I. Fouling Is the "Extra Thermal Resistance" During Operation
Recall the overall heat transfer coefficient 1/K = 1/h_hot + δ/λ + 1/h_cold. After a period of operation, a layer of scale grows on the wall, and the formula becomes:
1/K = 1/h_hot + δ/λ + 1/h_cold + Rf_hot + Rf_cold
Rf is the fouling thermal resistance. The thermal conductivity λ of scale and ash is often only one thousandth of that of metal, and a thin layer is enough to make K drop significantly. The common saying "new equipment exchanges heat well, gets worse with use" is mostly rooted in the continuous growth of Rf.
II. How the Five Types of Fouling Come About
| Type | Mechanism | Typical Scenario |
|---|---|---|
| Crystallization fouling | Calcium and magnesium salts in water reach supersaturation and precipitate (e.g., calcium carbonate) | Cooling water, hard-water side |
| Particulate fouling | Dust and silt deposit on the wall | Flue gas side, fluids with suspended matter |
| Biological fouling | Bacteria and algae form slime | Warm water circulation, cooling tower |
| Corrosion fouling | Metal corrosion products adhere | Mismatched material, poor water quality |
| Chemical reaction fouling | Fluids react with each other or the wall to form polymers, coke | High-temperature oil, viscous media |
III. Why "Boiling the Frog in Warm Water"
Fouling growth is often non-linear: slow at first, then accelerating once an "attachment substrate" exists. More troublesome is its positive feedback—the wall temperature rises due to insulation, which in turn promotes certain chemical reactions and scaling. So by the time heat transfer is clearly insufficient, the scale is usually already thick.
IV. Early Signals
- Rising pressure drop at the same load: The channel is squeezed narrower by scale, fluid more "crowded";
- Outlet temperature fails to reach design value: The same area transfers less heat;
- Abnormally high wall temperature: The insulation layer raises the hot-side wall temperature, accelerating aging.
Including these three items in operation inspection saves far more than "wait until it stops heating, then clean." The next article covers how to clean.