Emissivity (basic facts about heat radiation)

Roospike said:

BrotherBart said:

NY Soapstone said:

First, how much heavier do you suppose your PE stove is compared to whatever a low grade knockoff might weigh? Let's be generous - 250 lbs more?

-Colin

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One of the things I have been curious about. While damn sure not willing to call my Englander a "low grade knockoff" it is approximately the same size as Spike's Summit Classic but while the Summit has a 3/8" top and 1/4" sides and the NC-30 has 1/4" top and 3/16 sides, the Summit is only 20 pounds heavier? Does not make sense.

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PE Summit 3.0 cf firebox weight 475lbs. Dimensions: 25 1/2" W x 29 1/4" H x 23 1/2" D
vs
Englander 3.5 cf firebox weight 455lbs. Dimensions: 23 1/4" W x 27" H x 24 1/2" D

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There seems to be a good bit of the difference, enough to make up the weight?

BrotherBart said:

Spike nailed it in a PM. The Summit sits on nine inch legs and the NC-30 legs are six inch so the NC-30 stove body is taller.

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Hey now I brought up legs, ashpan & height damnit LOL.
Anyways, if both PE & Englander are willing to send me a Summit and 30-NCL & NCP for a controlled experiment. I would be more than happy to strip em down and weigh each part, compare and post my findings. Of course I cannot guarantee return of the stoves *evil grin* Now I think thats a fair offer, and Impartial also

Roospike said:

BrotherBart said:

NY Soapstone said:

First, how much heavier do you suppose your PE stove is compared to whatever a low grade knockoff might weigh? Let's be generous - 250 lbs more?

-Colin

Click to expand...

One of the things I have been curious about. While damn sure not willing to call my Englander a "low grade knockoff" it is approximately the same size as Spike's Summit Classic but while the Summit has a 3/8" top and 1/4" sides and the NC-30 has 1/4" top and 3/16 sides, the Summit is only 20 pounds heavier? Does not make sense.

Click to expand...

PE Summit 3.0 cf firebox weight 475lbs. Dimensions: 25 1/2" W x 29 1/4" H x 23 1/2" D
vs
Englander 3.5 cf firebox weight 455lbs. Dimensions: 23 1/4" W x 27" H x 24 1/2" D

Click to expand...

posted #'s are "stove size" and not the exact steel plate size but we'll run the #'s of the top plate as shown to show the weight difference of just 1/4" thick vs 3/8" thick ...............( you wood workers might be suprised )


PE Summit top plate Dimensions: 25 1/2" W x 23 1/2" D X 3/8" thick = 69.92 lbs

Englander top plate Dimensions: 23 1/4" W x 24 1/2" D X 1/4" thick = 41.50 lbs

Now if i took a heating torch to both steel plates, set at the same BTU it would take a lot longer to heat up the thicker plate and a lot longer to cool down , also , the end max temperature of each plate, at the set BTU of heat applied to each steel plate would be DIFFERENT temperatures of the end result on the different plates.

In the end this is why when fabricating these two pieces of steel you would have to have two different torch heat settings and also when welding these steel plates you would have to have higher amps and voltage to weld the thicker steel.

( this has nothing to do with one stove being better than another , the posted information is inline with the topic of heat transfer and the side topic of heat transfer with steel plate as an example)

Real world #'s of the exact same size steel plates 26" X 26" of each:

square steel plate -

26" X 26" X 3/16" thick = 35.49 lbs

26" X 26" X 1/4" ..thick = 47.32 lbs

26" X 26" X 3/8" ..thick = 70.98 lbs


***************************

Full Steel Plate -

4' X 8' X 3/16" thick = 241.92 lbs

4' x 8' X 1/4" ..thick = 322.56 lbs

4' X 8' X 3/8" ..thick = 483.84 lbs

I have deleted the off topic comments. Dylan, if you dont have something to add here besides antagonizing the members then go somewhere else (please).
Im putting this thread in time out.

OK folks, this thread is open for business.

