US3835296A

US3835296A - Improvement in industrial electric resistance heater

Info

Publication number: US3835296A
Application number: US00402092A
Authority: US
Inventors: W Middough; L Jaquay
Original assignee: Dravo Corp
Current assignee: Dravo Corp
Priority date: 1972-01-27
Filing date: 1973-10-01
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

An electric resistance heater for industrial use with high heat emmisitivity per unit of length and low energy loss has coaxially-extending non-magnetic resistance units with a terminal end and an end opposite the terminal ends where the two conductors, otherwise electrically separate, are joined, thereby connecting them in series. At the terminal end the inner conductor projects through an enlargement of the outer conductor and has an exposed terminal portion of the same diameter as the enlarged terminal end of the outer conductor.

Description

A United States Patent 1 Middough et a1.

[54] INDUSTRIAL ELECTRIC RESISTANCE HEA R [75] Inventors: William V. Middough, Shaker Heights, Ohio; Louis 11. Jaquay,

Bridgeville, Pa. [73] Assignee: Dravo Corporation, Pittsburgh, Pa. [22] Filed: Oct. 1, 1973 [21] Appl. No.: 402,092

Related US. Application Data [60] Division of Ser. Nos. 234,689, Jan. 27, 1972, Pat. No. 3,764,718, Continuation ofSer. No. 29,150, April 16,

1 l970,abandoned.

[52] US. Cl 219/553, 13/25, 219/536, 338/237, 338/272 [51] Int. Cl. H05b 3/10 [58]. Field of Search 13/16, 20, 22, 23, 25,

[56] References Cited UNITED STATES PATENTS 1,933,532 '10/1933 Nichols 13/25 1,947,793 2/1934 O'Neil et a1. 338/235 1,951,753 3/1934 George et a1. 13/20 2,271,838 2/1942 Hanawalt et a1 13/20 2,355,343 8/1944 Von Zeerleder et al. 13/20 X [11] 3,835,296 Sept. 10, 1974 2,623,080 12/1952 Young 13/16 2,858,403 lO/1958 Butler, Jr... 338/229 2,971,039 2/1961 Westeren... 13/25 3,147,331 9/1964 Brugger 13/23 3,281,517 10/1966 Hemmer et al.... 13/31 3,571,476 3/1971 Anthony 13/20 FOREIGN PATENTS OR APPLICATIONS 215,231 9/1941 Switzerland 13/25 450,959 7/1936 Great Britain 219/553 915,619 7/1946 France.... 13/25 1,315,187 12/1962 France 219/553 Primary Examiner-Velodymyr Y. Mayewsky Attorney, Agent, or Firm-Parmelee, Miller, Welsh & Kratz [5 7] ABSTRACT An electric resistance heater for industrial use with high heat emmisitivity per unit of length and low energy loss has coaxially-extending non-magnetic resistance units with a terminal end and an end opposite the terminal ends where the two conductors, otherwise electrically separate, are joined, thereby connecting them in series. At the terminal end the inner conductor projects through an enlargement of the outer conductor and has an exposed terminal portion of the same diameter as the enlarged terminal end of the outer conductor.

9 Claims, 17 Drawing Figures ass-5.296

' PATENTEDSEPI 01w sum 1 or 4 El i bu? PATENIEU SEP 1 0 1914 saw 3 OF H PAIENIEBSEP 1 01914 .JII

I INDUSTRIAL ELECTRIC RESISTANCE HEATER This is a division of application Ser. No. 234,689 filed Jan. 27, 1972, now U.S. Pat. No. 3,764,718, and which is a continuation of application Ser. No. 29,150, tiled Apr. .16, 1970, now abandoned.

This invention is for an electrical resitance heater, and more especially a resistance heater for use in certain industrial environments, particularly enclosed chambers, such as a molten metal degassing chamber, where a large capacity heater is necessary, and which is often subjected to especially destructive conditions.

Since the invention herein disclosed was developed especially for use in the preheating of a vacuum metal degassing chamber, it will be described in connection with such an apparatus, but without limitation to its use in other apparatus to which it is also applicable.

