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Flexible Heater Design: Watt Density, Input Power, and Heater Size

Epec Engineered Technologies
Written by Epec Engineered Technologies
Posted on November 5, 2021 at 11:06 AM

Flexible heaters allow customers to apply heat at targeted areas at any given moment. This procedure is desired for applications for a variety of reasons, such as preventing condensation to form on instrument panels, keeping the consistency of semi-fluid materials so that they may pass through pipe systems, or keeping systems warm during extreme environmental temperatures.

To design the right flexible heater for each application, there are three specific aspects to understand: watt density, input power, and heater size. Each of these factors plays an important role in getting a heater that is the optimal size for the heated area, obtaining the ideal temperatures, and using the correct voltage. These aspects also help to minimize potential issues that the flexible heater may experience, such as hot spots, on/off power cycling, delamination of materials, or oxidation of the elements that could lead to potential burnout.

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Watt Density

Watt density refers to the central concept that the wattage output of a heater and will be relative to its size. All flexible heaters, no matter their size, temperature range, or materials used, rely on this concept to ensure their proper design and function. The wattage output is expressed at watts per square inch (wpsi) in the United States.

So, when designing a flexible heater that will have a size of 10 inches by 10 inches and operate on 200 watts, the watt density for the heater will be 2 wpsi. Always keep in mind that, with all things equal, the watts per square inch will greatly influence the heater's operating characteristics for the application.

Input Power

When designing flexible heaters, they operate on input power and resistance. As voltage is applied along the heating elements, there is opposition to the current flow as the electricity is turned into heat. Raising the applied voltage will raise the power output of the flexible heater, and in turn, raise the amount of heat generated.

For a flexible heater, the elements – either wire wound or etched foil – will be placed in a thermal pattern to direct heat in the desired places. Not every flexible heater has to have even heat throughout the surface. Some heaters may have multiple heating zones where places are cooler than other areas. To control these temperatures, the heating elements may have varying thickness or varying spacing. In this manner, the heater can have different watt density levels while still operating on the same input power.

Heater Size

Keep in mind that even when compensating for the generated heat by changing the thickness of the heating elements, the size of the heater also comes into play. Having a large amount of voltage going through a small amount of space may cause hot spots as the materials themselves will fail. Delamination may also occur between the material's layers.

Various size silicone and polyimide flexible heaters

Various size silicone and polyimide flexible heaters.

On the flip side, having a low amount of voltage with a large heater size will have the heater struggling to reach the desired temperature. The flexible heater may also experience on/off cycling as the application's load requirements do not match the flexible heater's wattage. The heater will constantly run, increasing power usage and energy waste.

Heater size is also limited to the types of materials that can be used. Polyimide flexible heaters offer more flexibility for the application. They can come in very small to medium sizes as their temperature ranges are of up to 300°F to 390°F. The materials have resistive foil etched elements and are typically used for lower to medium heating applications. Silicon rubber heaters come in small to larger sizes and has temperature ranges of up to 450°F. They are less flexible than polyamide while having wire wound elements. These materials offer higher watt densities.

Additional Considerations

The conductive materials will also impact changes to the watt density. Some materials have more insulation resistance. This resistance may inhibit generated heat that cannot dissipate quickly.

An important consideration when it comes to watt density and heater size is how fast or slow the heat dissipates. If the surface or application for the flexible heater will cause the heat to dissipate at a rapid rate, then the heater may have a smaller size at a higher temperature and watt density without the worry of hot spots or damage to the materials or elements. A heat sink can improve the watt density of the flexible heater. The heat sinks will constantly pull and dissipate the temperatures quickly without the elements experiencing burnout.

Due to the varying flexible heater designs, materials, elements, and sizes, determining what is best for the application while keeping costs low requires understanding the temperature needs of the application and the input power that is available. In addition, the application may undergo certain environmental variables that may impact the flexible heater's functions.

Obtaining sample heaters may help a customer figure out how it will respond to the application as well as the environment. They may see for themselves how the polyimide or silicon rubber materials interact with the application's materials when attached. The customer may also determine whether the watt densities may need to be adjusted to generate the appropriate temperatures.


When considering a flexible heater for your operations, the watt density, size, and input power will have a direct impact on the application as well as the materials that are used for the heater. Contacting the manufacturer's engineering team, such as Epec, will allow everyone to go over the possibilities for the heater as well as the thermal pattern for the elements of the heater.

By evaluating the test results of the samples, we often provide suggestions regarding the size and materials to use. We also consider the heating elements, the thickness of the elements, and the spacing in the given size of the heater. Performing these tasks early on during the scope of the project helps everyone be on the same page regarding the heater's design. Then fewer changes will need to be made to the heater during the production stages, as the customer may obtain the flexible heater to their specified needs and deadlines.

Topics: Product Design, Flexible Heaters

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