CA2265674C - Polymeric immersion heating element with skeletal support - Google Patents

Abstract

Electrical resistance heating element, hot water heaters containing such elements, and method of preparing such elements are provided. The electrical resistance heating elements can be disposed through a wall of a tank for heating fluid, such as water.

They include a skeletal support frame (70), having a first supporting surface (69), thereon. They also include a resistance wire (66) wound onto the first supporting surface (69) and preferably connected to at least a pair of terminal end portions. The support frame (70) and resistance wire (66) are then hermetically encapsulated and electrically insulated within a thermally conductive polymeric coating (64). The skeletal support frame (70) improves injection molding operations for encapsulating the resistance wire (66), and can include heat transfer fins (62) for improving thermal conductivity.

Description

10152025CA 02265674 2003-08-0671576-135POLYMERIC INERSION HEATING ELEMENT WITH SKELETAL SUPPORTCross Reference to Related ApplicationsThis application is a continuation-in-part of U.S.patent application Serial No. 08/365,920 filed December 29,1994, and entitled “Immersion Heating Element With ElectricResistance Heating Material and Polymeric Layer DisposedThereon”, now U.S. Patent No 5,586,214.Field of the InventionThis invention relates to electric resistanceheating elements, and more particularly, to polymer—basedresistance heating elements for heating gases and liquids.Background of the InventionElectric resistance heating elements used inconnection with water heaters have traditionally been madeof metal and ceramic components. A typical constructionincludes a pair of terminal pins brazed to the ends of anNi-Cr coil, which is then disposed axially through a U-shaped tubular metal sheath. The resistance coil isinsulated from the metal sheath by a powdered ceramicmaterial, usually magnesium oxide.While such conventional heating elements have beenthe workhorse for the water heater industry for decades,there have been a number of widely—recognized deficiencies.For example, galvanic currents occurring between the metalsheath and any exposed metal surfaces in the tank can createcorrosion of the various anodic metal components of thesystem. The metal sheath of the heating element, which istypically copper or copper alloy, also attracts lime1015202530CA 02265674 2003-08-0671576-135deposits from the water, which can lead to premature failureof the heating element. Additionally, the use of brassfittings and copper tubing has become increasingly moreexpensive as the price of copper has increased over theyears.As an alternative to metal elements, at least oneplastic sheath electric heating element has been proposed inCunningham, U.S. Patent No. 3,943,328. In the discloseddevice, conventional resistance wire and powdered magnesiumoxide are used in conjunction with a plastic sheath. Sincethis plastic sheath is non—conductive, there is no galvaniccell created with the other metal parts of the heating unitin contact with the water in the tank, and there is also nolime buildup. Unfortunately, for various reasons, theseprior art, plastic—sheath heating elements were not capableof attaining high wattage ratings over a normal usefulservice life, and concomitantly, were not widely accepted.Sumary of the InventionAccording to the present invention, there isprovided an electrical resistance heating element capable ofbeing disposed through a wall of a tank for use inconnection with heating a fluid medium, comprising: (a) afirst flanged end; (b) a skeletal support frame having aplurality of openings therethrough and a first supportingsurface thereon; (c) a resistance wire wound onto said firstsupporting surface and connected to at least a pair ofterminal end portions at said first flanged end of saidheating element; and (d) a thermally-conductive polymericcoating disposed over said resistance wire for hermeticallyencapsulating and electrically insulating said resistancewire from the fluid medium.1015202530CA 02265674 2003-08-0671576-135This invention greatly facilitates moldingoperations by providing a thin skeletal structure forsupporting the resistance heating wire. This structureincludes a plurality of openings or apertures for permittingbetter flow of molten polymeric material. The open supportprovides larger mold cross—sections that are easier to fill.During injection molding, for example, molten polymer can bedirected almost entirely around the resistance heating wireto greatly reduce the incidence of bubbles along theinterface of the skeletal support frame and the polymericovermolded coating. Such bubbles have been known to causehot spots during the operation of the element in water.Additionally, the thin skeletal support frames of thisinvention reduce the potential for delamination of moldedcomponents and separation of the resistance heating wirefrom the polymer coating. The methods provided by thisinvention greatly improve coverage and help to minimize moldopenings by requiring lower pressures.According to another aspect of the presentinvention, there is provided a polymeric skeletal supportframe for supporting a resistance wire of an electricalresistance heating element, comprising: a plurality oflongitudinal splines including spaced grooves along theirlengths, a plurality of heat transfer fins extending fromsaid longitudinal splines, said longitudinal splinesintegrally connected by a plurality of longitudinally-spacedring supports.According to yet another aspect