MXPA99004709A - Improved immersion heating element with highly thermally conductive polymeric coating - Google Patents

Abstract

Electrical resistance heating elements (100) are provided which are useful in heating fluid mediums, such as air and water. The heating elements include an element body (100) having a supporting surface (10) and a resistance wire (14) wound onto the supporting surface (10) which is connected to a pair of terminal end portions (16 and 12). Disposed over the resistance wire (14), and over most of the supporting surface (10), is apolymeric coating (30) which hermetically encapsulates and electrically insulates the resistance wire (14) from the fluids to be heated. This thermally-conductive polymer coating (30) has a thermal conductivity value of at least about 0.5 W/mK. Improved properties are preferably provided by ceramic powder, aluminum oxide and magnesium oxide, and glass fiber additives.

Description

IMPROVED IMMERSION HEATING ELEMENT WITH THERMALLY DRIVING POLYMERIC COATING FIELD OF THE INVENTION This invention is concerned with electrical resistance heating elements and more particularly with resistance heating elements containing polymers for heating gases and liquids.

BACKGROUND OF THE INVENTION Electric resistance heating elements used in connection with water heaters have traditionally been made of metal and ceramic components. A typical construction includes a pair of terminal needles welded to the ends of a Ni-Cr coil, which is then axially disposed through a U-shaped tubular metal casing. The resistance coil is insulated from the casing. metal by a pulverized ceramic material, usually magnesium oxide. While such conventional heating elements have been the workpiece for the water heater industry for decades, there have been a variety of widely recognized deficiencies. For example, the galvanic currents that occur between the REF .: 29808 metal enclosure and any exposed metal surface in the tank can create corrosion of the various anodic metal components of the system. The metal envelope of the heating element, which is usually made of copper or a copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper pipes has become increasingly expensive as the price of copper has increased over the years. As an alternative to metal elements, at least one electrical plastic wrap heating element has been proposed in US Pat. No. 3,943,328 issued to Cunningha. In the described device, a conventional resistance wire and powdered magnesium oxide are used in conjunction with a plastic shell. Since this plastic enclosure is non-conductive, there is no galvanic cell created with the other metal parts of the heating unit that are in contact with the water in the tank and there is also no buildup of lime. Unfortunately, for various reasons, these plastic envelope heating elements of the prior art were not able to achieve high nominal powers in a normal useful service life and concurrently were not widely accepted.

BRIEF DESCRIPTION OF THE INVENTION This invention provides electric resistance heating elements for use in connection with the heating of fluid media, such as air and water. These elements include an element body having a support surface thereon and a resistance wire wound or wound on the support surface and connected to at least a pair of end portion portions of the element. Arranged on the resistance wire and the support surface is a thermally conductive polymeric coating that forms an airtight seal around the resistance wire. The thermally conductive polymeric coating has a thermal conductivity value of at least about 0.5 W / m ° K. The heating elements of this invention are designed to provide multiple rated powers from 1000 W to approximately 6000 W and higher. For gas heating, these elements can provide lower nominal powers of less than about 1200 W. The improved thermally conductive polymeric coatings of this invention provide thermal conductivity values that allow a fairly improved heat dissipation of the resistance wire. This property allows the described elements to provide efficient fluid heating without melting the relatively thin polymer coatings. Loads in the range of about 60-200 parts of ceramic material per 100 parts of resin in the polymeric coating are preferred. The lower limit is set by the amount of thermal conductivity needed to heat fluids and the upper limit is set to provide easier molding of these elements by standard processing, such as by injection molding. The fibrous reinforcement has also been useful in providing mechanical strength to the polymer coating to resist cracking and deformation during cyclic thermal loads, such as those experienced in a water heater. In further embodiments of this invention, the improved thermally conductive polymeric coatings are applied to conventional metal shell elements to reduce galvanic corrosion in water heaters, without substantially interfering with the heating efficiency of the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the description, in which: Figure 1 is a perspective view of a preferred polymeric fluid heater of this invention; Figure 2 is a plan view, on the left side, of the polymeric fluid heater of Figure 1; Figure 3 is a front plan view, which includes partial and sectional cross sectional views of the polymeric fluid heater of Figure 1; Figure 4 is a cross-sectional view, of the front plane, of an internal portion of the preferred mold of the polymeric fluid heater of the figure 1; Figure 5 is a partial cross-sectional view, of the front plane, of a preferred termination assembly for the polymeric fluid heater of Figure 1; Figure 6 is an enlarged partial front plan view of the end of a preferred coil for a polymeric fluid heater of this invention; and Figure 7 is a partial, enlarged front plane view of a double coil embodiment for a polymeric fluid heater of this invention; Figure 8 is a front perspective view of a preferred structural support frame or frame of the heating element of this invention; Figure 9 is an enlarged partial view of the preferred structural support frame or frame of Figure 8, illustrating a deposited thermally conductive polymeric coating; Figure 10 is an enlarged cross-sectional view of an alternative structural support frame or frame; Figure 11 is a side plan view of the structural support frame or frame of Figure 10; Figure 12 is a front plan view of the entire structural support frame or frame of Figure 10; and Figure 13 is a cross-sectional side view of an improved metal casing element equipped with a thermally conductive polymeric coating of this invention.

