RogueRose
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Mylar vs Al Foil IR hear reflectivity efficiency
I'm trying to figure out if there is a significant difference in the insulative properties between the two. a 4 ft^3 cube of Polypropylene (MP 266 -
340F) needs to be insulated to keep heat in. The high temp will be 160F at the very highest (where air enters cube) but will probably function
between 110 and 140 for the majority of the time. The outside of the cube will have either rigid foam insulation ~ 3/4" - 1.5" depending upon
location or possibly a fiberglass wrap (the stuff used between joists or studs - that "pink stuff"). The goal is a low temp "oven" with selective
humidity air at the input (from as low as possible to saturation).
The heat source will either be something like a heat gun, hair dryer or possibly a light bulb of some type (halogen on a dimmer for temp adjustment)
with a fan for circulation.
I'm not sure if the aluminum or mylar is particlarly necessary and the research I have seen stems mainly from grow houses in the new medical "green"
industry - where they are trying to make the most out of their light sources - not reflect heat back into the room, so the research wasn't geared
towards heat related results.
I was thinking that lining the inside with the mylar (almost identical to material used to make potato chip bags - thicker than emergency blankets) or
Al foil or putting it on the outside of the cube with the reflective surface facing in and then place the insulation on top of this.
This cube is going to be located inside and outside depending upon time of year and what the goal is at the time.
So, one of the foils better suited for the task. I just found that mylar is pretty stable from –94 to 302°F so temps shouldn't be an issue.
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Twospoons
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I think adding the foil will make bugger all difference, given the low temperatures. Heat transfer by radiation follows a fourth power law, so really
only comes into play at higher temperature differentials.
In your case convection and conduction will dominate, so your foam or fluff layer will be doing all the heavy lifting.
Helicopter: "helico" -> spiral, "pter" -> with wings
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RogueRose
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Quote: Originally posted by Twospoons  | I think adding the foil will make bugger all difference, given the low temperatures. Heat transfer by radiation follows a fourth power law, so really
only comes into play at higher temperature differentials.
In your case convection and conduction will dominate, so your foam or fluff layer will be doing all the heavy lifting. |
Excellent! That is kind of what I was thinking but other reports stated it was needed. Thanks for your opinion!
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Melgar
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I remember getting into a debate on some other unrelated forum, where someone insisted that aluminized mylar was superior, but it turns out, they're
essentially identical for light and IR reflectivity. Though, mylar would melt at any temperatures where a reflective cover would be advantageous, so
I suppose it stands to reason that you should always use foil and never mylar.
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Fulmen
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So why do emergency services use it to prevent/treat hypothermia?
Edit: A few years ago I did some work calculation passive cooling on a plastic foil line. With a dT of appr 20°C the emissive heat losses were
calculated to appr 85W/m^2.
[Edited on 20-1-17 by Fulmen]
We're not banging rocks together here. We know how to put a man back together.
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Twospoons
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Windproof, water proof, very compact when folded up, disposable. The biggest thermal advantage they have is holding a layer of air next to the
patient. Aluminizing the mylar squeezes the last little bit of thermal performance out of the material, for very little extra cost.
Blankets have none of those advantages for an emergency service.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Fulmen
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Good points. But nevertheless, radiant heat can have a significant contribution even at low temperatures. My work on plastic foil entailed 7m^2 of
horizontal PP-foil with a starting temp of 40°C and a final temp of 35°C. We measured a total heat loss of 1100W, of this the radiant heat was
calculated to 600W while the remainder was lost through natural convection. And as we were able to calculate the theoretical loss to within 2% of the
measured I'm fairly confident that these values were real.
We're not banging rocks together here. We know how to put a man back together.
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Melgar
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I meant it more for as insulation for a reaction vessel. For anything that won't burn cellulose, use cotton or paper towels. For anything that might
burn cellulose, better to use aluminum foil and/or fiberglass. If you're hitting temperatures that would greatly benefit from reflective insulation,
those temperatures are likely to melt mylar all over your glassware.
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Fulmen
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In that case I misread your post. You're right, as long as normal insulation is an option I see no reason for using mylar.
We're not banging rocks together here. We know how to put a man back together.
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