CO2 Cascade Intermediate Heat Exchanger (IHX) Excessive Temperature Deviation (-40°C Low-Temp System)

Question

Accepted Answer

Oil pooling on CO2 side of IHX → reduced heat transfer area, temperature deviation exceeds 8°C. Check: Check drained volume at IHX bottom oil drain valve + measure CO2 inlet/outlet temperature deviation (above 8°C confirms oil pooling) What to do: 1. Open IHX oil drain valve (bottom) and recover oil — record drained volume
2. Top up CO2-compatible refrigeration oil (polyol ester or PAG type) to proper level
3. Inspect oil separator efficiency — take measures to reduce oil carryover Insufficient refrigerant flow on high-temp side (NH3 or HFC) → intermediate stage evaporation temperature rises. Check: Check EEV step log on high-temp side + sight glass bubbles in liquid line → bubbles indicate low charge or EEV over-closed What to do: 1. Check for high-temp side refrigerant leak and recharge
2. Analyze EEV log and readjust superheat target to 4~6°C
3. Clean high-temp side expansion valve strainer Partial icing inside CO2 side of IHX → reduced flow, temperature uniformity deteriorates. Check: Measure frost thickness on IHX exterior (above 20mm is suspect) + check CO2 inlet/outlet pressure differential (rise of 0.3 bar above normal indicates icing) What to do: 1. Temporarily stop system and defrost IHX exterior with warm water (40~50°C) circulation or electric heater
2. Reset CO2 EEV superheat target to 4~6°C (corrects root cause)
3. Check and replace moisture filter (dryer) at IHX inlet CO2 low-temp circuit EEV superheat set too high (>8°C) → excessive superheat at evaporator outlet → insufficient enthalpy at IHX inlet. Check: Measure CO2 evaporator outlet temperature + calculate saturation temperature from suction pressure → compute superheat (outlet temp - saturation temp > 8°C = set too high) What to do: 1. Reset CO2 EEV superheat target to 4~6°C
2. After reset, run for 30 minutes and confirm IHX 4-point temperatures stabilize
3. Monitor superheat log for one week to check for recurrence Excessive frost on outer surface of IHX tubes → increased heat transfer resistance. Check: Visually check frost thickness on IHX exterior (immediate defrost needed if over 20mm) + measure air temperature before and after IHX to confirm reduced efficiency What to do: 1. Defrost IHX exterior with warm water (40~50°C) or electric heater — duration: 30~60 minutes
2. After defrost, confirm drain line is draining properly
3. Adjust defrost schedule — set automatic defrost trigger at 10mm frost thickness on IHX

CO2 Cascade Intermediate Heat Exchanger (IHX) Excessive Temperature Deviation (-40°C Low-Temp System)

Likely causes

Oil pooling on CO2 side of IHX → reduced heat transfer area, temperature deviation exceeds 8°C

Insufficient refrigerant flow on high-temp side (NH3 or HFC) → intermediate stage evaporation temperature rises

Partial icing inside CO2 side of IHX → reduced flow, temperature uniformity deteriorates

CO2 low-temp circuit EEV superheat set too high (>8°C) → excessive superheat at evaporator outlet → insufficient enthalpy at IHX inlet

Excessive frost on outer surface of IHX tubes → increased heat transfer resistance

Related cases