Failure analysis · Brazed plate heat exchanger
Connection Leakage in a Vapor-Injection Heat Pump BPHE
Field analysis linking flow restriction, elevated head pressure and thermal stress to fatigue-driven leakage.
Project summary
Challenge
A vapor-injection heat-pump exchanger developed connection leakage under unstable system conditions.
Engineering response
The review examined contamination, hydraulic imbalance, elevated condensing temperature and repeated thermal loading rather than treating the leak as an isolated joint defect.
Outcome
The corrective direction focuses on system cleanliness, balanced flow and control of pressure-temperature transients.
- Vapor-injection heat pump
- Flow restriction
- High head pressure
- Thermal fatigue
Original project record
Case Study: Connection Leakage in a Vapor Injection Heat Pump BPHE
1. Observed Field Condition
A brazed plate heat exchanger (BPHE) used in a vapor injection heat pump system developed leakage at the nozzle connection region. Field inspection revealed significant contamination within the refrigeration circuit.
- Severely clogged strainers (3)
- Oil sludge and metallic debris
- Sensor contamination affecting readings (4)
- Evidence of unstable refrigerant flow
These hydraulic abnormalities indicate that the exchanger was operating under non-design flow conditions prior to failure.
2. Flow Restriction and System Instability
When refrigerant mass flow is restricted, compressor discharge temperature increases. Reduced heat transfer efficiency leads to elevated condensing pressure, commonly referred to as a High Head condition.
Flow restriction often represents the initiating disturbance in the failure sequence.
3. Thermal Consequences of High Head Pressure
High head pressure directly increases condensing temperature. As condensing temperature rises, the internal metal temperature of the BPHE increases accordingly.
This results in:
- Elevated internal pressure load
- Higher metal temperature exposure
- Increased mechanical stress within brazed joints
4. Thermal Stress Mechanism
Thermal expansion is governed by:
ΔL = α · L · ΔT
In brazed plate heat exchangers, stainless steel plates and copper brazing material have slightly different thermal expansion coefficients. Under elevated and cyclic temperature conditions, differential expansion generates internal stress.
Repeated exposure to elevated condensing temperature leads to low-cycle thermal fatigue.
5. Stress Concentration at the Nozzle Region
The nozzle transition zone represents a geometry change area and a natural stress concentration point. Under combined pressure loading and repeated thermal cycling, fatigue cracking may initiate in brazed regions.
Contributing factors include:
- Repeated high-pressure cycles
- Localized thermal gradients
- Material expansion mismatch
6. Root Cause Sequence
Contamination → Flow Restriction → High Head Pressure → Elevated Thermal Stress → Brazed Joint Fatigue → Leakage
Based on observed field evidence, the leakage appears to be a secondary consequence of system instability rather than a primary manufacturing defect.