A great thread, gone a bit awry. Back to original post- I wonder this:
If the surface area of the radiating body (just infrared radiation for now) increases its BTU output with more surface area, WHY don't they attach HEATSINKS all over the outside of woodstoves? Ya know, a massive version of the heatsinks that attach with heat-conductive compound to all your micro-processors? Say that the heatsinks were painted flat black for max IR radiation efficiency. Would'nt that hugely increase the body's surface area and so increase its heat radiation? OK- so blow a fan over these heatsinks to increase the stove's convection, too?
Would'nt these stoves be UGLY to the eye, but crank out alot more heat for the size of stove? Of course they would have to be loaded with more wood more often to generate this heat. I read thru all the responses and must now go get some aspirin.
This is a really neat-o site. Thanks.

That original information is quite useful. It's good to know that I'll get twice as much heating if I get that surface temperature to 600 versus 400. But, do you think it will take twice as much wood in the box to raise it from 400 to 600 degrees?

Gark said:

A great thread, gone a bit awry. Back to original post- I wonder this:
If the surface area of the radiating body (just infrared radiation for now) increases its BTU output with more surface area, WHY don't they attach HEATSINKS all over the outside of woodstoves? Ya know, a massive version of the heatsinks that attach with heat-conductive compound to all your micro-processors? Say that the heatsinks were painted flat black for max IR radiation efficiency. Would'nt that hugely increase the body's surface area and so increase its heat radiation? OK- so blow a fan over these heatsinks to increase the stove's convection, too?
Would'nt these stoves be UGLY to the eye, but crank out alot more heat for the size of stove? Of course they would have to be loaded with more wood more often to generate this heat. I read thru all the responses and must now go get some aspirin.
This is a really neat-o site. Thanks.

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not sure of this , but anyway, heat sinks would give more surface area yes, but remembering that we have a finite amount of btu's imparted to the firebox area, heat sinks would reduce the overall temperature of the surface area (which is really a heat sinks purpose in life) if it were overdone as in too much surface area, wouldnt the heat level per square inch fall off? this would not allow the heat to be radiated as far ( i think someone pointed out that the higher the surface temperature the more active the transfer of heat would be?)so if the surface area didnt match the ability of the heat source inside the stove's capability wouldnt its effectiveness diminish? i dont know the science well enough to state this, so im asking instead , but it seems logical to me that it would. i could very well be wrong

Yes- a stove (heatsink- studded) WOULD need "more wood more often" to maintain its surface temp. at, say 500 F. But it seems that this is how stove makers could design a unit to heat a larger space with a physically smaller stove. It would only require the user to visit the stove more often to stuff it with fuel. After all, you can't get 50 million BTU of heat from fuelwood with intrinsic heat value of 30 million BTU's. The stove's efficiency would not be improved by heatsinks. Back to the drawing board.....

Sounds like you need to put a pot of stew on top of the bad~boy and take advantage of the heat.

Some light reading from MIT on Heat Transfer
http://mightylib.mit.edu/Student Materials/books/ahht.pdf

DriftWood said:

Some light reading from MIT on Heat Transfer
http://mightylib.mit.edu/Student Materials/books/ahht.pdf

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Good link - the example on page 79 helps illustrate one of the points I was making in this thread earlier. The concept of understanding relative resistances to heat transfer for convection versus conduction is very important when "optimizing" a device to either hold or transfer heat.

Too often people focus on thickness of conductive slabs and how conductive steel is vs stone, etc... and in fact, there are other much more rate-limiting factors that dictate the performance of a woodstove.

-Colin

BrotherBart said:

I would like to shove a big heat sink up Accuweather's butt, sideways. Two hours ago I loaded up the big ass heat sink that Mike sold me for the 22* low they forecasted. It is 43* outside and they changed their minds to 33* low. Grrrr....

Yo Mike! Where is the "low temp" button on the 30-NC?

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BB we only offer that in our pellet units, but if ya got ideas , i'll listen , once that beast is cooking its probably like trying to stop a brahma bull at a rodeo after he leaves the chute.

Gark said:

Yes- a stove (heatsink- studded) WOULD need "more wood more often" to maintain its surface temp. at, say 500 F. But it seems that this is how stove makers could design a unit to heat a larger space with a physically smaller stove. It would only require the user to visit the stove more often to stuff it with fuel. After all, you can't get 50 million BTU of heat from fuelwood with intrinsic heat value of 30 million BTU's. The stove's efficiency would not be improved by heatsinks. Back to the drawing board.....

