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A new look at the Bell. The 'hidden' advantage?  RSS feed

 
terry jones
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I posted this thread on donkeys, but thought it might be of interest here as well. Thoughts, corrections or additions are most welcome.




Most of us here use and are probably in love with our rocket heaters. We would also be aware of the many benefits, smoke free combustion, the lovely long term warmth (for those installations that incorporate mass storage) and of course, the vastly decreased fuel usage.

Almost all of us are able to give reasons for this vastly decreased fuel usage, clean burn means that all of the fuel is burned, the very low flue temperatures mean that we have trapped and stored the heat for long term usage, that is mostly achieved by the use of mass. Some use a long flue buried within a cob bench of some sort, others use the concept of a bell. Whatever the mechanism, the point is we gain extra efficiency by lowering the flue temperature.

Those who have lived with a mass heater (which naturally includes masonry heaters) will most likely attest to the gentle warmth and comfort they provide.

In this essay (?) I want to examine this from an aspect that I personally have not seen before. I am sure that there is more than a little measure of truth in it, and is perhaps a hitherto not recognised contributor to WHY rocket mass heaters are so effective and why we can achieve such seemingly low fuel usage.

By default I might use the term bell often, maybe only because that is what I use myself. It should be known that in this discussion it is a generic term for 'mass', however it is realised in a particular situation.



There would be many places to start from, but let us start from 'how do we get warm?'. Well, obviously we have a heat source in the room. This can be from a radiator fed with hot water, a normal wood heater in a fireplace, a standalone wood burner, a rocket heater. However that heat gets there is not the concern right now, simply that we have a heat source. What we are going to examine is the transfer of the heat from that heat source to us, the occupants. And this is where I have not seen any examination of this aspect regarding the effectiveness of the bell.

There are 3 forms of heat transfer: Conduction, Convection and Radiant heat. that is well known, but I will be using this reference with comments as applicable http://www.herschel-infrared.com/heater-fundamentals/radiant-versus-convection-heat/

This one is interesting.....'In a Comfort Heating situation, Conduction (physical transfer of heat from source to target by direct contact) is not an option, so whilst it is the most efficient method of the three (presuming a suitable medium to conduct of course), we’re left with Convection or Radiant heat.'

Well there ya go, those of you that do use a bench warmed by the exhaust DO have the most efficient form of heat transfer. I don't personally as I have a large brick bell (tho there are ways to have a bench with a bell, see this site http://batchrocket.eu/en/)

Conduction of course is the transfer of heat within a solid substance, heat one end of the spoon and after time you will feel it get hotter were you are holding it, the heat has moved from one end to the other within the material.

Convection is the transfer of heat by the movement of the material itself, gases and liquids. As the gas/liquid heats, its density falls and so rises to the top and is replaced by colder gases/liquids that, in turn, heat up and rise and get replaced by colder gases and liquids. This process continues in a never ending loop until all of the material reaches the same temperature, equilibrium.

The last heat transfer mechanism is radiation, where heat is given off as an electromagnetic wave. As such it transfers over a distance and once it hits an object, ourselves, the far wall, the kitchen bench, that electromagnetic wave induces the material to warm up. Once that kitchen bench warms up, it too then becomes an emitter (in this case of lower temperature) electromagnetic waves which will hit yet other objects in the room. Air is essentially transparent to these IR waves (infrared) so they pass through the air unaltered until they hit a solid object, think of a cold day on the ski fields when the air temperature itself is very cold, but you feel warm from the IR/light waves from the sun. If a cloud passes in front of the sun, you will suddenly feel cold again due to the low ambient air temperature.

So there is our basic background information to delve a little deeper into 'why the bell contributes to the efficiency of these heaters'.

Again, I think it worthwhile that you read the links provided.

It turns out that, just like light, IR is a spectrum, that is a range of wavelengths. It follows from that that these differing wavelengths have differing properties, ie not all IR rays are 'created equal'. It is clear that *our* interest lies solely in space heating, if I were an industrial manufacturer I would have a very different set of criteria. And those two different objectives would be realised by different IR radiation. 'It is IR' is insufficient to understand what is happening.

'IR radiation' is broken down further into three broad categories of wavelengths. See this link http://www.herschel-infrared.com/heater-fundamentals/preferred-wavelengths-comfort-heating/

These are:"Infrared -A , classified as the “hottest” Infrared with temperatures up to 2,700C and wavelengths of 0.7 – 1.4 microns and is also called “Short-wave” or “Near” Infrared;- Infrared – B is infrared with temperatures of 500 – 800C and wavelengths of 1.4 – 3 microns and is also called middlewave or “Medium” Infrared;- Infrared – C is infrared with temperatures of less than 500C and is the final and broadest waveband of 3 microns – 1mm and is also called “Longwave” or Far Infrared."

