Selection Guide part 3/4 - Calculation of the dehumidification load

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Dehumidification – Selection Guide
Part 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
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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
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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/hour
ρ = air density (kg/m3) = the value commonly used is approximately 1,2
kg/m3 at 15-25°C
V = 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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