US2007605A - Electric furnace - Google Patents

Description

Patented July 9, 1935 i UNITED STATES 2,007,605 Y ELECTRIC FURNACE Albert H. Heyroth and James I. Miller, Niagara Falls, N. Y., assignors to The Global' Corporation, New York Application January 16,

6 Claims.

This invention relates to the manufacture of ceramic articles and particularly to the annealing of such articles and to the apparatus for carrying out the process.

The object of the invention is to provide a new and improved process for removing the irregularities, such as warping, produced during the prior steps in the manufacture of long slender ceramic articles and to provide new and improved apparatus for accomplishing this purpose. Other objects of the invention will become apparent from the following specification and claims.

Long, slender, ceramic articles, such as hollow insulators for thermocouple wires and thermometer tubes, should be straight and uniform in their finished state, but inasmuch as the methods cmployed heretofore have produced a great many Warped and irregularly shaped articles it has been necessary to reject a large percentage of the articles. made and to use only the ones that came through the process in a satisfactory condition. There has been no way of reclaiming articles that become distorted.

By our process we are enabled to reclaim the majority of articles that would be rejected as unsuitable if made according to the prior methods.

Some of the principal problems that we have solved are the shielding of the ceramic objects to be treated from direct radiation, and the support of the ceramic objects in such a manner that they may be inserted and removed from the furnace simultaneously without seriously affecting the temperature conditions of the furnace.

The furnace which we have designed to meet these conditions is illustrated by the accompanying drawing in which:

' Figure 1 is a vertical section of the electric annealing furnace in a plane at right angles to the resistor rods used for heating the furnace; and

Figure 2 is a section on the line 2-2 of Figure l.

Referring to the drawing in detail, a heating chamber 2 is provided with a plurality of rigid resistor rods 3 which consist mainly of silicon carbide. These resistor rods are mounted at the sides of the heating chamber which is permanently closed on all sides excepting a portion of the top wall Where there is a removable cover piece 4. There may also be a small door near the base for the removal of dbris from time to time. The cover piece 4 is used to support a large number of ceramic objects 5 which are held in holes I0 made in the lower portion of the cover. The furnace has a number of similar cover pieces 4. Cover pieces not in use on the furnace are loaded with a plurality of ceramic articles 5 by inserting and Niagara. Falls, N. Y., va. corporation of 1931, Serial N0. 509,136

holding them in the holes provided for the purpose. The cover piece with its load is placed in such a manner as to bring the ceramic articles in the central portion of the heating chamber and close up the furnace as illustrated in the drawing. The ceramic articles hang in a position in which the action of gravity tends to straighten them out at high temperatures. The ceramic articles experience a gradual change in temperature on introduction to the furnace as they are shielded from direct radiation emitted by the resistor rods 3.

The shielding mechanism disclosed in the drawing consists of two banks of cylindrical rods or bars 5 which rest in a position inclined slightly to the vertical and so that they loosely Contact with each other. These rods are preferably made of silicon carbide because rods of this material, when properly made, retain a large part of their mechanical strength even up to a dazzling white heat. This is particularly true of rods made of recrystallized silicon carbide. While some radiation penetrates between rods 6 it is so scattered by reflection from the tangential surfaces of the rods 6 that the articles 5 are not subjected to appreciable radiation from the resistor rods 3. 'Ihis result follows also from the fact that the resistor rods 3 are at right angles to the screening rods 6.

The resistors 3 are made of reorystallized or bonded silicon carbide. They extend completely across the heating chamber, the end portions being supported in conical recesses 'l which extend slightly into the furnace wail. Mechanical support and electrical engagement are obtained by butt-end engagement with resiliently supported water-cooled terminals (not shown) such as are disclosed in the United States Patent to Shaw, N o. 1,742,286, January 7, 1930.

The cover 4 and inner section of the furnace wall 8 are made of high temperature refractory, such as fused alumina bricks or silicon carbide bricks. The high temperature refractory section of the wall is surrounded by a wall section made of insulating bricks 9 composed of porous clay refractory such as is described in the United States patent to Hartmann, No. 1,545,559, patented July 14, 1925. The porous clay refractory section is surrounded by another insulating section ll made, for example, of Silo-cel bricks. An outer section l2 of low temperature insulating material such as magnesia or asbestos can be provided to further reduce the heat losses. This material is contained within an outer casing I3.

On account of the large heat capacity of the banks of massive rods t and the refractory walls 8 (as well Ias the extensive provisions for insulation) the furnace is very economical of power and emcient in operation.