In the degassing of molten metal, such as steel, the molten metal is drawn from a receptacle such as a ladle up a tubular leg and discharged into a refractory-lined degassing chamber which is connected to a pump of some kind that maintains a vacuum therein. The reduced pressure in the degassing chamber releases gases that are entrained in the metal. The degassed metal may then be returned to the receptacle from which it was withdrawn or discharged into another vessel or into a continuous casting apparatus.

Before the degassing chamber can be put into operation, it must be preheated to bring the interior lining of the vessel up to a sufficiently high temperature that the molten metal will not be chilled and solidify therein. Preheating the degassing chamber is sometimes accomplished by burning has inside the chamber. However, electric resistance heaters are desirable, and the heater of the present invention is especially, but certainly not exclusively, for this purpose.

One type of resistance heater heretofore designed for this purpose is the straight through-type where a resistance element extends more or less diametrically through the interior of the degassing chamber with its terminals passing through the walls of the vessel and with electric connections at the opposed protruding ends of the element. This requires that therebe two opposite ports in the wall of the vessel through which opposite ends of the heater extend and both ports must be sealed against air leakage when the chamber is under vacuum. The seals must be adequately protected by the refractory lining of the vessel. In such constructions the resistor is usually left in place when the apparatus is op erating, exposing the resistor both to destructive gases and to being splashed by not metal. When such a heater is energized with alternating current, there may be substantial current losses due to the induction of eddy currents in the steel vessel or other metal objects adjacent the heater.

A second type of heater which requires only a single port in the vessel wall has a resistor of U shape or hair pin" loop, but it requires a complex seal and electric terminals and the resistor may become fouled and short-circuited by a build-upof metal, slag, or other foreign substances across the space between the two parallel legs of the resistor.

The present invention has for its principal objects to provide a resistance heater having a unique arrangement of resistor elements to reduce upkeep costs, avoid any external magnetic field, and in which the electric terminals are at the same end of the resistor, so that only one port is required in the furnace wall where the heater passes through the furnace wall of an enclosure. However it is not subject to short-circuiting by fouling of the resistor element. A further object is to provide with such a resistor a unique arrangement for connecting the same to a source of electric power. A still further object is to provide a resistor which may be withdrawn into a protected environment when it is not in use.

BRIEF DESCRIPTION OF THE INVENTION In general, the resistor of the present invention in its simplest form comprises an outer tubular element, and positioned within it is a coaxially-extending inner element, these elements being spaced from one another throughout their length, but they are connected at the inner end of the resistor by a conducting closure member which substantially excludes furnace gases from entering the interior of the heater. The other or outer end of the outer tube has a terminal portion to which a current input connector may be clamped. The outer end of the inner element also has a terminal portion that extends through and projects beyond the terminal portion of the outer member for attachment to the second current supply line. In a desirable installation, as for degassing, there is a tubular housing or shroud that is secured to the top of the degassing chamber, and the resistor may be lowered from a raised position where it is enclosed in the housing to an operating position where it extends down into the degassing chamber to radiate heat to the walls of said chamber. Spaced pairs of opposed clamping elements are so located on the housing that when the heater is lowered into operating position, one pair of clamps will engage the upper end or terminal portion of the outer sleeve element, and one pair of clamps will engage the protruding end of the inner resistor element to supply electric current thereto so that the two elements of the resistor are connected in series.

In most cases the outer tubular element and the inner element are both heat-generating resistors, but in some cases the outer member may be operated primarily as a conductor and as a radiant element that receives the heat generated by the inner resistor element and reradiates it to the exterior. In other cases there may be two or more resistors in the outer member.

My invention may be more fully understood by reference to the accompanying drawings, in which:

FIGS. 1A and 1B are complementary longitudinal sections through the resistor apart from the apparatus with which it is used, 1A being the outer end portion and B being the inner end portion;

FIG. 2 is a transverse section in the plane of line II-II of FIG. 1, but on a larger scale, included in FIGS. 3 and 4;

FIG. 3 is a transverse section in III of FIG. 1;

FIG. 4 is a transverse section in the plane of line IVIV of FIG. 1;

FIG. 5 is a side elevation of a molten metal degassing apparatus with the electrode and the accompanying apparatus mounted on the degassing Vessel;

