U-Bend Fatigue Cracking from Thermal Stress and Freezing

Question

Accepted Answer

Repeated start/stop cycles with refrigerant ΔT > 50°C. Check: copper U-bend expansion/contraction cycles → fatigue crack What to do: Confirm crack: apply liquid penetrant test (PT) on U-bend surface or pressurize with N2 at 0.8 MPa and bubble-test to locate crack Repeated freeze-thaw cycles (ice expansion ~9%). Check: plastic deformation accumulates in U-bend → eventual cracking What to do: Replace cracked U-bend: use deoxidized copper (TP2, C1220T), ensure bend radius R ≥ 1.5D, reconfirm with PT after brazing Insufficient bend radius (R < 1.5D). Check: stress concentration → accelerated cracking What to do: Absorb thermal expansion: install expansion loop or U-shape compensation tube in straight section → self-absorbs thermal growth Poor brazing. Check: capillary voids → crack initiation site What to do: Install vibration clamps: fix clamp within 150 mm of U-bend (isolates vibration transmission) Missing vibration clamps. Check: resonance causes repeated bending load on U-bend What to do: Prevention: during nightly shutdown, recover refrigerant from circuit + open drain valve to remove residual water

U-Bend Fatigue Cracking from Thermal Stress and Freezing

Likely causes

Repeated start/stop cycles with refrigerant ΔT > 50°C

Repeated freeze-thaw cycles (ice expansion ~9%)

Insufficient bend radius (R < 1.5D)

Poor brazing

Missing vibration clamps

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