US2020127A - Electric heating resistor and mounting therefor - Google Patents

Description

1U P... U nu nu nu nu N. R= 'sTANsEL ELECTRIC HEATING lREZSISTOR AND MOUNTING THEREFOR Filed Nov. 2, 1952 ig. lo.

Inventor Numan 'R StanSeI,v

Attorheg.

'H is Patented Nov. i 5," 1935 morale uname nssrs'ron AND nomme. runas-ron A smania. sum1, Schenectady, N. Y., minor to General Electric Company.. a corporation of New York scrum.. (cms-:si

My invention relates to electric' heating resistors, suchas used in electric furnaces and other electric heating devices; and has for its obiect a simple and reliable resistor and mounting there-f s for which for a given resistor temperature dissipates heat at -a greater aggregate rate than the forms of resistor now in use, whereby lower resistor temperatures can be used for a given heating .chamber temperature or conversely higher l heating chamber temperatures for a given resistor temperature.

In electric heating devices of the resistor type the resistor operates at a higher temperature than that of the heating chamber, the value of l this temperature gradient being determined by the surface area of the resistor. The heat re-` sisting property of .the material of the resistor it- `self is the fundamental factor which limits the maximum operating temperature of the resistor.

2 0 That is, for eachmateriai used as a resistor there is a temperature above which the resistor is either' destroyed or rendered unt for further use. For

. example, alloys begin to .soften and sag at a tem- Derature peculiar to each alloy. With the nickel chromium alloy which is in general use for resisters because of its feature of self-protectionagainst oxidation at elevated temperatures, this protection becomes less and less as the temperature oi' the resistor increases. Hence in all cases there is an upper limit of operating temperature for the Iresistor and when using the nickel chromiumjalloy it is desirable tooperate the resistor at as' low a temperature ,as will give the needed rate of heat transfer to the furnace. It is furthermore desirable tooperate a resistor at the lowest possible temperature because of the risk of overheating exposed projections and edges of the charge if the temperature of the resistor'is considerably in excess of the temperature of the 40 heating chamber., Furthermore, by thus decreasing the temperature gradient between the resistor and the heating chamber, i. e. between the resistor and the gaseous or other medium surroundingthe resistorfmy invention provides a means of using a higher heatingchamber temperature with a resistor of a given material.

In accordance with my invention I increase thel surface area of the resistor by means of'a pluralityof projections or fins whichare secured to the'resistor along its length in intimate' thermal relation with the resistor and in such manner as not to increase the cross sectional area 'of thev current carrying part of the resistor to any a preciable extent. That is. the 'surface area of e resistor-is increased without appreciabledecrease in its electrical resistance and, therefore, without appreciable increase in the rate of heat generation in the resistor for a given applied voltage. This increase in the surface area of the Aresistor without an increase in'its rate of heat' genera-A tion decreases the rate ci heat dissipation per unit surface area of the resistor surface and thus, all

. other conditions being the same, lowers the operating temperature of the resistor.`

The projections may be of the material of the 1| resistor or of another materialI as may be best suited in each case. For example. with a nonoxidizing atmosphere in the heating chamber and for temperatures say up to 1600 deg. F. the current carrying portion of the resistor may be a il nickel chromium alloy and the projections of copper to take advantage .of the comparatively high thermal-conductivityof the latter metal. Also the proportion between the added surface area and -the` original surface area of the resistor can 2| be variedbetwee'n wide limitsas may be'best suited in each case. y

My invention is not limited to any particular shape of the current carrying portion of the resistor, that is, the current carrying portion of the 2i resistor may be rectangular, round, or any par-- ticular shape desired.

