Heat Exchanger Fouling and Cleaning: A Practical Engineer's Manual
After a period of operation, various scale layers gradually deposit on the heat transfer surface, causing the overall heat transfer coefficient K to drop and pressure drop to increase, and in severe cases causing blockage or corrosion perforation. Statistics show fouling costs global industry hundreds of billions of dollars in extra energy loss and maintenance annually.
1. How to Determine if Cleaning is Needed
- Heat transfer efficiency drops >15% (insufficient heat transfer, outlet temperature below design)
- Pressure drop increases >20% over initial value (flow drop or blockage)
- Last cleaning exceeded 12-18 months (scheduled maintenance)
- Signs of corrosion perforation or leakage
2. Common Scale Types and Characteristics
| Scale Type | Appearance | Cause | Cleaning Difficulty |
|---|---|---|---|
| Calcium carbonate scale | White hard | Hard water heating, Ca2+ precipitation | Moderate (acid wash effective) |
| Silicate scale | Grey-white hard | Excess SiO2 | Relatively difficult |
| Biofilm | Black/green soft | Bacteria/algae growth | Easy (alkali wash + biocide) |
| Corrosion products | Reddish-brown | Iron ion deposition | Moderate |
| Process coking | Black hard | Organic high-temp decomposition | Hard (mechanical cleaning needed) |
3. Cleaning Method Selection
Chemical Cleaning (for detachable plate and shell-and-tube)
- Alkali wash: 2%-5% NaOH solution, 75-80C circulating 1-2 hours, removes grease and biofilm
- Acid wash: 2%-5% citric or nitric acid, removes calcium carbonate and rust (note: do not use hydrochloric acid on stainless steel)
- Neutralization rinse: after acid wash, neutralize with 0.5% NaOH, then rinse with water to pH 7-8
Physical Cleaning
- High-pressure water jet: 200-400 bar, suitable for tube-side cleaning of shell-and-tube exchangers
- Ultrasonic cleaning: for precision exchangers, effective on stubborn scale
- Disassembly manual cleaning: plate exchangers can be disassembled directly, cleaned with soft brush; never use a steel wire brush that scratches the plates
4. Fouling Prevention Recommendations
- Control circulating water quality (hardness <200 mg/L, pH 7.0-8.5, turbidity <20 NTU)
- Install filters and softening devices to reduce suspended solids and hardness
- Add corrosion inhibitor and scale inhibitor regularly
- Monitor pressure drop changes and maintain a maintenance log