My Attempt at an IFB Domed Furnace Lid

You must have one heck of a burner. I'm burning motor oil, but I guess a little over a gallon per hour, I really don't know how much. From a cold furnace it takes me about 20 minutes to melt 5# of aluminum and 30 minutes for the same volume of brass. The little bit of cast iron I did was about 45 minutes. Time to melt would be a good comparison. I know you are using an atomizing burner which should burn hot, and I thought you were burning 3 gph. I can turn mine up but don't because I feel like I have acceptable melt times, for what I do.

I just can't believe the flue gas is approaching 3,800F. It's just really hard to get to the ideal mixture and then the cool refractory and iron is sucking heat out of the gas stream. Have you tried holding a thermocouple in the flue stream and monitor the temperature rise until it gets to 2,000F?

I'll be really surprised if you're seeing anywhere near 3,830F. That's the theoretical maximum, for perfect combustion. Propane is only 200F lower at 3,623F. Does your crucible get pretty bright before your iron gets red? I don't see why the bottom of the furnace is much cooler if you have the ideal air/fuel mix to get above 3,200F bulk combusted gas temperature everything should warm up together. I think cast iron melts about 2,200F and you want a couple hundred degrees superheat to pour around 2,400F. Am I wrong on that? Pure elemental iron does melt at 2,800F but that is well above most steels even. The differential temperature is what controls the speed of the melt, I think 200F differential will get it meltid in reasonable time, but I may be wrong on that.

Mizzou and other hard refractorys have pretty low insulating values. When you rammed the Mizzou how did you keep from compressing the ceramic fiber? Of course it has very low insulating value if compressed tightly.

I have to believe red hot spots on the outside of the brick means the insulation has broken down for some reason, or you have gas bypass. Hot spots is the indicator, if the insulation were intact, we would see the entire shell get red pretty much at the same time. When I was melting iron my outside shell was below 300F, but my furnace is built backwards to yours, mine has brick inside, wool outside. Red color has to indicate a temperature of 1,100F at night and more like 1,200 or 1,300F in daylight. My experience has been that 2-1/2" of 2,600F firebrick will not get red on the outside with 2,500F inside. And 2,500F melts cast iron just fine.

I am truly not being contentious. It is frustrating when people experience different results. I'm trying to figure out why we are seeing things which appear to be in conflict.

 

If the thermocouple that was behind the hot face that was rated at 2,250 F melted, then a thermocouple would only last seconds in the flue stream (that is my thought anyway).

I don't have any way to measure this, but we know that is the theoretical maximum for light fuel oil with combustion air added, and I have no reason to think it would not approach that temperature. It will melt the end off of a steel rod no problem.

I burn 3 gal/hr.

The furnace interior gets red and then basically almost white hot, and dark lenses are necessary to look at it for any length of time without burning the eyes.
The ITC100 coating also reflects the heat back into the furnace, and it becomes a brilliant bright color.
The more heat that is reflected back into the crucible (instead of being transmitted through the refractory and outside the furnace), then the faster the melt will be, and the ultimate maximum temperature as well as the flue gas temperature may also be higher when using an ITC100 coating.

There is only 1" of hot face, and the total hot face refractory mass of the entire furnace is only 140 lbs (furnace and lid).
That is relatively little mass to heat up, and this furnace heats up considerably faster than my heavy furnace.

I seem to recall it was mid-way between what is considered insulating and non-insulating refractory.

The Mizzou was rammed between two sonotubes. There was no ceramic fiber involved.

Melterskelter has an iron pyrometer, so he can probably shed more light on this than I can.
He recently mentioned some pour temperatures.

I have heard pour temperatures in the 2,500 F range as I recall (I need to check the Navy Foundry Manual, there is a higher level for thin castings, and a lower level for chunky castings).
The Navy Foundry Manual says that if a 1/2 iron rod melts off after perhaps 30 seconds of stirring the melt, then the temperature of the iron is at 2,800 F, and I can easily do this.

The thing is, you have to have superheat because you have to add ferrosilicon and stir it in, then there is time to lift out the crucible, get the shank in place and locked, and time to go from mold to mold.
You must have enough pour temperature to fill the last mold, so this means the first mold(s) may get poured on the high side of the range, and the last molds get poured on the low side of the range.
You can waste several minutes easily after the crucible has been pulled out of the furnace.

Yes, it means the refractory is cracking or has cracked and has opened up significantly.
The cracks have to be patched after every melt while they are small.

I have been comparing my data with scavenger, ironsides, porositymaster, and melterskelter, and I am starting to see patterns, and things are much more predictable now that I can easily melt iron (easily as long as the furnace lid cooperates; my burner is never a problem).
Myfordboy had done at least one iron melt, but he no longer does them to my knowledge, and probably because they are so hard on the furnace, and so much significantly hotter than aluminum.

Everyone pretty much uses 3 gal/hr fuel rate to melt iron.
The lower the furnace mass, the faster the melt.
The coated ceramic blanket furnaces are going to win the speed contest hands-down.

My furnace with its 1" cast refractory hot face shell is a good compromise for me as far as melt speed (perhaps 25 lbs in 1 hour).
If I can hammer out all the nagging problems I am having, I feel like I will be good to go for a number of iron melts.
I am in the "debugging" phase of my furnace design, but I will get there, and I still believe in a low mass design, although I am not ready to try coated-ceramic-blanket-only yet.
I am not ready to try coated insulating fire brick either, but best of luck to melterskelter.
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Making steady progress toward completion:

Today I cut the vent hole and demolded the high density castable liner and fit the hole to the liner. In the top photo you can also see the “spider” that supports the base of the chimney. Also shown in that image is a coarse rasp used to trim the hole to fit.







