Sodium Chloride Brines

Sodium chloride (NaCl, common table salt) aqueous solutions serve as economical secondary coolants for moderate-temperature refrigeration applications. While offering lower cost than glycol solutions, NaCl brines present higher corrosivity and limited freeze protection compared to calcium chloride or glycol alternatives.

Physical Chemistry of NaCl Solutions

Sodium chloride dissolves in water through ionic dissociation: NaCl → Na⁺ + Cl⁻. The dissolved ions disrupt ice crystal formation, depressing the freezing point through colligative properties proportional to ion concentration.

Molecular interactions: Sodium and chloride ions hydrate with water molecules, requiring energy to remove water from the ionic solvation shells during freezing. This energy barrier lowers the chemical potential of liquid water relative to ice, depressing the freezing point.

Solubility limit: NaCl exhibits maximum solubility of approximately 36% by weight at 20°C. Concentrations approaching saturation risk precipitation during temperature excursions or evaporation.

Eutectic Composition and Freeze Characteristics

The NaCl-water system exhibits a eutectic point representing the lowest achievable freezing temperature:

Eutectic composition: 23.3% NaCl by weight at -21.1°C (-6°F). At this concentration, ice and NaCl·2H₂O (hydrohalite) crystals form simultaneously.

Below eutectic concentration: Solutions freeze over a temperature range. Ice crystals form first, concentrating the remaining liquid until reaching eutectic composition at -21.1°C.

Above eutectic concentration: Salt hydrate crystals (NaCl·2H₂O) precipitate first upon cooling. The remaining liquid concentrates in water until reaching eutectic.

Practical systems operate at 20-22% concentration to approach eutectic freeze protection while maintaining safety margin against salt precipitation.

Ice Rink Applications

Sodium chloride brine represents the traditional secondary coolant for ice skating rinks due to economic advantages and adequate freeze protection for ice maintenance temperatures.

Operating conditions: Ice surface temperatures of -4 to -7°C (25-20°F) require brine circulation at -8 to -11°C (18-12°F). NaCl concentrations of 18-22% provide adequate freeze protection with operational safety margin.

System configuration: Ammonia (R717) refrigeration plants chill NaCl brine in flooded or direct expansion evaporators. The brine circulates through embedded steel pipes in the concrete ice floor slab, extracting heat to maintain ice temperature.

Thermal mass benefits: The ice rink concrete slab acts as thermal storage, buffered by brine circulation. The system maintains ice temperature during load fluctuations from skater activity, lighting, and ambient conditions.

Brine volume: Typical ice rinks contain 1500-3000 gallons of brine providing thermal inertia. Large volume dilutes contamination and provides reserve capacity for leak makeup.

Industrial Refrigeration Applications

Industrial cold storage and process cooling applications employ NaCl brines where moderate temperatures (-15°C and above) suffice:

Defrost water cooling: Fish processing and food plants chill defrost water recovery using NaCl brine heat exchangers before wastewater discharge.

Product cooling: Immersion cooling of packaged goods in NaCl brine tanks provides rapid heat removal. Brine temperature uniformity eliminates hot spots.

Cold storage distribution: Large cold storage facilities distribute refrigeration capacity via NaCl brine loops serving multiple rooms and equipment, centralizing the ammonia refrigeration plant in a dedicated machine room.

Limited Freeze Protection Range

NaCl brines provide adequate protection only to -21°C (-6°F) at eutectic concentration. Applications requiring lower temperatures necessitate calcium chloride brines (eutectic -55°C) or glycol solutions.

Temperature limitations:

• Eutectic at -21°C limits absolute minimum temperature
• Practical concentrations (20-22%) provide freeze protection to -16 to -18°C
• Safety margins required for cold spots and startup transients
• Not suitable for ultra-low temperature applications (-30°C and below)

Alternative brine selection: Applications requiring temperatures below -15°C typically employ calcium chloride brines, potassium formate, or propylene glycol instead of NaCl.

Corrosivity Concerns

Chloride ions aggressively attack most metals through several corrosion mechanisms:

General corrosion: Chlorides disrupt protective oxide layers on steel, aluminum, and stainless steel, accelerating uniform metal loss.

Pitting corrosion: Localized chloride attack penetrates passive films, creating deep pits that perforate piping and vessels. Pitting represents the primary failure mode in NaCl brine systems.

Crevice corrosion: Stagnant areas trap concentrated brine, creating oxygen-depleted zones that accelerate corrosion.

Stress corrosion cracking: Chlorides induce cracking in stressed austenitic stainless steels (300 series). Welded joints and cold-worked areas particularly vulnerable.

Corrosion rates: Uninhibited NaCl brines corrode carbon steel at rates exceeding 100 mils/year (2.5 mm/year). Proper inhibition reduces rates to 2-5 mils/year (0.05-0.13 mm/year).

Corrosion Inhibition

Effective corrosion control requires multi-component inhibitor packages and careful water chemistry management:

Historical chromate inhibitors: Sodium chromate (Na₂CrO₄) provided excellent corrosion protection but environmental and health concerns eliminated use in most jurisdictions.

