US2662158A - Heating unit and method of making the same - Google Patents

Description

DCC 8, 1953 B. J. vALLoRANl ET AL 2,662,158

HEATING UNIT AND METHOD OF MAKING THE SAME Filed July 28 1951 Patented Dec. 8, 1953 HEATING Um'r AND METHOD OF MAKING `THE Ben. J. Vallorani, ,Melrose Park, and Clarence J- McConnell, Chicago, Ill., assignors to General Electricv Company, a corporation of New York Application July 28, 1951, Serial No. 239,118 6 Claims. (Cl. 219. 37)

This invention relates to an improved electric range surface heating unit and to the mlfllOd of making the same.

Range vsurface heating units .of present day design generally .utilize heating elements of :the sheathed conductor type. in the manufacture of such elements, a Ahelix of resistance wire 1s positioned coaxially within a metallic tube vor sheath, which is then filled with granular electrically insulating and heat conducting material such as finely powdered magnesium oxide, After forming the sheath into a hat spiral ,of -,desired radius, it is operated on to transform the cross section of Ithe Vsheath from its initially circular to a flat-topped shape. During :this llattening operation the magnesium oxide is compacted -into a hard, dense, body; and ,care is taken -to insure that `the density of lthe insulating material -is uniform throughout the length -of the sheath so that uniform dielectric strength will be ojbtained, `and the heat transfer from the helix to the sheath will be uniform. Patent No. 2,094,480 granted September 28, 1;937, v.to C). G. Vogel for Electric Heater, assigned to lour present assignee, explains such heating `elementconstruction `in greater detail. r[he :flattened coil is mounted on a spider or other support arm structure kand suitably -secured thereto.

It is obvious that the greater lthe :total vflattop area, the `:better will be the heat transfer to the cooking vessel yplaced thereon. Contemporary workers have sought increased heat ltransfer area by using ,sheath ,stock which gives great width .per unit kof length; but it 'has :been our experience that although .the eiciency of such elements is satisfactory .after they have ,reached operating temperature, the large mass iof the element retards its response, 'making Lit slow'to heat and slow to cool. In practicing our inF Vention, however, we utilize 'a smaller diameter sheath than :any which has heretofore .come :to our attention, and hatten it to a greater degree than has previously been thought advisable; fact, lthe heat transfer ,surface is ualmost 'equal to the inside diameter of the v shea-th. vfllhe small volume of :the heating funit VLper unit of mass and the Aextreme flatness .gives jamazingly quiol; heat response :and transfer to .a cooking vessel placed thereon. The .cross sectional shape of thesheath of our heating element vIna-kes the dat, spirally shaped element `inherently stables-,that is, it maintains its flatness with respect to the Support .Structure- We also insure '.fflatness AzWl1-i-1.e in actual operation by .securing :the lelement to the support arms et one @point instead of at .a plurality of points along its length as is now customary. This single point o f attachment permits expansion and contraction of the sheath during heating and cooling without danger of a portion between spaced securement vpoints Vlifting above the vcommon plane. We prefer to clamp the element to .the Support str-ucture by an an.- ohor member having jaws which grip the sides of the sheath. However, in our .construction the transition between the substantially semi-.circular .base portion of the sheath Vand the flat top is so abrupt that there is inadequate surface to be gripped by the jaws, and We specially shape a small portion of the .sheath to provide sloping shoulders which ,the jaws may engage. It' is one of the important featuresV of our invention that we produce this configuration while main.- taining the runiform density of .the insulation material.

Among the objects Vof our invention .therefore are: to provide a surface heating unit having a heating element which will rapidly attain Ydesired operating temperature and have 'superior heat transfer qualities; to provide a sheathed conductor heating element having a flat heat transfer surface of substantially maximum width relative rto the .original diameter of the sheath; to .provide a -heating .element .of the sheathed conductor type which may ybe for-med into Aa flat spiral and then flattened to such a degree that it is highly resistant to .departure from the flat condition; vto provide a method of vmanu-factur-'e of a tubular sheathed heating element wherebya very `small portion ,of the length of ythe heating element may be specially deformed, without materially Achanging the density of vthe granular insulation material therein Vwith respect to that of the remainder of the unit, to provide shoulder portions at which the unit may be secured to a supporting structure; and :to provide an improved means for securing a spirally arranged heating element to a `supporting structure to permit free expansion and contraction of the element.

