US3324280A - Insulated metal sheath heating element for electric water heaters - Google Patents

Description

June 6, 1967 F. E. CHENEY ET AL 3,324,280

INSULATED METAL SHEATH HEATING ELEMENT FOR ELECTRIC WATER HEATERS Filed Aug. 6, 1964 I NVENTORS ATTORNEYS United States Patent 3,324,280 INSULATED METAL SHEATH HEATING ELE- MENT FOR ELECTRIC WATER HEATERS Frank E. Cheney, 109 St. Martins Road, Cherry Hill, NJ.

08034, and Frank Kahn, 1865 Edmund Road, Abington, Pa. 19001 Filed Aug. 6, 1964, Ser. No. 387,923 6 Claims. (Cl. 219-544) ABSTRACT OF THE DISCLOSURE This invention comprises a rod-type copper-sheathed immersion heating element for an electric water heater, having a plating of nickel on the external surfaces of the copper. The nickel surface is sanded to provide a matte surface and heated to deposit an oxide coating. An external insulating coating of polytetrafluoroethylene is then applied. The purpose of this new heating element is to minimize the current flowing to the heating element from the galvanic anode used for cathodic protection of a glass-lined Water heater tank.

The present invention relates to heating elements for electric water heaters and more particularly to rod-type metal sheath immersion electric heating elements which are resin-coated to insulate the metal sheathing from contact with Water. The invention also relates to methods of electrically insulating the metal sheath of an electric immersion water heater element.

A purpose of the invention is to apply a resin-coating to the metallic sheath of an electric immersion Water heater element which will be capable of withstanding the severe heat shock in cyclic operation and remain serviceable for a long period of time.

A further purpose is to coat a nickel external layer of the metallic sheath by polytetraflu-oroethylene which will remain adhering thereto.

A further purpose is to nickel-plate the copper sheath of an electric water heater element and then apply polytetrafluoroethylene to the nickel-plating.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate one only of the numerous embodiments in which the invention may appear, selecting the form shown from the stand points of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

FIGURE 1 is a conventional central vertical section of 3,324,280 Patented June 6, 1967 sheath'which needs no cathodic protection because the corrosion of copper is ordinarily negligible in domestic water heaters. In the case of the tin plating, the current may be reduced by about but is still considered excessive.

The life of the sacrificial anode could be greatly prolonged and the useful life of the water heater could be proportionately extended if the protective current which is unnecessarily diverted to the bare copper sheath of the immersion heating element could be minimized. United States Patent No. 2,649,532 issued to Kenneth L. Woodman on Aug. 15, 1953, for Water Heater Apparatus, proposes to accomplish this by coating the entire surface of the sheath of the immersion electric heating unit with an insulating coating which is resistant to hot waer. This patent states that the insulating coating will be a varnish of any well known type, such as for example the varnishes then on the market under the trade names Micarta, Permolite or Heresite.

By contact with Westinghouse Electric Corporation, the assignee of said Woodman patent, it was learned that on practical tests none of these coatings were found to withstand service conditions and commercial exploitation of the invention of this patent was accordingly abandoned. Westinghouse Electric Corporation advised that its B-192 Micarta insulating varnish, a polyvinyl butyral resin product, was deemed to be most suited for this purpose but proved to be unsatisfactory. In order to confirm this conclusion, a sample of this Micarta varnish was obtained, along with a sample of Teresite, a phenol-formaldehyde resin marketed by Heresite Chemical Company, and a sample of Permolite, a soluble plasticized alkyd resin used as a finish for viscous and acetate fabrics, marketed by Arkansas Company, Inc.

Each of these varnishes was coated on a clean conventional bare copper sheath immersion electric water heating element in accordance with the directions of the manufacturer of the varnish. Effectiveness of the coating for the intended purpose was evaluated by making endurance tests of the electric heating elements immersed in tap water and cyclically energized at the rated voltage to provide a thermal cycle of water temperature between 150 F. and 180 F. with the following results: After 100 heating cycles the Micarta varnish had become disbonded in flakes over the entire surface of the heating element. The insulating resistance which was 300,000

an electric water heater embodying the principles of the invention.

FIGURE 2 is a diagrammatic enlarged section of the electric water heater element of FIGURE 1.

