Cooling coil issues - cold crashing

[DGuggenb]

I have two- 1bbl unitanks with the cooling coils from SSBrewtech. So far, they've been ok and have been able to get beer down to 38-40F. I'm just finishing up a fruited saison, fruited with raspberry and peach puree at about 2.5 lbs/gallon. With the added liquid, should bring the OG to about 11.1, FG 1.6. Glycol is usually set to 26F which hasn't caused issues before. This time, I go to cold crash and the beer gets stuck at around 58F and won't move. I grab a pump and start to do some recirculating and the temperature quickly drops to 41F or so over the course of an hour or so. I turn the pump off and the temperature begins to rise, quickly going back up to 50+ over the course of an hour, kick the pump back on and it goes down again.

At this point, based on some other threads I assume that maybe there's ice forming inside around the coils. I transfer the beer over to a 1bbl brite tank, open up the unitank and don't see anything. The brite tank which is also from SS brewtech, also has cooling coils and hasn't been great, but still manages to get the beer to 38F most of the time. Well, the exact same things starts to happen in there. At first it reads that the beer is coming in at 41F, but then it rises up to the low 50s. By this point I've raised the glycol temp to 32F to ensure that there's no freezing going on but this is still occurring.

Has anyone had something like this happen before?

[jebzter]

Water does a funny thing around these temperatures. You would think that normal convection would mean that the warmer liquid is on top, colder on the bottom, but the water starts to do different things with density as it moves colder. Water is at peak density at 39.2F, as it gets colder it gets less dense and as it gets warmer, it gets less dense. And what you end up with is a weird inversion where the coldest area is in a thin layer sandwiched in between layers of slightly warmer water. You will need to recirculate the beer in order to get it colder than that, or you need dual zone cooling to get the top and bottoms cooled. This will be a problem any time you try to go below 40F with a single coil that is in the top 1/3 of the tank. I suspect your temperature probe is stuck in one of the less dense areas and not measuring a true temperature. This can be exacerbated by lower alcohol beers as the effect is increased.

[DGuggenb]

Water does a funny thing around these temperatures. You would think that normal convection would mean that the warmer liquid is on top, colder on the bottom, but the water starts to do different things with density as it moves colder. Water is at peak density at 39.2F, as it gets colder it gets less dense and as it gets warmer, it gets less dense. And what you end up with is a weird inversion where the coldest area is in a thin layer sandwiched in between layers of slightly warmer water. You will need to recirculate the beer in order to get it colder than that, or you need dual zone cooling to get the top and bottoms cooled. This will be a problem any time you try to go below 40F with a single coil that is in the top 1/3 of the tank. I suspect your temperature probe is stuck in one of the less dense areas and not measuring a true temperature. This can be exacerbated by lower alcohol beers as the effect is increased.

That would definitely make sense, especially if alcohol plays a decent role in that. In general, the higher alcohol beers have done better, with more issues as the ABV has gotten lower, until this beer which is the lowest alcohol one yet, where it's completely out of wack. Anyway, I've asked SSBrewtech what they recommend, although I'm not sure I'll get a helpful response. In the meantime, I transferred it to my jacketed brite tank as a half-fill and it's doing fine in there.

Anyone have suggestions on how to prevent this? Just a couple days ago I was able to crash one of the tanks down to 38F in 6 hours or so and didn't have any issues. That was a 6% beer. With this approx. 5% beer, it immediately had issues with what I guess would be stratification or temperature layering. The part of the tank with the temperature probe never got below like 60F until I started recirculating. Crashing fast worked fine on the 6% beer, but do I need to step down lower ABV beers much slower?