To get static chill water to really do anything, you need constant agitation, otherwise, the water right next to the coils heats up and acts as a barrier to the rest of the cold water in the container. Agitation is greatly improves heat transfer, unfortunately, it also means the ice melts faster. In the summertime, be prepared to go through quite a bit of ice to cool your entire batch this way.
Personally, I've quite using ice in the summertime and just use my 78 F tap water through my counter flow chiller and get about 82 F wort. I then seal the carboy and toss it in the fermentation chiller for a few hours to bring it down the rest of the way.
Going through 20 lbs of ice on brewday was just the last straw for me.
Prosit!
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