Thermal transmission

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Underlying technologyHeat dissipationThermal transmission

 

Thermal energy is always transmitted from a medium with the higher temperature to one with a lower temperature. This becomes obvious if you consider that heat is the perceptible vibration of molecules (atoms). The stronger the vibration, the higher the perceived temperature. The energy of a molecules vibration can only be transmitted to a neighbouring molecule if that molecule's vibration energy is lower.

 

The thermal transfer within substances depends on their characteristics.

 

In the case of dielectric conductors (isolators) the electrons are fixed to the atoms and can therefore not transfer any vibration energy to neighbouring atoms. Instead, the vibration energy is transmitted via the vibrations in the lattice of atoms or molecules.

 

In the case of conductive substances the vibration energy is transmitted directly via the moveable electrons to the neighbouring atoms, and via the lattice of atoms or molecules. In the case of metals, the proportion of vibration energy transmitted via the electrons is higher than that via lattice vibration. This is also the reason why good electric conductors (such as copper) transmit heat much better than bad conductors (such as iron).

 

In the case of liquids and gases the vibration energy (heat) is also transmitted via lattice vibration. If, however, the increase in temperature causes a change in density in the liquid or gas, the heat can also be transmitted via convection (e.g.:cicon2 air circulation), which speeds up the thermal transmission.

 

Click to expand or collaps object.Example of thermal transmission in a control room

Below is a simplified description of the heat exchange between an HMI and surrounding components. The control cabinet has an integrated forced ventilation, which increases the convection.

Fgr_Principal_heat_transfer

The source of heat is inside the HMI. It transmits the heat via various thermal transmission media to the surrounding enclosure. Consequently, the only relevant aspect for the analysis of thermal dissipation is the differentiation between the different amounts of thermal energy emitted between front plate and enclosure in the control cabinet. In the case of the HMIs, this is about half via the front plate, and half via the enclosure.

 

The heat emitted via the front plate must be safely dissipated via thermal resistors Rth_2_1 and Rth_2_2.

The level of thermal resistance provided by Rth_2_1 is determined by the transmission of thermal energy from the front plate to the air and the resulting convection, as well as the transmission of thermal energy of the air to the cooler cabinet wall.

The thermal resistance provided by Rth_2_2 is determined by the thermal conductivity of the cabinet wall and its thermal dissipation to the surrounding air.

 

The heat transmitted via the enclosure must be safely dissipated via heat resistors Rth_1_1 to Rth_1_4.

The thermal resistance provided by Rth_1_1 is determined by the transmission of the heat of the enclosure to the air inside the control cabinet and the existing convection.

The thermal resistance provided by Rth_1_2 is determined by the transmission of heat from the air discharged from the control cabinet to the surrounding air.

The thermal resistance provided by Rth_1_3 is determined by the thermal transmission from the air inside the room to the outer wall.

The thermal resistance provided by Rth_1_4 is a result of the thermal conductivity of the outer wall and its thermal exchange with the surrounding air.

 

Hinweis

An optimised forced air circulation can reduce all thermal resistance by means of increased convection.

 

See also:

Kapitelseite Thermal resistance

Kapitelseite Thermal transfer resistance

 

 

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