[TRNSYS-users] Heat exchanger temperature control

[Mario Leroy Ortiz]

David,

Sadly, reducing the time step did not fix my problem. 
 Also, there is plenty of capacitance in the loop as it 
has a 30 m3 storage tank and a pipe.  I have tried 
changing the component order in the control card and in 
the loop itself.  I have added pipes between the tank and 
the pump, the pump and the HX and the HX and the tank.  I 
have tried making the controlled variable the outlet 
temperature out of the HX, the outlet out temperature of a 
pump and the outlet temperature of a pipe.  Nothing has 
given consistent results that I expect, which is for the 
cold side pump to mirror/shadow the hot side pump.

I will continue to try other things.

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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> Thermal Energy System Specialists (TESS), LLC David 
>BRADLEY                      22 N. Carroll Street - Suite 
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