FIG. 6 is a vertical longitudinal section showing the clamps for making electrical contact with the resistor elements when the resistor is in operating position;

the plane of line III- FIG. 7 is an enlarged fragmentary view of the apparatus shown in FIG. 6, but rotated 90 from FIG. 6, the view being a vertical section so that the operating linkage is not seen;

FIG. 8 is a transverse fragmentary section in the plane of line VIII-VIII of FIG. 7, but showing the clamp-operating linkage;

FIG.9 is a side elevation of the linkage shown in FIG. 8, the view being an elevation of the part of the structure shown in FIG. 7;

FIG. 10 is a vertical fragmentary section of the shroud on a larger'section than FIG. 6, showing the lifting and lowering connection at. the top of the heater;

FIG. 11 is a transverse horizontal section in the plane of line XIXI of FIG. 10;

FIG. 12 is an enlarged detail view in horizontal section of a single connector clamp and its mounting, it being an enlargement;

FIG. 13 is a more or less schematic modification of the heater shown in FIGS. 1A and 1B, the view being partly in elevation and partly in horizontal section;

FIG. 14 is a transverse section on line XIV-XIV FIG. 13;

FIG. 15 is a view' similar to FIG. 13 of another modification for use with a three-phase alternating current; and

FIG. 16 is a transverse section in the-plane of line XVI-XVI Of FIG. 15.

As previously indicated, resistance heaters as herein contemplated may be designed to operate at quite high temperatures, even approximating the melting point of steel. For industries, such as steel degassing, where production is reckoned in tonnage, they must be large. In the specific example hereinafter described, the overall length of the heater is of the order of around twentythree feet, but it may be longer or shorter, depending on the use to which it is to be put. To develop such high temperatures in resistors of this size, very-heavy currents are used and substantial current losses can occur in some environments if the heater in operation sets up an external magnetic field. v

A form of the resistance heater itself is shown apart from the apparatus or equipment with which it is used in FIGS. 1A and 18. Because of its length, and in order to show the construction, it has been necessary to show the outer or upper end portion of the resistor in FIG. 1A and the opposite end portion in FIG. 18. For economy of manufacture and repair or replacement of parts, very long heaters may be comprised of longitudinal sections which are coupled by separable couplings.

The heater as shown in FIGS. 1A and 1B is comprised of two elements or assemblies, an outer tubular assembly 2, an inner assembly 3, and there is a combined closure and connector element 4 at the inner end uniting both members.

The outer tubular assembly 2 has a terminal element 5 at the outer end, a tubular resistance element 6 having one end screwed in the terminal 5 at 7. While the tubular element 6 could be an integral tubing for the full length of the resistor, it preferably extends to a coupling sleeve 8, and a second similar tubular resistance element 9, also screwed into the coupling 8, extends from said coupling with its opposite end screwed into the terminal closure element 4. As seen in the drawing, the closure element 4 is of generally cup-like section with an internally-threaded sleeve portion 10 into which the member 9 is screwed.

The inner assembly 3 of the resistor comprises an outer terminal portion 11, preferably of a diameter about the same as the terminal portion of the outer member, enabling similar connector clamps to be used with both terminals; It has a conductor portion 12 of the substantial or solid section concentrically positioned in and spaced from terminal portion 5 of the outer member. As here shown it is made in two sections, the outer portion of which is integral with terminal 11, and the inner portion 12a is joined to the outer section in tight abutting relation thereto by a threaded internal coupling 13. The reduced inner end 12b of section 12a is connected at 14 to the inner resistor element 15 by an internally-threaded coupling 16. The resistor is here shown as a tubular element, but may be a rod. It is preferably made in two or more sections joined together in abutting relation by an internal threaded coupling, as indicated at 16. The innermost end of the resistor 15 is firmly connected to the interior end of the cup-like end member 4 by a solid screw plug coupling 17 in a central bor in the member 4, which is also screwed into the end portion 15 of the resistor. This screw coupling 17 holds the inner element centered in the outer one with the two spaced from each other throughout their length. The two assemblies 2 and 3 are thus spaced from and insulated from each other except for the end connection 4 which unites them electrically and mechanically.

