A new approach to fabricating electric heating elements promises to significantly change the way heat sources are designed into appliances and other products that employ electric heating. Developed by 2DHeat Ltd., the method involves spraying the heating element material onto a rigid substrate such as metal, glass, or ceramic. The creation of heating elements by surface coating may permit heating surfaces to be integrated within existing appliance components or housing walls and thereby eliminate the need to employ separate heating components, such as conventional coiled-wire elements.

This technology is currently being commercialized through various licensing arrangements with interested parties. Home appliances such as cooktops, ovens, and tumble clothes dryers will be the initial focus of commercialization efforts. Other targeted sectors will be pursued later in the year. (See Table 1.)

Enlarge this picture Fig. 1. CFD modeled close-up view of a flat blade element showing the localized temperature across the heated blade when in use. (The scale, in degrees Kelvin, is shown on the vertical axis on the left.) It can be seen that the impinging, turbulent airflow on the leading edge of the blade element (on left) shows initial cooling over the first ~10 mm of the blade, which rapidly reduces, leaving large, intimate, surface areas across both faces of the rest of the blade for heat transfer to the air flowing over them. (Images courtesy of C-Tech Innovations Ltd.)

A key advantage of the new type of heating elements is their ability to significantly reduce an appliance’s energy use, thereby increasing overall energy efficiency compared to a traditional heating appliance. This improved level of efficiency is made possible by two factors:

• The design of the elements allows them to operate at a lower temperature.
  
• The surface coating technique allows the elements to be incorporated within the appliance in a more thermodynamically efficient manner.

An illustration of the latter concept is to replace conventional open coiled-wire elements in tumble dryers with flat enamelled blades to which 2DHeat elements have been applied on both upper and lower surfaces. This concept was simulated by CFD modelling of resultant air-flow turbulence patterns, velocity, and pressure drop.

As a result, the prolonged and largely laminar airflow across the broader blades show significant heat-transfer efficiency potential, and at lower element operating temperatures, compared with the more turbulent and shorter contact of the airflow through the complex assembly of the conventional coiled element.

Enlarge this picture Fig. 2. Similar CFD modeled close-up view of a single section through a conventional coiled element, showing the initial cooling impact from the greater air turbulence of the impacting airflow with the leading edge of the more complex coiled structure, following which the turbulent air flow is thrown away from the much shorter following coil surface, resulting in less opportunity for further heat transfer to the air. The temperature scale in this figure is about 60 to 118 degrees Kelvin higher than in Fig. 1, indicating the higher energy levels required for heating the air to the same appliance exit temperature using this coiled configuration. (Images courtesy of C-Tech Innovations Ltd.)

Fig. 1 and Fig. 2 show simulated element operating temperatures (in degrees Kelvin) for a conventional coiled element versus a flat element, each located in an adiabatic duct 40 mm deep, with an air velocity of ~7m/sec and a power input of ~3.5/cm2. The lower temperatures of the flat element reflect the more effective heat dissipation to air.

The 2DHeat elements are applied as a thin thermal-spray coating directly to the surfaces to be heated. (See Fig. 3.) The coin illustrates the extreme thinness of the elements, which are typically between 150 to 250 microns thick, depending on design purpose and expected load.

For appliances such as ovens, this allows rapid and highly efficient heat transfer from the element to the inner oven wall itself, mainly by conductive heating rather than the traditional combination of radiation and convective heating from elements located within the oven wall cavity air space.

This resultant high heat-transfer efficiency to the inner oven walls, combined with the large surface area available, offers efficient radiation cooking of food, but from elements operating at significantly lower temperatures than traditional elements. Modelling suggests this difference could be in excess of 250 DegC between the respective elements; however, the precise difference will need to be determined by extensive cooking trials with a specific appliance design.

Fig. 3. Flame-sprayed elements are typically 150 to 250 microns thick.

As with the tumble dryer example above, an oven’s efficiency can be similarly improved by re-working the convective air circulation design within the oven, replacing the conventional double-annular element in the back of the appliance by flat elements applied directly to the rear of the fan shield plate.

Clearly, incorporating heating elements in this new manner brings new technical challenges that must be addressed. One of those involves the changes required to the enamel finish within the appliances. This surface now has to serve a dual purpose, firstly as the decorative, easy-clean finish, and secondly as a dielectric barrier coating between the applied live element and the appliance chassis. The solution requires the use of suitable enamel formulations with high-temperature dielectric-barrier properties resistant to working at the required appliance operating temperature.

Another critical consideration for utilizing this new system in such an application is ensuring that the thermal expansion coefficient of the surface applied elements broadly match that of the surface to which they are applied. 2DHeat has working solutions for many different substrates, including various metals, vitreous enamels, ceramics, and glasses. In addition, 2DHeat is exploring the use of thermal transfer compounds to provide a solution for glass-ceramic cooktops, possibly the most challenging substrate because of the negative expansion of the latter material as it heats up.

Fig. 4. A radiator panel sprayed as a uniform whole has the positive terminal contact strip split to give two separate heating zones of 325 watts each, which can be operated either separately or together.

Apart from the above functional design opportunities, the surface coated heating system also offers significant practical and aesthetic design potential. For example, the thinness of such elements helps to optimize effective internal volume of appliance cavities, such as with tumble dryers and ovens.

Other examples include the potential use of variable element geometries and zone heating in appliances. The term flat is used to differentiate the new elements from conventional wire wound elements. However, when fabricating such elements, the element materials can be sprayed onto surfaces that are flat, circular, tubular, elliptical, or potentially any other form that capable of being coated by a robotically controlled spray gun. So while the element is flat in respect to its attachment to the substrate, the material itself can be sprayed onto three-dimensional configurations, creating new design possibilities.

Furthermore, the technology permits the establishment of different power ratings across the single surface of an applied element. This variation can be achieved by a combination of using spray masks to spray certain sections of the element to a greater thickness than other sections, thereby reducing the electrical resistance properties of the element and increasing its power rating, or by deliberately conditioning sections of the element to give different power ratings.

Fig. 5. An example of the variable design geometry potential is illustrated by the application of a 2DHeat element applied to a 21 mm O/D diameter metal tube section.

This patent-pending conditioning process is a key part of the 2DHeat application process, whereby the sprayed elements are given a final, physical treatment using pulsed, high-voltage electrical energy to bring the power rating of each element to a specific, consistent operating level. Depending on the design of electrical contacts applied to the 2DHeat elements, this conditioning stage can be used to deliberately introduce intended zoned power rating differences within an element. Fig 4. shows a radiator panel sprayed as a uniform whole, but with the positive terminal contact strip split to give two separate heating zones of 325 watts each (at 13 A and 230/240 V), which can be operated either separately or together.

An example of the variable design geometry potential is illustrated in Fig. 5, which shows a 2DHeat element applied to a 21 mm O/D diameter metal tube section.

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This and various other operational features enabling design flexibility are listed in Table 2.

The 2DHeat application process itself has been designed with economy in mind from standpoints of both operational simplicity capital cost, as readily available components have been used to run a simple, straight-through automated process. Because the power rating of each element can be set as a final step in the 2DHeat manufacturing process, the whole production call-up and supply chain scheduling is vastly simplified, bringing major working capital benefits to the users.

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Finally, users of this fast, low-cost process have the added satisfaction of knowing that they are doing the environment a big favor by replacing conventional elements that require 100 times the energy to produce and consume more than 10 times more natural materials.

For more information, visit: www.2dheat.com