Glycol Solutions

Glycol-water solutions represent the predominant secondary coolants in HVAC applications, providing freeze protection for chilled water systems, ice storage, snow melting, and process cooling. The two primary glycols—propylene glycol and ethylene glycol—offer different toxicity profiles while maintaining similar freeze protection and thermal characteristics.

Glycol Chemistry and Structure

Glycols are dihydric alcohols with two hydroxyl groups enabling hydrogen bonding with water molecules. This molecular interaction depresses the freezing point through colligative properties while maintaining complete miscibility across the full concentration range.

Propylene glycol: C₃H₈O₂ (1,2-propanediol), molecular weight 76.09 g/mol. GRAS (Generally Recognized As Safe) status permits food-grade applications. Slightly higher viscosity than ethylene glycol at equivalent concentrations.

Ethylene glycol: C₂H₆O₂ (1,2-ethanediol), molecular weight 62.07 g/mol. Superior heat transfer properties but toxic (LD₅₀ ≈ 4700 mg/kg oral, rat). Requires isolation from potable water per plumbing codes.

Both glycols are hygroscopic, biodegradable under aerobic conditions, and thermally stable to 120°C. Oxidation in the presence of oxygen and metals produces organic acids (glycolic, formic, oxalic) that reduce pH and accelerate corrosion.

Freeze Point Depression

Glycol addition progressively lowers the solution freezing point through colligative freezing point depression. The relationship between concentration and freeze point is nonlinear, reaching a eutectic minimum before increasing at higher concentrations due to pure glycol’s higher freezing point.

Propylene glycol eutectic: Approximately 60% by weight at -60°C. Solutions above 60% exhibit increasing freeze points as concentration approaches pure propylene glycol (freezing point -59°C).

Ethylene glycol eutectic: Approximately 60% by weight at -52°C. Pure ethylene glycol freezes at -13°C, limiting usefulness above eutectic concentration.

Practical concentrations rarely exceed 50% by weight. Typical applications specify concentration providing 10-15°F safety margin below minimum operating temperature to account for:

• Local cold spots in stagnant piping
• Heat loss during shutdown
• Concentration drift from water loss
• Measurement uncertainty

Property Degradation with Concentration

While freeze point improves with glycol concentration, thermal and hydraulic performance degrades significantly:

Specific heat reduction: Pure glycol exhibits cp ≈ 2.5 kJ/(kg·K) versus 4.18 kJ/(kg·K) for water. A 40% propylene glycol solution retains approximately 85% of water’s specific heat, requiring 18% greater flow rate for equivalent capacity.

Viscosity increase: Glycol solutions exhibit 2-10× water viscosity depending on concentration and temperature. Viscosity increases exponentially with concentration and inversely with temperature, dramatically affecting pressure drop and pumping power.

Thermal conductivity decrease: Glycol solutions reach 70-80% of water’s thermal conductivity at moderate concentrations, reducing heat exchanger effectiveness.

Density increase: Solutions are 2-7% denser than water, affecting pump head calculations and thermal expansion.

Inhibitor Technology

Uninhibited glycol solutions corrode ferrous and aluminum metals through several mechanisms:

Oxygen corrosion: Dissolved oxygen oxidizes metal surfaces. Glycol oxidation products accelerate this process.

Galvanic corrosion: Dissimilar metals create electrochemical cells. Mixed metallurgy systems (steel/copper/aluminum) require careful inhibitor selection.

Concentration cell corrosion: Stagnant areas develop concentration gradients driving localized corrosion.

Modern inhibitor packages employ multiple components:

• Nitrite: passivates ferrous metals
• Molybdate: protects aluminum and yellow metals
• Silicate: forms protective glass-like layer (avoid with aluminum)
• Azoles (benzotriazole, tolyltriazole): copper corrosion inhibition
• pH buffers: maintain alkaline pH (8.5-10.5) for corrosion passivity

Inhibitor depletion occurs through:

• Oxidation and chemical breakdown
• Precipitation with hard water minerals
• Adsorption onto corrosion products
• Biological consumption

Annual testing verifies inhibitor reserve alkalinity above minimum protective levels.

Temperature-Dependent Properties

All glycol solution properties vary significantly with temperature, requiring evaluation at both design and extreme conditions:

Cold temperature (0 to -20°C): Viscosity increases exponentially, potentially reaching 10-20 cP compared to 1 cP for water at 20°C. This cold viscosity drives pump sizing and determines minimum startup temperature.

Operating temperature (5 to 15°C): Design point properties determine heat exchanger sizing, pressure drop calculations, and flow rates.

High temperature (>40°C): Accelerated oxidation and thermal degradation. Systems with boiler connections or solar collectors require high-temperature stable formulations.

Application-Specific Considerations

Chilled water freeze protection: Typical 25-30% propylene glycol provides freeze protection to -10 to -15°C for air-handling unit coils exposed to cold ambient conditions during shutdown.

Ice storage systems: 25% ethylene glycol solution circulates at -8 to -4°C for ice-on-coil or ice harvester systems. Lower specific heat requires 20-25% higher flow rates than water systems.

Snow melting: Embedded hydronic tubes use 30-35% propylene glycol for -15 to -20°C protection during extreme cold events when system continues operating.

Process cooling: Food-grade propylene glycol at 30-40% serves food processing, pharmaceutical, and beverage applications where incidental contact with product is possible.

Solar thermal: Propylene glycol resists high-temperature stagnation conditions (up to 180°C) in solar collector loops during summer no-load periods.

Fluid Maintenance and Management

Glycol solutions degrade over time through oxidation, thermal stress, and contamination:

Color change: Fresh glycol solutions appear clear to light yellow. Darkening indicates oxidation and degradation, signaling replacement need.

pH drift: Oxidation produces organic acids, dropping pH below protective range (pH < 8). Alkalinity buffer depletion accelerates corrosion rates.

Particulate contamination: Corrosion products, scale, and biofilm require filtration. Online filtration (5-10 micron) extends fluid life.

Concentration monitoring: Refractometer readings or density measurements verify freeze protection levels. Water loss through leaks concentrates solution; water addition dilutes it.

Service life: Properly inhibited and maintained glycol systems achieve 5-10 year service life. Contaminated or uninhibited fluids require replacement within 2-3 years.

Disposal and Environmental Considerations

Spent glycol solutions require proper disposal:

Propylene glycol: Biodegradable, low aquatic toxicity (LC₅₀ > 1000 mg/L). Many jurisdictions permit sanitary sewer disposal after pH neutralization and toxicity testing.

Ethylene glycol: Moderately toxic to aquatic organisms. Requires hazardous waste disposal or specialized recycling. Sweet taste attracts animals, causing poisoning incidents.

Both glycols exhibit high biological oxygen demand (BOD) and chemical oxygen demand (COD), potentially overwhelming small wastewater treatment plants if discharged in quantity.

Sections