Thermal radiation inside a cooler

  • #1
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.
stef3.gif

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?

Thanks in advance.
 
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  • #2
Charles Link
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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.
 
  • #3
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.
 
  • #4
russ_watters
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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.
 
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  • #5
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.
 
  • #6
Charles Link
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One time in college in a chemistry laboratory exercise on calorimetry, we did an experiment where we wrapped the system (in a glass bottle) in aluminum foil to insulate it from radiative heat transfer and observed the results. We still had considerable heat getting into the system. One of my classmates was clever and, as I recall, he put a layer of cotton around the layer of aluminum foil, and then put on a second layer of foil. He slowed the amount of heat that got into the system considerably. ## \\ ## In the case of thermos bottle design, I think some of the better ones use two layers of glass with silver coating with a vacuum in between them.
 
  • #7
pks
One time in college in a chemistry laboratory exercise on calorimetry, we did an experiment where we wrapped the system (in a glass bottle) in aluminum foil to insulate it from radiative heat transfer and observed the results. We still had considerable heat getting into the system. One of my classmates was clever and, as I recall, he put a layer of cotton around the layer of aluminum foil, and then put on a second layer of foil. He slowed the amount of heat that got into the system considerably. ## \\ ## In the case of thermos bottle design, I think some of the better ones use two layers of glass with silver coating with a vacuum in between them.
From what I have read so far that's because foil won't stop thermal conduction, just radiation. I'm going to have either a layer of cardboard, then foil, then foam or foil, then cardboard, then foam. I think foil on the outermost layer will work the best. It's just a little harder to make because you need two separate packaging pieces.
 

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