Casting your own steam engine castings?

[GreenTwin]

Have you made any posts about your furnace? interested in making one similar

There are a couple of approaches to furnaces that are common.
One is the coated (or sometimes non-coated) ceramic fiber blanket furnace, and the other is the cast refractory thin hotface furnace.

The consensus among the backyard casters I know is that an uncoated ceramic blanket furnace puts dangerous fibers into the air, and ultimately into your lungs. Despite the junk science that has been posted about ceramic fibers in the lungs, the truth is that they don't break down, and they do ruin your lungs.
The only safe ceramic blanket furnace is one that has the blanket surface fully coated with some high temperature material such as satanite.
The coating must be maintained throughout the life of the furnace, and the ceramic blanket not allowed to degrade.

The good part about ceramic blanket furnaces is that they are relatively easy and inexpensive to initially build.
The downside to ceramic blanket furnaces (in my opinion, and opinions differ on this) is that the are not very rugged, and don't stand up to abuse very well, such as bumping into the surface with a crucible or lifting tongs.

I use a 1" thick cast refractory hot face that uses a material called Mizou, and it stands up well to iron temperatures and iron slag.
It should be noted that cast refractory that is operated at iron temperatures will crack, and cracked refractory should not be confused with refractory that is crumbling/disentigrating.
Patching cracks in Mizou is a minor thing, and the cracks don't really affect the long term performace of the hot face, or the durability.

I used a layer of insulating fire bricks around my hot face, basically to give a rigid surface to somewhat support the cast refractory.
Outside of my insulating bricks is two layers of 1" ceramic blanket, and then a stainless steel metal shell.

With both insulating fire bricks and ceramic blanket, the cost goes up exponentially with the temperature rating.
The only affordable insulating fire bricks and ceramic blanket is in the 2,600 F range as far as operating temperature.
They do make 3,000 F insulating fire bricks and ceramic blanket, and basically sell as if they were gold.

Mizou I think it rated around 3,000 F, and thus it makes a good hot face.
Mizou is a dense refractory, and thus it is important to minimize the use of it in the furnace, since the speed at which a furnace will melt metal is directly related to its mass.

My first furnace used a 3" thick wall of dense refractory, and that was a blunder, since the high mass of the furnace took over 30 minutes to come up to iron melting temperaures, and thus 30 mintues of time and fuel are wasted with every new melt.

My second and current furnace uses 1" of dense refractory, and it brings iron up to pour temperature much more quickly (generally in about an hour).

So a word of warning to the wise; protect the lungs at all cost when doing foundry work, because the cost can be total disability or death.
When using/cutting/handling foundry sand, fire bricks, ceramic blanket, cast refractory, parting dust, etc., wear a commercial grade respirator that will totally block all of this material from getting into the lungs.

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[GreenTwin]

Many use a beer keg as a shell for a furnace, and that is the material I used.
I ended up expanding several beer kegs, and my final exterior shell ended up being the diameter of a 55 gallon drum.
If I had to do over, I would have just purchases a stainless steel 55 gallon drum, since welding stainless steel with a nickle rod is a bit tricky.

My recommendation is to select a crucible first, and then let that dictate the overall size of your furnace.
Usually a 2"-3" clearance is required around the crubible for iron melts, and sometimes you can get away with a little less, especially for aluminum.

The burner tube enters the furnace on a tangent, off to one side of the crucible, so as to prevent any direct impingment of flame onto the crucible.
The burner tube entry into the furnace is called the tuyere (I think pronounced tweer), and the tuyere should be at the bottom of the crucible.
The crucible should sit up on a plinth made of dense high temperature refractor that is about the size of the bottom of the crucibl.
The plinth should elevate the crucible up above the bottom of the furnace at least several inches, and to allow the centerline of the burner tube to be located at the bottom of the crucible.

Here is the layout I used for my furnace, which can use anywhere from a #10 to a #20 (possible a #30) crucible.
If you are going to use a crucible smaller than a #10, then the entire furnace can be scaled down proportionally.

Most folks turn a flange out at the top of the hot face, but since I used a layer of rigid insulating fire bricks behing my hot face, I omitted the flange.
I flange at the top of the hot face is probably the best idea though, and if I had to do over, I would probably add that feature.

A less expensive furnace that would perform as well as the one below would use three layers of 1"ceramic blanket around the hot face, in lieu of the insulating fire bricks and two 1" layers of ceramic blanket that I used.
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[GreenTwin]

Many people read about furnaces online, and become fixated with features that a furnace MUST HAVE.
A furnace only needs enough features to work efficiently and to hold up to the temperatures and metal/slag that will be melted in it.
Many/most people use a drain in the bottom of their furnace, and then they complain about the drain leaking hot gases and causing all sorts of problems.

The thing to remember is that the furnace interior is pressurized by the combustion air blower, and so any crack or opening will generally leak hot gasses, which can cut through things like exterior shells like a hot knife.
The fewer openings you can have in a furnace, the fewer hot gas leaks you will have.

The junction of the furnace lid and the top of the hot face will leak hot gasses; there is no way to permanently stop that, but leakage at this joint does not create problems as long as it is not excessive.

