US 3121154 A
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Feb. 11, 1964 J, A, MENzlEs ETAL 3,121,154
ELECTRIC HEATERS l ,6 F/G.3. &\W N'/ A INVENToRs l -James A. Menzles BY George D. Robertson ATTORNEY Feb. 11, 1964 Filed oct. so, 1959 J. A. MENZIES ETAL ELECTRIC HEATERS n [5| aq/. ww(
5 Sheets-Sheet 2 INVENToRs James AI Menzles iBY George D. Rober'rsor) ATTORNEY Feb. 11, 1964 J. A. MENzlEs ETAL 3,121,154
ELECTRIC HEATERS Filed Oct. 30, 1959 3 Sheets-Sheet 3 F/GS,
INVENT ORS ATTORNEY United States Patent O 3,121,154 ELECTRIC HEATERS James A. Menzies, London, England, and George D. Robertson, Glasgow, Scotland, assignors to Babcock & Wilcox, Limited, London, England, a corporation of Great Britain Filed Oct. 30, 1959, Ser. No. 849,775 3 Claims. (Cl. 219-19) This invention relates to electric heaters. ln a highly rated electric heater of the kind including an electric resistance and a covering means, for example, an enclosing sheath, difficulty is experienced in insulating the resistance from the sheath in a manner adapted to withstand the relatively high temperature and to permit high heat transfer rates to the covering means |when the latter is required to operate at temperature of the order of 900 F. The present invention enables highly rated electric heaters to be provided which have satisfactory insulation enabling particularly high rates of heat transfer from the resistance to the covering means to be achieved.
The present invention includes an electric heater having an electric resistance `and a covering means for the electric resistance wherein the resistance is insulated from the covering means by a thin, dense and coherent layer of oxidized aluminum.
ln a form of rod heater in accordance with the invention, the resistance is of helical form, its radially outer surface being in contact with an interior anodized aluminu-m film on a surrounding tube which may be of aluminum alloy. Suitably the helix is in the form of a ribbon with the faces of the ribbon facing radially inwards and radial-ly outwards. The ribbon may be of uniform area and shape, referred to cross-sections transverse to the ribbon, or, if it is desired that the heat output therefrom shall vary from point to point yalong its length, the crosssectional area may appropriately 'vary from point to point along its length, by variation, for example, of the radial dimension only of the ribbon along its length, the radial Kdimension being reduced in regions in which it is desired that the heat output per unit length of the heater shall be relatively greater.
The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of -the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying -drawings and descriptive matter in which we have illustrated and described preferred embodiments of the invention.
Of the drawings:
FIGURE 1 is :an elevation partly in section on its axis of one form of the invention;
FIGURES 2 and 3 are sectional elevations of parts of modified forms of the heater shown lin FIGURE l;
FIGURE 4 is an elevation of a tubular heater, partly in a section on its axis; and
FIGURE 5 is a section through a heater unit comprising `an assembly of heater plates.
Referring to FIGURE l of the drawings, -a heater for simulating a fuel element for a gas-cooled, natural uranium nuclear reactor core comprises a covering means such `as aluminum alloy tube 10, of the same length and outside diameter as the length and outside diameter of the can of the fuel element and housing a helical electric resistance heating element 12.
The helical heating element is formed by machining a helical gap 13 along the length of a tube of suitable metal, for example, .the iron-chromium-aluminum-cobalt alloy known as Kanthal A, or a stainless steel of a kind used for resistance heaters, to `form a helical ribbon with 3,121,154 Patented Feb. l1, 1964 the faces of the ribbon facing radially inwards and radially outwards. At its ends the ribbon is brazed or silver soldered to respective brass cylindrical blocks 14 which close the respective ends of the surrounding tube and also act as terminals through which the ribbon may be connected into an appropriate electric circuit.
The interior surface of the covering tube 10 is covered by a continuous, dense and coherent film 15, having a uniform thickness of the order of `0.002 inch, formed electrolytically by the treatment development for producing a particularly Wear-resistance surface on the alloy containing a sufficiently large proportion of aluminum and known as ha-rd anodizing. The edges 16 bounding the said interior surface are rounded-off and the anodized film extends a certain distance onto the rounded edges 16, whereby the tube surface uncovered by the film is spaced from the blocks 14 by a considerably greater distance than the thickness of the film.