.In the accompanying drawing, Which shows a present-1y ,preferred embodiment of our invention, Fig. f1` is a .plan view of a range surface heating unit, ycertain portions thereof vbeing broken -away to show underlying structure; Fig'. 2 -is a lside sectional elevation taken onv x lines 2-.-'2 of Fig. l; Fig. 3 lis a fragmentary end elevation showing the terminal portions of the heating element and the individual drip shields applied thereto; Fig. 4 is .a Aperspective of av form `,of hinge and .ground strap; Fig-5 is a somewhat J schematic view showing a method of flattening the coiled heating element; Fig. 6 is an enlarged sectional elevation of the heating element at the point of attachment thereof to the support spider; and Fig. 7 is an enlarged sectional elevation representative of the cross section of the remainder of the operative portion of the element.

Referring now to Figs. 1 and 2, the surface heating unit includes a first heating element I having the respective terminal portions 2 and 3, and a, second heating element 4 having the terminal portions 5 and 6. It will be seen that the operative portions of said heating element areA formed into flat spiral configuration, and that the respective elements are nested one within the other. Together they provide a substantial area of transfer surface substantially uniformly disposed over the area of a circle which may be of 6", 8, or even larger diameter. Six inch and eight inch surface heating units are relatively conventional on domestic electric ranges.

As indicated in Fig. 2, the respective terminal portions 2, 3, 5, and 6 are offset so as to occupy a common plane; and each has a projecting terminal to which is secured a terminal piece, respectively 2a, 3a, 5a, and 6a, for connection with the electrical wiring (not shown) of the range. Pursuant to one commonly known system providing for various circuit combinations of the units I and 4, theterminals 2 and 5 are interconnectedy as by the bridging strip 1.

The heating elements per se are of the sheathed conductor type such as described in the aforesaid Vogel Patent 2,094,480; briefly, they comprise a tubular metallic sheath B within which is a resistance wire element 9 which has been wound on a mandrel to provide desired thermal output per unit of length. The resulting helix is maintained in uniform spaced relation with respect to the sheath by a densely7 compacted mass I0 of electrically insulating and heat transmitting material such as finely powdered magnesium oxide.

The flat, spirally arranged, heating elements are supported on a frame or a spider I I formed in any suitable manner, as by interconnecting several radially extending plate members as shown in Fig. 2. The radially outermost ends of the members have tips I2 which project through suitable apertures provided in the downwardly depending wall I4 of a trim ring I5 which fits about an opening in the range cooking top I6 and rests upon said top. The range top opening has a downwardly extending flange I1 terminating in a horizontal wall portion I'Ia. The latter provides a support for the removable refiector pan IB, which has a wall opening I9 which permits the passage of the respective terminal portions of the heater elements. Thus, connections to the terminals may be positioned well behind the range top opening. Usually, the terminal connections are covered by a block (not shown) of insulation material. To prevent drippage along the sheath from reaching the connections, there are employed individual drip shields 2b, 3b, 5b, and 6b, tightly secured about the respective terminal sheathed portions. Each shield has a d ownwardly depending tip as shown in Fig. 3. The upper edges of the spider arms lie in a common plane and the engagement of the trim ring I5 with the cooking top I5 is such that said plane is horizontal.

Pursuant to con'entional practice, spider II is hingedly rrounted relative to the cooking top. A suitable hinge structure 20, Fig. 4, is more fully 'of a typical finished unit.

described in the presently pending application of Ben J. Vallorani, Serial No. 239,117, filed July 28, 1951 and assigned to our present assignee. Suffice it to say that a rigid hinge arm 2I is securely fastened to one of the spider plates IIa and is pivotally attached to a bracket 22 which fastens to the flange I'I of the cooking top opening by means of a sheet metal screw orequivalent 23, Fig. 2, passing between the upstanding legs 24,- 24al of said bracket. To ground the sheath of the respective heater elements I and 4, the hinge has a ground strap 25 attached to the arm 2|. Flexible metallic strips 26, 25a have downwardly extending ends which are electrically connected to the respective drip shields 2b and 5b. The hinge permits the entire unit to be swung upwardly as indicated in dotted line in Fig. 2.

