BenB
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| posted on 23/6/09 at 05:40 PM |
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Way off top: anyone know about thermodynamics?
If I'm using a copper coil full of cold water to cool a hot (but no longer heated) liquid is it best to have the coldest water entering the top
of the coil or the bottom....
My thinking is that the water at the top of the bucket will be hottest due to convection so you actually want the cold water entering the bottom of
the coil. It will gradually be heated as it goes up the coil but will ensure that there is a constant temperature differential between the stuff in
the bucket (okay, it's homebrew  ) and the water in the coil, ensuring maximum heat removal from the hot stuff.
Other people though say if you have the cold water going in the top it's best because then you've got the hottest water being cooled by
the coolest water. But equally by that rationale by the time you get to the bottom of the bucket the water in the coil may be at the same temperature
as the stuff in the bucket and so no cooling will take place.
Or do you work on the principle that there is X amount of heat energy within the bucket, that you are supplying Y amount of cold water at Z cc/second
and that convection and thermodynamics will take care of the rest to the extent that it won't make any difference whether it goes in the top or
the bottom..... In which case plan B is probably best (cold water in the top) as the convection will ensure good temperature mixing and no hot
spots...
My brain hurts....
[disclaimer: that's not my photo- my copper coil will be much nicer ]
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twybrow
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| posted on 23/6/09 at 05:42 PM |
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Maximum temperature gradient, will give you the best heat exchange.... I would say go in cold at the top (but I hated thermo at uni!).
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dinosaurjuice
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| posted on 23/6/09 at 05:45 PM |
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its the exact opposite of a central heating cylinder. they have hottest water entering coil at top, so i think putting coolest in at bottom would work
best. i doubt it makes a massive difference to be honest.
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Jon Hazan
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| posted on 23/6/09 at 05:46 PM |
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Im no expert but i would have thought cold water entering the bottom would be most efficient. Mainly due to natural convection within the copper pipe
will want to move the heat upwards.
Also if you run from top to bottom if the heat difference between top and bottom of the pan is enough you could actually end up heating the liquid at
the bottom of the pan!!
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twybrow
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| posted on 23/6/09 at 06:04 PM |
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But equally, if the brew does warm up as the bottom, it will rise to the top of the bucket and hit the cooler pipe.
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JoelP
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| posted on 23/6/09 at 06:15 PM |
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it makes no significant difference IMHO.
However, having a zip tie on the copper pipe means that some heat you've removed will go back into the bucket via the incoming cold! Insulate
between them! Though again, it wont really make a difference.
I say no difference because there will only be a very small temperature difference in the beer between top and bottom. However, assuming the pipe was
100% efficient and the coolant exits at the same temp as what it went past last, you would techincally want it to start in the coolest part and finish
in the hottest part.
However, a further complication is that you cant be sure its hottest at the top - afterall, most of the cooling will happen at the surface!
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nib1980
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| posted on 23/6/09 at 06:17 PM |
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think of it as a kettle in reverse.
BUT it works by the same principle i.e element in the bottom of kettle, therefore coldwater in the bottom first.
enthalpy, entropy, SFEE, etc etc
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matt_claydon
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| posted on 23/6/09 at 06:18 PM |
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If you assume the fluid in the bucket in proximity to the coil is flowing upwards, you essentially have either a parallel-flow heat exchanger (cold in
at bottom) or a counter-current heat exchanger (cold in at top).
Counter-current heat exchangers are well-known to be more effective. First para on Wikipedia article for heat exchangers has a diagram.
However, the convection current in the bucket will be complex and there will be significant cooling from the air/fluid interface at the surface to
complicate matters.
[Edited on 23/6/09 by matt_claydon]
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nstrug
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| posted on 23/6/09 at 06:20 PM |
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Interesting question - try an experimental approach. Run both ways and measure the temperature difference between the input and the output of the
cooling coil. Whichever direction gives you the largest temperature difference is the most efficient.
Nick
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mookaloid
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| posted on 23/6/09 at 06:43 PM |
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quote:
Originally posted by nstrug
Interesting question - try an experimental approach. Run both ways and measure the temperature difference between the input and the output of the
cooling coil. Whichever direction gives you the largest temperature difference is the most efficient.
Nick
Ah yes empirical analysis
"That thing you're thinking - it wont be that."
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indykid
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| posted on 23/6/09 at 06:45 PM |
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most importantly, what are you homebrewing?
looks like mead. mmmmm. mead. leave it slightly fizzy. it's delicious
my thermofluids lectures suggest cold should go in at the bottom for maximum efficiency
tom
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b16mts
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| posted on 23/6/09 at 07:35 PM |
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all this theory could be screwed up if we learn that the "brew" is first heated in the container it now resides, in which case
there'll be plenty of residual heat in the tub.
either way, i shouldn't worry too much.
who says you can't drive a kit car when you're 6'5"?
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craig1410
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| posted on 23/6/09 at 08:48 PM |
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I'd say it will make no difference if...the rate of flow of water through the copper piping is the same in both directions (top to bottom or
bottom to top).
However, unless your pump or source of pressure is much greater than you actually require then I suspect you will get slightly greater flow rate if
you feed cold water in at the bottom as this will gain the benefit of thermo-syphon effect.
You wouldn't want to connect your engine's top radiator hose to the bottom of the radiator as you would then be pumping hot water
downwards and then expecting it to rise through the radiator as it cools - not likely!
There may be other "secondary" effects involved in this but I'd say the primary effect is thermo-syphon and therefore cold feed to
the bottom is the way to go.
Cheers,
Craig.
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BenB
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| posted on 23/6/09 at 09:47 PM |
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quote:
Originally posted by nstrug
Interesting question - try an experimental approach. Run both ways and measure the temperature difference between the input and the output of the
cooling coil. Whichever direction gives you the largest temperature difference is the most efficient.
Nick
Oh you would not believe how tempted I am to do an experiment to find out. Strange how on the homebrew forums the answers to most questions appears to
be "who f#cking cares, open another beer and choose an option randomly". Whilst I appreciate the thinking I prefer the slightly anally
retentive "correct" answer to the usual reply I get of "you worry too much, just make the fricking beer"....
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Nash
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| posted on 24/6/09 at 07:07 AM |
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We need to establish more facts. Is the product succeptable to thermal shock? Does it react better to gradient cooling or quench cooling? Can you
adjust the flowrate of one or both mediums? Is there an optimum time to cool eg 1 degree per 10 seconds?
I think the cooling rate of the product will have and effect on final taste and texture?!
Failing that drink it warm and give yourself a much more enjoyable headache
Regards............ Neil
It's What You Do Next That Counts.
Build It, Buy It, Drive It:
Southern Kit Car Club
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BenB
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| posted on 24/6/09 at 07:33 AM |
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I just remembered that viscous liquids convect heat differently to thin ones as well. I seem to remember from "earth story" that golden
syrup convects in hexagonal cells....
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dinosaurjuice
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| posted on 24/6/09 at 07:42 AM |
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why not pass the beer through the copper pipe submerged in a saucepan of icey water. just to add ANOTHER option
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BenB
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| posted on 24/6/09 at 02:18 PM |
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That would be counter-flow-cooler design. Trouble there is trying sterilise the inside of 25m of copper tubing between brews..... but I like your
thinking
Certainly in the CFCoolers they (as the name suggests) flow in the opposite directions to ensure a constant temperature transfer gradient...
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