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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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@tess-inc.com Web Pages:
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