US1729673A - Electrical heating element - Google Patents

Description

@at l, 31929., A. J. Krakau-1ER ET AL LZQ ELECTRICAL HEATING ELEMENT Filed Dec. 15, 192e 2 sheets-sneek, l

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A. J. EQR'R ET AL ELECTRICAL HEATING ELEMENT @cih 2 Sheets-Sheet Filed Dec 13, 1926 3/9 [mimiHHHHHMHW yIl FILE .E

NYENTUHS Arf/7cm .7. Kem/rer lW//ia Wes/ ORNEYS Patented Oct. l, 1929v UNITED STATES PATENT OFFICE ARTHUR J'. KERCHER, OF BERKELEY, AND WILLIAM WESLEY-HICKS, 0F SAN FRAN- CISCO, CALIFORNIA ELECTRICAL HEATING: ELEMENT Application filed December 13. 1926. Serial No. 154,340.`

This invention relates generally to heating elements -for use .with electrical appliances.

Of the different forms of heating element being employed for electrical appliances, one

of the most practical consists of an insulated resistance conductor positioned within a groove or pocket formed in a heat absorbing body. Various means have been devised for retaining the conductor within the groove and for providing an insulation which `will not break downor ground the conductor to the metal body. Cement for embedding the conductor has proven unsatisfactory since 1t 1s soon broken by the expansion and contraction of the heat absorbing body due to changes in temperature. A better arrangement is to use contiguous beads of refractory material, but these beads must be retained in close thermal contact with the sides of the groove to prevent an abnormal heat lag. If cement is used together with the beads, it is difficult to secure a proper bond between the cement and the surfaces of the heat absorbing body.

It is an object of this invention to retain an insulated resistance conductor Within a groove by the use of a cement which interlocks with a portion of the heat absorbing body.

It is a further object of this invention to utilize an unvitrified refractory cement for retaining an insulated resistance conductor 1n position within a groove and to prevent the cement from coming into electrical contact with the conductor.

Further objects of this invention will appear from the following description in which we have set forth the preferred embodiment of our invention. It is to be understood that the invention is to be accorded a range of electrical and mechanical equivalents consistent with the state of the prior art.

' Referring to the drawings:

Figure 1 is a side view, one half in section, showing a heating appliance embodying a heating element constructed in accordance with this invention.

i Fig. 2 is a cross sectional view taken along the line 2 2 of Fig. 1. 50

Fig. 3 is an enlarged cross Sectional detail showing the construction of the heating element.

Fig. 4 is an enlarged cross sectional view taken along the line 4--1 of Fig. 3.

Fig. 5 is a side elevational view of a modi fied form of heating appliance incorporating this invention.

Fig. G is an enlarged cross sectional detail showing the construction of the heating element.

Fig. 7 is a bottom plan View of the heating appliance shown in Fig. 5, one-half of the inclosing casing being removed.

Fig. 8 Vis a plan view illustrating diagrammatically an electrical heating stove incorporating the heating appliance shown in Figs. 5 and 7. l

The invention consists generally of a heating clement which is adaptable for a variety of electrical appliances. It incorporates an insulated resistance conductor which is adapted to be positioned within a groove formed in l a heat absorbing body. The body is recessed along one or more walls of the groove and the insulated conductor retained in position by a refractory cement which fills the recess and thereby forms an interlocking engagement with the heat absorbing body. The cement which we prefer to use is notvitrified at the normal operating temperature of the heating element, so thatrif it is necessary to make renewals, the cement may be easily broken away to permit removal of the insulated conductor. The insulation for the resistance conductor is preferably in the form of a plurality of contiguous refractory beads which are constructed of a vitritied refractory substance having a relatively high resistivity at the operating temperature. As the cement used for retaining the refractory beads within the groove has a relatively lower resistivity at the operating temperature, it is proposed to keep this cement ont of electrical contact with the resistance conductor by forming the beads with intertting ends.

