Traditionally, electronic or electrical components installed in an enclosure are designed for a maximum operating temperature of +50 °C. However, how long the components can actually be used depends heavily on the temperature. However, how long the components can actually be used depends heavily on the temperature - a temperature 10 °C lower doubles the service life. For this reason, a temperature of +35 °C is generally selected inside the enclosure - as the ideal temperature in terms of the service life of the components and the cost of enclosure air conditioning.

Passive instead of active

In principle, there are two options for dissipating the power loss or heat from the enclosure: the use of a medium (air or cooling water) that transports the heat out of the enclosure, or convective heat transfer via the surface of the enclosure. In the first option, active cooling, additional devices are required - for example filter fans, cooling units or air-water heat exchangers. In the second case, passive cooling, heat is transported exclusively via the enclosure walls. As no additional devices are required here, the costs are lower. At the same time, users save on both energy and maintenance costs during operation. The system is also better protected against dust and moisture, as no additional openings are required in the enclosure walls. The EMC protection of a completely enclosed enclosure is also simpler. Another positive effect is that the formation of condensation, which can occur with active cooling, is ruled out. Last but not least, the load on the components due to temperature changes is lower than with active air conditioning, as the temperature in the enclosure remains constant while the power loss remains the same.

The benefits of a larger surface area

However, passive heat dissipation has limitations that are based on the physical principle. The lower the ambient temperature, the better this method works. The heat transfer coefficient of the enclosure material and the effective surface area of the enclosure are also decisive for heat dissipation.

In the example, the internal enclosure temperature is +43.9 °C and is therefore slightly above the recommended temperature range of +35 to +40 °C. It is therefore necessary to cool the enclosure using active air conditioning.

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DIN EN 0660-600-1 supplement 2/IEC TR 60890 specifies how this is calculated. The effective enclosure surface area is maximum for a given enclosure size if the enclosure stands alone and free in the room. It is reduced by baying several enclosures, wall mounting or covering the roof surfaces. If the power loss of the components in the enclosure and the ambient temperature are known, the average temperature inside the enclosure can be easily calculated.

Even if the calculated temperature is higher than the desired enclosure internal temperature, active cooling is not necessarily required - if a slightly larger enclosure is used, for example, this may be sufficient to get by with passive cooling. With small enclosures in particular, a slight increase in the enclosure surface area can lead to a significant reduction in the maximum enclosure internal temperature. This should be taken into account when dimensioning a control and switch cabinet with a low heat load.

Another way to manage without active air conditioning components is to install components with particularly high power losses, such as braking resistors, outside the enclosure. With clever planning of a control and switchgear system, costs for cooling can be saved. The size of the control cabinets, their installation and the positioning of the components with the highest power loss are decisive factors. The enclosure material also influences the enclosure climate control: While enclosures made of painted sheet steel or stainless steel with a heat transfer coefficient of approx. k = 5.5 W/(m^2 K) are traditionally used in mechanical engineering, the k value changes due to the design of double-walled or insulated enclosures for other industries or for outdoor applications.

Author: Heiko Holighaus is Head of Research & Development at Rittal in Herborn.