Sunday, February 1, 2015

Heat dynamics

Finally, I'm starting to understand heat dynamics. It's taken me at least 5 years, and there are still lots of things that don't make sense. And, as usual, just when I think I have it sorted, something pops out of the metalwork to show me that I don't.

For this reason, I am going to try to explain heat dynamics to you - kinda like a brain dump, so that I can nail what I know down for a bit. So this has nothing to do with  education - and everything to do with maintaining some degree of sanity. 

More art than science
Heat, as they say, is more art than science.  Art has a way of messing with one's mind, for the sake of anyone who has ever brushed up against said beast (art - not a hot oven). 

Heat is heat - plain and simple - only it behaves in unexpected ways. This can be quite a problem if, like me, you are untrained in the principles if heat exchange and wish to build an oven or three. 

The thermal engineers, in their semi scientific way, classify heat according to how it moves - conduction, convection and radiance. Physical transfer, gas transfer and heat emission. This last one  is when something fills up with heat and passes it out. The other three are fairly self explanatory.

I find these pieces of information as useless as teats on a bull, in practical terms. And kinda stupid, as they tend to overlap - radiant is both conductive and convective, or one, or the other - in the real world, at least. Particularly to a lump of dough, who wouldn't know the difference anyway. 

For the sake of a workable analogy, though, let's stick with the lump of dough for a minute, to explain a certain type of insulation principle.

Edible insulation?
Imagine you are a nice, ripe lump of dough, and the baker dumps you on the hot stone floor of his woodfired oven. Instantly, conductive heat finds it's way into you through the stones. Soon after,  you feel the heat all around you, convectively. All of this heat is being radiated via the hot stones. But in spite of all this absorption, your surface would be only warm to touch.

Your gluten bubbles are forced to expand quite rapidly, as the last bit of fermentation energy from the ripening process (proofing, in baker's jargon)  gives off carbon dioxide as a result of all this heat. You suddenly grow as you absorb all the heat. After a while, you solidify. You would now be quite hot to touch, as anyone who handles crusty hot bread straight out of the oven would be happy to testify. 

The baker flips you out if his oven onto the cold steel bench. You take your heat with you, and it passes into the bench via conduction. 

You have become a kind of edible insulator, and, inadvertently, a heat transfer device. The convective heat is now in your carbon dioxide bubbles, which are, in this case, made of gluten. They filled up quite rapidly (due to their gassy nature) with heat energy. After you are baked, this heat energy passes out of you gradually into the atmosphere - convectively, as it happens. And, while you are still hot, you have become a radiant heat source. And the bench you are sitting on is instantly hot, as conductive heat energy has made it so. It feels hot because it is quite dense, and instantly reflects this heat. 

Light and Water all rolled into one
The thing is, heat is both like light and like water. It reflects instantly when it comes up against a dense substance (like steel or brick), but then, slowly, these things absorb it. 

When heat comes up against a substance like bubbles, or something filled with gas, it is absorbed first, until it is the substance is full, and then the substance reflects the heat. 

So things with a lot of gas or air in them absorb heat first.  Then they fill up with it. Then they reflect it. And then the heat passes out if them quite quickly. 

Things with very little gas or air, (dense things), reflect heat first, then, via conduction, they fill up with it. When they are full, they too pass the heat out, but more slowly. 

So dense things become a good radiant heat source over time, whereas light, airy things don't. 

Up to this point you can see that heat is like water. Spongy substances soak up heat, while solid substances cause heat to bounce off. Like washing the car - the sponge absorbs water, while the steel bonnet bounces it off. 

But heat is also part of light energy. So a dense, dark coloured substance will tend to absorb heat - or even, I suppose, a substance that is in the dark (though I'm only guessing, as it was in the dark and I didn't see it).  Similarly, I light coloured substances will tend to reflect heat. But this is all theoretical at this point, because the heat spectrum we are in doesn't involve much in the way of light. 

All of these principles come into play in my world all the time as a baker, a Fournier, and as an oven designer. It's only recently that I feel I have achieved some sort of mastery over them - enough, at least, to control a woodfired oven when baking for long enough to make a couple of hundred good quality loaves if bread. 

And, of course, to design a woodfired oven that actually works reasonably well. 

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