US2725457A - Electric heating device - Google Patents

Description

Nov. 29, 1955 w. H. NORTON ELECTRIC HEATING DEVICE Filed July 1, 1954 ETFEYTZET NOI'L OZ'Z b L Hrn 75 2,725,457 .ELECIR Q HEA NG DEV CE William H. Norton, Chicago, Ill., 'assignor to Thermal,

' inc Chicago, 111., acorporationof Illinois Annl ca ita rlu y 9 4, .SeriaLNc- 4 9 Claims. (31. 219-38) This invention relates no electrical heating units, and more particularly to metal sheath units adaptedfor cooking orlsirnilar heating operations.

in general, .there are two types of cooking units employed. JOne type, known as the" surface type, involves the use of ajheating element mounted on a base of an open refractory support. This type of unit has interest and popularity with the development of refractory thermally conductive and electrically i-nsulative cement, but

United States Patent v :almost invariably this unit leaves something to be desired particularly because 0f thettendency for the heating element to separate from the cement ence s sin :thermal expansion. I "*Ehe other type'of -=unit is known as the metal sheath innit wherein -the hea-ting element 'is encased in and insuilated from a sheet of metal. Such units have enjoyed .ponsiderable populanity because of their comparatively highrefliciency and their-speed of operation. These units have teertain drawbacks, howeverfin that "they are expensive L10 manufacture and they have a tendency -to warp under usage. Both of these difiiculties-are caused to substantial extent tby :the problem of providing a suitable electrically insulative mounting an the resistor or elec- -,t;rical heatingelement.

'ilihe instant invention provides :an improved and simtplifiedmounting arrangement tor the resistor in a metal sheath limit. The assembling expense is materially re- ;duced in :the instant invention by the-provision of sleeve like units which retain ithe resistorhtherein (insulated atherqfirom rbytparticular mountingtmeans.) and -whichmay the shout .or otherwise .shaped as desired to conform with the tnltimate tshape "of :the metal sheath. These units are anounted'in the metalssheathembedded-lin amaterialsuch asgthe {electrically ginsulativerthermally conductive cement, which ;retained in ,the sheath solthat break down or rcraclcing thereof to railimite'd textent (due to the strains retreated byathermallenpansiom) twill in no way impair-the operation of atheunit. "The metal sheath serves to retain gthe .cement in substantially \its original form {and 'the icement senves 1:10 retain the sleeve units in substantially their loriginel tposition, notwithstanding expansion and mntraction during Cheating and .:cooling, respectively. In ,tt dditiwl heat transmission resisting lmeans are provided asp as :to direct the flow lof cheat toward ,a'Theat-i-ng surface iflftlhfirmetfilfiheathfirtoontainer.

it tis, gtherefore, ran importantlobject of this invention .ttoprovidezaniimproved electricrheater.

A'llQfllfil' object of :the tins tant invention is -to provide .Qan improved electric -gheater member which is durable, sturdy, well tprotectcd land -.which is capable of resisting rwarzpage and deterioration :due itD :thermal =expansionand {contraction duting repeatedzuse.

A tiurthertobject of :the ginvention sis to provide an imtpmved electric theater [comprising a resistor, an electritcallyfind :thermally conductive-retainer closely surrounding the :resistor, electrically tinsulative refractory mate- .n ia l {within :the ;retainer maintaining the resistor therein in fil fi y spa d :reletio 1.19 the meta'ner, a container 'rpl'O' "because of their differ- I heating device alloehir-al semi-annular erally aligned strap 2,725,457 Patented Nov. 29, 1955 2 adding a heating surface, and relatively highly thermally conductive refractory material in said container mounting said retainer in heat-exchange relationship with the heating surface.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art in the following detailed disclosure of a preferred embodiment thereof and the drawings attached hereto and made apart hereof.

In-the drawings:

F'gure *1 is a view in perspective showing the general arrangement of --a pair of heating members embodying-the invention assembled foruse;

Figure 2 is an enlarged detail sectional view-taken substantially along the line l I- II of figure 1;

:Figure '3 is a top plan view of a heating member embody-ing the instant invention, with parts broken away and parts shown in section;

Figure 41's a detailed view taken substantially-from the position I-V--IV indicatedin Figure 3;

Figure 5 -is an enlarged detail view,'-with parts shown in section and partsbroken away, --taken substantially along :the line -'V-V of Figure 3; and

Figure 6 is a view in perspective showing a heating member embodying the invention with thecover removed.

