US1784670A - Electric furnace - Google Patents

Description

Dec. 9, 1930.

A. IMBERY ELECTRU: FURNACE 8, 1928 2 Sheets-Sheet l Filed Oct C MW ATTORN EY A. IMBERY ELECTRIC FURNACE Dec. 9, 1930.

8, 1928 2 Sheets-Sheet 2 Filed OCT..

Patented Dec. 9, 1930 UNITED STATES- PATENT GFFICE .ARTHUR IMBERY, F HALIFAX, ENGLAND, ASSIGNOR TG GLOBAR CORPORATION, 0F NIAGARA FALLS, NEW YORK, A. COB-PRATION OF NEW YORK ELECTRIC FURNACE Application led October 8, 1928, Serial No. 310,981, and in Great Britain October 10, 1927.

This invention relates to an electrical furnace containing a plurality of heating elements and a lurality of muil'les, in Whose in,- terior it is esired to obtain a temperature which is made substantially uniform in the interior of the portions of the mullle subjected to heat radiations by (l) the symmetrical distribution of the mules With respect to the heating elements and by the screening effect of the highly conducting Walls of the inutiles which smooth out temperature differences due to Want of uniformity of temperature of the heating elements and their varying positions with respect1 to different parts of given muflles. My invention is useful for example in the heat treatment of mining drills Where a uniform temperature at the working ends is desired.

One difficulty in maintaining the temperature desired for annealingr and similar purposes has been the diiiiculty of: maintaining a plurality of inutiles at the same temperature owing to non-uniform heating as is particularly the case in combustion furnaces. Electrie furnaces are more easily regulated in this respect than combustion furnaces, but Where metallic heating elements are used in commercial practice they' are liable to burn out at points of high local resistance with the result that operations are interrupted during the period of repair and replacement. The upper temperature limit at which metallic resistors can be used is about H000 (lion account of the strong tendency to oxidation in metals that can be used in commercial practice. The teinicratures in a protected muilie heated by metallic resistors Will of course be somewhat l a inutile of large dimensions is used the temperatures of different parts of the 49 mulie Wall Will var considerably on account of their dierent distances from the resistor elements and the length of conducting,l path between a portion of the wall subjected to higher temperatures and another portion of thy. Wall subjected to lower temperatures.

l propose to overcome these difficulties by usingv a plurality of nouilles et small diameter in which (l l the thickness of the Wallis lar je with respec to the diameter and (2) the Wa ls 5@ are made et highly thermal conductingL material such as an iron alloy capable of withstanding temperatures up to 1100 C. or silicon carbide which will withstand temperatures up to 1400 C. under the furnace conditions to be described. The thermal conductivity of silicon carbide is about 0.03 calorie/c'm.3/C./sec at 1000o C. This value, however, depends on the temperature and bonding material. A. value as low as 0.0127 has been published.

I propose also to use non-metallic resistors of large size which will remain rigid at temperatures up to about lll-(lilo C. Resistors of graphite or some other form of carbon may be used. These resistors are however subject to rapid oxidation at MOGo l. and graphite is of too highconductivity for use in short rods of the size desirable for the furnace which I describe below. Silicon carbide is better suited for the purpose on account oit its greater electrical resistivity and its greater resistance lo oxidation. Silicon carbide may be ceramically bonded into rods Which are very strong and resistant to oxidation at high temperatures. Such resistors can be used to attain and maintain temperatures up to lll-00 C. 'l'lie non-metallic heating elements are placed in chambers or ovens of refractory material and :ire arranged in suitable proximity to tubes which er; tend into or through such ovens and receive the bars t0 be treated. These tubes may be made of silicon carbide or elimine iron containin about 27% chromium, or other l'nateriai which Will stand the temperature required. The furnace may be used on either alternating or direct current circuits. The electrical connections to the non-metallic heating elements are effected by metal alloy tern'iinals of a lower electrical resistance than the .poinmetallie resistors. These metal terminals may be so placed that the hout is practically all developed by the electric current `wi '1 the inutile or adjacent theV inner surface of the Inutile wall. The ends of' the non-metallic. resistors may also be enlarged or (it oi silicon carbide) impregnated with silicon te reduce the end resistance ot the resistor rods,

My improved fi :nace is .illustrated by the accompanying drawings in which:

, Figure 1 is a longitudinal vertical section taken on the line 1-1 of Fig. 2.

