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

[David BRADLEY]

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:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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