Water is the ultimate industrial cooling medium: it is cheap, safe, and has an exceptionally high specific heat capacity. However, water freeze-ups inside a chiller evaporator are catastrophic, usually cracking the copper tubes and ruining the compressor. When target process temperatures drop below 40°F (4.4°C), adding glycol (antifreeze) is non-negotiable. But doing so changes the physical properties of your coolant and de-rates your chiller.
The Glycol Sizing Penalty Explained
Glycol de-rates a chiller in two ways:
1. Lower Specific Heat: Glycol absorbs and carries less heat energy per gallon than pure water.
2. Higher Viscosity: Glycol is thicker than water, which creates a thicker laminar boundary layer inside heat exchangers (reducing the heat transfer rate) and increases piping pressure drops (loading the pump).
1. Ethylene vs. Propylene Glycol
Before running sizing calculations, you must select the type of glycol based on your facility's safety requirements:
- Ethylene Glycol (Industrial Grade): Offers the best heat transfer efficiency and lowest viscosity. However, it is toxic to humans and animals. It is the standard choice for plastics molding, machining, laser cutting, and heavy industrial loops.
- Propylene Glycol (Food Grade): Has a lower heat transfer efficiency and much higher viscosity than ethylene glycol, but is non-toxic. It is legally mandated for breweries, wineries, food packaging, and pharmaceutical processes.
2. Concentration & Capacity Correction Factors
To maintain freeze protection, you select a glycol concentration based on your lowest expected fluid temperature. A higher glycol percentage means a larger de-rating penalty.
| Glycol Concentration | Freeze Protection Limit | Ethylene Capacity Factor | Propylene Capacity Factor |
|---|---|---|---|
| 0% (Pure Water) | 32°F (0°C) | 1.00 | 1.00 |
| 20% Concentration | 18°F (-7.8°C) | 0.98 | 0.96 |
| 30% Concentration | 8°F (-13.3°C) | 0.97 | 0.94 |
| 40% Concentration | -8°F (-22.2°C) | 0.95 | 0.92 |
| 50% Concentration | -29°F (-33.8°C) | 0.93 | 0.89 |
3. The Viscosity & Pump Power Penalty
Because glycol is more viscous than water, it increases fluid friction in the pipes. This requires more pump head (PSI) to achieve the desired flow rate.
- Ethylene Glycol Impact: Running a 30% ethylene glycol mix at 20°F requires approximately 15% more pump horsepower compared to running pure water.
- Propylene Glycol Impact: Because food-grade propylene is much thicker at low temperatures, a 30% propylene mix requires 25% to 30% more pump horsepower and may restrict flow to the process if the pump is undersized.
The Heat Transfer Coefficient Hit
The convective heat transfer coefficient inside the evaporator drops by 15% to 30% with glycol. This means that even if a chiller has enough compressor displacement, the physical heat exchanger cannot transfer heat as fast, which de-rates the entire machine.
4. Real Sizing Scenario: Brewery Glycol Loop
Consider a commercial craft brewery that needs to pull heat from their fermentation tanks at a target temperature of 28°F (-2.2°C). The calculated peak fermentation heat load is 12.0 tons.
To prevent freezing at 28°F, the brewery must use a 35% propylene glycol mixture (food-grade requirement).
Sizing Math Breakdown
To deliver 12.0 tons of actual cooling under this glycol concentration and temperature profile, the chiller nominal capacity must be:
If the brewer had purchased a 12-ton chiller because their load was 12 tons, they would have been nearly 37% short of cooling capacity. Their fermentation temperatures would have drifted, spoiling the batch.
Best Practices for Managing Glycol
- ✓Do Not Over-Concentrate: Keep glycol concentrations to the minimum required to prevent freezing at your target temp plus a 10°F safety buffer. Running 50% glycol when 30% is sufficient introduces unnecessary cooling and pump de-rating penalties.
- ✓Always Audit Pump Head: When replacing water with glycol, verify that your chiller's internal or external circulation pump is rated to handle the increased viscosity and pressure drop.
- ✓Use Premixed or Distilled Water: Never mix glycol with raw tap water. Minerals in tap water bind with glycol inhibitors to form scale on heat exchanger surfaces, further de-rating heat transfer. Use distilled or deionized water.