DETAILED DESCRIPTION OF THE INVENTION This invention provides electric resistance heating elements and water heaters containing these elements. These devices are useful for minimizing galvanic corrosion in water and oil heaters, as well as the accumulation of lime and the problems of the reduced life of the element. As used herein, the terms "fluid" and "fluid medium" apply to liquids and gases. With reference to the drawings and in particular with reference to Figures 1-3 thereof, there is shown a preferred polymeric fluid heater 100 of this invention. The polymeric fluid heater 100 contains a resistance electrically conductive heating material. This resistance heating material may be in the form of a wire, mesh, ribbon or coil shape, for example. In the preferred heater 100, a coil 14 having a pair of free ends attached to a pair of end portions 12 and 16 are provided to generate resistance heating. The coil 14 is hermetically and electrically insulated from the fluid with an integral layer of a polymeric material resistant to high temperatures. In other words, the active resistance heating material is protected from the short circuit in the fluid by the polymer coating. The strength material of this invention is of a cross-sectional area, length, or thickness sufficient to heat the water to a temperature of at least about 48.9 ° C (120 ° F) without melting the polymeric layer. As will be evident from the discussion below, this can be accomplished through the careful selection of the appropriate materials and their dimensions. With reference to Figure 3 in particular, the preferred polymeric fluid heater 100 generally comprises three integral parts: a termination assembly 200, shown in Figure 5, an internal mold 300, shown in Figure 4 and a polymeric coating 30. Each of these subcomponents and their final assembly to the polymeric fluid heater 100 will now be further explained. The preferred internal mold 300, shown in Figure 4, is an injection molded, one-piece components made of a polymer resistant to high temperatures. The internal mold 300 desirably includes a flange 32 at its outermost end. Adjacent the flange 32 is a collar portion having a plurality of threads 22. The threads 22 are designed to fit into the internal diameter of a mounting opening through the side wall of a storage tank, for example in a tank 13 of the water heater. An O-ring (not shown) can be employed on the inner surface of the flange 32 to provide a more secure water-tight seal. The preferred internal mold 300 includes a cavity 39 for thermistor located within its preferred circular cross section. The cavity 39 for the thermistor may include an end wall 33 for separating the thermistor 25 from the fluid. The cavity 39 for the thermistor is preferably opened through the flange 32 to provide easy insertion of the termination assembly 200. The preferred internal mold 300 also contains at least one pair of conductor cavities 31 and 35 located between the cavity for the thermistor and the outer wall of the internal mold for receiving the conductive bar 18 and the terminal conductor 20 of the termination assembly 200. The internal mold 300 contains a series of radial alignment slits 38 arranged around its outer circumference. These slits may be unattached threads or ditches, etc., and must be spaced sufficiently to provide a seat for electrically separating the propellers from the preferred coil 14. The preferred internal mold 300 may be manufactured by using injection molding processes. The through-flow cavity 11 is preferably produced by using a long, hydraulically activated, long ram of 31.7 cm (12.5 inches) long, thereby creating an element that is approximately 33.02-45.72 cm (13-18 inches) of length. The internal mold 300 can be filled into a metal mold by using an annular orifice or gate positioned opposite the flange 32. The target wall thickness for the portion 10 of the active element is desirably less than 1.27 cm (0.5 inches) and preferably less than 0.254 cm (0.1 inches) with a target range of approximately 0.1016-0.1524 cm (0.04-0.06 inches) which is believed to be the current lower limit for injection molding equipment. A pair of hooks or bolts 45 and 55 are also molded along the developed portion of the active element 10 between consecutive threads or ditches to provide a termination or securing point for the propellers of one or more coils. Lateral core dampers and a central end ram through the flange portion can be used to provide the cavity 39 for the thermistor, the through flow cavity 11, the cavities 31 and 35 for the conductor and the openings or holes 57 of through flow during injection molding. With reference to Figure 5, the preferred termination assembly 200 will now be discussed. The termination assembly 200 comprises a cap 28 of the polymer end designed to accept a pair of terminal connections 23 and 24. As shown in Figure 2, the terminal connections 23 and 24 may contain threaded holes 34 and 36 to accept a threaded connector , such as a screw, for mounting external electrical wires. The terminal connections 23 and 24 are the end portions of the terminal conductor 20 and the conductive bar 21 of the thermistor. The conductive bar 21 of the thermistor electrically connects the terminal connection 24 with the terminal 27 of the thermistor. The other terminal 29 of the thermistor is connected to the conductive bar 28 of the