How do I smoothly cool a thermal chamber with cascaded compressors?

[LarryAndrews314]

Any help on this is much appreciated. I've tried everything I can, even calling the manufacturer for their PID settings and gotten no where on this. I took a Tenney TJR thermal chamber, removed the old Watlow controller and replaced it with a new Watlow F4T. The heating cycle practically took care of itself but the cooling cycle has been very difficult. It has cascaded compressors. The hi stage turns on, runs for about 15 secs and then the lo stage clicks on. I've set the compressors to be on for at least 30 seconds and off for at least 15 secs to avoid burning out the compressors. The problem is the temperature plummets a few degrees even with the compressors only on for about 40 secs(the graph is C degrees vs time). The profiles only asking for about a 0.5 C drop per a minute so the compressor turns on the heaters to battle the temperature drop. This results in a very jagged cooling profile that can not be the best this chamber can do. Am I misunderstanding something about how the compressors should be controlled? Any tips?

 

[IRstuff]

You seem to be making it worse by alternating the compressors, so why do that? You mention "hi" and "lo" stages, so they're intended for different operating regimes, but you've not stated how they're supposed to be working, and for sure, they ought not be alternately cycling.

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[lilliput1]

You have too much cooling capacity. One way would be to add a storage tank for the low temperature chilled water and a separate secondary chilled water loop with variable speed pump to modulate the chilled water flow to the thermal chamber as required to keep within the 0.5 C drop per minute. Size tank so chillers would operate above the minimum time recommended by the manufacturer. Say operate primary chilled water pump and the chillers in sequence till the water temperature in the tank drops to say -8 C then restart when it has risen to say -4 C. Operate the VFD secondary pump to draw water from the lower part of the storage tank (provide dirt leg in piping) and direct flow to the thermal chamber. Return secondary CHW to top of tank. Provide recirculation loop piping with normally closed valve to direct flow from secondary pump discharge to secondary pump inlet. When secondary VFD pump speed has dropped to about 20% hold pump at this minimum speed and modulate recirculation valve to maintain the -4 C drop per minute or the required minimum temperature of the thermal chamber. The secondary pump is required because the primary pump is off when chillers are off.