of the presentinvention, there is provided a water heater comprising: (a)a tank for containing water; and (b) a heating elementattached to a wall of said tank for providing electrical1015202530CA 02265674 2003-08-0671576-135resistance heating to a portion of the water in said tank,said heating element comprising: a skeletal support framehaving a plurality of openings therethrough and a firstsupporting surface thereon; a resistance wire wound ontosaid first supporting surface and connecting to at least apair of terminal end portions; and a thermally—conductivepolymeric coating disposed over said resistance wire and amajor portion of said skeletal support frame forhermetically encapsulating and electrically insulating saidresistance wire from the water.According to a further aspect of the presentinvention, there is provided a method of manufacturing anelectrical resistance heating element for heating a fluidmedium, comprising: (a) providing a polymeric skeletalsupport frame having a plurality of longitudinal splinesconnected by a series of spaced ring supports, saidlongitudinal splines comprising spaced grooves; (b) windinga resistance heating wire onto said spaced grooves, saidresistance heating wire having a pair of free ends joined toa pair of terminal end portions; and (c) molding a polymericcoating containing an additive for improving the thermalconductivity of said coating over said resistance heatingwire and at least 90 percent of said skeletal support frameto electrically insulate and hermetically encapsulate saidresistance wire from the fluid medium, whereby said skeletalsupport frame provides a plurality of openings forfacilitating the molding of the polymeric coating.According to a further aspect of the presentinvention, there is provided an electrical resistanceheating element capable of being disposed through a wall ofa tank for use in connection with heating a fluid medium,3a1015202530CA 02265674 2003-08-0671576-135comprising: (a) a polymeric skeletal support frame having aplurality of longitudinal splines connected by a series ofspaced ring supports, said longitudinal splines comprisingspaced grooves; (b) a resistance wire having a pair of freeends joined to a pair of terminal end portions, saidresistance wire being wound onto and supported by saidspaced grooves; and (c) a polymeric coating containing anadditive for improving the thermal conductivity of saidpolymeric coating disposed over said resistance wire and atleast about 90 percent of said skeletal support frame forhermetically encapsulating and electrically insulating saidresistance wire from the fluid medium, whereby said skeletalsupport frame provides a plurality of openings forfacilitating a molding of said polymeric coating.According to a further aspect of the presentinvention, there is provided an electrical resistanceheating element capable of being disposed through a wall ofa tank for use in connection with heating a fluid mediumcomprising: (a) a tubular, polymeric, skeletal support framehaving a first supporting surface thereon; (b) a resistancewire wound onto said first supporting surface and connectedto at least a pair of terminal end portions; (c) athermally-conductive polymeric coating disposed over saidresistance wire and a significant portion of said supportframe for hermetically encapsulating and electricallyinsulating said resistance wire from the fluid medium; and(d) a plurality of heat transfer fins disposed to extendfrom the surface of said heating element to provide moreefficient heating of the fluid medium.According to a further aspect of the presentinvention, there is provided a method of manufacturing an3b1015202530CA 02265674 2003-08-0671576-135electrical resistance heating element for heating a fluidmedium, comprising: (a) providing a tubular, polymericskeletal support frame having a first supporting surfacethereon; (b) winding a resistance wire connected to at leasta pair of terminal end portions onto said first supportingsurface; (c) molding a thermally—conductive polymericcoating over said resistance wire and a significant portionof said support frame for hermetically encapsulating andelectrically insulating said resistance wire from the fluidmedium; and (d) providing a plurality of heat transfer finsextending from the first supporting surface of said heatingelement to provide more efficient heating of the fluidmedium.According to a further aspect of the invention,there is provided a method of manufacturing an electricalresistance element comprising: (a) providing a supportstructure having a plurality of openings therethrough and asupport surface thereon; (b) disposing a resistance heatingwire on said support surface; and (c) molding a thermally-conductive polymeric material over said resistance heatingwire and a major portion of said support structure toelectrically insulate and hermetically encapsulate said wireand the major portion of said support structure, saidthermally—conductive polymeric material contacting saidwire, where the electrical resistance element is anelectrical resistance element for heating a fluid, thesupport structure is a skeletal support frame comprising aplurality of longitudinal splines, and said wire and themajor portion of said support structure are encapsulatedfrom the fluid, wherein step (a) comprises injection moldingsaid skeletal support frame, and step (c) comprisesinjection molding said thermally—conductive polymeric3C1015CA 02265674 2003-08-0671576-135material to encapsulate said wire and at least about 90percent of said skeletal support frame wherein the remainingportion