> -----Original Message-----
> From: Jim Thompson [mailto:jim at netgate dot com]
> Sent: Monday, 31 July 2006 6:31 a.m.
> To: Paul Dugas
> Cc: m0n0wall at lists dot m0n0 dot ch
> Subject: Re: [m0n0wall] Hardened Platform -40C to +85C
> 1) paint (or powder coat) it a light color. There is far less heat
> gain from solar heating for a box painted a light color than one that is
> painted a dark color.
Craig FALCONER wrote:
> Would shiny chrome (mirrorlike reflectivity) be any better than a
> white-painted case?
(We're now so far off the subject-matter of this list that I'm
embarrassed to be responding.)
The simple answer, "No."
The problem with chrome is that it has a low heat emissivity. Emissivity
is the ability for an object to release radiant heat. In layman's terms,
emissivity is the ability for radiant heat to leave the surface of an
object. It matters not what the density, mass or thickness of the
object, only the surface. The lower the emissivity, the more difficult
it is for heat to leave itsí surface. This why a chrome car bumper is
hotter than one painted black if left to sit several hours in the sun.
Most paints emit in the .90 range which is very high. (See below) Chrome
has an "E" value of .05. It will take that chrome bumper longer to get
hot due the high reflectivity value, but the low emissivity of chrome
"traps" the heat making it much hotter than the same bumper painted black.
Another example is leaving your toolbox open to the sun while doing car
repairs. Ever try to pick up a chrome socket or ratchet handle? How
about a chrome car door handle or the chrome ignition starter on the
steering column? The low 'E value' of chrome prevents the absorbed heat
from escaping making chromed items very hot when left in the sun.
This is also why black chrome solar hot water panels provide hotter
water than panels painted flat black. Black chrome will take a little
longer to get hot, but once it does, the low E selective surface traps
heat in the absorber which in turn transfers it through conduction into
the water passages.
In technical terms:
emissivity: The relative power of a surface to emit heat by radiation.
The ratio of
the radiant energy emitted by a surface to that emitted by a blackbody
at the same temperature
black body: an ideal body or surface that completely absorbs all radiant
falling upon it with no reflection and that radiates at all frequencies
a spectral energy distribution dependent on its absolute temperature
As can be seen below, emissivity (E factor), plays a significant role in
how heat moves into or out of our homes, our cars, our firewalls, and
... our bodies.
Material Emissivity value
Gold, polished .03
Metalized Film Radiant Barrier .04
Silver, polished .04
Aluminum, polished .04
Brass, polished .04
Iron, polished .21
Copper, polished .05
Human skin .98
EMISSIVITY OF BUILDING MATERIALS
Paint, average of 16 colors .94
Brick, common red .93
Plaster, rough coat .91
> (rice my firewall anyone?)
Not if you want the heat to get out. Now, if you were to encase your
firewall in human skin...