[TRNSYS-users] Strange error when using the equation proforma

[David BRADLEY]

Chun Kwong,
  You are correct in your understanding of how TRNSYS iterates to solve. 
However, it has to have somewhere to start so TRNSYS performs a check on 
all the equations before starting the simulation to make sure that it 
can solve them. If you wrote a set of equations that said:

EQUATIONS 2
A = B
B = A

there would be no way that TRNSYS could ever solve because it would have 
nowhere to begin the iteration.  My guess is that your T_CHWS and T_CHWR 
equations are too interdependent and therefore can't be resolved.
Best,
  david


On 12/13/2010 20:38, a8304506 at graduate.hku.hk wrote:
> Hello, everyone.
>
> I am trying to simulate a chiller plant with primary and secondary chilled water
> pump systems.  I use to equation proforma to calculate the chilled water flow
> and temperature at the supply and return header.  However, I get the following
> error messages:
>
> *** Fatal Error at time   :         0.000000
>      Generated by Unit     : Not applicable or not available
>      Generated by Type     : Not applicable or not available
>      TRNSYS Message    420 : The following variable defined in an EQUATIONs or
> CONSTANTs card could not be solved as this variable depends on either an
> undefined EQUATION or CONSTANT name or depends on another EQUATION or CONSTANT
> that cannot be solved. For example A=B and B=A would trigger this error
>      Reported information  : T_CHWS
>
>
> *** Fatal Error at time   :         0.000000
>      Generated by Unit     : Not applicable or not available
>      Generated by Type     : Not applicable or not available
>      TRNSYS Message    420 : The following variable defined in an EQUATIONs or
> CONSTANTs card could not be solved as this variable depends on either an
> undefined EQUATION or CONSTANT name or depends on another EQUATION or CONSTANT
> that cannot be solved. For example A=B and B=A would trigger this error
>      Reported information  : T_CHWR
>
> I am a little bit confused.  My understanding is that Trnsys performs the
> calculation in the equation proforma based on the input and the sequence of the
>
> equations as typed in the proforma, and the system performance at each time step
> is determined component by component which follow the order as indicated in the
> input file.  Hence, should there be any improper formulation, the consequence
> should generally be non-convergence.  Is my interpretation wrong, and how can I
> solve the problem?  Please advise.  Thank you!
>
> Best Regards
> LEE, Chun Kwong
> City University of Hong Knog
>
>
>
>
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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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