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I Efficacy of white/aluminum tarps at shedding heat

  1. Aug 3, 2017 #1
    Recently there was a discussion on a tree planting forum.

    Contractors in the field will put a tarp over boxes of seedlings to keep them cool.

    The tarp is white on one side, and aluminum coated mylar on the other. Discussion was:
    * do these work?
    * Silver side in or out?

    Whether white or aluminum, I don't see that it makes much difference. The ideal material would be perfectly white with an emissivity of close to zero. Whether is is white (random scattering) or silver (specularly reflecting) makes little difference.
     
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  3. Aug 3, 2017 #2

    berkeman

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    What if the outside layer were a very efficient mirror, reflecting 99% of the insolation? Would that change your answer? :smile:
     
  4. Aug 4, 2017 #3
    My assumption was that you had perfect silvering on one face, and perfectly white paint on the other.
     
  5. Aug 4, 2017 #4

    berkeman

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    I did a Google search on Reflectivity of White Paint, and got lots of good hits. Including ones that compare white paint to reflective surfaces... :smile:
     
  6. Aug 5, 2017 #5

    sophiecentaur

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    Radiation is not the only mode of heat transfer - particularly inside the tent. If the two surfaces have exactly the same reflectivity, it won't make any difference to the radiation situation but convection / conduction against the inner surface will make a difference to the inside temperature. The shiny surface would have less effective contact area (perhaps) so that could imply that it could be best inside. The ground will be the 'cold sink'.
    I wonder about this every week when I wrap a hot Sunday Roast in foil to rest. Shiny outside or inside?.
     
  7. Aug 5, 2017 #6
    About "resting" hot food. The two things I consider are: One, that any foil touching the meat is likely to draw more heat away through conduction, so I try to avoid that and 'tent' it. Hmmm, but if the foil is already hot from convection/radiation, maybe not a big added effect? But I would think more of an effect than shiny versus dull?

    Two, I hate wasting that aluminum, it takes a lot of energy to produce aluminum, and often the cooking aluminum doesn't end up in recycling. But even recycling takes some energy to melt it down, so I generally invert a large stainless steel bowl over the resting meat to avoid that issue. No conduction, and even if it does get a bit of fat on it, that washes right out.
     
  8. Aug 5, 2017 #7

    sophiecentaur

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    Yes. A 'tent' is the way to go, I think, with minimal contact.
    To minimise the effect of wastse, I keep the tent foil to wrap the meat when it's in the fridge (after cooling) and the foil is used until the meat is all gone. I do like the SS cover idea.
    I earn brownie points by putting as much of the fat as possible into the dustbin. Fatbergs are a menace for the sewage industry.
     
  9. Aug 5, 2017 #8
    Yes, keeping fat/grease/oil out of the sewer system is important, good that you pay attention to that, many don't.

    I am on a private well/septic, so this would have a direct affect on me in $$$ and inconvenience in case of a back up, or a failed septic field. So we routinely separate fat/grease/oils, wipe plates and pans with paper towels before washing, etc.
     
  10. Aug 5, 2017 #9

    sophiecentaur

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    Returning to OP matters. I really should experiment with the meat tent thing. I roast something nearly every week and it wouldn't be difficult to spend the last minutes before carving in measuring the temperature drop. I think any visiting Family would have some comments about that, though. "Dad - what aaaaaaare you doing?"
     
  11. Aug 5, 2017 #10
    Hah-hah! For extra "family eye-rolling effect", be sure to set a timer, and record the temperature every XX seconds, and enter the values into a spreadsheet. Added points for graphing the results as you go! Us geeks have to have our fun!

    I actually do this with the cooking Turkey, though there seem to be secondary effects as the meat looses moisture or something, it seems rather hard to predict based on a slope, or even to characterize from year to year. And yes, the kids laugh at me, which only encourages me!
     
  12. Aug 5, 2017 #11

    sophiecentaur

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    The parent strikes back.
     
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