Naked Science Forum
General Science => General Science => Topic started by: neilep on 16/10/2011 11:02:04
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Dearest Radiatorlogists and those studying radiatorology.
As a sheepy I of course luff radiators. radiators are my all time favourite metal panel wally things that have oil or water and radiate heat !...hey !!....what a coincidence with them being called RADIATors eh ?...I bet when the bloke who invented them called them that he did not realise the coincidental implications to the nature of their radiance practicalities !
look, here's a radiator.
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A radiator.
Now , following on from CZARCAR's thread here (http://www.thenakedscientists.com/forum/index.php?topic=41349.0;topicseen)
Can ewe tell me why most radiators are white ?...I have seen yellow and green and blue ones too !...so why the other colour choices ?...also..would it be silly to have a black radiator ?.....Surely a black radiator would get just as hot and that heat would have to be radiated yes ? *guffaws at the name coincidence again*..*le sigh* [::)]
So whajafink ?
Why Are Most Radiators White ?
huggy kuggy
mwah mwah
Neil
Please do respond about radiate
Elucidate and Elaborate
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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Strikes me radiators are usually white as that is the normal colour of radiator paint. I guess they would radiate more efficiently if painted black but would look awful. Possibly the colour has little effect as central heating radiators operate at low temperatures and give off most of their heat through convection and not radiation. Perhaps they should be called convectionators?
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maybe the white results in a more even heating of the radiator?
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A lot of the heat is directly coupled into the surrounding air by conduction. The heated air rises and circulates around the room by convection. I don't know what the ratio between radiation and conduction is, but I suspect most of the heat is removed by conduction. Same thing with the "radiator" in your car. Should it really be called a "conductor"?
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Possibly the idea is to alleviate the re-absorption of the room heat by the radiator when it is not carrying heat.
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Possibly the idea is to alleviate the re-absorption of the room heat by the radiator when it is not carrying heat.
Interesting idea, but you'd have to run really cold water through the radiator to remove much heat from the room. If the water is not flowing, there is nowhere for the heat to go.
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I painted one of the conductors brown in room number 6. [;D]
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Strikes me radiators are usually white as that is the normal colour of radiator paint. I guess they would radiate more efficiently if painted black but would look awful. Possibly the colour has little effect as central heating radiators operate at low temperatures and give off most of their heat through convection and not radiation. Perhaps they should be called convectionators?
Thanks MikeS. So, the colour makes little difference. Though, to call them convectionators would have an impact on the nomenclaturial labelling on the packaging and thus create the use of more ink and be non eco friendly [;D] (unless a smaller font would be used which could cause eye strain) [:D]
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maybe the white results in a more even heating of the radiator?
Well this is what I thought. Thanks CZARCAR...but would another colour* make it truly uneven ?
* Note to self: ewe know 'white' is not a colour.
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Possibly the idea is to alleviate the re-absorption of the room heat by the radiator when it is not carrying heat.
Thanks Grizelda....good thinking !!
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Possibly the idea is to alleviate the re-absorption of the room heat by the radiator when it is not carrying heat.
Interesting idea, but you'd have to run really cold water through the radiator to remove much heat from the room. If the water is not flowing, there is nowhere for the heat to go.
So, if I wanted to..I could turn my lounge into a fridge ! [;)] Thanks Geezer !
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I painted one of the conductors brown in room number 6. [;D]
Room 6 eh ?...well done ! [:D]...good job it was not room 5 !..it's important ewe know ! [;)]
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maybe the white results in a more even heating of the radiator?
Well this is what I thought. Thanks CZARCAR...but would another colour* make it truly uneven ?
* Note to self: ewe know 'white' is not a colour.
dunno, but water comes in @ 1 end & goes out the other? So I think more even + water prbly flows to rads downsream? Never seen rad like yours but it looks like 2 heated panels with a hole in the middle for convection? if so white would keep the surface facing the room not as hot so u dont burn urself as bad if touched?
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I checked out my radiators and they're all white, too! They're the old steam-heat type. The walls of my apartment are blue in the living room/bedroom, white in the bathroom, and yellow in the kitchen, but the radiators are white throughout. Weird.
I agree with Geezer that most of the heating is going to be by conduction/convection, so color isn't a big deal. The thickness of the paint could be a factor--if you have a really thick layer of paint it will insulate your radiator a bit and make it less efficient, so you don't want to layer on too much. But the effects of black/white paint on the thermal radiation from a normal radiator is going to be minimal--the radiator just isn't hot enough for it to be important.
