I have been setting aside material to build a furnace and I have done a lot of research on same. There's more out there on the internet than one can absorb. In all the things I have seen, I notice one common theme that has me confused. Why do all the furnaces have a big hole in the top to let the heat out? It seems like trying to bake a cake with the oven door open. I'm guessing that if one measured the temperature of the flame coming out the top it's pretty much the same as the temp. at the source of the flame. Well, maybe a bit less since some of the energy goes into the stuff you're trying to melt, but at the rate of "flame flow", not much. If I were to use an electric furnace (kiln) to melt the material, I'd surely close the lid to keep the heat in, but all the "bucket" furnaces I see seem to go to great lengths to let it out. Why is that?
Foundry furnace
- Thread starter Ed T
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OK, but if you didn't let as much out, you wouldn't have to let as much in. There are lots of gas fired kitchen ovens which get up to temp and stay there w/o exploding. There are gas fired kilns that don't explode and don't let all the heat out the top. It leaks out through the insulation and is replaced by the burning of additional fuel. The set up of most furnaces I see is kinda like holding the crucible in the flame of a big blow torch. It gets hot and the metal melts but most of the heat goes into the room, not into the metal. Most of the heat is wasted. This is not a big deal, I am just trying to understand why its the way it is instead of a way that seems more logical to me.
Hi Ed, Tels quite correct, to create the heat necessary, you need to burn the fuel, lots of heat requires lots of fuel (energy). To burn it you need large volumes of air, once its burnt its also expanded quite a lot so ...some air in ...lots of exhaust out.
The flame would simply run rich (if it would run at all) with a closed or overly restricted exhaust, to stop it running rich you would need to lean the air fuel ratio and at such a low setting it wouldnt build up the heat quickly (operative word 'quickly'). Yes you could build up the heat using lots less fuel but every time you opened up to check or deslag it would lose so much it would take ages to come back to temp. The kiln doesnt do the job in 30 mins like we want the foundry to do, and we never open the kiln it once its under way (like we have to do with the foundry), too much loss and too long to bring it back up to temp. Similar with the oven which isnt able to get anywhere near the temps.
Look at this pic. This is using the Reil style gas burner. Do you think this much air is coming in through that small hole at atmospheric pressure? No, its expanded, heated gas.
This pretty exagerated and its not operating conditions, in fact its nothing more than showing off (she does that a lot... naughty foundry...). When in melt mode there is teh merest hint of flame out the flu, thats at its most efficient.
Think car with restricted exhaust, will run but not quickly... same sort of thing here.. if you need anything specifically give me a yell any time.
The flame would simply run rich (if it would run at all) with a closed or overly restricted exhaust, to stop it running rich you would need to lean the air fuel ratio and at such a low setting it wouldnt build up the heat quickly (operative word 'quickly'). Yes you could build up the heat using lots less fuel but every time you opened up to check or deslag it would lose so much it would take ages to come back to temp. The kiln doesnt do the job in 30 mins like we want the foundry to do, and we never open the kiln it once its under way (like we have to do with the foundry), too much loss and too long to bring it back up to temp. Similar with the oven which isnt able to get anywhere near the temps.
Look at this pic. This is using the Reil style gas burner. Do you think this much air is coming in through that small hole at atmospheric pressure? No, its expanded, heated gas.
This pretty exagerated and its not operating conditions, in fact its nothing more than showing off (she does that a lot... naughty foundry...). When in melt mode there is teh merest hint of flame out the flu, thats at its most efficient.
Think car with restricted exhaust, will run but not quickly... same sort of thing here.. if you need anything specifically give me a yell any time.
tel
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To take your analogy a bit further Ed, fan forced kitchen ovens have to be vented. In fact most oven have some degree or another of venting.
My propane fired furnace has a 3" exhaust hole in the lid and that is just barely man enough for the job, even tho my burners (the 'Mikey' type) are naturally aspirated.Burning charcoal means injecting large volumes of air, and thus the problems begin.
My propane fired furnace has a 3" exhaust hole in the lid and that is just barely man enough for the job, even tho my burners (the 'Mikey' type) are naturally aspirated.Burning charcoal means injecting large volumes of air, and thus the problems begin.