of said skeletal support frame that is notencapsulated comprises a plurality of heat transfer fins.A Brief Description of the DrawingsThe accompanying drawings illustrate preferredembodiments of the invention, as well as other informationpertinent to the disclosure, in which:FIG. 1: is a perspective view of a preferredpolymeric fluid heater of this invention;FIG. 2: is a left side, plan view of the polymericfluid heater of FIG. 1;FIG. 3: is a front planar view, including partialcross-sectional and peel-away views, of the polymeric fluidheater of FIG. 1;FIG. 4: is a front planar, cross-sectional view ofa preferred inner mold portion of the polymeric fluid heaterof FIG. 1;3dWO 98124269l0l520253035CA 02265674 l999-03- 15PCT/US97/2171 1FIG. 5:View of a preferred termination assembly for the polymericfluid heater of FIG. 1;FIG. 6:the end of a preferred coil for a polymeric fluid heateris a front planar, partial cross—sectionalis a enlarged partial front planar view ofof this invention; andFIG. 7:a dual coil embodiment for a polymeric fluid heater ofis an enlarged partial front planar view ofthis invention;FIG. 8: is a front perspective view of a preferredskeletal support frame of the heating element of thisinvention;FIG. 9: is an enlarged partial view of the preferredskeletal support frame of FIG. 8, illustrating a depositedthermally-conductive polymeric coating;FIG. 10: is an enlarged cross—sectional view of analternative skeletal support frame;FIG. 11: is a side plan view of the skeletal supportframe of FIG. 10;FIG. 12 is a front plan view of the full skeletall0.andsupport frame of FIG.Detailed Description of the InventionThis invention provides electrical resistance heatingelements and. water heaters containing these elements.These devices are useful in minimizing galvanic corrosionwithin water and oil heaters, as well as lime buildup andproblems of shortened element life. As used herein, theterms "fluid" and "fluid medium" apply to both liquids andgases.With reference to the drawings, and.particularly withreference to FIGS. 1-3 thereof, there is shown a preferredfluid heater 100 of this Thefluid heater 100polymeric invention.polymeric contains an electricallyconductive, resistance heating material. This resistanceheating material can be in the form of a wire, mesh,ribbon, or serpentine shape, for example. In thepreferred heater 100, a coil 14 having a pair of free ends4WO 98/24269l01520253035CA 02265674 l999-03- 15PCT/US97/21711joined to a pair of terminal end portions 12 and 16 isCoil 14 ishermetically and electrically insulated from fluid with anprovided for generating resistance heating.integral layer of a high temperature polymeric material.In other words, the active resistance heating material isprotected from shorting out in the fluid by the polymericcoating. The resistance material of this invention is ofsufficient surface cross—sectionalarea, length orthickness to heat water to a temperature of at least aboutl20°F without melting the polymeric layer. As will beevident from the below discussion, this can beaccomplished through carefully selecting the propermaterials and their dimensions.With reference to FIG. 3 in particular, the preferredpolymeric fluid heater 100 generally" comprises threeintegral parts: a termination assembly 200, shown in FIG.5, an inner mold 300,shown in FIG. 4, and a polymercoating 30. Each of these subcomponents, and their finalassembly into the polymeric fluid heater 100 will now befurther explained.The preferred inner mold 300, shown in FIG. 4, is asingle—piece injection molded component made from a hightemperature polymer. The inner mold 300 desirably includesa flange 32 at its outermost end. Adjacent to the flange32 is a collar portion having a plurality of threads 22.The threads 22 are designed to fit within the innerdiameter of a mounting aperture through the sidewall of astorage tank, for example in a water heater tank 13. AnO—ring (not shown) can be employed on the inside surfaceof the flange 32 to provide a surer water-tight seal. Thepreferred inner mold 300 also includes a thermistor cavity39 located within its preferred circular cross—section.The thermistor cavity 39 can include an end wall 33 forseparating the thermistor 25 from fluid. The thermistorcavity 39 is preferably open through the flange 32 so asto provide easy insertion of the termination assembly 200.The preferred inner mold 300 also contains at least a pairof conductor cavities 31 and 35 located. between the5WO 98/24269101520253035CA 02265674 l999-03- 15PCT/US97l21711thermistor cavity and the outside wall of the inner moldfor receiving the conductor bar 18 and terminal conductor20 of the termination assembly 200. The inner mold 300contains a series of radial alignment grooves 38 disposedaround its outside circumference. These grooves can beand should bespaced sufficiently to provide a seat for electricallythreads or unconnected trenches, etc.,separating the helices of the preferred coil 14.The preferred inner mold 300 can be fabricated usinginjection molding processes. The flow—through cavity 1112.5thereby creating anis preferably produced using a inch longhydraulically activated core pull,element which is about 13-18 inches in length. The innermold 300 can be filled in a metal mold using a ring gateplaced opposite from the flange 32. The target wallthickness for the active element portion 10 is desirablyless than .5 inches, and preferably less than.04-.06 inches,believed. to be the current lower limit for injection.1 inches,with a target range of about which ismolding equipment. A pair of hooks or