What happens when a 30% ethylene glycol/water mix freezes?

Question

Specifically does such a mix have enough glycol in it to offset the expansion of the water or does it need to be stronger? Blockages from frozen fluids are not a concern, expansion is though.

Answer 1

Not sure what the "freezing point depression" for 30% vol / vol Ethylene glycol is. I used to know how to calculate that.

In any case, the water will tend to precipitate out of solution as needle-like crystals, their size depends on how fast it's cooled. You end up with a slushy mix of concentrated glycol and ice. The mixture will still expand in volume, of course, but as long as it's still semi-liquid, you can just install an overpressure reliever.

Expansion only becomes a problem, if you end up with trapped, sealed off pockets of fluid inside pipes. As the ice crystals grow, they take up more space than the original fluid did, and exert tremendous pressure on the liquid, which has nowhere to escape.

I think I'd recommend doing the same thing as you would with a car, go 50/50. That way, even if the water is completely frozen, the ice will still probably be too slushy to seal off liquid pockets.

Answer 2

When this mixture gets cold enough, ice will start to crystalize out of the solution. This ice will be nearly pure water, so the remaining solution becomes more and more concentrated in ethylene glycol as more and more of the water freezes out, and thus, the freezing point gets lower and lower. Eventually, an equilibrium is established where no more ice will crystalize out, or if the temperature gets below about -60F the whole mess freezes. In cases of partial freezing, you"ll get a slush. The frozen water will expand as ice does, but the remaining liquid might contract because it's concentration is changing, so it might counteract the expansion from freezing; If you make sure your solution is strong enough that no ice crystals form at the lowest temperature your system will be exposed to, you don't have to worry about the expansion on freezing.