As previously stated, we employ an unusually small diameter sheath and flatten it to maximum degree. For example, the sheath tubing 8 is initially .260 outside diameter, and of .018 wall thickness. In the manufacture of the heating element, the helix 9 of resistance wire is suspended within the vertically supported sheath tube in coaxial relation, whereupon the sheath is` charged with insulating powder, This work is performed by machines such as disclosed in the J. L. Andrews Patent 2,316,659, granted April 13, 1943, for Apparatus for Loading Sheath Wire Heating Units. The sheath intermediate the terminal portions is then rolled by suitable means to reduce the diameter to approximately .240", and the terminal ends are reduced, by swaging or other suitable means, to approximately .220 diameter. As a result of these operations, the magnesium oxide filling material I0 has become compacted and the loaded sheath may be handled through the next steps without danger that the helix will depart appreciably from its coaxial relationship.

After annealing, the straight sheath is coiled into spiral form, The spirally shaped sheath, which is still of substantially circular cross section, is placed within a die 28 having a continuous spiral groove by means of which the desired cross sectional shape is given to the sheath. The deformation of the sheath is acomplished by a press member 3I. Fig. 5 shows the extent of the deformation from initially round cross section, indicated in dotted line, to the cross section indicated in the full line in said figure.

Throughout substantially the entire length of the heater element intermediate the terminal portions the final shape of the sheath 8 and resistance element 9 is as shown in Fig. 7. The lower portion of the sheath is substantially semicircular with respect to the diametrical line D, which is usually somewhat below the flat top surface. The maximum outside diameter of the sheath is then of the order of .280z the final sheath is thus slightly larger in diameter than the initial tube stock. It will be noted particularly from Fig. 7 that the dimension d of the heat transfer surface 32 of the heating element is closely approximate the inside diameter of the sheath; it is in fact within a few thousandths of an inch of said diameter. Measurements of actual surface heating units constructed pursuant to our invention show that said surface dimension averages .220 in width whereas the maximum inside diameter is approximately .245. It will be understood that the completed sheath is not always truly symmetrical, and the foregoing figures represent average dimensions Surface 32 .is the medium through which the heat of the insulating material transfers to the bottom of a cooking vessel (not shown) placed on the unit, and it will be evident that our construction provides almost maximum area of direct contact of said heating surface 32 with the insulation mass I0 and the vessel, as compared with present constructions typified by the said Vogel patent. The improved heat transfer characteristic resulting from said large direct contact reduces the operating temperature of the resistance element and prolongs its life.

Another` important attribute resulting from this extreme flattening of the initially tubular sheath is that the spirally coiled and flattened sheath has remarkable resistance to departure from the flat state. Thus, when it is considered that the die member 28 accurately establishes the vertical dimension of the sheath, it is seen that when the coiled sheath is placed upon the supporting spider il its fiat heat transfer surface will lie in the same horizontal plane within very close limits.

The horizontal plane stability of the spiral units is extremely advantageous, in that contrary to earlier practices in which it was thought necessary to secure the sheath to the supporting spider at a plurality of places along the length of the sheath it is necessary to secure the instant sheath at only one point. This permits the heating elements to coil and uncoil relatively freely as they expand and contract with changes in operating temperature. This movement is not resisted in any practical degree by the terminal connections because it is apparent that the normal flexibility of the range Wiring at its attachment to the connections 3a and 6a particularly, will permit the coil movement. An additional important advantage of securing the heating elements at only one point is that there is no possibility for the elements to buckle intermediate their length as is the case When an element is secured at more than one point. Expansion of the elements between securement points may raise a portion of the heating element above the initially common plane and tilt the cooking vessel out of direct heat transfer relation with a large portion of the heating element. We prefer to stake the respective heating elements I and 4 to one of the arms l Ib of the spider I I because in our opinion such construction is more secure than hook-like attachments which must be welded to the sheath and brought through openings in the arms of the support spider. Accordingly, we Weld or otherwise affix to arm I Ib an anchor member 33 having the respective pairs of jaws 33a and 33h. It will be apparent from Fig. 6 that the very small radius of transition between the side wall and the top Wall of sheath 8 affords insufficient Wall area for engagement by the jaws of the anchor 33. Accordingly, at the location where the elements l and 4 are to be staked to the spider we deform the sheath to provide walls 34 which slope angularly from the upper surface of the sheath to the main body portion thereof.