'Ihe outside diameter of the tube from which the ribbon 12. is formed is slightly greater than the internal diameter of the covering tube 10 with its anodized film 15, e.g. the said outside diameter may be at least that of the internal diameter of the covering tube before the anodized film is formed; for this reason the helical ribbon, in place in the covering tube, is under strain and the radially outer face 17 of the ribbon is forced into intimate contact with the .anodized film 15. When inserting the ribbon into the covering tube, one cylindrical block 14 is temporarily turned with respect to the other so that the ribbon assumes a reduced diameter, whereby the longitudinal movements of the coils of the ribbon inside the covering tube easily move to take up positions in which the coils are evenly spaced. The diameter of the cylindrical blocks is substantially the lsame as the internal diameter of the covering tube before the anodized film is formed.
In the operation of the heater, heat generated by resistive heating of the coils of the ribbon 12 passes outwardly from the coils through the anodized film 15 and the wall of the covering tube 10 to whatever body is provided for accepting the heat. The reference numeral 18 denotes some of a number of fins, which could be integral with the covering tube, which might be provided vto assist in the transfer of heat to a cooling gas iiowing tween the surfaces intimately contacting it on its inner and outer faces respectively, whereby the heating ribbon runs at minimum temperature, but is thick enough to act as electrical insulation; the anodized film retains its electrically insulating properties in spite of high temperatures.
In a rod heater of the nature described, having -a covering tube of 15/16 inches external diameter, it has been found possible satisfactorily to effect heaty transfer at the rate of l5 kilowatts per foot length of heater with the covering tube at a temperature of about 900 F.
An oxidized aluminum film adapted for the ready passage therethrough of heat in large quantities while acting as electrical insulation may be deposited on metal surfaces by a suitable known spraying process. Thus, instead of an anodized film 15 on the interior surface of the covering tube 1t) in the rod heater described with reference to FIGURE l, an oxidized aluminum film may be used which has been deposited by spraying on the radially outer face 17 of the helical ribbon 12, and also on the cylindrical surface of the cylindrical blocks 14, insulating the blocks from the covering tube. In this modification it is appropriate to round-off, in the manner illustrated in FIGURE 2, the edges 19 of the helical ribbon 12 bounding the radially outer face 17 of the ribbon, and
:spectively or on the heating element.V
4the electric resistance heating element is in the form of a tube 22 of which the radially outer surface 27 is in *Contact with a hard anodized film which has been formed on the interior surface of the covering tube lil. The tubular heating element 22 and the covering tube 10 are of the same length; the terminals are constituted by cylindrical blocks 14 closing the respective ends of the tubular heating element, which blocks may be tackbrazed or tack-silver soldered to the tubular heating element after having been driven or screwed into position. 4The edges 16 bounding the interior surface of the tube 10 are rounded and the anodized film 1S extends a certain distance onto the rounded edges 16. If the tubular heating element 22 makes a push fit in the surrounding tube 1t) before the anodized film 15 has been formed on the interior surface of the latter, the tubular heating element may, after the hard anodized film of the order of 0.002 inch thick has been formed be driven into position within the surrounding tube 10 without damagaing the desired properties of the film and intimate contact relsults between the radially outer surface V27 of the tubular heating element 22 and the inner surface of the film ,'15, Alternatively, if desired, the tubular resistance element 22 may be expanded into intimate contact with the film 15, the expansion being made by hydraulic or meuchanical means or taking place by warming up after it has been inserted into the covering tube in chilled condition, or the outer tube may be shrunk about the resistance element by any of the means well known in the art.
FIGURE 4 shows a tubular heating element 31 of Kanthal A or stainless steel mounted in a tubular heater adapted for heating fluid flowing thorugh a central passage 32 of the heater as well as fluid flowing over the outside of the heater. The tubular heating element is held at its ends in respective similar terminal block assemblies 33 each consisting of an annular inner brass block 34 and an annular outer brass block 35. Each end length of the heating element lits over a cylindrical outer surface 36 of the appropriate inner block 34 and within a cylindrical inner surface 37 of the appropriate outer block 35, and the extremity thereof is brazed or silversoldered as at 39 to the inner end as at 3S Vto the outer brass block.
' Intermediate its end lengths the heating element 31 is supportedrby a closely-fitting inner covering tube 4@ and a closely-fitting outer covering tube 41 both of aluminum or aluminum alloy, which are also held by the terminal block assemblies, the former tube 4h tting at each end between the heating element and an annular collar 42 on the appropriate inner brass block 34, and the latter tube 41 fitting at each end between the heating element and an annular collar 43 on the appropriate outer brass block 33.