Describing in illustration but not in limitation and referring to the drawings:

In the production of steel tank storage electric water heaters provided with glass linings, the presence of imperfections in the glass is so common that it is ordinarily necessary to protect the steel exposed at such imperfec tions by a sacrificial anode, usually of magnesium or zinc, which provides cathodic protection. The anode supplies protective cathodic current to the exposed steel of the tank and is consumed in providing this protection. Once the anode is completely dissipated, rapid corrosion at exposed metal surfaces occurs. Most rod-type immersion heating elements for electric water heaters are sheathed in copper which is left bare or plated with a more electronegative metal such as tin. In such Water heaters with bare copper sheaths, up to about three-quarters of the electric current from the anode may flow to the copper ohms before the test had dropped to 50 ohms after the test.

In the case of the Heresite varnish, at the end of 20 heating cycles blisters began to form on the surface of the coating, at the end of 30 cycles pieces of the coating began to flake off, and at the conclusion of 300 cycles large areas of bare metal were exposed and the remaining coating was crazed and covered with blisters. The insulation resistance was 1,800,000 ohms before the test and 400 ohms after the test. In the case of the Permolite varnish, itwas found to be impossible to apply this material to the bare copper surface to obtain any practical insulating value. This is a product normally used in water solution for finishing various types of textile fabrics.

Similar tests were made without success on a substantial number of other insulating coatings. One of those tested was a silicone coating. In every case the coating disintegrated or disbonded from the copper under the testing procedure.

The most promising coating tried was polytetrafluoroethylene coated to a thickness of approximately 1 mil. This was applied directly to the copper surface by dipping the electric immersion heating element in a commercially available polytetrafluoroethylene coating marketed by E. I. du Pont de Nemours & Company under the trade name 3 Teflon 30 Aqueous Dispersion. The coating was dried and then sintered. This coating also blistered and disbonded under tests and was found to be unsatisfactory.

Further efforts were made to produce a satisfactory insulating coating of polytetrafiuoroethylene on a conventional copper sheath immersion water heater element using the Teflon 30 coating material above referred to and eventually these efforts were successful.

The coating in question is preferably made according to Ostal U.S. Patent 2,562,118, granted July 24, 1951, for Polytetrafiuoroethylene Coating Compositions, although optionally it can be made according to U.S. Patent 2,562,117, granted to the same inventor on the same date.

To prepare the surface of the copper to receive the polytetrafluoroethylene coating it has been found to be important to nickel plate the copper according to any conventional nickel plating process. This may be done by cleaning the copper surface by dipping it in a conventional bright dip. This is a brightening pickle composed of dilute nitric and sulfuric acid, followed by bufling with jewelers rouge, and washing in cold water. The copper is then electroplated with nickel to a thickness of about 0.1 mil to about 1 mil and then washed with cold water and dried.

The nickel provides a thermal conducting base which has the property of forming a firm adherence with the polytetrafluoroethylene. Conventional nickel plating is adequate for this purpose and it is not necessary that the nickel plating be of uniform thickness.

In the preferred embodiment the nickel surface is slightly roughened as by sand blasting with a fine grit such as aluminum oxide to give a smooth matte surface. The electrode is then heated to a temperature, preferably of the order of 700 to 750 F. to drive off any adhering organic material which may have become embedded in pores or seams and to deposit an oxide coating which improves the adherence of the polytetrafluoroethylene. The electrode is then allowed to cool to room temperature.

The polytetrafluoroethylene aqueous dispersion used is in the form of a slurry or water dispersion of very small particles of polytetrafluoroethylene stabilized with a nonionic wetting agent which decomposes cleanly at the fusing temperature of the resin. This material as supplied by the manufacturer is a hydrophobic negatively charged colloid, which comprises about 60% by weight of small particles (averaging of the order of 0.5 micron in diameter) of polytetrafluoroethylene suspended in water containing a wetting agent, suitably a non-ionic surface reactant such as octylphenoxypolyethoxyethanol amounting preferably to about 6% by weight of the polytetratfluoroethylene, although any wetting or dispersing agent may be used provided it is stable in the presence of the other ingredients. The concentration of the wetting agent is not critical. If desired, modifying agents such as pigments, fillers, or the like may be added provided they are compatible with and stable in the presence of the other ingredients and capable of withstanding temperatures in the region of 750 F.

