CA2500588A1

CA2500588A1 - Foam heater element

Info

Publication number: CA2500588A1
Application number: CA 2500588
Authority: CA
Inventors: Yonggang Y.Z. Zhang; Suqing S.Z. Zheng
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-03-21
Filing date: 2005-03-21
Publication date: 2006-09-21

Abstract

In place of the traditional coil of resistance wire, the heater element comprises a cellular matrix. The matrix is formed by plating a coating of electrically-resistive material (ERM), being metal or ceramic, onto a block of plastic (polyurethane) foam. The ERM forms an exoskeleton, which is mechanically rigid and robust, and yet has the desired heating characteristics when an electrical current is passed through. The block is slotted, to create and define a long narrow pathway. Terminals are formed at the ends of the pathway by plating e.g copper or nickel onto the foam.

Description

Title: FOAM HEATER ELEMENT
This invention relates to electrical resistance heaters. The invention provides an alternative heater material, which may be used in place of the conventional resistance wire.
BACKGROUND TO THE INVENTION
Resistance heaters are well known in many applications, from domestic handheld hair-dryers to sophisticated large industrial heaters and dryers.
Traditionally, the heater element comprises a length of metal wire, having a substantial electrical resistivity, whereby the passage of an electric current through the wire causes the wire to heat up, and in many cases to glow red hot.
In many applications, especially where the heater is used to heat a moving stream of air, the heater wire is arranged as a coil, and the wire itself is open to the air. In other applications, such as stove-top hotplates, or a water-immersion heater element, where the electrically-live element has to be covered, the heater resistance wire is embedded in insulation, and the insulation is encased in an outer tube.
The heater element of the present invention is mainly intended for use in the kind of application where the fluid to be heated is a moving stream -- especially a moving stream of air -- passing over and through 30 the heater element. If the invention is to be used in a case where the electrically-live element has to be covered, or insulated from the fluid to be heated, the element should be suitably encased or otherwise isolated.

In a preferred version, the invention makes use of plastic foam. The foam is produced with cells that are interconnected, i.e the foam is of the open-cell type. The porosity or cell-size of the foam preferably is such that air can pass through the foam, with only minimal resistance 40 to airflow.
In order to create the heater element, the foam is plated with an electrically resistive material (ERM) , such as a resistive metal. Thus, the form of the plastic foam is reproduced in the plated metal. The 45 metal is of the kind having a high resistivity, suitable for a heater element. The metal serves to conduct electrical current, and the passage of the current through the metal causes the metal to become heated.
The electrically resistive material ERM may also be a ceramic material .
50 The plastic foam itself serves merely as a base form, on which the metal or ceramic can be plated. Once the metal coating is in place, the foam is no longer needed. In fact, when the metal is heated (e.g to red heat) the foam decomposes as a structure.
55 Prior to plating, the foam may be spongy, i.e. may be easily compressed between a person's fingers. After plating, the ERM forms what may be described as an exoskeleton with respect to the foam. That is to say, the plated-on form is rigid in itself, and derives none, or substantially none, of its rigidity from the encased foam.
The thickness of the ERM deposition determines the electrical resistance of the heater element. The thickness of the ERM also determines the mechanical strength and rigidity of the heater element.
The plating would be too thin from a mechanical standpoint if the ERM
exoskeleton were too flimsy and fragile, mechanically, for its function as a heater element. The plating would be too thin from an electrical standpoint if the overall resistance of the ERM were so high as to inhibit the heating current.
The plating would be too thick electrically if the ERM were of such low overall resistance that a large current can pass through the ERM
without heating it. The plating would be too thick mechanically if the ERM started to fill up the pore space in the foam structure to the extent of impeding airflow through the structure.
It is recognised that there is a margin between too thick and too thin, in the above senses. That is to say, it is recognised that a plated-foam heating element can, on the one hand, have good characteristics mechanically, and can also, at the same time, have good characteristics of electrical resistivity. It is recognised that this advantageous thickness can be provided by ordinary conventional plating techniques, without having to resort to special processes or undue expense.
The foam form of the heater element is advantageous in that it provides a large surface area for a given quantity of material. Thus, heat can be transferred efficiently to a moving airflow at substantially lower temperatures than has been the case with traditional electrical heater elements.