These walls offer ample area to be engaged by the respective jaw members of the anchor and securely lock the elements against vertical separation. As Fig. 1 indicates, the sheath deformation at the staking points is localized, and the side wall of the sheath changes from the Fig. 6 to the Fig. '7 shape relatively quickly. Therefore, the sheath configuration almost immediately on either side of the anchor is incompatible withv the 6 angle and arrangement of the anchor jaws, and it is impossible for the sheath to shift transversely of the anchor member to any appreciable degree.

We have previously mentioned herein that the density of the insulation material should be uniform throughout the length of the heatingl element so as to maintain uniform dielectric strength. With particular respect to the oper'- ating portion of the heating element, uniform density is also necessary to insure uniform heat transfer from the helix to the sheath. It is a particular feature of our invention that we maintain such uniform density even at the anchorage points by specially shaping the sheath to maintain substantially uniform cross sectional area without change in the essential dimensions of the sheath.

As shown in Fig. 5, We provide the press member 3| with the shoulder-forming parts 3m and form the die groove 29 with a compound curvature characterized by shorter radius side wall portions and a narrow flat bottom portion. For example, the radius R of groove E@ is .140 representing a maximum diameter of .280 as previously noted. The radii R2 of groove 29 are .125, spaced uniformly on either side of the vertical center line of the sheath to produce the over-al1 transverse dimension of .286. In each case the vertical dimension of each sheath form is .181. These dimensions are held within close tolerances. So by modifying the configuration of the anchorage portion of the sheath while maintaining the same area and essential dimensions, the filling of granular insulation material within the anchorage area has the same density as in the remainder of the heating unit.

It is obvious that the shape of the sheath in Fig. 6 is slightly less eiiicient for .heat transfer than that of Fig. 7 in that there is smaller working surface in contact with the cooking vessel and the body of insulation, and it might be thought that a localized hot spot may be created. However, heat transfer from the shoulders of the sheath through the jaws supplements the otherwise deficient working surface and prevents any abnormal temperature rise in the securement area.

summarizing, we have devised a tubular sheath heating element of small volume per unit of mass and large effective transfer area; have provided a heating element that is dimensionally stable and resistant to departure from its fiat coil form; and have provided an improved means for securing the element to a supporting structure in a manner permitting coiling and uncoiling of the element as it undergoes temperature changes Without substantial departure of the heating surface from its established plane. These qualities result in rapid response of the sheath to the temperature rise of the resistance element during the heating up period; improve the heat transfer from the element to the cooking vessel; and maintains the heat transfer contact throughout the cooking period.

While there has been described what is at present considered to be the preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall Within the true spirit and scope of the invention.

We claim:

1. A surface heating unit, including a support structure having a plurality of radially arranged vertical plate members, the upper edges of which occupy a common plane, a tubular sheathed heating element of flat coil form supported on said upper edges, the major portion of heating element having a sheath of approximately semicircular cross section providing a flat heat transfer surface occupying a common plane and a very short length only of said sheath intermediate the ends thereof having shoulder portions sloping outwardly and downwardly from said flat heat transfer surface, and means for securing said sheath to one of said support plates, comprising an anchor memberl fixed to said plate and having upwardly and inwardly extending rectilinear jaws in tight surface engagement with said shoulder portions; the slope and extent of said shoulder portions establishing maximum vertical and transverse sheath dimensions maintaining the cross sectional area of said heating element at the point of securement equal to that of the remainder of the spirally formed element.