The water dispersion of the resin is applied to the heating element surface by dipping, flowing, sprayed or brushing to a thickness of about 1 mil. The water is then removed from the deposited polymer by drying at room temperature or at moderately elevated temperature, not over 200 F. Finally, the dried polymer is fused or sintered by baking suitable for about three minutes at a temperature in the range of 675750 F. Sintering of the resin takes place almost instantaneously on reaching the required temperature and during the remainder of the baking period the stabilizer wetting agent is volatilized and burnt off. The heating element is then cooled slowly to retain good adhesion.

In the drawings we show a conventional electrical water heater 20 consisting of a tank 21, which will suitably be internally glass lined (not shown) or otherwise coated for protection on the inside. The tank contains an inlet connection 22, conveniently located at the bottom and an outlet connection 23, suitably at the top. A sacrificial anode 24 is removably inserted as by a threaded connec tion at 2 5. The anode 24 will conveniently consist of magnesium, zinc or the like,

A looped electrical resistance heater element 26 extends into the tank, and is mounted on a suitable removable base 27 provided with an insulating support 28 and having electrical connections 30 and 31.

As best seen in FIGURE 2, the electrical resistance heater 26 desirably consists of an interior heating element or resistor 32 suitably of nichrome or other resistance wire, protected by a surrounding insulating layer 33, suitably magnesia or other refractory powder, surrounded by a copper sheath 34 as well known in the art. According to the present invention a nickel layer 35 surrounds the copper and then a polytetrafluoroethylene layer 36 according to present invention forms a barrier between the copper layer and the water. The interior 37 of the tank will suitably be filled with water, not shown.

Example 1 Three l500-watt copper sheathed heating elements were prepared for application of the polytetrafluoroethylene coating by first conventionally nickel plating them to a thickness averaging approximately 1 mil, lightly sand blasting, heating and allowing them to cool to room temperature as previously described. The elements were then each coated with two successive coatings comprising polytetrafluoroethylene aqueous dispersion, suitably a primer coat and a top coat, each separately applied and fused in the manner previously described.

The two coating mate-rials used were those commercially available from E. I. du Pont de Nemours & Co. under the designations 851-1204 Teflon TFE-Fluorocarbon Resin One Coat Green Enamel (primer coat) and 851- 205 Teflon TFE-Fluorocarbon Resin Black Enamel (second coat). The Du Pont 851-204 product is composed of about 48% of polytetrafluoroethylene resin soluble by weight in a Water medium and the 851-205 product is composed of about 41% polytet-rafluoroethylene solids by weight in a water medium, each containing a nonionic wetting agent as above set forth and a few percent of pigment which is optional. The dispersion was applied as a spray to the heating elements, which were supported by the terminal ends, the primer being coated to a thickness of about mil and the second coat to a thickness of about 1 mil. After fusing, the primer coat had a thickness which ranged from 0.1 mil to about 1 mil and averaged about 0.35 mil. The second coat after fusing had a thickness which ranged from about 0.1 mil to about 1 mil and averaged about 0.43 mil.

The coated electrodes were energized at rated voltage of 236 volts and subjected to 3,000 heating cycles. Each heating cycle consisted of 15 minutes with elements energized and 20 minutes with the elements de-energized. The water temperature ranged from F. to F. The resistances of the coating in ohms on the respective electrodw before test were 50,000, 60,000 and 70,000 ohms. After test the resistances of the coating in ohms were 30,000, 40,000 and 30,000 ohms. One of the elements was then energized at 360 volts (an over-voltage). This gave a watt density of 300 watts per square inch as compared to a normal watt density of 140 watts per square inch. The element was subjected to 136 cycles, consisting of 15 minutes energized and 30 minutes deenergized. Twice, the element was inadvertently energized for a period of 16 hours. The resistance of the coating before test was 20,000 ohms and after test was 25,000 ohms.

Example 2 The copper sheathing of the heating element is prepared by conventional nickel plating, light sand blasting, heating and allowing to cool to room temperature as previously described. A liquid primer coating corresponding to Example 7 of U.S. Patent 2,562,118 is applied to a thickness of mil and then dried at a temperature of 200 F. and then heated at approximately 750 F. for about 5 minutes to produce a fused coating. The heating element is then allowed to cool slowly to room temperature.

