Fluoride Application in Lithium Bromide Absorption Chiller Cavity Cleaning

I. Overview of Lithium Bromide Absorption Chillers and Cavity Scaling Mechanisms

The lithium bromide absorption chiller is a core piece of equipment in large central air conditioning systems and industrial refrigeration. It uses water as the refrigerant and lithium bromide solution as the absorbent, driven by thermal energy to execute the refrigeration cycle. The unit primarily consists of a generator, condenser, evaporator, absorber, and solution heat exchangers, with heat transfer achieved through tube bundles in each cavity. Due to the slow corrosive effect of LiBr solution on carbon steel and copper alloys at high temperatures, combined with scale deposition on the cooling water and chilled water sides, multi-layer composite scale layers form on the internal cavity walls and heat exchanger tube surfaces after long-term operation.

Deposits inside LiBr chiller cavities typically consist of three types of substances: First, water-side inorganic salt scale, primarily calcium carbonate, calcium sulfate, and magnesium silicate, originating from the concentration and precipitation of calcium and magnesium ions in circulating cooling water, with thermal conductivity only 1/30 to 1/50 that of carbon steel — the primary heat transfer obstacle. Second, corrosion products, including iron oxide (Fe₃O₄/Fe₂O₃) and copper oxide, generated by slow corrosion of metals by LiBr solution at high temperatures, appearing as black or reddish-brown loose adherent layers. Third, solution residues, including LiBr crystalline salts, lithium chromate corrosion inhibitor decomposition products, and microbial slime.

Measured data shows: when scale thickness on heat exchanger tube surfaces reaches 0.5mm, cooling capacity decreases by 8%–12%; at 1mm, cooling capacity drops by 15%–20% and energy consumption increases by 20%–30%; above 2mm, the unit experiences frequent high-pressure alarms, crystallization faults, and even forced shutdowns. Furthermore, under-deposit corrosion can cause localized metal perforation, severely shortening unit life. While traditional hydrochloric acid cleaning is effective against carbonate scale, it is nearly ineffective against silicate scale and exacerbates corrosion risks for copper tubes and stainless steel components in LiBr units. Fluoride cleaning agents, with their unique complexing scale dissolution capability, have become a critical technical approach for LiBr chiller cavity cleaning.

From a heat transfer perspective, the typical thermal conductivity values of deposits are: calcium carbonate scale 0.5–2.3 W/(m·K), calcium sulfate scale 0.6–2.3 W/(m·K), silicate scale 0.3–1.0 W/(m·K), and iron oxide corrosion products 0.6–1.2 W/(m·K), compared to copper heat exchanger tube thermal conductivity of 380–400 W/(m·K) and carbon steel at 45–55 W/(m·K). This means that just 1mm of silicate scale creates thermal resistance equivalent to over 300mm of copper tube wall thickness. For a unit with a cooling capacity of 2 million kcal/h, the energy cost increase from scaling can amount to hundreds of thousands of RMB annually in electricity or steam costs, far exceeding the investment in regular cleaning.

II. Complex Dissolution Mechanism of Fluoride Cleaning Agents

The core active ingredient of fluoride cleaning agents is fluoride ion (F⁻), whose mechanism of action during cleaning differs fundamentally from conventional acid cleaning. Conventional acid cleaning (such as hydrochloric acid, sulfuric acid) relies on H⁺ neutralizing carbonate scale: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂↑. This dissolution mechanism has limited effectiveness on silicate scale and iron oxide scale, because silicon dioxide and iron oxide have very low solubility in conventional acids.

The unique advantage of fluoride ion lies in its powerful coordination complexing ability. F⁻ has a small ionic radius (133pm) and high charge density, enabling it to form stable coordination complexes with various metal ions:

Silicate Scale Dissolution — Fluorosilicate Complexation Reaction: Silicon dioxide reacts with fluoride ion: SiO₂ + 6F⁻ + 4H⁺ → [SiF₆]²⁻ + 2H₂O. The resulting fluorosilicate ion [SiF₆]²⁻ has extremely high solubility in water, far superior to any acid dissolution product. This reaction is the core advantage that differentiates fluoride cleaning agents from conventional acid cleaning — the ability to thoroughly dissolve stubborn silicate scale layers into soluble complexes. For complex silicate scales such as magnesium silicate and calcium silicate, fluoride ion similarly achieves efficient dissolution by breaking Si-O-Si and Si-O-M (M=Ca/Mg) bonds.

