Recently there has been some discussion about using ice water with an immersion chiller to cool wort below tap water temperature. A couple of methods have been discussed: use of a prechiller submerged in a bucket of ice water to chill the tap water going to the IC; and recirculation of ice water through the IC and back to the cold water bucket or cooler by means of a pump. Either of these would presumably only be used after the wort has already been cooled to near tap water temperature by running tap water through the IC. Intuitively, I felt that the recirculation method would make more efficient use of the ice water, but I finally got interested enough to develop some equations in order to get a better handle on which method is best, and by how much. I wrote up my work in a pdf file which can be downloaded from here:
http://www.mediafire.com/?c9mkm19uxjiz9hj
I had to make some simplifying assumptions in order to be able to do this, so the equations developed won't be completely accurate, but should give a good idea of the relative benefit of the recirculation method over the prechiller method. I assumed complete heat transfer in both the prechiller and chiller; i.e., that the temperature of the liquid leaving the coil is the same as the temperature of the liquid in which it is immersed, and I avoided the complications of melting ice by assuming the use of cold water with no ice. I plotted the results of two "typical" situations, each with 6 gallons of wort at 90ºF, tap water at 80ºF, and starting with cold water at 35ºF with the only difference being the use of 4 gallons of cold water in one case and 8 gallons in the other. I hope some of you find this to be useful, or at least interesting. Sorry for the heavy math in the report, but it was unavoidable.
http://www.mediafire.com/?c9mkm19uxjiz9hj
I had to make some simplifying assumptions in order to be able to do this, so the equations developed won't be completely accurate, but should give a good idea of the relative benefit of the recirculation method over the prechiller method. I assumed complete heat transfer in both the prechiller and chiller; i.e., that the temperature of the liquid leaving the coil is the same as the temperature of the liquid in which it is immersed, and I avoided the complications of melting ice by assuming the use of cold water with no ice. I plotted the results of two "typical" situations, each with 6 gallons of wort at 90ºF, tap water at 80ºF, and starting with cold water at 35ºF with the only difference being the use of 4 gallons of cold water in one case and 8 gallons in the other. I hope some of you find this to be useful, or at least interesting. Sorry for the heavy math in the report, but it was unavoidable.