Engineering 101: Three Ways Heat Can Transfer

Q: What are the three ways heat can transfer, and why does that matter when you pick a heater?

A: Heat only moves when there is a temperature difference. It flows from hot to cold through three basic mechanisms: conduction, convection, and radiation. By understanding these mechanisms, heater selection becomes easier, as the goal is to transfer heat from the heater to the part or process efficiently, while maintaining safe operating temperatures and extending heater life.

Conduction is heat moving through a solid or across solid contact. Think of a cartridge heater in a steel block. If the bore is sized correctly and the surface finish is clean, heat can flow easily into the block, allowing the heater to operate at a cooler temperature.

Lower heater temperatures can translate into longer heater life. If the fit is loose or there are air gaps, the heater’s temperature can run hotter. This is because air is a poor conductor, and the heater can’t effectively transfer the heat. In conductive heating applications, good surface contact and material choice matter. Metals with higher thermal conductivity transfer heat faster into the workpiece, while low-conductivity materials act like insulation and should be strategically placed to retain heat within the system.

Convection is heat transfer between a hot surface (like an electric heater) and a moving fluid, such as a liquid or a gas. It can occur naturally, such as warm air rising from a heater’s surface, or it can be forced, using a blower or pump. Forced convection removes heat more efficiently, allowing higher watt densities on the heater surface without overheating it. The fluid properties are an important factor and should be considered in every heater design. For example, water with a high flow rate can accept a higher watt density than thick oils or heat-sensitive materials with a low flow rate.  

Radiation is energy in the form of electromagnetic waves emitted from a hot surface and absorbed by a cooler surface, with no direct heater-to-part contact required.  A critical consideration in radiation heating applications is the geometry, wavelength, and surface condition of the object being heated. If the target fills more of the heater’s view, more of the radiated energy can be captured. Emissivity, or the measure of the material’s ability to absorb electromagnetic energy, is important. For radiant heating applications, the target object’s emissivity should be as high as possible. Dull and black-coated surfaces with a high emissivity can absorb heat more effectively than shiny or reflective surfaces.  Radiant heating may not be the best choice for objects that are shiny or have low emissivity.

Choosing the right heater means evaluating how these three modes will work in your process. Look at surface contact quality for conduction, available flow or mixing for convection, and spacing and surface condition for radiation. From there, select a solution that keeps heater and sheath temperatures under control while delivering heat where needed. Sometimes, one approach is the clear, optimal choice.

Other times, two or even three approaches can work, and the final selection is determined by factors such as footprint, available power, controls, environmental conditions and ratings, or ease of maintenance. Let the fundamentals point to a solution, and when more than one option meets the requirements, choose the one that is simplest to operate and easiest to implement.