thermistor which is designed to fit into the cavity 35 for the conductor along the lower portion of FIG. 4. To complete the circuit, a thermistor 25 is provided. Optionally, the thermistor can be replaced with a thermostat, a solid state TCO or only a grounding band that is connected to an external circuit breaker or the like. It is believed that the grounding band (not shown) could be located close to one of the portions 16 or 12 of the terminal end to be shorted during melting of the polymer. In the preferred embodiment, the thermistor 25 is a spring-action thermostat / thermoprotector such as the W series model sold by Portage Electric. This thermoprotector has compact dimensions and is suitable for 120/240 VAC loads. It comprises a bimetallic conductive construction with an electrically active box. The end cap 28 is preferably a separate molded polymer part. After the termination assembly 200 and the internal mold 300 are manufactured, they are preferably assembled together prior to the winding of the described coil 14 on the recesses 38 of alienation of the portion 10 of the active element. In doing so, care must be taken to provide a complete circuit with portions 12 and 16 of the terminal end of the coil. This can be ensured by brazing, welding or spot welding of the portions 12 and 16 of the terminal end of the coil to the terminal conductor 20 and the conductive bar 18 of the thermistor. It is also important to properly locate the coil 14 on the internal mold 300 prior to the application of the polymeric coating 30. In the preferred embodiment, the polymeric coating 30 is molded to form a thermoplastic polymeric bond with the internal mold 300. Similarly to the internal mold 300, central rams can be introduced into the mold during the molding process to keep the apertures 57 open. through flow and through flow cavity 11. With respect to Figures 6 and 7, embodiments of a single resistance wire and double resistance wire for the polymeric resistance heating elements of this invention are shown. In the single-wire embodiment shown in Figure 6, the alignment slots 38 of the internal mold 300 are used to wrap a first pair of wires having helices 42 and 43 in a helical shape. Since the preferred embodiment includes a folded resistance wire, the end portion of the fold or propeller term 44 is crowned when folded around the needle or bolt 45. The needle or bolt is ideally part of and injection molded together with the inner mold 300. Similarly, a double wire resistance configuration can be provided. In this embodiment, the first pair of propellers 42 and 43 of the first resistance wire is separated from the next consecutive pair of helices 46 and 47 in the same resistance wire by a secondary coil winding term 54 wrapped around a second bolt or needle 55. A second for helices 52 and 53 of a second resistance wire, which are electrically connected to the secondary coil winding term 54, are then wound around the internal mold 300 next to the helices 46 and 47 in the next attached pair of alignment slits. Although the double-coil assembly shows alternating pairs of helices for each wire, it will be understood that the helices can be wound into groups of two or more helices for each resistance wire or in irregular numbers and winding forms as desired. their conductive coils remain insulated from each other by the internal mold or some other insulating material, such as separate plastic coatings, etc. The plastic parts of this invention, such as the polymeric coating 30, the structural support frame or frame 70 and the internal mold 300, preferably include a polymer "resistant to high temperatures" which will not deform significantly or melt at average temperatures of fluid of approximately 48.9-82 ° C (120-180 ° F) and coil temperatures of approximately 232-343 ° C (450 - 650 ° F). Thermoplastic polymers having a melting temperature greater than 93.3 ° C (200 ° F) and preferably higher than the coil temperature are more desirable, although certain thermosetting ceramics and polymers could also be useful for this purpose. Preferred thermoplastic material may include: fluorocarbons, polyaryl sulphones, polyimides, bismaleimides, polyphthalamides, polyetheretherketones, polyphenylene sulfides, polyether sulfones, and mixtures and copolymers of these thermoplastics. The thermosetting polymers that would be acceptable for such applications include polyimides, certain epoxies, phenolics and silicones. Liquid crystal polymers ("LCP") can also be used to improve the properties of high temperature resistance. In the preferred embodiment of this invention, polyphenylene sulfide ("PPS") is more desirable because of its high service temperature, low cost and easier processability, especially during injection molding. The polymers of this invention may contain up to about 5-60% fiber reinforcement. Thermoplastics and thermoplastics that reinforce the fiber greatly increase the resistance. For example, short glass fibers at a load of about 301 by weight reinforce the tensile strength of the design plastics by a factor of about two. Preferred fibers include fragmented glass such as glass E or glass S, boron, aramid, such as Kevlar 29 or 49, graphite and carbon fibers in which high modulus of tensile graphite is included. Other desirable fibers include polyphenylene benzobisthiazole (PBT) and polyphenylene benzobisoxozole (PBO) fibers heat treated