Name: Al A.
Date: August 2004
Why are most radiators painted white, and not black?
Was asked on your site and the answer given was that a black radiator
would make a better radiator than a white one.
However, I need to understand this further because to me the answer is
ridiculous. Is my understanding of black completely wrong? - black must
indicate an inability to radiate energy - its black like a black hole.
Black holes can only radiate heat through the process of virtualparticles
created at the event horizon - therefore they are not truly black either.
By what process would my truly black radiator transfer energy to the air
If a radiator is painted with black paint the pigment in the paint gives
the radiator colour. Black paint after all is an illusion. Therefore the
question can only be true if we use the words black paint rather than
black colour (which in English is an oxymoron - black is not a colour)
Can you please let me know our thoughts on this as the topic has caused a
major discussion in our office?
Several points are being confounded here:
1. Is there data on what the dominant color of radiators is? (I assume you
mean automobile radiators, not the ones in older houses.)
2. The color "black", and in the sense used in paints and coatings it IS
considered a "color", has nothing to do with "black holes". In the case of
"black holes" the term is used metaphorically.
3. The idea behind painting a radiator white, if in fact they are, is that
white paint would reflect incident light making the radiator more efficient.
However, in the "real world" you could paint it pink, orange or green,
because the dominant mechanism of heat exchange of an automotive radiator
and the atmosphere is convection not radiation. It just does not get hot
enough to radiate a lot of heat by radiation. Fans are installed in most
cars models to increase air flow to increase convection, a car will quickly
overheat if the fan belt breaks, a car in stop-and-go traffic for a long
time will overheat too.
4. "Black body" radiation does not depend upon the color of the body. If you
heat a white bar or a black bar to sufficiently high temperature that it
"glows" red, you get the same "glow" because that depends only on the
temperature. At room temperature a "black body" is still "glowing" but it is
"glowing" in the infrared, not the visible region of the electromagnetic
spectrum so we cannot see the "glow" with our eyes.
Click here to return to the Engineering Archives
Update: June 2012