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Smart ways to deliver enclosure climate control

Author : Karl Lycett is Rittal’s Product Manager for Climate

02 February 2018

All electrical components inside control enclosures generate heat and this heat needs to be efficiently dissipated from the enclosures to protect them against excessive temperatures. But an active climate control solution such as a cooling unit is not always needed.

Passive dissipation without active climate control components such as cooling units is also possible depending on the enclosure’s heat loss and size.

Depending on the heat loss and the size of the enclosure, passive heat dissipation may be sufficient.

The lifespan of electrical and electronic components depends to a large extent on how well the ambient temperature within an enclosure is managed. Typically, this should be below 50°C; dropping the temperature by 10°C will double their service life. The usual rule of thumb is to aim for an internal enclosure temperature of 35°C as the perfect compromise between a component’s service life and the degree of enclosure climate control.

Benefits of passive heat dissipation

In principle, there are two ways of dissipating the (lost) heat from an enclosure – a medium (air or cooling water) that conveys the heat away from the enclosure or convective heat transfer via the enclosure surface. 

The first option – active cooling – requires additional equipment such as fan-and-filter units, cooling units or air/water heat exchangers. 

Maximum enclosure temperatures at an ambient temperature of 25°C based on the effective enclosure surface area for various heat loads with purely passive dissipation

In the case of passive heat dissipation, heat is conveyed exclusively via the enclosure panels. The benefits of this are clear. The initial outlay is lower because no additional equipment is required, and users also subsequently save on both energy and maintenance costs. No additional openings in the enclosure panels means the system is better protected against dust and moisture. What’s more, a completely closed enclosure facilitates EMC protection and eliminates the risk of condensation that can be associated with active cooling. 

Uniform heat loss also means a constant enclosure temperature, so components are subject to lower stresses associated with changes in temperature than in the case of active climate control.

Larger surface areas improve heat dissipation

Passive dissipation does have its limits, however, because of the physical principle involved. 

The lower the ambient temperature, the more effectively this method works. The enclosure material’s heat transfer coefficient and the effective enclosure surface area are also key dissipation factors. DIN EN 0660-600-1 supplement 2 / IEC TR 60890890 shows how the latter is calculated. Based on a defined enclosure size, the effective enclosure surface area is at a maximum with a free-standing enclosure.  Meanwhile enclosure baying, wall mounting or covering the roof surfaces reduces this area. If the enclosure components’ heat loss and the ambient temperature are defined, it is easy to calculate the average temperature inside the enclosure. 

Provided example

However, even if the calculations show the temperature will be too high inside the enclosure, active cooling is not necessarily required. Using a slightly larger enclosure, for instance, may mean passive dissipation is adequate after all. A slight increase in surface area - particularly for small enclosures - can significantly reduce the maximum internal enclosure temperature. This should be taken into account when dimensioning a control enclosure with a low heat load.

Another option is to install any components with particularly high heat losses – such as braking resistors – outside the enclosure. Skilful planning of controls and switchgear is therefore a highly effective means of cutting dissipation costs. The size of enclosures, their installation and the positioning of components with the highest heat losses play a key role in this respect.

The enclosure material also has an influence on climate control. Spray-finished sheet steel or stainless steel enclosures with a heat transfer coefficient of approx. k=5.5 W/(m^2  x K) has been predominate in mechanical engineering applications, but this coefficient changes according to the design, for example with double-walled or insulated enclosures used in other sectors or in outdoor applications.

In the example provided, the internal enclosure temperature is 43.9°C and thus slightly above the recommended temperature range of 35 to 40°C. Active climate control is therefore necessary to ensure adequate heat dissipation in the enclosure. The disadvantages associated with active dissipation can be avoided by making the relevant enclosure slightly larger. Under identical installation and ambient conditions, the effective enclosure surface area for an enclosure measuring 1000mm·2000mm·600mm (B·H·T)  is 6.60m², which gives a maximum internal enclosure temperature of 38.8°C. The internal temperature is now within the recommended temperature range, thereby enabling cost-efficient purely passive dissipation via the enclosure surfaces.   

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