Next a top coat of liquid coating composition E as set forth in said Patent 2,562,118 made as described in U.S. Patent 2,613,913 is applied to a thickness of 1 mil. This is dried as above and fused as above and then allowed to cool slowly to room temperature.

An adhering coating of high insulation value is obtained.

Example 3 The procedure of Example 2 is followed except that the top coat liquid composition is modified to introduce about 2% of compatible black pigment, suitably carbon black producing a black surface. Good adherence and good insulating value is obtained.

Example 4 The coating is prepared as in Example 2, except that the primer coat conforms with coating composition C of said U.S. Patent 2,562,118. Good adherence and insulating values are obtained.

The resin coatings are poor thermal conductors. It is therefore desirable to apply them as relatively thin coatings to prevent buildup of excessive internal temperature for specific heating element Watt changes. The electric potential on the heating element surface to be insu lated against by these coatings is only of the order of 1 volt and since polytetrafiuoroethylene is practically an ideal insulating material adequate electrical insulation is provided by extremely thin polytetrafiuoroethylene coatings. The important criterion is that the coating thickness be such as to assure adequate adhesion and integrity and be within the capabilities of practical commercial application.

In view of our invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the. art to obtain all or part of the benefits of our invention without copying the composition and method shown, and we therefore claim all such insofar as they fall within the 4 reasonable spirit and scope of our claims.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. An immersion electric water heating element for cyclic operation in an electric water heater having a corrosion protective anode, for reducing deterioration of the anode and adapted to extend into the tank of the water heater, comprising a looped electric resistor, a resistor electric insulating layer surrounding said electric resistor, a metallic sheath surrounding said resistor insulating layer, said sheath having an exterior surface of nickel, a sheath electric insulating layer of polytetrafiuoroethylene covering said nickel surface and adhering thereto, electric connections to the two ends of the electric resistor, and an insulating support for the electric connections.

2. A water heating element of claim 1, in which said sheath is of copper, having an exterior layer of nickel plated thereon.

3. An immersion electric heating element of claim 1, in which the exterior surface of said nickel surface has a matte finish.

4. An immersion electric heating element of claim 1, in which the exterior surface of said nickel surface has a coating of nickel oxide.

5. An immersion electric heating element of claim 2, in which the external surface of said nickel coating has a matte finish.

6. An immersion electric heating element of claim 2, in which the external surface of said nickel coating has a coating of nickel oxide.

References Cited UNITED STATES PATENTS 2,181,484 11/1939 Harris 338-236 2,649,532 8/1953 Woodman 219-316 2,700,212 1/1955 Flynn et al 11771 X 2,814,710 11/1957 Schuetze 219-245 X 2,816,207 12/1957 Boggs 219-523 X 3,055,402 10/1961 Starger et al 156-583 X 3,103,446 9/1963 FitzSimmons 11775 3,136,680 6/1964 Hochberg 16l189 RICHARD M. WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

Claims (1)

1. AN IMMERSION ELECTRIC WATER HEATING ELEMENT FOR CYCLIC OPERATION IN AN ELECTRIC WATER HEATER HAVING A CORROSION PROTECTIVE ANODE, FOR REDUCING DETERIORATION OF THE ANODE AND ADAPTED TO EXTEND INTO THE TANK OF THE WATER HEATER, COMPRISING A LOOPED ELECTRIC RESISTOR, A RESISTOR ELECTRIC INSULATING LAYER SURROUNDING SAID ELECTRIC RESISTOR, A METALLIC SHEATH SURROUNDING SAID RESISTOR INSULATING LAYER, SAID SHEATH HAVING AN EXTERIOR SURFACE OF NICKEL, A SHEATH ELECTRIC INSULATING LAYER OF POLYTETRAFLUOROETHYLENE COVERING SAID NICKEL SURFACE AND ADHERING THERETO, ELECTRIC CONNECTIONS TO THE TWO ENDS OF THE ELECTRIC RESISTOR, AND AN INSULATING SUPPORT FOR THE ELECTRIC CONNECTIONS.

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