[TRNSYS-users] TRNSYS-users Digest, Vol 49, Issue 40
ghetu mariana
ghetu_mariana at yahoo.co.uk
Mon Jan 26 05:05:09 PST 2009
Trnsys users,
I have
problems with type56. I’m simulating a heating installation with different time-base
and time-step. For a time-base of 20 min, 40 min, 50 min, 100 min (time-step:
2min, 5min, 10min,etc.), I get an error message saying that the temperature for
zone 3 surface 32 does not converge. I looked up zone 3 surface 32 in the “.bui” file but I
can’t find it, it doesn’t exist!
If I start
the debugger on the “.dck” file I get nothing, it’s like I have no errors.
Have any of
you had this problem before?
Thanks,
Mariana Ghetu
--- On Sat, 24/1/09, trnsys-users-request at cae.wisc.edu <trnsys-users-request at cae.wisc.edu> wrote:
From: trnsys-users-request at cae.wisc.edu <trnsys-users-request at cae.wisc.edu>
Subject: TRNSYS-users Digest, Vol 49, Issue 40
To: trnsys-users at cae.wisc.edu
Date: Saturday, 24 January, 2009, 8:03 PM
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Today's Topics:
1. Re: 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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>> TRNSYS-users at cae.wisc.edu
>> https://www-old.cae.wisc.edu/mailman/listinfo/trnsys-users
>>
>
> -- ***********************************************************************
> Thermal Energy System Specialists (TESS), LLC David BRADLEY
22 N. Carroll Street - Suite 370 Partner
Madison, WI 53703 USA
> P: +1.608.274.2577 F: +1.608.278.1475
> E-mail: bradley at tess-inc.com Web Pages: http://www.tess-inc.com and
http://www.trnsys.com
>
> ***********************************************************************
>
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