US3541293A - Muffle furnace - Google Patents

Description

NOV. 17, 1970 MaCQNALD ETAL 3,541,293

MUFFLE FURNACE Filed Oct. 29, 1968 2 Sheets-Sheet 1 INVENTORS.

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f a UL A TTOEA/(GVE United States Patent ()1 3,541,293 Patented Nov. 17, 1970 3,541,293 MUFFLE FURNACE Ronald MacDonald, 9140 Brookshire, Downey, Calif. 90240, and Elliott E. La Frienier, 9615 Brighton Way, Beverly Hills, Calif. 90210 Filed Oct. 29, 1968, Ser. No. 771,409 Int. Cl. F27b /14 US. Cl. 219-390 19 Claims ABSTRACT OF THE DISCLOSURE A mutfie furnace having a unitary core supporting a readily-replaceable heating unit. The core and heating unit assembly utilizes a blanket wrapping of heat insulating material to support the assembly centrally of a furnace housing. Refractory end members, one of which is equipped with an access port, provide closures for the furnace. The core is fabricated from thin-walled molded refractory members fused into a rigid assembly having tubular passages through which a flexible heating element is threaded.

This invention relates to mufile furnaces and more particularly to an improved core for supporting a flexible electric heating element and to a simple mode of supporting the core and heating element removably within a furnace housing in such manner that the heating element can be quickly serviced and replaced by the user.

It has been customary heretofore in constructing muffle furnaces to imbed the electric heating element in a tubular clay shell providing a heating chamber for a crucible and surrounded by suitable heat-insulating material. Such constructions are subject to certain disadvantages and shortcomings, a chief one being the fact that the clay core must be destroyed to gain access to the heating element for servicing or replacement. Accordingly, it has not been feasible heretofore to replace a damaged or ununserviceable heating element.

Other serious disadvantages of prior mufiles include non-uniform heat distribution, slowness in coming up to operating temperature and in cooling after being deenergized, and the presence in the refractory of impurities harmful to the heating element. A distinctly superior construction utilizes a strictly inert refractory free of impurities harmful to the material to be heated or capable of causing premature failure of the heating element as by causing a hot spot and overheating of the element or bridging of adjacent convolutions. The refractory should also be reasonable thin walled and of uniform thickness in the interest of high efiiciency and quick response to initial heating and to quick cooling after power cut-off. The heat conduction properties are of importance and differ widely between different refractory materials.

These and other desirable characteristics are satisfied by this invention for an improve mufiie of uniquqe construction preferably but not necessarily fabricated from molded quartz components. These components include a plurality of thin walled tubes fused at their ends to end loops of the same refractory material and cooperating to form a central heating chamber. The tubes are sized to provide a close sliding fit for a flexible spiral coil of resistance wire threated through the tubes in seriesserpentine fashion and with the return bend portions between tube ends supported by the wide refractory end loops of the muffle. All refractory components are fused into a single unitary structure. The heating element coil is free to expand and contract but is snugly supported for maximum protection against shock and vibration. Owing to the high heat conductivity characteristics of molded quartz and its semi-transparency thereby minimizing dispersion losses of radiant energy, the furnace heat-up and cool-down periods are approximately 40 percent of those characteristic of conventional furnaces.

Accordingly, it is a primary object of the invention to provide a new, improved, simplified, rugged muffle furnace readily disassembled for inspection and servicing and reassembled for use without need for any except simple commonly available tools.

Another object of the invention is the provision of a muffle furnace featuring an improved, unitary supporting core structure for the heaing element.

Another object of the invention is the provision of a unitary support for an electric heating element formed of thin-walled high-purity components fused together.

Another object of the invention is the provision of a muffie furnace having very substantially shorter heat-up and cool-down characteristics.

Another object of the invention is the provision of a mufiie unit having a readily replaceable heating element and designed to closely support the major length of the element and protect it against shock and vibration.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of the invention is illustrated.

FIG. 1 is a side elevational view of one preferred embodiment of the invention muffie furnace having a chargereceiving port at one end;

FIG. 2 is a cross-sectional view on an enlarged scale taken along line 22 on FIG. 1;

FIG. 3 is a vertical sectional view taken along line 33 on FIG. 2;

FIG. 4 is a perspective exploded view of the core element prior to fusing the rear endplate in place;

FIG. 5 is a side elevational view of a second preferred embodiment having the charging port at its lower end;

FIG. 6 is a cross-sectional view on an enlarged scale taken along line 66 on FIG. 5;

FIG. 7 is a vertical sectional view taken along line 77 on FIG. 6; and

FIG. 8 is an exploded view in perspective of the core unit prior to fusing the top endplate in place.