A s-shown on "the" drawings:

-Figure 1 shows a heating device indicated se etai y the reference numeral 10 which comprises a pair of heating members 11 and "12 matingly'mounted ;to define an annulus for receiving an article (not shown) to be heated. Such article may be a cooking pot which fits rather snugly in the annular 1-0 and is supported thereby or may be an article which is supported on a plate (not shown) beneath the annular device 10, The heating members '11' and 12 are separa-bly secured together by a periph- 13 of steel or similar refractory materialwhich *is clipped, or =bo l-ted together by suitable means not shown),

It will be noted that electrical leads -14 and -15 for the members 11 and 12,-respectively, extend outwardly from -adjacent ends of the semi-annular heating members '11 and 1-2; and these leads-are, of course, connected to suitable source of electrical energy (not shown) for operation of the instant device. As will be explainedhreinafter, an advantage of the present device' 10 is that the {leads 14 and 15 are closelyiadjacent when the device 10 is assembled in the desired manner (although the'leads a14andq15 are shown here inreduced size).

Referring nowto Figures 2 and 3, it will be seen that the member 12 comprises a resistor 16a, an electrically and thermally conductive retainer 17a made of suitable metal such ga S steel, electrically insulative refractory {hate- -rial 18a (magnesia) within the retainer 17a maintaining the resistor 16a therein in closely spaced relation to the retainer 17a, acontainer 19 proyidinga heating surface -1 9a, and relatively highly thermally conductive refrac- --tory material 2;!) 'in the container {19 mounting the re- -t-ainer zlflajn heat exchange relationship with the hea in surface 1 9 a. Actually, the res stor 16a,

r e Hear insulative;material'18a form part of a unit (X I- igure 6 whichhas-the shape of an elongated "letter L] with the legs "X8 and Xb arcu ately curved to define generallya semiannulus. The cross sectional viewin Figure 2 of thejleg member'Xb shows alsothe corresponding portions .of the resistor 16b, sleeve retainer 17band insulative material 1 817. The container 19 'is actually an open stainless steel -trough the bottom 19a of which forms the heating surface and this open-trough 1-9'is also arcuately curved to conform with thesemi-annul-ar shape of the unit *X there- ;by-provitling the inside semi-annular wall 19a as'the heating surface. In.Eigure-.1,the heating surface IQaean- U not be seen, but the corresponding heating surface H for the member 11 is shown.

A refractory metal cover 21 for the container 19 de fines the outside semi-annular wall 21a for the container 19 and may be considered the back surface of the container assembly 1921. The cover 21 is also preferably made of stainless steel, with extending end fasteners 21b and 210.

The cover 21 is provided with heat transfer resistant means, which include thermally insulative means 22 in the form of sheets of asbestos and heat reflector means 23 in the form of A1 foil sheets, the reflector sheets 23 and the insulator sheets 22 being alternately stacked or superimposed to provide an assembly of heat transfer resistant means positioned intermediate the retainer or sleeve 17a17b and the back surface 21a. Actually, the heat transfer resistant means are stacked againt the back surface 21a in the form of the sheets 22 and 23 here indicated; and the Al foil sheets 23 are so mounted that they present bright reflective surfaces facing the retainer 17a- 17b.

As will be seen from Figures 5 and 6, the electrical leads a and 1512 which are insulatively mounted on the ends of the units legs Xa Xi: and electrically connected to the ends of the resistor legs 16a and 16b extend radially outwardly from the semi-annular member 12 through the cover member 21 and out beyond the back surface 21a. By having both leads 15a and 15b closely adjacent and at the same end of the annular member 12 (and also closely adjacent corresponding leads 14, 14 of the annular member 11) electrical connection is greatly facilitated. It will also be appreciated that the instant assembly is not limited merely to the use of a U shaped unit X, since a serpentine unit presenting a plurality of elongated closely adjacent bodies (comparable to the legs Xa and Xb might be employed. In general, it would be preferable to have an even number of passes" or elongated legs so that the ultimate ends of such a serpentine member would both be at the same end of the semi-annular member, that is, they would be in substantial longitudinal alignment with respect to such semi-cylindrical member surface. As here shown, however, only two passes or legs X9. and Xb are employed and the thermal conductivity of the cement matrix affords a very substantial advantage for the instant member 12 in that it is capable of providing a relatively uniform heat for the heating surface 19a without the necessity for employing any more than two passes or legs X9. and Xb for a unit of this size.