By way of further explanation of the invention, exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Fig 1 is a diagram showing a basic heater element.
95 Fig 2 shows a heater element of spiral configuration.
Fig 3 shows a heater element of a coiled configuration.
Figs 4a, 4b, 4c are close-ups showing examples of end terminals for use with the elements.
Figs 5a, 5b are photographs showing the cellular structure of the elements .

The apparatuses shown in the accompanying drawings and described below are examples which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by specific features of exemplary embodiments.

Fig 1 shows a basic rectangular element for a heater, and shows air passing through the element. The airflow may be natural, or may be fan-induced, etc.
110 The element 20 was first made as a simple rectangle, and then slots 23 were cut into the rectangle. After the cuts, the resulting structure defines a long, narrow, zig-zag pathway 25.
Suitable electrodes or terminals 27 were provided at the ends of the 115 pathway 25, whereby electricity can be applied between the ends of -S-the pathway.
Fig 2 shows an element 29, which has been formed to a spiral configuration.
Again, the slot 30 is cut into the element form, to define the long 120 narrow pathway 32.
Fig 3 shows another element 34, which has been formed to a coiled configuration, again by suitably cutting a slot 36. The element 34 is in the form of a hollow tube.

Figs 4a,4b,4c are close-ups showing examples of the manner in which the end terminals 27 may be structured.
In the element of Fig 1, the element has been manufactured by first 130 providing a rectangular block of plastic foam, in this case of polyurethane .
The polyurethane material is porous; that is to say, the material forms walls or ligaments, which define pores or cells. The foam is of the open-cell, or interconnected-cell, form. The foam is soft and flexible, to the extent that the foam can be squeezed flat by pressing 13S it between the fingers.
A foam would be unsuitable for use in heating air passing through the foam if the pores are so small as to impede the passage of the air, or so large that air passes through without being substantially heated.
140 The preferred range of pore sizes, for typical heater element applications, is about '~ mm to about ten, or perhaps fifteen, mm. The pore size may be measured as the average number of pores per (linear) inch, or PPI, which preferably lies in the range about five to about fifty PPI.

It may be noted that the designer has available a number of properties of the block of foamwhichmaybe tailored to suit the particular application:
the size and pitch of the pores; the thickness of the plating; the area, thickness, and configuration, of the block of foam; and so on.

