[TRNSYS-users] Heat exchanger temperature control

[Mario Leroy Ortiz]

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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