Referring initially and more particularly to FIGS. 1 through 4, there is shown a horizontally-arranged muffie furnace designated generally 10 embodying the principles and features of this invention. The tubular housing 11 is supported on a stand 12 and its charging port 14 may be closed when in use as by a block of refractory 15.

The unitary core forming an important feature of the invention is best shown in FIG. 4 prior to fusing endplate 17 in place. It will be understood that all components of core unit 16 are formed of a suitable high-purity, inert refractory capable of use at temperatures of at least 2,300 degrees Fahrenheit. Among the refractories found satisfactory are alumdum, molded aluminum silicate. silicon carbide, molded quartz, and others. Semi-transparent molded quartz is especially satisfactory because of its many beneficial characteristics and properties including its purity, uniformity, ease of assembly using fused quartz joints, outstanding heat conductivity, and attendant fast heat-up and cool-down characteristics, resistance to shattering and to vibration and shock. The individual pieces employed in fabricating core unit 16 include rear endplate 17, bottom plate 18, identical U-shaped members 19, 20 and a plurality of channeled members or tubes 21. These various members are heat-fused together with a suitable material such as alumdum cement or, in the case of quartz elements, by quartz to form a core unit 16. Tubes 21 have an internal diameter providing a close or snug sliding fit with the flexible heating element 22, with the result that vibration and shock do not adversely affect the heating element while permitting the heat element to expand, particularly when the latter is energized. It is also pointed out that the major width of the end rings supporting tubes 21 project beyond the ends of these tubes to provide supporting shelves for the return bend portions 23 of the heating element. Further support for these portions of the heating element is provided by the blanket 25 of fibrous heat-insulating material loosely wrapped about the heating unit assembly, comprising core 16 and heating element 22. This heat insulating blanket may comprise a long thin blanket wrapped several times about the heating unit assembly to a diameter having a snug fit within housing 11. It will therefore be understood that this blanket serves to support and center the core assembly within the furnace housing.

Assembly of the furnace is accomplished by inserting the heating element, core unit and its enshrouding insulating blanket 25 through one end of the housing and then inserting disks of refractory material 26 and 27 from the opposite ends. If desired, the layer of fibrous insulating material 28 may be inserted either inwardly or outwardly of refractory material 26, 27. Usually the opposite ends of the furnace are closed by asbestos disks 29, 30. These may be held in place by assembly screws or any suitable fasteners, not shown.

The terminal ends of the heating element extend through passages in the insulating material to terminal bolts 33 at the rear end of the furnace. Usually it is desirable to provide the heating chamber with a thermocouple of any suitable construction and mounted in a suitable, closefittings port in the rear endwall in accordance with conventional practice.

In use, a crucible or other container for material to be heated is inserted through the charging port 14 and slid rearwardly along the fiat horizontal surface of bottom plate 18 of the core assembly, it being noted that the upper surface of this plate preferably lies flush with the lower edge of board 14.

Referring now to FIGS. through 8, there is shown a second preferred embodiment of the invention wherein the axis of the core and heating unit assembly is arranged vertically of the upright tubular housing 11. The same or similar components of this embodiment are identified by the same characters as in the first-described embodiment but distinguished therefrom by the addition of a prime. In this embodiment, the end rings 19, 20' are circular and connected to one another only by refractory tubes 21', the upper rim edge of ring 19 being closed by a refractory endplate 17.

The components of the embodiment are assembled in the same manner as described above in connection with FIGS. 1 to 4, the assembled furnace being supported upright on stand 12 with its charging port 14' opening downwardly through a central opening 35 in stand 12. The assembled furnace is employed in the same manner as the first embodiment, it being understood that a crucible is supported on refractory material extending upwardly through charging port 14.

Should either of the described furnace constructions require servicing, it is a simple matter to withdraw the refractory closure rings at either or both ends of main housing 11 and then remove core assembly 16 along with its enshrouding blanket 25. If the heating element is in need of replacement, it is quickly Withdrawn from tubes 21 and replaced by a new one, after which the components are reassembled in the manner first described.

Owing to the heat transfer efficiency and the low heat capaciity and other properties of quartz, it is feasibly to make a very inexpensive furnace admirably suited for the fast or slow multistep cycling to precisely selected temperature and time control without need for pyrometcrs or other temperature controls. It sutfices to use a Simple timer controlled variable power supply.

While the particular muille furnaces herein shown and disclosed in detail are fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims.

We claim:

1. An article of manufacture comprising a unitary onepiece support for the electrical heating element of an electric furnace, said support being formed essentially of molded quartz including a plurality of elongated openended channeled quartz members arranged in side-by-side relationship to embrace a central heating chamber and cooperable to support a coil-like heating element threadable through a serpentine path lengthwise of the first one and then another of said channeled members, and said one-piece molded support including molded quartz ring means fused to each of said channeled members and effective to rigidly interconnect the same circumferentially of said central heating chamber.

