I. Central AC System Structure and Scaling Mechanisms

A central air conditioning system consists of the refrigeration main unit (LiBr absorption chiller or electric-driven centrifugal/screw chiller), cooling water circulation system, chilled water circulation system, and terminal air handling equipment. Cooling water absorbs refrigerant heat in the condenser and is recirculated after heat dissipation via cooling towers. Chilled water is cooled in the evaporator and then delivered to terminals for cooling supply.

Cooling water operates in an open loop, continuously evaporating and concentrating, causing Ca²⁺, Mg²⁺, and HCO₃⁻ concentrations to steadily increase. When CaCO₃ saturation exceeds the Langelier Saturation Index (LSI>0), CaCO₃ crystallizes and precipitates on the inner walls of condenser heat exchange tubes, forming scale. Meanwhile, cooling towers exposed to the outdoor environment allow dust and microbial spores to continuously enter the system, forming biofilm on heat exchange surfaces.

Chilled water operates in a closed loop, where the primary issue is carbon steel pipeline corrosion caused by dissolved oxygen. The resulting corrosion products (Iron Oxides, Fe₂O₃/Fe₃O₄) deposit on evaporator tube bundle surfaces, reducing heat transfer efficiency.

II. Condenser Chemical Cleaning — Cleaning Agent Selection

Condenser heat exchange tube materials are typically copper (B10/B30 copper-nickel alloy or TP2 phosphorus-deoxidized copper). Chemical cleaning must select a cleaning agent system with controllable copper corrosion.

2.1 Preferred Solution: Sulfamic Acid + Citric Acid Combined Cleaning

Sulfamic Acid (NH₂SO₃H) is the preferred primary agent for copper tube condenser cleaning, at a concentration of 5%–8%. It rapidly dissolves CaCO₃ scale, with a copper corrosion rate only 1/8 to 1/10 that of HCl, and contains no Cl⁻, eliminating the risk of chloride pitting on stainless steel components (such as condenser water chamber linings).

Citric Acid (C₆H₈O₇) serves as an auxiliary cleaning agent at 2%–3%, dissolving iron oxides through chelation while offering the environmental advantage of natural biodegradability.

Corrosion Inhibitor System: BTA (Benzotriazole) 0.15%–0.25% + Sodium Molybdate (Na₂MoO₄) 0.05%–0.10%. BTA is a specific copper corrosion inhibitor, forming a Cu-BTA complex protective film on copper surfaces through the nitrogen atom lone pair electrons. Sodium Molybdate acts as an anodic corrosion inhibitor, promoting the formation of a stable oxide film on metal surfaces.

2.2 Heavy Scale Solution: HCl + Urotropine System

For heavily scaled condensers with scale thickness ≥2mm, Hydrochloric Acid (HCl) 5%–7% combined with Urotropine (Hexamethylenetetramine) 0.3% corrosion inhibitor can be used for ambient-temperature circulation cleaning. HCl offers the fastest descaling speed, but cleaning time (4–6 hours) and inhibitor concentration must be strictly controlled. After cleaning, immediately neutralize with 0.5% Na₂CO₃ solution to prevent acid residue-induced copper tube corrosion.

2.3 Stainless Steel Condenser Specialized Solution

Stainless steel tube (TP304/TP316) condensers can be cleaned using Nitric Acid (HNO₃) 5%–10% with Lan-826 0.3% corrosion inhibitor. HNO₃ not only dissolves scale but also simultaneously passivates the stainless steel surface, forming a dense Cr₂O₃ passive film. Cleaning media containing Cl⁻ are strictly prohibited.

III. Evaporator and Chilled Water System Cleaning

Evaporator heat exchange tube materials are primarily copper and carbon steel. The main contaminants on the chilled water side are iron oxides and small amounts of oil (from possible trace compressor oil leakage). It is recommended to use Sodium Hydroxide (NaOH) 1%–2% + Sodium Carbonate (Na₂CO₃) 1% for alkaline degreasing cleaning at 50–60°C, circulating for 4–6 hours. Then rinse with clean water to neutral, followed by Citric Acid 3% + EDTA (Ethylenediaminetetraacetic Acid) 1% for acid cleaning to remove iron, with pH controlled at 3.5–4.5.

IV. Cooling Water System Quality Stabilization Treatment

After cleaning, a water quality stabilization treatment program must be established to prevent rapid re-scaling and corrosion:

  • Scale Inhibitor: HEDP (1-Hydroxyethylidene-1,1-Diphosphonic Acid) 5~10mg/L + Polyacrylate Dispersant 3~5mg/L, preventing CaCO₃ crystallization and deposition through lattice distortion and dispersion effects
  • Corrosion Inhibitor: Zinc Sulfate (ZnSO₄) 2~3mg/L (cathodic type) + Sodium Molybdate 3~5mg/L, synergistically controlling carbon steel and copper alloy corrosion
  • Biocide: Isothiazolinone 50~100mg/L, shock dosing, controlling total heterotrophic bacteria in cooling water to ≤1×10⁵ CFU/mL
  • Biodispersant: Quaternary ammonium salt dispersant, stripping biofilm from tube walls

V. Special Maintenance for LiBr Absorption Chillers

In addition to routine heat exchange tube cleaning, LiBr chillers require the following specialized maintenance:

  • LiBr Solution Regeneration: After long-term operation, LiBr solution absorbs CO₂ and corrosion products, causing pH decline and performance degradation. Use Ion Exchange Resin or Electrodialysis technology to regenerate and restore performance
  • Internal Cavity Cleaning: Chemical cleaning of the unit's internal cavities (absorber/generator shells) to remove corrosion products, using Citric Acid 3%–5% + BTA 0.1% circulating cleaning, temperature ≤60°C, protecting copper tubes and carbon steel shells
  • Vacuum Maintenance: After cleaning, evacuate to an absolute pressure ≤6 mmHg to ensure unit thermal efficiency

VI. Typical Engineering Case Study

In summer 2025, a company's LiBr absorption chiller (cooling capacity 2 million kcal/h) experienced severe cooling water-side scaling, with condensing pressure rising to 0.09 MPa (normal value 0.06 MPa) and cooling capacity decreasing by approximately 30%. After on-site inspection by Blue Star Cleaning's technical team, the copper tube condenser was cleaned using Sulfamic Acid 6% + Citric Acid 3% + BTA 0.2% + Sodium Molybdate 0.08% formulation at 40–50°C circulating for 6 hours. The evaporator chilled water side received NaOH 1.5% + Na₂CO₃ 1% alkaline degreasing followed by Citric Acid 3% acid cleaning for iron removal. After cleaning, condensing pressure recovered to 0.062 MPa and cooling capacity recovered to 97% of design. The client simultaneously established a HEDP + Zinc Sulfate + Isothiazolinone water quality stabilization treatment program, with no scaling issues after one year of operation.

Danyang Blue Star Anti-corrosion Cleaning Co., Ltd. — Professional Central AC Chemical Cleaning & Water Quality Stabilization

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