Iron Oxide Scale Dissolution — Fluoroferrate Complexation Reaction: Iron oxide reacts with fluoride ion under acidic conditions: Fe₂O₃ + 12F⁻ + 6H⁺ → 2[FeF₆]³⁻ + 3H₂O. The resulting hexafluoroferrate complex ion [FeF₆]³⁻ has a stability constant as high as 10¹⁶, remaining completely undissociated in solution, effectively preventing Fe³⁺ re-precipitation. Compared to conventional acid cleaning, fluoride complex dissolution is 2–3 times faster and produces no loose Fe³⁺ hydrolysis products.

Synergistic Solubilization Effect: In fluoride-containing cleaning solution systems, F⁻ and H⁺ exhibit significant synergistic solubilization. H⁺ first disrupts the crystal lattice structure of carbonate scale, exposing the encapsulated silicate and iron oxide components; F⁻ then complex-dissolves these exposed components. This "acid dissolution + complexation" dual mechanism gives fluoride cleaning agents excellent removal effectiveness on the multi-layer composite scale commonly found in LiBr chillers, simultaneously removing carbonate scale, silicate scale, and iron oxides in a single cleaning operation, eliminating the complexity of staged cleaning.

Comparative Advantages Over Conventional Acid Cleaning: The following table compares the dissolution capabilities of typical cleaning agents for scale layers commonly found in LiBr chillers:

The table clearly shows that single cleaning agents each have their shortcomings — hydrochloric acid and sulfamic acid cannot handle silicate scale, and citric acid is insufficient for carbonate scale. Only a compound formula based on ammonium bifluoride can comprehensively address all three scale types, and when combined with corrosion inhibitors, reduces corrosion to safe levels, making it the optimal technical solution for LiBr chiller cavity cleaning.

III. Fluoride Cleaning Agent Types and Formula Selection

The fluoride cleaning agents commonly used in industrial cleaning today mainly include the following types, with significant differences in scale dissolution capability, corrosivity, and operational safety:

Ammonium Bifluoride (NH₄HF₂) — Primary Cleaning Agent of Choice: White crystalline solid, readily soluble in water, hydrolyzing to produce HF and NH₄F, providing sustained release of F⁻ and H⁺. Usage concentration 3%–6%, solution pH 3.5–4.5, belonging to a mildly acidic system. Advantages include mild acidity, with corrosion rates on copper alloys and stainless steel of 0.3–0.8 g/(m²·h) and 0.1–0.3 g/(m²·h) respectively, far below hydrochloric acid's 3–8 g/(m²·h). Solid at ambient temperature for convenient storage and transport, with safe and convenient on-site preparation.

Fluoroboric Acid (HBF₄) — Auxiliary Enhancer: Colorless transparent liquid, commercially available typically at 40%–50% concentration. Adding 1%–3% to the cleaning solution significantly enhances the dissolution rate of silicate scale. HBF₄ slowly hydrolyzes in aqueous solution to release F⁻: HBF₄ + 3H₂O → H₃BO₃ + 4HF, forming a long-acting fluoride ion sustained-release system. Particularly suitable for treating stubborn scale with silicate content exceeding 30%.

Compound Formula Design Principles: Based on the composite scale characteristics of LiBr chiller cavities, the following compound formula system is recommended:

Strictly Prohibited: Pure hydrofluoric acid (HF), high-concentration hydrochloric acid (>5%), sulfuric acid (H₂SO₄), strong alkaline solutions (pH>12). Although hydrofluoric acid has strong scale dissolution capability, it is highly toxic to humans and excessively corrosive to metals; its use in LiBr chiller cleaning is absolutely prohibited.

IV. Corrosion Inhibitor System Design — Key to Protecting Copper Tubes and Stainless Steel Components

LiBr chiller cavities contain large quantities of copper tubes (absorber, condenser, and evaporator heat exchanger tubes) and stainless steel components (solution pumps, valve bodies). Corrosion inhibition protection is critical to the success of fluoride cleaning. Pure fluoride cleaning solutions can corrode copper alloys at rates of 1.5–3 g/(m²·h), exceeding the national standard GB/T 25146-2010 safety limit of ≤1.0 g/(m²·h). Corrosion rates must be controlled within safe limits through corrosion inhibitor compounding.

