Post Mortem on Thick Dense Castable Furnace

Today I broke up and carted off my retired original dense castable furnace. Taking it apart revealed a few interesting to me facts. The furnace probably had 75 to 100 melts.

First, the flat lid castable had only a few radial cracks beginning at the outer edge and extending toward the center only a few inches. The lid had sagged only a quarter inch or so despite being flat. It took quite a few solid blows with a ten-pound maul to break it into 4 inchish chunks. The castable was about 1.25” thick backed by two one inch layers of 2600 deg wool. The wool was just slightly crispy on the surface nearest the heat but was otherwise soft and could be reused.



The vertical walls were also 1.25 thick and had a few cracks extending from the top edge down about 6 or 8 inches. The underlying wool was again just a bit crisp on the first surface. The cracks had allowed no evident jets of flame to burn the wool. (You will see gasket wool in the photo on the top edge of the furnace body)




The floor was the most interesting. It originally was 1 inch of castable but most of that was replaced by some sort of iron-castable amalgamation. In a few places it was completely replaced by black crystalline amalgam. That stuff was dense and tough.



The wool under it was a single 1” layer and was pretty cooked.

Lessons learned:
1)Dense castable is very refractory to heat and reactive atmospheres

2) Iron spills and drips break down and burn into dense castable despite its refractory qualities. Avoid drip and spills. Don’t try to pour with 90% -95% full crucibles as spills and slops are likely.

3) 2600 deg wool is well protected by a thick dense face.

4) Drain holes sound good in theory but mine was soon blocked by drips/slag.

5).I would be tempted to use a 1/2” thick castable face if using a castable face. 1.25 is needlessly thick and has too much thermal mass. I would also use 1/2” in the lid but make it dome-shaped for better clearance and that shape would promote strength.

FWIW,
Denis

 

Good review!

Looks like the lid was radially cracked by the center being hotter. No surprise.

Gas jets should not happen if your enclosure is tight and your flue opening is adequate to prevent pressurizing the firebox. I can't see the value of trying to pressurize your furnace.

I keep looking for someone who has a drain which functions as they hoped. Some must, but I'd like to hear of a cracked crucible where the drained metal actually ran out of the furnace.

It would be very interesting to see whether a 1/2" thick hot face would survive that many heats. If your castable is insulating like Mizzou then you might damage the wool more.

Although the floor was eaten up with drips, had that stopped and the trash on the floor was protecting it? My aluminum trash does protect the floor in my mostly aluminum melting furnace.

 

The floor “composite” was containing whatever was being accidentally added to that point. I suspect where it was full thickness it would have eaten into and through the wool. It had started to eat in. (Come to think of it I think it was board not wool in the floor)

However, all that floor mass/mess had to heat up each time. I’m guessing fifteen pounds of ferrous something or other.

The main reasons I retired that furnace was it was too short inside and had too much thermal mass.

I’d be tempted to go pretty thin on the walls and lid as the castable has strength comparable to stoneware or porcelain. There was no evidence of thinning of the walls. Thinner walls would insulate/protect the wool less well, but dense castable even at 1.25 insulated poorly. Also use 3k wool.

Denis

 

The only problem I have had with a thin Mizzou hot face against ceramic wool is that it cracks. I don't see any way to prevent it.

My furnace method of choice is a 3/4 inch Mizzou hot face against a two inch rammed up Clay/sand backing. I have a furnace I built that way which has been melting bronze for over ten years with not a single crack.


Richard

 

What do you think is the mechanism by which the sand/clay backing prevents cracking vs the wool? Does the sand clay backing significantly slow initial heating of the furnace? Are you using an external combustion burner? Is the cracking in the wool furnace functionally problematic or can you patch and keep on using it?

Denis

 

Probably. Which ones?

Denis

 

I guess it might also have to do with how much differential heating occurs at say the area of flame impingement vs up near the lid early on. The thin low mass setup might allow larger heat gradients from low or mid levels to lid and floor for example. Kinda hard to wrap my head around that one. Thinking about radial heat differences in startup, a thin wall should reduce differences and reduce internal stresses. At the extreme a bubble should have approaching zero radial gradients. So, kinda hard to figure.

Surely gradients are playing a role, but which ones? What about at shut down? Are those gradients the culprit? Why?

 

I use a Lionel designed waste oil burner, so insulation is not a concern. The fuel is free, and with an adequate blower I could produce enough heat to melt the furnace.

As long as the cracked hot face doesn't collapse, cracks are not a problem. Put a bit of furnace cement in the cracks if you want to. Or not. I haven't bothered to patch them in my big furnace.

I suspect the cracking of a thin hot face has to do with the rate of cooling. The extreme example is when you overheat a glass baking dish and pour in some cold water. My clay/sand backed furnace cools slowly. My large ceramic wool furnace with a thin hot face must cool a lot faster. Too fast in fact.

Richard

 

I use wool furnace lids with no hot face on the lid. The 2600 F. wool deteriorates pretty quickly. It's easy to replace so I don't care.

That does lead me to believe that a wool lined furnace with no real hot face would not hold up for long when subjected to bronze melting temperatures. My 3000 F. Mizzou takes a beating, as you mentioned above, where the flame hits it.

Richard

 

So, I am curious as to how intense a “blast” of air your burner might use. In other words there should be an air/fuel ratio that once provided should liberate a set amount of heat. Blowing air faster than just enough to achieve that mix would not increase flame temperature or would it? I am not sure about that.

