translational motion of the door toward and away from the container, and pivotal motion of the door with respect to the container. In this way the improved support and contact for the heater element can be mounted on the inside of the door without requiring an excessively large furnace in order to permit opening and closing of the door.

Another object of the present invention is the same as the preceding object wherein the support structure for the door is thermally isolated from the container.

Another object is to provide a furnace having a heating element mounted on the inside of the furnace door, and means for energizing the door-mounted element by forcing its current leads into intimate contact with current leads inside the furnace upon closure of the door, thus providing reliable current connections without using heavy electrical cables and feedthrough terminals on the door.

Briefly described, the invention comprises a furnace having a heater element including an electrical lead arm extending therefrom. A support member provides mechanical support and electrical connection for the lead arm. The support member includes spring means for maintaining continuous tight contact between the support member and the heater arm regardless of differential thermal expansion of the heater arm and support member. The invention includes an arrangement in which at least one heater element is mounted on the inside of a door which is hinged to the furnace. 'In order to be able to open and close the door without making an excessively large door opening, the door is so supported that it may be moved in a straight line away from the furnace and then turned about its hinge. In addition, one embodiment of the invention provides one arrangement whereby the spring which assures contact between the heater arm and its support member is compressed by the force exerted in closing the door.

Other advantages of the present invention will become obvious from the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspecitve view partly broken away showing a vacuum furnace embodying features of the present invention;

FIG. 2 is a fragmentary top view showing in more detail the operation of the novel door and support structure of the present invention;

FIG. 3 is a cross sectional view taken on line 33 of FIG. 1 and showing one embodiment of a novel support member for a heater element according to the present invention;

FIG. 4 is a fragmentary cross sectional view similar to FIG. 3 and showing another embodiment of a support structure for a heater element according to the present invention; and

FIG. 5 is an enlarged top view partly broken away taken along the lines 55 of FIG. 4.

Referring now to FIGS. 1 and 2, there is shown one 0 embodiment of a furnace employing the novel features of this invention. The furnace comprises a cylindrical container 1 which can have conventional thermally insulated wall structure. Ring-shaped vacuum sealing flanges 2 and 3 are attached at each end of container 1 by welding, brazing or the like. Rear cover flange 4 is fastened to a base 5 by welding or brazing. Flange 3 is adapted to mate with rear cover flange 4. A vacuum seal is formed when flange 3 is tightened by bolts or clamps (not shown) against rear cover flange 4 with a gasket therebetween. Rear cover flange 4 is provided with a large central tubulation 6 which passes through rear cover flange 4 and is connected to vacuum pump 7 for evacuation of container 1.

A support structure consisting of a bushing bearing block 8, rods 9 and hinges supports a door 11. The door 11 comprises a flange plate 11a and an outer cover shell 11b. Bushing bearing block 8 is mounted upon base 5 by welding, brazing or with bolts.

Bearings 12, for example, ball bushings are mounted within apertures in bushing bearing block 8 for receiving rods 9 and permitting substantially frictionless longitudinal motion of rods 9. Hinges 10 are attached to rods 9 and door 11 to permit rotation of the door. Handles 13 are provided for ease in moving door 11. Stop rings 14 mounted in grooves near the end of each rod 9 limit the travel of rods 9.

A- guiding pin 15 attached to door 11 passes through bearings 16 in a bushing bearing block 17 upon closure of door 11, providing precise alignment between a flange surface 18 on door 11 and the mating surface of flange 2 Clamps 19 are used to provide the large closure forces required to compress a gasket (not shown) between the flange surfaces to provide reliable vacuum sealing.

A cold wall 20 is mounted upon door 11 by means of a bracket 21. Liquid coolant tubes 22 pass through coolant feedthroughs 23 in door 11 and are brazed to cold wall 20.

Similarly, a cold wall 24 is supported within container 1 upon brackets (not shown) and provided with liquid coolant tubes 25 which pass through coolant feedthroughs 26 on rear cover flange 4.

A semi-cylindrical heater element 27 is mounted upon door 11 and within cold wall 20 by liquid cooled support members 28 to which the electrical lead arms 29 of heater element 27 are connected. Similarly, liquid cooled conductors 30 support a second semi-cylindrical heater clement (not shown) by its electrical lead arms 31 within cold wall 24. The heater element 27 is divided by a gap 32 which extends from the top of element 27 to a point adjacent the bottom of the element. In this way, current coming through one lead arm 29 is required to pass down through the heater element on one side of gap 32 and up on the other side of the gap. The heater element in cold wall 24 is constructed exactly the same as heater element 27 in cold wall 20. When door 11 is closed, the two heater elements form a cylindrical hot zone surrounded by a cylindrical thermal insulating barrier formed by

cold walls

20 and 24.

