Thermal emissivity of materials in the framework of heating and construction

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Principle of thermal emissivity

The emissivity of a material (ε) is its capacity to absorb and reemit energy by radiation. This physical value indicates how much a surface emits thermal infrared radiation compared to an ideal body called “black body”. The “black body” is the most efficient in terms of heat emission by radiation, ε=1=maximum.

Emissivity of materials for emitting radiation

Emissivity of materials for emitting radiation

Difference in emissivity between 2 materials (sleek-reflective white IR / rough-absorbent black IR)

Difference in emissivity between 2 materials (sleek-reflective white IR / rough-absorbent black IR)

In the example thereagainst, we bring into light a higher emissivity (capacity to radiate) for a non-polished black metal material, coated with an enamelled layer.

Thermal emissivity in the context of radiation heating

radiant power emissivity formula

Contribution of emissivity in the radiant power

The emissivity of a surface varies according to several parameters (which makes it difficult to measure):

  • the nature of the material.
  • surface condition. The rougher (as opposed to sleek) the surface of the emitting material, the better the emissivity.
  • the colour. A black pan will have a better emissivity than a silver pan.
  • the temperature of the material. The higher the temperature, the better the emissivity.

A few indicative values of emissivity of materials used to transmit heat in the context of heating:

  • ε (anodised aluminium) =0.77
  • ε (polished aluminium) =0.05 (the surface being therefore a significant non-negligible part)
  • ε (glass) =0.93
  • ε (smooth metal) =0.25
  • ε (metal coated with specific emissive powder to maximise the emissivity) =0.96 – sources www.degxel.co.uk

In clear terms, when we talk about the diffusion efficiency of radiation by a heater, the knowledge of the material is not sufficient to draw a conclusion. Other factors described above are essential.

Infrared thermography of an infrared heater with an emissivity ε of 0.96 (max = 1) –

Infrared thermography of an infrared heater with an emissivity ε of 0.96 (max = 1) – Source www.degxel.co.uk

 

Thermal emissivity in the context of construction

Thermo-detector of emissivity of the measured material (wall)

Thermo-detector of emissivity of the measured material (wall)

Some common emissivity values of building materials:
  • ε (stone)=0.93
  • ε (frosted glass)=0.93
  • ε (frosted tiles)=0.92
  • ε (non-smooth wood)=0.90
  • ε (red brick)=0.90
  • ε (plaster)=0.90

The surface emissivity is taken into account when measuring the surface temperature of a wall using an infrared thermo-detector. The gun captures the surface temperature of the material without contact. We can thus deduce thermal bridges and the risk of mould.

Other topics that may be of interest to you

To know more about The best insulators for construction

To know more about The best heaters

To know more about The differences between the ambient temperature and the perceived temperature

To know more about Identifying thermal bridges and preventing mould

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Summary
Thermal emissivity of materials in the framework of heating and construction
Title
Thermal emissivity of materials in the framework of heating and construction
Description
THERMAL EMISSIVITY OF A MATERIAL IN THE FRAMEWORK OF RADIATION. Principle of thermal emissivity. Thermal emissivity in the context of radiation heating. Thermal emissivity in the context of construction.
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