and carbon / graphite fibers at 2% tension. These polymers can be mixed with various other additives to improve the properties of thermal conductivity and release of the mold. The thermal conductivity can be improved with the addition of metal oxides, nitrides, carbonates or carbides (hereinafter sometimes referred to herein as "ceramic additives") and low concentrations of carbon or graphite. Such additives may be in the form of powder, flakes or fibers. Good examples include oxides, carbides, carbonates and nitrides of tin, zinc, copper, molybdenum, calcium, titanium, zirconium, boron, silicon, yttrium, aluminum or magnesium or mica, glass ceramics or fused silica. The charges in the polymer matrix for these thermally conductive materials are preferably in a range of about 60 and 200 parts of additive to 100 parts of resin ("PPH") and more preferably about 80-180 PPH. These additives are generally non-electrically conductive, although conductive additives, such as metal fibers and flake powders of metals such as stainless steel, aluminum, copper or brass and higher concentrations of carbon or graphite, could be used if they are after This is molded or coated with a polymer layer more electrically insulated. If an electrically conductive additive is used, care must be taken to electrically isolate the core to prevent short circuiting between the coils.

It is however important that the above additives are not used in excess, since it has been known that an overabundance of the fiber reinforcement or metal or metal oxide additives deteriorate the molding operations. Any of the polymeric elements of this invention can be made with any combination of these selective polymeric or materials of these polymers can be used with or without additives for various parts of this invention depending on the end use for the element. This invention specifically contemplates that many combinations of polymer resin, glass fiber and different thermally conductive fillers in various percentages will be employed in polymeric compositions to provide desirable thermal conductivity values for the heating elements of various nominal powers. In addition to the thermally conductive reinforcements and fillers, the plastic compositions of this invention may also contain mold release additives, impact modifiers and thermooxidant stabilizers that not only improve the performance of the plastic parts and extend the life of the heating element , but also help in the molding process. The compositions listed in Table 1 below were prepared by combining polyphenylene sulfide with the aforementioned amounts of aluminum oxide, magnesium oxide and glass fiber in fragments, according to methods well known in the art. Pills or pellets (or pellets) of these materials were injection molded to produce ASTM test samples that were tested according to ASTM procedures to provide tensile strength, flexural strength, flexural modulus, and tensile strength data. Notch impact shown in Table 1. The thermal conductivity values were obtained similarly. It was found that comparative example 1 had too low a thermal conductivity to be useful in water heating elements. When the material of Example 8, which had the highest thermal conductivity, was injection molded onto a wound core to form the water heating element of this invention cracks and breaks occurred for wall thicknesses less than 0.0762 cm (0.030 inches) ). However, wall thicknesses greater than 0.0762 cm (0.030 inches) will allow such higher loads. This is evidence that tensile strength and flexural strength, as well as impact resistance, are adversely influenced by the addition of powdered ceramic additives, but variations in the design of the element and resins can be used to overcome the effects of high loads. Ideally, the tensile strength of the coating should be at least about 492 Kg / square centimeter (7,000 pounds / square inch) and preferably about 527-703 Kg / square centimeter (7,500-10,000 pounds / square inch) provided that a satisfactory thermal conductivity is maintained. The flexure modulus at operating temperatures should be at least about 35,150 Kg / square centimeter (500,000 pounds / square inch) and preferably greater than 703,000 Kg / square centimeter (1,000,000 pounds / square inch). Finally, of all the materials in Table 1, it was found that those materials corresponding to Examples 6 and 7 were more suitable for water heating elements because they had the best balance of structural properties and thermal conductivity. Of course, ceramic loads of approximately 60-200 PPH are to increase the thermal conductivity as much as possible without interfering with the molding operations. The thermal conductivity of the resulting coating should be at least about 0.5 / m ° K, preferably about 0.7 W / m ° K and ideally greater than about 1 W / m ° K. These compositions are presented by way of example and not by way of limitation. Nevertheless, for those skilled in the art, it should be clear that there are innumerable combinations of various conductive fillers with reinforcing fibers in resins which can also be optimized to function properly in the device of this invention. Such combinations could include LCP or PEEK resin resistant to high temperatures with boron nitride and glass additives in fragments, for example or if the cost is a matter, a resin of PPS and A120 ^ or MgO and glass additive in fragments.