Conductors 30 pass through vacuum sealed feedthroughs 33 in rear cover flange 4 and are provided with tubes 34 for connection to a source of coolant (not shown). A power supply 35 is connected to conductors 30 to provide current to both heater elements when door 11 is closed as will be more fully explained below.

In FIG. 3 there is shown an enlarged cross sectional view of one of the liquid cooled support members 28 mounted on door 11. A coolant tube 36 passes through an insulating bushing 37 of Teflon, for example, and

extends beyond the flange plate 11a to permit connection of a source of liquid coolant to

holes

38 and 39. The liquid coolant flows through flexible tubing (not shown) with the tubing extending through the far side of cover shell 11b in FIG. 1 and being connected to the

holes

38 and 39. Liquid coolant enters through hole 38, flows to the left through a small pipe which is coaxially supported within coolant tube 36, leaves pipe 40 at the left end, flows to the right within the space between pipe 40 and coolant tube 36, and leaves coolant tube 36 through hole 39. Plug 41 seals the right ends of coolant tube 36 and pipe 40.

A sealing collar 42 is welded or brazed within an opening in door 11. A retainer ring 43 is joined to sealing collar 42 with a circle of bolts such as the bolt shown at 43a. A gasket 44 is inserted between the chamfered sealing surfaces 45 and is compressed to form a vacuum tight seal when retainer 43 is tightened against sealing collar 42. An outer sleeve 46 is fastened to retainer 43 by welding or brazing. An inner sleeve 47 is welded or brazed to an adapter ring 48, and is dimensioned to slide freely within outer sleeve 46. A helical drive spring 49 is compressed between a spacer ring 50 and adapter 48 to force adapter 48 to the left. Spacer 50 is held in place by the force of compressed drive spring 49. A dowel pin 51 is press fitted within a hole in outer sleeve 46 and extends into an elongated recess 52 in inner sleeve 47 to limit the travel of inner sleeve 47 in a leftward direction. A metal bellows 53 is fitted over outer sleeve 46 and welded or brazed at its ends to retainer 43 and adapter 48. A collar 54 is attached to adapter 48 by welding, brazing or the like. A ceramic insulator ring 55 has its ends metalized and then brazed to metal sealing rings 56 and 57. Sealing ring 56 is welded or brazed to collar 54. A washer 58 is welded or brazed to scaling cup 57 and to a clamp block 59. Coolant tube 36 is welded or brazed within a recess in clamp block 59. The electrical lead arms 29 of heater element 27 have dovetail-shaped end portions (as shown in FIG. 3). The end of each support arm 29 is held in a dovetail recess in its respective clamp block 59 when a cap plate 60 is tightened in place by screws 61. Similarly, each electrical lead arm 31 is held in a dovetail recess in its respective clamp block 62 on condutor 30 by a cap plate 63 and screws 64.

In operation, the furnace as detailed in FIGS. l-3 is opened for the insertion of test specimens by pulling door 11 away from container 1 until stop rings 14 strike bushing bearing block 8. The door is then swung aside on hinges 10 as in FIG. 2 for convenient access to the interior of the furnace. When the test specimen has been inserted in the furnace, the door is closed again with a gasket of, for example, soft copper wire in place between flange surface 18 on door 11 and sealing flange 2. Closure of door 11 brings the mating surfaces of heater element lead

arms

29 and 31 into contact as shown in FIG. 3. Pin 15 is at the same time inserted into bearings 16 in bushing bearing block 17 thereby assuring alignment of flange surface 18 with the mating surfaces on flange 2. Clamps 19 are then positioned around the periphery of sealing flange 2 and door 11 as seen in FIG. 2. When clamps 19 are tightened the gasket is compressed to form a vacuum seal. At the same time, drive springs 49 are compressed so that when the clamps are fully tightened, the force of spring 49 pressing the mating surfaces of the electrical

lead arms

29 and 31 together is approximately 250 lbs. This force provides intimate electrical and thermal contact not only between the electrical lead arms but also between the electrical lead arms and the clamp blocks into which they fit. The result is that rigid mechanical support and low electrical resistance contact is maintained at the connections to the heater elements, and good thermal conductivity to the liquid coolant is also maintained, all despite possible slight softening, warping and different expansion rates of the various parts at high temperatures. It is particularly important that the electrical lead arms =be rigidly supported because the large electric currents flowing in the heater elements produce a magnetic field which exerts forces in a direction to pull the electrical lead arms on each heater element toward each other, causing vertical gap 32 to close and resulting in short circuiting the upper portions of the heater elements.