pins 45 and 55 arealso molded along the active element development portion10 between consecutive threads or trenches to provide atermination point or anchor for the helices of one or morecoils. Side core pulls and an end core pull through theflange portion can be used to provide the thermistorcavity 39, conductor cavities 31and 35,flow—through cavity 11,and flow—through apertures 57 during injectionmolding.With reference to FIG. 5, the preferred terminationassembly 200 will now be discussed. The terminationassembly 200 comprises a polymer end cap 28 designed toaccept a pair of terminal connections 23 and 24. As shownin FIG. 2, the terminal connections 23 and 24 can containthreaded holes 34 and 36 threadedsuch as afor accepting aconnector, screw, for mounting externalelectrical wires. The terminal connections 23 and 24 arethe end portions of terminal conductor 20 and thermistorconductor bar 21. Thermistor conductor bar 216l0l520253035WO 98/24269CA 02265674 l999-03- 15withThe other thermistor terminal 29electrically connects terminal connection 24thermistor terminal 27.is connected to thermistor conductor bar 18 which isdesigned to fit within conductor cavity 35 along the lowerportion of FIG. 4. To complete the circuit, a thermistor25 is provided. Optionally, the thermistor 25 can bereplaced with a thermostat, a solid—state TCO or merely agrounding band that is connected to an external circuitbreaker, or the like. It is believed that the groundingbandthe terminal end portions 16 or 12 so as to short—out(not shown) could be located approximate to one ofduring melting of the polymer.In the preferred environment, thermistor 25 is asnap—action thermostat/thermoprotector such as the ModelW Series sold by Portage Electric. This thermoprotectorhas compact dimensions and is suitable for 120/240 VACloads. It comprises a conductive bi—metallic constructionEndpreferably a separate molded polymeric part.with an electrically active case. cap 28 isAfter the termination assembly 200 and inner mold 300are fabricated, they are preferably assembled togetherprior to winding the disclosed coil 14 over the alignmentgrooves 38 of the active element portion 10. In doing so,one must be careful to provide a completed circuit withthe coil terminal end portions 12 and 16. This can beassured by brazing, soldering or spot welding the coilterminal end portions 12 and 16 to the terminal conductor20 and thermistor conductor bar 18. It is also importantto properly locate the coil 14 over the inner mold 300In thecoating 30 isprior to applying the polymer coating 30.the polymerover—extruded to form a thermoplastic polymeric bond withthe inner mold 300. As with the inner mold 300,pulls can be introduced into the mold during the moldingtheflow—through cavity 11 open.preferred. embodiment,coreprocess to andkeep flow—through apertures 57With respect to FIGS. 6 and 7, there are shown singleand double resistance wire embodiments for the polymeric7PCT/US97/21711101520253035WO 98/24269CA 02265674 l999-03- 15resistance heating elements of this invention. In thesingle wire embodiment shown in FIG. 6, the alignmentgrooves 38 of the—inner mold 300 are used to wrap a firstwire pair having helices 42 and 43 into a coil form.Since the preferred embodiment includes a foldedresistance wire, the end portion of the fold or helixterminus 44 is capped by folding it around pin 45. Pin 45ideally is part of, and injection molded along with, theinner mold 300.Similarly, a dual resistance wire configuration canbe provided. In this embodiment,42 and 43 of theseparated from the next consecutive pair of helices 46 andthe first pair ofhelices first resistance wire are47 in the same resistance wire by a secondary coil helixterminus 54 wrapped around a second pin 55. A second pairwhichare electrically connected to the secondary coil helixof helices 52 and 53 of a second resistance wire,terminus 54, are then wound around the inner mold 300 nextto the helices 46 and 47 in the next adjoining pair ofalignment grooves. Although the dual coil assembly showsalternating pairs of helices for each wire, it isunderstood that the helices can be wound in groups of twoor more helices for each resistance wire, or in irregularnumbers, and winding shapes as desired, so long as theirconductive coils remain insulated from one another by theinner mold, or some other insulating material, such asseparate plastic coatings, etc.The plastic parts of this invention preferablyinclude a "high temperature" polymer which will not deformsignificantly or melt at fluid medium temperatures ofabout l20—180°F. Thermoplastic polymers having a meltingtemperature greater than 200°F are most desirable,although certain ceramics and thermosetting polymers couldalso be useful for this purpose. Preferred thermoplasticmaterial can include: fluorocarbons, polyaryl—sulphones,polyimides, polyetheretherketones, polyphenylenesulphides, polyether sulphones, and mixtures andcopolymers of these thermoplastics. Thermosetting8PCT/US97/217111O1520253035WO 98/24269CA 02265674 l999-03- 15polymers which would be acceptable for such applicationsinclude certain epoxies, phenolics, and silicones.Liquid—crystal polymers can also be employed for improvinghigh temperature chemical processing.In the of thispolyphenylene sulphidepreferred embodiment(uppsn)of its elevated temperature service, low cost and easierinvention,is most desirable becauseprocessability, especially during injection molding.The polymers of this invention can contain up toabout 5-40 wt.