At the cooler terminal end of the heater a composite insulating bushing structure 18 surrounding the portion 12 of the inner element and secured in the outer end of the outer terminal member 5 keeps the two resistor elements in concentric spaced relation. Because of the small scale of the drawing the structure of this bushing arrangement is only indicated in FIG. 1A but is shown in greater detail in FIG. 10 and will hereinafter be more fully described. The bushing 18 also encloses the space inside the heater against free flow of air into it. There are shown radial spacing pins 19 arranged in groups of three at intervals along the length of the heater, these also serving to keep the inner and outer resistor elements in concentric spaced relation. In most cases they may be unnecessary, but where they are used they are located where their outer ends pass through the resistor element and bear against the inner surface of a coupling, as at 7 or 8. Arranged in this way they are located for convenient assembly or replacement.

The resistor elements and terminal portions and the plug 4 and external and internal couplings may be formed of carbon, silicon carbide, or a metal or metal alloy of a type commonly used in resistance heaters or combinations thereof. The terminal 11 of the inner element is here shown with an end socket in which is secured insulation'20 around a connector 20a into which is screwed a lifting eye 21 for use where the resistor is suspended in a vertical position, as hereinafter described.

The heater, like a hair-pin type of .heater, has both terminals at one end, but it is of advantage over a hairpin type in that with one resistor leg is centered within the other and the space between the two legs of the resistor is enclosed so that metal or slag can never build up and bridge across the two resistor elements of this invention as it may and does do with a hair-pin loop type of resistor. In operation, substantially all of the heat generated by the inner resistor element is radiated to and through the outer one, which also radiates heat generated within itself, providing a heaterwith a relatively large heat radiating surface which is also 'a resistance heater with a smaller diameter internal resistor to increase or boost the overall temperature. As above pointed out, the coaxial arrangement of the two resistors eliminates any external magnetic field, thereby re ducing energy loss to induced fields in the surrounding structure. With the inner resistor substantially protected from corrosive atmospheres to which the heater is exposed, or from the free circulation of air about it, it may be operated at higher temperatures or for longer periods of time or both than a resistor freely exposed to the surrounding atmosphere. The outer resistor of course has no similar protection, but it is cheaper to replace one resistor from time to time than two of them, as is the case with a hair-pin type of resistor heater or two conventional resistors. Also the outer resistor of the present invention may be of a more expensive and corrosive or oxidation-resistant material than the inner one, or may have a protective surface composition thereover that has a softoning point higher than the operating temperature of the resistor and does not react with the resistor.

One apparatus for which the heater is especially, though not exclusively useful, is one in which it is used in a vertically elongated generally cylindrical enclosure, such as a hot metal degasser. In a degasser the heater is used to preheat the interior of the enclosure into which molten metal is drawn from a ladle by vacuum, one such apparatus being shown in FIG. 5.

Referring to FIG. 5, 22 designates a typical vacuum degassing vessel having one or more depending legs 23. Where there is a single leg, molten metal is alternately sucked up from a ladle into the vessel 22, retained in the vessel for degassing, and then discharged into the ladle. More frequently the vessel has two depending legs 23, one behind the other as viewed in FIG. 5, and molten metal is sucked up one leg and discharged from the other, as is well understood in the .art. There is a duct 24 leading from the upper portion of vessel 22 through which gases are drawn to maintaina vacuum in the degassing chamber by means of a pump of some type (not shown). In a typical installation,and for purposes of illustration, the degassing chamber may be of a height of around 22 feet with a diameter of perhaps 9 or 10 feet, while, as above noted, the heater has an overall length of around 2l to 22 feet.

The heating apparatus of the present invention is mounted on top of the degassing chamber and is designated generally bythe numeral 25 in FIGS. 5 and 6. The top of the degassing chamber has a port 26 therethrough above which is a flanged fitting 27 to which a housing 28 is fixed. This housing as here shown has spaced walls so that it may be cooled, either with water or air, and is of an upwardly-increasing flat-sided eliptical section. There is a tubular extension 28a bolted or sealed to the top thereof. At the top of the tubular extension there is provided an internally-offset gas-tight casing 29. There is a sheave 30 inside this casing, and a second idler sheave 31 in the casing offset to one side of the sheave 30, and a motor-driven winch 32 is contained within an extension at the top of the casing 29. It is driven-by a motor and reducing gear mounted on the outside of the casing 29, but which is not shown in the drawings. A cable 33 from the winch 32 is reeved under the sheave 31 and over the sheave and passes centrally down through the shroud and extension 28, and is attached to the lifting eye 21 at the terminal 11 of the resistor for moving the entire'heater unit up and down in the shroud as hereinafter more fully explained.

