Concentration Effects

Glycol concentration profoundly affects both freeze protection and thermal-hydraulic performance. The concentration selection process balances adequate freeze protection against performance degradation, pumping power penalties, and heat exchanger sizing impacts.

Freeze Point Depression Curves

Freeze point depression follows nonlinear concentration relationships documented in ASHRAE Fundamentals Chapter 31. The freeze point decreases with increasing concentration until reaching a eutectic minimum, beyond which further concentration increases the freeze point.

Propylene glycol concentration-freeze point relationship:

Ethylene glycol exhibits similar but slightly lower freeze points at equivalent concentrations.

The freeze point represents onset of ice crystal formation. Slush formation occurs over a temperature range, with complete solidification requiring significantly lower temperatures than the tabulated freeze point.

Eutectic Composition

The eutectic point represents the lowest achievable freeze point for the glycol-water system. At eutectic concentration, solution and ice crystals form simultaneously during freezing.

Propylene glycol eutectic: 60% by weight at approximately -60°C (-76°F). Concentrations above 60% provide less freeze protection due to the -59°C freezing point of pure propylene glycol.

Ethylene glycol eutectic: 60% by weight at approximately -52°C (-62°F). Pure ethylene glycol freezes at -13°C (9°F), limiting utility of high concentrations.

Practical HVAC systems rarely approach eutectic concentration. Most applications use 20-40% solutions balancing adequate freeze protection (typically -10 to -25°C) against thermal performance degradation.

Specific Heat Capacity Reduction

Specific heat capacity decreases approximately linearly with glycol concentration, directly impacting system flow rate requirements.

Propylene glycol specific heat at 20°C:

The flow rate increase required to maintain capacity follows:

ṁ_glycol / ṁ_water = cp_water / cp_glycol

A 40% propylene glycol solution requires 12% higher flow rate than water for equivalent heat transfer capacity. This increased flow rate propagates through the entire system, affecting:

• Pipe sizing for equivalent velocity
• Pump selection and power consumption
• Heat exchanger effectiveness
• Temperature differences across loads

Viscosity Increase with Concentration

Viscosity increases dramatically with glycol concentration, exhibiting exponential temperature dependence per the Arrhenius relationship:

μ = A · exp(B/T)

where A and B are empirical constants varying with concentration.

Propylene glycol dynamic viscosity at 20°C:

Viscosity effects compound at low temperatures. A 40% propylene glycol solution at -10°C exhibits viscosity approaching 30 cP—30 times that of water at room temperature.

Pressure drop implications:

Pressure drop in turbulent flow scales with viscosity per the Darcy-Weisbach equation:

∆P = f · (L/D) · (ρV²/2)

where friction factor f depends on Reynolds number Re = ρVD/μ. Higher viscosity reduces Re, increasing friction factor and pressure drop. The combined effect increases pressure drop by factors of 1.5-4× compared to water at equivalent flow rates.

Laminar flow (Re < 2300) becomes more likely in small diameter pipes with glycol solutions. Laminar flow exhibits f = 64/Re, making pressure drop directly proportional to viscosity with ∆P ∝ μV.

Thermal Conductivity Degradation

Thermal conductivity decreases with glycol concentration, reducing convective heat transfer coefficients in heat exchangers:

Propylene glycol thermal conductivity at 20°C:

Reduced thermal conductivity affects the Prandtl number:

Pr = cpμ/k

Prandtl number increases with glycol concentration (water Pr ≈ 7, glycol solutions Pr ≈ 20-100), reducing convective heat transfer coefficients per correlations like Dittus-Boelter:

Nu = 0.023 Re^0.8 Pr^0.4

The net effect reduces heat transfer coefficients by 20-40% compared to water, requiring 25-60% additional heat exchanger surface area for equivalent performance.

Density Effects

Density increases approximately linearly with glycol concentration:

Propylene glycol density at 20°C:

Higher density slightly increases pump head requirements for equivalent system elevation changes. The density increase is small compared to viscosity and specific heat effects.

Prandtl Number and Heat Transfer

Prandtl number Pr = cpμ/k represents the ratio of momentum diffusivity to thermal diffusivity. Glycol solutions exhibit higher Prandtl numbers than water due to increased viscosity and reduced thermal conductivity:

Propylene glycol Prandtl number at 20°C:

Higher Prandtl numbers indicate thicker thermal boundary layers relative to velocity boundary layers, reducing convective heat transfer coefficients. The Pr^0.4 dependency in turbulent flow correlations means a 5× increase in Prandtl number reduces heat transfer coefficient by approximately 35%.

Optimal Concentration Selection

Optimal concentration balances competing objectives:

Minimum concentration requirement:

• Freeze point < minimum system temperature - 10°F safety margin
• Account for stagnant areas, shutdown conditions, cold spots

Performance penalties above minimum:

• 10% concentration increase → 2% specific heat reduction
• 10% concentration increase → 30-50% viscosity increase
• 10% concentration increase → 5% thermal conductivity reduction
• Combined effect: 5-10% heat transfer coefficient reduction per 10% concentration

Economic optimization: The lifecycle cost optimum typically occurs at minimum concentration satisfying freeze protection requirements plus a safety margin. Over-concentration increases:

• Initial heat exchanger capital cost (larger required area)
• Pumping energy cost (higher viscosity, higher flow rate)
• Fluid replacement cost (more expensive fluid volume)

Pumping Power Increase

Pumping power follows:

P_pump = (ṁ · ∆P) / (ρ · η_pump)

For glycol solutions versus water at equivalent capacity:

• ṁ increases by (cp_water/cp_glycol) factor due to reduced specific heat
• ∆P increases by 1.5-4× due to combined viscosity and flow rate effects
• Net pumping power increases by 1.8-5× for 30-40% glycol solutions

This parasitic power penalty persists throughout system operating life, making minimum adequate concentration economically optimal.

Over-Concentration Disadvantages

Concentrations exceeding freeze protection requirements impose penalties without benefits:

Performance degradation: Exponentially increasing viscosity reduces heat transfer and increases pressure drop.

Economic waste: Glycol costs $3-8/gallon. Excess concentration adds unnecessary fluid cost.

Freeze protection reversal: Concentrations above eutectic (60%) actually reduce freeze protection.

Inhibitor depletion: Higher glycol concentrations accelerate inhibitor consumption in some formulations.

Operational difficulties: Very viscous solutions (50%+) exhibit difficult cold-start behavior, require oversized pumps, and may cause flow measurement errors.

Best practice specifies concentration providing required freeze protection plus 10-15°F safety margin, avoiding over-concentration beyond this requirement.