For a more complete understanding of., my invention, reference should be'had to the accompanying drawing in which Fig. l is a sectional 3( view of an electric resistor furnace embodying my invention; Fig. 2 is a. sectional view of Fig. 1 takenalong the line 2-2 looking'in the direction of the-arrows; Fig. 3 is an enlarged fragmentary sectional view sho details of con- 3i struction of the resistor shown in Figs. 1 and 2: Fig. 4 is a sectional view taken along the line I--4 of Fig. 3 looking in the direction of the arrows:

. Fig. 5 is an enlarged fragmentary sectional view showings modmed construction simmer to that 4 of Figs. 3 and 4 but with the current carrying element of the resistor mounted on,` the inner surface of the wall of the heating chamber: Fig. 6 is a sectional view taken along the line 6-6 of Fig. 5 looking in the direction of the arrows; a Figs. '1 and' 8 are enlarged fragmentary sectional views similar toll'igs. 3 and 4 showing another arrangement of the projections or nns. while Figs. 9 and lil are similar enlarged fragmentary sectional views showing the application of my W invention to the conventional resistor mounted on the inner surfaces of the -heating chamber. Referring to Figs.- l-4, inclusive, of the drawing, Ihave shown my invention in'one form in connection with an electric furnace comprising wall of the furnace.

,the temperature of the furnace.

AAs shown, the resistor is embedded in the side wall, although it may also be applied to the top or bottom walls. The resistor comprises an elongated conductor I2 of -uniform [cross sectional area throughout its length in which heat is generated; This conductor I2 has a rectangular cross section, as shown in Fig. 4, i. e. it is ribbon-like` in shape, although other shapes such as a round wire may be used; As shown, this resistor., formed in vertical loops, is placed in a closely fitting recess, or groove, in the side wall of -the furnace so that its outer surface is hush with the inner Heretofore, such mountings for the resistance conductor have been prohibitive because of the obstruction o'ered by the furnace wall to heat dissipation from the faces of the resistor in contact with the wall, vby reason of which these embedded surfaces operate at higher temperatures than the inner surface exposed to the interior of the furnace chamber from which heat is readily dissipated. As a result, the furnace temperature is limited to that temperature which can be obtained by operating the embedded portions of the resistor at their maximum permissible temperatures. The portion of the sur face of the resistor exposed to the interior of the furnace will thus operateA at a lower temperature than the muffled portion. It is obvious, therefore, that if heat were dissipated as freely from the portions in contact with the furnace wall as from the exposed portion, the entire resistor could be operated at its maximum permissible temperature with consequent increase in In order to increase the rate of heat dissipation from the embedded portion -of the heating resistor, I provide a plurality of projections Il on this embedded portion, which projections are in direct contact, or otherwise in good thermal relation with the furnace wall. Asshown. these projections are rectangular in shape, having a thickness not greater than the thickness of the resistor ribbon l2, so as to providefor a large surface area for heat dissipation forv4 a given vol'- ume of metal. These projections are preferably integral .with the resistor ribbon. and they are amused in va series in alinement and'spaeed `apart along the center ofthe rear surface of the,

resistor ribbon and extending throughout 'its' length. The resistor may be constructed, for ex mph. by' nrst forming a resistor of continuous T section, short portions of the upright bar of the T being thereafter cut out to form the spaces and thus leave the projections I3. Alsothe resistor may be constructed by ilrst forming the ribbon I2 as a separate part and thereafter welding or otherwise suitably securing the projections to the ribbon in good thermal relation therewith and at the desired spaced intervals.y 1/

- It will be observed that the projections, while very greatly increasing the surface area of the' embedded portion of the resistor, donot appreciably decrease the total resistance of the resistor for the reason that any current flowing infthe shunt path formed by each projection has a longer' path than through the ribbon. and consequentlyv a path of greater resistance than through the Y v concerned the lining together with the exposed ribbon itself, for equivalent cross-sectional areas.

It will further be observed that thearea of the point of junction between each heat-radiating projections I3 and the conductos l! is small as compared with the area of that p0rtion of the conductor extending between the ends of each of the members. Consequently. while theoretically there will be some current flow through the projections, it is comparatively small in amount. vIn other words, by reason of the ins creased length of the conducting path through the projections the current density in the projectionsis very low as compared with that in the ribbon. The surface area afforded by the projections may be increased by increasing the height of the projections, that is their dimension in a direction vtransverse to the resistor and without y appreciable decrease inits resistance. As shown the total length of the projections measured lengthwise of the resistor-is substantially greater l5 than one-half the length of the resistor. This relation facilitates the conduction of heat from all parts of the heat generating ribbon portion to the projections.