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dang good thought though gark, its interesting enough for me to think about it, ive thought about putting heat sinks in the exhuast/ room air interface with our pellet and corn units for that very purpose, if they were aligned right so that they didnt obstruct convection ?"room" air , might just boost output, havent had time to "play" with that yet, but another summer approaches. i might just have to do some sketching

heat sinks are for air. they transfer most heat (or are designed to transfer heat) to air through convection. that is why they have fins. the fins dont' help as much for radiation, because energy radiated from one fin is absorbed by the neighboring fin. only the surfaces that "aim" away from the stove and other hot parts of it radiate energy away.

(epic post worthy of elkimmeg)

This thread is an interesting read if for no other reason than to gauge people’s perceptions of how a fire heats a house. Early on in the thread I mentioned that there are really three modes of heat transfer and was essentially shot down and basically told that unless I am touching the stove, ‘radiation pretty much covers it’.

I knew nothing more than a flame war could ensue so I backed out and let the thermodynamics experts have their say and enjoyed the read. No that the thread is somewhat back on course I’d like to continue the discussion. I’ll put a disclaimer in that I am no thermodynamics expert…just some old guy that cuts wood and knows how to punch a few keys on a calculator now and then. So here is my latest try at understanding what goes on in a wood stove:

First, it occurs to me that anecdotal evidence would suggest that emission can’t ‘pretty much cover it’ because I don’t burst into flame or get severe burns on my skin every time I am within site of a wood stove. In fact, even when my stove was the hottest I had ever seen it, I could comfortably walk up and stand a foot of the front of the stove with no ill effects! Now, notice I said the front of the stove, the top gets a little tricky because that is where the three main modes of heat transfer all come in to play.

So I began to wonder what equations might quantify individual mores of heat emission versus conduction/convection. I searched a bit on the internet and found:

http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon

This site has a couple of online calculators where you can punch in various parameters and get a relatively simplistic idea of what is happening with the thermal transfer (note the one of the calculators is wrong, but we can save that for a different thread) So I began to wonder what a simplified model of a stove top (say a 2’ x 2’ steel plate maintained at 500F) would yield and punched in the given parameters:

Atmosphere / heat parameters

77F room temp
500F stove top

Stove top parameters

24” x 24” steel plate (576 in square)
3/8” thick
.95 Emissivity (nice flat black color)
26 BTU/hr ft^2 F thermal conduction (approximate value for rolled steel sheet)


I will save all the boring equations, but punching those numbers in reveals approximately 5,000 BTU/hr radiation / emission from the “stovetop” while almost 16,000 BTU/hr is carried away by thermal conduction/convection. What do you know…conduction beats emission by over a 3:1 margin! Maybe this is why thermos bottles use a vacuum as insulation…with no air, there can be no conduction and no convection…the only thing left is emission. But compare cooling a cup of coffee on a tabletop where all modes of heat transfer are active to coffee in a thermos and it again seems to support the fact that not only is there more to heat transfer than emission – but the other modes seem to outweigh emission by a substantial margin.

Corey

I agree that convection will beat radiation until ludicrous temperatures are reached. radiation is driven by the difference of the 4th powers of the absolute temperatures. Convection is driven by the differences of the temperatures (to the first power). when the temperature gets way up there, the radiation can take over, transferring more heat. this is why nuclear weapons burst in the air tend to fry things they "shine" upon. large radiation due to high temperature.

However, the convection from the flat stove top is not the best. convection is a consequence of gravity, and the vertical sides of the stoves are where the big convection will happen.

you may recall one of my comments in other posts "...like those silly thermoelectric fans atop the stove..." or whatever I said.

The "forced" convection of the silly fan is miniscule compared to the heat transfer of the vertical plume of hot air that the vertical sides (and to some extent the stove top and bottom) cause through free convection. A "down" ceiling fan on the other side of the room completes the circle and you can keep a whole big room homogenous with a ceiling fan on low.

The stove top example did not say whether it was for horizontal or vertical, but it still makes the point the convection is a bigger heat transfer mode than radiation at the relatively low temperatures of stove operation.

also, I should reiterate--radiation is driven by difference of forth power temperatures. this means that while convection may handily beat radiation at 500F (I did not check the math and may be guilty of propagating an error here...), the radiation transfer may overtake the convection transfer surprisingly quickly. I don't have time to calculate where it happens, but there is enough info to figure it out in the link above, or check -Fundamentals of Heat and Mass Transfer, Incropera and DeWitt, Wiley-(one of the best texts on the subject, IMHO). it may be 600F, 700F, 4700F, I don't know--but worth checking if anyone is interested in continuing this discussion.