Our furnace is especially adapted t'o the mass production of ceramic articles which require uni- Vform heat treatment under conditions which extended lengths as showny inthe drawing. After the heat treatment is completed, such extended bodies are cut into suitable lengths. Inthisway it is possible to manufacture short objects of regular shape and sightly appearance. These results would be difcult to obtainv if these shortr objects were heat treated in short lengths.

The silicon carbide shields or bales 6 have many advantages in addition to those already mentioned.4 |'ihey provide a larger radiating surface than would beprovidedby baflies composed of bricks or tiles. They also permit close temperature control of the objects 5 Vby reason of the good thermal conductivity of,y silicon carbide andV its high ernissivity.V They also provide a more uniform distribution of vthe. heat from' the resistors and are readily replaced. r

'The furnace which we have described is especially easy to operate from the fact that when it is opened itV throws the radiation which escapes on removal of the cover in an upward direction where it does not disturb workmen in the Vicinity.

i The ceramic objects to be heat treated can therefore be inserted or removed with a minimum of inconvenience. Deb-ris which falls from objects to be heat treated collects on thev floor of the heating chamber and may be removed through a door iii indicated in Figure 1.

Thev method which we have described may be applied 'to any kind of ceramic' object which is intended to have a substantiallyuniform crossl section. Thermometer tubes and baked clay cyl- 45` inders may be cited Vas examples. Clay bonded articles are vitried previously to the straightening process. The straightening temperature is carried to the initial softening point of the'glass or clay bond. AVVinstead of Vglass or'clay other bonding materials of simpler vcharacter may be used such as calcium fluorideV or other alkaline earth halide, aluminum fluoride, etc. Binary mixtures may also be used as bonds,"such as a mixture of lime, 25-35 per cent and titanium oxide, 65-75 per cent. Bonds of this character are particularly resistant to reducingatmospheresy at temperatures in the neighbourhood of 1,300 C.

The advantages of our method of heat treatment of cylindrical ceramic objectsare particularly obvious when this method'is compared with the usual method of heat'treatment .of such objects while they are supported in a horizontal `position in contactv with horizontally disposed hearths.A En the case of. our' method, on the contrary, ,the portions of the ceramic objects which are to be later used are supported in isolated Vpositions where they are heated only by radiation and their'sides are in contact with gaseous material only, thereby eliminating the liability of injury or disgurementof the objects during insertion or removal.

We claim:

l. A furnace for the heat treatment of ceramic objects comprising a heating chamber, electrical heating resistors within said chamber, a removable cover from which objects to be heat treated are supported in vertically downward position by attachment of their upper ends to the covenand a screen of refractory rods interposed between the electrical resistors and the ceramic objects under heat treatment.

2. A furnace for the heat treatment of ceramic lobjects comprising a heating chamber, electrical heating resistors Within said heating chamber, a removable cover from the lowerV surface of which extend objects to be heat treated while secured to said cover to extend into vertically downward position in said heating chamber, a bank of refractory bars of high thermal conductivity andhigh emissivity placed between said resistors and the objects to be heat treated, a v

wall 'of high temperature refractory surrounding said heating chamber, and Walls of insulating material surrounding said high temperature refractory. Y Y

3. A furnace 'for the heat treatment of ceramic objectscomprising a heating chamber, electrical heating resistors horizontally disposed adjacent to sido walls of said heating chamber, aremovable coverof highly refractory material to which are attached ceramic objects to be heat treated i in a vertical position in said heating chamber; a bank of refractory bars of high thermal conductivity and'high emissivity placed between said resistors and the objects to be heat treated, a wall of high temperature refractory surrounding said heating chamber, and walls of insulating material surrounding said high temperaturek refractory.

4. In an electricy furnace, a furnace chamber, a rod shaped resistor extending along a side wall of the said chamber, and a series offrefractory rods of high thermal conductivity interposed between the resistor and the ware being treated in the furnace, the said rods being adapted to shield the warefrom' the direct radiation of the resistor and being positioned so that their principal axes are Vin a direction substantially atright angles tothe principal axis of the resistor, the rods being adapted to receive heat from the resistor and reradi'ate it from Van area substantially wider than the lateral area of the resistor.

5. In an electric furnace, a furnace chamber, a rod-shaped resistor extending horizontally along a side wall of the said chamber, and a series of refractory rods of substantially circular cross section adjoining each other and interposed between the resistor and the ware being treated in the furnace, the said rods being adapted to shield the ware from direct radiation from the resistor and being in an uprightV position so as to reradiate theV heat from the resistor from an area substantially wider than the lateral area of the resistor.

6. The furnace described in claim 5, in which thev rods shielding the ware from'the resistor are composed of silicon carbide.

ALBERT n. HEYROTH. JAMES I. MILLER.

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