# The Naked Scientists Forum

#### CZARCAR

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« Reply #25 on: 19/10/2011 14:15:08 »
foil reflects radiant heat the same as a flashlight. Zeroth law sayeth black absorbs radiant best so it has to radiate best also. What are u trying to do?

#### JP

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« Reply #26 on: 19/10/2011 14:34:28 »
Actually, I'm not convinced radiation isn't important (though the paint color is probably fairly unimportant).  A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m2 radiated energy.  That's fairly significant, thought not huge.  It's peak wavelength would be at 7700 nm, which is in the infra red.
http://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Ok, but what does the zeroth law have to do with what I was pointing out?

#### CZARCAR

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« Reply #27 on: 19/10/2011 15:11:46 »
Actually, I'm not convinced radiation isn't important (though the paint color is probably fairly unimportant).  A back-of-the-envelope calculation for a radiator operating at 100 C in a 20 C room shows that it could maximally emit about 680 Watts/m2 radiated energy.  That's fairly significant, thought not huge.  It's peak wavelength would be at 7700 nm, which is in the infra red.
http://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Ok, but what does the zeroth law have to do with what I was pointing out?
"paint color is fairly unimportant"?

#### JP

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« Reply #28 on: 19/10/2011 15:43:47 »
Ah, but the zeroth law is about thermal equilibrium between two bodies.  The direct statement of emission being equal to absorption is Kirchoff's law: http://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiation

Regardless, paint color is still going to be fairly unimportant because we're talking about emission primarily in the thermal range of infra red, while the color of paint has to do with the visible spectrum--two completely different ranges of radiation.  Paint which appears white in the visible spectrum still tends to be highly absorptive in the infra red.

#### CZARCAR

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« Reply #29 on: 19/10/2011 16:00:34 »
i was thinking solar panel where black is prefered color for absorption & the implied inverse for radiation?

#### JP

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« Reply #30 on: 19/10/2011 16:10:59 »
Quite true true, but a solar panel is operating to absorb light from the sun, which is strongly peaked in the visible range: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png  Black paint looks black because it absorbs well in the visible.  Much of the sun's energy is in the visible range, which is why black things left out in the sun get very hot, while white things don't.

For a steam/hot water radiator, most of the radiation will be in the infra red (way off the right-hand side of that chart) which is far from visible radiation, so I doubt paint "color" matters much (since color refers to what we see).  I believe most house paints absorb fairly well in the infra red, though I'm sure there are special paints that are designed to reflect it.  If you polished your radiator so it was highly reflective, it would, however, be very poor at radiating heat.

I could be completely wrong, though--maybe white paints are also reflective in the infra red.  If you do a search, though, you find companies selling specialized paint of various colors that reflects in the infra red.

#### Geezer

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« Reply #31 on: 19/10/2011 17:45:25 »
To settle this one, I've set up an experiment:

I have suspended a piece of sliced bread one inch from a radiator. We should be able to get a pretty good idea of the amount of IR the radiator emits by seeing how long it takes to toast one side.

#### Bored chemist

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« Reply #32 on: 19/10/2011 19:27:28 »
Quite a lot of things that look black- like soot for example, are also pretty black in the IR. However things that reflect white light well are often fairly good absorbers (and emitters) of IR.
The colour of the paint is only an indication of how well it absorbs or reflects visible light and since radiators don't do that, the colour isn't important.
Polishing the radiator to a mirror finish would be a bad idea but I don't expect any real effect from any other change in the surface finish.

#### CZARCAR

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« Reply #33 on: 19/10/2011 20:33:42 »
To account for a body's outgoing radiation (or its emissive power, defined as the heat flux per unit time), one makes a comparison to a perfect body who emits as much thermal radiation as possible. Such an object is known as a blackbody, and the ratio of the actual emissive power E to the emissive power of a blackbody is defined as the surface emissivity e,

By stating that a body's surface emissivity is equal to its absorption fraction, Kirchhoff's Identity binds incoming and outgoing radiation into a useful dependent relationship,
Found this & aint it saying that if black absorbs best it has to radiate best also?

#### Bored chemist

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« Reply #34 on: 19/10/2011 21:01:13 »
Yes,black is the best absorber and therefore the best radiator. It's just that the difference isn't that big and the things we call radiators actually transfer quite a lot of the heat by convection.(which explains the design of them- they are usually set up to have quite a lot of surface area with ribs + ribs etc. if they were pure radiators this wouldn't help.) The complicated bits of this, fairly typical, design
are all facing eachother in the middle of the radiator. Since they face eachother they can't radiate heat out to the room, but they can transfer a lot by convection.

