Four Selection Considerations for PTC Heaters
Positive temperature coefficient heating elements have some unique properties that are taken into consideration when deciding which type to use for industrial applications. These are small scale heating applications for both industrial, commercial, and residential applications.
- Temperature & Controls
- Voltage & Watt Density
- Space & Safety
How Does the Application Impact Design Considerations?
We find PTC thermistor elements in almost every industry. They are primarily known as PTC heating elements or PTC ceramic heating elements utilized for creating heat. Used with industrial applications, these are small scale heating applications, not furnaces or ovens.
Space or Air Heating
Using a PTC element heater for space heating is very common. These units are in:
- Household space heaters
- Cockpits or driver cabins of heavy equipment
- Augmenting heating in airflow ductworks
- Outside enclosures
The household space, truck cabin, and ductwork heaters require a thermostat of some time to control the enclosure temperature for human comfort. Outside enclosure environments may use a thermostat or hydrostat with maximum temperature in mind.
The element heating output, while controlled by the thermostat, still offers some desirable features over traditional resistant heaters. Maximum temperature can be designed into the unit to act as a safety feature for both circuit, fire prevention, and overheating.
PTC air heating element automatically adjusts circuit resistance based on the ambient temperature. In some applications, this makes it possible not to require additional control circuitry.
If the required temperature in the air heated space is 75 degrees, the PTC element itself can be designed not to shut off when the ambient temperature reaches 75 degrees. Often electric heaters are difficult to set if the control circuit doesn’t include a temperature-sensing device.
We define surface heating as transferring heat generated by the PTC heating element directing to another surface. A PTC flange heater is ideal for this purpose because the element is in an extruded aluminum flange that attaches to the surface to be heated.
These surface heaters are fastened with mechanical fasteners, screws, and bolts and use a heat-conductive silicone or similar material to close any air gaps between the surfaces. Typical applications:
- Manufacturing equipment
- Personal care devices
- Condensation protection
- Heat conduction
- Outside electronic enclosures
PTC finned heaters are surface heaters. Their aluminum fin structure radiates heat from the device to the air within the cabinet.
Cartridge Heaters, Embedded or Immersion Heating
Embedded heaters are inserted into solid surfaces such as equipment dies or molds to maintain a specific temperature. PTC cartridge heaters come in tubular shapes. The PTC heater cartridge transfers heat uniformly to the circular tube conducts the heat to the adjoining surfaces.
Immersion heaters are cartridge heaters with electrical connections protruding through a mounting device on one end of the tube. The mounting inserted into the wall of a container tightly to prevent leakage.
A typical application would be similar to a household water heater on an industrial scale. The immersion element heats the contents, these heaters heat from within more effectively.
PTC convection heaters transfer heat as ambient air passes over their fins. Heating via the chimney effect and design for a specific temperature, they simplify control circuitry and reduce cost. A finned air heater is an excellent example of a convection heater.
The heaters come in a variety of orientations and configurations, attaching to electrical enclosures simply with rack mounting directly to DIN rails. These units are electrically isolated from the PTC heater circuit and safe to touch.
PTC air heaters are similar to the convection heaters with more surface area that transfers heat to air forced through the fin or perforated structure. With more surface area, heat transfers with low outlet volumes.
The combination of PTC heating element independent temperature limiting effect and the adjustment of air volume makes these superior for controlling temperature.
Space heating for trucks or farm equipment cabs, ships, or outside enclosures exposed to extreme weather conditions is one used for these heaters. They also replace traditional resistant heating elements found in HVAC systems.
PTC fan heaters produce a higher volume of heated air than required, and a built-in fan is more desirable than separate forced air. In these cases, the fan is incorporated directly into the heating unit—separate circuits power the fan and heating current.
Typically the heating output efficiency is more significant than conventional heating or air heating units. They also make installation into cabinets easier with built-in panel mounts. These heaters can are for heating any enclosed space similar to other heaters.
Not just for industrial applications, the fan heaters can be incorporated into consumer products such as electric fireplace or space heaters. The fans have built-in finger guards and voltage isolation for home use or transportation such as trucks and equipment cabs
Temperature & Controls
Electric heating systems
Electric heating systems require control circuitry to set and monitor the temperature. Applications utilizing PTC heating elements may or may not require control systems due to the self-regulating nature of the elements.
