Why condenser inlet temperature get so high just changing CT outlet setpoint temp?

I am investigating how cooling tower outlet temperature setpoint affect to cooling consumption using the model with water cooled chiller+cooling tower.
In this study, I tried to change cooling tower setpoint from 21degC (case1) to 32degC (case2) using SetpointManager:Scheduled.
Even though case1 was operating as I expected checking chiller condenser inlet temperature, chiller condenser inlet temperature get over 40degC in winter season which I cannot explain why. (climate is Hong Kong) I checked cooling tower heat transfer rate, but seems working without problem.(refer to second image) Could someone tell me what could be the reason causing this operation?
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katsuya.obara

asked 2018-08-27 23:01:32 -0500

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This is interesting because for Case 1 the set point is maintained in winter while the CT outlet temp increases in summer with the outdoor wet-bulb temperature. For Case 2 the CT outlet temp is maintained in summer, since 32 C is probably higher than the outdoor wet-bulb temperature in summer, yet cannot maintain the CT outlet temperature in winter. I suspect the cooling tower model calculations and would like to see the input file for Case 2 to diagnose this further. Please send the EnergyPlus input file (*.idf) to energyplus-support@gard.com

I ran your file and the CoolTools cross flow tower model has known problems, one of them being the maximum range (Tin - Tout) and approach temperature (Toutlet - T,OA,wb) is 11.1 C. At 32 C set point and ~10C outdoor wet-bulb temp, the approach temperature is 22 C. So the operation of this plant is outside the allowable range of the tower model inputs. The tower works as expected when the outlet water temperature set point is within 11 C of the outdoor wet-bulb temperature.

So you could either:

1) limit your study to outlet water temps in a more limited range,

2) try the YorkCalc model where the maximum range = 22 C and max approach = 40C,

3) change the tower Model Type to CoolToolsUserDefined and then enter the coefficients using the CoolingTowerPerformance:CoolTools object (the coefficients are in the Eng. Ref.) and you would have control over the range and approach minimum/maximum temperature ranges. Maximum range and approach of 25C might get you where you need to be for this study, or

4) use a different tower model, e.g., single- or two-speed.