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Dehumidification – Selection GuidePart 3/4 – Calculation of the dehumidification load

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Dehumidification falls into 2 main categories. One category relates to problems mainly concerning excess water content in the air. The second category concerns drying out the water content in different kinds of materials.

It is important to distinguish between these 2 categories when determining the model type of mobile dehumidifier that needs to be used.

The 2 categories of dehumidification problems

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Excess water content in the air

The humidity of the air affects both people, electronic equipment, machinery and various materials in the room.The table is showing a list of limit RH-values that indicates when various negative effects of excess water content in the air set in.

Please note that the values listed are merely indicative as there are situations in which even lower RH-values might cause problems.

For instance you should keep the RH-value below 40% when dealing with large cold surfaces.

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In all cases concerning continuously high levels of relative humidty it is advisable to look into the actual reasons for the problem – not just cure the effects.When you are determining the dehumidification load, you need to consider several parameters before calculating the required dehumidification load and choosing the right dehumidifier for the job.

Different parameters will be explained on the following slides.

Assessment of the need to dehumidify

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First you have to get hold of some general meterological information for your geographic area. Temperature and RH-values change from region to region and they also vary a lot during the year.Statistics are available for most geographical areas and can be obtained locally. (See Table 4 for an example of how much outside conditions fluctuate during a year in Denmark).To make sure that you always have sufficient load you should normally enter the temperature and RH-values as worst case scenario for water content (July level for Denmark).

Meteorological data

Notice that even with a high RH-value in a cold winter month the water content of the air is relatively low, whereas the hot summer months normally constitutes worst case scenarios with a relatively low RH-value and high water content due to the fact that hot air holds more water.

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The size of the room or the building has an indirect influence as it is the amount of water in the air that determines the actual dehumidification load required.

However, you should calculate the volume of the room in cubic metres to see how much air it will hold.

Size of the room

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Category 1 Air change

The air change, n, is very important as outside air contributes to both the temperature and RH-values inside the room.Reasearch has shown that in most cases problems concerning excess water content in the air are caused by air change problems.You must determine or estimate how many times pr. hour the air of the room is changed. The air change is indicated by, n.This ventilation might occur naturally because the room is not completely tight or it might be forced due to mechanical ventilation and by doors or windows being opened from time to time.

The additional water content introduced into the room by the air change measured in kg water/hour is calculated by using this formula:

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W = g water/hourρ = air density (kg/m3) = the value commonly used is approximately 1,2 kg/m3 at 15-25°CV = room volume (m3)n = air change in the room (time -1)x1= water content in the outside air (worst case) (g water/kg air)x2= water content in the inside air at the required RH-value (g water/kg air)

Explanation of the labels used in the formula by air change

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Finally you have to consider the humidity coming from people, processes, products and other sources.

Other sources

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For further information about your dehumidification need you are welcome to take a look at our Selection Guide, click here.If you would like to know more about what we have to offer click here.

Further information

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