In any shell-and-tube or finned-tube heat exchanger, the tube wall temperature should stay inside a narrow, predictable band defined by the hot and cold side design conditions. When engineers report that some tubes run dangerously hot while neighbouring tubes in the same bundle stay cold, the root cause is almost never the process fluid itself. In the overwhelming majority of field cases, the real reason is uneven fouling: a non-uniform deposit layer on the heat transfer surface.
Because fouling does not build up evenly along the tube length or across the bundle, the thermal resistance of each tube differs. Tubes with a thicker scale layer cannot remove heat efficiently, so their metal temperature climbs. The result is an uneven temperature distribution across the same heat transfer surface, which is the direct trigger of two failure modes: localized corrosion and thermal stress.
Fouling deposits such as calcium carbonate scale, corrosion products, polymerised organics, coke or biological slime act as an insulating layer. Their thermal conductivity is typically one to two orders of magnitude lower than that of the base metal, so even a fraction of a millimetre changes the local heat transfer coefficient significantly. Three mechanisms then work together:
Once a stable temperature gradient and a stagnant, chemically aggressive environment form beneath a deposit, classic localized corrosion mechanisms appear: under-deposit pitting, crevice corrosion, galvanic attack at the scale-metal interface, and stress corrosion cracking in susceptible alloys. Chloride concentration cells under scale can reach several times the bulk concentration, accelerating pitting of stainless steel and copper alloys.
Temperature differences of 30–80 °C between adjacent tubes, or between the hot and cold side of a single tube wall, generate restrained thermal expansion. Where the tube is fixed in the tubesheet, in baffle holes or at rolled joints, that restrained expansion converts directly into stress. Repeated start-up and shut-down cycles then produce low-cycle fatigue, tube-to-tubesheet joint leakage, baffle fretting and, in severe cases, tube cracking or rupture.
| Symptom | Likely Cause | Recommended Action |
|---|---|---|
| Hot spots on individual tubes with cold neighbours | Non-uniform scale thickness in the bundle | Inspect with IR thermography, plan bundle cleaning |
| Falling outlet temperature with unchanged flow | Progressive fouling on the heat transfer surface | Measure fouling factor, schedule chemical or mechanical cleaning |
| Rising pressure drop across the shell side | Deposits narrowing the flow passages | Check flow distribution, clean and verify tube ID |
| Pitting or perforation under a deposit ring | Under-deposit corrosion and concentration cells | Tube inspection (EC/IRIS), plug or retube affected tubes |
| Leakage at rolled tube-to-tubesheet joints | Thermal fatigue from cyclic temperature gradients | Re-roll or re-expand joints, review operating cycles |
Local overheating or underheating of heat exchanger tubes is a diagnostic signal, not a random event. Uneven fouling raises the local thermal resistance, distorts the temperature distribution across the heat transfer surface and opens the door to localized corrosion and thermal stress. Regular inspection, correct water treatment and condition-based cleaning keep the tube wall temperature uniform, extend tube life and protect the availability of the whole exchanger.
If your exchanger shows hot spots, falling performance or repeated tube failures, our engineering team can help you analyse the fouling pattern and select the right tube, cleaning and water treatment solution.