Table 1 Ex Comp 1 Ex 2 Ex E 4 Ex Ex6 Ex7 Ex_8 Oxide of - 44 - - 37 69 129 208 Aluminum (PPH *) Oxide of - - 34 82 Magnesium (PPH *) Fiberglass 25 - 34 41 47 57 25 35 (PPH *) Resistance to 1188 (16,900) 689 (9,800) 815 (11,600) 597 (8,500) 1012 (14,400) 956 (13,600) 724 (10,300) 548 (7,800) traction Kgcm2 (psi) Resistance to 1870 (26,600) 1160 (16,500) 1357 (19,300) 1111 (15,800) 1441 (20,500) 1420 (20,200) 1146 (16,300) 766 (10,900) bending Kg / cm2 (psi) NJ Module 79439 (1,130) 56240 (800) 94905 (1,350) 125837 112480 133570 123025 170829 Bending Kg / cm2 (1,790) (1,600) (1,900) (1,750) (2,430) (Kpsi, 25 ° C) Scalping 1.08 0.40 0.52 0.44 0.53 0.50 0.31 0.25 Izod (foot-pound- / inch) Conductivi- 0.24 0.36 0.37 0.61 0.40 0.51 0.84 1.2 thermal input (W / m ° K.) * All additive measurements are in parts per hundred parts sulfide matrix polyphenylene.

With the use of the above polymeric materials of this invention, it is possible to coat the metal shell of the heating elements by conventional electrical resistance to avoid many of the problems previously experienced with such elements. It has been known that such enclosures include copper and stainless steel. Additionally, this invention contemplates the use of non-corrosion resistant materials for the shell, such as carbon steel. For corrosion resistant materials, the coating should be relatively thinner than for non-corrosion resistant materials and this should require coatings of at least about 0.0254 cm (10 mils) and higher thermal conductivity values. An improved version of a conventional electric resistance heating element 201 is shown in FIG. 13. This element 201 has a resistance heating wire disposed axially through a shell 220 of U-shaped tubular metal with material 230 of pulverized ceramic between wire 210 and shell 220 of metal. The shell 220 is then coated with a highly thermally conductive polymeric coating 240 of this invention to prevent the occurrence of galvanic currents between the metal shell and any exposed anode metal components of the system. The excellent thermal conductivity of the polymeric materials, in particular with the additives described herein, allows the heating elements to reach the high nominal powers necessary to heat the water efficiently at temperatures of more than 120 ° C without melting the coating. The polymeric coating can be applied to the metal shell, which contains for example copper, brass, stainless steel or carbon steel, either by injection molding or by dip coating the metal shell in a fluidized bed of the polymer. agglomerated or pulverized, such as PPS, PEEK, LCD, etc. The resistance material used to conduct electric current and generate heat in the fluid heaters of this invention preferably contains a resistance metal that is electrically conductive and heat resistant. A popular metal is Ni-Cr alloy although certain alloys of copper, steel and stainless steel may be appropriate. It is further contemplated that conductive polymers, containing graphite, carbon or metal powders or fibers, for example, used as a substitute for the metallic strength material, so long as they are capable of generating sufficient resistance heating to heat fluids, such as Water. The remaining electrical conductors of the preferred polymeric fluid heater 100 can also be manufactured by using this conductive material. As an alternative to the preferred internal mold 300 of this invention, a structural support frame or frame 70, shown in Figures 8 and 9 has been shown to provide additional benefits. When a solid internal mold 300, such as a tube, was used in injection molding operations, improper filling of the mold sometimes occurs due to heater designs that require thin wall thicknesses as low as 0.0635 cm (0.025 inches) and exceptional lengths of up to 35.56 cm (14 inches). The thermally conductive polymer also presents a problem since it desirably includes additives, such as glass fiber and ceramic powder, aluminum oxide (Al20- <;) and magnesium oxide (MgO), which cause the molten polymer to be extremely viscous. As a result, excessive amounts of pressure would be required to properly fill the mold and sometimes such pressure caused the mold to open. In order to minimize the incidence of such problems, this invention contemplates the use of a structural support frame or frame 70 having a plurality of openings and a supporting surface for retaining the resistance heating wire 66. In a preferred embodiment, the structural support frame 70 includes a tubular element having approximately 6-8 spaced longitudinal wedges running along the entire length of the frame. The wedges 69 are held together by a series of ring supports 60 spaced apart longitudinally over the length of the tube-like element. These ring supports 60 are preferably less than about 0.127 cm (0.05 inches) thick and more preferably approximately 0. 0635 - 0.0762 cm (0.025 - 0.030 inches) thick. The wedges 69 are preferably approximately 0.3175 cm (0.125 inches) wide at the top and desirably are tapered to a fin 62 heat transfer fin. These fins 62 should extend at least about 0.3175 cm (0.125 inches) beyond the internal diameter of the final element after the polymeric coating 64 has been applied and as much as 0.635 cm (0.250 inches) to effect maximum heat conduction to the fluids such as water. The external root surface of the wedges 69 preferably includes slots that can accommodate a double helical alignment of the preferred resistance heating wire 66.

