Radiators, Low Temperatures, Radiation, Convection

Mike T., Swampeast MO

http://www.callisto.si.usherb.ca/~fluo2000/PDF/Fl_010.pdf

Found this while looking for nice radiator air vents.

This one goes in the bathroom.

Steve Ebels

When I saw the title of the post................

Somehow????, I knew it would be Swampy Mike. Where were you looking when you found this gem? Very interesting results as far as the amount of radiant vs. convective energy. Looks like the lower the surface temp the higher the proportion of radiant energy. Nice. Just what we had thought all along.

Mike T., Swampeast MO

I was DEEP inside a Kartoo "Meta-Search"

http://www.kartoo.com/en/kartoo.html

looking for radiator air vent valves of all things.

Just scanned through it and found I wasn't the only kook with "strange" ideas about radiation... Looked like this came from a REAL laboratory instead of just my house...

Printed it and put it in my "reading" bathroom--the ONE in the house with a high-rate flush and great cleaning action that I use for #2 and to give a nice "slug" for the OLD pipes under the basement floor...

There was another interesting statement found on a European website, "...since TRVs are nearly universal..."

Mike T., Swampeast MO

Now if only

I can somehow explain why this particular shape seems to radiate more than it is "supposed" to radiate...

Even accounting for my crude methods there REALLY is something about this shape!

Look at the shadow on the floor and how the depth/height of the curves changes with perspective...

Uni R

Mike, have you seen these?

http://www.heatprofile.co.uk/NewFiles/home.html

It really makes the "BB" more truthful for HWBB. The right half of my brain thinks they are on to something really good, while my left lazy suspicious side says if it were so good, why wouldn't they be more universal already?

Eric Taylor_37

Apples and oranges again

The heat emmitter that they used for this whole study is a flat panel radiator. It dosen't look anything like a cast iron radiator. Read how they did the study very carefully and remember that the results presented only apply to this set of test conditions.


"Radiation represents about 70 percents of
the total heat flux issued from the heater.

It shows a predominance of the heat transferred by
radiation among the global heat flux issued from THIS TYPE OF RADIATOR.

The fraction
of heat exchanged by each transfer mode (2/3 for
radiation, 1/3 for convection) remains unchanged if
smaller water temperatures are used."


They also didn't talk about USEFUL energy, just the energy emmitted from the test subject. Radiating onto the wall behind the radiator dosen't seem to help ME very much when I'm on the other side.

Don't get me wrong, I like radiation. I also just love being immersed in a warm fluid. ;-)

Eric

Mike T., Swampeast MO

"They also didn't talk about USEFUL energy, just the energy emmitted from the test subject."

BUT since this is a "steady state" study where the transfer of heat to the adiabatic surfaces (those at the same temperature as the air temp where temp loss/gain is at parity) it seems rather evident that ALL of the emmitted energy is USEFUL energy in that it is not increasing the OVERALL heat loss of the space.

The energy radiated to the back wall is NOT lost--it still contributes to the heating of the rest of the objects in the space be they solid or fluid.

Yes, it only applies to a certain type of panel in a certain size in a certain shaped and certain space with heat loss controlled in a certain manner. No, they're not going to use cast iron radiators because a) they're not particularly common in new installations b) the construction of the panel used is VERY similar to the ORIGINAL radiator on which all subsequent radiators are supposed to be based c) the mathematics involved with column or tube iron radiator would be unbelievably complicated.

Specifics in a contrived space do not directly reflect reality--REAL spaces have too many variables. The BEST that such can provide are trends and generalities.

Of particular interest (to me at least) is the demonstration that the convective portion of output drops compared to radiative as the surface temp of the panel drops (3rd page, bottom right); the high percentage of radiant output (numerous tables); and the way that the "standard" way of determining what surface temperature should achieve steady-state is too low at moderate outside temperatures. There seems to be a correlation here with the drop of convective percentage of output.

You can never forget that these tests are based on steady-state--the condition that proportional control strives to achieve. Were this the test of raw power in a radiator (the ability to RAISE temperature) the convective/radiative proportions would likely be MUCH different.

Also remember that proportional flow control is not the only way to APPROXIMATE steady-state. The extreme mass of tube-in-slab does this quite well despite significant changes in supply temperature (i.e. digital control). At the opposite end would be digitally controlled fin baseboard. Other digitally controlled emitters like dimensionally large (but of less mass) radiant panels, iron radiators; convectors; iron baseboards would fall somewhere between these extremes.

Thermal comfort is exceptionally related to radiation. Our body WANTS to loose heat via radiation from the skin--ALWAYS. To a certain degree we can control this radiation loss from our skin--and we do everything (first sub-consciously then consciously) to achieve this. Do you really thing it cooincidence that skin (regardless of "color") is exceptionally emissive?