I've read through these posts and seen a lot of gum flapping, opinions, generalizations and few posts based on basic physics explaining the difference between radiation and the other two ways heat reaches equilibrium.

Here's an old goat's interpretation on how part of our world works.

First, realize "heat" is the result of motion between minute particles of matter and that it is constantly being exchanged in an endless circle wanting and seeking thermal equilibrium. "Temperature" measures intensity of heat and, indirectly, the amount of motion in the particles.

Gases, like the air we breathe, exchange heat mostly by convection, a means of flow, whereby cold gas particles have less movement and thus are closer together (read more dense) than warm air particles (less dense) accounting for the fact warm air rises and cold air sinks. When a warm air particle (molecule) contacts a cold air particle, its higher heat energy in the form of molecular vibration is transferred to the cold air particle, which then becomes warmer and rises with its less dense cousins. This energy transfer, per molecular vibrations, moves like a wave through the cooler particles as they warm up. There is significant space between air molecules, so this process of molecular collisons passing on energy takes time and causes air currents as warmer molecules rise and cooler ones sink. In the process, as the wamer particles contact fixed objects in the room, these absorb the vibration energy and begin to warm. This method of heat transfer (convection) is indirect since the heat transfer goes through air containing much space between molecules.

This continual movement of air from a hot stove causing convective air currents can result in drafty indoor conditions with temperature zones (aka "indoor weather").

Liquids also exchange heat chiefly via convection. Particles are closer together than in air but the same phenomenon occurs explaining why cooler water is near the bottom of a lake and warmer water on the surface. But in this example, wind can blow warm surface water away resulting in the temporary surface feeling cold as colder water rises to replace the displaced warmer surface water.

Solids exchange heat by radiation and, with direct contact, conduction. In its quest for equilibrium, heat transfers via conduction when two solid objects are in contact. Vibrations from the warmer object molecules spread by contacting the cooler objects molecules thereby increasing their vibrations and heat which, spread like a wave through the object. The degree of vibrations, from the amount of heat, can be substantial. If a very hot object (hot metal stove) with considerable heat energy (lots of vibrating molecules) is touched by a finger, the molecules on the surface of the finger can vibrate so fast that those molecules separate resulting in a burn to the finger.

Friction is another way to increase particle movement resulting in heat by conduction. For example, experience the warmth generated by rubbing your hands together.

Heat transfer by radiation is a slightly different story. To understand it, realize that any matter with a temperature above Absolute Zero (0* K, or -273* F) gives off "infrared radiation", not seen in the visible spectrum of light, but explained by quantum mechanics as a stream of extremely small photons having properties of waves and particles, maybe both, maybe alternating between the two. So small are these photons that it is debated (when I was in school, please, update me) whether they are pure energy or particles, either travelling at phenomenal speeds. When these photons collide with other molecules or particles of matter, they cause increased particle movement and more heat.

As mentioned, gases, like air, have relatively enormous amounts of space between the molecules of the gas. Liquids have less space between molecules and solids even less (solids being generally more dense than liquids or a gas). The tighter the molecules are packed together in a piece of matter, the easier it is to absorb any radiant heat photon which may strike it, making it warmer. Conversely, in air, since the molecules are far apart making the chances of a radiant energy photon hitting the gas particle much smaller.

Hot objects radiate photons of greater amplitude (like a sine wave) than cool objects. A photon from a very hot source has a greater chance of colliding with a particle of matter, like an oxygen or nitrogen molecule in room air, in a given distance than a photon from a cooler source.

This explains why a hot metal stove at 550* F tends to heat the air around it and induce convection air currents versus a cooler stove at 180* F which will radiate photons with less amplitude, have less chance of colliding with air molecules over a given distance and can heat objects further away from the source than the hot metal stove.

That's the story and I'm stickin' to it: why radiant heat has been heralded as "more healthy" (like the heat from the sun) and the air remains still at a more even temperature than convection heat which causes draftiness, temperature zones, etc.

Aye,
Marty

Albert Einstein once said, "Nothing happens, until something moves."

Thanks guys- I'm much more 'enlightened' now after reading thru this thread............