Although this invention discloses the heat transfer fins 62 as part of the structural support frame 70, such fins 62 may be adapted as part of the ring supports 60 or the molded polymeric coating 64 or from a plurality of these surfaces. Similarly, the heat transfer fins 62 can be provided on the outside of the wedges 69 to pierce past the polymeric coating 64. Additionally, this invention contemplates the provision of a plurality of irregular or geometrically shaped protrusions or depressions along the length of the ribs. the internal or external surface of the heating elements provided. It is known that such heat transfer surfaces facilitate the removal of heat from surfaces to liquids. They can be provided in a variety of ways, including injection molding thereof to the surface of the polymeric coating 64 or fins 62, etching, sand spraying or mechanical tilling of the outer surfaces of the heating elements. this invention. In a preferred embodiment of this invention, the structural support frame 70 includes thermoplastic resin, which may be one of the "high temperature resistant" polymers described herein, such as polyphenylene sulfide ("PPS"), with a small amount of glass fibers for the structural support and optionally ceramic powder, such as Al0- < or MgO, to improve thermal conductivity. Alternatively, the structural support frame may be a cast ceramic element, in which one or more of alumina silicate, A120, MgO, graphite, ZrO.-., Si (Nj, Y..0 ,,, SiC, Si02, etc., or a thermoplastic or thermosetting polymer that is different from the "high temperature resistant" polymers suggested to be used with the coating 30. If a thermoplastic is used for the structural support frame 70, it must have a thermal deflection temperature greater than the temperature of the molten polymer used to mold the coating 30. The structural support frame 70 is placed on a wire winder and the preferred resistive heating wire 66 is folded and wound into a double configuration helix around the structural support frame 70 on the preferred support surface, that is, spaced grooves 68. After this, the structural support frame 70, fully It is placed in the injection mold and then molded with one of the preferred polymeric resin formulas of this invention. In a preferred embodiment, only a small portion of the heat transfer flap 62 remains exposed to contact with the fluid, the rest of the structural support frame 70 is covered with the resin molded on the inside and outside if it is tubular in shape. This exposed portion is preferably less than about 10 percent of the surface area of the structural support frame 70. The open cross-sectional areas, which constitute the plurality of openings in the structural support frame 70, allow an easier filling and greater coverage of the resistive heating wire 66 by the molded resin., while minimizing the incidence of bubbles and hot spots. In preferred embodiments, the open areas should comprise at least about 10 percent and desirably more than 20 percent of the entire tubular surface area of the structural support frame 70, such that the molten polymer can flow more easily around of the supporting frame 70 and the resistance heating wire 66. An alternative structural support frame 200 is illustrated in Figures 10-12. The alternative structural support frame 200 also includes a plurality of longitudinal wedges 268 having spaced slots 260 for accommodating a wrapped resistance heating wire (not shown). The longitudinal wedges 268 are preferably held together with spaced annular supports 266. The spaced ring holders 266 include a "wagon wheel" design having a plurality of spokes 264 and a hub or center 262. This provides increased structural support with respect to the structural support frame 70, so long as it does not substantially interfere with the preferred injection molding operations. Alternatively, the polymeric coatings of this invention can be applied by immersion of the described structural support frames 70 or 200 and the core 10 wound with wire, for example, in a fluidized polymer bed or powder, such as PPS. In such a process, the resistance wire must be wound on the structural support surface and energized to create heat. If PPS is employed, a temperature of at least about 260 ° C (500 ° F) must be generated before submerging the structural support frame to the agglomerated polymer fluidized bed. The fluidized bed will allow intimate contact between the agglomerated polymer and the heated resistance wire to substantially uniformly provide a polymeric coating completely around the resistance heating wire and substantially around the structural support frame. The resulting element may include a relatively solid structure or have a substantial number of open cross-sectional areas, although it is assumed that the resistance heating wire must be hermetically isolated from contact with the fluid. It will be further understood that the structural support frame and the resistance heating wire can be pre-heated, instead of energizing the resistance heating wire, to generate sufficient heat to melt the polymer pellets or pellets on its surface. This process may also include the post-heating of the fluidized bed to provide a