In the particular example, the foam is to be coated with a layer of nickel-chromium alloy, whereby the walls and ligaments of polyurethane acquire a coating or layer of the solid metal. It will be understood that the manner of applying plating to the walls and ligaments that 155 constitute the foam material is conventional. Plating may be done by dipping the plastic foam in a bath of liquid, by electroplating, etc.
Where the plating of the ERM is to be done by dipping the foam in a bath of molten metal, e.g carbon may be used as the foam precursor material, instead of polyurethane. Alternatively, plating may be done 160 by dipping plastic foam in a bath of ERM powder slurry, followed by sintering.
In making the end connections 27 as shown in Figs 4a, 4b, 4c, the designer may prefer to provide cold ends 36 to the electrical pathway 32. Thus, 165 where most of the length of the pathway 32 is plated with e. g the resistive metal nickel-chromium, the ends 36 may be plated with e.g copper or nickel; these metals being conductive, the ends 36 remain cold while the main length of the pathway 32 becomes heated. The conductive metal at the ends 36 may be plated to a heavier thickness than the resistive 170 metal, to give the terminals an enhanced mechanical strength and electrical conductivity, if desired.
As shown, a copper strap 38 is wrapped around the plated-on cold end, being brazed or soldered on, and/or mechanically clamped tight, as 175 desired. Cables or wire leads (not shown) may then be bolted to the straps 38, or otherwise secured, as desired.
Alternatively, instead of plating the foam with a different (more conductive) metal, the resistive metal (or ceramic) ERM itself may 180 be plated on more thickly at the ends. The extra thickness serves to reduce the electrical resistance of the end, whereby the end remains cool. Both techniques may be used together, i.e the resistive metal is plated over the ends of the pathway, and then a conductive metal is plated over the resistive metal at the ends.
185 The porosity of the plated-on matrix of ERM should be small enough that the air passing through the matrix is properly heated. On the other hand, the porosity should not be so small as to impede the airflow through the matrix. The preferred degree of porosity may be defined as follows. If a block of the plated foam material is held to light, 190 a dark-area of the total area of the plated foam material is the area that is visually obscured by the material, while light is visible through the remaining light-area of the total area: considering a one-cm-thick block of the plated foam material, preferably the porosity should be such that the light-area is not less than about 2~ of the total area, 195 and the dark-area is not less than about 20~ of the total area.
The gaps or slots 23 should be as narrow as can conveniently be formed.
The slots would be too wide if the airflow tended to pass through the gaps, and not through the cellular matrix, and thus tended not to be 200 heated. It is recognised that a block of the plated foam can readily be made to such dimensions, and to such porosity characteristics, that enough of the airflow passes through the cellular matrix, i. a not through the cut slots or gaps 23, as to heat the air. The purpose of the gaps 23 is to define the electrical pathway 32, not to provide an avenue 205 through which air can pass without being heated. The gaps 23, of course, should not be so narrow that the material of the matrix might accidentally touch and make an (electrical) short circuit.

_g_ However, in some cases, the gaps or slots 23 may be as wide as desired 210 to meet the requirement of a heater design.
The gaps 23 may be made e.g by saw-cutting the finished ERM-plated cellular matrix. The polyurethane foam material preferably is a simple block prior to plating, i.e is not pre-slotted. If the polyurethane 215 block were pre-slotted, then the metal plating might tend to form a surface skin, in the region of the slots, whereby the final metal-plated matrix might be less (or more) porous at or near the gaps. When the gaps are saw-cut, the porosity of the final matrix can be expected to be the same in the centre of the matrix as at the surface of the 220 matrix. In fact, preferably, the whole form of the matrix is machine-cut, all over, after plating, to ensure that all surfaces are free from skinning. However, in the invention, in suitable cases the plating may be carried out on foam material that has already been slotted.
22S The matrix block that is the basis of the heater element may be provided in thicknesses in the range of five to fifty millimetres. However, these dimensions are more by way of illustrations of typical usage than technical limitations. Essentially, the matrix should be sized such that the fluid to be heated can pick up the desired amount of 230 heat as it passes through the matrix, within the limits of mechanical robustness and temperature as can be accommodated by the material.
The cross-sectional area of the electrical pathway 32 preferably should be more or less uniform throughout the length of the pathway. That 235 is to say, the maximum cross-sectional area CSAmax, if not equal to the minimum cross-sectional area CSAmin, should not be more than about twice the minimum cross-sectional area. The greater the cross-sectional area, the smaller the current density over that area, and thus that area would run cooler. On the other hand, for some reason a designer 240 might wish to provide that different areas of the matrix run hotter or cooler; but generally the preference is for the heating to approximate to the ideal of being uniform and even through the whole pathway.
The electrical pathway 32 should be of a long/narrow configuration.
245 One of the benefits of a long/narrow configuration is that the electrical current density may be regarded as being more or less constant over the whole cross-section and the whole length. Thus, the more long/narrow the configuration, the greater the expectation that the degree of heating will be constant over the whole volume of the matrix, and predictable.

