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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