Heat Radiation in Industrial Furnaces

Heat radiation often plays a dominant role in the total heat-transfer process between furnace gases and heated workpieces. In several types of furnaces—such as resistance furnaces and muffle furnaces—radiation from heating elements and furnace walls becomes the primary and sometimes the only means of transferring thermal energy to the material being processed.

What Is Heat Radiation?

The term heat radiation refers to a complex process composed of multiple continuous stages.
It begins when the thermal energy stored within a material is converted into radiant energy. This radiant energy then propagates in all directions, and once it encounters another object, it is fully or partially transformed back into heat.

From a physics standpoint, thermal radiation behaves identically to visible light radiation. Both are electromagnetic waves; the only difference lies in their wavelength. Therefore, many laws that govern visible light—such as reflection, refraction, and propagation—also apply to heat radiation.

Key physical characteristics include:

• Thermal radiation travels in straight lines through homogeneous and isotropic media.

• The intensity of radiant energy decreases with the square of the distance from the radiation source.

• Laws of reflection and refraction apply equally to thermal radiation.

Although we describe radiation or absorption as occurring “on the surface” of a material, this is a conceptual simplification. The surface itself is a geometrical boundary; the actual energy transformation occurs within the material’s structure.

All Heated Bodies Emit Radiation

Except at absolute zero, all materials emit radiant energy when heated to a specific temperature—even if the surrounding space is at a higher temperature.
The amount of radiant heat emitted increases sharply with rising temperature.

An object with a perfectly black surface (known as a blackbody) emits radiant energy across a continuous spectrum of wavelengths. However, not all wavelengths appear with the same intensity:

• Radiation intensity is zero when wavelength is zero

• Intensity increases across the spectrum

• The distribution depends on temperature and surface properties

This understanding is fundamental for furnace design, thermal efficiency analysis, and the selection of heating methods in industrial processes such as metallurgy, heat treatment, and material forming.