[TRNSYS-users] Heat exchanger temperature control

[Mario Leroy Ortiz]

TRNSYS users,

I found what was wrong.  My cold side HX pump was working 
well while my solar loop temperature was close to its 
controllers set value.  However, during the shoulder 
season, when the heating load is well below the heat 
collected by the solar collectors, the solar loop 
temperature rises above the set point.  The solar array 
exit temperature varies widely during the shoulder season 
when heat collected is much less than heat used for 
heating/cooling load.  This is due to the solar loop pump 
running at maximum flow rate to try to reestablish the 
solar array exit temperature to the lower set temperature. 
 Thus, the solar array outlet temperature varys with the 
load and insolation.  I had allowed the heat exchanger set 
point to vary at 5 oC below the solar array exit 
temperature.  Thus, my controller for the cold side heat 
exchanger pump was confused by the ever changing set 
temperature.

The moral of the story is that a constantly changing set 
point may cause controller issues that are unbearable.

Thank you,

Mario Ortiz

On Wed, 14 Jan 2009 10:01:59 -0600
  David Bradley <bradley at tess-inc.com> wrote:
> 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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>BRADLEY                      22 N. Carroll Street - Suite 
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