Special conditions call for special solutions
ELSTA Mosdorfer is one of the pioneers in the field of preventing or minimising condensate in distribution cabinets that stand outdoors to ensure the maximum possible operational reliability and functionality, even under persistently negative outside influences. Find out more!
Condensation is a consequence of surfaces or condensation nuclei dropping below the dew point. This occurs with high humidity and low temperature. The condensate appears as dew on surfaces or as a mist or fog on condensation nuclei. This phenomenon is known, for example, from kitchens (vapour), or from cars when dewdrops precipitate on the windshield in the morning hours at sunrise. Condensation outdoors is a common weather phenomenon.
Distribution cabinets outdoors also form a “microclimate” on the inside. This microclimate is essentially determined by the environmental conditions such as humidity and outside temperature. In freestanding cabinets (or pillars) on a ground-mounted base there is also rising soil moisture. Other influencing factors are additional heating caused by sunlight or thermal radiation from nearby hot surfaces (e.g. asphalt), cooling by the wind or precipitation, etc. Unsealed cable ducts with a direct connection in a damp cellar or direct water ingress during heavy rain can also increase the humidity on the inside of a cabinet and thereby facilitate the formation of condensate.
The humidity is very high on humid days or nights. Since the cabinets described here are not gastight (up to and including IP54), the humidity in the cabinet is generally similar to the humidity of the environment. Thus in humid weather conditions, the humidity in the cabinet is also high. So if there is sudden cooling, for example from a thunderstorm with cool rain, the onset of night, or a load change with decreasing waste heat from power dissipation, the surface temperature of the cabinet drops on the outside and inside. Even a few degrees of temperature difference will result in condensation (droplet formation) on the inner cabinet surface if there is high internal humidity.
The probability of the occurrence of dew is the greatest when the air above the soil is saturated with moisture and is warmer than the ambient temperature. This is particularly the case in the early morning hours. The air has cooled down significantly during the night and is cooler than the moist air rising from the soil. The dew “drops”, i.e. droplets settle on blades of grass etc.
…CONDENSATION INSIDE THE DISTRIBUTION CABINET
Exactly the same thing happens in a cabinet that sits directly above the soil. Here as well, the inside cabinet temperature drops at night (with a slight delay compared to the outside temperature and possibly to a lesser extent if there is waste heat through power dissipation).
In this state, the cabinet inner surface is coolest and has approximately the temperature of the ambient air. The soil in the cabinet is exposed; warm moist air can rise freely and precipitates out as dew on the inner surfaces.
Why does warm air rise? Because humid air is lighter than dry air, and because warm air is less dense than cold air.
Unlike outdoors, neither wind nor sun can provide rapid drying. The humid air cannot escape. Only the warming up of the surroundings or additional heating of the cabinet surface from sunlight leads to evaporation of the dew drops.
Whether for fittings, electrical components, copper rails etc., fogging up depends on whether these components are heated by electrical losses. If they are not, they assume the temperature of the air inside the cabinet. On very cool nights, this might result in condensation on installed components and also live parts.
Due to condensation, enough water can accumulate on the cabinet surfaces that the dewdrops suspended on the top of the cabinet detach and drip down. The dew drops can run down the side walls. Whether these processes cause problems depends on the application and the fittings in the cabinet.
When there are free-standing cabinets on a ground-mounted base, the rising ground moisture can lead to high humidity on the inside of the cabinet. As a result, in unfavourable conditions this can result in condensation on the inside of the cabinet’s outer surfaces .
Base fillers are used to reduce this rising of the soil moisture. A widely-used filling material is expanded clay.
However ELSTA Mosdorfer recommends a hydrophobic filler, which is additionally covered with a cold foam mat, also called a foundation cushion. Handling is easy due to the low weight and comparable protective effect at a lower filling level (about 1/3 less filling volume compared to expanded clay).
How base fillers work
It is true of both base fillers that they separate the interior of the cabinet from the (moist) soil in such a way that moisture running off the cabinet is not impeded. In this way, the cabinet can dry out again after water gets in (e.g. from heavy rain or from condensation on the cabinet walls due to sudden cooling after thunderstorms). No damp/condensing cabinet climate occurs, such as can arise, for example, with an IP54 base plate without any additional measures.
Expanded clay base filler
This is a granulated bulk clay made of fired clay. Through the addition of organic material that decomposes during the firing process, the clay expands and traps hollow spaces. This leads to a moderately good insulation value. Filling the base with expanded clay creates a barrier that inhibits the rising of air enriched with soil moisture that is close to the ground. The moisture condenses on the surface of the expanded clay balls. Expanded clay can absorb water to the extent of up to 20% of its own mass and therefore acts as a buffer that reduces moisture peaks (the water is absorbed and released again at a later time). The recommended fill level in the base is 30 cm.
Hydrophobic base filler
This involves a bed of foam glass. Foam glass has very good insulation properties (λ = 0.04 W/mK, which is three to four times better than expanded clay). A relatively fine grain size is used as a base filler. The moist air which has formed over the soil condenses on the foam glass balls due to the temperature gradient between the soil and the internal cabinet temperature. Therefore the foam glass balls act similarly to a distillation column with a corresponding number of packings. The condensate formed in this way flows back into the soil. Since the foam glass itself does not absorb water (hence the term “hydrophobic”), the water cannot be released as vapour at a later time.
We were able to achieve the best results in our tests with fine-grain foam glass. The recommended fill level in the base is 20 cm.
The foundation cushion (also called absorption mat) consists of flexible polyurethane foam with a thickness of 100 mm, which is precisely adapted to the base dimensions. The mat is intended to separate the cabinet from the soil so that even if wetness is present (damp soil, base filler wet or damp), parasitic voltage does not occur in the event of a fault. Therefore the absorption mat is tested for electrical breakdown in a wet state (5 kV/9mm). In addition, tests have shown that the absorption mat also causes a further improvement in the moisture barrier created by the base filler.
When the temperature drops, or during sudden lowering of the temperature, the humid air is already inside the cabinet. In this case, condensation on the inside surfaces of the cabinet cannot be prevented or limited by the base filler. In order to keep the impact of this phenomenon to a minimum in critical applications (e.g. electronic components), Elsta Mosdorfer recommends either a second inner roof (roof panel) or the use of an anti-condensation fleece. This fleece prevents the formation of dewdrops due to its surface texture.
Binding and evaporating condensate
For liquids to form drops on surfaces, the surface tension of the liquid, the cohesive forces of the liquid and the adhesive forces between the liquid and the surface must be in a specific proportion to each other. Water cannot form any drops on the anti-condensation fleece up to saturation, but rather it is absorbed by the capillaries of the fleece. The water bound in this way evaporates gradually, but does not drip down as a liquid. Sensitive components are effectively protected against dripping water in this way.
Positively tested functionality
To test the anti-condensation fleece, observations were carried out on cabinets equipped with the fleece under condensing conditions. Condensation droplets were visible on the plastic surfaces and particularly evident on the fastening elements of the roof. It was only on the anti-condensation fleece that no drops formed.
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