In a glycol system a chiller cools the glycol–water mixture; pumps send this mixture through pipes to the room coolers, tanks and process lines. The mixture does not evaporate in the cooler; it only warms up and returns. The refrigerant circuit stays short and factory-built; the charge is 70–90% lower than in a direct expansion system and the risk of leaks is confined to the machinery room. It allows refrigerants such as ammonia or propane to be used without entering the food area. Adding a new room or cooling point is simply a matter of running pipes.
In food plants propylene glycol, which is non-toxic, is used; in other applications ethylene glycol, whose heat transfer properties are better. The freezing point depends on the concentration: 30% propylene glycol by volume protects down to about −13 °C and 40% to about −21 °C; with ethylene glycol the same concentrations give about −15 and −24 °C. The concentration is chosen to protect to 5–8 K below the lowest evaporating temperature. For lower temperatures salt solutions such as potassium formate, or pumped CO₂, are used.
The price is efficiency. Because one more heat exchanger comes in between, the evaporating temperature must be 4–6 K lower than with direct expansion, and the pumps draw electricity continuously; total consumption usually rises by 10–20%. Glycol thickens as it gets colder: 40% propylene glycol at −10 °C is several times more viscous than water, and the pump and pipe diameter are calculated accordingly. The concentration is not kept higher than necessary; heat transfer falls and the pump load rises.
In practice ready-made glycol with corrosion inhibitors is used; uninhibited glycol attacks steel and copper. Galvanised pipe is incompatible with glycol. Soft or treated water is chosen for the mixture. The concentration is measured with a refractometer at least once a year and the pH value is checked; a system that has lost fluid is topped up not with water but with mixture of the same concentration. The pipes are covered with insulation that has a vapour barrier, and the system has an expansion tank and air vents. Leaked glycol makes the floor slippery; the pump seals are checked regularly.