I could probably show a distinct correlation between the material taught in heat engineering classes and increased energy company profits. Would it be valid?

Radiation to a space no more than 50% EVER? Pah!!

TRVs "Mickey Mouse"? Lunacy!

Mike T., Swampeast MO

No--haven't seen

A "Thermofin" baseboard with a copper lined conduction route to the water...

EGAD!

MOST IMPORTANT thing I saw in the specifications was "TRV Included"!!!!!!!

To put their output ratings into U.S. terms it appears that they claim an output of 190 btu/hr per lineal foot at 180° average supply water.

Each lineal foot (INCLUDING the top) has about .54 square feet.

This would equate to about 360 btu/hr output per square foot of the radiating surface @ 180° average temperature.

Granted I believe in the power of radiation but 360 btu/hr per square foot @ 180°? Such would only be possible if the cold surfaces (the walls/ceiling) were EXTREMELY cold AND significantly more BTUs than "normal" were available in the pipe.

In a 10x12 room with a 3' door that would put make about 22 square feet of radiating surface. Start "covering" the baseboard with furniture and the radiation/conduction/radiation gets REALLY strange and you'll have a problem with rapidly changing outdoor conditions--the system simply won't be able to transfer heat (primarily radiation) rapidly enough as the truly cold surfaces will be "blocked" from view.

---------------------

Remember that the temperature in the U.K. (and even much of Europe) is VERY moderate compared to much of the U.S.

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THOSE OF YOU WEST OF THE SIERRA MADRE SHOULD INVESTIGATE THIS PRODUCT!!!!!!!!!

Eric Taylor_37

get real

"BUT since this is a "steady state" study where the transfer of heat to the adiabatic surfaces (those at the same temperature as the air temp where temp loss/gain is at parity) it seems rather evident that ALL of the emmitted energy is USEFUL energy in that it is not increasing the OVERALL heat loss of the space.

The energy radiated to the back wall is NOT lost--it still contributes to the heating of the rest of the objects in the space be they solid or fluid. "

Steady state dosen't mean adiabatic! Adiabatic means no heat is transferred. In the real world all exterior walls lose heat. Look at that diagram again on page 4 they assumed no heat loss through only two of the walls and the floor and ceiling. The ENTIRE wall opposite the emmitter is single pane glass and the wall behind it is an insulated exterior. Look at the difference in the radiation numbers from front to back as the outdoor temp climbs. In mild weather almost all the radiation is being lost out the back through the exterior wall!! (3rd page, bottom right) And when it gets cold out that glass window starts sucking the energy out of the panel. Where is that energy going? OUTSIDE!!!!! Rember, they assumed that the floor, ceiling, and two of the walls were not transferring heat, so the heat load of the space was minimal resulting in very little heat required to maintain the internal air temp. Hence small convection numbers. Did you see the change when they added a small amount of ventilation to the mix? They used therotical and numerical methods to investigate this. The two DON'T match! They rarely do, but their's are off by quite a bit. As much as 25%. This says they need to rework some of the assumptions and recalculate.
As for the ratio change with lower temps, don't forget that as things get smaller your error becomes more and more significant.

"Radiation to a space no more than 50% EVER? Pah!!

TRVs "Mickey Mouse"? Lunacy!"

I might get 15% radiation out of my baseboards, I sure can't feel it unless I'm inches away from it and I just LOVE the TRVs that I put in. My floors are a little cold on the first floor (workin' on that , but I am extreamly happy with 85% convection. There is absolutely no way you are getting more than 30% radiation out of your collumn rads mike. Again its the area ratio thing. Model the old iron as a stack of vertical plates and it looks just like a stack of fins on a baseboard. They call 'em radiators but to me something that puts out more than 50% convection is a convector.

Eric

Eric Taylor_37

ahem

That vertical plate must be convecting quite a bit to move that much heat me thinks.

Mike T., Swampeast MO

I worded something poorly there. Wasn't trying to say that "steady state" at a specific height above the floor meant that the entire space itself was adiabatic. I had to look up the word "adiabatic" when I wrote the above. In such a model truly adiabatic would be impossible but they seem to saying that it appears as such because any heat that transfers "in" (air in places warmer than the water behind the wall) equals heat transfered out (air in places cooler than the water behind the wall). Thus the statement about the water in those walls staying constant.

Granted that is a strange construction with no heat loss through either the floor or the ceiling and with heat loss occurring only at opposing ends. BUT, if related in truth, such IS the "standard" by which radiant panels are rated in Europe. I'm assuming that such a construction is done to try to eliminate as many variables as possible.

"two of the walls were not transferring heat"--oh yes they were--radiation occurrs between ALL objects unless both are absolute zero in temperaure.

"glass window starts sucking the energy out of the panel" Yep. Just like ANY other object--as it gets colder it just "sucks" more. This one gets strange but remember that glass is opaque to energy in the infrared range. It's loosing heat via radiation to the outdoors from the OUTER surface, not through the glass. Granted it's a good conductor and poor insulator but there IS a difference as the heat had to be conducted through the glass.

