The theory behind dehumidification

The basic functional principles of dehumidification and dehumidifiers are fairly straightforward. The psychrometric calculations involved in the dehumidification process, however, are quite complex. Several interrelating parameters need to be taken into consideration.

The Mollier hx-diagram is a graphical representation of the interrelation of the temperature and the relative humidity of the air. This diagram is key to determining the various parameters required to calculate the dehumidification load required for any situation.

This is an introduction aiming to help you understand how this basic tool works. Chapter 4 of our "Selection guide for mobile dehumidifiers" contains a number of examples on how to calculate specific dehumidification loads referring to the Mollier hx-diagram and using the terms and quantities found in the diagram.

Download our “Selection guide for mobile dehumidifiers” for professional guidance on how mobile dehumidifiers work and how you choose the right one for your project.

Using the Mollier diagram

At first glance, the Mollier diagram may seem rather confusing with all of its curved, diagonal and slanting lines, but it is actually a quite easy and useful tool once you get the hang of it. Identify the easily measured temperature and the relative humidity of the air inside the room, and you are all set.

Let us start with a simple example:

We want to calculate the enthalpy or heat energy needed to raise the temperature in a given room with a relative humidity of 60% RH from 20°C to 30°C.

Start off by finding the 20°C point on the pink axis to the left. Now follow the slightly upward slanting horizontal gridline to the point where it crosses the 60% RH green curved line. If you follow the purple diagonal line to the point where it crosses the green 100% RH line, you will see that h=42 kJ/kg.

Now go back to the point indicating 20°C/60% RH. Raise the temperature vertically until you cross the 30°C gridline. You will notice that the relative humidity drops to about 35% in the process. But as we are interested in the enthalpy needed to raise the temperature to this point, you should again follow the purple diagonal line to the point where it crosses the green 100% RH line. Now you should get h=52 kJ/kg.

The rest is easy: h= (52-42) = 10 kJ/kg air heat energy must be added to the air in the room in order to raise the temperature from 20°C to 30°C.

 

Download our “Selection guide for mobile dehumidifiers” for professional guidance on how mobile dehumidifiers work and how you choose the right one for your project.

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