Does the color of a radiator matter?

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Does the color of a radiator matter?

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RustyRad

Can you please explain the mechanism by which metallic paint damps heat radiation? I don't see how the metal flakes could act as an aggregate radiant barrier and reflect the radiator's heat back to it--because, with no air gap between the paint and the metal, the paint should become a heat conductor. And since metal is a good conductor, the reduction in radiation seems counterintuitive. Green Building Advisor has lots of articles about the inappropriate use of radiant barriers in applications where no uniform (preferably horizontal) air space is maintained. So, what is the real mechanism for this heat-squelching effect?

DanHolohan

I think it's fully explained in the above 1935 article from the National Bureau of Standards. The metal flakes in the paint affect the way the radiator radiates. It has no affect on convection.

Jamie Hall

Perhaps a little more technically - radiation of heat from a surface is determined by two things: the temperature of the surface, as one might expect, but also a property called emissivity of the surface. And the emissivity of more or less pure metals is very low -- hence if the outermost surface is a more or less pure metal -- such as those metal flake paints -- the rate at which energy is radiated (not, as @Dan Holohan emphasizes, convection) will also be very low.

Hence such things as space and rescue blankets...

Leonard

Haven't read about flake paints. But have read in my heat transfer book that the COLOR of the radiator has an effect on emissivity.

A given shape at a given temperature colored Black radiates more BTU/hr than same shaped colored gray. However not many people will accept the look of a black colored anything in their house, so for some reason gray color became standard. Should be similar effect for other colors. (look up blackbody and graybody radiant heat transfer).

Not much important with finned tubing, since each fin mostly just "sees" the adjacent fin ( view factor). And mostly any heat RADIATED from one fin just heats the other fin, so color is not very important. Suspect a coating of paint on fins would be a bit bad, (insulation)

Jamie Hall

True enough, @Leonard -- but the emissivity of paint, even glossy, is much higher (60% and up) than metal flake paint (3%, maybe) regardless of colour -- although a high emissivity dead flat black would be best. I suspect the spousal acceptance factor needs to be considered too, though...

(somewhat off topic, but a special dead flat black, very high emissivity finish was used on the SR-71 -- not for stealth, but to dissipate at least some of the aerodynamic heating)(the SR-71 at speed was hardly stealthy -- it glowed brilliantly in the infrared!)

The blocking from fin to fin is also a factor, but in most radiators it isn't as bad as one might think; look at a radiator and see how much of the tubes one can see from even a fairly flat angle (keeping in mind that if the surface isn't specular -- mirror -- the radiation is nearly Omni-directional). It is a factor in fin tube radiation, however, if one starts to really get precise.

Gordy

Actually the SR-71s paint scheme served multiple purposes. Dissipate internal, AND external heat gain, and help absorb radar waves along with its aerodynamic shape. The cockpit windows were 2" thick quartz since glass would deform at 600 degrees, and give the pilot a not so good view of their surroundings.

At the time of the SR-71s high use, heat signature was hardly an issue since missile technology did not possess the speed, guidance, or altitude to intercept. Interception of a moving target at 2100 mph (approx. 3000 FPS) , and an altitude of 16 miles was near impossible at the time.

Jamie Hall

But oh they were beautiful, eerie birds, @Gordy -- I managed to get up close and personal to them a few times, a long time ago... I remember watching a night takeoff of one, somewhere "over there" -- and the pilots flicked off the lights just after takeoff, and all you could see was the shock diamonds from the afterburners, disappearing in the sky...

Gordy

What I find inspiring is the era from development to production. Kelly Johnson proclaimed this would be the fastest plane into the 21st century. He was right.

There is a SR 72 in the works. Unmanned Mach 6 hypersonic. Unmanned takes the thrill out of it. May as well call it a drone. In which case again the A12 of which was the predecessor of the sr 71 did do a drone launch off the back of an A12. It didn't go well as both aircraft were lost, and one of two pilots. Just something about our excellerated advances in aircraft after WW II. Makes you wonder what we don't know about now, like we didn't know about the sr71 then. That's what's in the stove pipe........

