Double-Pipe Heat Exchanger Online Chemical Cleaning Technology

1. Structure and Scaling Mechanism of Double-Pipe Heat Exchangers

The double-pipe heat exchanger is a fundamental device in the industrial heat transfer field, consisting of two concentric tubes of different diameters — the inner tube carries one fluid, while the annular space between the inner and outer tubes carries another fluid, achieving counter-current or co-current heat transfer through the tube wall. Its structure is simple, easy to manufacture, and capable of withstanding high pressure (up to 10 MPa and above), making it particularly suitable for low-flow, high-temperature, high-pressure, and solids-containing process medium heat exchange, with wide applications in chemical, petrochemical, pharmaceutical, and food industries.

However, the narrow annular flow passage of double-pipe heat exchangers (typically only 5–15 mm clearance), combined with the 180° return bends at both ends, makes them high-risk equipment for scaling and blockage. Ca²⁺, Mg²⁺ and other ions in the fluid precipitate as solubility decreases during heating, forming a carbonate scale layer primarily composed of Calcium Carbonate. Meanwhile, suspended particles in the process medium, corrosion products (Iron Oxide), and microbial slime also deposit in the lower-velocity bend areas. Measured data indicates that when the annular flow passage cross-sectional area is occupied by more than 30% scale, the overall heat transfer coefficient K-value can drop by 40%–60%, causing the outlet temperature to fail to meet process requirements. Traditional shutdown disassembly cleaning requires draining materials, removing flanges and bends, and clearing section by section — the shutdown cleaning of a single DN50/DN80 double-pipe heat exchanger takes at least 2–3 days, during which the entire production line is forced to halt.

2. Process Principles of Online Chemical Cleaning

Online Cleaning (or Cleaning-in-Place, CIP) refers to introducing a prepared cleaning solution into the equipment interior through a temporary circulation piping system without disassembly or shutdown, utilizing chemical reactions between the cleaning agents and the scale layer to achieve descaling. The essence of online cleaning is temporarily transforming the "heat exchanger" into a "chemical reactor" — using the equipment's own flow passages as reaction space, driving the cleaning solution to circulate repeatedly between the tube side and shell side via a circulation pump, removing the scale layer through the synergistic action of chemical dissolution and physical scouring.

Acid Dissolution — for Carbonate Scale: Using Sulfamic Acid (NH₂SO₃H) as the primary cleaning agent, it undergoes a double decomposition reaction with Calcium Carbonate: CaCO₃ + 2NH₂SO₃H → Ca(NH₂SO₃)₂ + H₂O + CO₂↑. Sulfamic Acid is a solid acid, appearing as white crystalline powder at room temperature, convenient for storage, transport, and on-site preparation, with far lower corrosivity to carbon steel than Hydrochloric Acid, making it a relatively safe pickling agent in industrial cleaning. Used at 5%–8% concentration, the reaction rate is moderate at 50–60°C, and a 1–2 mm thick carbonate scale layer can be completely dissolved within 4–8 hours.

Chelation Dissolution — for Iron Oxide Scale: Supplemented with Citric Acid (C₆H₈O₇) at 1%–2%, utilizing the three carboxyl groups (-COOH) in its molecule to form stable iron citrate chelates with Fe³⁺, converting insoluble Iron Oxide into soluble complexes dissolved in the cleaning solution. Citric Acid also provides pH buffering, maintaining the cleaning solution pH in the weakly acidic range of 3.0–4.5, reducing erosion of the metal substrate.

Corrosion Inhibition Protection System: Corrosion inhibitors must be added to acid-containing cleaning solutions. BTA (Benzotriazole) is the most commonly used corrosion inhibitor for copper alloys/carbon steel, used at 0.15%–0.25% concentration. BTA molecules form coordination bonds with metal surface atoms through the lone pair electrons on nitrogen atoms, self-assembling into a dense [Cu(I)BTA]n polymer protective film that effectively blocks the diffusion of H⁺ and cleaning agent anions toward the metal substrate. Combined with 0.05%–0.10% Sodium Molybdate as an anodic passivation-type auxiliary inhibitor, the carbon steel corrosion rate can be controlled below 1.0 g/(m²·h), complying with GB/T 25146-2010 Quality Acceptance Specification for Chemical Cleaning of Industrial Equipment.