The inner and outer covering tubes 40 and d1 are electrically insulated from the heating element, and also from the terminal block assemblies 33, by electrolytically or. spray deposited aluminum oxide layers or im44land 45 on the inner and outer covering tubes 40 and 41 re- Thus the film 44 extends over the'whole length of the outer surface, facing the inner surface of the heating element, over the ends 46, suitably rounded, and over endlengths, longer 1 than the collars 42, of Vthe inner surface of theV inner covering tube 40. The film 45 also extends over the whole length of the inner surface, facing the heating element, over the ends 037, suitably rounded, and over end lengths, longer than the collars 53, of the outer surface of the outer covering tube 41.
Each inner block 34 provides an aperture 43 leading to the passage 32 within the tubular heating element. The terminal block assemblies 33 serve for leading electric current to and from the heating element, heat from which passes through the aluminum oxide films 44 and 45 and through the inner and outer covering tubes 40 and 41 to the duid to be heated.
FIGURE 5 is a cross-section of a modified heater umt comprising a plurality of flat heating element plates 81 of aluminum, aluminum alloy or zirconium assembled into a group, parallel to one another, with uniform gaps between their adjacent faces, tot form a heater unit adapted for heating gaseous lluid. The plates have opposite sides entered into slots 82 equidistantly spaced on opposed faces on respective support plates 83 of -aluminum or aluminum alloy forming par-t of or secured to some suitable supporting structure for the heating unit as a whole. O11 said opposite faces of the support plates including the surfaces of the slots, a dense, coherent film of alumina of at least 0.002 inches thick has been formed by a hard anodizing process. Alternatively or in addition, when l the resistance elements are of aluminum or aluminum resistance, a thin dense vand coherent film of oxidized block assembly.
alloy the end portions 36 of the resistance elements which enter the slots may be formed with hard anodized lms. Electric current may be led to and from the plate ltype heaters through one or'more terminal strips such as 84 at one end of the heater unit each -welded at that end to the edges of all the heating element plates, and a similar terminal strip or strips at the other end of the heater unit.
While `in accordance with the provisions of the statutes we have illustrated and described herein the best forms and modes of operation of the invention now known to us, those skilled in the art will understand that changes may be made in the forms of the apparatus disclosed without departing from the spirit of the invention coveredV by our claims, and that certain features of our invention may sometime be used to advantage without -a corresponding use of other features.
l. An electric heater having a plurality of electric resistance plates, said resistance plates positioned in paralel relationship to one another, a pair of supporting plates oppositely arranged transversely of said resistance plates and on opposite sides thereof and having a plurality of parallel slots on the opposed faces thereof, said resistlance plates yadapted to fit in said slots in said supporting Vtubular resistancea second covering tube interior of said tubular resistance, a thin dense and coherent lm of oxidized aluminum in direct contact with ,and between said resistanceA and said first Vand second covering tube,
and at least one terminal block assembly adapted to sup-V port at least one end of said tubular resistancel and said coveringtubes, said film of oxidized aluminum further disposed between said covering tubes and said terminal 3. An electric heater comprising an electrical resistance of `tubular shape, a covering tube enclosingV said aluminum in direct contact withand between said resistance and said covering-tubefandat least one terminal block assembly adapted to support at least one endrof said tubular resistance and said covering tube, said lm of oxidized aluminum further ldisposed between said covering tube and said terminal block assembly.
References Cited in the le of this patent UNITED STATES PATENTS Appleyard Oct. 4, 1892 Lohmann Nov. 19, 1929 Fanzer Aug. 9, 1932 Fekete Aug. 3, 1937 Doczekal Mar. 8, 1938 Walker NOV. 8, 1938 Duston Nov. 2, 1943 6 Osterheld Sept. l2, 1944 Osterheld Oct. 10, 1944 Osterheld Dec. 28, 1948 Lennox Mar. 7, 1950 Horsfall et al Mar. 6, 1951 Huck Feb. 21, 1956 Glaser May 15, 1956 Hatch Feb. 4, 1958 Norton Feb. 24, 1959 Sawyer Dec. 8, 1959 Fenu Nov. 8, 1960 Kawalle Aug. 8, 1961