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Modelica Buildings MixingVolumeMoistAir

asked 2017-03-12 12:58:33 -0500

raymondsg gravatar image

Hi,

I am trying to develop a cooling and dehumidifying coil and would like to use Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir.

I wonder why this model uses enthalpyOfWater for calculating QLat_flow instead of enthalpyOfVapotization which I've found to be the standard for calculating latent heat flow? I am surely missing something theoretical here.

The issue is that since I am calculating mWatFlow from Sensible and Latent heat calculations, the Q_flow calculated by the MixingVolumeMoistAir is different from the ones I calculate internally giving me big errors in enthalpy of outflowing air.

Many thanks in advance for your help.

Bests regards,

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answered 2017-03-13 17:38:18 -0500

The reason for using enthalpy of water is as follows: The energy balance has two different components that cross the thermodynamic boundary of the volume: The incoming and leaving moist air, and the water.

For the moist air, water is assumed to be in vapor form. However, for the water, mWat_flow is the water mass flow rates that is drained from the components (in your case, drained from the dehumidifying coil). This drainage flow is in the form of liquid water. Hence, the leaving enthalpy flow needs to use the enthalpy of liquid water.

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Hi Michael,

Many thanks for your reply. I was intending to use mWat_flow to denote water, in vapor form, removed from the air. Hence the issue (the info does says it has to be liquid form and I overlooked it, my fault).

I'll have a think.

Bests regards, Raymond

raymondsg gravatar image raymondsg  ( 2017-03-15 07:14:57 -0500 )edit

Raymond,

If you need to add (or remove) water vapor, you could use the same mWat_flow (in kg/s), and add the difference in enthalpy flow rate (in Watts) between the liquid and vapor form to the heat port of the volume. This will then yield the correct mass and energy balance for your situation.

Best,

Michael

Michael Wetter gravatar image Michael Wetter  ( 2017-03-15 08:40:26 -0500 )edit

Excellent! I was taking a similar approach but this is definitely cleaner.

Many thanks. Raymond

raymondsg gravatar image raymondsg  ( 2017-03-15 09:03:05 -0500 )edit

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Asked: 2017-03-12 12:58:33 -0500

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Last updated: Mar 13 '17