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pks
I am trying to calculate how much a foil liner helps keep the inside of a package cool. I have calculated the rate of conduction but am now concerned with radiation.

Above is the equation I am using. The emissivity for the material is 0.05, the area is 1 m^2, the outside air temp is 295 degree K, the air temp inside the package is 280 degrees K. This gives a value of 4 watts. Is using the air temp an over simplification or should I be using the outside surface temp of the package? I am trying to create a mathematical model that is as close as possible. Is there anything else I should be looking at?

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Homework Helper
Gold Member
Your calculation is reasonably good. Is it simply a single layer of foil, or do you have a layer of insulation right below that? If the outside temperature of the foil liner (which is the number you put in for ## T_C ##) is at a temperature closer to 295 K, rather than at 280 K, it would aid considerably in keeping the inside cool, both from a radiation standpoint, as well as from thermal conduction. Meanwhile, I presume the package is not sitting in direct sunlight. Even some indirect sunlight could cause the numbers to go up.

pks
I am testing several different materials like foam and bubble wrap with a foil coating. The foil will face outward toward the heat so it should be warmer. The package in testing won't be in direct sunlight but in real world it will at times. So there's nothing else to consider besides conduction and radiation then? Thanks for your response.

russ_watters
Mentor
Based on your title it sounds like you want to wrap something that is already in a cooler with foil. In that case, the temperature difference is approximately zero and the radiation therefore also approximately zero.

pks
Based on your title it sounds like you want to wrap something that is already in a cooler with foil. In that case, the temperature difference is approximately zero and the radiation therefore also approximately zero.
I am determining the effectiveness of foil coatings on foam and different products and how they keep packaging contents like food cool during transit. Sorry for the confusion in the title.