Two aspects are at the forefront of modern control cabinets: the smallest possible space requirement and a high packing density. At the same time, a wide range of physical conditions must be taken into account, which have a significant impact on both energy efficiency and the service life of the installed components.

Today, industrial electronics are generally designed to be robust and can easily withstand vibrations or vibration and shock tests, for example. In addition, components and systems can be designed in such a way that neither moisture nor humidity can penetrate.

Desuperheating through circulation flow - even without active cooling.

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The situation is different with heat: it is still the great enemy of all electronics! Efficiencies of over 90 % in some cases are often standard for electronic or electrical devices and machines. It is therefore no problem to use compact and condensed designs; the space required per watt installed is reduced. However, even if the energy efficiency is very high thanks to high efficiency, the heat is concentrated in a very limited area and can quickly reach very high, critical values there. In addition, the compact design results in less surface area, which reduces heat dissipation to the environment. This increases the risk of heat nests or hotspots, which is exacerbated with increasing packing density.

One way of counteracting this risk is to use ductless wiring systems, such as those offered by Lütze. The elimination of cable ducts results in a space saving of 30 %; in addition, natural convection is supported by a reduction in flow-calmed zones. Another thermodynamically important feature of ductless wiring systems is the division into a mounting side and a wiring or cable routing side.

In close cooperation with the Institute for Thermodynamics and Thermal Engineering at the University of Stuttgart and the Fraunhofer Institute, Lütze has brought together the results of various research projects on the subject of 'Thermal behaviour in the enclosure' in the form of analyses, field tests and simulations, thus implementing new ideas for an optimized and balanced enclosure climate. One of these ideas combines the clear division into a front mounting level and a rear level for wiring and cable routing on the one hand and the elimination of horizontally mounted cable ducts in particular on the other. This enables an actual circulation flow and stimulates and supports natural convection. This positive effect also works without active cooling and is measurable. The use of an air conditioning unit also measurably increases cooling efficiency.

Gaining space, what now?

The high packing density in a smaller housing requires a higher cooling capacity, which drives up costs and has a negative impact on energy efficiency. Targeted positioning of large heat loads close to the cooling air inlet can alleviate the thermodynamic problems somewhat, but not eliminate them. The same applies if the placement of components is consistently optimized according to power losses.

... the cooling capacity can be reduced.

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The hardware planner can also take the opposite approach: At least some of the space gained can be used for a so-called 'loosened packing density' by spreading the components out and arranging them more generously. This avoids or reduces flow-calming zones in the upper part of the enclosure. This supports natural air convection or at least hinders it less, and the cooling capacity can be reduced.

This effect can be further enhanced. The current 2nd generation of the 'Airstream' ductless wiring system from Lütze offers a curved core structure for the profiles, which increases torsional strength.

The advantage of curved profiles

The air flow around curved profiles ensures optimum rear ventilation.

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Heavier and larger components can be mounted securely and stably on the DIN rail without additional effort. The thin profiles slim down the enclosure and are installed in such a way that they are exposed to the cooling air flow and optimally ventilated.

However, components with high power loss and correspondingly high heat dissipation, such as frequency converters or power supply units, deserve special attention. The enclosure designer should specifically cool such heat-critical components in order to increase the service life (MTBF) or efficiency (no de-rating). Airblade' can be used to precisely direct the air flows in the enclosure: Part of the airflow is redirected directly to the components with high power loss and the air circulation inside the enclosure is optimized.

Is an air conditioning unit necessary?

Cooling and power loss in relation: The influence of air conditioning components on the maximum temperature in the free air volume is clearly recognizable.

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The aforementioned studies and simulations have shown that an active cooling unit is essential from an installed power loss of >600 W, regardless of whether the enclosure is positioned free-standing or as a wall-mounted unit. If the enclosure is air-conditioned using ambient air as a heat sink, as in the case of free cooling, the air temperature inside the enclosure is always higher than the ambient temperature.

This raises the question of which type of air conditioning unit and which output is to be used and where this unit is to be installed, whether laterally, frontally or as a roof-mounted unit. The question of whether an active cooling unit can be dispensed with by loosening the packing density and avoiding dead flow areas also requires more detailed investigation.

In many situations, it may be sufficient to simply support convection with a circulating air fan - for example with the 'Airblower'. This low-power blower is hooked into the upper part of the enclosure from the front without tools and locked in place. The heated air is sucked in and blown downwards at the rear, generating a continuous circulation flow. In addition, heat is constantly dissipated via the side walls. The fans can be switched on and clocked individually. With an air volume of 510 m³/h, the temperature layers are homogenized in just a few minutes and the temperature level can be reduced by up to 10 K. Three sensors placed in the control cabinet record the temperature, a parameterizable control unit takes over the signal processing and supplies the fan with power (24 V DC).

The company's own series of measurements have shown that using the Airblower is sufficient for cooling the enclosure in many cases. As only the air inside is moved in a controlled manner and no external air is drawn in, such enclosures can also be built completely sealed in accordance with protection class IP67 and can therefore also be used in hygienically critical applications - such as in the food or medical sectors.

Authors:
Peter Burger is Branch Manager at Lütze in Switzerland;
Michael Bautz is Product Manager Cabinet at Friedrich Lütze in Weinstadt.