The burner tube should be a tight fit to the tuyere, to prevent any significant hot gases from blowing out past the burner tube.
Any hot gases that blow past the burner tube will heat it up and perhaps overheat it.
The burner tube should operate cool to the touch, or perhaps warm, when operating the furnace.

You shoud use a quality crucible such as a Morgan Salamander Super clay graphite, and don't ram the scrap metal in it prior to a melt, because the metal will expand and crack the crucible. If you take care of your crucibles, and use quality crucibles, you should never have one fail.
If the crucuble wall gets too thin, stop using it.

If your crucible full of molten metal for some reason does break inside the furnace, you should remove the burner to prevent it from running into the burner tube. My plan if this ever happens is to turn my furnace on its side and run the burner, and let the metal melt and drain out.
I don't anticipate ever breaking a crucible inside the furnace though.

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[GreenTwin]

There are a large number of foundry burner videos on ytube, and the debate about who has the best mousetrap/burner can get very contentious to say the least.

The best burner for your furnace is the one that:
1. Works efficiently.
2. Has few or no moving parts.
3. Does not degrade over time.
4. Brings the metal you are melting to pour temperature in a reasonable amount of time.
5. Does not require adjustment during the operation of the furnace, even if the fuel tank level changes significantly.

For me, other critical concerns with a burner (used to melt iron) are that it starts easily using diesel only, and propane it not required at all, the burner must have fine and immediate control via a needle valve, and the burner can be brought up to full power in perhaps 15 seconds after starting without becoming unstable.
My siphon nozzle burner achieves all of the objectives listed above.
Very few other burner designs (perhaps none) will achieve all of the above.

The only change I am going to make is to convert my burner from a siphon-nozzle style to a pressure nozzle type, using a small gear pump.
This will allow me to operate the burner without using compressed air. A gear pump uses a very small amount of 120 VAC power, and a small fractional horsepower motor.

Some folks seem to just like to endlessly experiment with burner types, and I have done my fair share of that.
After trying perhaps 10 different burner types/configurations, I finally settled in on the Delavan siphon nozzle arrangement, but will convert to the Delavan pressure nozzle type as soon as I get the time. I have purchased the gear pump and a fractional 120VAC motor to power it.

There is one burner that uses hot gasses out of the furnace to impinge back on the burner tube, thus heating it red hot.
This is a terrible design, but the people who use it swear that it is the best.
Again, I go back to the items listed below, and ask of any burner design will it achieve them.
If not, you are perhaps using a sub-optimal burner design, that used gimmicks when they are not required.

Burner designers tend to be like snake oil salesmen; their burner will always cure any ill, and also save the whales.
It all goes back to the items listed above though, and whether you want a burner that operates consistently without degredation over time.

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[dnalot]

Buy the book Complete Casting Handbook by John Campbell.

This is the furnace I built after reading Mr Campbell's book. I cast the floor first and then the walls using a cardboard tube as a form. I cast the walls in two steps. The first casting I used a large tube that made a 1" liner with a low temp refractory and then I used a second smaller tube to to cast a second 1" layer of high temp refractory. It has held up well for many years (no cracks or crumbling) mostly casting brass and bronze. Heat is provided using a naturally asperated propane torch. I have also melted cast iron by adding a blower. A propane torch requires a bell shaped end. I cast the bell shape into my refractory walls and that has worked great.

Mark T

 

[GreenTwin]

Most people seem to have the a lot of trouble with their furnace lid, and that part of the furnace is probably the hottest, with a lot of combustion air turbulence.
The bottom of the furnace is the coolest, and thus one of the reasons you want your crucible elevated on a plinth.

My solution to a furnace lid is to use Mizou (dense castable refractory) to make a domed lid with a short chimney.
If a domed lid cracks, it will not colapse into the furnace, since the shape acts like a Roman arch, and is self supporting.

Making a domed lid from cast refractory is not the easiest thing to do, but for long term durability, especially with iron melts, it is somewhat necessary.
The other option is to use a coated ceramic blanket on the lid, and expect to replace it frequently.

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[GreenTwin]

This is the furnace I built after reading Mr Campbell's book. I cast the floor first and then the walls using a cardboard tube as a form. I cast the walls in two steps. The first casting I used a large tube that made a 1" liner with a low temp refractory and then I used a second smaller tube to to cast a second 1" layer of high temp refractory. It has held up well for many years (no cracks or crumbling) mostly casting brass and bronze. Heat is provided using a naturally asperated propane torch. I have also melted cast iron by adding a blower. A propane torch requires a bell shaped end. I cast the bell shape into my refractory walls and that has worked great.

Mark T

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I like that design a lot.
There are many good features to it.