Applications such as remote outdoor control panels or other self-standing devices like ATM or even medical devices may not need a control system. The PTC ceramic elements are designed specifically for a set temperature.
Once at the design temperature, the nature of the positive temperature coefficient device is to dramatically increase the resistance of the circuit and stop current flow. Overheating is not a concern.
Also characteristic of this positive temperature coefficient, PTC heaters is their ability to respond to ambient temperature. As the ambient temperature goes down, the resistance in the circuit goes down, generating more heat. As the temperature increases, the device increases in resistance, reducing the temperature.
Temperature and Power Control
Where the temperature needs a control circuit, PTC heating elements may still be more energy efficient than traditional resistance heaters. Fan or forced air units move heat away from the elements into adjacent space that need control devices.
An HVAC or vehicle cabin heater needs a temperature sensor that detects the heat or temperature level in the heated space. The temperature sensor provides feedback to a power controller or resistance that reduces current to the circuit.
This control device monitors humidity and sends control signals to the power controller, adjusting the temperature for the humidity in the air. Humidity can destroy electronics and other scientific components, where temperature sensing is not enough to maintain the right environmental conditions.
Voltage & Watt Density
PTC heaters come in standard voltages of 12V, 24V, 120V and 240V. Most engineers know that watts produced are a function of voltage and current.
The wattage of the heater
Watts = Current X Voltage or amps X volts. Wattage is the power produced by the device-dependent on the voltage and current. Power is another term used to describe voltage and current. Power creates heat in a resistor, and in a PTC ceramic heating element is transferred to the material that encloses it.
The temperature of the heater
Calculating the temperature created by the power is a little more complicated. The heat produced by the heating element raises the temperature. Heating is measured in BTUs or British Thermal Units. A BTU is a heat needed to raise the temperature of a pound of water by a Fahrenheit degree.
Are you confused? I bet you think that a pound is a measure of British currency, and liquid measured in liters! Not relevant to determining the temperature but made you consider it.
To further understand temperature, you would need to know about the specific heat capacity of a material and its weight in grams, not pounds! And the number of joules in a BTU. All of which is beyond the scope of this article.
Watt density of the heating assembly
Watt density is expressed as watts per square inch or meter, depending on which side of the pond you are on. Watt density = power/surface area. The efficiency of the power transmitted to the surface area is a big factor in calculating the effective watt density.
PTC heating elements enclosed inside the material that transfers heat to the surface or air with silicone or other conductive materials to improve the watt density. Ideally, for an application, the lower the watt density to reach a set temperature is better.
Another term used for watt density is heat flux. It is the amount of heat emanating from the surface area. An example of a very focused watt density would be a plastic sealer. The flux required to seal plastic is very focused.
Safety of employees and customers
Always a concern when using heating equipment is the safety of the devices. Temperature, voltage, and current always has the potential for injury. Safety is an essential consideration in industrial, commercial, and residential applications.
Heaters voltage and current should be isolated from the heat transfer material. Aluminum fins make good conductors of electricity if the heating elements are not adequately insulated. The terminations for connecting the devices to electrical sources should be factory-made and insulated correctly.
Do the fans employed for heating need finger guards or specific mounting considerations to ensure people cannot come in contact with the blades?
The surfaces of the heating assemblies by their nature can be hot. Do you need an assembly that shields the heating surfaces from human contact?
PTC heating elements and devices have a built-in safety feature in that they shut off when reaching the desired temperature. Take this into consideration when selecting a temperature in units that can come into contact with humans or animals.
While space the heater is mounted or the space to be heated seems to be an obvious consideration. The more space available for the heater, the lower watt density for the application. Lower watt densities are better than higher densities for efficiency.
The space that requires heating is always a consideration. There are online calculators available to help you determine proper wattage for a given space and enclosure materials. When in doubt, check with an application specialist.
PTC heating components come in a variety of sizes for space availability. Choose the one that best fits your needs. Don’t try to stick a square unit in a round hole!