CO2 Dry Ice Blockage in Expansion Valve (Operation Below Triple Point)

Question

Accepted Answer

Low-pressure cut-out set too low (below 6 bar) — CO2 solidifies to dry ice below triple point (-56.6°C, 5.2 bar). Check: Check low-pressure cut-out setpoint — if below 7–8 bar, adjust immediately. Check controller low-pressure trip history What to do: Raise low-pressure cut-out to 7–8 bar to maintain triple-point safety margin. If dry ice blockage confirmed: run hot-gas defrost for 10 min (hot gas ~40°C) → blockage clears, then restart Expansion valve over-opening → evaporator pressure drops sharply → below triple point. Check: Check EEV minimum opening limit (electronic expansion valve minimum step ≥20). Review evaporator pressure log What to do: Set EEV minimum opening limit to step ≥20. Test solenoid valve for leakage when closed (observe pressure hold for 30 min) Moisture present — below 5°C, water ice and dry ice form simultaneously. Check: Check sight glass moisture indicator. Perform acid test What to do: After acid test, replace filter drier with CO2-compatible molecular sieve type. Recover all refrigerant → flush with dry nitrogen → evacuate → recharge

CO2 Dry Ice Blockage in Expansion Valve (Operation Below Triple Point)

Likely causes

Low-pressure cut-out set too low (below 6 bar) — CO2 solidifies to dry ice below triple point (-56.6°C, 5.2 bar)

Expansion valve over-opening → evaporator pressure drops sharply → below triple point

Moisture present — below 5°C, water ice and dry ice form simultaneously

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