BTA — Preferred Copper Alloy Corrosion Inhibitor: BTA is a nitrogen-containing heterocyclic compound that self-assembles on copper surfaces via Cu-N coordination bonds to form a dense polymeric protective film [Cu(I)BTA]n. This film is approximately 5–10nm thick and highly hydrophobic (water contact angle >90°), effectively blocking diffusion of F⁻ and H⁺ toward the copper substrate. In fluoride cleaning solution systems, 0.2% BTA can reduce copper alloy corrosion rates from 2.5 g/(m²·h) to below 0.3 g/(m²·h), achieving over 88% inhibition efficiency. Note that BTA undergoes thermal decomposition and loses effectiveness at temperatures above 60°C, so the operating temperature of fluoride cleaning solutions must be strictly controlled below 50–55°C.

Sodium Molybdate — Anodic Passivation Protection: Sodium molybdate is an anodic inhibitor that promotes the formation of a MoO₄²⁻/Fe₂O₃ composite passive film on metal surfaces. This film has self-healing capability — when the passive film is locally damaged, free MoO₄²⁻ in solution migrates to the damaged site and re-participates in film formation. In fluoride systems, sodium molybdate's passivation effect is not disrupted by F⁻ because MoO₄²⁻ has much stronger chemisorption affinity for metal surfaces than the competitive adsorption of F⁻.

Urotropine — Secondary Protection for Carbon Steel Substrate: Urotropine (hexamethylenetetramine) hydrolyzes in acidic media to produce formaldehyde and ammonia; formaldehyde reduces on metal surfaces to form an adsorbed protective layer while neutralizing excess acid in localized micro-zones. Adding 0.10%–0.15% urotropine to fluoride cleaning solutions can additionally reduce carbon steel corrosion rates by 40%–50%, particularly suited for protecting unit shells and tube sheets made of carbon steel.

Corrosion Inhibitor Synergy: The ternary compound system of BTA + sodium molybdate + urotropine exhibits significant positive synergistic effects in fluoride cleaning solutions. Coupon test data shows: single BTA inhibition efficiency 88%, BTA + sodium molybdate binary compound improves to 93%, and ternary compound achieves 96% or higher, with copper alloy corrosion rates reduced to 0.15–0.25 g/(m²·h) and carbon steel corrosion rates to 0.3–0.5 g/(m²·h), fully meeting national standard requirements.

Critical Effect of Temperature on Corrosion Inhibition: In fluoride cleaning operations, temperature control is a sensitive parameter affecting inhibition effectiveness. When cleaning solution temperature exceeds 60°C, the adsorption equilibrium constant of BTA molecules on copper surfaces drops sharply, with protective film coverage declining from over 95% at 50°C to below 60% at 65°C, causing a corresponding sharp decline in inhibition efficiency. Additionally, sodium molybdate accelerates hydrolysis above 65°C, reducing passive film density. Therefore, in engineering practice, cleaning temperature is strictly controlled within the 48–55°C range, ensuring sufficient chemical reaction rates (for every 10°C increase, scale dissolution rate increases approximately 1.5–2×) while maintaining corrosion inhibitor protection system effectiveness. When ambient temperature is high or cleaning pump friction causes solution temperature rise, temperature must be controlled within safe limits via heat exchangers or intermittent shutdown.

V. Complete Fluoride Cleaning Process Flow for LiBr Chiller Cavities

Stage 1 — Shutdown Inspection and System Preparation: Switch the unit to shutdown state, close steam valve and cooling water/chilled water inlet/outlet valves, and completely drain residual LiBr solution from the unit into a storage tank. Conduct a comprehensive visual inspection of the unit, measuring and recording pre-cleaning operating parameters such as heat exchange temperature differential and condensing pressure as baseline references. Set up a temporary cleaning circulation system, including a fluoride-resistant plastic cleaning pump (head ≥20m, flow ensuring heat exchanger tube velocity ≥0.5m/s), PE cleaning solution storage tank (volume ≥1.2× unit water capacity), and acid-resistant hoses and valves. All circulation loop metal components must be 316L stainless steel or PP/PE material; carbon steel or copper alloys must not directly contact acidic cleaning solutions.

Stage 2 — Fresh Water Flushing and Leak Test: Circulate industrial fresh water through the entire cleaning circuit for 30 minutes, then drain the flush water. The purpose of flushing is to remove loose deposits and residual LiBr solution from the cavity, preventing these from consuming subsequent cleaning agents. After flushing, perform a system hydrostatic test by filling with water to 0.3–0.4 MPa and holding for 30 minutes, checking all flanges, valves, and hose connections for leaks.

