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Supercooler Physics
Old 09-09-2011, 01:16 PM   #1
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Supercooler Physics

Consider the TromboneAl supercooler, which has an outer layer of reflectorized bubble wrap:

Supercooler.jpg

If the cooler is covered with something opaque, such as a sleeping bag, does the shiny aluminum material make any difference for keeping the interior cold? IOW, does the reflector help, even if the cooler isn't in the direct rays of the sun?
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Old 09-09-2011, 01:24 PM   #2
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Is your cooler a conspiracy theorist too? Tin foil hats and now this...
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Old 09-09-2011, 01:33 PM   #3
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If it reflects infrared as well as visible light then, yes.
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Old 09-09-2011, 02:06 PM   #4
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If it reflects infrared as well as visible light then, yes.
The foil can only be helpful if it reflects some rays that strike the cooler, which would make the cooler hotter if not reflected. Since the sleeping bag is opaque, any such rays won't be transmitted by the sleeping bag, but as the sleeping bag heats up, it could emit infrared rays which would heat up the cooler, if not reflected by the foil. But the problem with this is that the cooler would also be heated conductively by the sleeping bag and the small air pocket between the cooler and the bag, and my guess is that this conductive heating would swamp any reflective effect of the foil. So, I'm saying no, the foil will not help keep the cooler cool when it's covered up by the sleeping bag.
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Old 09-09-2011, 02:14 PM   #5
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Al, underneath the sleeping bag the layer of bubble wrap will act as insulation and assist in keeping the contents cool. I don't think the reflective foil will help if it isn't on the surface to reflect sunlight.
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Old 09-09-2011, 02:20 PM   #6
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I retract what I said above. I read a little on this page, The physics of foil: how radiant barrier stops heat gain & loss in buildings., on the use of foil in insulation, and it now seems to me that the foil between the cooler and the sleeping bag would be analogous, and would have an insulating effect.
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Old 09-09-2011, 03:58 PM   #7
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I never thought I could think such a thing but I believe it is time for you to go back to work Al.
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Old 09-09-2011, 04:47 PM   #8
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If you want to keep things cool, add the insulation to the bottom. Cold sinks. Added insulation on top will make little difference.

-ERD50
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Old 09-09-2011, 05:09 PM   #9
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If you want to keep things cool, add the insulation to the bottom. Cold sinks. Added insulation on top will make little difference.

-ERD50
That's true for convection, but I think radiation and conduction are relevant here as well. If you put a hot pancake on a bunny's head, he/she will feel the heat. Or if you put your hand on the top of a block of ice, it will feel cold.

Note that this reflectix stuff that I bought is meant to go in walls. So it either reflects heat, or is a scam. OTOH, the Internet tells me that "Infrared radiation can only travel through empty space or transparent objects."

Anyway, it probably won't make much difference, but I've always been curious about this. The cooler will be surrounded by pillows and sleeping backs, etc.
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Old 09-09-2011, 05:11 PM   #10
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The cooler will be surrounded by pillows and sleeping backs, etc.
Hope those sleeping backs aren't wet...
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Old 09-09-2011, 05:16 PM   #11
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That's true for convection, but I think radiation and conduction are relevant here as well. If you put a hot pancake on a bunny's head, he/she will feel the heat. Or if you put your hand on the top of a block of ice, it will feel cold.
Sure, heat will travel (conduct, convect and radiate) down and cold will travel up. But they travel one way at a far higher rate than the other. In true engineering fashion, you want to optimize the use of limited resources. For cold, apply most of your resources (insulation) to the bottom.

A typical cooler (not all) has no insulation in the lid, since it is meant to keep things cool. Beer brewers often use these coolers as a 'mash tun', where the grain and water are kept at ~150F for an hour, and they want to maintain that temperature very closely (within a few degrees) over that time. They end up shooting the lid full of foam (Great Stuff type) and that helps a lot.

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Old 09-09-2011, 07:08 PM   #12
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Al,
The dark colored tape might absorb some light, and creat heat. They make some very good foil tapes used for HVAC duct work that you might want to consider instead.
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Old 09-09-2011, 07:51 PM   #13
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Ok lets start out with some definitions.

A) Heat flows from hot to cold, not the other way. This is the second law of thermodynamics. Don't break it, there are unintended consequences.