Figure 2 is a vertical section taken on the line 2-2 of Fig. 1;

Figure 3 is a vertical section of a modied arrangement in which the resistor elements extend in two mutually perpendicular directions and Figure 4 is a section taken on the line 4-4 of Fig.

In the drawing the reference character A indicates non-metallic heating elements, B the chamber or oven, C the tubes or mulles for the reception of the drills to be heated. The oven or chamber B is constructed of refractory material b. This may be of tire.

clap or silicon carbide brick. In the example shown there are three superposed rows of non-metallic heating elements A which extend in a longitudinal direction through the oven. These elements are shown as made in the form of rods and are supported by alloy terminals a which are stationary at one end and maintained in Contact with the nommetallic resistor rods at the other end through the action of springs a to provide for expansion and contraction through heating and cooling The said alloy terminals are as already indicated of lower resistivity than the non-metallic rods, so that the heat developed in them is not so great as in the resistors. The oven or chamber so formed together with the special alloy terminals may be embedded in a brick work or other setting of refractory materials 291 mounted in a metal casing b2 supported on a stand ba. The refractory materiai hl may be a porous clay refractory to give the Walls good insulating qualities. Extending transversely between the rows of nou-metallic heating elements are the tubes C which also extend through the brick work or other refractory setting and through the outer metal casing and are there provided with hinged Haps or doors c which normally close on the tubes by gravity. The bore of each tube is closed at its central portion c1 to provide a stop Vtor the drills, and the length of the tubes together with the thickness of the brickwork setting is such that it is only the centrai portions of the alloy tubes that are heated to the maximum temperature with the result that loss of heat by radiation and through the tube openings (in which the driiis are inserted) is reduced to a minimum. In alternating current circuits a transformer may oe employed with a number of secondary taps in order to provide an adjustable voltage across the heating elements to compensate .for auf change in resistance that may take pia the high operating temperatures to material is subjected and give an life to the heating elements 'with a c iseouent lower maintenance cost. In som.Vx an induction regulator .may be empioyed instead of the transformer to give the desirable adjustments of voltage. The equipment may be provided with automatic temperature control to enable uniform results to be obtained even when the equipment is operated by unskilled labor.

In the modification shown in Figures 3 and 4 two sets of mutually perpendicular resistor rods are used.` Here `certain diaphragms of the tubes C are shown as lying in the plane of one set of resistor rods while each diaphragm is also symmetrically placed with respect to four resistor rods. The tubes may be made of chrome iron (27 per cent chromium) and with walls about 1/2 inch thickness in a tube of 2 inches mean diameter so that temperature differences due to one part of a diaphragm being nearer one rod than another or due to uneven temperature distributions in the rods are smoothed out by the comparatively easy conduction of heat in the metal.

To get some quantitative idea of the effect .of the thickness of the Walls of the tubes as compared with their distance from the axis we cite the following example:

In the case of a cylindrical tube in which the mean radius of the walls is 3 centimeters and their thickness is 1 centimeter the thermal resistance from a square centimeter outer surface exposed to maximum radiation from a resistor rod (for example directly above the unit area) to a unit area on the outer lateral surface (90 from the first` unit area) would be about four times the thermal resistance to flow of heat directly across the wall. If now, the mean radius being kept the same, the thickness of the wall be reduced to l millimeter, the above ratio becomes about 100 times as great, because the length of path for flow directly across the wall is reduced t0 one tenth and the cross-section of the path vfrom one unit area to the other unit arearemoved ninety degrees is reduced to one tenth. Both of these effects work together to increase the ratio referred to. This example shows the larje stiect that thickness of the walls of the tu es have in evening out the temY peratnres of the walls,q different portions of which are exposed to different intensities of radiation.