After the door 11 is closed, vacuum pump 7 is started and a suitable vacuum is created within the interior of container 1. During the evacuation process the walls of container 1 may be heated by any suitable means, for example, electrical heater elements (not shown) distributed over the surface of container 1. This heating, called bake-out in the vacuum art, is used to free gas molecules which have been absorbed by the walls of container 1. The cracking and leakage which sometimes occurs around the hinges or other parts which are welded to the container in prior art furnaces is avoided in the present invention because the support structure for the door is thermally isolated from the container wall. More specifically, the support blocks 8 and 17 are mounted on the base 5 rather than on the container 1. When the desired vacuum has been achieved, the flow of liquid coolant can be started and power supply 35 switched on to provide current to the heating elements. Current flows through liquid cooled conductors 30 and through both heater elements which are electrically connected in parallel because of the close contact between the electrical leads 29 and 31. Current is prevented from flowing to the door 11 through liquid cooled support members 28 by the

insulators

55 and 37.

FIGS. 4 and 5 illustrate an alternate embodiment of 'a liquid cooled current conducting support 65 for heater elements.

In the alternate embodiment, a clamp block 66 is provided with a recess having a vertical front side 67 with a short lip-like projection 68 extending over a portion thereof. An electrical lead arm 29 from the heater element 27 is held in place by projection 68 to prevent tilting of the lead arm 29'. The lead arm 29 is the same as lead arm 29 except that the end of arm 29' is rectangular in cross section instead of being dovetailed. The rear side 70 of the recess is angled, and a wedge 71 having an identically angled rear face is forced into the recess by

helical springs

72 and 73, thus tightly pressing electrical lead arm 29' against front side 67 to provide continuous electrical contact therebetween. A cover plate 74 is tightened down with screws 75 to compress

springs

72 and 73. The arrangement of wedge and spring provides continuous high pressure on electrical lead arm 29 even at the high temperatures usually encountered. The lead arms 29 and 29', and the clamping blocks 59 and 66 are normally made of metals having different coefficients of expansion, such as tungsten for the lead arms and copper for the clamping blocks. As a result there is substantial difference in expansion of the lead arms and clamp blocks, and the required tight contact between the lead arms and clamp blocks cannot be maintained without the

springs

49 and 72, 73.

The remainder of the liquid cooled support 65 is similar to liquid cooled support 28 as shown in FIG. 3 except that it, of course, does not include spring 49. A conductor tube 76 having an internal pipe similar to pipe 40 in FIG. 3, is supported within an outer housing 77 by insulating bushing 78 of Teflon, for example. Conductor tube 76 extends into a recess (not shown) in clamp block 66 and is welded or brazed therein as in FIG. 3. Insulating bushing 78 and a ceramic ring 79 (similar to ring 55 in FIG. 3) prevent current flow in outer housing 77. Outer housing 77 is joined to a flange 80 by welding or brazing. A flange 81 is welded or brazed to a tubulation 82 which passes through door 11 and is welded or brazed thereto.

Flanges

80 and 81 are bolted together with a gasket (not shown) therebetween to form a vacuum seal. Metal sealing rings 83 and 83a are brazed to ceramic insulator 79. Ring 83a is welded or brazed to outer housing 77. Washer 84 is welded or brazed to ring 83 and clamp block 66. Holes 85 and 86 are provided for connecting liquid coolant as described for

holes

38 and 39 in FIG. 3. Liquid cooled current conducting support 87 is constructed similarly to support 65. Supports 65 and 87 are positioned with their ends close together when door 11 is closed. The exposed right end of conductor tube 76 is connected to a source of current. Similarly the corresponding conductor (not shown) of liquid cooled support 87 is connected at its left end to a source of current.