2. A surface heating unit for an electric range or the like, including a support structure having a plurality of radially arranged members having wall portions which occupy a common plane, a f

plurality of tubular sheathed heating elements supported on said wall portions, the major por tion of each said heating element being arranged in flat coil form and having a sheath of approxi-- mately semi-circular cross section providing a g5,

flat, co-planar, upper surface on which to sup-- port a cooking Vessel for heat transfer thereto and a very short length only of each sheath intermediate the ends thereof having relatively long, straight, shoulder portions on dialnetrically 5;,

opposite sides of said sheath, and means for securing said sheath at one point only to one of said support members, comprising upwardly and inwardly extending jaws fixed to said member and engaging said shoulder portions, terminal ends of said heating elements being physically independent of each other and therefore relatively unrestrained for movement induced by expansion and contraction of said coiled heating element during heating and cooling thereof.

3. A surface heating unit for an electric range flat heat transfer surface substantially as wide as the inside diameter of the sheath and a single, short length of said sheath within said coiled portion having at least one relatively long, straight, shoulder extending outwardly and downwardly from a relatively narrower flat heat transfer surface, said heat transfer surfaces occupying a common plane, and means for securing said sheath to one of said support plates, ccmprising an anchor member fixed to said plate and having rigid jaws engaging opposite side wall portions of said sheath, one of said jaws engaging said outwardly and downwardly extending shoulder throughout the length thereof; said sheath configuration at each side of said last-named point of engagement returning rapidly to said substantially semi-circular cross section, whereby the changing wall configuration on each side of the jaws becomes incompatible with the jaw configuration and substantially prevents displacecompacted mass of electrically insulating and Iheat-transmitting material lling said sheath to maintain said resistance element in insulated relation to the wall of said sheath; said heating element having a first heat-transfer portion of substantially semi-circular cross section providing a flat heat-transfer surface, the transverse dimension of which is approximately equal to the inside diameter of the sheath, a second heattransfer portion integral with said first portion and characterized by a flat Iheat-transfer surface of substantially less transverse dimension and shoulder portions extending angularly from said transfer surface, and a third heat-transfer portion integral with said second portion and having i a substantially semi-circular cross section as in said iirst heat-transfer portion; the length of said second portion being only a small fraction of the length of either the first or the third portions and the flat heat-transfer surfaces of all of the portions continuing from one to the other without interruption.

5. In the manufacture of heating elements of the type having a metallic sheath, an electrical resistance element disposed therein and a mass of electrically insulating and heat transmitting granular material compacted about said resistance element to maintain the same uniformly spaced relative to said sheath While providing for transmission of heat thereto; the method which includes the steps of forming said sheathed element into coil form, placing said coil in a die, and pressing said coil to give to the major part of the length thereof a substantially semi-circular cross sectional shape having a continuous flat heat transfer surface occupying a common plane and to a very small length intermediate the ends thereof a cross section characterized by relatively long, angularly disposed, shoulder portions While maintaining the cross sectional area of said small length equal to that of the remainder of said major portion of the sheath, whereby the compaction of the granular material during said press operation will be substantially uniform throughout the length of the coil.

6. In the manufacture of heating elements of the type having a metallic sheath, an electrical resistance element disposed therein and a mass of electrically insulating and heat transmitting granular material compacted about said resistance element to mintain the same uniformly spaced relative to said sheath while providing for transmission of heat thereto; the method which includes the steps of forming said sheathed element into coil form, placing said coil in a die, and pressing said coil to give to the major part of the length thereof a substantially semicircular cross sectional shape having a continuous fiat heat transfer surface occupying a common plane and concurrently therewith to give to a very small length of said sheath intermediate the ends thereof a cross section characterized by relatively long shoulder portions sloping outwardly and downwardly from a flat upper surface in said common plane, said last-named cross section having a maximum width and height equal to aeeaib A 3, 9 10 I that of the remainder of the coil and a wall con- References Cited in the file of this patent guraton below said shoulders establishing the UNITED STATES PATENTS cross sectional area of said small length of said sheath equal to that of the semi-circular portion Number Name Date major part of the length of the coil. 2,328,113 Tuttle et al ug- 31 1943 2,456,343 Tuttle Dec. 14, 1948 BEN J. VALLORANI;

y l 10 2,490,611 Backer Dec. 6, 1949 CLARENCE J. MCCONNELL;

2,490,934t Vogel Dec. 13, 1949

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