Stage 3 — Alkaline Degreasing (As Needed): If significant oil or organic contamination is present on the cavity interior walls, perform alkaline cleaning first. Prepare an alkaline cleaning solution containing 0.5%–1% sodium hydroxide and 0.2%–0.3% non-ionic surfactant, circulate at 55–65°C for 2–4 hours to thoroughly remove grease and organic deposits. After alkaline cleaning, repeatedly rinse with fresh water until effluent pH ≤8.

Stage 4 — Fluoride Chemical Cleaning: Prepare the fluoride cleaning solution according to the formula. Addition sequence: first fill with clean water to 80% of system volume, then sequentially add calculated amounts of sulfamic acid, citric acid, corrosion inhibitors (BTA, sodium molybdate, urotropine), stir to dissolve uniformly, and finally add ammonium bifluoride. Adjust cleaning solution pH to 3.5–4.5, control temperature at 48–55°C, and circulate for 6–10 hours. During cleaning, sample and analyze the following indicators every 30 minutes:

When Fe³⁺ and Ca²⁺ concentrations stabilize over two consecutive tests (fluctuation ≤5%) and cleaning solution color no longer deepens, the cleaning endpoint is reached. Drain spent acid solution into a dedicated waste liquid collection container.

Stage 5 — Rinsing and Neutralization: Repeatedly rinse the system 2–3 times with fresh water, each cycle 15–20 minutes, until effluent pH ≥6.0. Then circulate 0.3%–0.5% sodium carbonate solution for 30–45 minutes to thoroughly neutralize residual acid. Finally, rinse with fresh water until effluent conductivity matches the supply water (deviation ≤50μS/cm).

Common Issues and Solutions During Cleaning:

Issue 1 — Rapid pH Rise in Cleaning Solution: If pH rises rapidly from 4.0 to above 5.5 within 1 hour of circulation start, this indicates high carbonate content in the scale layer with high acid consumption. Response: supplement ammonium bifluoride and sulfamic acid by 1%–2% each, re-adjust pH to 3.5–4.5, and appropriately extend cleaning time by 2–4 hours.

Issue 2 — Abnormally Elevated Iron Ion Concentration: If Fe³⁺ concentration exceeds 5000 mg/L after 2 hours of cleaning and continues to rise, this may indicate insufficient corrosion inhibitor concentration or localized acid short-circuiting. Response: immediately supplement BTA to 0.3% and sodium molybdate to 0.15%, reduce cleaning temperature to below 45°C, and inspect circulation piping to ensure uniform solution distribution.

Issue 3 — Turbidity or Precipitation in Cleaning Solution: White flocculent precipitate appearing late in cleaning is typically calcium fluoride formation (reaction of F⁻ with excess Ca²⁺). Response: small amounts of white flocculent do not affect cleaning effectiveness; if heavy precipitation blocks piping, drain the cleaning solution, rinse with fresh water, and prepare fresh solution to continue cleaning.

Stage 6 — Passivation Treatment: Prepare 1%–2% trisodium phosphate or 0.5%–1% sodium nitrite passivation solution, circulate at 50–60°C for 2–4 hours to form a uniform, dense phosphate or passive film on metal surfaces. After passivation, drain the passivation solution and perform a quick fresh water rinse. Post-passivation unit interior surfaces should exhibit a uniform silver-gray or light gray appearance with no secondary rust spots.

VI. Safety Protection and Environmental Waste Treatment

Personnel Safety Protection: Fluoride cleaning operators must wear neoprene or PVC chemical-resistant gloves, full-face acid-resistant face shields (or safety goggles + acid-resistant masks), acid-resistant work clothing, and acid-resistant rubber boots throughout the operation. Ammonium bifluoride solid dust is irritating to the respiratory tract; solid chemical preparation must be conducted in well-ventilated areas with dust masks. Emergency eyewash stations and safety showers must be available on-site, with clear safety warning signs and evacuation routes posted.

Waste Liquid Environmental Treatment: Fluoride-containing cleaning waste liquid must never be discharged directly. Treatment proceeds in two steps:

Step 1 — Fluoride Ion Precipitation: Add excess calcium chloride solution or lime milk (calcium hydroxide) to the waste liquid to convert F⁻ into insoluble calcium fluoride precipitate: 2F⁻ + Ca²⁺ → CaF₂↓. Each gram of fluoride ion theoretically requires 1.47g Ca(OH)₂; in practice, dose at 1.5–2.0× the theoretical amount, stir and react for 30–60 minutes, then allow to settle. Supernatant F⁻ concentration should drop below 10 mg/L (GB 8978-1996 Class I standard).

