PDA

View Full Version : dumb Reflectix question



sierraDoug
09-15-2005, 00:55
So I go and buy some Reflectix to make a pot cozy. After I start cutting it up I wonder why it only has foil on one side. I go the Reflectix website and they've got it with foil on one side, on both sides, and double bubble thickness each way. (Mine is single bubble.)

So I ask all you experts, if I put the foil on the outside so it will do a good job of reflecting the radiant heat from the pot... will the white plastic on the inside melt or otherwise be a problem?

Thanks mucho.

hiker5
09-15-2005, 09:25
I'm no expert, but I would think your best bet would be to keep the reflective side on the inside of your cozy. If you put it on the outside (and we assume it is the reflective part of the material that is providing the benifit with regards to radiant heat transfer) the heat from your pot would have to travel through the rest of the cozy material before being reflected back. So more of your radiant heat energy is being use to heat the cozy instead of being reflected back to your pot.

I also think that the reflective material on the outside would speed the convective heat transfer from your cozy to the air. Sort of like a heat sink.

The only possible negative effect I can see with putting the reflective side on the inside of your cozy, is the fact that if the pot is sitting on the reflective material, it may be conducting heat away from the pot faster than it would otherwise. The conduction can be minimized by leaving a small air gap between the cozy and the pot. A relaxed fit if you will. Also, the bottom surface that must contact the pot could be covered with something to reduce the heat loss through that surface.

I think the radiation and convection considerations trump the possible loss due to conduction (particularlly if you take some of the precautions I mentioned), but I haven't done any calculations to confirm that. I've considered doing these calculations, but have not taken the time to dig out my heat transfer textbook.

In case there was any doubt, why yes I am and engineering nerd.

Youngblood
09-15-2005, 10:17
Following heat transfer mechanisms (there are three: conduction, convection and radiation) in many applications is a bit like trying to follow the pea in the old shell game. One needs to realize that it takes physical contact at the molecular level for conductive heat transfer to take place, it takes fluid movement for convective heat transfer to take place and it takes wave propagation (which is kind of the opposite of physical contact) for radiation to take place... and all three are likely occuring at some point with the cozy system and all three are likely occuring on the outside surface of the cozy simultaneously, but one or two of the heat transfer mechanisms may be overwhelmed (or made irrelevent) by one or both of the others.

The foil in Reflectix is aluminum which is a good conductor so the solid sheet of aluminum will do a reasonable job of staying at the same temperature (don't think that particularly matters in this application but in some applications it does). The foil is also a good reflector of radiant heat so it won't absorb much radiant heat from warmer objects... this also makes it a poor emittor of radiant heat so it doesn't lose much heat by radiation to cooler objects. The bubble wrap is so-so insulation for conductive heat transfer (I say so-so because it is not as good as closed cell foam of the same thickness).

Okay, enough of the shell game... if your single sided foil pot cozy is making physical contact with the pot, it will insulate best with the aluminum on the outside... if it isn't making physical contact it probably doesn't matter. But, you may melt the plastic if it is in contact with a hot enough pan. In reality, you will likely have enough insulation for your application no matter how you do it as you are just trying to maintain some heat for minutes, not hours. So, I 'vote' for putting the aluminum on the inside to help protect the plastic from the hot pan.

Youngblood

sierraDoug
09-15-2005, 13:06
Thanks for the great input. If I weren't sorry about spending the money on something twice I'd go out and get the double foil Reflectix. In the mean time I may as well try this stuff. I would think the foil on the outside would keep it warmer since the bubbles create the air gap to the foil, but don't want to melt the plastic to my cook pot. So I guess I'll try the foil on the inside. Live and learn.

Dances with Mice
09-15-2005, 14:00
Thanks for the great input. If I weren't sorry about spending the money on something twice I'd go out and get the double foil Reflectix. In the mean time I may as well try this stuff. I would think the foil on the outside would keep it warmer since the bubbles create the air gap to the foil, but don't want to melt the plastic to my cook pot. So I guess I'll try the foil on the inside. Live and learn.Is the plastic easily punctured without the foil? I'd put the foil on the inside (won't melt) and wrap the outside with duct tape.

hiker5
09-15-2005, 14:34
I think Youngblood hit the nail on the head. Any of these setups will likely be more than enough to get the job done.

I made mine out of a windshield sun shade (instead of buying a roll of reflectix) such that it would hold a Heiniken Keg can so I could store may kitchen in side of it. But when I cook, I use quart sized zip-loc bag in the cozy. After the meal has simmered, I pop off the lid, open the bag, and roll the edge of the bag back over the rim of the cozy. This way the cozy holds the bag wide open and I can hold the still very hot bag in my hand.

When I find the time I plan on trying some different stove and pot stand combinations with the keg can pot. The first stove I used for this setup was made from two small SPAM spread cans. The cans were just the right size so I could put the keg can right on top of the stove without a pot stand. This seemed to pressurized the stove somewhat (since it covered the filling hole on top with pretty snug fit) and cause the stove to burn hotter and faster than it otherwise would. The result was burning through a couple of oven liner windscreens. Oops.

sierraDoug
09-15-2005, 15:02
I just boiled some water in my cook pot on my apartment stove (gas) and set it on two scraps of Reflectix, one foil side up, the other plastic side up. Left pot for ten minutes. No melted plastic. Life is good. I'll make the cozy with the foil on the outside and be a happy camper.

Lesson to this story... if you get home with material for a DYI project and something looks wrong, don't cut it up before finding out what's what.

Youngblood
09-15-2005, 17:01
I just boiled some water in my cook pot on my apartment stove (gas) and set it on two scraps of Reflectix, one foil side up, the other plastic side up. Left pot for ten minutes. No melted plastic. Life is good. I'll make the cozy with the foil on the outside and be a happy camper.

Lesson to this story... if you get home with material for a DYI project and something looks wrong, don't cut it up before finding out what's what.
That's great. Made me think of something else though... it might slide easier against the foil side than the bubble-wrap side, if that is an issue.

sierraDoug
09-15-2005, 17:59
That's great. Made me think of something else though... it might slide easier against the foil side than the bubble-wrap side, if that is an issue.
I'm making it a little loose to make life easier getting the pot in. And I might make the bottom foil in, the sides foil out, and the top double layer with foil out and in (heat rises).

Thanks again for the ideas and info. Science is fun.

dougmeredith
09-15-2005, 18:12
heat rises
I think that only applies to convection, not radiation or conduction.

Doug

sierraDoug
09-15-2005, 21:32
I think that only applies to convection, not radiation or conduction.

Doug
Good point.