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Old 08-01-2010, 03:18 PM   #11
spriolo
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Quote:
Originally Posted by rgray58 View Post
Agree. The science is interesting, but definitely over my head. I figure to build a chiller based on a flexible combination of my limited knowledge, reasonable sounding anecdotal evidence cited by others, within my budget, good money value, job size-matched with expansion capability, and doable within my limited skills. From all indications, what I plan will work and I am sure I can modify in different ways and it will still work.
Sounds... complex.



 
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Old 08-03-2010, 11:16 AM   #12
ni*
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The question isn't as complicated to answer as it might initially appear. Whether additional tubing length results in faster cooling is reducible to a much easier empirical question: Using a 20' cooler, is the exit water cooler than the wort? If it is, additional length will give faster cooling.

In practice, the exit water on a 20' cooler is going to be cooler than the wort (at least after the initial few minutes) by at least a few degrees. The real question is not whether additional length will result in faster cooling (it definitely will), but whether it will result in a big enough improvement in cooling rate to justify the expense and effort (as alluded to above).

As everyone else has mentioned, this depends strongly on the particular details of your setup, but it's still easy enough to answer. Simply take a 20' coil, measure the exit water temperature and compare it to the wort temperature. If it's quite significantly cooler, more length would help. Of course, if you're using the bathtub method, this experiment is a bit tough to perform.

For what it's worth, if I were you I'd go with the 20', unless you're planning to make batches larger than 5 gallons or have some reason to suspect you're going to have trouble cooling. 20' is fine for everyone else with 5 gallons, and it will almost certainly be fine for you.



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Old 08-03-2010, 05:06 PM   #13
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WOW, that answer and method of deduction is so simply prefect. I will be doing batches of approximately 10 gallons so i think I'll end up going with a longer coil anyway. I love the info that comes up in these threads. HBT wins a gold star again.
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Old 08-03-2010, 05:14 PM   #14
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if the exit water of the chiller is hotter than the wort temperature, you have violated the laws of thermodynamics.

 
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Old 08-03-2010, 05:18 PM   #15
ni*
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Not quite (there is friction between the water and pipe, and within the water itself, although it's miniscule), but yes, effectively. You've misunderstood, though: the question isn't about whether it's cooler or hotter, but whether it's cooler or equal. And it's always going to be at least marginally cooler (again, thermodynamics) so the question is really (as I said above) about how much cooler it is.

 
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Old 08-03-2010, 06:30 PM   #16
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Quote:
Originally Posted by ni* View Post
Not quite (there is friction between the water and pipe, and within the water itself, although it's miniscule), but yes, effectively. You've misunderstood, though: the question isn't about whether it's cooler or hotter, but whether it's cooler or equal. And it's always going to be at least marginally cooler (again, thermodynamics) so the question is really (as I said above) about how much cooler it is.

This is a true statement. What annoys me right now is it seems like I would have been able to solve a problem like this when I was in my Thermal Fluid Dynamics class, but I am drawing a complete blank when I think of how I would go about setting up this problem. I keep tossing around words that I remember from the class like laminar and turbulent flow, as well as thinking that you could set up a model equation and differentiate to find the min/max's which would allow you to determine an optimium length. However I don't get far when actually trying to do it. The above description by ni seems like the most appropriate way to consider the issue. I do wish I wasn't so long out of college (only a few years) so I would have a chance at remembering how to approach a question like this.
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Old 08-04-2010, 10:56 AM   #17
spriolo
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Doesn't it all boil down to what temp the source water is?

All other variables (including the boil temp of the wort) are predictable. The volume of the wort is important, but that scales up and down predictably too.

What temp is your source water? You can guesstimate your length requirement by evaluating your source temp. The colder your source the more you can save on length, and visa versa, if you live south of the Mason-Dixon line you might need more copper in the pot.

 
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Old 08-19-2010, 08:24 PM   #18
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Quote:
Originally Posted by ni* View Post
The question isn't as complicated to answer as it might initially appear. Whether additional tubing length results in faster cooling is reducible to a much easier empirical question: Using a 20' cooler, is the exit water cooler than the wort? If it is, additional length will give faster cooling.

In practice, the exit water on a 20' cooler is going to be cooler than the wort (at least after the initial few minutes) by at least a few degrees. The real question is not whether additional length will result in faster cooling (it definitely will), but whether it will result in a big enough improvement in cooling rate to justify the expense and effort (as alluded to above).
Yes, exactly.

 
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Old 08-19-2010, 10:22 PM   #19
rgray58
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Quote:
Originally Posted by spriolo View Post
Doesn't it all boil down to what temp the source water is?

All other variables (including the boil temp of the wort) are predictable.
Close. For the most part, the source water temp is a known value coming out the tap. But that source water temp increases while passing through the chiller and the wort temp drops. The temp differential (TD) between chiller water and wort water is directly proportional to the heat transfer rate (HTR) between the two. The greater the TD, the greater the HTR and the faster the wort temp drops - which is the goal. The smaller the TD, the lower the HTR, and as chiller water temp increases and wort temp decreases, the wort temp drop is going slower and slower. The question then becomes a comparision between heat transfer efficiency and cost. As ni* indicates, as long as the chiller discharge temp is lower than wort temp, some heat transfer is occurring. 100' of coil would probably still allow for that heat transfer at the end, but the cost would be prohibitive and the heat transfer rate toward the end of such a long run would be low. I stick with my idea that two 25' coils will work better than a single 50' coil (or similar setup) because each coil will maintain a higher temp differential throughout its run. At the start of each 25' run, chiller water is as low as source water. Anecdotal evidence is that a 25' run discharge is pretty hot. With a 50' run, chiller water is as low as source water at the start and after 25 feet that temp is again, "pretty hot". (Sorry, but I haven't actually done measurements.) If that second 25' run was using water at source temp like a dual coil setup, the heat transfer rate would be higher because the temp differential is higher. But a dual coil setup adds financial and labor cost that some may find prohibitive.

 
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Old 08-19-2010, 10:47 PM   #20
prosper
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the simple answer is that it's all about surface area. More = faster cooling, provided you're able to flow enough coolant water over it to absorb the heat.


There are other factors, but mainly it comes down to surface area and coolant capacity.



 
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