Immersion Type Heating Element With A Plastic Head For A Storage Water Heater Tank

Abstract

An improved electric immersion heater wherein the metallic heating element sheath is mechanically and electrically secured to a dielectric element head rendered conductive by suspending carbon or metallic conductors in the dielectric to produce a predetermined resistance. This allows anodic current to flow from the heating element sheath to ground when the conductor is installed in a water tank without the need for external resistors or tinning of the heating element sheath.

Description

BACKGROUND OF THE INVENTION

This invention relates to electric heaters for water tanks, particularly an improved heating element head construction for an immersion heater as used in electric water heaters.

Typical electrically heated water tanks employ an electric immersion heater. Most lined water tanks constructed of ferrous metal also contain a sacrificial anode such as magnesium, aluminum or zinc. The anode is in electrical contact with the tank and serves to cathodically protect the tank from corrosion by electrolysis.

To prevent the sacrificial anode from being sacrificed at an excessively rapid rate, the prior art has recognized that the uncoated copper sheathed heating element cannot be directly grounded to the ferrous tank. For example, if the heating element sheath and ferrous tank are in perfect electrical contact, a substantial electropotential difference will exist between the copper sheath and the sacrificial anode. On the other hand, it has been established that the element sheath will corrode if the element is electrically isolated from the ferrous tank. Therefore, there must be an electrical connection between the sheath and the tank. The amount of current flow, however, has to be controlled to prevent excessive anode depletion.

One solution employed by the art is to embed the heating element sheath in a steel head as illustrated in FIG. 2 accompanying this application. In this figure, a copper heating element sheath 7 is mechanically and electrically secured to steel head 10 having a plastic center portion 11 to retain wire leads 6. Steel head 10 is supplied with male threads 14 designed to engage the female threads in steel coupling 13 welded to the tank wall 4. To prevent excessive depletion of magnesium anode 2 due to the intimate electrical contact between sheath 7 and tank coupling 13 through steel head 10, copper sheath 7 and steel head 10 are plated with a metal 12 (thickness of metal 12 is greatly exaggerated for purposes of illustration) of less nobility in the electropotential series than copper such as tin. Because the gap in the electropotential series between tin and magnesium is less than the gap between magnesium and copper and because the tin coating has the tendency to polarize, the magnesium anode will not be depleted at an excessively rapid rate despite the fact that sheath 7 is mounted in direct electrical contact with the tank. While this solution to the anode depletion problem is satisfactory in most cases from a technical standpoint, it requires the expense of tin plating. Further, the tin plating may dissolve in certain waters thereby leading to rapid anode depletion.

Another device used by the art to control excessive anode depletion is illustrated in FIG. 3. In this device, the heater head or flange comprises copper faced steel plate 21, dielectric plate 22 and steel face plate 23. Copper face plate 21 and steel face plate 23 are not in direct electrical contact and are separated by dielectric material 22. This plate assembly or element flange is mounted on the wall 4 of water tank 1 by inserting sheath 7 through wall opening 25 and connecting the plates to the tank flange 27 through bolts 20. An insulating gasket 26 separates tank flange 27 from plate 21. Electrical connection between copper face plate 21 and steel plate 23 is provided by a 500-600 ohm resistor. This value resistor allows sufficient current to pass from the sheath to ground (the tank) to prevent electrolytic corrosion of the sheath while at the same time preventing rapid anode deterioration. While this solution does not require the tin plating of the copper sheath, it does require a more complex heating element head and the need for an external electrical circuit with resistor.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an electric heating element for water tanks that has a conductive plastic element head connected to the metal heating element sheath which does not require a less noble metal coating to prevent excessive anode deterioration.

It is an object of this invention to provide a conductive plastic element head for an electric water heater that does not require an external electric circuit to prevent excessive anode deterioration.

It is a specific object of this invention to provide a conductive plastic heating element head that is quickly and inexpensively manufactured.

According to this invention, there is provided an improved all plastic heating element head for a conventional electric immersion heater having a heating element enclosed in a metallic sheath. This improved head is mechanically and electrically secured to the metallic sheath and preferably eliminates the need for external electric circuits or plating of the metallic sheath with a metal less noble than the sheath metal while at the same time controlling anode deterioration. This improved head comprises a dielectric solid such as a plastic resin, having suspended therein carbon or a metallic conductor in an amount sufficient to allow current to flow from the sheath to ground (the tank) through the conductor suspended in the dielectric when the heater is installed in a water tank.

Other objects and embodiments will be found in the following more detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view illustrating a conventional water tank and the placement therein of an anode and a water heating element.

FIG. 2 is a detailed sectional side view of a conventional combination steel and plastic element head wherein the steel portion of the head is electrically grounded to both the metallic sheath and metallic tank coupling thereby requiring tin plating of the copper sheath to prevent excessive anode depletion as discussed earlier.