For uniting the inner and outer assemblies and allowing for relative thermal expansion and contraction, there is an eleptical flange-plate 35 at the end of terminal 11 of the inner member, but which is insulated therefrom by insulating sheet 35'. The connector 20a and the shank of the screw-eye 21 pass up through the center of the flange plate. The composite bushing structure 18 at the outer end of the outer assembly comprises applate 37 with a center sleeve portion screwed into the outer end of terminal 5 and with an insulator ring 37a therein between the sleeve and portion 12 of the inner assembly. There are headed tie'rods 38 passing upwardly through the plate 37 with insulators 39 separating them from the plate 37. These rods pass through plate 37 and their upper ends pass through and are adjustably secured in a plate 40. This plate rests on a Belleville spring assembly 41 that is confined against a fixed abutment 42 on the shank of the lifting eye below plate 40.

The plate 35 carries a short tubular extension 35a which, as seen in FIG. 10, alwaysprojects up into the tubular housing or shroud, even when the heater is in its lowermost position, as hereinafter described to guide the heater, and it is raised or lowered inside the shroud. This extension 35a has peripherally-spaced pins- 35b on the exterior thereof which move in guideways on the interior of the shroud to hold the heater from twisting, or from becoming snagged when the heater is lowered to its fullest extent. The shroud has double walls providing a space between them for circulation of cooling water.

The housing 28 contains the parts through which electrical connection is made with the terminal of the resistor when the heater is in its lowermost or operating position. At this time the major portion of the heater projects to its fullest extent down into the degassing vessel with its lower end spaced only a few feet at most from the bottom of the degassing chamber. When the heater is thus lowered, the resistor terminals will be inside the housing 28, but the guide section 35b will still be inside the shroud.

Electrical connection is made to the terminal 11 of the inner resistor element by a pair of opposed clamps and 50a respectively and to the terminal 5 of the outer resistor element by a similar pair of opposed clamps 51 and 510, these being directly under the respective clamps 50 and 50a. The larger diameter of the terminal portion 5 of the outer assembly with its corresponding increase in area and the enlarged portion 11 on the conductor 12-120 of the inner heating element provide enlarged areas for contact with the electrical connector clamps to thereby lower the contact resistance between the clamps and the conductors leading to the resistance heater elements themselves. Also it enables each pair of connector clamps is the same and each individual clamp is the same, so that one clamp of each pair and one pair will be described, to apply to all. The clamps 50 and 50a for example may be formed of copper or graphite. Each has a curved inner face to conform to the surface of the resistor terminal which is contacts. Each contact element or shoe is at the inner end of a reciprocable conductor stem 52. This stem is slidably guided in insulating bushings 53 with a supporting sleeve 54 carried on a water-cooled supporting assembly 55 welded to and passing through the housing 28. On the stem 52 beyond the end of the sleeve 54 there is a fixed disk or'plate 56. A metal bellows 57 abuts against and is joined to the inner face of this plate through a layer of insulation 58. As thus assembled, each Connector can move toward and away from the terminal of the resistor which it is adapted to contact, and each one is insulated from the housing 8, but the bellows provides aseal to prevent leakage of air from outside the bellows into the supporting structure 28,

. vessel 22 and all passages opening into it.

