Moisture in enclosures – how to prevent condensation and short-circuits

What is condensation and how does it come about?

Condensation is a widespread everyday phenomenon. When you breathe on a cold window and it mists up, or when water droplets form on a cold drinks can, that’s down to the process of condensation. The droplets themselves are also often referred to as condensation, but also as condensate. The condensation process is triggered when air reaches its dew point – a specific temperature below which the air can no longer hold the water vapour it contains. The relative humidity of the air at this point is 100 percent, and excess water vapour condenses on surfaces in the form of tiny droplets. Since cold air can generally hold less water than warm air, condensation occurs whenever the temperature drops far enough.

Dew point – a dynamic value

One important point to bear in mind is that the dew point is a dynamic value. The exact dew point temperature at a certain time and in a specific location always depends on the air humidity, air temperature and air pressure. The relevant calculations are based on the “Magnus formula”.

The above table shows how air humidity affects the formation of condensate. At 80 percent humidity, even slight cooling is sufficient for condensate to form.

Condensation in enclosures

Condensate most commonly forms in enclosures when warm internal air comes into contact with cooler enclosure surfaces. Electrical components in particular cause temperatures inside an enclosure to rise quickly. If the colder enclosure housing cools this warmer air to its dew point, condensate forms on the housing’s internal surfaces.

This problem becomes particularly acute when the environmental conditions also increase the likelihood of high humidity in the enclosure. The higher the humidity, the faster the dew point is reached when the temperature drops. Air with a high moisture content can get into enclosures if their doors are open during maintenance work or their seals are not up to the task.

Depending on the application, there are essentially two recommended approaches when it comes to preventing condensate from forming in enclosures.

Cooling and condensate removal

In many cases, enclosures must be actively cooled. This is necessary to protect the installed electronic or electromechanical components from overheating. In the case of high thermal loads and/or sealed enclosures, cooling units such as the ones in the Blue e+ series from Rittal are therefore mostly used.

Heating and the prevention of condensation

Another way of preventing condensation from occurring in enclosures is to use an enclosure heater. This approach is a good option, for example, if the thermal load resulting from the installed equipment isn’t all that high for the majority of the operating period and the enclosure is located in areas with low external temperatures. The heater then consistently keeps the internal temperate at a sufficiently high level to ensure it doesn’t drop below the dew point.

Automated heating with a hygrostat

Needs-based regulation ensures an enclosure heater benefits from particularly energy-efficient operation. This is possible using a hygrostat, for example, which regularly measures the humidity of the air in the enclosure and activates the heater as soon as a preset threshold value is exceeded. In this case, the heater only comes on when the temperature is close to the dew point and there is an acute risk of condensate forming.

A question of location

An enclosure’s location is another factor that determines the most appropriate way to prevent condensation in a particular application. For example, an enclosure heater is preferable when external temperatures are low or fluctuate a lot, whereas a cooling unit with a condensate evaporator is preferable when external temperatures are high.

Sealing enclosure housings

In locations with high air humidity, care should be taken to ensure enclosure housings are highly airtight. Even the tiniest gaps in a seal – around doors, cable glands or lock mechanisms, for instance – can allow humid air into the enclosure. Seals should therefore be checked regularly as part of any routine maintenance. When selecting an enclosure, it is also advisable to ensure a high protection category of at least IP 55. Enclosures with this kind of protection category don’t just offer protection from dust – they also prevent moisture ingress.

Preventing pressure differentials

Even with maximum airtightness, the ingress of humid air can never be completely ruled out. In locations where external temperatures fluctuate a great deal, for example, pressure differentials can occur between the internal and external air. If this creates a vacuum inside the enclosure, even tiny openings allow humid external air to be drawn in.

However, there is a solution to this problem, too. Stainless steel or plastic pressure relief stoppers are available for certain Rittal enclosure models (including for Hygienic Design applications). The integral sealing membrane allows the air to circulate but keeps moisture out. This prevents pressure differentials and moisture ingress. When combined with a heating element, it even ensures warmer, humid air is expelled.