%graphite, glass or polyamide fiber. These polymers can bepercent fiber reinforcement, such. asthermalThermalconductivity can be improved with the addition of carbon,mixed with various additives for improvingconductivity and mold—release properties.graphite and metal powder or flakes. It is important,however, that such additives are not used in excess, sincean overabundance of any conductive material may impair thetheAny of the polymeric elementsinsulation and corrosion—resistance effects ofpreferred.polymer coatings.of this invention can be made with any combination ofthese materials, or selective ones of these polymers canbe used with or without additives for various parts ofthis invention depending on the end—use for the element.The resistance material used to conduct electricalcurrent and generate heat in the fluid heaters of thisinvention preferably contains a resistance metal which iselectrically conductive, and heat resistant.A popularmetal is Ni—Cr alloy although certain copper, steel andstainless—steel alloys could be suitable. It is furtherenvisioned that conductive polymers, containing graphite,carbon or metal powders or fibers, for example, used as asubstitute for metallic resistance material, so long asthey are capable of generating sufficient resistanceheating to feat fluids, such as water.The remainingelectrical conductors of the preferred polymeric fluidheater 100 can also be manufactured using these conductivematerials.PCT/US97/21711. ..W.«............a...«..w....... .._....,...... V V7--M . ..i._...,...,.. ,. , ..101520253035W0 98l24269CA 02265674 l999-03- 15As an alternative to the preferred inner mold300 of this invention, a skeletal support frame 70, shownin FIGS. 8 and 9 hasadditional benefits.been demonstrated to provideWhen a solid inner mold 300, such asa tube, was employed in injection molding operations,improper filling of the mold sometimes occurred due toheater designs requiring thin wall thicknesses of as lowas 0.025 inches, and exceptional lengths of up to 14inches. The thermally—conductive polymer also presenteda problem since it desirably included additives, such asglass fiber and ceramic powder, aluminum oxide (Al§%) andmagnesium oxide (MgO), which caused the molten polymer tobe extremely viscous. As a result, excessive amounts ofpressure were required to properly fill the mold, and attimes, such pressure caused the mold to open.In order to minimize the incidence of such problems,this invention contemplates using a skeletal support frame70 having a plurality of openings and a support surfacefor retaining resistance heating wire 66. In a preferredthe skeletal includes aembodiment, support frame 70tubular member having about 6-8 spaced longitudinalsplines 69 running the entire length.of the frame 70. Thesplines 69 are held together by a series of ring supports60 longitudinally spaced over the length of the tube-likemember. These ring supports 60 are preferably less thanabout 0.05 thick,0 . O2S—O . 030 inches thick.about 0.125 inches wide at the top and desirably areThese fins 62should extend at least about 0.125 inches beyond the innerinches and more preferably aboutThe splines 69 are preferablytapered to a pointed heat transfer fin 62.diameter of the final element after the polymeric coating64 has been applied, and, as much as 0.250 inches, toeffect maximum heat conduction into fluids, such as water.The outer radial surface of the splines 69 preferablyinclude grooves which can accommodate a double helicalalignment of the preferred resistance heating wire 66.Although this invention describes the heat transferfins 62 as being part of the skeletal support frame 70,10PCT/US97/21711lO1520253035WO 98/24269CA 02265674 l999-03- 15such fins 62 can be fashioned as part of the ring supports60 or‘ the overmolded polymeric coating 64, or from aplurality of these surfaces. Similarly, the heat transferfins 62 can be provided on the outside of the splines 69so as to theAdditionally,piercethisbeyond polymeric coating 64.invention envisionsproviding aplurality of irregular or geometrically shaped bumps ordepressions along the inner or outer surface of theprovided heating elements. Such heat transfer surfaces areknown to facilitate the removal of heat from surfaces intoliquids. They’ can. be provided i11 a number of ways,including injection molding them into the surface of thepolymeric coating 64 or fins 62, etching, sandblasting, ormechanically working the exterior surfaces of the heatingelements of this invention.In a preferred embodiment of this invention, theskeletal support frame 70 includes a thermoplastic resin,which can. be one of the "high temperature" polymersdescribed herein, such as polyphenylene sulphide ("PPS"),with a smallamount of glass fibers for structuralsupport, and optionally ceramic powder, such as Al§§ orMgO, for improving thermal conductivity. Alternatively,the skeletal support frame can be a fused ceramic member,including one or more of alumina silicate, Algk, Mgo,ZrO2, Sifig, Y§%, SiC, SiO2, etc., or athermoplastic or thermosetting polymer which is differentgraphite,than the "high temperature" polymers suggested to be usedwith the coating 30. If a thermoplastic is used for theskeletal support frame 70 it should have a heat deflectiontemperature greater than the temperature of the moltenpolymer used to mold the coating 30.The skeletal support frame 70 is placed in a wirewinding machine and the preferred resistance heating wire66 is folded and wound in a dual helical configurationaround the skeletal support frame 70 in the preferredThe fully woundskeletal support frame 70 is thereafter placed in thesupport surface, i.e. spaced grooves 68.injection