In FIGS. 8 and 12 the contact shoe 50 is retracted from and out of contact with the terminal 11, and at the same time all the other contact shoes 51 and 51a are retracted. To operate the contact shoes 50-50a to press tightly against the terminal 11, there is a fluid pressure cylinder 60, preferably designated to be operated by air pressure. One end of this cylinder is pivotally connected to one end of a lever 61 that pivots about a fixed post 53, and which has its other end pivotally connected at 63 to the outer bifurcated end of the stem 52 through a short link 61a pivoted to 52 at 59. The cylinder has a piston and piston rod 64 connected to a similar lever 65 that pivots about post 66 and is similarly connected to the stem 52 of the shoe 50a, the arrangement being such that when air under pressure is applied to cylinder 60, both levers will be rocked to move their respective shoe-supporting stems 52 toward each other to clamp terminal 11 between them. To equalize their respective movements, lever 61 has a bushing thereon from which extends a lever'68 and there is a similar lever 69 connected with lever 65. Links 70 connect the

levers

68 and 69 to opposite ends of a horizontally pivoting equalizing bar 71 so that each shoe moves the same distance and the pressure of each against the terminal 11 is equalized.

As above stated, the contact shoes 51 and 51a are arranged and operated in the same way. Since each shoe is insulated from the housing 28, the upper contact shoes are insulated from the lower ones, so that the current path, must be from one heater resistor terminal, through the full length of one resistor across plug 4 and back through the full length of the other resistor.

-When' the degassing apparatus is operating, the heater is not required, and it is drawn by a winch 32 and cable 33 up into the tubular housing or shroud 28a, at which time of course the clamping electrodes are retracted. At start-up, however, when the degassing vessel is cold, theheater is lowered by the winch to the full length of the cable 33, at which time the terminal 11' of' the inner resistor member will bepositioned between the upper clamping electrodes 50 and 50a and the terminal of the outer resistor will be positioned between the lower clamping electrodes 51-5la. .The several fluid pressure cylinders are energized to move and desirable to have three resistor'elements, which could be concentric, or two parallel resistors may be positioned inside a third element or outer tube which may be a resistor, or simply a current-carrying element and re-radiator of heat generated in the two resistors inside it. Such an arrangement is shown in FIGS. 13 and 14. In these figures 70 designated an outer tube having a terminal 71 at one end. Extending lengthwise through the tube 70 are two

parallel resistors

72 and 73 which may be tubes or rods. The tube 70 and

resistors

72 and 73 may be made in section with couplings as shown in FIGS. 1A and 1B.

In FIGS. 13 and 14 the two

resistors

72 and 73 have their inner ends secured in a conducting plug or body 74 that is screwed into or otherwise fixed in the end of tube 71 and in addition to forming a current conducting path from tube 70 to

resistors

72 and 73, forms a closure for the lower or inner end of the tube 70..At the opposite or outer end of the tube 70 the

resistors

72 and 73 pass through a ceramic insulating bushing 75 that serves to support them in parallel relation and insulate them from terminal 71. The outer ends of the re sistors 72 and 73 are connected into a conducting terminal 76.

In this arrangement current may flow between

terminals

71 and 76 through a path comprising tube 70, plug 74 and

resistors

72 and 73. If desired, tube or element 70 may be a resistor, or it may be primarily a current conducting enclosure for

resistor elements

72 and 73. Whether 70 is a resistor or primarily a conductor, heat generated in

resistors

72 and 73 is radiated to element 70 and re-radiated from element 70 to the surrounding environment.

FIGS. 15 and 16 show another modification in which the outer element is simply an enclosure for three resistor elements which generate the heat that is radiated from the resistors. In these figures 80 is a tubular outer member, which may here be non-conducting, and within it are three parallel

separated resistors

81, 82 and 83. They are connected electrically at the lower or inner end by a'conducting closure plug 84 that excludes air and furnace gases from freely circulating inside the tube 80. The upper ends of the three resistors pass through an insulating closure at the top or outer end of the tube 80. This closure keeps the upper ends of the resistors spaced from each other and prevents the free circulation of air or gases into the interior of the tube.

Each resistor has a terminal portion 81a, 82a and 83a respectively at its outer end.

This arrangement is especially suitable for use with a three-phase alternating current where the current paths may be from 81 through 84 and 83, or 81 through 84 and 82, or through 82 through 84 and 83.

To the extent applicable, any arrangement or description shown in anyof the several heaters may be utilized in any of the others.