I intend to seal leaks around the liner with a gasket of wool.

Denis

 

Looking really good! The brick cuts like a dream, doesn't it?

I like the idea of the liner. Mine is just painted with Satanite. You could also seal the liner in with Satanite, just around the lip, and it will crack so it can come out easily.

 

Looks good.
The liner is definitely going to save some wear and tear on one of the hottest parts of the furnace.

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Yes, that is the hope. I have come to believe that the various areas attacked by hot gases are affected proportional to the gas velocity at those areas. High velocity=vigorous attack. The vent would certainly qualify as high velocity! The vent liner protrudes about 3/4” into the space beneath the lid vent and will hopefully greatly reduce the flow of gas over the corners of the brick there as well as separating the brick from the gases in the vent channel. Having the chimney in place tends to complicate the flow out the vent as well. The vent liner will be in two equal pieces being split on the diameter. That is to accommodate inevitable cracking and to control it.

Tonight I will fire the liner in my pottery kiln over a nearly 24 hour period and then install it. Hoping to burn by the weekend.

As near as I can tell the welding and fussing over the lid and various improvement to the lid lifter are completed.

Denis

 

Here is a chance to visit my shop as I de-mold a vent liner after initial curing and prior to firing it in the kiln. The video is 12 mins long and may provide some morning entertainment.



Denis

 

Both. I used a small dowel to ram in the rather dry mixture and the rapped on the base board many time to coalesce the aggregate as evidenced by it developing a wet skim coat

And yes I am guilty of using materials I had at hand. Having NOT lost my soul to lost foam, I have none in stock and buying 4x8 sheets of different thicknesses and then using a tiny fraction of a sheet just doesn’t seem like the best choice in my shop. Sounds like a good method overall though.

It sounds like both you and I have concluded that castable is very sticky stuff and attention to release agents is important. And we both do not rely on draft plus release agents to allow sliding molded refractory from a rigid mold. Instead I peel and you break. I have used past wax and packing in the past also, but in the case of this mold, brushed on canning wax bridged gaps. The tape/wax combo I also found to work well.

This was Vesuvius brand dense castable whose number designation I have forgotten. If it is important, I can retrieve that info.

The liner is presently in the kiln holding at 250 for a couple hours before ramping to 600 and subsequently ramping to 1700.

I expect to make at least two liners, so I hold onto my mold parts.

Denis

 

Here is the resulting liner after firing to 1750.



Today is final assembly of the lid, application of the bubble alumina and prepare for weekend firing.

Denis

 

I have not tried to separate the halves. I too got to thinking about the AL shim being consumed by its neighboring alumina. I think that should be unlikely since the alumina is already fully oxidized and don't see where the alumina would want to give up its oxygen. But that right there was mostly ignorance talking.

I did not try to split the halves as the shims did not form perfectly complete separation of the the left from right half. So, by subjecting the liner to a shock I might get splitting not necessarily at the parting line. The same may be true in use, but it seems more intuitive to put the heat to it and expect that the joint will provide a "fault" that will guide splitting.

Denis

 

No, you got it right. I am just planning on letting the heat/cool stress crack them and hopefully crack them on the expansion joint.

Denis

 

Good plan, and if they don't crack, so be it.

 

Well, I fired her up today. Melted 40+ pounds of iron and did a limited post-heat inspection. It was too hot to get under the lid and really look it over carefully. But, what I saw did not disappoint. It looks like the Bubble Alumina held up very well. I see no areas of loss of the 1/8" coating I applied. The 2600deg insulating bricks also held up very well showing no signs of deterioration.



Incidentally, those bricks really insulate, they were 460 degrees on the outer surface 40 mins into the melt.

I am hopeful this lid will hold up for a while. I always had to patch the wool/satanite lid to some degree after each melt. That is not such a big deal, but this one looks more durable. Time will tell.

I also liked my "new and improved" lid lifting mechanism.

Overall a very good (long) day that started with making a replacement sod silicate core, packing a mold for the 18" straight edge (takes 80 mins if things go smoothly, they did), making various repairs and upgrades to furnace and blower, melting and pouring two 18" str edges, and general cleanup of the aftermath. I'm tired but HAPPY!

Denis

 

Congratulations!

by the way, why does the bottom of the lid look flat?

 

I guess it is because of "flat light" ;-) It really is domed with the bricks inclined ten degrees (still!).

See above ^^

Thanks to both of you. I do feel some sense of relief as I truly was prepared for the BA to just sheet off and the bricks to melt part way through or collapse.

Denis

 

The lid interior looks pristine.
What is the poop on that bubble alumina stuff ?
A million dollars a gallon, or reasonable?

Sprayed on I would guess.

That is pretty much how ITC100 works.
It seems to completely shield the hot face refractory, and also seems to be impervious to oil burner flames with no spalding at all (if sprayed on in a thin coat or two).

I am interested in satanite too and wonder what temps it is rated for.

If the bubble stuff is cheap enough, and if it remains in place over time, then it may be reasonable to just build the entire furnace out of IFB's (perhaps 3,000 F rated, or maybe as low as 2,600 F), and spray the interior with the bubble stuff.
This would totally eliminate the cast refractory, and would be sort of an intermediate solution between coated ceramic blanket and cast refractory.

Can you do 20 more melts this week so we can see how it holds up ? (just kidding)

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