Also, the rated power 1360 W is much higher than the possible radiated power especially as one face of it can only radiate towards a wall, and once the wall warms up there's much less net heat radiated in that direction so the item only has about half a square metre of effective area. It only could radiate a few hundred watts, even if it were run at 100C.
« Last Edit: 19/10/2011 21:07:25 by Bored chemist »

#### damocles

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« Reply #35 on: 19/10/2011 21:24:50 »
For a genuine radiator -- i.e. maximum radiant heat output -- the best colour is black. I suspect that the reason that the water-based heaters you are describing are mostly white, or occasionally pastel colours, has to do with the aesthetics of their not clashing with wall colours. (as lightarrow suggests).

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

Radiative efficiency is exactly the same as absorption efficiency. Something which absorbs all visible light (black) will be a 100% efficient radiator of visible light. But a device operating at 100°C or less will not emit a significant amount of visible light anyway! The bottom line is that in terms of radiation efficiency for a water-heating device, it makes absolutely no difference what colour it is, as far as visible colour is concerned! The important thing is that it should be "black" (that is, totally absorbent) in the infrared.

#### Geezer

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« Reply #36 on: 19/10/2011 22:01:52 »
I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

I sort of doubt it. Haven't the water filled variety been around longer?  Also, water cooled IC powered vehicles have radiators too, and it's pretty clear they work because of conduction

Where are all the etymologists when you need them? (Out catching butterflies as usual?)

#### CZARCAR

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« Reply #37 on: 19/10/2011 22:12:32 »
For a genuine radiator -- i.e. maximum radiant heat output -- the best colour is black. I suspect that the reason that the water-based heaters you are describing are mostly white, or occasionally pastel colours, has to do with the aesthetics of their not clashing with wall colours. (as lightarrow suggests).

I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

Radiative efficiency is exactly the same as absorption efficiency. Something which absorbs all visible light (black) will be a 100% efficient radiator of visible light. But a device operating at 100°C or less will not emit a significant amount of visible light anyway! The bottom line is that in terms of radiation efficiency for a water-heating device, it makes absolutely no difference what colour it is, as far as visible colour is concerned! The important thing is that it should be "black" (that is, totally absorbent) in the infrared.
my non-frostfree fridge has a black heat dissipation grid mounted up the outer backwall of the fridge & the compressor + all freon piping is black. The fridge's main job is to remove heat from its interior, not to heat my kitchen

#### damocles

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« Reply #38 on: 19/10/2011 22:16:22 »
I think the reason these water-based heating devices are called "radiators" even though they mainly work by conduction/convection is because they perform the same function as genuine radiators: the electric devices that make a ceramic bar glow red hot (or a wire coil in some older models).

...(snip)... Also, water cooled IC powered vehicles have radiators too, and it's pretty clear they work because of conduction ,,,(snip)...

#### damocles

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« Reply #39 on: 19/10/2011 22:40:40 »
my non-frostfree fridge has a black heat dissipation grid mounted up the outer backwall of the fridge & the compressor + all freon piping is black. The fridge's main job is to remove heat from its interior, not to heat my kitchen

Although the main (intended) job of your fridge is to remove heat from the cold box, it actually does a slightly better job of heating your kitchen.

From the Law of Conservation of Energy:

Heat released into kitchen = heat removed from cold box + heat released due to the conversion of mechanical energy as the result of friction in the compressor.

In terms of logic, the fact that your heat exchanger or that "all" freon piping is black cannot be taken as evidence that black is a more efficient colour for low temperature radiators, particularly when theory would tell us that colour (in the visible part of the spectrum) is irrelevant.

#### JP

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« Reply #40 on: 19/10/2011 22:43:56 »
Heatsinks on your computer work similarly, and they're quite effective.  Mostly, they're left with a metal surface, which means the designer isn't particularly worried about the low radiation efficiency.

Also, if you check out the wiki link on radiators, they say what most of us are saying: they don't get hot enough to heat efficiently by radiation.

If you've ever shopped for space heaters, though, you'll find they do get hot enough to radiate, and many of them are designed specifically with reflectors to "beam" the heat in one direction.  If you've ever used one of these, you can actually feel the difference between getting in the path of the heat radiation and just standing near it and being heated by convection.