Leonard

Read in my engineering magazines about some Mach ~15 birds in development.
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Was thinking more about flake paint on radiators this morning. Seem to remember from my heat transfer book that a surface that radiates heat well also absorbs well. And vice versa, a poor radiating surface also poorly absorbs.

On space craft they use shinny gold coatings to stop space craft from absorbing sun's radiant heat , so gold is a poor absorber.
And similarly the emergency thermal blankets for hikers are just shiny metalized plastic ( to reflect the heat back to you)

So it's reasonable a shinny flake surface would be a poor radiator of IR heat.


I'm GUESSING a steam house radiator only delivers 1/3 to 1/2 of it's BTUs by radiant heat transfer, the rest by natural convection heating of air flowing over it. So painting a radiator with shiny flake paint reduces it's radiant heat output, but shouldn't reduce convection output if paint is thin (not an insulating layer).

Gordy

an oldie, but a goodie from 1924.


https://nvlpubs.nist.gov/nistpubs/nbstechnologic/nbstechnologicpaperT254.pdf

Fred

Link just opens a very faded, completely unreadable PDF file @Gordy. Not sure what it is???

Gordy

Hmm it opens for me. Did you let it load? There is 19 pages of material.

Jamie Hall

No problem for me, either, @Gordy .

Leonard

Page down a bit, begining has unreadable ghost like image. Later pages the full text is easily readable.

Interesting report, basically says metal surfaces and "flake" paint are poor radiators. A few interesting quotes from the conclusions section at end:

"Another example worth citing was given by a former colleague interested in the manufacture of Portland cement. By applying a coat of aluminum paint to the outside of the kiln he was astonished to find that the saving of fuel could be measured in tons of coal. " (leonard... but then again cement roasting kilns run VERY VERY hot.)


"steam-heated radiator is in reality a convector of heat, the heat dissipated into the room by radiation from the sides being relatively of secondary importance. For example, in a 2 –column radiator of 13 sections, Allen and Rowley found that of the total heat dissipated about 30 per cent was lost by radiation and 70 per cent by convection. When this radiator was covered with aluminum paint, it dissipated only about 81 per cent as much heat as the nonmetallic covering. (leonard..... thin paint only effects the radiation portion)"

leonard...... Also interesting that I've seen roofing companies sell aluminum paint to apply to new tar roofs to lower roof temps in summer so tar roofing doesn't deteriorate as quickly over the years. Guess lower temps slows tar's outgasing, so takes more years to become brittle and bad.

Fred

Strange. Document loads fine but the entire document is all "ghost" print and unreadable on what looks like age yellowed paper. Oh well, no worries.

Leonard

Strange. I clicked on the link again and found the whole thing ghostly and unreadable.

When I tried to load it the first time my computer was sluggish, so I copied the link and pasted it into my browzer instead, it worked fine. (My computer has been getting sluggish for about a month or 2)

I'm using a windows XP operating system and firefox browser

Fred

I'm using Vista and Firefox. ???

Gordy

I don't know works every time for me. Second page is blank. There is some ghosting on the pages because the scanned pages had print on the back side. However it is perfectly legible.

1Matthias

Working fine for me. Ubuntu Linux, chrome and reader. Took a while to load though.

Leonard

Most interesting part is conclusions at end.

In a radiator with "regular" paint ~ 30% of heat output is by radiation and 70% by convection.

Painting with aluminum paint reduces that to ~ 80%.

That means the radiation portion of output is reduced by ~ 60% just by changing the paint........significant. The change sounds reasonable as emisivity likely changes that much.


Interesting thing is study talks so much about painting underside of roofs that it must have been conducted try to find a way to cool Quanset hut type Army buildings down south

C10

Apologies for coming in late, and getting all theoretical. Hope I don't start any flame wars.