In assembling the instant heating member 12, the elongated helical resistor Wire 16 is suitably positioned within the sleeve member 17 and the magnesia particles or similar particulate refractory electrically insulative materials are forced into the sleeve 17 and compacted therein in order to substantially fix the position of the resistor 16 in spaced relation from the inside walls of the sleeve 17. Magnesia is a preferred material for this purpose because it is refractory and electrically insulative although relatively highly thermally conductive. The sleeve 17 may be bent before or after insertion of the resistor 16 and insulative material 18 therein, but preferably the bending is carried out after the resistor 16 and insulative material 18 have been added to the assembly. This then provides the unit X which is a fixed permanent unit for retaining the resistor wires 16. The leads 15a and 15b may be connected at this time. Repeated heating and cooling of the unit X during operation does not appreciably alter the relative positions of the elements therein. The particulate magnesia will itself have little or no thermal expansion or contraction, because it is in the form of individual particles (even though compacted) and these particles will permit a slight amount of relative movement between the magnesia and the heating wire 16 as well as the tube 17. Such relative movement does not, however, cause structural deterioration of the unit X. Also, the wire 16 and tube 17 are maintained in closely spaced relationship so that the bulk of the heat generated in the wire 16 is transferred immediately to the tube 17, which itself is made of a metal or similar heat conductor.

The terms insulator and conductor" with respect to both heat and electricity are well understood by those skilled in the art, notwithstanding the fact that all materials are generally considered to have at least limited heat and electrical conductivity and no material is considered to be a perfect insulator. For the purpose of simplification in this specification, it will be noted that heat conductors are generally considered to have a thermal conductivity constant k which is defined as the quantity of heat (B. t. u. or calories) that flows in a unit of time (one hour or one second) through a unit area of plate (one sq. ft. or one sq. cm.) of unit thickness (one ft. or one cm.) having unit difference of temperature (1 F. or 1 C.) between its faces. The thermal conductivity of different material varies greatly. For metals and alloys, k is high, while for certain insulating materials such as asbestos, cork, and kapok, k is very low. Herein heat conductors will be considered to have a thermal conductivity constant k of as much as about 10 (in the case of highly resistant alloys used as heating elements) and preferably of as much as about 30; whereas the ordinary insulating materials have heat conductivity constants k" of less than 1 and usually less than 0.1.

In the electrical arts, electrical conductors are usually understood to have resistivity which is defined as the resistance of a sample of the material having both a length and cross section of unity. The resistance of a centimetercube and circular-mil-foot are the two most common units of resistivity. Herein electrical conductors are understood to have resistivities, in micro-ohms per cm. of not more than about 100 (as in the case of the Nichrome" heating elements) and preferably not more than about 20; whereas the resistivity of the insulators are usually considered to be at least about 10 ohms per cm. and preferably 10 ohms per emf.

After the unit X has been assembled, it is placed in the open trough provided by the container 19 (which is also preferably provided with screen elements 24) (Figure 2) extending along the side walls to assist in retaining the cement 20. The container 19 is, of course, also curved or bent so as to be in the form of the desired semi-annular shape. With the unit X mounted therein, the cement 20 is poured into the container 19.

The cement used may be any of the ordinary types of commercially available cements adapted for this use, in that they are electrically insulative but thermally conductive to substantial extent. An example is a composition which is made from parts by weight of zirconia particles, 20 parts of zirconia powder, 8 parts of phosphoric acid and 8 parts of water. Such a cement is ordinarily set at a relatively high temperature, but low temperature cements may also be used including the various high silica cements. Also, it is not necessary to use a cement which is thermally conductive per se, if the cement employed contains embedded therein a substantial amount of particulate thermal conductors. For example, the cement may preferably contain from 5 to 50% aluminum filings or particles in order to enhance the thermal conductivities thereof. Since the resistor wire 16 is spaced from the walls of the sleeve or tube 17, it is not really necessary to employ a cement which is an electrical insulator, but this is preferred for safety reasons.

