TRVs and towel rads

[John D v2.0]

how a coating of paint lets more heat out of a metal tube (25% more) than a coating of chrome

Radiation is electro magnetic radiation the same as light.
Same way a mirror doesn't let in or out any light of its own, but a black surface lets all the light in but not out.
If you stand in the sun in black clothes you get a lot hotter than wearing white clothes. If you could somehow paint your hand black and the other silver then heated them to the same temperature, the black hand would appear red hot and radiate the heat and the silver hand would still be silver and not radiate much.

But that can't be so because the heat is being convected into the chrome coating just the same as it would into a paint coating

It's being conducted into the coating the same, then conducted into the surrounding air which is then causing convection.
Only the electro magnetic radiation part is affected. In a way your question could be rephrased as "why does a black item look darker than a white item" as visible light is also a scale of emissivity of electromagnetic radiation.

 

[fixitflav]

Do you know why? I didn't beleive, it but having checked, yes it seems that ratings for white rads are better for the same size chrome rad.

I'm curious how a bit of pipe filled with hot water should release more heat if covered in paint than if covered with chrome - especially when neither paint or chrome is a particularly good insulator and the heat it mainly convected. It just seems daft.

As others have said, part of the heat is due to radiation. I recently estimated heat output due to convection, with water 80°C, air at 20°C and surprised to get only ~ 0.4kW/m^2, about 1/2 typical total output figure from a rad. Then I remembered radiation, and that gave another 0.4kW/m^2 (can't remember exact figures, it's on another computer). That was assuming a painted surface, with emissivity 90% of a black body. You can check it from the Stefan-Boltzmann law.

But Engineering Toolbox site says

Surface Material Emissivity Coefficient
- ε -
Chromium polished 0.058
Oil paints, all colours 0.92 - 0.96

and that would reduce the radiation output to ~ 0.024kW/m^2, so quite a substantial drop in total output. More than I would have expected, but there you are!

 

[^woody^]

I'm still struggling. Yes I can see how chrome does reflect radiated heat, but that is when heat is radiated towards the chrome - from across an air void.

Where I am coming from is that the heat from the water travels to the chrome, to the chrome material via conduction, via direct contact between the hot water the steel tube and into the chrome. This is not radiated heat, so how come the heat is not conducted into the chrome plate and out the other side?

 

[John D v2.0]

I can see how chrome does reflect radiated heat, but that is when heat is radiated towards the chrome - from across an air void

"emissivity" works both ways exactly the same. The reflecting that we intuitively know about things not being able to "receive" electromagnetic radiation, also works the opposite way, not being able to "emit" electromagnetic radiation. We just don't have that intuitive sense, because most things don't emit visible light as they aren't hot enough, only infra red.

the heat from the water travels to the chrome, to the chrome material via conduction, via direct contact between the hot water the steel tube and into the chrome. This is not radiated heat,

Correct, that's conduction and happens the same.

so how come the heat is not conducted into the chrome plate and out the other side?

It certainly is, that's conduction into the air surrounding the radiator, and that air moves away from the rad to be replaced by more. That is called convection. These happen the same in any colour radiator.

However radiators also radiate as well as conduct/convect. This radiation is the part that's affected. The heat is conducted out of the chrome plate into the air, but it is not radiated to the surrounding surfaces.

Interestingly, while we're on the topic, if you had 3 rads - one chrome, one black and one white, then you fitted them all into identical perfectly ZERO (edited) emissive 100% mirrored rooms (expensive wallpaper?!), the heat output would be identical, and would not involve any radiation component. Therefore the rad would be less efficient than any calculations. This is because any radiated heat would not be absorbed by the mirrored surfaces due to their emissivity.

also interestingly, your standard rads would be slightly less efficient if they had a layer of grime on them, but your chrome would improve in efficiency (slightly)

 

[D_Hailsham]

if you had 3 rads - one chrome, one black and one white, then you fitted them all into identical perfect emissive 100% mirrored rooms (expensive wallpaper?!), the heat output would be identical, and would not involve any radiation component. Therefore the rad would be less efficient than any calculations. This is because any radiated heat would not be absorbed by the mirrored surfaces due to their emissivity.

