[TRNSYS-users] Heat exchanger temperature control

[David Bradley]

Mario,
 I have not gotten a chance to look at the attached files but chances 
are that there are some convergence problems associated with the 
iterative feedback controller. As I mentioned in an email yesterday, the 
simplifying assumptions that often get made in building models of 
hydronic loop components (pumps, heat exchangers, etc.) neglect the 
thermal capacitance of the fluid in the devices, which in turn leads to 
very fast reactions to control decisions. In some cases, putting time 
delays on controllers helps solve the convergence problems but often, it 
just masks an underlying issue. My recommendation would be to make sure 
that you have at least one timestep of storage in your loop (in other 
words, model the pipes with some realistic thermal losses), reduce the 
time step and tighten the tolerances to at least 0.0001 (the default is 
0.001 I think).

 Alternatively, there is a model of a heat exchanger that controls the 
outlet temperature using an internal bypass available in the TESS 
Hydronics Library.
Kind regards,
 David


Mario Leroy Ortiz wrote:
> TESS and TRNSYS users,
>
> Please help me with the attached files...
>
> I am having some problems controlling the outlet temperature of the 
> cold side of a type 5b counter flow heat exchanger.
>
> I am modeling a plate and frame heat exchanger where the heat source 
> is a solar thermal array and the cold side charges a storage tank.  
> Heat is used for a building load.
>
> My example project file is simple.
>
> Water at a constant temperature (65C) is supplied to a heat exchanger 
> with a varying flow rate (low in morning and afternoon, high at 
> midday) to mimic what might be supplied by a solar array.
>
> I am trying to maintain the heat exchanger cold side temperature (tank 
> heat supply) at 60C using a variable speed pump and an iterative 
> feedback controller.
>
> Heat is drawn from the tank to the building load.
>
> There are 7 days in the simulation.  Day 4 and day 7 seem to work right.
>
> On the other days, during the last hour of time when solar thermally 
> heated water is supplied to the heat exchanger, the cold heat 
> exchanger outlet temperature seems to drop prematurely due to the cold 
> side pump speeding up.  Under certain unknown circumstances (days 4 
> and 7), the cold side pump flow mirrors the hot side pump flow and the 
> cold side temperature is maintained for most of the time that solar 
> thermal heat is available.  For unknown (to me) reasons, at other 
> times the cold side pump flow is sporadic (speeds up rapidly, 
> decreasing the cold side outlet temperature), especially late in the 
> afternoon.
>
> Attached are the project file and input file for my tank.
>
> This is an example of the problems I am seeing in a larger project 
> file.  This project does not show it, but what I see in the larger 
> project is the cold side pump turning off on me when the hot pump is 
> running at full capacity, causing my solar loop to get very hot.
>
> Any assistance with this problem is appreciated.
>
> Thank you,
>
> Mario Ortiz
>  
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>
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Thermal Energy System Specialists (TESS), LLC 
David BRADLEY                      22 N. Carroll Street - Suite 370 
Partner                            Madison, WI 53703 
                                   USA
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F: +1.608.278.1475
E-mail: bradley at tess-inc.com 
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