The cover 21 is also an open trough of the size necessary to slip down over the top of the container 19. The cover 21 is also made of stainless steel preferably and in assembly it is bent to the desired shape and the alternate sheets 22 and 23 of insulating material to limit heat conduction, such as asbestos, and highly reflective sheets of material to return to the source the radiant transfer of energy, such as aluminum or tin foils, polished steel, etc., are stacked against the top of the cover 21 and the cover 21' is then placed over the container 19 filled with swash-57 .Qement 30. ,1: will also be noted that the cement .20 xten s pwardly above the 111111 X a distance of ab teonehalf the thickness .of the tUllit X, but the cement 120 forms only a thimfilm (preferably) between the :heating surface .191: and the :unitX. Actually, better heat distribution is obtained by employing this slight film of cement 20 he- .tmeen :the unit X and the heating surface 194 (than sit the unit touched the .back of the heating surface .1-9a;) because 10f the ,heat conductive character of cement '20.

The cement ,20 is retained in the container 1 9 *sozthat .it remains :as :a monolithic body, ';but;acertain amount of cracking or other moderate deterioration, which could not be tolerated in an :open surfaced unfit, may .take place while the cement is retained in the metal sheath 19-21 without causing :a failure of the cement 72010 carry out its various functions. The various strains *which may be created through thermal expansion and contraction .of the .unit X and the sheath member '19'21 and in the cement 20 itself may thus be relieved without causing a breakdown of the unit.

:It will be \understood that modifications and variations maybe effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. In an electric heater, in combination, a resistor, an electrically and thermally conductive retainer closely surrounding the resistor, electrically insulative refractory material within the retainer maintaining the resistor therein in closely spaced relation to the retainer, a container providing a heating surface and a back surface, thermally insulative means positioned adjacent said back surface, and relatively highly thermally conductive refractory material in said container mounting said retainer in heatexchange relationship with the heating surface.

2. In an electric heater, in combination, a resistor, an electrically and thermally conductive retainer closely surrounding the resistor, electrically insulative refractory material within the retainer maintaining the resistor therein in closely spaced relation to the retainer, a container providing a heating surface and a back surface, relatively highly thermally conductive refractory material in said container mounting said retainer in heat exchange relationship with the heating surface, and heat transfer resistant means in said container intermediate said retainer and said back surface.

3. In an electric heater, in combination, a resistor, an electrically and thermally conductive retainer closely surrounding the resistor, electrically insulative refractory material within the retainer maintaining the resistor therein in closely spaced relation to the retainer, a container providing a heating surface and a back surface relatively highly thermally conductive refractory material in said container mounting said retainer in heat-exchange relationship with the heating surface, and heat transfer resistant means in said container intermediate said retainer and said back surface, including thermally insulative material and heat reflector means facing said retainer.

4. In an electric heater, in combination, an elongated resistor element, a refractory metal sleeve closely surrounding the resistor, compacted particulate refractory electrically insulative and relatively highly thermally conductive material retaining the resistor in the sleeve in closely spaced relation to each other, a refractory metal container providing a heating surface, a cover on the container providing a back surface therefor, a refractory electrically insulative and relatively highly thermally conductive matrix in said container retaining the sleeve embedded therein in heatexchange relationship with said heating surface, and heat transfer resistant means in said container intermediate said sleeve and said back surface including a thermally insulative sheet and a heat reflector facing said sleeve.

5. In an electric heater, in combination, an elongated resistor element, a refractory metal sleeve closely surrounding the resistor, compacted particulate refractory el tr cally, nsulative and relative ;)'hi hl thermally conductive material retaining the resistor in in c osely p c d relation to each othe a retracted metal container Providing a hea ng surfa ea cover th cont iner prov ing a ack surfa e here r, at e- .fractory electrically insulative and relatively high t-lermally c nd c ive m rix in said container re aining th sleeve embedded therein in heat-exchange relationship said heating surface, and heat transfer resistant means in said container intermediate said sleeve and said back surface including superimposed asbestos fibettts nd m l foil sheet Pr senting hnf-gh surfaces fa ing the sleeve.