The embodiment of FIG. 4 is simpler and cheaper to manufacture than the embodiment of FIG. 3. However, it should be understood the embodiment of FIG. 4 requires that a current carrying lead pass through the door, whereas FIG. 3 does not. The reason that the embodiment of FIG. 3 does not require current connection to tube 36 is that spring 49 holds the

heater lead arms

29 and 31 in continuous tight contact so that the current for both of the

arms

29 and 31 is supplied through the conductor 30. It should be understood that the embodiment of FIG. 4 is useful in furnaces where the arrangement is such that closure of the furnace door will not supply compressive force for a spring such as the spring 49 of FIG. 3.

Since many changes can be made in the disclosed construction and many apparently widely different embodi ments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electrical furnace comprising a container having an open portion, a door for closing said open portion, support means providing a sliding connection between said door and said container for permitting straight line motion of said door toward and away from said container, said support means including a hinge operatively connected between said door and said container to provide pivotal motion of said door relative to said container, a first resistance heater element, first attachment means attaching said first heater element to the inside of said door, said heater element and said attachment means being movable together with said door, a second resistance heater element, second attachment means attaching said second heater element to said container internally thereof, each of said heater elements having an electrical lead portion, one of said attachment means comprising a sup port member, and spring means holding said electrical lead portion of one of said heater elements in constantly biased connection with said support member when said door is closed.

2. An electrical furnace as claimed in claim 1 in which one end of said support member is attached to the inside of said door, said spring means is axially positioned along said support member to expand the support member away from said door, said electrical lead portion of the first heater element is attached to the end of said support member, and the electrical lead portion of said second heater element is positioned to be engaged by the electrical lead portion of said first heater element upon partial closure of said door.

3. An electrical furnace as claimed in claim 2 further including current carrying means for both of said heater elements connected to said electrical lead portion of said second heater element.

4. An electrical furnace as claimed in claim 1 further including means connected between said door and said container for aligning said door with said open portion of said container.

5. An electrical furnace as claimed in claim 4 wherein said means comprises a guiding pin connected to said door and a bushing member connected to said container, said bushing member having an aperture therein adapted to receive said pin as said door is slid to the closed position.

6. An electrical furnace as claimed in claim 4 further including support means thermally isolating said hinge and said aligning means from said container.

7. The combination comprising a resistance heater and support member for electrical furnaces, said resistance heater having an electrical lead arm extending therefrom,

said support member having a recess therein forming a wall, said recess receiving said arm extending therein in a first direction, said recess including a projection preventing tilting of said arm, said combination also comprising spring biased means exerting a force in a second direction non-parallel to said first direction and forcing said lead arm against said wall of said recess.

8. A resistance heater and support member as claimed in claim 7 in which said spring biased means comprises a wedge received in said recess with one side of the wedge in engagement with said lead arm and the other side in engagement with said wall of said recess, said wedge being tapered to have the smallest thickness between its said sides toward the bottom of said recess, and said spring biased means further comprises a spring constrained to force said wedge into said recess.

9. A resistance heater and support member as claimed in claim 7 in which said spring biased means comprises two members moveable relative to each other along a longitudinal axis, a spring positioned along said axis to cause relative movement between said members along said axis, and abutment stop means for limiting the extent of such relative movement.

10. A furnace comprising a container having an open portion, means for heating the interior of said container, a door adapted to close said open portion, and means external to said container for supporting said door comprising: means operatively connected between said door and said container for providing translational motion of said door toward and away from said container, and means operatively connected between said door and said means for providing translational motion for providing rotational motion of said door with respect to said container, whereby said door can be translated away from .said container and then rotated, said means for supporting said door being positioned to remain external to said container when said door is moved to fully closed position.

11. A furnace as claimed in claim 10 wherein said means for providing translational motion comprises at least one rod member and one bushing bearing member having an aperture therein adapted to receive said rod member, and said means for providing rotational motion comprises a hinge member.

12. An apparatus according to claim 10 further including means for thermally isolating said means for supporting said door from said container.

13. An electrical connection member for a heater element having an electrical lead arm, said connection member comprising: a supporting member having a recess for receiving said lead arm extending in a first direction into said recess, a lip portion extending over a part of said recess to prevent tilting of said lead arm, a wedge received in said recess with one side in engagement with a wall of said recess spaced from said lip portion, said wedge being tapered to have the smallest thickness between its sides toward the bottom of said recess, and means forcing said wedge into said recess in a second direction non-parallel to said first direction.

14. The electrical connection member according to claim 13 wherein said forcing means comprises a spring constrained to force said wedge into said recess.

References Cited UNITED STATES PATENTS ROBERT K. SCHAFFER, Primary Examiner.

M. GINSBURG, Assistant Examiner.