Fouling is deposit growth on the heat-transfer surface: scale, particulates, biofilm, corrosion product, or reaction coke. It cuts U and raises ΔP. Design allowances (TEMA Rf) keep the unit on duty until the planned clean; they do not stop the deposit. Reducing fouling is a combination of process chemistry, velocity, temperature, metallurgy, and configuration.
Know which mechanism you are fighting
| Mechanism | What actually slows it |
|---|---|
| Crystallisation / inverse-solubility scale | Lower wall temperature, better water chemistry, avoid stagnant zones |
| Particulate settling | Velocity above deposition threshold; strainers; avoid dead areas behind baffles |
| Biofilm | Biocide programme, velocity, temperature out of the growth window, clean metallurgy |
| Corrosion product | MOC and water chemistry; do not 'design around' ongoing corrosion with Rf alone |
| Coking / polymerisation | Control wall temperature; minimise residence in hot zones; allocation of the reacting fluid |
Velocity on the fouling side
Most deposit mechanisms slow as wall shear rises. Cooling-water tube-side velocities that sit too low are a classic reason river-water and tower-water coolers slime and silt. The limit on the high side is erosion, vibration, and ΔP. Pass count and tube diameter are the levers. Shell-side, baffle spacing and sealing strips reduce bypass that leaves some lanes slow enough to foul while the rated ΔP is still met in the windows. 'Low ΔP at any cost' is often a fouling specification in disguise.
Surface temperature
Calcium carbonate and similar salts precipitate faster on hot walls. Coking and polymerisation rates rise with metal temperature. Thermal design should check wall temperature, not only bulk outlet temperature. Sometimes the dirty or reacting fluid belongs on the side where the wall is cooler; sometimes steam should be desuperheated before the exchanger. These are process decisions that belong on the datasheet as constraints, not after the first year of operation.
Put the dirty fluid where you can clean it
If mechanical cleaning is the plan, the fouling stream belongs on the tube side of a straight-tube, removable-cover exchanger. If the shell side will foul and must be blasted, you need a removable bundle — U-tube in Ohm's manufacture, or a purchaser-specified floating head from a shop that builds that rear end. Fixed-tubesheet units with a dirty shell side are a chemical-clean (or live-with-it) choice. Allocation that ignores cleaning is how plants hydroblast the wrong circuit.
Write cleaning method on the same sheet as fouling factors. Rf without access is an unplanned shutdown.
Chemistry and upstream equipment
Cooling-tower programmes (pH, hardness, biocides, filtration) do more for a water cooler than a larger Rf. Process-side, strainers, coalescers, and control of oxygen or chlorides may be the real fouling project. Metallurgy that corrodes into the stream (carbon-steel tubes on aggressive water) manufactures its own deposit. Ohm will build the MOC you specify; we will not pretend a carbon-steel bundle is a water-treatment plant.
Do not stack margins instead of designing
Conservative TEMA Rf, plus 10% extra area, plus a low assumed U, produces a large, slow exchanger that fouls in the bypass lanes and may vibrate when clean. Specify expected Rf, ask for Uc and Ud, and keep overdesign explicit and modest. If the service is known to be worse than TEMA tables, say so with plant data — that is better than a hidden multiplier.
RFQ: fouling as a process input
Send fluid identities and contaminants, expected Rf or plant standard, cooling-water analysis if relevant, wall-temperature limits, cleaning method, and whether bundle pull is required. Use /rfq?product=heat-exchangers. Thermal layout (velocity, allocation, TEMA type) will follow those constraints for the tubular types Ohm manufactures.
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