Antifreeze fluids

Ethylene and propylene glycol protect cooling circuits against frost, but only if the concentration is right and the corrosion inhibitors are still active. This guide explains how to choose, test and replace glycol in chiller and dry cooler systems.

A glycol mixture is a working fluid, not a one-time filling. It is diluted by top-ups, it ages with heat and oxygen, and it slowly loses its corrosion protection. Many coil and evaporator failures we see in service start with glycol that nobody checked for years.

Ethylene glycol and propylene glycol

Both fluids lower the freezing point of water and are supplied as ready mixtures or as concentrates with inhibitor packages. The choice depends mainly on safety requirements and on the operating temperature.

Ethylene glycol (MEG)

Ethylene glycol has better heat transfer properties and lower viscosity than propylene glycol at the same freezing point. Systems with ethylene glycol therefore need slightly less pump power and achieve better heat exchanger performance. It is toxic when swallowed, so it is used in industrial and technical circuits where contact with food or drinking water is excluded.

Propylene glycol (MPG)

Propylene glycol has low toxicity and is used in food and beverage processing, pharmaceutical plants and wherever a leak could reach products or drinking water. Its viscosity rises strongly at low temperatures, which increases pressure drop and reduces heat transfer. For the same freezing point a slightly higher concentration is needed.

Freezing point and concentration

The table gives approximate freezing points for mixtures with water, expressed as percent glycol by volume. Values for commercial products differ slightly depending on inhibitors and additives, so the manufacturer datasheet should always be checked.

The freezing point marks the start of ice crystal formation. Below it, the mixture first turns into slush, and burst protection extends lower than the freezing point. For circulating systems, however, slush already blocks heat exchangers and pumps, so the design should be based on the freezing point, not on burst protection.

How much glycol is enough

• Choose a freezing point at least 5 K below the lowest expected fluid or ambient temperature.
• For outdoor dry coolers in Central Europe, freezing points of -20 to -25 °C are common.
• For process chillers with low supply temperatures, check the evaporating temperature as well: the fluid near the evaporator plates is colder than the supply temperature.
• Do not use concentrations above about 50 to 60 % by volume: protection does not improve further, while viscosity and cost rise.
• Concentrations below about 20 to 25 % offer weak corrosion protection in many products, as the inhibitor content becomes too low.

Our glycol calculator computes the concentration for a target freezing point and the amount of concentrate needed to correct an existing filling.

Corrosion inhibitors

Pure glycol is not a corrosion protection. When it ages, especially at high temperatures and with oxygen ingress, it oxidises into organic acids. The pH drops and the fluid starts attacking steel, cast iron, copper and particularly aluminium and zinc. Inhibitors buffer the pH and form protective layers on metal surfaces.

• Inorganic inhibitors (silicates, phosphates, nitrites) act fast but are consumed and may form deposits.
• Organic acid technology inhibitors last longer and form fewer deposits.
• Hybrid packages combine both approaches.

Mixing products from different manufacturers or with different inhibitor systems can cause precipitation and gel formation. Top-ups should use the same product, or the circuit should be fully replaced.

Testing on site and in the laboratory

On site

A refractometer with scales for ethylene and propylene glycol shows the freezing point and concentration within a minute. It is important to use the correct scale: reading a propylene glycol mixture on the ethylene scale gives a wrong result. We also check the pH with test strips or a meter. Fresh inhibited glycol mixtures usually have a pH of about 8 to 9.5; values below about 7.5 indicate ageing and loss of inhibitors. The appearance of a sample, its colour, smell, turbidity and sediment, tells a lot as well: a brown or black fluid with particles points to corrosion inside the system.

In the laboratory

When the on-site results are borderline, or when an expensive system is involved, a sample can be sent to a laboratory or to the glycol supplier. Typical analyses include reserve alkalinity, inhibitor content, metal ions (iron, copper, aluminium) and degradation products. The result shows whether a correction with fresh concentrate is enough or whether the whole filling must be replaced.

Replacement and disposal

Glycol is usually replaced when the pH is low, the inhibitors are exhausted, the fluid is visibly contaminated, or after a repair that introduced large amounts of water. A typical replacement includes:

1. Draining the old fluid into containers and recording the volume removed.
2. Flushing the circuit with water, cleaning strainers and, where needed, using a cleaning agent compatible with the materials of the system.
3. Filling with a pre-mixed solution or mixing concentrate with demineralised or softened water in a tank before filling.
4. Venting at all high points, pump start-up and repeated venting.
5. Measuring concentration and pH after full circulation and recording the values.

Used glycol is waste that must be handed over to an authorised waste management company; it must not be discharged into the sewer or the ground. The waste documentation should be kept with the system records. More about this service is on our page about glycol testing and replacement.

Effect of glycol on system performance

Glycol reduces specific heat capacity and increases viscosity. At the same flow rate, a 30 % mixture transfers noticeably less heat than water, and the pressure drop across heat exchangers and pipes rises. When a water system is converted to glycol, or when the concentration is increased, the pump capacity and the chiller capacity at the new conditions should be checked. The glycol dilution table gives further values.

Frequently asked questions

Can I mix ethylene glycol and propylene glycol?

It is not recommended. The freezing point of the mixture becomes hard to measure, and the inhibitor packages may not be compatible. If the type of glycol in a system is unknown, a sample should be analysed before any top-up.

Can I top up with water when the system loses pressure?

Only in very small amounts. Every litre of water lowers the concentration. After a larger leak or repair, the concentration must be measured and corrected with concentrate.

How often should glycol be checked?

At least once a year, ideally in autumn before the first frost, and always after a repair or top-up. Systems with oxygen ingress or high temperatures need more frequent checks.

Is car antifreeze suitable for a chiller?

No. Automotive coolants contain additives designed for engines and may form deposits in plate heat exchangers. Use glycol products intended for heating and cooling installations.

How long does a glycol filling last?

Well inhibited fluids in closed systems commonly last several years. The actual life depends on temperature, oxygen ingress and top-ups, so regular measurement is more reliable than a fixed replacement interval.