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

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@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

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