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# Cooling Capacity of Air-Chiller Decrease as the Temperature Rises (hottest simulation day)

Hello evryone,

I am modeling a real-case refrigerated warehouse that is composed of 13 zones, 12 walk-in coolers, and a loading zone.

I defined 2 refrigeration systems: the east and west systems.

Eastern System:

• All 6 eastern air-chillers with a cooling capacity of 32 kW each + Loading zone air-chiller with 20.9 kW
• 1 condenser
• 5 compressors of 28.4 kW each
• 1°C as setpoint

The same thing applies to the Westers System except for the Loading zone air-chiller.

I run the model on the hottest day of the year, where the maximum temperature reaches 43°C.

The problem is that the cooling capacity of the loading zone's air-chiller decries during the day from 11 a.m. to 18 (figure below), and I have no idea why.

I thought that if the compressors work at maximum capacity and if the condensing temperature rises, maybe it affects the system cooling capacity, so I plotted the eastern compressor group power consumption (figure below), and the group is not working at maximum.

Fig2: Estern Compressors Power Consumption

...

I then added another compressor to the group + augmenting the air-chiller capacity from 20.6 kW to 32 kW, and the problem was solved, but the model is not representing the real-case warehouse.

Fig3: Loading Zone Total cooling Rate and Condencing Temperature after adding a compressor N° 6 and Changing the air-chiller cooling capacity

Fig4: Estern Compressors Power Consumption after adding a compressor N° 6 and Changing the air-chiller cooling capacity

What I did then was change the simulation day to a day less hot, and the problem was solved without adding a compressor or augmenting the cooling capacity of the air-chiller.

Fig5: Loading Zone Total cooling Rate and Condensing Temperature on a day less hot

I don't know why this problem occurs and why the compressors do not work at maximum (especially compressor 5 CP5) to cover cooling capacity.

Thanks!

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As a guess, I've had a similar issue before with heat pumps for cooling and I found the problem to be related to minimum temperature setpoints in loops/setpoint managers.

For a refrigerant flow of X m3/s (The maximum for all compressors) you may need the refrigerant to be at -10oC for the heat balance to be closed and the cooling demand met that hottest day. However if a setpoint somewhere (loop or manager) says -8oC mimimum that last compressor would have it's capacity limited when your Xm3/s of refrigerant is cooled to -8oC. (-8oC is an imaginary number for the sake of the explanation)

This could explain why: a) By adding an extra compressor if you also adjusted the maximum refrigerant flow when adding this fifth compressor now you have a max flow of X+X/5 which in this imaginary scenario would now only need to be at -7.8oC and the setpoint minimum would not apply.

b) A day with lower temperatures would reduce the cooling demand and now your X m3/s of max refrigerant flow could meet the demand at only -7.5oC.

Hope this helps, PMP

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