View Full Version : Transcritical CO2 question

28-07-2017, 08:13 PM

I am working on a transcritical CO2 system with one compressor rack for both low temperatures and medium temperatures, common liquid line and 2 suction lines (one MT and one LT). The max pressures (the set point of the relief valves on the rack) are 60 bar on common liquid line, 30 bar on LT suction line and 50 bar on MT suction line.

The LT evaporators are designed at the max pressure of 34 bar (working pressure on liquid line is 38 bar). We did pressure tests on liquid line (with the shut off valves before the evaporator closed) at 66 bar and it was no problem, I studied the types of stainless steel pipes and K65 copper pipes before designing the piping diagram. I have Danfoss CCMT and AKVH electronic expansion valves on the evaporators.

I have a question about the liquid line: If something happens with the expansion valves (as far as I know if there's a power failure the valve goes normally closed), would i get 38 bar in the evaporator? I know they are tested at a higher pressure than 34 bar, but I am worried if something happens in the system and at some point i will have maybe 59 bar (lower than the relief valve set point) and one expansion valve is broken and stays open...would the evaporator break? Or it's just a matter of a few seconds to go from 59 bar to 60 bar and the relief valves will activate?

Sorry if this sounds like paranoia, but this is my first transcritical CO2 system and I am a little worried..

Thank you.

Have a good day and weekend!


Glenn Moore
28-07-2017, 09:39 PM
The AKVH valves fail safe closed. The LT and HT suction lines are connected to the evaporator outlets and therefore any overpressure in either the LT or MT Evaps are protected by their own safety valves on their suction lines so no problem.

29-07-2017, 09:09 AM
I don't have safety valves on the evaporators, only on the receivers on the rack and on the main pipes that go from/to the rack to the consumers (LT suction/ MT suction/ liquid).

Glenn Moore
29-07-2017, 05:38 PM
The LT suction and the MT suction relief valves protect their respective connected evaporators ie the suction pipes are part of the evaporators, so the evaps are protected by their common LT or MT suction relief valves

07-08-2017, 06:48 AM
Are you fans set to go off on Evaps when your pack goes into blackout when the AKV valves close. If your using Danfoss 550 controllers parameter o90 should be set to "no" for fans to go off on refrigeration shutdown. It does help reduce the increase in pressure whilst pack is sat doing nothing. Most instances you would vent from LT suction first usually especially with a 60 bar receiver PRV.