Flexible heaters bring with them a wide range of advantages that cannot be ignored for most applications. For starters, they offer superior versatility (hence the name) when alternative types of heaters and can have very thin form factors as well.
They offer excellent thermal stability and can operate at very high temperatures. They are easy to bond with just about any surface. This, coupled with their overall cost efficiency, make flexible heaters a very popular choice in a lot of situations.
Flexible heaters are also inherently customizable, meaning it is possible to design your own based on whatever application you're working on. If this is an option you choose to pursue, there are a few important things when it comes to design that you'll need to be aware of moving forward.
How Does a Flexible Heater Work?
A flexible heater generates heat in different ways - either through electricity or through a chemical reaction. A typically flexible heater consists of a heating element, either etched foil or wire wound. These heating elements are sandwiched between layers of flexible materials that allow for heat transfer from the materials to the application. Some form of resistance is used within the elements to create heat.
There are a few different types of flexible heaters for you to choose from depending on your needs, such as polyimide, silicone rubber, and polyester.
Example of large and small polyimide flex heaters.
(For size reference, we lined it up next to a penny)
Polyimide options are typically used to heat both rigid and curved areas. Because of how they function, these types of heaters are regularly employed for applications that need precise temperature control. Polyimide heaters can be designed very thin, from 0.0045" to 0.010". Due to this thickness, etched foil elements are found in polyimide heaters.
The next is called a silicone rubber heater, which is known for its chemical-resistant properties. Here, a circuit that is attached to power leads is used to heat two layers of silicone rubber. These types of heaters are commonly used in healthcare environments, along with in the military. Silicone heaters can have etched foil or wire round heating elements. The materials are less flexible than Polyimide heaters, as they are used more for straight and flat applications as polyimide heaters are used for curved surfaces. Silicone heaters come in thicknesses of 0.015” to 0.060”.
Finally, you have polyester wire heaters. These are very common in the types of commercially available heated blankets that people can buy. Because these types of heaters are normally contained inside some type of waterproof compartment, they are ideal for wet or otherwise damp environments in particular. Any type of heated commercial product likely has a wire heater inside.
Using Ohms Law to Determine Output Wattage Based on Input Voltage
One important factor to be aware of when it comes to flexible heater design has to do with output wattage. Having the correct wattage output ensures that no matter the size of the heater, enough resistance, voltage, and current will be generated to provide the right amount of heat throughout the flexible heater without heat sinks or cold spots. To find the right output wattage, you use a calculation known as Ohms Law.
Here, you'll need to know the precise input voltage and the amps. Multiply the volts by the amps and the number you arrive at is the output power, measured in watts.
Wattage output is normally expressed as watts per square inch (wpsi). So, if you want a 10-inch by 10-inch heater designed and know that the wattage used is 200 watts, the wattage density would be 2 wpsi. Ohm's Law is an important factor in the design of the flexible heater to ensure enough heat is generated for the application without going over the maximum allowable tolerances for either the application or the heater's components.
You can also use Ohms’s Law calculations to determine other factors about the electric heater, too. If you wanted to find the voltage, you would need to know the amps and the ohms. Multiply one by the other and what you are left with are the volts.
What is the Highest Temperature That These Heaters Can Be Used For?
The highest temperature that flexible heaters can be used for will vary a bit depending on a few different factors. For starters, one must consider the fact that these heaters can get very small - as little as 1/2" in certain situations. On the other end of the spectrum, you can have them as big as 72" for larger environments and industrial settings.
Also, keep in mind that the temperature range may be impacted by the heater's materials as well as the type of heating element used. Polyimide heaters with etched foil elements have a lower temperature threshold than silicone heaters using wire wound elements. A polyimide heater may have a temperate range from 300-390 degrees Fahrenheit while silicon flexible heaters can obtain maximum operating temperatures of 450 degrees Fahrenheit.
Can I Test the Heater in Free Air?
Another common question that many people have when it comes to flexible heaters has to do with whether or not they can be tested in free air - meaning that the heater itself is not coming into direct contact with anything.
The answer is yes; flexible heaters can be tested and used in free air and are in fact done so regularly in certain applications. In an aerospace environment, for example, it's not uncommon for equipment to expand and contract as this occurs. If left unchecked, doing so would potentially damage the fragile components and parts. Therefore, flexible heaters are often used to carefully regulate the temperatures of these environments, eliminating the significant expansion in parts and components that would otherwise occur.
Do Flexible Heaters Need to Be Square?
While it's common to see flexible heaters that are square in shape, this is not a hard and firm requirement when it comes to your design. Rectangular and even round heaters are common as well. But when you find the right partner to help with your design like the team at Epec, there really are no limitations in terms of shape or even size. Having said that, as always, the eventual application will likely dictate what shape of flexible heater makes the most sense.
Can You Place Cutouts in the Middle of a Flexible Heater?
Yes, it is common to place cutouts in the middle of a flexible heater. Cutouts allow the flexible heater to fit around objects on the application or to leave spaces open that will not become heated. Sometimes this is also done for the purposes of mounting, which is again something that will be dictated by the environment. Other times, it is something done to make it possible to have multiple heating zones in the heater itself.
Keep in mind that cutouts can greatly impact the thermal results of the flexible heater. Heat loss can occur near the cutouts based on how the heating elements are run between the polyimide or silicone materials. A heat sink may happen when cutouts are close to the edges or corners of the heater, or when there are multiple cutouts near each other.
For more even temperatures throughout the materials, the thermal pattern for the heating elements is modified. The thermal pattern involves running the elements in a certain direction with a specific amount of space between each line of the etched foil or wire wound element. The size and thickness of the elements may also be altered. These factors ensure that the wattage output is maintained, and the temperature is uniform around the cutouts.
Summary
Flexible heaters are valuable to pieces of equipment that serve a purpose in a variety of contexts. When in the design phase, there are a few careful considerations that need to be made to help the equipment function as efficiently - not to mention as safely - as possible.
If you'd like to find out more information about the basics of flexible heater design, or if you'd just like to go over your own project with a flexible heater engineer, contact the team at Epec today.