2. An article of manufacture as defined in claim 1 characterized in that said channeled members are shaped to embrace closely substantial lengths of an elongated helically coiled heating element and protect the same against shock and hold the element against vibration while energized.

3. An article of manufacture as defined in claim 1 characterized in that said channeled members are tubular and enclose a substantial length of a helically coiled heat element between the terminal end portions thereof.

4. An article of manufacture as defined in claim 1 characterized in that at least a substantial number of said channeled quartz members are exposed directly to the central heating chamber along the major portion of their respective lengths.

5. An article of manufacture as defined in claim 1 characterized in that said tubular quartz members are arranged generally parallel to one another and in a ring to provide a tubular central heating chamber the sides of which are defined in major part by said quartz tubes.

6. An article of manufacture as defined in claim 5 characterized in that a plurality of the tubes along one longitudinal side of said heating chamber lie in a common plane, and a quartz plate bonded to said last-mentioned tubes and adapted to provide a bottom for the heating chamber to support material to be heated.

7. An article of manufacture as defined in claim 6 characterized in that one end of said heating chamber is closed by a quartz end plate bonded to adjacent quartz portions of said article of manufacture.

8. An article of manufacture as defined in claim 1 charcterized in that said elongated channeled quartz members are supported beneath their respective ends by an associated quartz ring.

9. An article of manufacture as defined in claim 8 characterized in that said ring supports for said tubular means are relatively thin and wide and have a major portion of their width projecting avially beyond the adjacent ends of said tubes to provide a shelf-like support for por tions of a heating element when threated in series through said tubes.

10. An article of manufacture for use as the heating unit of a high temperature electrical furnace, said article comprising a unitary tubular core formed in one piece from thin-walled molded refractory material and including a plurality of tubes arranged in spaced-apart side-byside relationship to form a major portion of said tubular core, and a continuous flexible electrical heating element threaded through said tubes and having a close sliding fit with the interiors thereof.

11. An article of manufacture as defined in claim 10 characterized in that said refractory tubes are so spaced and arranged that substantially all except return bend portions of said heating element are enclosed within and supported thereby.

12. An article of manufacture as defined in claim characterized in the provision of relatively wide, thin refractory end rings at the opposite ends of said refractory tubes with the adjacent inner edges of said support rings fuse bonded to the end edge portions of said tubes with the remaining width of said end rings serving as protective guard means for the return bend portions of said heating element.

13. A muffie furnace comprising a unitary mufile having a core formed essentially of thin-walled molded refractory of generally uniform thickness and including a plurality of tubes arranged in spaced apart side-by-side relationship, electric heating element means threaded through and having a close sliding fit within said tubes and embracing a heating chamber, housing means for said muffie, a blanket of heat insulating material surrounding said mufile and filling the space between the housing and said mufile, end plugs of heat insulating material supported by said housing at the opposite ends of said muflle, one of which is formed with a charging opening, and terminal means extending from the ends of said heating element to the exterior of said furnace.

14. A muifie furnace as defined in claim 13 characterized in that said heating element is flexible and readily replaceable by withdrawing said muffle unit and threading a substitute heating element through said tubes after withdrawing an unserviceable heating element therefrom.

15. A muffle fumace as defined in claim 13 characterized in that said core is formed of molded quartz including quartz tubes arranged to receive and support a flexible serpentine heating element therealong and forming an enclosure for a centrally disposed muffie chamber.

16. A mufiie furnace as defined in claim 15 characteri zed in that said blanket of heat insulating material embraces said muffle chamber and provides means supporting said quartz core and heating element within said housing.

-17. A muffle furnace as defined in claim 16 characterized .in that said end plugs include rigid refractory members shaped to telescopically fit into the opposite ends of said housing.

18. A mufile furnace as defined in claim 16 characterized in that said heating element is rendered accessible for servicing and replacement by the simple expedient of withdrawing the mutfie from one end of said housing and unwrapping said blanket.

19. A mufile furnace as defined in claim 13 characterized in that said muffle core is closed at one end by a re fractory plate and open at the opposite end thereof, and a horizontally-disposed refractory plate along the lower side thereof providing a support for a crucible.

References Cited UNITED STATES PATENTS 1,234,499 7/1917 Smalley 219-390 2,034,892 3/1936 Berryman et a1 219-390 2,476,613 7/1949 McKinley 219-530 X 2,894,107 7/1959 Letbure 219-553 X 2,954,826 10/1960 Sievers 219-553 X 2,992,286 11/1961 Smit et a1 "13-22 X 3,114,822 12/1963 Boland 219-395 3,307,017 2/1967 Horstmann 219-553 3,346,723 10/1967 MOhn et a1. 219-553 3,353,005 11/1967 SiSSOn et a1 219-411 VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R.

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