6.. ,In an electric -heater, in combination, an elongated resistor element, a refractory :metal sleeve closely sur- 0unding the resistor, compacted particulate Iififractory electrically insulative and relatively high-Ly tthq tmflil gy conductive material retaining the resistor ,in the sleeve in closely spaced relation to each other, a refractory metal container providing a :heating surface :3 cover on the container providing a back surface therefor, an a {refractory electrically insulative and rel ti ely vh h y thermally conductive cement matrix containing aluminum particles embedded therein in said container retaining the sleeve embedded therein in heat-exchange relationship with said heating surface.

7. In an electric heater, in combination, an elongated resistor element, a refractory metal sleeve closely surrounding the resistor compacted particulate refractory electrically insulative and relatively highly thermally conductive material retaining the resistor in the sleeve in closely spaced relation to each other, said sleeve with the resistor and the material mounted therein forming a unit having the shape of an elongated letter U with the legs arcuately curved to define generally a semi-annulus, a refractory metal container of semi-annular shape presenting an inside semi-annular wall as a heating surface, a refractory metal cover for the container defining the outside semi-annular wall therefor, a refractory electrically insulative and relatively highly thermally conductive matrix in said container retaining the sleeve embedded therein in heat-exchange relationship with said heating surface, and heat transfer resistant means lining the inside of said cover.

8. In an electric heater, a pair of allochiral semi-annular heating members matingly mounted to define an annulus for receiving an article to be heated, each of said members comprising an elongated helical resistor, a refractory metal sleeve closely surrounding the resistor, compacted particulate refractory electrically insulative and relatively highly thermally conductive material retaining the resistor in the sleeve in closely spaced relation to each other, said sleeve with the resistor and the material mounted therein forming a unit having the shape of an elongated letter U with the legs arcuately curved to define generally a semi-annulus, a refractory metal container of Semi-annular shape presenting an inside semi-annular wall as a heating surface, a refractory metal cover for the container defining the outside s'emi-annular Wall therefor, a refractory electrically insulative and relatively highly thermally conductive matrix in said container retaining the sleeve embedded therein in heat-exchange relationship with said heating surface, and heat transfer resistant means lining the inside of said cover.

9. In an electric heater, a pair of allochiral semi-annular heating members separably secured together by a peripherally aligned strap to define an annulus for re ceiving an article to be heated, each of said members comprising an elongated helical resistor, a refractory metal sleeve closely surrounding the resistor, compacted particulate refractory electrically insulative and relatively highly thermally conductive material retaining the resistor in the sleeve in closely spaced relation to each other, said sleeve with the resistor and the material mounted therein forming a unit having the shape of an elongated letter U with the legs arcuately curved to define generally a semi-annulus, a refractory metal container of semi-annular shape presenting an inside semiannular wall as a heating surface, a refractory metal cover for the container defining the outside semi-annular wall therefor, electric leads extending outwardly through one end of the cover from both ends of the legs of said resistor, a refractory electrically insulative and relatively highly thermally conductive matrix in said container retaining the sleeve embedded therein in heat exchange relationship with said heating surface, and heat transfer resistant means lining the inside of said cover, said members being positioned so that the electric leads of each are at adjacent ends.

10. In an electric heater, in combination, an elongated resistor element, a refractory metal sleeve closely surrounding the resistor, compacted particulate refractory electrically insulative and relatively highly thermally conductive material retaining the resistor in the sleeve in closely spaced relation to each other, said sleeve with the resistor and the material mounted therein forming a unit presenting a plurality of closely adjacent elongated bodies lying in a generally semi-cylindrical surface with both ends of the unit being in substantially longitudinal alignment with respect to such semi-cylindrical surface, a refractory metal container of semi-annular shape presenting an inside semi-annular wall as a heating surface, a refractory metal cover for the container defining the outside semi-annular wall therefor, a refractory electrically insulative and relatively highly theremally conductive matrix in said container retaining the sleeve embedded therein in heat-exchange relationship with said heating surface, and heat transfer resistant means lining the inside of said cover.

References Cited in the file of this patent UNITED STATES PATENTS

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