Process cooling is a system that uses chilled water or a water-glycol mixture to draw heat from a production process. In plastic injection, food, beverage, chemical and pharmaceutical facilities, these circuits often run below 0 °C or on lines at risk of freezing in winter. In that case, adding glycol to the water becomes mandatory.
The problem is that in most facilities the glycol ratio is set with the logic of "a bit extra won't hurt". But excess glycol has a cost too.
Which temperature determines the glycol ratio?
The glycol ratio is set by the lowest temperature the circuit may encounter. This temperature comes from two sources, and whichever is lower is used.
- Process temperature: The lowest set value of the fluid leaving the chiller evaporator. The evaporator surface drops a few degrees below this value.
- Ambient temperature: For pipes left outdoors, and for an air-cooled chiller or dry cooler in the open, the region's lowest winter temperature.
In common engineering practice, the freezing point of the mixture is chosen to be generally 5–6 K below this lowest temperature. For example, for a process running at -5 °C, a mixture with a freezing point of around -10/-11 °C is targeted.
Let's explain with an example: In a beverage facility, the chiller outlet water is set to -3 °C, the unit is on the roof, and in winter the outdoor air drops to -15 °C. Here the deciding factor is not the process but the outdoor air temperature. Considering a fault or power outage that could stop circulation, the freezing point should be chosen at around -20 °C. If the same chiller were in an enclosed machine room, the target could stay around -9/-10 °C.
Ethylene glycol or propylene glycol?
Ethylene glycol has better heat transfer and lower viscosity, but because it is toxic it is used in circuits with no risk of food contact. Propylene glycol, because of its low toxicity, is preferred in food, beverage and pharmaceutical facilities.
Propylene glycol generally requires a somewhat higher ratio for the same freeze protection and is more viscous. For this reason, pump and heat exchanger selection should be verified for the glycol type. For both types, an industrial-grade product with corrosion inhibitors should be used.
Which ratio gives which freezing point?
As the glycol ratio increases, the freezing point of the mixture drops, but the relationship is not linear. The values below are approximate for ratios by volume; exact values should be taken from the manufacturer's table for the product used.
| Glycol ratio by volume | Ethylene glycol (approximate freezing point) | Propylene glycol (approximate freezing point) |
|---|---|---|
| %20 | -8 °C | -7 °C |
| %30 | -15 °C | -13 °C |
| %40 | -24 °C | -21 °C |
| %50 | -36 °C | -33 °C |
The glycol ratio should not be dropped below 20%. At very low ratios the inhibitor concentration also becomes insufficient and the mixture becomes open to bacterial growth.
Why is excess glycol harmful?
A glycol ratio higher than needed lowers the system's capacity and efficiency. Glycol has a lower specific heat and a higher viscosity than water.
- Heat transfer decreases: The amount of heat carried at the same flow rate falls, and chiller and heat exchanger capacity is reduced by a correction factor.
- Pump energy increases: A viscous fluid creates more pressure drop, and the difference is especially noticeable in cold operation.
- Flow drops: With the pump curve fixed, the flow decreases, and low-flow alarms may appear at the evaporator.
The size of the capacity loss depends on the glycol type, the ratio and the operating temperature; it stays limited to a few percent at low ratios, but can rise to significant levels at high ratios. This difference shows up as the process being unable to reach the desired temperature in a facility running at full load in summer.
Have your glycol ratio measured
Let's measure your circuit on site and report the ratio, inhibitor and capacity status in writing.
That is why the ratio should be chosen as needed, rather than on the basis of "the more, the safer". When selecting a chiller, the actual capacity should also be checked by entering the glycol type and ratio in the manufacturer's software.
How is the glycol ratio measured and maintained?
In the field, the glycol ratio is usually measured with a refractometer that has a scale matching the glycol type. Measurements should be taken with samples from different points of the circuit while the system is running. If the results are recorded in the same format at every maintenance visit, changes in the ratio over time are easy to track.
During filling, glycol and water should not be pumped into the circuit separately; they should be mixed in a tank beforehand and then filled. Otherwise the mixture will not be evenly distributed in the circuit and measurements will be misleading. Softened or demineralized water should be used if possible; hard water depletes inhibitors and leaves scale.
The ratio changes over time: water make-up after leaks dilutes the mixture. For this reason, if the system has automatic water make-up, a meter showing the make-up volume is recommended. The following checks should also be done regularly:
- Are the pH value and inhibitor level within the range recommended by the manufacturer?
- Has the color or smell of the mixture changed, is there any sediment?
- Is sludge building up in the filters and strainers?
Automotive antifreeze must not be used in process circuits. Its additives can build up in heat exchangers. Galvanized pipe should also be avoided in glycol circuits; inhibitors may be incompatible with zinc coating.
What else should be done in winter?
The right glycol ratio alone is not enough; insulation of outdoor lines, heat tracing and keeping circulation running are also part of freeze protection. We explain the winter preparation steps in detail in our chiller winter preparation guide.
How do we work on glycol circuits?
At YOİ Mühendislik, we calculate the glycol ratio in process cooling plants based on the lowest operating and ambient temperature of the facility, and record the measurement results in the maintenance report. When needed, we carry out mixture replacement, circuit cleaning and chiller capacity checks.
If a new circuit is being installed, the most economical approach is to determine the glycol ratio together with pump and heat exchanger selection at the project stage. In an existing facility, the ratio and condition of the current mixture are measured first, and then it is decided whether to correct the ratio or replace the mixture entirely. If the mixture needs to be replaced, the old glycol must be disposed of in compliance with environmental regulations; it must not be poured down the drain.
For details, you can visit our chiller service page or request a site survey for your facility.
Frequently asked questions
What should the glycol ratio be in process cooling?
The freezing point of the mixture is chosen to be generally 5–6 K below the lowest fluid or ambient temperature in the circuit. In most process applications this corresponds to a ratio of 25–40% by volume.
Which glycol should be used in a food facility?
In food, beverage and pharmaceutical facilities, inhibited propylene glycol, which has low toxicity, is preferred. Ethylene glycol is used in circuits with no risk of food contact.
What happens if the glycol ratio is too high?
Viscosity rises, heat transfer and chiller capacity fall, and pump energy goes up. That is why the ratio should be chosen as needed.
How is the glycol ratio measured?
In the field it is measured with a refractometer with a scale suited to the glycol type. Measurement should be done while the system is running, with samples taken from different points.