Thus referring to the drawings, there is shown in Figs. l to 4- inclusive an electrical appliance of the type commonly known as an electrical hot plate, having a hi at alf s' `bing body inthe form oi t fiat pla-tk i() 'l :h is

formed with an upper augmented heat transmitting surface 11 and a lower augmented heat absorption surface which is in thermal contact with one or more heating elements. In order to form the heat absorbing area and for the purpose of positioning the resistance element, there is formed one or more ribs 12 'providing one or more' grooves 13 upon the body. For a purpose later to be described,

the ends of the beads are adapted to have intertitting engagement with each other. Thus one end ot each bead is formed to provide a spherical convex surface 17 while the opposite end is formed to provide a concave surface 18, the convex surface 17 of one bead interitt-ing in the concave surface 18 of an adjacent bead. The material from which the beads are constructed is preferably a vitriied refractory substance having a relatively high resistivity at the operating temperature of the heating unit, say 6000 ohms per cubic centimeter at 1000 degrees centigrade. .A resistivity of this value will practically obviate all grounding of the conductor to the metal heat absorbing body so that a voltage less than say 6 volts will occur between the plate and the ground when the device is in operation. A suitable material having these properties is vitritied alundum which also has the inherent property of having unusual mechanical strength when made up into insulating beads or tubes.

- For retaining the insulating conductor in position within the groove we prefer to employ a relatively small amount ot refractory cement 21 which is filled into the crevices between the ribs 12 and the sides of the insulating beads 16, after the beads have been positioned within the groove. For interlockingthe cement with the body of the heating element, the ribs 12 are provided with recesses or grooves 22 which extend about the side walls of the groove 13 in spaced relationship with the end of the ribs 12. These recesses serve to etectivelyanchor the cement 21 to prevent removal of the insulated resistance conductor, even though the bond between tlie'cement andthe ribs 12 should become broken. Good results have been obtained by using a cement formed from powdered alundum mixed with a suitable bonding material. Since a cement of this kind will not be vitritied at the operating temperature of the ordinary heating appliance, it will have a substantially lower resistivity than the vitritied insulating beads 16. tact the `resistivity may be sutlieiently low to form a serious leakage between the conductor 14 and the metal absorbing body in case the cement comes into electrical contact with the resistance conductor. The intertitting nature of the beads however serves to prevent such contact between the cement and the conductor. In. practice the cement may be poured upon the insulating beads` in a semi-fluid condition with suiicient plasticity to prevent it from scoping' between the insulating beads and coming into contact with the resistance conductor. It the beads were made with ordinary abutting ends which were not interfitting, the cement might come into electrical contact with the resistance conductor with the result that at the normal operating temperature of the device, an appreciable voltage drop would be obtained between the metal body member and a user might obtain a shock. As the cement is unvitritied by the normal operating temperature it may easily be broken away to remove the insulating beads and resistance conductor, in case the conductor becomes burned out and it is necessary to insert a new resistance element.

The complete appliance shown in Figs. 1 and 2 which incorporates the heating element described above, may be completed by providing a shallow metal casing 23 covering the lower tace of the plate and having its upper edge secured to the periphery of the plate by suitable means such as screws 24. A baille' wall 26 is provided for minimizing transfer of heat from the plate 10 to the bottoni wall of the casing. Suitable terminal connectors 27 serve to make electrical connections with the terminal wires 28 leading from the sections of the heating element.