The radiation is naturally "sucked" by its points of greatest loss because the warm surface is receiving less than it is transmitting to the cooler surfaces.

No matter how deep the text, every writing on thermodyamics prefaces itself with some version of "radiation is the most poorly understood of the heat transfer processes". I don't claim to know even a fraction of what those who say such understand. What I do know though is just because it's "poorly understood" I WILL NOT try to claim that it is "inconsequential" JUST BECAUSE I DON'T FULLY UNDERSTAND IT!

The description of the construction behind the radiator confused me a bit as well. I believe though that the "cold" walls were consructed similarly to the others (water behind) but that there was insulation directly behind the radiator in an approximation of a "real" space.

Eric Taylor_37

first principals

"radiation occurrs between ALL objects unless both are absolute zero in temperaure."

False. Radiation occurs whenever there is a DIFFERENCE in temperature between two objects. No temperature difference, no radiation. Adiabatic means no heat transfer of any kind. For those two side walls in this study they set them as adiabatic because they want them to behave as interior walls with heated space behind them. If both sides of a wall are at the same temperature then no heat can be transferred through the wall and it is therefore adiabatic. Given that both sidewalls are maintained at the same temperature in the room there can be no radiation exchange between them.

I don't really care HOW the heat gets through the window. All I care about is that it gets through and is lost to the outside. How can this be good?

"The radiation is naturally "sucked" by its points of greatest loss because the warm surface is receiving less than it is transmitting to the cooler surfaces."

I agree with that completely. So my panel rad sees a cold surface of an exterior wall and radiates like mad at it. That cold wall sucks up the radiation and conducts it to the outside where it is lost. What good is that? Don't give me that garbage about re-radiation either. The exterior wall is still colder than everything else in the room including my skin. The exterior wall is ALWAYS colder than the interior wall also, so it is ALWAYS a heat sink, not a source.

Speaking of skin, you said that humans are designed to radiate. What are all of those sweat glands for then? To make us smell nice?

You think I am discounting radiation because I don't understand it?!? BAH! You are overemphasising the radiation because YOU don't fully understand first principals. There is no magic here. Yes, the math is really messy, but all can be reduced to first principals for the basis of COMPARING two emmitters. The panel rad radiates and convects from ALL surfaces while the collumn rad has a large proportion of it's surface area INTERNAL to the rad. These surfaces CAN NOT RADIATE because they "see" only surfaces at the SAME temperature. Again, no temperature difference, no radiation. The air in contact with the internal (and external) surface of the collumn rad IS at a lower temperature, so it WILL convect.

Eric

Mike T., Swampeast MO

So star "A" doesn't radiate to star "B" and vice-versa?

No alpha and beta particles (and everything "lower" as well) going between them even if their mass and composition (thus temperature) are identical?

I don't think you discount radiation any more than I try to say that it's the ROOT of more heat transfer than many believe.

The sweat glands are there to cool us when our attempt to loose heat via radiation is inadequate and the air movement across the skin is also insufficient. Even though you may not think about it we really do try to loose heat first via radiation by exposing as much of the surface of our skin as possible to the environment.

In real-world construction the radiation from the back of such a panel goes into a wall. Generally a wall of heavy mass that is typically quite conductive. The radiation doesn't just radiate to the outside from the back side of the plaster/gypsum/wood etc., it spreads through it via conduction.

We can't let the walls be the temperature of the great outdoors and still be comfortable except in the most contrived of circumstance like a mylar survival blanket.

RB_2

as you were saying...

Eric Taylor_37

I like how you think.

You and I need to sit dow and have a few beers sometime Mike.

Do you ever get out to Taxachusetts?

Thermal radiation and matter radiation are very different animals. I really doubt that stars interact much on any level given the distances involved. Point source radiation intensity (W/m^2) falls off with the cube of the distance. The nearest star is about 100 light years away. That's an area of over 4 million square light years for our little star to shine at. Why is it so darn cold on pluto?

The root of all heat transfer is temperature difference. Nothing more. Space IS at abslolute zero, or at least as close as you need to get, so objects radiate into space at T^4th. It was you who reminded me that exchange between two bodies was (T1^4th-T2^4th) remember?

Exposing our skin to reject heat will always result in BOTH types of heat transfer. We sweat when THEY aren't enough. I can't radiate and NOT convect.

I could never get my walls to equal the outside temp without shutting off all heat sources anyway, so a mylar blanket would be very welcome. How much conduction do you get through the edge of the wallboard versus through the face? Seems to be a rather large surface area ratio there--OOPS there we go again with the whole area thing. Maybe the root of all heat transfer is temperature difference AND surface area. Hmmmm.

Eric