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issue: October 2003 APPLIANCE Magazine

Heating Elements
New Thin Film Heating Elements For Rapid, Even Heating Of Large Surface Areas


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by Jeff Bernthisel and Brian Biller, EGC Enterprises, Inc., Chardon, OH, U.S.

A unique thermoelectric thin-film heater technology was developed with NASA, but has numerous potential commercial and industrial applications.

A unique thermoelectric thin-film heater technology was developed by EGC Enterprises, Inc. - working in conjunction with NASA and aircraft manufacturers - to solve in-flight icing problems on leading edge surfaces in small aircraft. The technology has numerous potential commercial and industrial applications as well. This paper focuses on the principles and capabilities of the technology, and its use to improve the efficiency and reduce the time required to heat large surfaces.

I. INTRODUCTION

The advantages of flexible graphite thin film heaters over conventional metallic element resistance heaters include faster thermal response, improved temperature stability, and operating efficiency. In addition, flexible graphite material is easily converted into the element shape. The resistive element is comprised of flexible expanded graphite foil, a homogeneous sheet material that is an excellent conductor of electricity and heat. The graphite foil is bonded in a laminate structure between outer layers of various dielectric materials. The outer materials are selected based on their thermal conductivity, dielectric strength, and upper temperature threshold. As each of these materials can be easily handled and assembled, Q¥Foilª graphite element heaters are efficient, cost-effective solutions to heat large surface area applications.

II. DISCUSSION

A. Properties Of Flexible Expanded Graphite Foil.


Figure 1. Typical Q¥Foilª Heaters.

1. Material processing and composition. Also known as vermiform graphite, flexible expanded graphite foil is a homogenous sheet material produced from natural graphite flake with no binding materials. The foil is made by first heating the graphite flakes with an oxidizing agent. The resulting reaction expands the graphite into a low density, multi-layer form. This material is then calendared or compressed into sheets that are 0.0030-in to 0.005-in thick. The sheet materials are cut to the element shape for specific heater applications.

2. Electrical, thermal, and mechanical properties. The electrical resistivity of the graphite ranges from about 1 to 8 x 10-4 ΩÐin. This is roughly eight times higher than that of metallic heater materials such as nickel-chromium-plated (nichrome) steel wire. In addition, flexible graphite has an extremely low thermal capacitance. Unlike metallic elements, which have a positive temperature coefficient of resistance, the graphite has a slightly negative temperature coefficient. In most cases this allows the element to increase in temperature at a more rapid rate up to its maximum, based on the designed watt density. These attributes result in heating elements that can repeatedly cycle up and down in temperature very rapidly.

The thermal conductivity of flexible graphite, at up to 225 W/mK in the plane of the material, approaches that of brass. The anisotropic nature of the material, to conduct heat more quickly in the plane than through the plane, creates a heat spreading effect and adds to the efficiency of graphite elements.

Flexible graphite is a relatively inert material that is unaffected by most chemicals. This property, coupled with its upper temperature threshold of 6,000¡F in non-oxidizing atmospheres, enables flexible graphite heater elements to withstand harsh environments.

To date, the widest use of flexible graphite remains in high-temperature gasket, seal, and packing applications to replace asbestos-based materials. Because of its extremely low coefficient of thermal expansion, excellent conformability, and resilience, it is used globally for sealing severe service connections in industrial, aerospace, and automotive applications.

Based on material properties and performance characteristics, many diverse applications for vermiform graphite are being pursued. Some of these include:

  • thermal interfaces to improve heat transfer across a connection
  • electrically conductive plates for fuel cells
  • EMI/RFI shields for electrical and electronics enclosures
  • resistive and electromagnetic heating elements

B. Graphite Element Design


Figure 2. CLICK to see large graphic.

The watt density of flexible graphite resistive elements can be manipulated using four variables: width, length, thickness, and density. Having four variables allows more latitude in the design of the heating element. Thus, the element can be extremely wide compared to the spacing between each segment of the element. This provides for normal coverage of 75 percent or more of the surface area to be heated. This surface area coverage is much greater than typical metal elements provide. The primary advantage is improved heat distribution across the entire surface area.

Flexible graphite heaters can be easily zoned to vary the watt density and thermal performance of different regions within the same element. Thus, if edge losses due to convection are expected around the perimeter of a large heated surface area, the element is simply configured to achieve a higher watt density in that particular zone.

