A Guide to Flexible Heaters, Their Types, and Selection Criteria
What Are Flexible Heaters?
Flexible heaters feature flexible heating elements, which can easily conform to any surface. Flexible heaters can be screen printed, etched, or wire wound and are made of different materials. The material of construction also contributes to different properties. The flexibility, ruggedness, and reliability of these heating elements are the three factors that have contributed to their widespread use across industries. Owing to their different materials, these flexible heaters exhibit a wide range of properties, such as chemical resistance, moisture resistance, and so on. They can be operated in harsh to vacuum environments and can withstand continuous temperatures of up to 392°F or 500°F intermittently.
Types of Flexible Heaters Based on Designs & Materials
Flexible heating elements get their names from their designs and materials. Based on their designs, the heating elements are distinguished as:
· Wire-Wound Elements: These feature a resistance wire of a specific diameter. The wire may possess multiple strands or a single strand placed onto the fiberglass core. If the wire possesses multiple strands, then they are woven together before being placed on the core. The wire is inserted in between two layers of a substrate. The substrate typically used by Birk Manufacturing is silicone rubber. Wire-wound flexible heaters ensure great physical strength and flexibility, which makes them popular in dynamic applications. These heaters can be bent and conformed to custom geometries without being damaged. They can also be placed in long patterns in applications where large heaters are needed.
· Etched Foil Elements: Unlike wire-wound heaters, etched foil heaters feature no wires; instead, a thin alloy metal is etched onto the substrate material. Usually, these thin foils are acid etched to create the appropriate circuit pattern. The foils can be made as thin as possible and can appear similar to regular aluminum foils. Flexible etched foil heaters are widely used in applications where weight and size can be major concerns. These heaters also possess complicated heat distribution properties, which enable manufacturers to achieve better temperature control.
Today, heating solutions are available in various materials. The following are a few common materials used in flexible heaters.
· Silicone Rubber: Flexible heaters of this type are made of high-quality silicone rubber, which stands out due to its durability, ruggedness, and flexibility. In addition to this, these heaters are also known for their properties, such as:
o Excellent chemical and scratch resistance
o Excellent zone and weather resistance
o Non-toxicity and flame retardance
o High thermal stability
Owing to the wide range of design flexibility that they offer, silicone rubber flexible heaters are offered as part of a sub-assembly or an integrated assembly. They can be easily integrated with different components as per the application requirements. Silicone flexible heaters can be easily vulcanized onto surfaces to ensure excellent bonding.
· Kapton®: Kapton® is a polyimide film that was developed by DuPont. The heaters made of this material are known as Kapton® flexible heaters. To create these heaters, an etched foil element 0.0001" thick is encapsulated between layers of polyimide film. Polyimide insulated flexible heaters possess the thinnest profile and can easily fit into small spaces. However, having the thinnest profile doesn’t mean having a weak tensile strength. When compared to other flexible heaters, Kapton® heaters have a higher tensile strength. They ensure excellent performance in all types of environments. Kapton® insulated flexible heaters are available in various shapes, wattages, and sizes, and they can withstand a wide range of temperatures.
· Mica: These heaters comprise mica paper that is infused with a polymer that can resist heat. Mica heaters are suited for applications that demand high watt densities and quick temperature recovery times. They can be shaped into 3D geometric patterns, but once the heater is put to use, the flexibility is lost.
Tips for Choosing the Right Flexible Heating Elements for Your Application
Flexible heating elements differ in terms of their properties and applications. So, while choosing them, the manufacturers need to perform a comprehensive evaluation of the requirements. The following pointers will make the selection easy if you are considering it for the first time.
· Performance Requirements: When it comes to addressing the performance requirements, you cannot ignore the following:
o Temperatures: Extreme heating temperatures and low temperatures are two challenges that manufacturers need to address while choosing the material for their flexible heating elements. Extreme temperatures may impair the heating elements, while low temperatures may break them. So, one needs to address them wisely.
o Maximum Operating Temperature: This is the maximum temperature that a heater can sustain without damage. Silicone heaters can withstand high temperatures, but polyimide heaters can work across a wide range of temperatures (from -100°F to +300°F).
o Lowest Operating Temperature: This is the minimum temperature that a heater may be exposed to. Polyimide and silicone heaters are ideal for the lowest operating temperatures. Silicone heaters can be operated in temperatures between -80°F and 70°F. The heaters may malfunction if exposed to temperatures below this. Polyimide heaters can withstand minimum operating temperatures from -200°F to -320°F.
o Heating Capacity: Measured in kilowatts, this indicates the amount of thermal energy delivered by the device over time.
o Watt Density: This is the output of the heater per square inch. It is expressed in watts per square inch of the surface area of the heater. It helps determine the operating temperature of the heater for a set of conditions. You can find heating elements with high and low watt densities. High watt densities may lead to failure due to material heating, fluid carbonization, and the breakdown of the element. High-watt-density heaters are not recommended for poorly circulated materials, viscous materials, or volatile materials due to the high risk of fire. Heating elements with high watt densities are chosen for high operating temperatures. Heaters with the lowest watt density are expensive, as they add to the heater life.
o AC Voltage: This refers to the minimum alternating current, which is required to operate the heater.
o Ambient Temperature: This refers to the immediate temperature of the surroundings where the heater may be stored. Gaining an understanding of ambient temperatures will help prevent the device from being damaged or malfunctioning.
· Surfaces: Flexible heating elements can conform to a wide range of surfaces. However, not all heating elements may be suitable for all surfaces. The following information will help further your understanding.
o Silicone rubber heaters can easily conform to large and flat surfaces.
o Polyimide and polyester heaters are suited for irregular and curved surfaces.
o Transparent polyester heaters are the best choice when aesthetics are important.
· Sizes: The heater dimension is an important consideration when choosing flexible heating elements. They must be sized properly to fit and perform in the application.
· Need for Sensors and Temperature Controllers for Flexible Heaters: Most of the time, controls and sensors are built into flexible heaters. Surface mount sensors are one of the important types of sensors used with flexible heaters. RTDs, thermistors, and thermocouples are also used on the surface to be heated. Thermostats are used alongside flexible heaters in many basic applications.
Applications of Flexible Heaters
Custom flexible heating elements are used in various industries. Silicone flexible heaters and polyimide insulated flexible heaters are integrated into the following equipment.
· Incubators and sterilizers
· Freeze-dry equipment
· Handheld outdoor devices
· Wastewater monitoring
· Automated weather equipment
· Food preparation and delivery stations
· Condensation and evaporation equipment
· Surgical tools
· DNA sequencing
· Vending machines
· Air horns
Conclusion
Flexible heating solutions add value to every application. It is important to choose the right material and design for your application. Today, it is quite easy to find these flexible heaters in standard specifications; however, they may not work for your application. Hence, it is always best to partner with a manufacturer who specializes in custom flexible heating elements. Considering all of the factors discussed in this post will make your selection easier and help you make the right decision.