The long/narrow preference may be defined in the following terms. The heated pathway length HPL of the matrix element is the shortest length as measured between the terminals or electrodes. In this sense, the cold ends 36 of the pathway are not included as portions of the heated 255 pathway length. In this sense also, if there is a portion of the matrix that has a cross-sectional area of more than twice CSAmin, that portion likewise is not included as a portion of the heated pathway length.
The long/narrow preference is that the heated path length HPL be preferably greater than ten, and certainly greater than five, times 260 the square root of the minimum cross-sectional area CSAmin.
The mechanical strength and rigidity of the matrix come from the plated-on metal coating, not from the polyurethane foam. The thicker the deposition, the stronger and more rigid the matrix will be, and the designer should 265 see to it that the thickness that will give the right electrical characteristics will also give the desired mechanical characteristics.
It is recognised as a feature of the invention that this will usually be possible.

270 However, in some cases, it may be required that the coating be very thin, for electrical reasons -- that is to say, the coating may have to be so thin for electrical reasons that the coating will not be robust enough mechanically. If the resultingfragility cannot be accommodated by other design aspects, the designer may prefer to use a more rigid 275 foam material, - carbon foam, for example - or may prefer to provide a preliminary coating of an electrically-insulative material onto the (soft) plastic foam, just to provide mechanical strength, and then to plate the resistance-heating material over the layer of insulative material.

The electrically-resistive metals that may be considered suitable candidates for selection as the ERM for use in the invention include alloys such as Ni-Cr, Ni-Cr-Si, Ni-Cr-Fe-Si, Fe-Cr-A1, Fe-Cr, and the like. Coatings of these metals can be applied to polyurethane foam 285 by the usual deposition techniques, including electrical deposition, andincluding chemical vapour deposition, chemical vapourinfiltration, and the like.
The ERM may be a ceramic material, for example MoSi2, SiC, or the like.
290 These ceramic materials may be plated onto the foam by chemical vapour deposition, powder vapour deposition, and the like. Both metal and ceramic foams may be produced by means of powder slurry sintering.
The thickness of the metal or ceramic deposited on the plastic foam 295 is preferably in the range from ten micrometres to half a millimetre.
The designer must see to it that the deposition is of the right thickness to become hot enough, in use, for proper heat transfer, and yet not so hot as to be damaged by excess temperature.
300 As mentioned, the plated-on thickness of the electrically resistive material should be uniform over the whole area of the matrix block, to minimise differences in current density over the regions of the plating.
305 Figs 5a,5b are close-ups, to the scales as shown, of the walls and ligaments that define the foam material, with the coating of ERM applied.
Preferably, the coating should be complete, i.e. there should be no gaps, and the coating should be uniform in thickness. The thickness of the ERM coating reduces the pore size of the foam material, of course;
310 the designer, in designing for a given airflow / heating effect, should set the pore size of the foam accordingly. Thus, the pore size and PPI measurements are not in direct relationship, in that thicker plating will reduce the pore size while not affecting the PPI.
315 The following table details the relationships between the various measures, in a typical case.
Foam Pore SizeApprox. Pore Approx. Pore Approx.Number Diameter in of Cells per in PPI Inch Diameter in Square Inch mm 0.160 4.064 25 0.080 2.032 100 0.040 1.016 400 0.030 0.762 900 0.020 0.508 1600 60 0.015 0.381 3600 80 0.010 0.254 6400 100 0.005 0.127 10000 As shown in Figs 5a,5b, The cells of the foam are generally twelve-320 to fourteen-sided polyhedrons, whose pentagonal or hexagonal faces are formed by five or six ligaments. The open window of each of these faces defines the pore diameter, which is expressed in terms of pores per linear inch standard pore sizes run from 5 to 100 ppi.
325 The electrical resistance of the foam element is dependent on several parameters such as the resistivity of the ERM, the deposited thickness, the pore size, and the cross-section (measured perpendicular to the pathway). Especially where different models of the heater elements are being made in batches on the same production line, designers may 330 find it more convenient to vary the cross-sectional area of the pathway (by cutting the slots appropriately), rather than to vary the type of ERM, or the deposited thickness, pore size, etc, of the foam.

