Ultra-Low Temp (-50°C) Two-Stage Compression Intermediate Pressure Control Failure

Question

Accepted Answer

Intercooler float valve seized — intermediate pressure deviates from normal range (3.5~4.5 bar for R404A). Check: Check intermediate pressure gauge 30 minutes after startup — suspect float valve if fluctuation exceeds 0.5 bar or deviates from design value What to do: 1. Recover refrigerant and disassemble intercooler float valve for inspection
2. Remove corrosion or debris from valve seat and ball, reassemble; replace if needed
3. Confirm intermediate pressure restored to design value (R404A: 3.5~4.5 bar, R22: 4.0~5.0 bar) Low-stage discharge temperature exceeds 120°C → intermediate pressure rises. Check: Check low-stage compressor discharge temperature sensor — if above 120°C, check low-stage expansion valve superheat setting What to do: 1. Readjust low-stage expansion valve superheat to 8~12K
2. Check low-stage compressor cooling water (water-cooled) or oil cooler
3. Monitor discharge temperature every 30 minutes during operation Intermediate pressure regulating valve (EPR) set point drifted or stuck. Check: Compare EPR set pressure with actual intermediate pressure — if deviation exceeds ±0.2 bar from design intermediate pressure, EPR adjustment or replacement needed What to do: 1. Readjust EPR set point using adjustment screw (within ±0.2 bar of manufacturer design intermediate pressure)
2. If EPR seized, recover refrigerant then disassemble, inspect and replace
3. After adjustment, verify intermediate pressure with 30-minute stabilization run Oil accumulation in intercooler — reduced heat transfer area causing intermediate temperature rise. Check: Check drained volume when opening intercooler oil drain valve — more than 100mL/month indicates excessive oil accumulation What to do: 1. Drain oil from intercooler — record volume recovered
2. Check and clean oil return line for blockage
3. Verify oil separator efficiency (replace if oil capture rate has decreased) High-stage suction check valve leaking — backflow causing irregular intermediate pressure fluctuation. Check: Observe intermediate pressure backflow during high-stage compressor start/stop — if intermediate pressure rises at stop, check valve leak confirmed What to do: 1. Recover refrigerant and disassemble high-stage suction check valve for inspection
2. Replace if seat worn (same specification check valve)
3. After reassembly, reconfirm intermediate pressure stability during start/stop

Ultra-Low Temp (-50°C) Two-Stage Compression Intermediate Pressure Control Failure

Likely causes

Intercooler float valve seized — intermediate pressure deviates from normal range (3.5~4.5 bar for R404A)

Low-stage discharge temperature exceeds 120°C → intermediate pressure rises

Intermediate pressure regulating valve (EPR) set point drifted or stuck

Oil accumulation in intercooler — reduced heat transfer area causing intermediate temperature rise

High-stage suction check valve leaking — backflow causing irregular intermediate pressure fluctuation

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