A chemical plant lost a PTFE immersion heater in a nickel plating tank after only seven months of service. The sheath showed a pattern of fine, intersecting cracks along the lower heated section. The cracks were not concentrated at the edges or the liquid line. They were distributed across the area that faced the tank's recirculation inlet. The plant's circulation pump was programmed to cycle on and off every fifteen minutes to save energy. Each pump cycle imposed a thermal shock on the heater surface.
Thermal shock cracking occurs when a PTFE immersion heater surface experiences rapid temperature changes that exceed the polymer's ability to accommodate the resulting strain. The cracks form because the surface expands or contracts faster than the underlying material. Over time, the cyclic stress causes fatigue cracking.
How Pump Cycling Creates Thermal Shock
When the circulation pump is running, the fluid velocity at the heater surface is high. The boundary layer is thin, and heat transfer is efficient. The sheath surface temperature stays within 10–15°C of the bulk fluid.
When the pump stops, the fluid velocity at the heater surface drops to near zero. The boundary layer thickens, and heat transfer efficiency collapses. The sheath surface temperature rises, sometimes by 20–30°C within minutes. The thermal expansion of the PTFE increases.
When the pump restarts, the high-velocity flow rapidly cools the surface. The PTFE contracts. The underlying metal, which has not cooled as quickly, constrains the contraction. The PTFE experiences tensile stress. Repeated cycles of heating and cooling cause fatigue cracking.
| Pump Cycle Period | Surface ΔT During Off Cycle | Surface ΔT During On Cycle | Stress Amplitude | Time to Crack |
|---|---|---|---|---|
| 30 minutes on/off | 15–20°C | 10–15°C | Moderate | 18–30 months |
| 15 minutes on/off | 20–30°C | 15–20°C | High | 8–16 months |
| 10 minutes on/off | 25–35°C | 18–25°C | Very high | 4–10 months |
| 5 minutes on/off | 30–45°C | 20–30°C | Severe | 2–6 months |
Pump cycling frequency and its effect on thermal shock cracking
Why the Cracks Concentrate Near the Inlet
The cracks form where the thermal shock is most severe-the area that faces the recirculation inlet. When the pump restarts, the inlet flow impinges directly on that area. The surface there cools faster than the rest of the heater. The thermal gradient between the impinged area and the surrounding surface creates the highest stress.
The cracks in the failed heater were concentrated in a band approximately 150 mm wide, centered on the inlet flow path. The rest of the heater showed only minor surface discoloration.
The Difference Between Thermal Shock and Thermal Fatigue
Thermal shock and thermal fatigue are related but distinct. Thermal fatigue occurs when the heater cycles between two temperatures over a long period. The temperature change per cycle is moderate, and the stress is low. The cracks develop slowly, often over years.
Thermal shock occurs when the temperature change per cycle is large and rapid. The stress is high, and the cracks develop quickly. The cracks are often more severe and more widely distributed than in thermal fatigue.
Preventing Thermal Shock Cracking
The most effective prevention is to avoid rapid changes in flow at the heater surface. A continuously running pump eliminates the thermal shock cycle. If energy savings are required, a variable-speed pump that maintains minimum flow is better than an on/off pump.
If pump cycling is unavoidable, the cycle period should be long enough to allow the heater surface temperature to stabilize between cycles. A 30-minute cycle imposes less stress than a 10-minute cycle. The minimum cycle period depends on the thermal mass of the heater and the tank.
A heater with a thicker PTFE sheath and a lower watt density has more thermal mass and a lower surface temperature. It responds more slowly to changes in flow and experiences lower thermal shock stress.
| Prevention Measure | Effect on Thermal Shock | Implementation | Cost |
|---|---|---|---|
| Continuous pump operation | Eliminates shock cycle | Operational change | Energy cost |
| Variable-speed pump | Maintains minimum flow | Pump upgrade | Moderate |
| Longer cycle period | Lower stress amplitude | Control change | Low |
| Lower watt density | Lower surface temperature | Heater design | Moderate |
| Thicker sheath | More thermal mass | Heater design | Moderate |
Prevention measures for thermal shock cracking
When Thermal Shock Cracking Indicates a Design Problem
A heater that develops thermal shock cracks despite continuous pump operation indicates that the heater design or the tank layout is creating localized thermal stress. A custom PTFE immersion heater configuration with lower watt density, a thicker sheath, and a flow-optimized position addresses the root cause of thermal shock. The specification process requires the pump operating schedule, the flow velocity at the heater, the bath chemistry, and the required wattage. For facilities with intermittent circulation, this application-specific approach reduces thermal shock cracking and extends heater service life.
