Fouling in heat exchangers is the accumulation of unwanted deposits on heat transfer surfaces. These deposits add a thermal resistance that reduces the overall heat transfer coefficient (U-value), decreasing thermal performance. At the same time, deposits reduce the flow area, increasing pressure drop. Together, these effects mean the heat exchanger delivers less heat duty while consuming more pumping power — a double operational penalty.
The Five Mechanisms of Fouling
| Fouling Type | Mechanism | Common Examples | Key Driver |
|---|---|---|---|
| Crystallisation (Scaling) | Dissolved salts exceed solubility limit and precipitate on surface | Calcium carbonate (CaCO₃), calcium sulphate, silica in cooling water; evaporator crystallisation | Temperature (inverse solubility salts deposit faster at high surface temp); concentration |
| Particulate / Sedimentation | Suspended solids settle on surfaces, especially low-velocity zones | Sand, clay, rust particles in river water; catalyst fines in process streams | Fluid velocity (low velocity = deposition); particle size and density |
| Biological (Biofouling) | Micro-organisms (bacteria, algae, biofilm) colonise heat transfer surfaces | Cooling tower water; once-through seawater cooling; river water | Temperature (optimal 20–35°C); nutrient content; biocide treatment effectiveness |
| Corrosion Fouling | Corrosion products (oxides, sulphides) form on metal surface | Iron oxide (rust) in carbon steel tubes; copper corrosion products | Metal corrosion rate; pH; dissolved oxygen content; velocity (erosion-corrosion) |
| Chemical Reaction / Polymerisation | Chemical reactions in the process fluid produce solid deposits on surface | Coking in crude oil; polymerisation in ethylene/propylene streams; protein denaturation in dairy | Surface temperature (elevated surface temperature accelerates reaction); fluid composition |
How Fouling Degrades Performance
The overall heat transfer coefficient (U) for a heat exchanger in service (dirty) is related to the clean coefficient (Uc) and the total fouling resistance (Rf_total = Rf_tube-side + Rf_shell-side) by:
1/Ud = 1/Uc + Rf_total
As fouling builds, Ud decreases. For example: if Uc = 1,000 W/m²K and Rf_total = 0.000352 m²K/W (two TEMA normal fouling services):
1/Ud = 1/1000 + 0.000352 = 0.001352 Ud = 740 W/m²K — a 26% reduction in heat transfer coefficient
To maintain the same heat duty with a 26% lower U, the exchanger requires 26% more heat transfer area (assuming the same LMTD). This is why fouling allowances are added at design stage — the exchanger is purposely oversized when clean to ensure it meets thermal duty after fouling deposits form.
TEMA Standard Fouling Resistances
TEMA 11th Edition provides recommended fouling resistance values by fluid and service type. These are design allowances — they do not represent the maximum fouling that will occur; they represent a reasonable design margin for normal operation.
| Service / Fluid | TEMA Rf (m²·K/W) |
|---|---|
| Seawater (velocity < 0.9 m/s) | 0.000176 |
| Seawater (velocity > 0.9 m/s) | 0.000088 |
| Cooling tower water (treated) | 0.000176 |
| River water (minimum) | 0.000352 |
| City or well water | 0.000176 |
| Boiler feedwater (above 50°C) | 0.000176 |
| Boiler feedwater (below 50°C) | 0.000088 |
| Steam (oil-free) | 0.000088 |
| Steam (oil-bearing) | 0.000176 |
| Refrigerant vapours (clean) | 0.000176 |
| Organic solvents (clean) | 0.000176 |
| Light hydrocarbons (below 50°C) | 0.000176 |
| Crude oil (velocity > 0.6 m/s, above 120°C) | 0.000528 |
| Fuel oil | 0.000881 |
| Chemical process streams (light) | 0.000176 |
Preventing Fouling Through Design
- Velocity control: Most fouling mechanisms are suppressed at higher velocities. For tube-side fouling services, design for tube-side velocities of 1.5–3.0 m/s. Low velocities (< 0.9 m/s) are the primary cause of rapid particulate and biological fouling in cooling water services.
- Surface temperature management: For inverse-solubility scaling (CaCO₃, CaSO₄), keep tube-wall surface temperature below the precipitation threshold. For coking and polymerisation, the shell-side fluid should be placed on the hot side (not tube-side) if higher surface temperatures are more tolerable on the shell side.
- Material selection: Use electropolished stainless steel or titanium for biological fouling resistance. Carbon steel tubes corrode and produce iron oxide deposits that compound fouling.
- Allocation of fouling fluid: Place the heavier-fouling fluid on the tube-side (easier mechanical cleaning) rather than shell-side, unless there is a pressure or corrosion reason to do otherwise.
- Specifying cleaning access: Ensure the rear head type allows tube access. Type A (removable channel cover) for tube-side cleaning. Floating-head or U-tube for shell-side bundle removal.
When and How to Clean
Cleaning is warranted when: heat transfer performance has dropped (outlet temperatures deviating from design), pressure drop has increased significantly (> 1.5× design), or during planned turnaround regardless of performance data.
| Cleaning Method | Applicable To | Effective Against | Limitations |
|---|---|---|---|
| Chemical CIP (Clean-In-Place) | Both tube-side and shell-side (circulate chemical through closed loop) | Scaling (acid), biological fouling (biocide/caustic), light corrosion deposits | Chemical selection must not damage tube or shell material; may require neutralisation step |
| High-pressure water jetting | Tube-side (tubes must be straight — not U-tube) | Particulate, soft deposits, biological fouling | Cannot reach fouling inside U-bend; water disposal required |
| Mechanical rodding / drilling | Tube-side straight tubes | Hard deposits, coke, polymerised material | Risk of tube damage if not done carefully; not suitable for thin-walled tubes |
| Air/water lancing | Shell-side after bundle removal | Particulate, soft fouling | Limited penetration into dense tube bundle |
| Ultrasonic cleaning | Tube-side, external on shell | Scaling, biological fouling | Limited to smaller exchangers; effectiveness varies with deposit type |
| Bundle replacement | Any | All fouling | High cost — only warranted when tubes are corroded or mechanically damaged |
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