mold and then is overmolded with one of the11PCT/US97/2171 1l01520253035WO 98/24269CA 02265674 l999-03- 15preferred polymeric resin formulas of this invention. Inone preferred embodiment, only a small portion of the heattheremainder of the skeletal support frame 70 is covered withtransfer fin 62 remains exposed to contact fluid,the molded resin on both the inside and outside, if it istubular in shape. This exposed portion is preferably lessthan about 10 percent of the surface area of the skeletalsupport frame 70.The open cross—sectional areas, constituting theplurality of openings of the skeletal support frame 70,permit easier filling and greater coverage of theresistance heating wire 66 by the molded resin, whileminimizing the incidence of bubbles and hot spots. Inpreferred embodiments, the open areas should comprise atleast about 10 percent and. desirably greater‘ than 20percent of the entire tubular surface area of the skeletalsupport frame 70, so that molten polymer can more readilyflow around the support frame 70 and resistance heatingwire 66.An alternative skeletal support frame 200 isillustrated in FIGS. 10-12. The alternative skeletalsupport frame 200 also includes a plurality oflongitudinal splines 268 having spaced grooves 260 foraccommodating a wrapped resistance heating wire (notshown). The longitudinal splines 268 are preferably heldtogether with spaced ring supports 266. The spaced ringsupports 266 include aof spokes 264 and a hub 262."wagon. wheel" design. having aplurality This providesincreased structural support over the skeletal supportframe 70, while not substantially interfering with thepreferred injection molding operations.the thisinvention can be applied by dipping the disclosed skeletalAlternatively, polymeric coatings ofsupport frames 70 or 200, for example, in a fluidized bedof pelletized or powderized polymer, such as PPS. In sucha process, the resistance wire should be wound onto theskeletal supporting surface, and energized.to create heat.If PPS is employed, a temperature of at least about 500°F12PCT/US97l217 ll101520253035W0 98/24269CA 02265674 l999-03- 15should be generated prior to dipping the skeletal supportThefluidized bed will permit intimate contact between theframe into the fluidized bed of pelletized polymer.pelletized polymer and the heated resistance wire so as tosubstantially uniformly provide athepolymeric coatingandThesolidentirely around resistance heating wiresubstantially around the skeletal support frame.resulting element include acan relativelystructure, or have a substantial number of opencross—sectional areas, although it is assumed that theresistance heating wire should be hermetically insulatedfrom fluid contact. It is further understood that theskeletal support frame and resistance heating wire can bepre—heated, rather than energizing the resistance heatingwire to generate sufficient heat for fusing the polymerpellets onto its surface. This process can also includepost-fluidized bed heating to provide a more uniformcoating. Other modifications to the process will bewithin the skill of current polymer technology.The standard rating of the preferred polymeric fluidheaters of this invention used in heating water is 240 Vand 4500 W, although theconducting coils 14 canlength and wire diameter of thebe varied to provide multipleabout 6000 W,between about 1700 W and 4500 W. For gas heating,wattages of about 100-1200 W can be used.ratings from 1000 W to and. preferablylowerDual, and eventriple wattage capacities can be provided by employingmultiple coils or resistance materials terminating atdifferent portions along the active element portion 10.From the foregoing, it can be realized that thisinvention provides improved fluid heating elements for usein all types of fluid heating devices, including waterheaters and oil space heaters. The preferred devices ofthis invention are mostly polymeric, so as to minimizeexpense, and to substantially reduce galvanic actionwithin fluid storage tanks. In certain embodiments of thisinvention, the polymeric fluid heaters can be used in13..,‘...». w...............,..x, ..PCT/US97/21711CA 02265674 l999-03- 15WO 98/24269 PCT/US97l2l71 1conjunction with a polymeric storage tank so as to avoidthe creation of metal ion—related corrosion altogether.Alternatively, these polymeric fluid heaters can bedesigned. to be used separately as their own storagecontainer to simultaneously store and heat gases orfluid. In such an embodiment, the flow—through cavity 11could be molded in the form of a tank or storage basin,and the heating coil 14 could be contained within the wallof the tank or basin and energized to heat a fluid or gasin the tank or basin. The heating devices of thisinvention could also be used in food warmers, curlerheaters, hair dryers, curling irons, irons for clothes,and recreational heaters used in spas and pools.This invention is also applicable to flow—throughheaters in which a fluid. mediunl is passed through apolymeric tube containing one or more of the windings orresistance materials of this invention. As the fluidmedium passes through the inner diameter of such a tube,resistance heat is generated through the tube’s innerdiameter polymeric wall to heat the gas or liquid.Flow-through heaters are useful in hair dryers and in"on-demand" heaters often used for heating water.Although Various embodiments have been illustrated,this is for the purpose of describing and not limiting theinvention. Various modifications, which will becomeapparent to one skilled in the art, or within the scope ofthis in the attached claims.l4