more uniform coating. Other modifications to the process will be in the reach of the current polymer technology. The standard classification of the preferred polymeric fluid heaters of this invention, used in water heating, is 240 V and 4500 W, although the length and diameter of the wire of the conductive coils 14 can be varied to provide nominal powers of 1000 W at approximately 6000 W and preferably between approximately 1700 W and 4500 W. For gas heating, powers of less than about 100-1200 W can be used. Dual and even triple rated power capacities can be provided by using multiple coils or resistance materials ending in different portions along the portion 10 of the active element. From the foregoing, it will be noted that this invention provides improved fluid heating elements for use in all types of fluid heating devices, which include water heaters and oil space heaters. Most of the preferred devices of this invention are polymeric, to minimize expense and substantially reduce the galvanic action in the fluid storage tanks. In certain embodiments of this invention, polymeric fluid heaters can be used in conjunction with a polymeric storage tank to completely avoid the creation of corrosion related to metal ions. Alternatively, these polymeric fluid heaters can be designed to be used separately as their own storage container for simultaneously storing and heating gases or fluids. In such an embodiment, the through flow cavity 11 could be molded in the form of a storage tank or container and the heating coil 14 could be contained in the wall of the tank or container and energized to heat a fluid or gas that is in the tank. the tank or container. The heating devices of this invention could also be used in food heaters, curling tongs, hair dryers, hair tongs, clothes irons and recreational heaters used in recreation centers and swimming pools. This invention is also applicable to through flow heaters in which a fluid medium is passed through a polymeric tube containing one or more of the windings or resistive materials of this invention. As the fluid medium passes through the inner diameter of such a tube, heat is generated by resistance through the polymer wall of the inner diameter of the tube to heat the gas or liquid. Through-flow heaters are useful in hair dryers and "on-demand" heaters often used to heat water. Although several embodiments have been illustrated, it is for the purpose of describing and not limiting the invention. Various modifications will become apparent to those of ordinary skill in the art or within the scope thereof in the appended claims. It is noted that, with regard to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (25)

1. Claims Having described the invention as above, the content of the following claims is claimed as property: 1. An electric resistance heating element for use in connection with heating a fluid medium, characterized in that it comprises: (a) a body elemental having a support surface thereon; (b) a resistance wire wound on the support surface and connected to at least a pair of terminal end portions of the element; and (c) a thermally conductive polymeric coating disposed on the resistance wire and the support surface to hermetically encapsulate and electrically insulate the fluid resistance wire, the polymer coating comprises a thermally conductive ceramic additive, which is not electrically conductive. The heating element according to claim 1, characterized in that the polymeric coating has a thermal conductivity value of at least about 0.5 W / m ° K. 3. The heating element according to claim 2, characterized in that the polymeric coating comprises a thermoplastic resin having a melting point greater than 93.3 ° C (200 ° F). 4. The heating element according to claim 3, characterized in that the polymeric coating comprises a fiber reinforcement. The heating element according to claim 4, characterized in that the fiber reinforcement comprises glass, boron, graphite, aramid or carbon fibers. The heating element according to claim 1, characterized in that the ceramic additive comprises a nitride, oxide or carbide. The heating element according to claim 6, characterized in that the polymeric coating comprises a charge of approximately 60-200 parts of the ceramic additive per hundred parts of the polymer in the polymeric coating. The heating element according to claim 7, characterized in that the polymeric coating is injection molded. The heating element according to claim 1, characterized in that the resistance wire is completely encapsulated in the polymeric coating during a molding operation. 10. A water heater characterized in that it comprises: (a) a tank for containing water; a heating element attached to a wall of the tank to provide heating by electrical resistance to a portion of the water in the tank, the heating element comprising: a support frame or frame; a resistance wire wound on the support frame and connected to at least a pair of terminal end portions; and a thermally conductive polymeric coating disposed on the resistance wire and a main portion of the support framework for hermetically encapsulating and electrically insulating the fluid resistance wire, the polymer coating includes a thermally conductive, non-electrically conductive additive to provide a value of thermal conductivity of at least approximately 0.5 W / m ° K. The water heater according to claim 10, characterized in that the polymeric coating comprises a fibrous additive to improve the mechanical strength and the thermally conductive, non-electrically conductive additive comprises a ceramic additive containing a nitride, carbide or oxide . 