In Figs. 5 and 6 the heating element has been shown as incorporated with a diii'erent type of electrical hot plate. In this instance the heat absorbing body 110 is rectangular in shape and is provided with a plurality ot spaced parallel ribs 112 upon its lower tace for forming a heat absorbing area, and for providing an augmented heat transmitting area the upper tace is provided with a plurality of spaced parallel grooves 31, the ribs 31 beine* at an angle to the ribs 112. A hot plate oiD this type is especially applicable as a heavy duty unit to be used in cooking ranges for restaurants. The lower face of the plate is shown as inclosed by a rectangular casing 32 within which is disposed a sheath 33 of a substance which is a poor conductor ot' heat.

l GLI lill One suitable material for example is a fire clay formed with relatively large pores or air cells. This may be constructed by mixing the clay with a combustible material such as sawdust or straw before firing. Suitable terminal connectors 34 are provided for making electrical contact with the terminal wires 36. In Fig. 8 we have shown a plan view of an electrical range incorporating a plurality of hot plates of this type. The complete hot plate units designated generally at 37 are supported upon the stove `top 38 as by means of flanges 39. The forward portion of each plate is preferably maintained at a higher temperature than the rear portion. This may be accomplished either by dividing the heating element into dierent sections so that the rear portion of the plate is heated to a higher temperature than the forward portion, or the convolutions of the resistance conductor 14 may .be spread farther apart along the rear portion of the plate while they are disposed relatively close together 4near the forward portion. With this construction the conductor will impart a greater amount'of heat to the forward portion of the plate than to the rear portion.

We claim:

1. A heating element comprising a heat absorbing body having a groove, an insulated resistance conductor positioned within said groove, said body -having a recess in one side wall of said groove, and a refractory cement in intimate contact with the insulation of the conductor and interlocking in said recess.

2. A heating element comprising a heat absorbing body havin a groove, a resistance conductor, insulating beads strung upon said conductor, said body having a recess formed in one side wall of the groove and a refractory cement in intimate contact with the insulating beads and interlocking in said recess.

3. A heatinnf element comprising a heat absorbing body having a ribbed surface, an insulated resistance conductor disposed between adjacent rib portions, onerib portion having a recess formed therein, and a refractory cement in intimate'contact with the insulation of said conductor and interlocking in said recess.

4. A heating element comprising a heat absorbing body having a ribbed surface, an insulatedresistance conductor fitted between adjacent rib portions, one rib portion havingI a recess formed therein, and a refractory cement in intimate contact with the insulation of said conductor and vinterlocking in said recess, said ribs being so formed that the insulated conductor may be removed upon breakin of the cement.

5. A eatin element comprising a heat absorbing bo y having a groove formed therein, av resistance conductor disposed in said groove, interfitting refractory beads strung upon said conductor, said beads having hi h resistivity at the operating temperature of said element, and a refractory cement in intimate contact with the insulating'beads and adjacent walls of said groove, said cement having a substantially lower resistivity at said operating temperature and being kept out of electrical contact with the conductor by the interiitting relationship of the beads.

6. A heating element comprising a heat absorbing' bod-y having a groove formed therein, a resistance conductor disposed in said groove, a plurality of insulating beads strung upon said conductor, said beads being of vitritied refractory material, said groove having a recess formed in one wall thereof, and an unvitried cement'in intimate contact with said beads and interlocking in said recess for retaining the beads and conductor within the groove.

7. A heatin element comprisinga heat absorbing bo y having a groove formed therein, a resistance conductor disposed in said groove, a plurality of interitting insulating beads strung upon said conductor, said beads being of vitrified refractory material having high resistivity at the operating temperature of the element, said groove having a recess formed in one wall thereof, and an unvitrited cement in intimate contact with said beads and interlocking in said recess for retaining the beads and conductor within the groove, said cement having an appreciable conductivity at the operating temperature of the element.

8. A hea-ting element comprising a metallic heat absorbingbody, a plurality of spaced ribs formed upon one face of said body, said ribs defining a groove, a convoluted resistance conductor disposed in said groove, the spacing between said ribs being substantially greater than the external diameter of the convolutions, insulating beads strung upon said conductor and disposed within said groove, one of said ribs having a recess formed in one face thereof, and refractory cement serving to retain said conductor and beads within said groove, said cement being within the groove and interlocked with said recess.

In testimony whereof, we have hereunto set our hands.

ARTHUR J. KERCHER. WILLIAM WESLEY HICKS.

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