Claims (22)

CLAIMS:

1. An electrical resistance heating element capable of being disposed through a wall of a tank for use in connection with heating a fluid medium, comprising:

(a) a first flanged end;

(b) a skeletal support frame having a plurality of openings therethrough and a first supporting surface thereon;

(c) a resistance wire wound onto said first supporting surface and connected to at least a pair of terminal end portions at said first flanged end of said heating element; and (d) a thermally-conductive polymeric coating disposed over said resistance wire for hermetically encapsulating and electrically insulating said resistance wire from the fluid medium.

2. The heating element of claim 1, wherein said skeletal support frame comprises a plurality of longitudinal splines.

3. The heating element of claim 2, wherein said longitudinal splines comprise a plurality of grooves for supporting said resistance wire.

4. The heating element of claim 3, further comprising a plurality of ring supports connecting said longitudinal splines.

5. The heating element of claim 4, wherein said skeletal support frame further comprises heat transfer fins disposed to extend into the fluid medium.

6. The heating element of claim 1, wherein said skeletal support frame comprises a generally tubular shape wherein said plurality of openings represent at least about percent of the entire surface area of said tubular shape for facilitating a molding of said thermally-conductive polymeric coating over said resistance wire.

7. The heating element of claim 6, wherein said skeletal support frame comprises a plurality of longitudinal splines having a series of spaced grooves for receiving said resistance wire.

8. The heating element of claim 7, wherein said skeletal support frame and said thermally-conductive polymeric coating comprise a common thermoplastic resin.

9. A polymeric skeletal support frame for supporting a resistance wire of an electrical resistance heating element, comprising:

a plurality of longitudinal splines including spaced grooves along their lengths, a plurality of heat transfer fins extending from said longitudinal splines, said longitudinal splines integrally connected by a plurality of longitudinally-spaced ring supports.

10. A water heater comprising:

(a) a tank for containing water; and (b) a heating element attached to a wall of said tank for providing electrical resistance heating to a portion of the water in said tank, said heating element comprising:

a skeletal support frame having a plurality of openings therethrough and a first supporting surface thereon;

a resistance wire wound onto said first supporting surface and connecting to at least a pair of terminal end portions; and a thermally-conductive polymeric coating disposed over said resistance wire and a major portion of said skeletal support frame for hermetically encapsulating and electrically insulating said resistance wire from the water.

