General Category > Zymurgy

chiller performance: sep/oct 2013

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--- Quote from: hopfenundmalz on August 23, 2013, 02:02:18 PM ---
--- Quote from: kramerog on August 23, 2013, 01:20:35 PM ---I was a chemical engineer, now a patent attorney.  I have a copy of Perry's where the data they use comes from.  The article is purely a paper study.  Their calculations are reasonably accurate for doing comparisons among common cooling options.  I would not assume that they are accurate for other purposes.    I do not consider what they are doing "BS," but I'm not surprised that one person's real world data does not coincide with an estimate from a paper study.

Theoretically, counterflow cooling should always be faster than batch immersion cooling for same coolant flow rate and same heat transfer area assuming proper design because in counter flow the coolant exiting the cooler is cooling wort at the initial temperature.  In immersion cooling, very little of the coolant is cooling wort at the initial temp.

--- End quote ---
Old MechE here. In Table 2, he has the counter flow with slightly higher Area for the study, the U is roughly a wash. My quandary is how does the immersion suddenly perform better than the counterflow with just a 5F change in cooling water? I think it has to do with the definition of the Delta T avg for an immersion. I might have to pull my dusty Heat Transfer book off of the shelf.

--- End quote ---

I think you are onto something.  I would expect that the curve for the immersion cooler would curve upwards a lot more given a 3 degree F temperature approach.  I'm fairly sure that the immersion cooler equation is incorrect unless there is a very complicated equation for Delta T avg because the immersion cooler operates at non-steady state.

It's that "delta T avg" that points to the problem.  It is apparently an attempt to get around the problem of thermal gradients within the wort; the simplified linear equations assume the only gradients are between wort and cooling water.  For plate chillers this assumption is probably justified, as the distances between plates are small and the wort velocity probably results in complete mixing (within each channel).  For counterflow chillers it may be untrue but probably not far enough off to cause major error.  But for an immersion chiller, the assumption is not justified unless the wort is completely mixed at all times, such that the temperature at any point is very close to the average temperature.  Unless you stir really vigorously and continuously, this is far from true; there is a significant thermal gradient within the wort (from the center of the kettle to the chiller coils) and this greatly increases the time to chill the wort.  (The time required for heat to travel through the wort toward the chiller coils is nowhere accounted for in the equation; and the actual gradient at the coil/wort contact is much lower than the equation assumes.)

Failure to recognize, identify, and examine all the assumptions behind a mathematical model is the #1 cause of model failures.  That is the best and most lasting lesson my hydrogeology prof (Michael Campana) taught me many years ago.


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