Modern inhibitor systems:

• Sodium nitrite (NaNO₂): Anodic inhibitor passivating steel surfaces (typical 1500-3000 ppm)
• Sodium tetraborate (borax): pH buffer maintaining alkaline conditions (pH 9-10)
• Sodium silicate: Forms protective silicate films (100-200 ppm SiO₂)
• Sodium mercaptobenzothiazole (MBT): Copper corrosion inhibitor if copper alloys present

pH control: Maintain pH 9.0-10.5 for corrosion passivity. Lower pH accelerates corrosion; higher pH risks calcium/magnesium hydroxide precipitation if makeup water contains hardness.

Dissolved oxygen control: Minimize oxygen ingress through closed system design, vapor barriers on brine tanks, and nitrogen blanketing. Oxygen accelerates corrosion despite inhibitors.

Material Selection

Material choices must account for chloride aggressiveness:

Carbon steel: Acceptable with proper inhibition. Requires minimum wall thickness accounting for corrosion allowance. Schedule 40 pipe minimum; Schedule 80 preferred for threaded connections.

Stainless steel: Avoid 300-series austenitic grades due to stress corrosion cracking. Use 316L with temperature limitation or duplex grades (2205) for superior chloride resistance.

Copper alloys: Acceptable for heat exchangers with proper inhibitors. Admiralty brass (arsenical) resists dezincification. Cupro-nickel (90-10, 70-30) offers superior corrosion resistance.

Aluminum: Unacceptable. Chlorides rapidly pit aluminum, leading to perforation failure.

Plastics and FRP: PVC, CPVC, polypropylene, and FRP exhibit excellent chloride resistance. Limited by temperature and mechanical strength.

Gaskets and seals: Use chloride-resistant elastomers (EPDM, fluoroelastomers). Avoid natural rubber which degrades in brine.

Marine and Seawater Applications

Seawater (approximately 3.5% NaCl plus other dissolved salts) serves as naturally-occurring secondary coolant for coastal installations:

Once-through cooling: Power plants, LNG facilities, and marine terminals use seawater for condenser cooling with direct discharge after temperature rise.

Closed-loop seawater: Desalination plants and offshore platforms circulate treated seawater in closed loops, adding corrosion inhibitors and biocides.

Temperature limitations: Seawater freezes at approximately -2°C (28°F). Applications in cold climates require dilution with freshwater or alternative coolants.

Thermal Properties

NaCl brines exhibit property variations with concentration and temperature:

Specific heat: Decreases with increasing salt concentration. 23% NaCl solution: cp ≈ 3.1 kJ/(kg·K) compared to 4.18 kJ/(kg·K) for water. Requires 35% higher flow rates than water for equivalent capacity.

Density: Increases approximately linearly with concentration. 23% NaCl: ρ ≈ 1178 kg/m³. Higher density increases pump head requirements and affects sump sizing.

Viscosity: Increases with concentration and decreases with temperature. 23% NaCl at -10°C: μ ≈ 3.5 cP. Viscosity remains lower than equivalent glycol solutions.

Thermal conductivity: Decreases slightly with concentration. 23% NaCl: k ≈ 0.53 W/(m·K). Reduction less severe than glycol solutions.

System Design Considerations

NaCl brine systems require specific design provisions:

Materials of construction: Specify corrosion-resistant materials throughout. Carbon steel pipe acceptable with inhibition; stainless steel or plastic preferred for valves and fittings.

Corrosion monitoring: Install corrosion coupons and electrical resistance probes for continuous monitoring. Target corrosion rates below 2 mils/year.

Brine makeup: Provide deionized or softened makeup water. Hard water introduces calcium and magnesium that consume inhibitors and form scale.

Strainers and filtration: Install Y-strainers at pump suctions. Cartridge filters (25-50 micron) remove corrosion products extending component life.

Expansion accommodation: Size expansion tanks for thermal expansion and contraction. Brine coefficient of expansion approximately 0.0004/°C.

Leak detection: Chlorides aggressively attack building materials and equipment. Leak detection and containment critical for long-term facility protection.

Maintenance Requirements

NaCl brine systems demand regular testing and chemical treatment:

Nitrite testing: Monthly minimum. Maintain 1500-3000 ppm sodium nitrite reserve. Replenish as depleted by oxidation and corrosion reactions.

pH monitoring: Weekly testing. Adjust pH to 9.0-10.5 range using sodium hydroxide or additional borax buffer.

Density/concentration: Monthly verification using hydrometer. Concentration drift indicates water loss (concentration increases) or dilution from leaks (concentration decreases).

Corrosion rate monitoring: Quarterly inspection of corrosion coupons. Weight loss measurement determines corrosion rate in mils per year.

Sludge removal: Annual tank draining and cleaning removes accumulated corrosion products, scale, and biological growth.

Inhibitor replenishment: Add inhibitor package per manufacturer recommendations, typically semi-annually or per corrosion monitoring results.