FIG. 3 is a detailed, sectional side view of a multipiece prior art element head or flange that requires an external electric circuit to control anode depletion as discussed earlier.

FIG. 4 is a detailed partial sectional side view of the plastic element head of this invention as positioned in a water tank.

FIG. 5 is a detailed, sectional side view of the element head illustrated in FIG. 4 as detached from the water tank.

FIG. 6 is an end view of the heating element illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 4 and 5 illustrate the improvement of the present invention, the prior art having been discussed relative to FIGS. 1 through 3. FIGS. 4 and 5 illustrate a rigid plastic head 30 having intimately connected thereto a conventional tubular, untinned copper heating element sheath 7. Tubular sheath 7 houses a conventional high resistance heating wire (not illustrated) maintained in a spaced relation from the sheath 7 by a suitable insulating material such as magnesium oxide. Head 30 is provided with suitable male threads 33 adapted to engage the female threads in tank coupling 13 affixed to wall 4 of the heating tank.

As illustrated in FIG. 5, sheath 7 is firmly embedded in plastic head 30. Electrical leads 6 extend from the high resistance heating wire (not shown) in sheath 7 through non-conductive plastic center piece 35 concentrically positioned within a plastic matrix 31 of head 30. Leads 6 may be connected to a suitable external power source.

The portion of head 30 comprising the plastic matrix 31 is made, for example, from a phenolic plastic. Uniformly distributed within matrix 31 are electrically conductive carbon or metallic particles 32. The nonconductive center piece 35 is comprised of an insulator for the leads 6 and preferrably is made from the same plastic as matrix 31. Of course the conductive particles 32 are excluded from center piece 35.

The amount of particles 32 per unit volume of matrix 31 will generally determine the resistance of the matrix 31 of the head 30. High amounts of conductive particles 32 generally result in lower resistance and lower amounts result in higher resistance. Alternatively, particles 32 can comprise discrete conductive units dispersed in a polymerized resin, each discrete unit comprising an aggregate of conductive particles bonded together by the resin matrix 31. Conductive resins comprising conductive carbon or metallic particles suspended in a resin matrix are well known to those trained in the art and need not be described in detail herein. See for example U.S. Pat. No. 3,056,750.

In general, any type of solid resin of either an inorganic or organic nature, natural or synthetic, which is capable of serving as a dielectric material may be used in the practice of the present invention.

Preferably, the matrix 31-particle 32 combination has a resistance value sufficient to allow a small amount of current to flow from the sheath 7 to the tank so as to eliminate the need for tinning the copper sheath 7 or using an external resistance. Typically, the matrix 31-particle 32 combination has a resistance of about 500-600 ohms although a resistance range of 200 to 1,000 ohms has been found satisfactory.

The resin for matrix 31 used must have sufficient stability to stand exposure to water temperatures on the order of 100.degree.-180.degree. F. as encountered in ordinary water heaters.

A preferred resin would be a single stage phenolic compound with fillers added to improve resistance to temperature, cracking, chemicals, etc.

The conductive particles suspended in the dielectric matrix consist of either carbon particles or metal particles or a mixture of both. Accordingly, any metal powder may be used as the conductive material of the present invention, such as, for example, copper, iron, zinc, aluminum, magnesium, tin, antimony, silver, chromium, etc. In addition, conductive oxides of metals may be used.

With respect to carbon, certain forms offer lower resistance values, such as crystalline carbon (graphite), whereas the amorphous forms of carbon (carbon black) afford higher values of resistance. Lamp black, having a specific resistance less than carbon black and greater than graphite may be used to obtain resistances of an intermediate value.

The invention is therefore defined and limited only by the following claims.

Claims

I claim as my invention:

1. In an electric immersion water heater element for a water heater tank which includes a heating element enclosed in and electrically insulated from a metallic sheath, and an element head for attachment to a water heater tank with the heating element adapted to project to the interior of the water heater tank said element head being mechanically and electrically secured to the metallic sheath, the improvement which comprises an element head including a dielectric solid portion in contact with said sheath and to contact said tank, said dielectric portion having suspended therein conductive carbon or metallic particles in an amount sufficient to allow current to flow from said sheath through the dielectric portion to ground on said tank, said head also including a nonconducting portion through which external leads are provided for connecting said heating element to a power source.

2. An electric immersion heater according to claim 1 wherein the heating element comprises an untinned copper sheath and said head contains a carbon or metallic conductor in an amount sufficient to eliminate said tinning without the necessity of an external resistor.

3. An electric immersion heater according to claim 1 wherein the carbon or metallic conductor is present in an amount sufficient to provide a resistance of 200-1,000 ohms between the sheath and ground.

4. An electric immersion heater according to claim 1 wherein said solid portion is a plastic resin.

5. An electric immersion heater according to claim 1 wherein said nonconducting portion comprises a plastic resin concentrically positioned with the dielectric portion.