Overrsized evaps and other questions for cascade system.

[g.mccormick]

I am looking at one of our systems mainly from a controls aspect. This system is on an environmental chamber designed to drive a vehicle into. The system has two high stage DX coils, a DX brazed plate hx, and 2 low stage DX coils. The high stage is 404A and low stage is 508.

The system has 2 air handlers, each housing 1 high stage and 1 low stage evap. There are liquid line solenoids that allow the use of high stage DX evaps, or low stage DX evap and high stage cascade hx. The compressors are:
HS:
Bitzer 6FE-50Y-2NU
404A
11.5TR @ -20F SST / 105F SCT


LS:
Bitzer 6B6462PH-2NU
508B
25.9TR @ -80F SST / -10F SCT



Per the prints, the evaps/HX have:
High Side:
2 evaps each 13.8TR

1 hx 360 kBTU/Hr


Low side:
2 evaps each 15.9TR


Gas cooler for low side discharge gas not rated.



Condenser for 404A:
Base rating 282.7 kBTUhr @ 105F SCT


It seems to me that the compressors are quite undersized for 2 evaps running. I see this in running that the suction pressure is quite high. I can turn off one evaporator so that system load decreases and that brings suction pressure and temperature down. I can also decrease load by slowing down fans.


Is it better to let the system run overloaded and hopefully bring down the suction pressure or should I only run 1 evaporator at a time? I don't know if the original intent was to have 2 evaporators running or if it was done this way so that 1 evaporator could be defrosting while the other is running?

[Rob White]

.

For the most part, normally systems are designed for the running temp and they will have a high back pressure
while the room is pulling down to temp. If you have a variation in temperatures and if you are swapping from
high to low temp then the designer would normally oversize the compressor and system respectively.

But the question is, does it work?
If it works then maybe they got the design right?

Rob

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