Flexible graphite foil used as the resistive element, laminated within various insulating materials, provides up to 35-percent faster thermal response than typical metallic heater elements. Figure 2 shows time-to-temperature curves for Q¥Foil and etched metal element heaters. Both heaters were constructed with polyimide film insulating layers, and were identical in size and watt density.

The rapid thermal response rate of flexible graphite, at up to 100¡F/sec, along with the high surface area coverage, allows the flexible heating elements to use far less energy than any previous type of thermoelectric heater. These attributes, coupled with the extremely low thermal capacitance of graphite foils, allow the elements to excel at heating up large surface areas rapidly and evenly. The temperature stability achieved with flexible graphite heating elements is typically +/- 3 percent across the entire surface. Shown in Figure 3 and Figure 4 are infrared images of Q¥Foil and etched metal element heaters taken in the same time lapse. Both heaters were constructed with polyimide film insulating layers and were identical in size and watt density. Note the more uniform temperature distribution over the entire surface of the Q¥Foil image.

The graphite resistive elements can be designed for either a.c. or d.c. variable input voltages up to 480 V a.c. Maximum watt densities up to 100W/in2 can be achieved, although 40W/in2 and below is generally enough for most applications.


Figure 3.

Figure 4.

C. Flexible Graphite Heater Construction

1. Laminate Construction. The die cut element and insulating materials are bonded together in a high-temperature vacuum lamination process using adhesives, which are typically fluoropolymer resin films.

Due to the laminate construction and bonding process, the selection of outer dielectric materials is extremely broad. Some of the more common outer layer materials include: polyimide film such as Kapton(R); polymer films such as polyester and polyethylene; fluoropolymer films such as TFE, PFA, and FEP; quartz; ceramic; refractory material; and metal and aluminum foils.

Many of these materials are flexible, allowing the heater to conform to most curved surfaces. When using polymeric materials the heaters can also be molded to specifically fit and be applied to a three-dimensional surface.

The thin profile of the heaters will allow for the reduction of the mass required by other, more bulky designs such as cartridge heaters. This reduction of mass, in turn, facilitates the ability to reach desired process temperature more quickly.

2. Terminations. Terminations of thin perforated metal foil are mechanically attached to the resistive element prior to lamination. These attachment points are also captured during lamination by the outer insulating materials and adhesive, thus providing a more secure bond to the element. Various leads can then be connected to the metal terminations via welding, soldering, riveting, or other mechanical means.

3. Temperature Sensing and Control. Temperature sensors can be readily incorporated into the heater laminate. Thermostats can be wired into the circuit to provide cost effective control of the heated system.

4. Thermal Transfer Interface Pads. Based on the excellent thermal conductivity of flexible graphite and its ability to conform to irregular surfaces, ThermaFoil(R) flexible graphite can also be used as a compliant interface between the heater and the heated surface. This material is referred to as ThermaFoil TIMª (Thermal Interface Material). For higher watt density heater applications, this results in improved surface area contact and increased thermal transfer rates.

III. CONCLUSIONS

Based on development and testing performed by EGC Enterprises, Inc. and NASA, it is apparent that this technology offers benefits to heat large surface area applications (quantified as 1 to 30 sq ft per element) more efficiently than metal element technology.

Primarily, the graphite elements high resistivity and thermal conductivity, low thermal conductance, and high surface area coverage provide for more rapid temperature rises and improved temperature stability across the entire surface.

In comparison to other thermoelectric heaters, the greater efficiency of Q¥Foil translates two ways. One, given that each element has the same watt density, the graphite element will reduce the time required to achieve process temperature and will provide more even heating over the entire surface. Two, to achieve the same set of process performance parameters, graphite elements can normally be designed with a lower watt density than other thermoelectric heaters would allow yet resulting in the same time-to-temperature performance. In either case, graphite heating elements can decrease energy usage and can reduce process cycle times.

This information presented in this paper is based on a broad review of flexible graphite heating element technology. This technology has proven beneficial for numerous large surface applications, but as with any technology, may not be the best solution for all.


About the Authors

Jeff Bernthisel received his BS in marketing management from the University of Toledo in Toledo, OH, U.S. His professional career includes management of sales, marketing, and engineering organizations providing engineered solutions for a wide range of applications and environments. He is currently the general sales manager for EGC Enterprises.

Brian Biller received his BS in mechanical engineering from the State University of New York at Buffalo, in Buffalo, NY, U.S. His professional career includes an intensive research project covering thermal contact conductance of various graphitic materials, resulting in an article published in Carbon Magazine. He is currently the lead engineer for heating and thermal transfer systems for EGC Enterprises.

 

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