US2901521A

US2901521A - Electric oven core block

Info

Publication number: US2901521A
Authority: US
Publication date: 1959-08-25
Anticipated expiration: 1976-08-25

Description

Aug. 25, 1959 Filed April 19, 1955 J. B. YODER ELECTRIC OVEN CORE BLOCK 2 Sheets-Sheet 1 Inventor 5:

Joe B. Yoder,

Their John Cappello, y M x 4 ttorney Aug. 25, 1959 J. B. YODER -v ELECTRIC OVEN CORE BLOCK 2 Sheets-Sheet 2 Filed April 19, 1955 Y Po e Se n Pd P 00W 0 tYa t n. t e e vmaw l h T y b United States Patent Cleveland, Ohio, assignors to General Electric Com- ;pany, a corporation of New York Application April 19, 1955, Serial No. 502,294

'1 Claim. (CI. 13-25) This invention relates to core blocks of ceramic or refractory materials for supporting the heating elements in electric furnaces. The core blocks of the instant invention are particularly suited for use in cylindrical electric ovens adapted to surround tubing or rodlike material in order to heat same.

The present invention is particularly useful in connection with ovens used in the manufacture of tubular fluorescent lamps of circular configuration and commonly known as circline lamps. These lamps are made from straight vitreous tubing on combined exhausting and forming machines. At the station of the machine where the forming occurs, the straight lamp envelope is located in a vertical heating oven wherein the glass is heated to a plastic state. The lamp is then rolled up, while still plastic, into a circular form. The oven in question is made up of semicylindrical segments or halves which are moved forward and then pivoted together to close about the straight envelope at the forming station of the machine, as disclosed in Patent 2,494,'9'23-Yoder et al. When the segments are pivoted together, they form a cylindrical oven which completely encircles the tube.

The oven segments consist of semicylindrical external supporting frames or casings within which are mounted the semicylindrical refractory core blocks which in turn support the electric heating-elements formed of helically coiled resistance wire. The length or vertical height of the oven will vary with the length of the lamp being formed, and at the present time ovens up to five .feet in height are being used. It is necessary to provide clearance holes in the top and bottom of the oven as well as clearance between the walls of the tube and the inside walls of the core blocks. The chimney effect in an oven of this height is considerable and the inrush of cool air at the bottom tends to cool it, whereas leakage of hot air tends to overheat the top end. Accordingly, a very flexible zone control of heating elements 'is needed in order to heat the lamp uniformly from top .to bottom to insure that the lamp will form into a substantially perfect circle.

In the core blocks which have been used up until the present time, the resistance wires are embedded in longitudinal, that is, vertically extending, slots in the interior core walls. The ovens are required to heat the tubes until plastic enough to stretch of their own weight. At this temperature, if the glass should accidentally touch the heating element, it melts completely and runs onto the resistance Wire, causing accelerated failure of the element. In manufacture of :circular fluorescent lamps, it has been found that approximately 70 percent of element failure of the electric ovens is caused by such melting of glass onto the resistance wires. It will be appreciated that the loss involved includes not only the cost of the core block elements themselves and of labor for replacement, but also the down time 'or tie-up of the machines while repairs arebeing effected.

An object of the invention is to provide a new and improved typeof refractory core "block for electric ovens.

Patented Aug. 25, 1959 Another object of the invention is to provide a refractory core block for electric ovens of the vertical type which gives more effective protection of the heating elements.

A more specific object of the invention is to providea refractory core block for vertical electric ovens particularly suitable for the heating of vitreous tubing preparatory to forming thereof.

Yet another object is to provide a suitable apparatus and method for forming core blocks of refractory material for electric ovens and particularly suitable for making the core block configurations in accordance with the instant invention.

