[TRNSYS-users] (no subject)

[Javier Vega via TRNSYS-users]

Jeff,

First of all, thanks for your response.

I was analysing different parameters of the model. I realised the
coefficients for the Nusselt calculation on the heat exchanger of the
storage tank are critical.

Nu=C*(Rayleigh)^n

A while ago, I set them as C=0.4 and n=0.25. I did this based on some data
I had from my Heat Transfer class in University.

I know the coefficients are by default as: C= 0.6 and n=0.6 . If I set the
model with these parameters the problem is solved and the heat pump can
operate below 50 (°C)

Nonetheless, now I was starting a little research on the subject, I notice
there is plenty of different results in the literature to define C and n
for horizontal tubes in natural convection. Now I'm not sure if the
cofficients I used are the correct ones, I'm continuing my research on it
to properly understand it.

Are the Default values proven to be the best suited?

Kind regards,
Javier


PS: I will attach the Screen Capture you asked. Please, tell me if you need
more details or info.



2017-05-08 12:09 GMT-03:00 Jeff Thornton <thornton at tess-inc.com>:

> Javier,
>
> Please post a screen capture of your system so we can see the flow
> connections.  I have an idea as to what may be wrong.
>
> Jeff
>
> Sent from my iPhone
>
> On May 6, 2017, at 5:37 PM, Javier Vega via TRNSYS-users <
> trnsys-users at lists.onebuilding.org> wrote:
>
> Hi TRNSYS users,
>
> I'm experiencing a weird issue. I'm testing a simple system where I have a
> heat pump (Type 941) and solar collectors (Type 538) coupled  to a Vertical
> Storage Tank *via heat exchangers*, modelled with Type 534. The tank is
> connected to a closed loop to a radiant floor.
>
> I have the control system setted this way:
>
> -The collector's circuit works if they can gain energy. ON/OFF strategy
> -The Radiant Slab Loop operates if the thermostat of the thermal zone
> requires it (ON/OFF strategy)
> -The heat pump works if the required temperature for the radiant slab loop
> isn't satisfied by the collector's energy gain. The temperature set for
> heating is *35 (°C) in the top of the storage tank. *ON/OFF strategy
>
> For some reason, when the heat pump is active its operating temperatures
> are *above 60 (°C) *at the heat pump's inlet,* even if the tank is at 35
> (°C)*. I've checked and the heat exchanger seems to transfer all the heat
> pump's energy to the storage tank. I don't know what is causing the
> temperature raise.
>
> Is this what is supposed to happen? Is it a weird simulation unrealistic
> error?
>
> My heat pump has a nominal capacityof *9 kW. *The heat load transfered to
> the radiant slab is above 7 kW max.
>
> My heat pump's performance map has operating data just to 50 (°C) inlet
> water temperature. If I force the heat pump to work below this, the heat
> pump operates less often and the system can't supply enough energy to keep
> the radiant floor water at 35 (°C).
>
> Here are two pics of the situation. First is the heat pump operating
> "naturally" at high temperature. Water inlet in the heat pump is in pink.
> Water temperature to the radiant slab is in blue. Heat output in the
> radiant slab is in green . Red is for the airnode temperature.
>
> <image.png>
>
> <image.png>
>
> I would appreciate some help, thanks in advance.
>
> King regards,
> Javier Vega
>
>
> _______________________________________________
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> TRNSYS-users at lists.onebuilding.org
> http://lists.onebuilding.org/listinfo.cgi/trnsys-users-onebuilding.org
>
>
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