Ever since I was a kid, I can remember people wrapping cold soda cans with foil in their lunches under the pretense that it keeps them colder longer. This seems bunk to me and a waste of time. Isn’t metal a good conductor of heat, thereby diminishing its insulating properties? We don’t line our attics with it.
Generally speaking, yes. Also, the cans themselves are aluminum, so adding more aluminum around the can will have no effect on maintaining the internal temperature. I could understand that trapped air pockets in sufficiently crumpled foil might have a slight effect, but I’d hazard to guess that it will be minimal.
Metals in general, and aluminum in particular are very good conductors of heat. Nevertheless, I suspect wrapping a can in aluminum foil slightly slows down the rate at which it warms up, because you’re trapping a thin layer of air between the foil and the can. I doubt if it’s a significant difference, however.
Heat can travel by radiation, conduction and convection. Tin foil reduces radiation losses [Heat Transfer 101 lecturer’s hat off]
Oops, cite
[sigh, premature post-ilation]
Heat is transferred from the outside to the can by conduction (your hand touching it ), convection (air currents) and radiation (infra-red). By wrapping it in foil, you have reduced the amount of heat flowing from you hand to the drink (it’s another object to block heat flow and it traps air, another insulator) and also reflects most of the infra-red energy.
There are few finer points though:
- The amount of energy lost through radiation is going to be small compared to the amount of heat lost via the other two methods. The upshot is that the amount insulation you get from foil may end up not being worth the hassle compared to other means. It’s better to use a dry teatowel. It’ll be reusable too.
- If you add too much foil, it is possible to increase how fast the drink is heating up (see Critical Radius for a anal-retentive analysis)
- Errr, hi Opal!
I can see it working if you put the shiny side in and try not to crease it at all. Then put the whole thing in something to block the other types of heat loss, like a towel or a cooler.
Good reflectors are terrible radiators. Either remove the paint from the cola can, or wrap it in unpainted foil, either way this should maintain cooling.
I’ve heard that multiple layers of aluminized (silvery) plastic with air between them is a VERY good insulator, but don’t recall where. The air layers would insulate, but the aluminum layers, once heat had flowed to them, wouldn’t radiate much energy to the next layer in line. Simple plastic sheets wouldn’t do this. A metal might be a good thermal conductor if it touches an object, but a layer of metal hanging in space is a good thermal insulator since it reflects heat back, and also since it’s not a good radiator itself.
Very true. This is the principle behind modern dewar flasks. Aluminized mylar (‘space blanket’) is wrapped around the inner flask to reduce heat loss. In an ideal case the air between the layers is pumped out to reduce convective heat loss as well. Now you have a sandwich construction with aluminium-mylar-aluminium-space-aluminium-mylar-aluminium etc, where the ‘space’ in an ideal case is vacuum. The mylar is poor at conducting heat, and the aluminium (being so shiny) is very poor at radiating it across the gap. Altogether you have a material with a very low heat conductivity. (At least perpendicular to the sheets. I would assume that the heat conduction within the plane is pretty good - but that’s beside the point.)
Have you noticed that all satellites look like they are covered in aluminum or gold foil? They are covered by Multi-Layer Insulation (MLI) which is made up of many layers aluminized plastic sheets separated with layers of synthetic fabric. The highly reflective surfaces minimizes both heating by the sun and radiative heat loss to space.
I believe that’s why emergency blankets for hiking, etc. are called space blankets. I don’t know if they are multi-layered or not, but the principle is the same: a highly reflective outer surface minimizes radiative heat loss.