Step 2 — Comprehensive Treatment: Adjust the supernatant pH to 6–9 after precipitation, then combine with other cleaning waste liquids for disposal by qualified hazardous waste treatment facilities. Calcium fluoride precipitate sludge is dewatered by filter press and disposed as solid waste. Waste liquids containing BTA and sodium molybdate must not be discharged to biological treatment systems, as BTA inhibits activated sludge microorganisms.

VII. Engineering Case Analysis

Case Background: In August 2025, a large chemical fiber enterprise in Zhejiang experienced severe cooling capacity degradation in a 2 million kcal/h LiBr absorption chiller. Operating records showed: chilled water outlet temperature had risen from the normal 7°C to 12°C, condensing pressure was 25% above normal, unit energy consumption had increased 35% year-over-year, and solution crystallization alarms occurred frequently. Upon shutdown and cover opening inspection, the absorber and condenser heat exchanger tube outer walls were covered with 1.5–2.5mm thick off-white hard scale, and the evaporator water chamber had substantial reddish-brown corrosion product accumulation.

Scale Sample Analysis: XRD and chemical analysis of scale samples showed composition: calcium carbonate approximately 58%, magnesium silicate approximately 22%, iron oxide approximately 12%, organics and LiBr crystalline salts approximately 8%. With silicate content as high as 22%, conventional hydrochloric acid cleaning could not effectively remove it.

Cleaning Solution: Danyang Blue Star Cleaning developed a fluoride compound cleaning solution based on scale sample analysis results. Cleaning solution formula: ammonium bifluoride 5% + sulfamic acid 4% + citric acid 1.5% + BTA 0.2% + sodium molybdate 0.08% + urotropine 0.12%, pH 4.0–4.3, temperature 50–53°C, circulation cleaning for 8 hours. During cleaning, Fe³⁺ and Ca²⁺ concentrations were monitored every 30 minutes; both indicators stabilized after 6.5 hours, and circulation continued for 1.5 hours to confirm endpoint before concluding chemical cleaning.

Cleaning Results: Post-cleaning cover opening inspection showed heat exchanger tube surfaces were clean, exposing bare metal. Silicate scale and iron oxides were completely removed. Copper tube corrosion rate from coupon testing was 0.21 g/(m²·h), far below the standard limit. After the unit resumed operation, chilled water outlet temperature recovered to 7°C, condensing pressure decreased by 22%, energy consumption decreased by 30%, cooling capacity recovered to 97% of rated value, and all unit operating parameters returned to normal.

Economic Analysis: The total cost of this fluoride cleaning project (including chemicals, labor, equipment rental, and waste liquid treatment) was approximately RMB 38,000. After cleaning, the unit's cooling capacity was restored. Based on 5,000 operating hours per year and an industrial electricity price of RMB 0.8/kWh, annual electricity savings amount to approximately RMB 185,000, yielding an investment payback period of only 2.5 months. Additionally, production losses from unplanned shutdowns due to continuing unit efficiency deterioration were avoided. This case fully illustrates: for composite scale layers with high silicate content, the fluoride compound cleaning solution is not only technically feasible but also economically significant, making it the optimal choice for deep cleaning maintenance of LiBr chillers.

VIII. Quality Acceptance Standards

Quality acceptance of LiBr chiller fluoride cleaning should include the following indicators:

① Visual Inspection: Heat exchanger tube surfaces clean, free of residual scale and corrosion spots; copper tubes exhibit uniform metallic luster; stainless steel components show no pitting traces.

② Residual Scale Thickness: Select no fewer than 5 representative points and measure residual scale thickness with a thickness gauge: ≤0.1mm.

③ Corrosion Rate: Coupon corrosion rates ≤1.0 g/(m²·h) (copper alloys) and ≤2.0 g/(m²·h) (carbon steel), compliant with GB/T 25146-2010 standard.

④ Operating Parameter Recovery: Chilled water outlet temperature recovers to within ±1°C of design value; condensing pressure decreases ≥15% from pre-cleaning level; cooling capacity recovers to ≥95% of rated value.

⑤ Surface Passivation Quality: Copper sulfate spot test ≥30 seconds without significant displacement reaction, indicating a complete and dense passive film. Test method: apply 1–2 drops of acidic copper sulfate solution (CuSO₄·5H₂O 41g/L + HCl 13mL/L) to the passivated metal surface and observe whether reddish-brown metallic copper displacement precipitation appears beneath the droplet. No precipitation reaction within 30 seconds = pass.

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