This is shown in the following graph (from the same link)








What is to be taken from graphs like this are a few things, that a 'blackbody' (as these emitters are known as-no need to delve into that) radiates some energy at all wavelengths, as clearly seen above. However, that does not mean they radiate equally at all wavelengths. We can clearly see that as the temp rises the peak energy emitted goes to shorter and shorter wavelengths, and that peak rises in intensity dramatically. The converse-and where we are heading in this little 'essay'-is also true, as the temperature of the emitter falls, the peak shifts to the right (longer wavelengths) and the emission becomes more and more consistent over those longer wavelengths.

This graph ends arbitrarily at 10 microns, that is not necessarily the 'end of the IR spectrum'. From http://www.noritake.co.jp/eng/products/eeg/support/heat/far_infrared_character.html I found the following data.."Wavelengths of 3 μm to 1,000 μm within the infrared range of electromagnetic radiation (light) are known more specifically as far-infrared. Because many materials, including water, plastic, paint, and foods have an absorption range between 2 μm and 20 μm, far-infrared radiation is effective at penetrating and transferring heat." (my emphasis)

So this introduces two things, that IR is 'extended' to 20 microns, and more importantly it introduces the WHY of choosing different wavelengths. Water absorbs certain wavelengths far better than others.

this is a graph of the absorption factor of water at varying IR wavelengths







Note how the absorption of water shown here relates to the breakdown of IR into IR A(near IR), IR B (mid IR) and IRC (far IR). The absorption of water is very high indeed across the IR B and IR C wavelengths.

The human body is roughly 80% water, and so we can see that for the most effective heating of the body it would make sense to use the most appropriate IR wavelengths, namely IR B and IR C.

This leads us back to the term used before, blackbody. It turns out that we can calculate the wavelengths emitted by a body, yes even the human body tho of course we are referring to a bell here, and that is dependent on the temperature of the body emitting the radiation. (Don't get hung up on, or assume 'body' here means a human or animal body, it is just a 'thing')

From wikipedia "Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body."

Working in reverse as it were, and knowing the wavelengths best absorbed by the body for warmth, it is clear that we can use that knowledge to determine the optimum temperature of the blackbody, or in the real world, our bell.

From the above graph and earlier data, we know we want IR B and IR C, or wavelengths between 3 and 20 microns. And that we see very high and almost constant absorption of those wavelengths by water (ie the human body).

There is a nifty little interactive graph on this page http://www.pveducation.org/pvcdrom/properties-of-sunlight/blackbody-radiation which although it loses all detail at the frequencies we are interested in DOES show the points raised above, that as the temp rises the peak sharpens dramatically at higher temps and the amount of radiation in the wavelengths we are interested falls greatly. But just above that interactive graph is a calculator that we can plug numbers into.

So let's make a few assumptions about the temperature of the bell. We could go max and min, and plug them in. I'll let you play with that. I will simply, and quite arbitrarily assume we have a 'constant temperature of 60 degrees celsius'. Yes, that will go up and down, but the whole idea of using mass is to keep a relatively constant temperature. You can plug different values into the calculator and see for yourself.

So 60 degrees celsius is about 325 degrees kelvin if my memory serves. (degrees celsius plus 273 gives degrees kelvin-without checking)

The value given for that temperature is '8.9231 um'.

Slap bang right in the middle of the peak absorption range for water.

Plugging in temperatures (ie max and min) for the bell either side of that arbitrary value does nothing, they all fall within the optimum range for absorption by water, that is they all emit IR radiation of type B and C.

You can now browse the links by Herschel (given above), they go into why radiant heating is better than convection heater, and how much energy is saved by using heat that falls into the most optimum wavelengths. And all of those arguments explain why the bell, or mass, is such a perfect way to provide warmth.

To sum up, the bell has 'only' been viewed as a way to capture heat, store it and release it later. Yes, that certainly contributes to the efficiency of the heater. I feel that the TYPE of heat released, ie the IR radiation it releases has not been previously considered. I do not know how to calculate it, but it seems to me, as fortuitous as it might be, that it releases the *perfect* heat for humans must also contribute in some way to the efficiency of these heaters. Any and all illustrations used by Herschel in their promo of the increased efficiency gained by using radiant heat apply equally well with our heaters.

I hope you found this as interesting and enlightening (?) as I did.
 
I agree. Here's the link: http://stoves2.com
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