Modelling a Dehumidifier with Simscape Fluids System Level Heat Exchanger(TL-MA) not possible?

Hey guys,

I'm trying to model a dehumidifier. For this purpose i've got 2 heat flow rate sources (evaporator/condenser) with given power from a datasheet. Those are connected to 2 constant volume chambers(TL) for hot and cold and then each connect to a system level heat exchanger (TL-MA). The one with cold TL should dehumidify meanwhile the other should heat up the air, just like with a real dehumidfier.

I'm getting the following error:

Thermal Liquid 1 inlet temperature at nominal operating condition must be greater than Moist Air 2 inlet temperature at nominal operating condition.

I essential understand this as, it's not possible to cool down moist air to dehumidify with an system level heat exchanger(TL-MA), since the fluid entering the heat exchanger should be colder than the moist air in order to condensate water, am i wrong?

I'm not super sofisticated with thermodynamics and simscape, therefore it could probably be that i'm simply making a simple mistake. But sofar i can't seem to find it. I've tried to choose parameters for the blocks which correspond to datasheets of units.

I'm using 2025a if that helps.

I'm really thankful for all the help i can get.

Answer 1

[Yifeng Tang]

The error is complaining that the parameters you've set don't make sense :p

For example, you've set the operation condition to be "Heat transfer from Thermal Liquid 1 to Moist Air 2", but your nominal condition has thermal liquid at 20 degC and Moist Air at 28 degC. Sounds like violation of 2nd law of thermodynamics? Check you nominal conditions more carefully. Typically, you would use a steady-state case to set the nominal.

And don't rush to close the loop on both side before the heat exchanger is correctly parametrized. Build a harness model similar to the one in this example: https://www.mathworks.com/help/hydro/ug/parameterize-a-heat-exchanger-using-parameter-estimation.html Set up the boundary conditions and nominal flow, make sure you get the right amount of heat transfer and, if any, condensation.

Once you've done that, close the loop one at a time.

Starting with the MA loop: the moist source is reasonable, but watch for condensation in the volume. The heat flow into the volume is OK, if you know that's the right value. I found it safer to use a temperature source (say 30 degC) and connect to the H port of the chamber via a thermal resistance with a tiny value. This way, the chamber is always almost at the temperature you prescribed. You can switch back to heat flow later.

The TL loop usually needs a tank or accumulator to stablize the overall pressure. Check out some TL examples in Simscape Fluids and look for such component. Without those, you'll likely get an error of unreasonable (too high or too low) a pressure or temperature.

[Moritz]

Thanks for the answer! Changing the heat transfer from TL->MA to MA->TL solved my problem.

It's embarassing, but i'm glad it works now :) You really helped me there, i already checked all the parameters but somehow didn't see this one.

And the idea of building a harness was good too, but my model with closed loop already works with the changed parameter. I'm using a heat flow source to understand how much heat power is necessary... but i guess for a fast approach it's easier with a temperature source.

Cheers for the help

[Moritz]

Hello, i've stumbled upon another issue...

Coud you please take a look?

https://de.mathworks.com/matlabcentral/answers/2177220-control-of-dehumidifier-with-system-level-heat-exchanger?s_tid=prof_contriblnk

Would mean a lot to me. Don't know how i should proceed.

Thanks for the help :)