#### CZARCAR

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« Reply #41 on: 19/10/2011 23:12:23 »
efficiency aint @ the radiator, its @ the furnace so the rad is filled with hot water & disconnected. Q= which color will heat the room quicker & 2=whch color will heat the room more efficiently BONUS Q= why?

#### damocles

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« Reply #42 on: 19/10/2011 23:30:29 »
If you disconnect the radiator and leave it with hot water in it, and presume, for simplification purposes only, that the room has no heat losses, then
Q1: Visible emissivity (= colour) will make no difference. If IR emissivity is the same for all cases then any colour will heat the room equally fast.
Q2: Any colour will heat the room equally efficiently because the total heat content of the room is fixed, and the eventual result is a radiator at room temperature, and a room temperature slightly higher than it otherwise would be because of the quantity of heat stored in the radiator (which does not vary with colour).
BONUS Q:  I hope has been adequately answered with the explanations attached to the other questions.

#### CZARCAR

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« Reply #43 on: 19/10/2011 23:40:22 »
Kirchoff? room nonheatloss is unrealistic

#### Geezer

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« Reply #44 on: 19/10/2011 23:53:07 »
I propose a second experiment: We suck all the air out of CZARCAR's kitchen to see if his fridge still works. We could put the fridge into orbit instead, but it might be a bit difficult to find a long enough extension cord.

#### CZARCAR

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« Reply #45 on: 20/10/2011 00:14:49 »
I propose a second experiment: We suck all the air out of CZARCAR's kitchen to see if his fridge still works. We could put the fridge into orbit instead, but it might be a bit difficult to find a long enough extension cord.
need 2 cords= i have a small chest freezer with matching coluors

#### neilep

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« Reply #46 on: 20/10/2011 00:58:42 »
I propose a second experiment: We suck all the air out of CZARCAR's kitchen to see if his fridge still works. We could put the fridge into orbit instead, but it might be a bit difficult to find a long enough extension cord.
need 2 cords= i have a small chest freezer with matching coluors

I have a spare power cord..it's very long....about 15 metres !...so..that's half the problem solved already !

THANK YOU ALL for the great replies here...it makes for very interesting reading..I am learning a lot...

Thanks again.

#### Bored chemist

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« Reply #47 on: 20/10/2011 06:56:57 »
If you disconnect the radiator and leave it with hot water in it, and presume, for simplification purposes only, that the room has no heat losses, then
Q1: Visible emissivity (= colour) will make no difference. If IR emissivity is the same for all cases then any colour will heat the room equally fast.
Q2: Any colour will heat the room equally efficiently because the total heat content of the room is fixed, and the eventual result is a radiator at room temperature, and a room temperature slightly higher than it otherwise would be because of the quantity of heat stored in the radiator (which does not vary with colour).
BONUS Q:  I hope has been adequately answered with the explanations attached to the other questions.

Eventually, the room will reach the same temperature, but it will do it faster with a black radiator than with a silver one.

The interesting point is that, because of the way our eyes are made, we cannot tell a black radiator from a white one.

#### Don_1

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« Reply #48 on: 20/10/2011 09:04:40 »
Most interesting replies, but all very wrong.

I know exactly why radiators are white, its quite simply so that 'er indoors can regularly tell me to repaint them because they have gone off colour. White shows up this fenom phenon phonen phenonimum phenominum phenomenon better than any other, even with the best paint, which usually smells of formaldehyde. If anything needs preserving around here, its me, not the bloody radiator.

#### Geezer

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« Reply #49 on: 21/10/2011 02:37:33 »
Most interesting replies, but all very wrong.

I know exactly why radiators are white, its quite simply so that 'er indoors can regularly tell me to repaint them because they have gone off colour. White shows up this fenom phenon phonen phenonimum phenominum phenomenon better than any other, even with the best paint, which usually smells of formaldehyde. If anything needs preserving around here, its me, not the bloody radiator.

I have a great excuse scientific reason why you should not do that Don. The reason Mrs Don_1 is always telling you to turn up the thermostat is because all the layers of paint on your radiators are preventing the heat from getting out of the bleed things. They work mainly by conduction, and paint is not a very good conductor (unless it's loaded with heavy metals like lead and stuff), so the more paint you put on them, the less effective they are at heating your house.

Soooooo, what you should really do is strip all the paint off, and give them a spray with some acid, or even salt water, so they develop that nice dull iron-oxide finish that seems to be quite popular with the arty crowd these days.
« Last Edit: 21/10/2011 02:55:27 by Geezer »