3. Process Parameters and Control Points

Temperature Control: Cleaning temperature is the most critical parameter affecting the reaction rate. 50–60°C is the optimal working range for the Sulfamic Acid system — for every 10°C increase in temperature, the descaling rate increases approximately 1.5–2 times. However, the temperature must not exceed 65°C, because BTA begins thermal decomposition above 60°C, and its corrosion inhibition efficiency drops sharply once the protective film structure is destroyed. By utilizing the double-pipe heat exchanger's own operating temperature (process medium temperature typically 60–80°C), after closing the cold fluid side valve, the hot fluid side temperature naturally maintains 50–60°C without requiring external heat source heating — achieving significant energy savings.

Flow Velocity and Circulation Mode: The cleaning solution flow velocity in the annular passage is controlled at 0.5–1.0 m/s. When velocity is too low (< 0.3 m/s), the flow regime is laminar, chemical mass transfer efficiency is poor, and bend area cleaning is inadequate; when velocity is too high (> 1.5 m/s), scouring force is excessive, potentially accelerating mechanical wear of the inhibitor film. Forward/reverse alternating circulation is recommended — switching the circulation pump direction every 30 minutes to ensure both side walls of bends receive thorough scouring. The circulation pump head is calculated based on equipment height and piping resistance, typically selecting a chemical centrifugal pump with 20–30m head.

Endpoint Determination: Ca²⁺ and Fe³⁺ concentrations in the cleaning solution serve as monitoring indicators. Sample and test every 30 minutes; when two consecutive test results show Ca²⁺ concentration no longer rising (change < 5%) and total Fe³⁺ concentration stabilizing, this indicates the scale layer has essentially dissolved completely. Circulate for an additional 30–60 minutes to confirm the endpoint, then conclude the chemical cleaning stage. Endpoint determination is more scientific than fixed cleaning time, avoiding both insufficient and excessive cleaning.

Safety Isolation: The greatest risk in online cleaning is cleaning solution leakage into the process side. Strict isolation verification must be performed before cleaning: close all valves connected to the process system and install blinds; perform a hydrostatic test on the cleaning loop (test pressure at 1.5 times cleaning working pressure); install pH online monitoring and conductivity alarm devices at the process side outlet. If pH < 5 or abnormal conductivity increase is detected on the process side, immediately stop the cleaning pump and initiate the emergency draining procedure.

4. Applicable Conditions and Contraindications

Applicable Conditions: Cases where the scale layer is primarily carbonate scale (> 70% proportion) and silicate content is < 10% are most suitable for Sulfamic Acid online cleaning. Equipment materials can be carbon steel or stainless steel, with tube walls showing no evident corrosion perforation or severe thinning (remaining wall thickness > 60% of original). Equipment operating parameters should be stable, capable of maintaining the temperature and flow conditions required for cleaning.

Contraindications and Alternative Solutions: For scale layers with silicate scale proportion > 30%, Sulfamic Acid is nearly ineffective — switch to an Ammonium Bifluoride (NH₄HF₂) complexing cleaning system. For copper tube heat exchangers, replace BTA with an MBT (2-Mercaptobenzothiazole) inhibition system, as the film formed by BTA on copper surfaces lacks sufficient stability under high-velocity scouring. Equipment with existing tube wall perforation or wall thickness reduction > 40% must first be shut down for repair before scheduling cleaning. Equipment with severely fluctuating operating parameters (frequent start-stop, unstable temperature and pressure) is not suitable for online cleaning. Additionally, for equipment with stress corrosion cracking risk (such as austenitic stainless steel heat exchangers operating long-term in Cl⁻-containing environments), trace Cl⁻ in Sulfamic Acid may induce stress corrosion, requiring prior material assessment and sensitivity testing.

Special Case Handling: When scale layer thickness exceeds 3 mm, a single online cleaning may not achieve complete removal — after the outer scale layer detaches, the exposed area of the inner scale layer increases, and Ca²⁺ concentration may exhibit a "secondary rise" phenomenon. In such cases, staged cleaning should be performed: after the first stage reaches Ca²⁺ concentration stabilization, discharge a portion of the waste solution and replenish with fresh cleaning agent for the second stage. The total duration of both stages is approximately 8–12 hours, achieving thorough removal of scale layers exceeding 3 mm thickness.