In accordance with a preferred embodiment of the invention, the core blocks used in the vertical cylindrical ovens for the heating of fluorescent lamps are of semicylindrical form and are provided with a plurality of internal circumferential grooves adapted to hold coiled resistance wire wound back and forth through the grooves. The grooves are relatively closely spaced and are separated by undercut fins for holding the resistance wire in place. In the preferred embodiment, the fins are extended into lips or ledges which incline downwardly for the purpose of protecting the resistance wires. Thus, the lips serve to shed any foreign material, such as molten glass, which might happen to come in contact with .the core block.

The method of the invention for molding such core blocks comprises pressing a measured quantity of refractory molding mix between a semicylindrical mold section corresponding to the outer wall of the core bloc]; and a rotatable core provided with circularly curving fins corresponding to the grooves and lips desired in the inner wall of the core block. After this forming operation, the rotatable core is turned degrees, lateral cover plates having openings registering with the fins being provided to prevent any distortion of the moldblock as the core is rotated. When fully rotated, the core is free of the core block which may then be removed and fired in the usual manner.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings of a preferred embodiment and of a preferred apparatus for producing same. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

Fig. 1 is a pictorial view of a vertical electrical oven used for heating fluorescent lamps and incorporating the core blocks of the instant invention, and shown in conjunction with a partly diagrammatic representation of a circline lamp forming machine;

Figs. 2 and 3 are vertical elevation and plan views, respectively, of a refractory core block embodying the invention in preferred form;

Fig. 4 is an exploded view of the component parts of a mold assembly used in making core blocks in accordance with the invention.

Referring to Fig. 1, there is shown a vertical electric oven 1 for heating a vertically suspended fluorescent lamp 2 which is supported and carried into position by a turrettype exhaust machine whereof a fragment is shown at 3 supporting an exhaust head 4 which moves in a circular path indicated at 5. The oven is made up of two semicylindrical casings 6 hinged on vertical shafts 7 mounted on a movable carriage 8. The turret rotates intermittently and successively indexes lamps into the station presently occupied by lamp 2. During the dwell interval of the lamps at this station, carriage 8 is caused to advance radially toward the lamp as indicated by arrow 9, the oven casings being meanwhile pivoted open. At thefor- Ward limit of advance of the carriage, the two casings are pivoted together to close about the lamp, thereby constltuting a cylindrical oven in which the lamp is axially encompassed. The lamp is heated to the point where the glass has become plastic and the casings are then pivoted open and the oven withdrawn to its original position. The lamp is then quickly rolled up into a circular shape, before it has had a chance to cool, by means of the rollers diagrammatically represented at 10.

Within the casings 6 are mounted semicylindrical heating elements comprising the core blocks 11 supporting the electrical resistance wires which are the source of heat, the core blocks being held in place by side strips 12 fastened to the casings. In the core blocks used prior to the instant invention, the resistance wires extended vertically through longitudinal grooves in the internal faces of the core blocks. Since the oven is required to heat the lamp to a temperature high enough to cause the glass to become plastic so that the lamp will stretch of its own weight, should the lamp envelope accidentally come into contact with the core block, the glass melts and flows as a liquid. Unevenness of heating of opposite sides of the lamp, for instance as a result of drafts, causes it to bow, so occasional contact of a lamp with the oven is practically impossible to avoid. With the prior art core blocks, the melted glass would run onto the heating wires, and since glass at these high temperatures has a relatively high conductivity, failure of the heating element would follow shortly.

In accordance with the invention, the improved core blocks 11 shown in Figs. 2 and 3 are provided with circumferentially extending grooves 13 in the internal annular surface. The grooves are generally circular in cross section in order to accommodate the helically coiled resistance wires 14, which may be of a nickel chromium alloy, commonly known as Nichrome. The circular cross section of the groove extends over more than 180 degrees whereby to provide an undercut in the refractory fins 15 separating the grooves. The resistance wires are thus locked in the grooves in the sense that they cannot be removed edgewise, but are at the same time free to expand circumferentially when heated. In the preferred embodiment of the invention illustrated, the separating fins 15 are extended into downwardly inclined lips 16 which serve to protect the resistance wires by shedding any molten glass. The ends of the fins 15 are cut back at 17 between pairs of adjacent grooves alternating on opposite sides of the block to provide a passageway for the resistance wire. This permits winding the resistance wire back and forth between adjacent grooves across the whole longitudinal extent of the block.