Claims

CLAIM 1. A procedureformanufacturing an electrical resistance heating apparatus, which includes:
[2] providing a body of porous foam, the foam comprising walls or ligaments of material, which define the pores;
[3] depositing an electrically resistive material (ERM) onto the body of porous foam, thereby forming a coating of the ERM around the said walls or ligaments;
[4] the ERM is of such resistivity that the ERM becomes hot when an electrical current passes therethrough;
[5] depositing and forming the ERM to the shape of a long narrow pathway, having two ends;
[6] providing electrical terminals at or near those ends;
[7] so forming the terminals as to be suitable for receiving an applied electrical current therebetween, and for passing same along the long narrow pathway.

Claim 2. Procedure of claim 1, which includes so depositing the ERM
that the deposited ERM forms an exoskeleton.

Claim 3. Procedure of claim 2, which also includes depositing the ERM to such thickness that the ERM exoskeleton is structurally robust and rigid, whereby the ERM exoskeleton derivessubstantially none of its robustness and rigidity from the foam material.

Claim 4. Procedure of claim 2, wherein the pores of the foam are so open and interconnected that the body of foam provides substantially no resistance to the passage of an airflow through the body of foam.

Claim 5. Procedure of claim 4, which also includes depositing the ERM only to such thickness that the open porosity characteristic of the foam is substantially retained in the ERM exoskeleton, in that the exoskeleton provides substantially no resistance to the passage of an airflow through the exoskeleton.

Claim 6. Procedure of claim 1, including [2] forming the terminals by plating an end-layer of a metal, e.g copper, that is substantially more conductive than the ERM, such that the end-layer is in intimate electrical contact with the ERM;
[3] attaching a terminal structure, suitable for attachment of a wire or cable thereto.

Claim 7. Procedure of claim 1, including [2] incorporating the heater element into an appliance;
[3] providing a powered fan in the appliance, arranging it to blow air through the element;
[4] passing a current through the element, of sufficient magnitude as to cause the element to become hot, and to be cooled by the airflow, whereby the airflow is correspondingly heated.
Claim 8. Procedure of claim 1, wherein the foam material is a soft plastic, preferably polyurethane.
Claim 9. Procedure of claim 1, wherein the foam material is rigid, and is preferably carbon.
Claim 10. Procedure of claim 1, wherein:
[2] the heater element is so structured as to be suitable for heating a fluid passing through the element;
[3] the body of foam, after deposition of the ERM, comprises a block having a thickness T measured in the direction of passing of the fluid through the block;
[4] the block of plated foam includes slots:
[5] the slots are formed right through the thickness T of the block;
[6] and the slots are so configured as to define the said long narrow pathway.
Claim 11. Procedure of claim 10, including forming the slots in the block of plated foam by cutting the slots after the ERM has been plated onto the foam material.
Claim 12. Procedure of claim 10, wherein the block of plated foam is a rectangle, and the slots are arranged to define the said long narrow pathway as a zig-zag.
Claim 13. Procedure of claim 10, wherein the block of plated foam is a circular disc, and the slots are arranged to define the said long narrow pathway as a spiral.
Claim 14. Procedure of claim 10, wherein the block of plated foam is a long cylinder, and the slots are arranged to define the said long narrow pathway as a helix.
Claim 15. Procedure of claim 1, wherein the foam material is itself non-conductive electrically.
Claim 16. An electrical resistance heater element, which has been manufactured by a procedure that falls within the scope of claim 1.
Claim 17. An appliance having an electrical resistance heater element, and having a means for passing a fluid to be heated through the element, wherein the element is an element that has been manufactured by a procedure that falls within the scope of claim 1.