How to transfer heat from a PTC heating element

PTC element heat transfer

The ceramic heating element designed with PTC, positive temperature coefficient, characteristics require a heat transfer medium to move the heat from the component to the material requiring heat.  The heat transfer design is dependent on the material, distance, and surface area.

Heat transfer is heat moving from a warmer object to a cooler thing.  Heat is a measure of the kinetic energy possessed by an item.  The heat measure of an object changes with time until equilibrium between the warmer and cooler things.

Conductive heat transfer coefficient

q = (k/s) A dt

Where 

q = heat transfer (W, J/s, Btu/hr)

k = thermal conductivity of material (W/m K or W/m°C, Btu/(hr°F ft2 /ft))

s = material thickness (m, ft)

A = heat transfer area (m2, ft2)

U = k/s Coefficient of heat transfer

dT =t1 - t2  Temperature Gradient

Disk to Bonding Agent - Bonding Agent to Aluminum - Aluminum to Container - Container to Material

Heat transfer calculations are complicated

Each connection from the source of heat to the intended object to be heated impacts the efficient transfer of heat.  When a PTC thermistor is the heat source, heat energy must move through multiple materials.  An engineer must take the heat transfer characteristics of each material into consideration for effective heat transfer.

Embedding or bonding the PTC element onto a heat transfer material

When manufacturing or installing, it must take special care to minimize heat transfer resistance when mounting the ceramic disk to aluminum or other housing for heat transfer.  A bonding agent with high heat-conducting properties is used to attach the disk to the heat sink material.

The PTC elements come from the factory attached to aluminum fins for surface or air transfer of the heat to adjoining surfaces.  The aluminum fins are bolted or bonded to the adjoining surface to be heated for surface transfer or the heat.   The fins can also be placed such that natural or forced air convection can transfer the heat

Immersion heaters have PTC elements bonded in silicone or other heat transfer material that distribute the heat evenly against a circular tube's inner surface.  The tube's outer shell is in direct contact with the liquid or other material to be heated.  The goal is to minimize heat losses.

Heat transfer efficiency

Each connection from the source of heat to the intended object to be heated impacts the efficient transfer of heat.  When a PTC thermistor is used as a heat source, heat is transferred through multiple materials.  The heat transfer characteristics of each material must be taken into consideration for effective heat transfer.

One of the most important of the PTC heating elements is their high power density.  It makes them ideal for a wide range of applications requiring electric heating.  They are a much better alternative to resistive wire.  Proper mounting to maintain these advantages is important.

The PTC ceramic disk mounting

The thermistor is mounted or bonded to a heat sink material such as aluminum extrusion or fins. Proper bonding to the heat sink material is essential. If there are air gaps between the disk and the heat sink, the heat transfer efficiency goes down.  

The bonding agent itself must be highly heated conductive and resist the heat generated at the PTC element source. Heat energy is transferred by the process of conduction in this example. The less resistance to conduction, the faster the heat will move from one material to another.

Heat energy transferred through radiation is much less efficient due to air's heat transfer coefficient. The thermal conductivity of air is 0.0151 (BTU/(hr ft) at 77°F, while a silicon resin used for bonding is 0.185, which is much more efficient.  

Thermal conductivity is the quantity of heat transmitted through a unit thickness of material due to a unit temperature difference.

The bonding is essential because of the high watt density of the individual PTC ceramic elements to be uniformly distributed to the heat sink.

Extruded aluminum heat transfer characteristics

Aluminum's thermal conductivity is 20.8, which is significantly higher and more efficient than air or silicon bonding agents.  This increased conductivity is why extruded aluminum is used for heat sink fins and other structures designed for heat transfer.

The heat sink material transfers heat more effectively over the longer distances and area required for moving the heat to the target material.  Extrusions with fins are used to transfer heat uniformly to the surrounding air by convection of forced air.

Surface heating applications to transfer heat directly to another surface are designed with a larger surface area that can be bonded directly to the surface of a vessel containing material to be heated or the surface itself.

The goal is an equal distribution over the surface contact area.  Physical attachment of the heat sink by fasteners or specialty glue designed for efficient heat transfer is the next barrier to heat transfer.

Target material or surface to be heated

The mass and volume of the target material is the last variable in the equation.  Is it just the surface area in direct contact with the heat sink, or does a larger surface area need to be heated?

The nature of conductive heat transfer works to continue transferring heat energy from the hottest component to the coolest.  Heat energy continues to move from the heat source through the heat sink.  This process will continue as long as there is a temperature difference.

One of the material's heat reaches the same temperature as the heat sink & PTC source; the PTC element will reach the target temperature and resist further current flow.  The target material doesn't produce heat energy; it only consumes it.

The only other variable is the ambient heat surrounding the system.  The ambient temperature essentially creates another baseline in the temperature resistance chart.  Or less of a delta between the target temperature and the starting point for heating.

How does the PTC heating element know when to increase current?

When there is a difference in temperature between the heat sink and the PTC element lower than the element's set-point, the resistance decreases, and the current again flows through the device.

This state of equilibrium through the heat transfer path will continue to keep the system at the targeted temperature without additional sensors or circuit controls.  As long as power is applied to the PTC element, the system will maintain the temperature.

When power is applied, the temperature delta between the PTC element and the heated material is high.  The PTC element resistance will be the lowest, and the maximum current will flow for a rapid heating system.

These characteristics are what makes PTC heating elements and their components favored for precise heating processes. 

Test your heat transfer calculations

Off-the-shelf PTC heating components are available for testing your assumptions and calculations.  The components solve the first two equations by coming from the manufacturer embedded into heat sink configurations.  What remains is to attach the manufacturer’s heating component to the surface of the material or vessel to be heated.

The supplier can offer you recommendations on how to best fasten the heating element to the surface to be heated.  Many of the devices come with mounting systems for surface or immersion heating.

Want to learn more or have a specific application?

DBK USA has experts standing by to answer your questions.  Specialists in PTC heating elements and applications can help you select the right components for your application.

Feel free to call our custom wiring engineers directly at 1-864-607-9047