The reason I ask is that I use a siphon burner that works well enough. It does start very conveniently, but it could become a drip burner after it is hot if I just turn way down or off the compressed air. I am going to test that today. (Using th same volume of combustion air to replace that volume of compressed air air would theoretically reduce heat absorption by compressed air that it sucks up when it expands). But I also use a low power derated from 20 v to 12v weed blower for combustion air that makes a modestly rapid air stream but not a stream that is “intense.” My question is if my fuel burn is 2.5 gal per hour as is and remains 2.5 gal/hr, will flame temp increase with a more powerful blower?

Denis

 

Do you wear a respirator when around the furnace? In industry we've been warned repeatedly that fried ceramic fiber is nearly as bad a asbestos fibers to inhale. Of course it's a hazard which may never show up (lots of asbestos workers were never damaged) but is a severe risk. Do you spray it with rigidizer?

Eager to hear Richard's response. Are you asking if you have full combustion of 2.5 gal per hour of fuel does it matter where the air comes from? If you do the math I think you'll find that the chill of the compressed air expanding is a very minute value compared to the heat being released by combustion. I don't know what to call a pressurized drip burner, but to me that is the ideal drip, just a little pressure to insure even flow. As long as the furnace is hot enough to vaporize the fuel and it burns before exiting the furnace it seems the lower velocity the better. You can't get more than complete combustion out of the fuel.

 

I use a 5 gallon shop vac with a standard household lightswitch dimmer to control the speed. With my Lionel Hot Shot, (with a one inch pipe) full speed is a little too much.


With the Brute (with a two inch tube) I could use a bit more air than the shop vac can deliver.

With a drip burner, the more air you can push through it, the more oil you can drip. But you have to keep the air/fuel mixture correct. I test the flame by holding a clean piece of galvanized sheet metal over the vent hole for five seconds or so. If it discolors just a little I know I am slightly on the rich side of neutral. (I do not want an oxidizing flame for melting bronze.)

I use a ball valve as a shut-off valve, but it is not precise enough for controlling the oil flow. I set the air flow first and then use a needle valve to control the oil flow to get the right flame.

Richard

 

There are two questions. One flame temperature vs total energy released. Obviously nearly complete combustion will have a clear upper limit of energy to be released. But it is the temperature difference that melts the iron. So, if a higher flame temperature (with necessarily decreased flame volume) could be achieved, faster melting would result. My suspicion is that higher combustion air velocity will not help matters much, but I would like to know Richard’s experience and thoughts.

And, yes, the amount of heat absorbed by the expanding compressed air is probably small, but could have a disproportionate effect if it reduces the initiating temp of the flame.

Denis

 

Ah, you were posting while I was blathering at the keyboard.

Interesting about the zinc testing of the flame. I am going to incorporate that. It sounds much more precise than the visual inspection that I have been using—-looking to see when the exhaust shows just a faint bit of smoke.

Thanks for your comments!

Denis

 

No. I am a sculptor; I only cast my own work, so I melt infrequently. Also I am seventy-six years old. I have been around long enough to have endured many waves of widespread cultural hysteria, everything from McCaethy's witch hunt for communists to the hysteria over second-hand smoke. (I have to wonder about people who are willing to raise their kids in the poison gas they call air in major American cities, and then complain about second-hand smoke.) I figure I will be dead of something else before either silica particles or second-hand cigarette smoke get me.

Richard

 

You've only got six years on me. I'm not hysterical about ceramic fibers but having lots of refinery safety training has colored my views. I seal all my wool and wear a respirator when melting brass even though I like milk. It is amusing the people who want only organic foods will drink alcohol to excess. I think I'm past the age of concern over silicosis as well. However I'm new to casting and am enjoying melting as much metal as I can. Surely that will slow down but right now it's an absolute blast.

 

I believe you get the most energy released with complete combustion, which is also highest flame temperature. We generally burn everything with incomplete combustion, so adding air gets closer to complete combustion but generally also adds velocity. That velocity is counterproductive but it's what we see in a campfire, etc. So we associate high velocities with high temperatures. Well designed propane stoves do not have high velocities. One way to look at it is compare not heating nitrogen. The nitrogen in air has a major effect on flame temperature since the nitrogen has to be heated. Propane flame temperature is about 3,600F in air, but in oxygen it's 5,100F. So if you burned oxygen you could slow the flow dramatically to release the same number of BTU's per hour into the furnace. Instead of 1-1/2 gph you would need about 1 gph, and only discharge about 21% of the vapor with oxygen that you would have with air. Saving all those BTU's going up the flue also means there is more time for the hot gas to conduct and radiate it's energy to the furnace. I'm not suggesting you burn oxygen but it helps me visualize why slower flow is better.

You have to think of a furnace as a box with a hot gas flame in it, with cold walls, and a cold crucible, and cold air coming in and hot air going out the top and a black hole at the top that absorbs any radiation which goes up. By insulating the shell you can get the temperature of the hot face up and it can radiate to the crucible. The crucible can never get to the temperature of the gas surrounding it. It only approaches it. Same for the hot face. The longer the residence time of the hot gas in the furnace the more it can transfer energy to the walls and crucible. If you are burning close to the bottom and absorbing energy from the flame as it spins around the crucible three times you are getting energy out of the gas. It takes energy to heat the crucible. It takes time to transfer energy. My flue gas is usually a few hundred degrees above the melt temperature (I can only measure on aluminum melts).

By the way, if you want to minimize melt times you add charge to the crucible as soon as there is room, keeping the crucible as cool as possible so it will absorb as much energy as possible, assuming you have the flame as hot as you can get it. Avoiding chilling the melt only keeps the crucible hotter so it can't absorb as much energy.