The number of core blocks 11 which are mounted in each of the semicylindrical casings or shells 6 will depend upon the length of the lamp required to be heated. Present commercial circline lamps use tubes from two and a half to five feet long. For the five-foot long tubes, of the six-inch long core blocks illustrated in Fig. 2 are stacked end-to-end in the casings 6, being retained by side strips 12. Due to the difference in expansion between the core blocks and the outer metal shells or casings of the oven, positive fastening of the core blocks in place is not feasible. There is considerable vibration due to the continuous opening and closing of the oven casings and the back and forth translation which tends to dislocate the core blocks and the core block elements. This vibration also causes breaking of the leadin wires which are a continuation of the Nichrome resistance wires, cracking of the ceramic fins separating the resistance wires, and powdering of insulation material disposed between the core blocks and the casing.

According to another feature of the invention, the above disadvantages of the prior art core blocks are substantially reduced by an improved mounting arrangement. To this end, grooves 18 are provided in the end faces 19 of the core blocks; these grooves are of a generally square cross section and extend circularly the entire extent of the core and terminate in laterally extending portions at 20. When core blocks are stacked end-toend in the semicylindrical casings 6, the slots accommodate curved metal strips or keys 21 which are inserted into them in order to lock adjacent core blocks together. The keys are preferably made of a high temperature resistant metal, such as Nichrome. A pair of keys, each extending over approximately 60 degrees of arc, may be used at each juncture; the keys have offset ends accom: modated in lateral slot portions 29 and are thereby locked in place when the end strips 12 are fastened in place on the oven casing 6. When the end strips are removed, the keys may be withdrawn with the adjacent core block elements in position; this arrangement facilitates removal and replacement of defective core blocks. Furthermore, the interlocking of the adjacent blocks provides a decided reduction in vibration and breakage. The ends of the resistance wire 14 are conveniently brought out through inclined slots 22 in the edge walls of the core.

An important advantage of the circumferentially extending groove feature of the instant core blocks, as applied to a vertical oven, is that the resistance wire is supported throughout its entire length. This removes any tendency to sag, as occurs when the wires run vertically in longitudinally extending grooves. Another advantage of the invention is that it makes possible painting the protective lips or ledges without running any paint over onto the resistance wires. It is desirable to paint the internal annular wall of the core block with Alundum cement: this aids in stripping off any glass bulbs that accidentally come into contact with the core blocks inasmuch as the paint wets to the glass but pulls loose fairly easily from the ceramic. With the prior art elements wherein the resistance wires and the separating lips ran vertically, it was found to be practically impossible to keep the paint off the resistance wires, and as the paint layers built up, radiation characterisics changed and control of temperature in the elements wired in parallel was lost. These disadvantages are substantially eliminated by the present core block construction.

The process of making a refractory core block in accordance with the invention and having the preferred configuration illustrated in Figs. 2 and 3 is as follows, reference being made to Fig. 4. A generally rectangular thick-walled molding box 23 (shown in phantom for convenience) is provided: this box must be able to withstand relatively high molding pressures and is preferably made of cast iron or steel. In the lower half of the box is placed a metal block 24 provided with a semicylindrical cavity 25 of a radius at least as great as that of the grooves 13 desired in the core block. The sides of the box are provided with apertures adapted to accommodate the shaft 27 of a mold core 28. The mold core is of generally cylindrical form and is composed of a series of discs 29 stacked on the shaft 27 and held together by tie bolts 31. In the assembly of the molding apparatus, the mold core is positioned within the box and shaft 27 is then inserted through one of the apertures in the sides of the box and is keyed to the mold core. A crank handle may then be fitted on the square end 32 of the shaft to enable the mold core to be rotated within the box. The discs 29 of the mold core have semicircular peripheral fins 33 (some being shown partly cut away) which serve as core elements to form the grooves 13 and the spaces between the fins 15 and lips 16 of the core block. Cover plates 34 are fastened to the upper edges of block 24. At least one of the cover plates has apertures or openings 35 therethrough corresponding to and registering with the fins 33 in mold core 28. In addition, the cover plates may be provided with bosses 36 corresponding to the cut-backs 17 provided in the fins between pairs of adjacent grooves on alternate sides of the core block.

