TechArchive

This article was originally written in the period 1995-2000

This guidance from Sarel is aimed at users of electrical switchboards, but it is equally applicable to the housing for any major item of electronic equipment.

Miniaturisation of components, the widespread use of electronics and the appearance of new power electronic base products make temperature management a constraint similar to IP, which must more and more frequently be taken into account in the design of electrical and electronic housings.The life of a component such as a switchboard inside a large enclosure depends upon the temperature and humidity conditions. The ideal temperature is between 10 and 45degC, with a relative humidity of 30-90%. There are various solutions. In some cases, it is sufficient to oversize the enclosure and use fans. In more extreme cases where the ambient temperature is very high, the refrigerating principle (cooling units) must be used by installing air/water exchangers or air coolers.

Natural ventilation

A supply of cool outside air through the ventilation louvers can improve heat dissipation by natural convection. However, this solution can only be used when the dissipated power is low and when the environment is not polluted by dust.

Circulating

Using a fan to circulate air within the enclosure can even out the temperature and prevent hot spots which could be harmful to some components.

Forced ventilation

Many fans are designed to remove large quantities of heat generated by components in electrical switchboards. The increases component life span significantly. They provide an efficient solution and are easy to implement and maintain. They can be used in industrial settings as well as in services

Cooling by air/air exchangers

Warm air within the enclosure and cool air from the surroundings are made to flow by two fans. They circulate on either side of sealed separation walls which stop dust and humidity from getting into the enclosure. Warm air from the switchboard heats these walls, which are cooled by the colder outside air. The transfer always takes place, of course(!), from the hotter side to the colder one, which is why these devices can only be used if the ambient temperature is lower by at least 5Cdeg than the required inside temperature.

The aluminium exchange cassette is the heart of the system. This element is easily removed and can be cleaned very easily. The continuous running of an internal fan prevents hot points in the switchboard. The temperature control system of the device turns the external circuit fan on or off.

Air/water exchangers

The working principle here is the same as in air/air exchangers: the cool air is replaced by cold water supplied by a network installed by the industrial facility. Due to this change of fluid, much larger quantities of heat can be removed and temperature within the enclosure can also be decreased below the level of the ambient air. The temperature within the enclosure is controlled by modulating the air flow. The water circuit is protected by a supply shut-off device.

Cooling units

Cooling units can be used in the most severe environments, where temperatures may be as high as 55degC. These units are particularly suitable when the required ambient temperature in the enclosure is below the ambient temperature or when large quantities of heat are to be removed. As in the case of exchangers, they do not affect the switchboard IP.

Thanks to the filter located at the inlet of the outside air circuit, they can work even when the ambient air is loaded with dust or oil particles. The filter is easy to replace and inexpensive. Units have an enclosure temperature control system and an alarm function which is set off in the case of improper working.

The thermal balance, whereby the power dissipated by the equipment is compared to the power which is spontaneously exchanged by the walls of the enclosure, is used to calculate the temperature reached within the enclosure when there is no temperature control device. With a knowledge of the dimensions and thermal characteristics of the enclosure, the power dissipation by the operating components and the likely characteristics of the ambient air, it is possible to determine the maximum and minimum internal temperatures likely to be reached without a temperature control system. This is fully explained in the Sarel brochure Thermal Control for Electrical Switchboards.

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