What d you mean by "perfect emissive"?

 

[John D v2.0]

What d you mean by "perfect emissive"?

Err sorry zero emissive
Perfect emissive would be a "black body" and the opposite of what I meant

 

[fixitflav]

"emissivity" works both ways exactly the same. The reflecting that we intuitively know about things not being able to "receive" electromagnetic radiation, also works the opposite way, not being able to "emit" electromagnetic radiation. We just don't have that intuitive sense, because most things don't emit visible light as they aren't hot enough, only infra red.

That's right, the emissivity of a body (actual emission/emission from an ideal, black-body emitter) is the same as the absorptivity (

actual absorption/absorption by a black-body absorber). This is Kirchoff's law (the one about radiation, not the electrical one)​

Interestingly, while we're on the topic, if you had 3 rads - one chrome, one black and one white, then you fitted them all into identical perfect emissive 100% mirrored rooms (expensive wallpaper?!), the heat output would be identical, and would not involve any radiation component. Therefore the rad would be less efficient than any calculations. This is because any radiated heat would not be absorbed by the mirrored surfaces due to their emissivity.

also interestingly, your standard rads would be slightly less efficient if they had a layer of grime on them, but your chrome would improve in efficiency (slightly)

To be slightly pedantic, the efficiency (heat into room per quantity of fuel used) doesn't change. An (eg) chrome plated or grimy rad needs more area to output given kW than a black one, but doesn't need more gas.

 

[fixitflav]

"emissivity" works both ways exactly the same. The reflecting that we intuitively know about things not being able to "receive" electromagnetic radiation, also works the opposite way, not being able to "emit" electromagnetic radiation. We just don't have that intuitive sense, because most things don't emit visible light as they aren't hot enough, only infra red.

Correct, that's conduction and happens the same.

It certainly is, that's conduction into the air surrounding the radiator, and that air moves away from the rad to be replaced by more. That is called convection. These happen the same in any colour radiator.

However radiators also radiate as well as conduct/convect. This radiation is the part that's affected. The heat is conducted out of the chrome plate into the air, but it is not radiated to the surrounding surfaces.

Interestingly, while we're on the topic, if you had 3 rads - one chrome, one black and one white, then you fitted them all into identical perfectly ZERO (edited) emissive 100% mirrored rooms (expensive wallpaper?!), the heat output would be identical, and would not involve any radiation component. Therefore the rad would be less efficient than any calculations. This is because any radiated heat would not be absorbed by the mirrored surfaces due to their emissivity.

also interestingly, your standard rads would be slightly less efficient if they had a layer of grime on them, but your chrome would improve in efficiency (slightly)

That's right, the emissivity of a body (actual emission/emission from an ideal, black-body emitter) is the same as the absorptivity (
actual absorption/absorption by a black-body absorber). This is Kirchoff's law (the one about radiation, not the electrical one)

That was meant to be part of my reply, I haven't got the hang of multi-quote yet!


Read more: https://www.diynot.com/diy/threads/trvs-and-towel-rads.500707/page-2#ixzz5BzXjXnEa

 

[John D v2.0]

To be slightly pedantic, the efficiency (heat into room per quantity of fuel used) doesn't change.

Totally agreed, I meant the efficiency of the radiator as an emitter (at converting the heat in the water into heat in the room) not the efficiency of the overall system in converting gas into heat (which would probably not change in any meaningful way)

 

[John D v2.0]

I haven't got the hang of multi-quote yet!

It's a pain using this forum sometimes! Just try using it on a phone, it's worse, as the little selector buttons cover up the multi quote button so you have to just end up typing it manually to avoid posting a load of rubbish (which I do anyway sometimes!)