The Lesson of Water-Glycol Chillers


Roughly every eighteen months, I have to re-learn this lesson. I'm gonna write it down this time. Maybe that'll help me remember.

Reactor chillers are really cool. You take a jacketed flask or reactor, hook it up, put some heat transfer fluid in the chiller, and turn it on. You can control the temperature of the jacket, and then the contents of the reactor, using the chiller. The fancy ones can do temperature programs and ramps and stuff if you're doing crystallization work. No oil baths, no dry ice, no heating mantles. They're awesome.

That heat transfer fluid, though. My favorite is syltherm, which is a polysiloxane material. You can get it really hot and really cold, at least in the range that organic chemistry is typically performed. But it burns, which isn't great. For very cold systems, like cryogenic condensers for vacuum distillations, one of my coworkers at my first process job introduced me to something really simple: ethanol. Very low viscosity compared to the other options when you're in cryogenic conditions. Watch out for the vapors, though. Again, the burning.

But the most commonly used heat transfer fluid is a water / glycol mixture. I've seen both 50:50 water : ethylene glycol and : propylene glycol as pretty common options. You can get them reasonably warm and reasonably cold. It's about as cheap as you can get for something like this. No big toxicity issues. And with all that water, flammability shouldn't be a problem.


You have to use your brain. When it's heated up, or sits there in the chiller for a while, that water has an appreciable vapor pressure. But the glycol doesn't. So one day, you open up the tank and notice it's low. You grab your pre-made bottle of 50:50 and dump some in. Congratulations, your 50:50 mix is now like 40:60 water : glycol. Next time, it's 30:70. And soon, because it doesn't take much, you have a very viscous mixture that stops flowing very well. Your pump says it's at -5 °C, but the fluid isn't making it to your condenser fast enough.

And if you're me, today, you suddenly remember all of this when the DCM you're trying to distill starts to fly right past your condenser.

Sigh. This time I'll remember. Certainly. Just add water to the chiller.