Thursday, December 28, 2017

Fire Down Below

Because my propane-fired kiln is outside, I am often asked how I can fire in the winter. Doesn't the cold prevent the kiln from heating up? Internally I chuckle at these inquiries. The difference betweencomfortable and uncomfortable, for people, is less than 100 degrees, and the first 100 degrees are the easiest for the kiln to achieve. I'm regretting those chuckles now, because, at -5°, the winter cold does in fact make s difference. Not to the ability of the flame to heat the kiln; that's still a negligible effect. The problem is getting enough propane to the burners to burn! Propane's phase shift from liquid to gas is -44°. It's stored as a liquid by keeping it under great pressure. When some of the pressure is relieved by opening the valve, some of the propane turns to gas and escapes out the valve, down the pipe and to the burner, where it (hopefully!) is burned to create heat for the kiln. This phase change - from liquid to gas - is endothermic, meaning it consumes heat energy to happen.

The closer the external temperature is to the liquid-point (there's some chemistry term for that, can't think of it right now) of propane, the less pressure is required to keep it in a liquid state. The tank loses pressure as it empties anyway, and the endothermic phase shift makes the tank even colder than the air around it.

If the air around the tank is -9°, as it was when I got up this morning, I would still be able to grill a steak using my propane tanks. The firing, however, takes hours - 11 hours even if everything goes perfectly. The tank is getting colder and losing pressure the whole time. The burners wouldn't stop burning, but it would be as if I were turning the valves down, down, down.

I might be able to finish the firing... I do have completely full tanks, which matters (pressure!). But it's possible that I will attempt to do the firing and just burn a bunch of propane and stall out before I reach temp.

Also - not that this affects my plans! - loading in the sub-zero suh-hu-huhuhucks. The my hands hurt, the wadding keeps freezing, the wax on the bottoms is brittle and flakes off.

So, firing this week is a big ol' nope. Instead, let's enjoy some Sonny Boy Williamson!

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