In the operation of the furnace the resistor is 20- connected to a suitable supply source in the con- 'ventional manner whereby current is passed through it sufllcient to heat it to the desired temperature. The projections I3 by reason of their increase in the surface area of the resistor pro- 2,5 vide for increased ability in the resistor to dissipate heat by conduction to the furnace wall. In other words, assuming a given amount of heat to be dissipated the increased area aorded by the projections serves to decrease the amount of 3,0 heat dissipation per unit area of the resistor, the result being that the resistor-operates at a lower temperature than would be the case without the projections, assuming the same watts input. to the resistor in each case. 35 One object of my invention is to increase the extent of the diffusion within the inner lining by transfer of vheat from the resistor to the inner lining by conduction, a feature which is accom fpllshed by 'mounting the resistor so as to obtain 40 contact of the projections with the material of the inner lining. `It will be noted that the formation of an envelope of heat in this manner is not anected bythe'volume of the charge. Hence less -carein the loading of the charge is required and the working capacity of a heating chamber of given dimensionsis increased. Obviously the higher tbe thermometric conductivity (or diffuslvlty) ofthe material of the inner lining of. the heating chamber the more edective is the above W To this end the furnace walls are constructed in two layers, an outer layer or wall Il of thermal insulating material, such as diatomite, and

an inner lining l5 in'which the resistor is mount- 55 edmade of a material having relatively good thermal conductivity as compared with the outer wall, such as silicon carbide or fire brick. (The voltage used for the resistor is selected to correspond with the electrical conductivity of the par- 60 surface of the resistor forms a heat radiating envelope' which( is not affected by the disposition or size of the charge.

In Figs. 5 and 6 I have shown a form ofmy 7l invention in which the ribbon pm i1 of the resistor is mounted against the inner face ofK the applied to a ribbon resistor 22 which is supported in the furnace in spaced relation to the furnace wall Il, as shown forv example in the McFarland Patent No. 1,661,842, dated March 6, 1928. In this case the projections 24 are provided on the inner edge of the ribbon, i. e. on the e'dge adjacent to the heating chamber, and consequently the projections are free to dissipate heat to the same surrounding medium as the ribbonl por,- tion 22.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made. and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

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

1. In an electric furnace, an electric heating resistor comprising a continuous elongated conductor made of resistance heating material, a plurality of heat dissipating projections secured to said conductor and extending laterally therefrom to provide increased heat dissipating area for said conductor without substantially increasing the rate of heat generation ofv said resisten and electrically insulating supporting means for l its surrounding medium. l

2. In an electric heating device, outer walls forming a heating chamber made of thermal insulating material, an inner lining for said outer walls formed of a material'having a relatively good thermal conductivityv as compared with said outer walls. an elongated resistance heating conductor at least partially imbedded lin said liningv in good thermal relation therewith. heat-dissipating projections on said conductor spaced apart intermediate the ends of said conductor, said projections extending into said inner lining to thereby increase the rate of heat transfer from said resistance conductor to said wall without substantially decreasing the resistance of said conductor, said lining and the exposed portion of said resistor forming a heat-radiating envelope for the charge in said chamber.

3.l In an electric heating device, heat refractory walls forming a heating chamber, a lining for at least one of said walls made of a material having relatively good thermal conductivity as compared with said walls, an elongated resistance heating conductor mounted on said lining, and a plurality of heat-dissipating projections spaced at intervals on said conductor intermediate its ends in intimate thermal relation therewith andextending into said lining, -sald projections being arranged to extend laterally into said lining so as to provide increased area for the dissipation of heat'from said conductor to said lining without substantially increasing the heat-generating area of said conductor. i

NUMAN n. s'rANsm..

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