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Dehumidification– Selection Guide

Part 2/4 – Using the Mollier hx-diagram

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What is the Mollier hx-diagram?

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The Mollier hx-diagram is the keyto determing the variousparameters required to calculatethe dehumidification load requiredunder any given circumstance.The Mollier hx-diagram is a simple and easy tool when you know howto use it. The only data you needto know are the temperature and relative humidity.

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How to use the Mollier diagram?

Start off by finding the 20°C point on the pink axis to the left.

Now follow the slightly upwardslanting 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 thath=42kJ/kg.

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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.

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Continued..

Now go back to the point indicating20°C/60% RH. Raise the temperature vertically untill 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 enthalpyneeded to raise the temperature to thispoint you should again follow the purplediagonal line to the point where it crosses the green 100% RH line. Now youshould get h=52 kJ/kg.

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

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Continued…

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In this example we establish thaton a hot dry summer’s day in Denmark a drop from 20°C daytime temperature to 0°C nighttime temperature inside a 80 m3 room would result in almost half a litre of water beingcondensed out of the air, and thiswater would form on couldsurfaces.

The condensation starts as soonas the temperature reaches the dew point.

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Continued…

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To establish the dew point at 20°C and 60% RH find the 20°C point on the pink axis. Follow the gridline to the 60% RH point. Now go down the vertical gridlineuntil it meets the green 100% lineFrom this point follow the horizontal gridline to the left to read a dew point temperature of 12°C Between this temperature and 0°C the water content in the air willcondense into water inside the room.

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Continued…

Follow the vertical line from 20°C and 60% RF point all the waydown to the horizontal light blueaxis at the bottom to read x=8,5 g water/kg air water content in the air.Do the same reading down from the 0°C og 100% RF, which shouldread x=3,5 g water/kg air.From these readings you caneasily calculate that 5 g water/kg air (8,5-3,5) has condensed and formed into condensation insidethe room. In an 80 m3 room thisequals 0,48 litre.

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To calculate your dehumidification need using the Mollier hx-diagram click here.

For further information we refer to our Selection Guide. You canfind it here.

You are welcome to contact our sales department for dehumidification on tel. +45 96143700 in any cases.

Further information

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