Using Flexible Heaters
Flexible heaters have been used in an array of niche products and applications for many years. They can be found in consumer appliances, electronics, medical diagnostic equipment, agricultural products, automotive, military applications, semiconductor and industrial processing. Today, flexible heaters are made from a range of materials, including:
- Wire elements encapsulated in silicone rubber.
- Etched copper foil laminated between sheets of Kapton.
- Carbon fiber consolidated in thermoplastic films.
- Heat rope sandwiched between layers of aluminum foil.
All products have their benefits and drawbacks, and new products get introduced every year with tighter temperature profiles and better reliability. When deciding upon a specific type of flexible heater to use, keep the following 10 tips in mind.
Choose the Right Material for Your ProcessAs your starting point, always determine the maximum operating temperature for your application and select a heater that matches it. As with most products, overengineered components can increase the cost of the final product and reduce sales margins. If your product provides process heat in the 120oF (49oC) range, why select a flexible heater that can operate well past 400oF (204oC)? Higher temperature flexible heaters typically are manufactured from silicone rubber, Kapton film, polyetherimide (PEI) film or aluminum foil. Lower temperature heaters can be found made from polyurethane and other thermoplastic films.
Know Your Thermal ProfileSome applications require very tight temperature uniformity across the surface of the heater (figure 1). Examples include semiconductor processing, medical diagnostics and food processing. Products that can deliver tight temperature uniformity have large continuous heated areas. Carbon-fiber based heaters are constructed using a continuous matrix of fibers that provide very uniform heat. Wire-wound silicone rubber and etched copper foil elements also provide uniform heat and are suitable for many applications. Foil-rope heaters provide the least uniform heat but are probably the least expensive product.
Determine How the Heater Will Be Attached to the Final ProductMost flexible heaters can be attached to the heated substrate by use of a pressure-sensitive adhesive (PSA). These PSAs will bond to most surfaces, including metals, plastics, woods, marble and rubber. If the application requires the heater to be removable, many heaters can be attached using mechanical fasteners such as Velcro, springs, clips or straps. How you attach the heater to the material to be heated is determined by whether it needs to be removable and repositionable.
Decide the Shape of the HeaterSome products such as wire-wound silicone rubber and etched foil can be manufactured in complex two-dimensional shapes -- triangles, trapezoids or almost any other shape. Some manufacturers can even make three-dimensional shapes from wire-wound silicone rubber. Typical applications for these products include heating jackets for large valves and pipe joints. The final shape of your heater is determined by the material to be heated and the desired heat profile.
Compare the Size of the Heater vs. the Operating VoltageMost heaters are built to a specific resistance as determined by Ohm's Law (see graphic).
If the application requires a small heater operated from line voltage (120 V) or higher, many technologies will not be compatible. The resistance of wire or foil is very low, and there often is not enough room in a small heater to package the needed amount of resistance. Some technologies such as carbon-fiber elements have the capability of varying the resistance of the active matrix by increasing or decreasing the amount of fibers or metal plating.
In addition, many flexible heating technologies have a current-carrying limitation. For example, it would be difficult for any flexible heating product to generate 500 W of power from a 12 V source -- the required current would exceed 41 A.
Examine Design Stability When Selecting a Heater DesignSome technologies are more flexible and more capable of handling multiple design cycles than others. Etched foil products are well suited for high volume applications but require a new etch mask to be created for every design change. The design of silicone rubber heaters does not require specific artwork, but the manufacturing process is labor intensive. The design of carbon-fiber based products is flexible and does not require any artwork or specific tooling.
Know When to Go Standard and When to Order CustomDo you require a standard or custom product? Many manufacturers stock hundreds to thousands of standard heaters. Often, these designs have been compiled over the years from their customer base and offered to others with similar requirements. In many cases, you can select an off-the-shelf design. For other applications, you might still never find the exact size, shape and output power that you desire. Almost all suppliers will make custom heaters, but expect to pay more than you would for an off-the-shelf design. However, if your process requires your exact heating profile, the added expense of a custom heater is offset by reduced operating headaches.
Control Surface TemperatureAll heating products have a maximum continuous operating temperature that should not be exceeded. Therefore, the user must maintain the surface temperature below this limit. Most suppliers can provide snap-action thermostats, thermistors, thermocouples or other types of sensors that will allow you to measure and monitor surface temperature.
An alternative to adding temperature sensing devices is to design the heater installation so active control is not required. This can only be accomplished with careful engineering analysis as it requires that the heat removal under normal and abnormal conditions will maintain the heater below its temperature limit. Consider active control for temperature-sensitive, hard-to-control and custom heater applications.