My brew club was asking if anyone ever used dry ice as a method of chilling the boiled wort.
Why not? It’s just solid carbon dioxide, and should drop the temp quickly. Simply drop several chunks into the wort at the end of the boil.
Are my friends crazy? Have they lost their minds?
It took us 1 hour to cool 5.5 gallons down to 74 degrees. Using a 1/2" 50’ copper immersion coil. And we used a 25’ copper coil in an outside ice chest as a pre-cooler. It still took 1 hour to get to 74 degrees. We wanted to get down to 65.
You could use dry ice, but you would need to stir constantly so you don’t get frozen wort around the outside of the dry ice chunks which would insulate them from doing any more cooling. Dry ice is denser than water and wort (SG = 1.400 - 1.600)) and will sink to the bottom of your kettle, which exacerbates the freezing problem. Breaking the dry ice into small pieces would work better than large chunks, but too fine a powder will just make a massive cloud of fog when you dump the dry ice into the kettle, like a Halloween show. It would be a fun experiment to try, but rather pricey.
When using an immersion chiller you need to make sure that either the chiller or the wort are in constant motion. Otherwise you get a boundary layer of cool wort next to the coil that inhibits heat transfer. At that point you are relying on diffusion for heat transfer, which is much less effective than conduction. If you jiggle the coil constantly or stir the wort constantly it will break the boundary layer and you will get much faster cooling because you will be bringing fresh hot wort into contact with the coil all the time. If you have a pump or whirlpool mechanism that is the least tedious way to keep your wort moving. If not, just jiggle the coil up and down and side to side a bit until you are bored, then have someone else take over. You might find that it only takes half the time to cool than if you leave the coil still.
We did an initial whirlpool to settle a nice filter bed. Then started the water flow thru the coil chiller. I never circulated the wort for fear of disturbing the filter of hops that formed on the bottom of the boil kettle, on top of my filter screen.
When I used an immersion chiller, I would swirl it in the boil kettle to eliminate the boundary layer. Swirling it will also preserve thee trub cone at the bottom of the kettle. A better way is to use a pump to circulate the wort around the coil This will accomplish the same thing and also keep you arm from falling off from fatigue!
I stir constantly but very slowly and gently during immersion chilling, just to keep up enough movement to avoid setting up the boundary layer Richard describes. I run tap water until the temperature is around 90°F (fairly quick, with stirring – no matter the temperature of the tap water, the differential between it and the wort is always significant at this point) and then switch to recirculating ice water with a utility pump. I can get 6-7 gallons from boil to lager pitching temperature in around half an hour to 40 minutes, including time spent switching over to the pump and filling the ice reservoir. (I have a new immersion chiller on the way that should be even faster! Another topic.)
After chilling, I let it settle for around ten minutes. Though the lighter break material settles more slowly, whole cone hops settle very rapidly. Since I rack from underneath them and they constitute a filter bed, I don’t need to wait for the trub itself to settle in order to send crystal clear wort to the fermentor.
Part of our problem is 50% of my copper chiller is above the wort, so we are only getting half efficiency.
Looking at doing a re-wind of the coils to make it both a larger diameter, and shorten the height.