B) There are three modes of heat transfer:
  1. Conduction: this is heat transfer thru a medium be it solid, liquid or gas. The rate of heat flow thru solid > liquid > gas. This is why double/triple thermopane windows lose less heat than single panes.
  2. Convection: this is heat transfer due to a flowing fluid (gas or liquid) convecting away heat from a surface (you can also have convective heating, but the term is defined as cooling, another one of those silly laws by that thieving calculus stealer Newton)
  3. Radiation: I'll let a physicist explain it, prepare for equations. Suffice to say all bodies above 0K, including you, emit radiant energy. When this energy falls on a surface three things can happen. The energy can be absorbed by the surface, it can be reflected or it can be transmitted thru.
So you have your bubble wrap with the reflective coating covered by a blanket or pillow. There is essentially no convection at the Al/blanket interface. The incident solar radiation will mainly be absorbed and a small amount will reflected by the blanket. The absorbed energy will result in an increase in temperature in the blanket's surface. This heat will be conducted thru the blanket to bubble-wrap. However, the blanket itself and the bubble-warp with or without the Al foil coating will act as insulators especially if the blanket is a quilt, think of all the air pockets where the conduction is low.

The minor effect of the radiation shield provided by the Al (that's another lecture but a simple equation) will be overshadowed by the insulating affect of the blanket and bubble-wrap.

If you really want to super-cool your cooler I suggest salt. 1 small bag of ice chilled to about 4F in the freezer, about 1 cup of salt, 2 cups water to form a slurry, stir and mix. Toss in 6 bottles of room temp Heineken, stir every few minutes (convective cooling). The Heinies will freeze in about 20 minutes. Throw in a small coke as a canary, it will start to solidify first. But who wants frozen beer anyway.
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Old 09-09-2011, 08:16 PM   #14
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In addition to what has already been said: The bubble wrap has two "foils" separated by the plastic bubbles. The top foil face (directly against the sleeping bag) will not serve as an effective heat reflector--the heat will flow from the sleeping bag fabric to the foil very effectively by conduction, which will be the primary heat transfer mechanism. Now, the back side of that piece of foil is shiny, so it is not a good radiator of heat (i.e. it has low emissivity, which is just the opposite of reflectance in most cases and most wavelengths). The bottom foil is also shiny on the insde, so it's not going to pick up much of the radiated heat. The top foil will give up heat to the other foil face via conductance (through the walls of the plastic bubbles) and convection (through the air in the bubbles). IIRC, with Reflectix this low inter-foil heat transfer is worth about an R-2. The bottom side of the bottom foil also has low emissivity, but it's right up against the cooler lid, so again direct conductance is the primary heat transfer mechanism and the shininess doesn't buy you any benefit. If you could get a small air space there it would help.
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Old 09-10-2011, 07:55 AM   #15
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Wouldn't it be simpler to just add more ice?
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Old 09-10-2011, 08:48 PM   #16
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Quote:
Originally Posted by TromboneAl View Post
...(snip)...
If the cooler is covered with something opaque, such as a sleeping bag, does the shiny aluminum material make any difference for keeping the interior cold? IOW, does the reflector help, even if the cooler isn't in the direct rays of the sun?
Just drink that 6 pack in the cooler before you start out. End of physics problem.
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Old 09-10-2011, 09:14 PM   #17
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Wouldn't it be simpler to just add more ice?
Do you know how much ice costs? What, 99 cents for ten pounds or something exorbitant like that?

This is Al we're talking about.

Spend money on ice, pshaw.
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I never thought I could think such a thing but I believe it is time for you to go back to work Al.
Imagine what he would be up to if he hadn't spent the last week chopping down a national forest into kindling!
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Old 09-10-2011, 10:04 PM   #18
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I never thought I could think such a thing but I believe it is time for you to go back to work Al.


Yeah, I think you engineering types should all get a life, too! Or stop speculating and test the hypothesis!
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Old 09-11-2011, 09:08 AM   #19
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Wouldn't it be simpler to just add more ice?
Well, the objective is to not have to get ice as often (dig out the cooler, pour off the water, repack). Also, sometimes getting ice might mean an extra trip into town or an extra stop on the highway. Sometimes ice simply isn't available.
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Old 09-11-2011, 10:30 AM   #20
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I think you might have better luck creating a perfect vacuum in the cooler - perhaps via a bicycle pump.
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