Initial Diagnosis: Multiple large compressors seized and were initially blamed on manufacturing defects.

Root Cause: After 5 site inspections, I traced the root cause not to the machine, but to the system: incidental liquid slugging caused by a flawed -45°C evaporator design.

Resolution: We fixed the system, saving the compressors.

Background

A low-temperature refrigeration plant serving a −45°C process duty experienced repeated large-compressor seizures. The owner and OEM initially treated each event as a discrete machine defect. Site access was intermittent; instrumentation logs were incomplete; and spare machines were swapped without a system-level failure tree.

Symptoms

Reported symptoms included sudden current spikes before trip, metallic noise at start/stop, and progressive bearing distress on more than one compressor body. Oil level sight-glasses often looked “normal,” which delayed suspicion of liquid return or lubrication collapse.

Initial hypothesis

Early hypotheses focused on manufacturing tolerance, bearing metallurgy, and oil grade. Those hypotheses were plausible for a single machine—but they failed the multi-unit, multi-visit pattern.

Measurements

Across five visits we reconstructed suction superheat trends, evaporator circuit balance, and start-up liquid hold-up. Quantitative plant tags were incomplete; where numeric logs were missing, we relied on repeatable qualitative checks (frost patterns, circuit ΔT feel, and trip chronology) rather than inventing missing sensor values.

Root-cause chain

The failure chain was system-side: evaporator design at −45°C duty allowed intermittent liquid carry-over → suction line liquid slug → sealing-line / bearing film collapse → seizure. The compressor was the victim, not the root cause.

Corrective actions

Corrective work focused on evaporator circuiting / liquid management and suction protection—not on serial bearing swaps alone. After the system-side fix, remaining compressors were preserved rather than condemned as defective stock.

Verification

Verification criteria were operational: repeat start/stop without liquid-noise signature, stable suction superheat on the repaired circuits, and no recurrence across a multi-week observation window agreed with the owner. Exact COP or power numbers are not published here because they were not part of the sealed failure narrative.

Prevention checklist

  • Treat multi-unit seizures as a system failure tree before blaming OEM metallurgy.
  • Map evaporator circuit balance at the design minimum temperature.
  • Require suction superheat / liquid-return checks in commissioning and after major overhauls.
  • Do not accept “oil level looks OK” as proof of an oil film under liquid-slug risk.

Engineering boundary

This note is a field-engineering postmortem for learning. It is not a PE-stamped design review, not a warranty determination, and not a substitute for OEM service manuals or local pressure-vessel / refrigeration codes.

Anonymization note

Plant identity, exact capacity, OEM model strings, and customer names are withheld. Temperatures and qualitative failure modes are retained where they are essential to the engineering lesson and were established on site.