At the beginning of the molding operation, the mold core 28 is angularly positioned as shown in the drawing, that is, with the fins 33 located in the upper half of the molding box. End plates 37 having a raised semicircularly extending embossment or ridge 38 on their inside faces are placed between the end walls of the molding box and the mold core. The embossments 38 correspond to the semicircular grooves 18 desired in the end faces 19 of the core block. A measured quantity of a refractory molding mix corresponding in weight to the finished weight of the core block is then placed in the molding box. The molding mix may consist of alumina and silica or other suitable clays with sufiicient moisture to make them amenable to molding and with other binding agents, if desired. Prior to pressing, the refractory mix may have a volume considerably greater than that of the molded core block and may substantially fill the upper half of the molding box above the mold core 28. An upper mold section 39 provided with a semicylindrical cavity 41 corresponding to the outer annular wall of the core is then placed in the molding box. The upper mold section is pressed down by suitable means, such as a screw cable of exerting a force of several thousand pounds on the rod 42 and the refractory mix is thereby compressed to the desired configuration.

The pressure against the upper mold section 39 is now relaxed slightly so as to reduce the binding of the refractory mix. Mold core 28 is then rotated 180 degrees to carry the fins 33 through the openings in one of the cover plates 34 and into cavity 25 of the lower block 24: the fins of the mold core are thereby disengaged from the grooves in the core block. Upper mold section 39 is then lifted from the molding box, shaft 27 is withdrawn, and block 24, along with mold core 28, is forced up from the bottom of the box in order to bring out the molded core block and end plates 37. The end plates may then be removed from the block which is now ready for baking and firing in the usual manner.

While a certain specific embodiment of the invention has been shown and described, it will be understood that various modifications may be made without departing from the invention. Likewise, as regards the pre- 6 ferred molding apparatus and process which have been described in detail, suitable modifications for making the process more adaptable to various sizes and specific configurations of core block will readily occur. The appended claim is intended to cover any such modifications coming within the true spirit and scope of the invention.

What We claim as new and desire to secure by Letters Patent of the United States is:

A core block for an electric oven comprising a refractory insulating body of generally semi-cylindrical hollow configuration, said body having a plurality of circumferentially extending grooves in its inside surface for accommodating coiled electric resistance wire, said grooves having a substantially circular cross section over more than degrees and being relatively closely spaced and separated by comparatively thin undercut fins extending into downwardly inclined lips for restraining said resistance wire within the grooves and protecting it by shedding foreign material coming into contact with the inside wall of said body, terminal end portions of the fins being removed between adjacent grooves alternating on opposite sides of the body to allow winding the resistance wire back and forth in successive grooves.

References Cited in the file of this patent UNITED STATES PATENTS 1,177,240 Gates Mar. 28, 1916 1,597,900 Keene Aug. 31, 1926 1,682,402 Mulvany Aug. 28, 1928 1,768,865 Smalley July 1, 1930 1,924,079 Campbell Aug. 22, 1933 2,001,316 Shipley May 14, 1935 2,035,306 Fannin Mar. 24, 1936 2,139,201 Nordstrom Dec. 6, 1938 2,234,391 Taylor Mar. 11, 1941 2,303,303 Schleicher Nov. 24, 1942 2,494,923 Yoder et al. Jan. 17, 1950 2,744,946 Lewicki May 8, 1956 FOREIGN PATENTS 511,515 Great Britain Aug. 21, 193.9