I claim:

1. An electric resistance heater unit providing a large heat-generating capacity in respect to its diameter comprising coaxially-extending resistance heating elements, one positioned within the other, the inner one being mechanically supported by the outer one and being electrically connected to the outer one only at oneend of the "heater unit, the outer heating element having a cylindrical terminal portion extending axially beyond both heating elements at the other end of the heater unit, the inner heating element having a terminal portion at said other end that extends axially through the terminal portion of the outer heating element and which projects beyond the end of the outer heating element, but which is electrically insulated from the outer heating element, said heating elements being detachably secured to their respective terminal portions and comprising of sections detachably secured together in end-to-end relation,

said sections comprising the outer heating element being joined by external sleeve couplings and the sections of the inner heating element being joined by internal axially-extending connectors,

and electric insulator means inside the outer heating element at intervals therealong supporting the innter heating element concentrically within the outer one.

2. An electrical resistance heating unit as defined in claim 1 wherein the inner heating element is mechanically supported by and electrically connected to the other one at said first end through an electrically conductive gas-excluding plug.

3. An electric resistance heater unit as defined in claim 1 wherein the terminal portion of the inner element which projects beyond the outer element has an enlarged cylindrical portion of a diameter substantially the same as the diameter of the terminal portion of the outer element whereby terminal clamps of like shape and design and range of movement can be used to supply current to the respective terminals of the two elements.

4. An electric resistance heater unit as defined in 30 first plate member secured to the free end of the terminal of the outer element and which has an insulating bushing therein through which the terminal of the inner element is slidably guided, a second plate member connected with the first through tie rods, and a compression spring operatively interposed between said second plate member and the free end of the terminal of the inner element.

7. An electric resistance heater unit as defined in claim 1 wherein there is a housing surrounding that end of the heater from which the conducting terminal of the inner resistor protrudes, two axially-separated terminal clamping arrangements carried by the housing having contact shoes movable radially toward and away from the heater, one such terminal clamping arrangement being positioned to clamp the protruding end of the terminal of the inner element, and the other being at a position to contact the end portion of the terminal of the outer element, said terminal clamping arrangements being electrically insulated from each other.

8. An electric resistance heater unit as defined in claim 1 wherein there is a housing surrounding the terminal portion of the two elements, a pair of opposed upper contact shoes supported by the housing for movement into and out of clamping engagement with the outer terminal portion of the inner resistor, a second pair of opposed contact shoes below the upper ones, also supported on the housing for movement into and out of clamping engagement with the terminal portion of the outer element, and fluid pressure means for operating said shoes into and out of clamping engagement with the respective terminals, the outer terminal portion of the inner resistor and the exterior of the terminal portion of the outer resistor being cylinders of substantially the same diameter.

9. An electric resistance heater unit as defined in claim 8 in which there is a shroud carried on and fixed to said housing, and means for raising the heater unit into said shroud when the shoes are retracted from clamping engagement with said terminals.

Claims

1. An electric resistance heater unit providing a large heatgenerating capacity in respect to its diameter comprising coaxially-extending resistance heating elements, one positioned within the other, the inner one being mechanically supported by the outer one and being electrically connected to the outer one only at one end of the heater unit, the outer heating element having a cylindrical terminal portion extending axially beyond both heating elements at the other end of the heater unit, the inner heating element having a terminal portion at said other end that extends axially through the terminal portion of the outer heating element and which projects beyond the end of the outer heating element, but which is electrically insulated from the outer heating element, said heating elements being detachably secured to their respective terminal portions and comprising of sections detachably secured together in end-to-end relation, said sections comprising the outer heating element being joined by external sleeve couplings and the sections of the inner heating element being joined by internal axially-extending connectors, and electric insulator means inside the outer heating element at intervals therealong supporting the innter heating element concentrically within the outer one.

4. An electric resistance heater unit as defined in claim 3 wherein means are attached to said enlarged cylindrical terminal portion for suspending the heater in a vertical position.

5. An electrical resistance heater unit as defined in claim 1 having means mechanically connecting the terminal portions of both elements arranged to resiliently compensate for unequal thermal expansion of the elements but to keep said terminal portions electrically separate.

6. An electric resistance heater unit as defined in claim 4 wherein said connecting means comprises a first plate member secured to the free end of the terminal of the outer element and which has an insulating bushing therein through which the terminal of the inner element is slidably guided, a second plate member connected with the first through tie rods, and a compression spring operatively interposed between said second plate member and the free end of the terminal of the inner element.