12. A method of manufacturing an electrical resistance element for heating a fluid, characterized in that it comprises: (a) providing a support frame; (b) winding or winding a resistance heating wire on the support frame; (c) applying a thermally conductive, non-electrically conductive polymer on the resistance heating wire and a substantial portion of the support frame for electrically insulating and hermetically encapsulating the fluid wire, the thermally conductive polymer coating has a thermal conductivity value of at least approximately 0.5 / m ° K. 13. The method according to the claim 12, characterized in that the application step (c) of the thermally conductive, non-electrically conductive polymer comprises injection molding. The method according to claim 13, characterized in that the thermally conductive polymeric coating comprises about 60-200 parts of a ceramic additive per one hundred parts of the polymer. The method according to claim 12, characterized in that the polymeric coating comprises a thermoplastic resin, a ceramic powder and glass fibers in fragments. The method according to claim 15, characterized in that the thermoplastic resin comprises polyphenylene sulfide (PPS) and the thermal conductivity value is greater than about 0.7 W / m ° K. 17. The method according to claim 15, characterized in that the thermoplastic resin comprises a liquid crystal polymer (LCP). 18. The method of compliance with the claim 12, characterized in that the application step (c) comprises the immersion of the resistance heating wire and the support frame to a fluidized bed. 19. An electrical resistance heating element, capable of being disposed through a wall of a tank for use in connection with the heating of a fluid medium, characterized in that it comprises: (a) a polymeric support frame; (b) a resistance heating wire having a pair of free ends attached to a pair of terminal end portions, the resistance heating wire is wound onto and held by the support frame; and (c) a non-electrically conductive polymeric coating, containing an electrically insulating, electrically conductive ceramic additive, for improving the thermal conductivity of the coating, the coating is disposed on the resistance wire and a portion of the support frame for hermetically encapsulating and electrically insulating the fluid resistance wire, the polymer coating has a thermal conductivity value of at least about 0.5 W / m ° K. 20. The heating element according to claim 19, characterized in that the ceramic additive comprises an aluminum or magnesium oxide. 21. The heating element according to claim 20, characterized in that the polymeric coating also comprises glass fibers in fragments. 22. An electrical resistance heating element for use in connection with heating a fluid medium, characterized in that it comprises: (a) an elementary body having a support surface thereon; (b) a resistance wire wound on the support surface and connected to at least a pair of terminal end portions of the element; and (c) a polymeric, non-electrically conductive, thermally conductive coating disposed on the resistance wire and a substantial portion of the support surface for hermetically encapsulating and electrically insulating the fluid resistance wire, the polymer coating comprises an additive of Ceramic that is not electrically conductive, thermally conductive, to obtain a thermal conductivity value of at least about 0.5 W / m ° K through the coating. 23. An electrical resistance heating element for use in connection with heating a fluid medium, characterized in that it comprises: (a) an electrical resistance wire; (b) a ceramic material that surrounds and electrically insulates the wire; (c) a metal enclosure surrounding the ceramic material and the electrical resistance wire; and (d) a thermally conductive polymeric coating disposed on the metal shell to hermetically encapsulate and electrically insulate the metal shell of the fluid, the polymer coating has a thermal conductivity of at least 0.5 W / m ° K. 24. An electrical resistance heating element for use in connection with heating a fluid medium, characterized in that it comprises: (a) an electrical resistance wire; (b) a ceramic material that surrounds and electrically insulates the wire; (c) a metal enclosure surrounding the ceramic material and the electrical resistance wire; and (d) a thermally conductive polymeric coating disposed on the metal shell to hermetically encapsulate and electrically insulate the metal shell of the fluid, the polymeric coating comprises a non-electrically conductive, thermally conductive ceramic additive. 25. The heating element according to claim 24, characterized in that the polymeric coating has a thermal conductivity value of at least about 0.5 W / m ° K.