I. Introduction
Shell and tube heat exchangers (tubular heat exchangers) are the most widely used heat exchange equipment in industries such as chemical, power, petroleum, pharmaceutical, and food processing, accounting for over 60% of all industrial heat exchangers. Their structure consists of a shell, tube bundle, tube sheet, and head. One fluid flows inside the tubes (tube side), while another flows outside the tubes (shell side), with heat exchange occurring through the tube walls.
However, after long-term operation, scaling on tube walls is inevitable — Ca²⁺ and Mg²⁺ ions in circulating water precipitate as CaCO₃ and Mg(OH)₂ scale in high-temperature zones, carbon steel tubes oxidize to form Fe₂O₃ and Fe₃O₄ rust scale, and microbial metabolism in cooling water produces bio-slime. These deposits cause a significant decline in heat transfer coefficient, sharply increased energy consumption, and in severe cases, tube perforation and leakage due to under-deposit corrosion. Starting from scaling mechanisms, this article systematically expounds the formulation principles, process flows, and engineering practice experience of chemical cleaning for shell and tube heat exchangers.
II. Technical Analysis
2.1 Scaling Mechanisms
Scaling in shell and tube heat exchangers is mainly classified into three types:
Scale Deposition: Ca²⁺ and Mg²⁺ ions in circulating cooling water precipitate in high-temperature zones of heat exchange tubes due to decreased solubility, forming hard scale dominated by CaCO₃ with minor Mg(OH)₂. When water temperature exceeds 60°C, the CaCO₃ precipitation rate multiplies, and the scale layer thickens at a rate of 0.2–0.5 mm per month.
Rust Scale Accumulation: Carbon steel tubes undergo electrochemical corrosion in water, generating Fe₂O₃ (red rust) and Fe₃O₄ (black rust). Rust scale is loose and porous, which not only reduces heat transfer efficiency but also adsorbs more scale through its porous structure, forming iron-calcium mixed scale — the most common and most difficult-to-remove scale type in shell and tube heat exchangers.
Bio-slime: Algae, bacteria, and other microorganisms in cooling water, along with their metabolic products and suspended particulates, deposit in low-flow-rate zones, forming biofilm. Biofilm not only impedes heat transfer but its metabolic organic acids also exacerbate localized corrosion of tube materials.
2.2 Key Parameters
| Parameter | Description | Typical Range |
|---|---|---|
| Scale Thickness | Deposit thickness on tube inner wall | 0.5–5 mm |
| Heat Transfer Coefficient Reduction | K value change ratio before/after scaling | 20%–60% |
| Cleaning Agent Concentration | Active ingredient mass fraction in pickling solution | 3%–10% |
| Circulation Flow Rate | Cleaning solution flow rate in tubes | 0.5–1.5 m/s |
| Cleaning Temperature | Chemical cleaning solution operating temperature | 40–65°C |
III. Cleaning Solutions
3.1 Chemical Cleaning Formulation
For typical carbonate/rust mixed scale on carbon steel shell and tube heat exchangers, the following formulation is recommended:
| Chemical | Concentration | Function |
|---|---|---|
| Sulfamic Acid | 5%–8% | Primary pickling agent, dissolves CaCO₃ scale |
| Citric Acid | 2%–3% | Chelates iron ions, removes rust scale |
| BTA (Benzotriazole) | 0.1%–0.3% | Corrosion inhibitor for copper tubes |
| Urotropine | 0.3%–0.5% | Corrosion inhibitor for carbon steel pickling |
| Surfactant | 0.1% | Penetration and wetting, accelerates scale layer detachment |
3.2 Process Flow
- Water Flushing: Remove loose deposits from tubes, confirm pipeline flow is unobstructed
- Alkaline Cleaning (Optional): Use Na₂CO₃ + Surfactant solution at 60°C circulating for 2 hours to remove oil and grease contaminants
- Acid Cleaning: Pump the prepared Sulfamic Acid + Citric Acid + corrosion inhibitor solution into tubes, control temperature at 50–60°C, circulate for 4–6 hours
- Neutralization: Drain acid solution, flush with clean water to pH≈7, add Na₃PO₄ solution to neutralize residual acid
- Passivation: Use NaNO₂ or Sodium Molybdate solution circulating for 1 hour to form a protective film on tube walls
- Acceptance: Inspect tube wall cleanliness via borescope, test corrosion rate using coupon method (must be ≤6 g/(m²·h))
IV. Engineering Case Study
Chemical Cleaning Case for a Chemical Group Company's Shell and Tube Heat Exchanger
Equipment Parameters: BEM700 shell and tube heat exchanger, heat exchange area 180 m², carbon steel shell/tube bundle, tube-side medium: circulating cooling water, shell-side medium: process material, design inlet-outlet temperature difference 15°C. Equipment had operated continuously for 3 years without chemical cleaning.
Pre-Cleaning Condition: Heat exchange tube scale thickness 2–3 mm, inlet-outlet temperature difference dropped from design value of 15°C to 7°C, heat transfer efficiency at only 47% of design value. Steam consumption increased 35% year-on-year, additional annual energy costs approximately ¥220,000. Sample analysis showed scale composition dominated by CaCO₃ (72%) and Fe₂O₃ (18%), with SiO₂ (3%) and organics (7%), confirming typical iron-calcium mixed scale.
Cleaning Solution: Adopted 6% Sulfamic Acid + 3% Citric Acid + 0.4% Urotropine + 0.1% Surfactant formulation, cleaning temperature controlled at 55±3°C, circulation flow rate 1.0 m/s, circulation cleaning for 5 hours. Acid concentration and iron ion concentration sampled every 30 minutes; cleaning endpoint determined when iron ion concentration stabilized.
Cleaning Results: Borescope inspection showed over 95% base metal exposure on tube walls, descaling rate 98%. Coupon corrosion rate measured at 3.2 g/(m²·h), far below the standard requirement of 6 g/(m²·h). After cleaning and resuming operation, inlet-outlet temperature difference recovered to 14°C, heat transfer efficiency recovered to 93% of design value, annual steam cost savings approximately ¥180,000, cleaning investment payback period under 3 months.
V. Summary and Recommendations
The effectiveness of chemical cleaning for shell and tube heat exchangers depends on three critical aspects:
First, scale sample analysis must be precise. Hard scale dominated by CaCO₃ is suitable for the Sulfamic Acid system; iron scale-dominated deposits require increased Citric Acid or addition of EDTA for enhanced chelation; silica-containing scale requires NH₄HF₂ auxiliary dissolution.
Second, corrosion inhibitor selection must strictly match tube material. Urotropine for carbon steel tubes, Sodium Molybdate for stainless steel tubes, BTA or MBT must be added for copper or copper-nickel tubes — using the wrong inhibitor is more dangerous than using none, as certain inhibitors may accelerate corrosion on dissimilar metals.
Third, post-cleaning passivation must not be omitted. Especially for equipment restarted after shutdown maintenance, the passivation film is the last line of defense against rapid re-rusting. It is recommended to use NaNO₂ + Na₃PO₄ composite passivator, pH controlled at 9.5–10.5, circulating for 1–2 hours.
Comprehensive recommendation: Shell and tube heat exchangers should undergo periodic chemical cleaning every 12–18 months, combined with online circulating cooling water treatment (scale inhibitor + corrosion inhibitor + biocide), which can extend effective operating life by over 30% and reduce comprehensive energy efficiency costs by 15%–20%.
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