Excellent. GT, What would you adjust to get a clean flame (not a reducing flame as you need to eliminate oxygen from the melt)? I am familiar with Pottery kilns using reducing gas fired flames for the same reason as in your furnace. (It creates brighter colours in the glaze than an "oxidising" atmosphere).
A steam boiler (IMHO) needs a balanced air-fuel ratio at (close to) stoichiometric ratio to be clean of CO without excess air that simply reduces boiler efficiency? (Power stations have monitors for CO and O2, just like the car exhaust systems, and feed-back to the air blowers to regulate the mixture - or they did back in the 1970s!).
The significant differences between a boiler and furnace I think are as below:
Furnace - uses refractory insulation so any direct flame impinging on the surface heats above CO combustion temperature, so maintains the combustion to completion of all the O2 or fuel.
But: The boiler firebox surfaces are below CO combustion temperature (and water-cooled to steam temperature by the boiler water) therefore any flame impinging on boiler firebox walls is immediately quenched to stop the CO combustion. This can be tested by taking a CO meter into the exhaust, or extracting a sample of exhaust and igniting it in air at the end of the sample tube. (CO burns with a near invisible dark blue flame).
The furnace is so hot, the surplus heat and exhaust is vented rapidly. The main aim is adequate power from combustion and insulation so all the combustion heat is concentrated - and lower temperature gases post combustion would cool the melt, not helping the melt.
In a boiler, all the heat should be captured - down to steam temperature by boiler-tubes, etc. - or below steam temperature by the addition of feed water economisers. "Efficient" boilers start with high temperatures of combustion - for high heat flow into the water - and long passages - to extract as much heat as possible to the water - before the exhaust leaves the boiler. Furnaces use "just the hottest bit from combustion" and exhaust cooler gases where there is no further combustion.
Boilers that have superheaters in "post-boiler" exhaust that can raise the steam temperature above boiling temp. are actually passing HOT (wasted) exhaust up the chimney. And boilers are all about efficiency...
I'm sure there are other differences, but that is just my view on the significant points for Toymaker to consider?
Hope this "meddle" helps a bit... just my ideas...
K2
There are some interesting points made here, and I will attempt to comment as best I can.
Keep in mind that pretty much everything I know about burners and burner combustion comes directly from Art B, and some information from a white paper written by the lead engineer at Delavan.
If Art B disagrees with anything I say, you can rest assured that Art is correct, and I am wrong.
I can verify that what Art B says about tuning a burner to achieve maximum temperatures is true in practice with my furnace and burner.
I have heard of situations where an oxidizing flame may be desirable, but I don't recall the circumstances.
The interior of the furnace does get very hot, and I think the heat transfer from the burner is first via heating the walls and lid of the furnace, and then via IR radiation, heating the crucible.
I don't think the flame itself necessarily does much heating of the crucible, since the flame never impinges directly on the crucible.
The crucible manufacturers state that direct flame impingement on a crucible will cause it to quickly fail.
In a video made by a crucible manufacturer, they said that the plinth (the support pedestal that the crucible sits on top of) should be made of a material that easily conducts heat into the bottom of the crucible. The only plinth material I have found to hold up to iron temperatures for repeated melts is Mizzou, but there are surely other materials that will withstand iron temperatures.
I am not sure how well Mizzou conducts heat into the bottom of the crucible, but the last thing you want is for your plinth to crumble in mid-iron melt.
They do make spray on ceramic coatings for a furnace interior, such as ITC-100, that are suppose to help reflect heat back into the crucible.
I have tried ITC, but can't tell any difference in melt time. I have never gotten scientific about measuring it though.
The slag on top of an iron melt tends to get whipped up in the air stream, and it splatters onto the walls and lid inside the furnace, so any interior coating gets covered in slag relatively quickly, which is why I stopped using ITC.
One online source says that crucible furnaces are 7-19% efficient, which means that most of the heat energy is going out the furnace lid.
I have seen designs for recuporators, and they consist of a heat exchanger located in the exhaust stream of the furnace, and the combustion air is preheated in the recuporator to perhaps 1,000 F before it enters the burner tube.
I seriously considered using a recuporator, but decided against it because of the coking/sludge problems you can have when operating a hot burner tube/nozzle. The plumbing required for a recuporator is significant, and could seriously obstruct the furnace operation and lid opening method.
There is no amount of efficiency gain from a recuporator that would justify my use of a recuporator in a backyard casting setting.
Any steel that is exposed to high temperatures does not last very long, and thus another reason to not use a recuporator.
The recuporator would have to be made from a refractory material in order to last any amount of time.
I have experimented with dual oil burners, with the burners located at 180 degrees.
This arrangement is found on several commercial furnace designs, in sizes in the range of my furnace, or somewhat larger.
A few things I noticed about my dual burner arrangement was:
1. The velocity of combustion air to each burner tube was reduced by 1/2.
2. The flames were very evenly spread around the bottom of the furnace, as opposed to a single burner tube with higher combustion air velocity, where the high speed combustion air causes the flame to climb up the back of the furnace at a 45 degree angle (you can see these effects when the burner(s) are first started, and the furnace lid is open).
I am of the opinion that the lower the combustion air stream velocity, the longer the combustion gasses have to transfer their energy to the furnace walls and crucible.
To achieve the temperatures required for an iron melt, you have to pump a given amount of combustion air and fuel into a furnace, and so reducing that velocity by 1/2 by using two burners I think would speed up the melt, especially in the warmup phase of furnace operation.
I did not know how to tune burners when I was experimenting with my dual burner experiments, and so it was impossible to tell if they worked better than a single burner.
I do intend to try dual burners at 180 degrees again.
Based on what others have reported recently, I think I will have good results with a dual 180 degree arrangement that is correctly tuned, with a noticeably shorter time to reach pour temperature with iron.
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