5. Typical Engineering Case Study

A chemical enterprise installed a DN50/DN80 double-pipe heat exchanger at a thermal oil-to-process medium heat exchange station, with carbon steel inner tube material, operating pressure 2.5 MPa, temperature 180°C/120°C (hot side/cold side). After 2 years of continuous operation, the outlet temperature dropped from the design value of 145°C to 118°C, unable to meet downstream process requirements. Shutdown inspection revealed: 1–2 mm thick grayish-white hard scale layer on the inner tube outer wall, with the most severe blockage at the two 180° bend locations in the annular space, where the flow passage cross-sectional area was reduced by approximately 40%.

Scale sample XRD analysis showed: Calcium Carbonate 78%, Iron Oxide 15%, Magnesium Silicate and organics 7%. This was a typical carbonate-dominant composite scale layer suitable for Sulfamic Acid system cleaning. Since this heat exchanger was a critical station in a continuous production line, a 3-day shutdown would halt four upstream and downstream process steps, with direct production loss estimated to exceed CNY 250,000.

Danyang Blue Star Cleaning developed an online cleaning plan without shutdown: temporary circulation piping (DN40 stainless steel hose) was connected at the heat exchanger inlet/outlet flanges, using a composite cleaning solution of Sulfamic Acid 6% + Citric Acid 1.5% + BTA 0.2% + Sodium Molybdate 0.08%, maintaining 55±3°C using the equipment's own operating temperature, circulation pump velocity 0.8 m/s, forward/reverse alternating circulation (switching direction every 30 minutes). Total cleaning duration was 6.5 hours, with endpoint determined when Ca²⁺ concentration change was < 3% over two consecutive tests. After cleaning, the heat exchanger outlet temperature recovered to 143°C (98.6% of design value), with production uninterrupted throughout. Cleaning cost was approximately CNY 6,000 (including chemicals CNY 2,200 + labor/equipment CNY 3,000 + monitoring CNY 800) — a mere fraction of the shutdown loss.

This case fully demonstrates that for double-pipe heat exchangers in continuous production chemical plants, online chemical cleaning is not only technically feasible but also highly economical — a CNY 6,000 cleaning investment avoided CNY 250,000 in production loss, achieving an investment return ratio exceeding 1:40. Subsequently, the enterprise incorporated all double-pipe heat exchangers into an annual online cleaning plan, and over 3 years, no unplanned shutdowns due to heat exchanger blockage have occurred.

6. Economic Benefits of Online Cleaning

Compared with traditional shutdown cleaning, the true value of online cleaning lies not in chemical cost savings (both are similar, approximately CNY 2,000–3,000), but in avoiding production loss. For continuous production enterprises, an unplanned shutdown of a single heat exchanger can lead to: upstream/downstream process waiting (2–5 steps affected), product batch scrapping or downgrading, restart commissioning time (4–8 hours), and contractual delivery delay breach risks. Comprehensive assessment shows that the total economic loss from a 3-day double-pipe heat exchanger shutdown typically ranges from CNY 50,000 to 250,000, far exceeding the cleaning project cost itself.

For chemical plant sections equipped with multiple double-pipe heat exchangers, establishing a preventive online cleaning plan is recommended: schedule routine cleaning every 12–18 months, maintaining equipment heat transfer efficiency consistently above 90%, while establishing cleaning maintenance records to track each unit's scale growth rate and cleaning intervals for scientifically optimized maintenance scheduling. From a full lifecycle management perspective, preventive online cleaning can extend equipment overhaul cycles from 3–5 years to 6–8 years, reducing total lifecycle maintenance costs per unit by 30%–40%.

7. Quality Acceptance Standards

After online cleaning completion, the following acceptance items should be performed: ① Equipment outlet temperature recovered to ≥ 95% of design value; ② Overall heat transfer coefficient K-value recovered to ≥ 90% of initial value; ③ Coupon corrosion rate < 1.0 g/(m²·h) (carbon steel), compliant with GB/T 25146 standard; ④ Cleaning waste liquid after neutralization treatment with F⁻ concentration < 10 mg/L, pH 6–9 before compliant discharge; ⑤ Provide complete cleaning project report including before/after data comparison, chemical usage records, and monitoring curves.

After acceptance, it is recommended to set a heat transfer efficiency warning value in the operating parameter monitoring system — when efficiency drops by more than 15%, trigger a reminder to promptly schedule the next online cleaning, avoiding passive treatment after severe efficiency degradation. This proactive maintenance model truly achieves the "preventive treatment" philosophy of equipment management — using small, regular cleaning investments to eliminate costly emergency shutdowns and equipment damage risks.

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