11. The water heater of claim 10, wherein said skeletal support frame comprises a plurality of longitudinal splines integrally connected by ring supports to provide a series of side wall apertures for facilitating molding of said thermally-conductive polymeric coating over said resistance wire.

12. A method of manufacturing an electrical resistance heating element for heating a fluid medium, comprising:

(a) providing a polymeric skeletal support frame having a plurality of longitudinal splines connected by a series of spaced ring supports, said longitudinal splines comprising spaced grooves;

(b) winding a resistance heating wire onto said spaced grooves, said resistance heating wire having a pair of free ends joined to a pair of terminal end portions; and (c) molding a polymeric coating containing an additive for improving the thermal conductivity of said coating over said resistance heating wire and at least 90 percent of said skeletal support frame to electrically insulate and hermetically encapsulate said resistance wire from the fluid medium, whereby said skeletal support frame provides a plurality of openings for facilitating the molding of the polymeric coating.

13. The method of claim 12, wherein said skeletal support frame and said polymeric coating comprise a common thermoplastic resin.

14. The method of claim 12, wherein said providing step (a) comprises injection molding of said skeletal support frame, and said molding step (c) comprises injection molding of said polymeric coating to encapsulate said resistance heating wire and the at least about 90 percent of said skeletal support frame.

15. An electrical resistance heating element capable of being disposed through a wall of a tank for use in connection with heating a fluid medium, comprising:

(a) a polymeric skeletal support frame having a plurality of longitudinal splines connected by a series of spaced ring supports, said longitudinal splines comprising spaced grooves;

(b) a resistance wire having a pair of free ends joined to a pair of terminal end portions, said resistance wire being wound onto and supported by said spaced grooves;
and (c) a polymeric coating containing an additive for improving the thermal conductivity of said polymeric coating disposed over said resistance wire and at least about 90 percent of said skeletal support frame for hermetically encapsulating and electrically insulating said resistance wire from the fluid medium, whereby said skeletal support frame provides a plurality of openings for facilitating a molding of said polymeric coating.

16. The heating element of claim 15, wherein said skeletal support frame comprises a generally tubular shape.

17. The heating element of claim 16, further comprising heat transfer fins disposed on an internal surface of said tubular shape being provided.

18. An electrical resistance heating element capable of being disposed through a wall of a tank for use in connection with heating a fluid medium comprising:

(a) a tubular, polymeric, skeletal support frame having a first supporting surface thereon;

(b) a resistance wire wound onto said first supporting surface and connected to at least a pair of terminal end portions;

(c) a thermally-conductive polymeric coating disposed over said resistance wire and a significant portion of said support frame for hermetically encapsulating and electrically insulating said resistance wire from the fluid medium; and (d) a plurality of heat transfer fins disposed to extend from the surface of said heating element to provide more efficient heating of the fluid medium.

19. A method of manufacturing an electrical resistance heating element for heating a fluid medium, comprising:

(a) providing a tubular, polymeric skeletal support frame having a first supporting surface thereon;

(b) winding a resistance wire connected to at least a pair of terminal end portions onto said first supporting surface;

(c) molding a thermally-conductive polymeric coating over said resistance wire and a significant portion of said support frame for hermetically encapsulating and electrically insulating said resistance wire from the fluid medium; and (d) providing a plurality of heat transfer fins extending from the first supporting surface of said heating element to provide more efficient heating of the fluid medium.

20. A method of manufacturing an electrical resistance element comprising:

(a) providing a support structure having a plurality of openings therethrough and a support surface thereon;

(b) disposing a resistance heating wire on said support surface; and (c) molding a thermally-conductive polymeric material over said resistance heating wire and a major portion of said support structure to electrically insulate and hermetically encapsulate said wire and the major portion of said support structure, said thermally-conductive polymeric material contacting said wire, where the electrical resistance element is an electrical resistance element for heating a fluid, the support structure is a skeletal support frame comprising a plurality of longitudinal splines, and said wire and the major portion of said support structure are encapsulated from the fluid, wherein step (a) comprises injection molding said skeletal support frame, and step (c) comprises injection molding said thermally-conductive polymeric material to encapsulate said wire and at least about 90 percent of said skeletal support frame wherein the remaining portion of said skeletal support frame that is not encapsulated comprises a plurality of heat transfer fins.

21. The method of claim 20, wherein said longitudinal splines comprises a plurality of grooves for receiving said wire.

22. The method of claim 20, wherein said skeletal support frame and said thermally-conductive polymeric material comprises a common thermoplastic resin.