This is an interesting topic, with a lot of mythology floating around. People who think radiators mostly radiate just need to put a hand a couple of feet above and a couple of feet to one side of a hot radiator in a still room. (This includes a few Ph.D. physicists I've worked with, who simply could not believe the evidence of their own senses vs. the notion that a "radiator" must be radiating.) And of course putting an open-top or high-louvered cabinet around a radiator can greatly improve heat transfer via the chimney effect.

Getting a little further into the science, another source of confusion comes from the wavelength range in question. Most people use "emissivity" in reference to visible light. Since sunlight's "white light" energy curve peaks about where visual sensitivity does, ca. 0.5 micron (or "micrometer," for younger folks), that makes sense for most applications.

But at the temperature of low-pressure steam or hot water, in accord with Wien's displacement law, maximum energy emission is at ca. 8 microns in the mid- to long-wave infrared. In the mid-IR, almost everything except metals absorbs pretty strongly. -So much so, in fact, that a few thousandths of an inch of a typical paint-binding polymer is nearly "black" at those wavelengths. Metallic paint will have extremely low emissivity only if there are a lot of flakes relative to polymer, and with a very shiny, flat surface. More on that here, among many other places: https://www.flir.com/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/.

For those actually interested: clean snow is one of the whitest substances accessible to most people. But the same tiny overtone absorption that gives thick ice a blue tint becomes more effective with multiple scattering. So snow is also one of the "blackest" substances - right down there with soot - in the mid/far IR.

Another source of confusion is that most laypeople think of plastics as good insulators for heat, as they are for electricity. They are not, unless compared with metals or crystalline carbon. As the Stefan–Boltzmann law says, emission goes as the fourth power of temperature. As the Engineering Toolbox indicates, steel and copper have much greater thermal conductivity than acrylic-type paint binders. But with both convection and radiation fairly inefficient at such a low temparture gradient (what matters is "absolute" temperature, say 370K fins vs. 300K room), a few thousandths of an inch of paint won't slow transfer much. Paint on fins reduces the sensation of heat to a finger, but doesn't so much slow heat flow to air.

I've rebuilt a number of home heating systems, but am not a pro. So if real life differs materially from what this science says, I would appreciate being educated.

Gordy

Radiators do both. Convective, and radiant heat transfer. Any hot object displaced some radiant heat transfer.

CBRob

It's a fascinating subject.
Im guessing most paints are going to show a minimal effect on the heating you get from s radiator.

An interesting display of the reduced heat radiation from a metallic surface is a chrome wrench left in the sun.
The shiny metal is a great absorber of short wave radiation and a poor emitter of long wave radiant heat.
A chrome tool left in the sun will get far hotter than a black tool

DavidinKenai

C10 offers lots of good information, which I would summarize as: All paint is black paint for radiant purposes (unless it has metal flakes in it).

I was playing with stove and pot efficiency for ultralight backpacking and it seemed clear that a pot bottom painted black would absorb (instead of reflecting) more IR from the cherry-red steel burner head below. And it does. But so did white paint. Because the paint was only white in the visible while functioning like black paint in the IR range.

I heated an aluminum pan uniformly and measured the apparent temperature (using a non-contact IR thermometer) of differently-painted sections:

Black: 197F
White: 197F
Red: 185F
Unpainted (bare metal): 98F Yes, 98F like body temperature. I didn't drop a "1" on that.

The sections were all the same temperature - they had the same hot air blowing on the other side and each section evaporated a water film similarly. And yet the bare metal emitted IR like it was at 98F while any color of paint emitted like it was 90-100F hotter.

When metal is really shiny, it functions as a mirror in the IR - the non-contact thermometer reading you get will just be a reflection of everything else in the room. What color do you see in a mirror if the room is painted green? Green. What "color" (temperature / IR emissions) do you "see" (feel) from shiny metal if the room is 70F? 70F.

Here's the write-up I did on the painted pot experiment (it starts 9 posts in): https://backpackinglight.com/forums/topic/57444/

Motorapido

David, I've enjoyed reading your ultralight backpacking forum posts for years. Glad to see you sharing your knowledge on this site, too. Small world.