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To a very good approximation, radiators don't radiate.
In particular, since they do not radiate visible light, their colour cannot matter much.
They are generally made of metal (which is quite a good conductor) and filled with flowing water (which has a high heat capacity) so their temperature is fairly even- there's a small drop in temperature between the input and the output. A radiator with a completely even temperature wouldn't be delivering any heat to the room.
People seem to like white, and it's cheap.
Grizelda's suggestion could only make sense if the room was warmer than the radiator.
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You can detect a bit of heat coming off them with the back of your hand. I always thought that was infrared radiation. If it is, there's not much of it.
The old fangled cast iron ones had a large surface area in a relatively compact volume and they could really chuck out the heat! However, in the UK at any rate, they gave way to the sleek, modern, pressed steel variety. I installed some "skirting board" radiators in a house because I hated the appearance of those big steel slabs. They consisted of a copper pipe that had lots of alumium* fins on it inside a natty teak and steel casing (this was in the seventies, so it had to be teak!)
*TNS preferred spelling to avoid transatlantic hostilities
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infrared gun thermometer, does the colouur of the surface affect the temp reading?
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The old fangled cast iron ones had a large surface area in a relatively compact volume and they could really chuck out the heat! However, in the UK at any rate, they gave way to the sleek, modern, pressed steel variety. I installed some "skirting board" radiators in a house because I hated the appearance of those big steel slabs. They consisted of a copper pipe that had lots of alumium* fins on it inside a natty teak and steel casing (this was in the seventies, so it had to be teak!)
The ones in our place now are the cast-iron. It's an old Victorian-era house. The ones where I grew up (built in 1985) had those awful copper pipe with fins. As kids, we'd constantly bend the fins out of shape by falling into the radiators or "accidentally" smashing toys into them.
One advantage of the fin variety is that it was much harder to burn yourself, whereas my brother managed to get a nasty burn once from a cast-iron radiator.
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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.
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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 seems to be about consistent with my back-of-the-hand measurement [:)]
If your bro was able to burn himself, those radiators must have been using really hot water, or even steam, like the ones in NYC. We were able to sit on top of the ones in our house in the UK.
I think I know the type of finned baseboard radiator you are referring to. Pretty ugly things, and not at all like the superior Scandinavian designed (did I mention they were enclosed in teak) variety that I installed in our bungalow in Renfrew.
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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 seems to be about consistent with my back-of-the-hand measurement [:)]
If your bro was able to burn himself, those radiators must have been using really hot water, or even steam, like the ones in NYC. We were able to sit on top of the ones in our house in the UK.
I think I know the type of finned baseboard radiator you are referring to. Pretty ugly things, and not at all like the superior Scandinavian designed (did I mention they were enclosed in teak) variety that I installed in our bungalow in Renfrew.
Yeah--the ones we had (and have now) are steam. The radiators we had as kids were the baseboard variety with fins.
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All the places I have lived didn't run radiators anything like as hot as 100C. We didn't want to get burned so 50 or 60C was probably the upper limit.
That makes a difference to the power output and, presumably, our perception of whether they radiate or convect..
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Why Are Most Radiators White ?
Maybe because most walls are the same colour.
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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
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The radiator in room 5 ewe sea is white like the rest. I am sticking stuff up behind all the radiators that is like about 5mm thick foam covered in a layer of foily substance from Germany. It states that it reflects 95% of something back into the room instead of into the wall underneath the window. What is the best colour for a radiator?
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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?
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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?
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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"?
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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.
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i was thinking solar panel where black is prefered color for absorption & the implied inverse for radiation?
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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.
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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.
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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.
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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?
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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
http://www.wickes.co.uk/double-universal-radiator/invt/201484/
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.
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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.
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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 [:D]
Where are all the etymologists when you need them? (Out catching butterflies as usual?)
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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
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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 [:D] ,,,(snip)...
Telling point, Geezer! I had temporarily forgotten about automotive radiators. I stand corrected.
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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.
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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.
http://en.wikipedia.org/wiki/Radiator_(heating)
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.
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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?
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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.
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Kirchoff? room nonheatloss is unrealistic
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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.
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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
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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.
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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 point about radiators is power transfer, not energy.
The interesting point is that, because of the way our eyes are made, we cannot tell a black radiator from a white one.
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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.
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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.
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Strange you should say that Geezer, I keep telling the silly old bat that if I paint the walls much more, there'll be no space in the rooms for us to get in!