The central portions of the inutiles are shown supported in the lining of the oven. The outer portions are not shown as integral with the central portions for the reason that l propose to make the outer portions of the tubes (i of firewclay or some other material of thermal coi'uiuctivity ktess than 0.005 calorie/'cm/OC./sec [n this Way the longitudinal conductim ot heat from the middle portion of given inutile can be greatly reduced and the miitormity of temperature ot' the cent-ral portion increased. My furnace vis also well adaptcf tor continuous operation so that the inner portions of the refractory walls are maintained at a temperature sub stantially the same as that of the oven wall b, especially when that is made of silicon carbide. In these various ways the uniformity of the temperature of the muliles is increased.

The furnace which I have described is particularly adapted to give uniform temperature conditions for the cutting ends of drills which are used in large numbers and are of uniform material. The furnace as shown in Figures 1 and 2 affords accommodation for forty rods with the use of any six non-metallic resistors. The drills are protected from the intrusion of foreign material (while being heated) by the surrounding tubes C. These surrounding tubes have their middle portions each at equal distances from four resistor rods so that the medial diaphragms of each of the forty tubes are likely to acquire substantially the same temperature with the possible exception of those nearest the oven walls, subject of courser also to possible variations in the resistor rods from point to point. By making the rods of large diameter and with careful attention to uniformity of material in the manufacture good results may be obtained.

I claim:

1. In an electrical furnace an enclosed chamber, a plurality of rigid resistors each extending substantially across said chamber, and a plurality of muiiles also extending across said chamber, the middle portion of eachmuiile being equidistant from a plurality of said resistors distributed around it.

2. In an electrical furnace for the heat treatment of metal drill rods, an enclosed chamber, a plurality of conducting tubes extending across said chamber and constituting heating chambers for said drill rods, and a plurality of non-metallic heating elements each extending across said chamber and placed adjacent said conducting tubes.

3. The furnace described in claim 2 in which the conducting tubes are all parallel to each other and in which the heating elements are arranged at right an les to the conducting tubes, the middle portion of each conducting tube being centrally placed with respect to the nearest heating elements.

4. The furnace described in claim 2 in which the heating elements are arranged equidistant from each other in parallel rows and in which the conducting tubes are arranged transversely to the heating elements, diaphragms closing the middle portions of the bore of each conducting tube to correctly position the ends of two drills under treatment, said diaphragms being centrally placed with` respect to all of the nearest conductors.

5. In an electrical furnace for heating a plurality of drills, heating chambers for the drills having central diaphragms, resistor rods disposed transversely to the heating chambers, each diaphragm being centrally located with respect to four resistor rods.

.sides of said tubes, diaphragms in each of said tubes locating the central portion of the heated tube and acting as a stop to position -two drills under heat treatment.

7. An electrical furnace containing a plurality of muflles in substantially arallel arrangement, a set of unitary, sel -sustaining resistor elements substantially at right angles to said muilles and extending between terminals of higher conductivity, and a second set of similar resistor elements which extend substantially at right angles to said muflles and to said first set of resistor elements.

8. The electric furnace described in claim 7 in which each metal tube has a central diaphragm'symmetrically situated with respect to a plurality of resistor rods, said diaphragms serving as stops for drill ends to be heat-treated.

9. In an electrical furnace for heatin a plurality of objects, a main heating cham er surrounded by refractory walls; tubes of good thermally conducting material for supporting the objects under treatment within the main heating chamber, whereby the objects in any one tube may be replaced without disturbing materially the objects in the remaining tubes; and a plurality of rigid resistor elements each of which extends across said main heating chamber, said resistor elements being on opposite sides of said conducting tubes.

In testimonyl whereof I aix my signature.

ARTHUR IMBERY.

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