1. Introduction

Plate heat exchangers are widely used in refrigeration, heat pumps, industrial cooling, and energy recovery due to their compact structure and high heat transfer efficiency. However, under low-temperature conditions, improper control logic or abnormal operating conditions may lead to freezing of the process medium inside the exchanger.

Freezing is a high-risk failure mode. It often occurs suddenly, causes severe mechanical damage, and is typically irreversible. Once internal structure deformation occurs, cross-leakage between circuits may develop.

2. Conditions That Lead to Freezing

Medium Temperature Below Freezing Point

• Cooling water or softened water used instead of antifreeze
• Antifreeze concentration lower than required
• Actual freezing point higher than design assumption

Insufficient or Uneven Flow

• Flow rate below minimum specification
• Localized blockage or fouling
• Uneven channel distribution
• Low velocity during start-up or shutdown

Control System Failure

• Excessive refrigerant supply
• Delayed temperature valve response
• Antifreeze interlock not activated

Even if outlet temperature remains above 0°C, localized plate surfaces may reach sub-zero temperatures. These “cold spots” are the most common but frequently overlooked freezing trigger.

3. Damage Mechanism of Freezing

3.1 Volumetric Expansion of Ice

Water expands approximately 9% during freezing. Inside narrow plate channels, this expansion generates extremely high localized stress.

Because plate thickness is typically 0.3–0.6 mm, freezing can cause:

• Direct mechanical loading on plate surfaces
• Plastic deformation
• Microcracks at stress concentration points

3.2 Permanent Deformation and Loss of Sealing Integrity

Freezing usually initiates locally and propagates along low-flow or stagnant regions. This may lead to:

• Plate bulging or indentation
• Permanent change in channel spacing
• Excess stress on brazed or welded joints

After thawing, plates do not return to original geometry, creating latent leakage risk.

3.3 Leakage Characteristics

• Damage limited to one or a few plates
• Leakage localized rather than random
• Initial leakage may be minor
• Leakage worsens during continued operation

In brazed or fully welded plate heat exchangers, freezing damage is generally not repairable and requires full replacement.

4. Why Freezing Damage Appears Sudden

Unlike corrosion or fatigue, freezing is an instantaneous mechanical failure mechanism.

• Can occur during a single abnormal event
• Total running hours may be low
• Leakage may only become visible after thawing

Therefore, leakage shortly after commissioning does not exclude freezing as the root cause.

5. Typical Contributing Factors

• Operating medium differs from design basis
• Insufficient antifreeze concentration
• Excess cooling capacity with delayed regulation
• Frequent start-stop cycles
• Internal fouling reducing local flow

6. Prevention and Engineering Recommendations

Verify Freezing Point

• Confirm antifreeze type and concentration
• Account for real operating conditions

Ensure Minimum Flow Rate

• Never operate below manufacturer minimum flow
• Avoid stagnant zones

Improve Protection Logic

• Low-temperature interlocks
• Automatic shutdown of cooling source
• Forced circulation pump operation

Maintain Adequate Safety Margin

• Design outlet temperature with margin above freezing
• Avoid continuous operation near freezing point

Key Conclusions

• Plate heat exchangers are inherently more sensitive to freezing than shell-and-tube heat exchangersdue to structural characteristics.
• Design parameters can significantly amplify or mitigate freezing-related leakage risk.

Design Factors Directly Related to Freezing Risk

1. Plate Thickness

Typical plate thickness of 0.3–0.6 mm improves heat transfer but reduces resistance to mechanical stress. Ice expansion can exceed yield strength and cause permanent deformation.

2. Channel Gap and Chevron Geometry

• Typical channel gap: 2–4 mm
• Numerous contact points concentrate stress
• Expansion force acts directly on plate surface
• High stress at chevron peaks and brazed joints

3. Flow Distribution Zone

Distribution areas near ports often have lower velocity. These regions are prone to cold spots and freezing initiation, especially under low-flow conditions.

4. Pass Arrangement and Operating Velocity

• Multi-pass: higher velocity but higher pressure drop
• Single-pass: lower pressure drop but more sensitive to low-load operation

If actual operation frequently occurs at low flow, freezing risk increases even when design is theoretically valid.

5. Freezing Safety Margin

• Outlet temperature too close to 0°C increases risk
• Antifreeze selected near theoretical limit reduces margin
• Instrumentation error and control delay may cause local freezing

If adequate safety margin and minimum flow verification are not included, freezing risk becomes embedded at the design stage.

Final Conclusion

Freezing-related leakage in plate heat exchangers is a mechanically induced failure caused by phase change. Its occurrence is closely linked to operating conditions, control logic, and exchanger design.

Once freezing occurs, irreversible structural damage may already exist. Therefore, freezing must be treated as a critical engineering risk during both design and operation stages.