[TRNSYS-users] Active Layer (Slab) Flow Requirement Conflicts with Type 1669 Control Scheme

[Jean Marais]

I can't aggree more. If your flowrate falls below the turbulant thresholds,
then you may as well switch the equipment off completely. Usually the
manufacturer's information will contain schemes on how to divide up and
connenct up the active areas so hit the right balance between retaining
minimum pressure loss whilst maintaining turbulant flow. Also make sure
that your poor pump is not opperating at an inefficient turndown point for
90% of the time.

Lastly, and very importantly, if you are using the active surface to cool,
keep a very tight eye on the surface temperatures. If they fall closely
within the range of the air dewpoint, you may get condensation problems.
Real systems ramp the fluid temperature in combination with sensors to
avoid this, but for you it is another very good reason to split the active
surface out exactly in the model otherwise this effect won't be captured.
You will find also that a lot less surface is active (and often at less
extreme temperatures) thereby reducing the ability to transfer heat with
the zone air. This is often forgotten in enery models.

Kind regards,

Jean Marais

2015-10-05 18:32 GMT+02:00 David BRADLEY <d.bradley at tess-inc.com>:

> Brad,
>   There is an example of using a PID to control flow through a radiant
> floor (SunSpace - Floor heating PID controller.tpf). You'll probably need
> to put a lower limit on the flow rate that your controls decide should go
> through the radiant floor. However, far from that being a workaound, it is
> a physical reality that pumps have minimum turndown ratios and the flow
> through the pipes in the floor must stay turbulent in order for there to be
> heat transfer. The requirement of maintaining turbulent flow is what causes
> the minimum flow rate in the first place. Lastly if you need to use a lower
> flow rate then you need to go into TRNBuild and autosegment the floor. What
> this does is to break the floor into smaller pieces that are in series with
> one another instead of being in parallel from a liquid flow point of view.
> regards,
>  David
>
>
> On 10/04/2015 21:52, Bradley Painting wrote:
>
> I received a suggestion to use a Type 1669 controller for a radiant slab,
> and it seems to work well (thanks Ben Heymer). However, right now I am
> using it to modulate temperature, but what I would really like to do is
> modulate the flow rate through the slab because I have a variable
> (uncontrolled) input temperature. The problem is that TRNSYS will not allow
> me to decrease the flow rate much below 2,000 kg/hr based on the size of my
> radiant floor.
>
> (Error Message: "Specific fluid mass flow of active layer in surface 9
> below minimum value of 6.74 kg/m2h Please, increase the minimum mass flow
> rate or use the autosegmentation feature of TRNBuild for subdividing the
> surface").
>
> The proportional controller will inevitably fall below this rate because
> it will ramp down to zero when no heating (or cooling) is needed. Is there
> a workaround for this? I looked into the autosegmentation feature and it
> doesn't seem to subdivide the surface at all, and I'm not sure how this
> would help. Thanks again.
>
> --
>
> Brad Painting
>
> Master of Science Candidate, Renewable Energy Engineering
>
> Department of Technology & Environmental Design
>
> Appalachian State University
>
> Boone, NC  28608
>
>
> _______________________________________________
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>
>
> --
> ***************************
> David BRADLEY
> Principal
> Thermal Energy Systems Specialists, LLC
> 22 North Carroll Street - suite 370
> Madison, WI  53703 USA
>
> P:+1.608.274.2577
> F:+1.608.278.1475d.bradley at tess-inc.com
> http://www.tess-inc.comhttp://www.trnsys.com
>
>
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