Heat transfer question

zmike6 · science forum beginner Joined: 04 Apr 2006 Posts: 2

I've got a basic heat transfer question, appreciate any insights.
Consider a black, plastic box (closed). Inside the box is an object
you wish to keep cool. The box is placed in sunlight on a hot day, so
it's exposed to both radiant heat (sun) and convective heat (hot air).
Would wrapping the box with a few layers of typical aluminum foil
accomplish any of the following?

1. The foil wrap keeps the object in the box cooler for a longer
period of time, relative to an un-wrapped box, but the equilibrium
temperature the object reaches is the same; or,

2. The foil wrap slows down the rate of temperature increase inside
the box, and reduces the final equilibrium temperature of the object
inside (compared to no wrap).

Or, even option 3: can wrapping the box in a fairly heat-conductive
metal foil actually result in accelerated heat transfer, compared to
the heat transfer across the air-plastic interface of an un-wrapped
box?

Brent Cullimore · science forum beginner Joined: 29 Apr 2005 Posts: 4

If the foil were placed on the inside of the box (outside of the
object), then it would slow down heat transfer but wouldn't change the
final temperature.

HOWEVER, you say the foil is placed on the outside, and that changes
everything. Now the solar radiation sees metal and not black plastic.
You might be surprised to find out that the equilibrium temperature of
the foil is likely to be very much hotter than that of black plastic.
(For black plastic, the IR emissivity is about the same as the solar
absorptivity, whereas for the foil the absorptivity is low but the IR
emissivity is usually even lower, so it gets much hotter.)

So with foil on the outside, it will slow the rate at which the object
heats, but will raise its final temperature above that of an unwrapped
box.

On Tue, 04 Apr 2006 22:14:43 GMT, zmike6 <zmike6@*SPAMBLOCK*yahoo.com>
wrote:

steve@nospam.com · science forum beginner Joined: 16 Sep 2005 Posts: 9

Bit of UFI:

Painting a bullet cylinder (pressurised storage for LPG) white can reduce
it's daytime temperature by up to 7 degrees celcius - at least in Australia,
where it can otherwise get VERY hot due to radiative heat transfer from the
sun.

- Steve.

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