Heat Exchanger Tube Overheating & Underheating: Uneven Fouling, Localized Corrosion and Thermal Stress

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:

• Local heat flux distortion. Clean areas carry a higher heat flux and run hotter on the hot side, while scaled areas fall behind, producing hot spots and cold spots on adjacent tubes.
• Flow maldistribution. Deposits reduce the free flow area, so the coolant or process fluid is pushed toward the cleaner passages. The starved passages overheat, the flooded passages overcool.
• Under-deposit boiling and dry-out. In evaporators and steam generators, scale traps liquid, causing localized dry-out, salt concentration and a sharp rise in wall temperature under the deposit.

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.

• Control water chemistry with scale inhibitors, dispersants and continuous blowdown to keep deposition rates low and uniform.
• Clean on a condition-based schedule rather than a fixed calendar, using fouling factor trending and infrared surveys to find hot spots early.
• Verify flow distribution after every cleaning and after any baffle or nozzle modification; maldistribution is the fastest way to re-create hot spots.
• Use the right cleaning method — chemical circulation for soft scale, high-pressure water jetting or mechanical tube cleaners for hard deposits, always followed by passivation where required.
• Design for thermal flexibility with expansion joints, U-tube or floating-head configurations.

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.