US 3384852 A
Description (OCR text may contain errors)
May 21, 1968 J. H. BECK ETAL 3,384,852
HIGH TEMPERATURE ELECTRICAL FURNACE 2 Sheets-Sheet 1 Filed Feb. 16. 1966 IN VENTOR,$ JACOB HOWARD BEcKm-aa P085191 6. 6060M! ATTORNEY.
y 968 J. H. BECK ETAL 3,384,852-
HIGH TEMPERATURE ELECTRICAL FURNACE 2 Sheets-Sheet 2 Filed Feb. 16. 1966 I NVENTORS'. JACOB HOWARD BECK Raw K05A F 8. 806'055 ATTORNEY.
United States Patent O 3,384,852 HIGH TEMPERATURE ELECTRICAL FURNACE Jacob Howard Beck, Newton, and Robert B. Bogosh,
Burlington, Mass., assignors to BTU Engineering Corporation, Waltham, Mass., a corporation of Delaware Filed Feb. 16, 1966, Ser. No. 527,992 Claims. (Cl. 338-316) ABSTRACT OF THE DISCLOSURE A high temperature electrical furnace having a resistance coil supported by ceramic spacers in slots. The ceramic spacers are free to move in the slots when there is thermal expansion of the coil during operation.
This invention relates to a high temperature electrical furnace. In particular, it relates to furnaces achieving temperatures in the order of magnitude of 1300" C. and utilizing a heavy gauge metal wire as the resistance heating element.
Electric furnaces achieving temperatures of 1300 C. suffer from the disadvantages that the metal wire used as a resistance element has very little strength at such temperatures and is in a semi-plastic state. It has been also asserted that at such temperatures superficial layers may form on the surface which tend to protect the underlying metal from deterioration. Further, at lower temperatures, where the wire is strong and hard, the differential expansion between the wire and the rest of the heater structure can generate large forces. As a general rule, the materials used to support and insulate the heater wires are ceramics which are intrinsically brittle and of low tensile strength. Accordingly, the effective life of a high temperature electrical furnace is limited.
One object of this invention is to provide a novel support for the electrical resistance wire which will permit the wire to move without rupturing the ceramic and also prevent undesirable loads on the heat softened wire.
Other objects and advantages of this invention will be apparent from the specification and claims which follow taken together with the appended drawings wherein:
FIG. 1 is a perspective view of a heating unit made in accordance with this invention.
FIG. 2 is a partial longitudinal section of FIG. 1.
FIG. 3 is an enlarged perspective view with partial breakaway of a portion of the interior of the unit illustrated in FIG. 1.
FIG. 4 is a partial interior longitudinal section of the unit of FIG. 1.
FIG. 5 is a schematic view of the unit illustrating the leads and connections.
The invention comprises generally a hollow ceramic cylinder having on its exterior surface a plurality of longitudinal grooves for locating the heater leads and on its inside surface a plurality of longitudinal grooves having an undercut. A plurality of ceramic spacers are provided adapted to seat in the undercut interior grooves and support the heating coil. The dimension of each spacer in the longitudinal direction can be selected as desired. This variation of dimension permits a variation of the spaces between the turns of the heater coil over a very wide range and in a very accurate manner. The spacers are free to move longitudinally but not radially. Because of this freedom of motion in the longitudinal direction, the thickness of the heater wire is not restricted 3,384,852 Patented May 21, 1968 by a fixed dimension between spacers. When thermal expansion occurs, the spacers move with respect to the core, avoiding the stresses that have hitherto ultimately broken the core. Further, this structure of the spacers does not impose any additional load on the heater wire when it is in a hot plastic condition. It tends to support the wire and maintain the protective surface coating in intact condition.
Referring now to the drawings, there is illustrated therein a hollow ceramic cylinder formed from abutting sections 11, 11a, 11b and 110, etc. On the exterior surface of the cylinder are longitudinal grooves 13, 13a and 13b to accommodate electrical leads such as 21, 22, 23, 24. On the interior surface of the tubes are longitudinal slots 12, whose interior width is greater than the opening width of the slot.
The hollow ceramic cylinder 11, 11a, 11b, 11c supports a helical electrical heating element 19 by means of arrays of spacers 14 which are slid in from the ends of the interior grooves 12. Each spacer is characterized as having a substantially wide base 17 cooperable with the interior portion of a slot and tapering side and end faces 15 and 16 forming a top portion protruding through the slot opening. The coil and spacers are adjusted so that a portion of the coil is in between the top portions of two adjacent spacers of appropriate dimension. Accordingly, the same core or hollow ceramic cylinder can accommodate coils of varying pitch and diameter by proper dimensioning of the spacers used.
As can be seen in FIG. 4, the spacers 14, although they are close to one another at their bases 17, normally provide a small space 18 so that as and when the coil 19 moves because of expansion or otherwise at high temperatures, the spacers can slide to accommodate the change in position of the coil.
It should be noted that the side tapering 15 is preferred, although not essential for permitting the desired spacial adjustment of the heating coil 19. The end tapering 16 is also preferred, although not essential, for maintaining the base 17 in a relatively stable position. However, the spacers can be of other design, such as dumbbell shaped whereby the wire is also supported by the exterior wide end of the dumbbell. The spacers can also be designed to support an inner tube as well as the coil.
The leads 2124 and 21a2 la to the coil are illustrated as running from the exterior grooves 13 through openings 25-28 in the tube 11, 11a and 1112.
1. An electrical heating unit comprising: a hollow ceramic cylinder whose inner surface has a plurality of longitudinal undercut slots; a plurality of spaced ceramic spacers in each of said slots longitudinally slidable in said slots and having top portions extending transversely out of said slots; and an electrical heating coil spaced within said cylinder and supported by said top portions of said spacers; said unit being characterized such that when thermal expansion of said coil occurs said spacers move longitudinally in said slots, relieving excess stress on said cylinder and minimizing the additional load on the coil when it is in a hot plastic condition.
2. The electrical heating unit of claim 1 wherein said ceramic cylinder has a longitudinal groove on its outer surface adapted to accommodate an electrical lead.
3. The electrical heating unit of claim 1 wherein said top portions of said spacers have tapering sides.
4. The electrical heating unit of claim 1 wherein said top portions of said spacers have tapering ends.
3 4 5. The electrical heating unit of claim 1 wherein said 1,997,198 4/1935 Ogg 338-322 X spacers are of dumbbell shape. 2,035,306 3/1936 Fannin 13- 25 References Cited FOREIGN PATENTS 125, 8 UNITED STATES PATENTS 5 93 10/1947 Ammha 1,432,442 10/1922 Collins 13 25 DARRELL L. CLAY, Primary Examiner.
1,614,321 1/1927 Smalley 13-25 E. GOLDBERG, Assistant Examiner.