If I were building it (everyone has their favorite features, and every furnace is a bit different depending on what individual features you want/like), I would:
1. Sit it on the ground and not elevate it.
2. Omit the drain hole in the bottom.
3. Omit the small second lid, and just have a single opening in the main lid, perhaps 4" in diameter.
4. Use a clay graphite crucible, although a steel crucible will work pretty well with aluminum.
5. Make the top of the plinth the same diameter as the bottom of the crucible.
6. I would use a hinged/swivel lid design, so that the lid remains horizontal when opened, and you don't get the heat from the lid reflecting directly onto your body (not really critical for this size furnace, but gets critical for a larger iron furnace).
7. The centerline of the burner tube is generally kept exactly at the bottom of the crucible, at least that is what I have seen on commercial furnace designs.
8. Put a slight amount of clearance/slop into the lid hinge, so that the lid can close evenly with no gaps. My experience with a rigid/fixed hinge is that it creates a gap on one side at the lid/furnace junction that leaks hot gasses excessively.

This is probably the best 20 lb cylinder furnace design I have seen.
Very nice, and no doubt highly functional.

A belled end on a propane burner is important if the burner is used outside of a furnace, or with a forge, but actually you don't need a belled end with a furnace. The commercial furnaces do used a belled tuyere though, and no doubt the bell assists with propane flame propagation, and helps keep a propane burner lit, especially during startup.

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[GreenTwin]

If I were going to build a furnace today, I would make the hot face from 3,800 F plastic refractory.
I would roll the plastic refractory out like cookie dough, perhaps 3/4" thick, and then wrap it around a waterproofed sonotube, with a flange/lip at the top.

Diddo for the bottom of the furnace.

For the lid, I would put 3/4" thick plactic refractory over a piece of metal that had a dome shape.

I discovered plastic refractory a few years ago, and while it has to be purchased at a foundry supply store and may be a bit tricky so source, it really changes everything as far as simplifying making furnaces, and also patching furnaces.

I ran across plastic refractory used by the art-iron folks to patch the interior of their cupolas, and they basically just hand-pack new refractory in place to repair the interior of their cupolas, as needed. You can keep a furnace or cupola in operation indefinitely using plastic refractory to patch it, and it totally eliminates the need to ever rebuild a furnace.

I will probably never use cast refractory again, as long as I can get 3,800 F plastic refractory.
I think it does have a shelf life though, so don't purchase more than you can immediately use.

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[dnalot]

If I were building it (everyone has their favorite features, and every furnace is a bit different depending on what individual features you want/like), I would:

You have made some very good points. My drawing was from a time when I had no experience in building a furnace. I never needed the hole in the bottom. I never used a steel crucible but did buy several graphite crucibles. I did as you suggest making a plinth the diameter of my crucible. The crucible I use for bronze is gazed and I did put a layer of cardboard between it and the plinth to keep it from sticking but ultimately it did stick. I did have problems with the top not sealing do to the hinge but fixed that by replacing the hinge pin with a smaller diameter. I have had mixed feelings about the second lid. I normally use it to feed the crucible and then close it as that reduces the noise quiet a bit. Sounds like a jet engine otherwise. To help with melting cast iron I added a 2" thick ceramic blanket and covered it with a sheet metal shell. And I added some wheels for moving the furnace around.

I admire you guys that have successfully mastered casting cast iron. For my little furnace it is just a little out of reach. I had never seen plastic refractory and will keep that in mind. I shaped some foam insulation material to cast the top. So far the top is holding up well and will probably last the rest of my years.

Mark T

 

[dazz]

The book is rather pricey, but the 10 rules for good castings can be found online, so I would recommend waiting on purchasing the book until you really are deep into castings things, and even then you may not need the book, but instead just use the 10 rules.

The book is pricey but good value for the $$$. I figure a book like that will help get me far enough up the learning curve to have a reasonable chance of success with metal casting.

At present, I am trying to clear a backlog of unfinished projects before starting metal casting.

 

[GreenTwin]

Here is the link I was looking for the other day, and this link contains John Campbell's 10 rules for good castings.
Hit the "next" button at the bottom left to go to the next rule.

http://www.atlasfdry.com/10rules.htm
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[GreenTwin]

You have made some very good points. My drawing was from a time when I had no experience in building a furnace. I never needed the hole in the bottom. I never used a steel crucible but did buy several graphite crucibles. I did as you suggest making a plinth the diameter of my crucible. The crucible I use for bronze is gazed and I did put a layer of cardboard between it and the plinth to keep it from sticking but ultimately it did stick. I did have problems with the top not sealing do to the hinge but fixed that by replacing the hinge pin with a smaller diameter. I have had mixed feelings about the second lid. I normally use it to feed the crucible and then close it as that reduces the noise quiet a bit. Sounds like a jet engine otherwise. To help with melting cast iron I added a 2" thick ceramic blanket and covered it with a sheet metal shell. And I added some wheels for moving the furnace around.

I admire you guys that have successfully mastered casting cast iron. For my little furnace it is just a little out of reach. I had never seen plastic refractory and will keep that in mind. I shaped some foam insulation material to cast the top. So far the top is holding up well and will probably last the rest of my years.

Mark T

I tried a dual-lid design, and it leaked so much that I abandoned it.
I do like the dual lid design a lot though, and so I have not give up on it completely, and if I can figure out a good seal, I will start using it again.
There are many advantages to opening a smaller lid like yours, and not opening the big furnace lid.
I can skim and add scrap through a 4" diameter lid opening, but an 8" 2nd lid is infinitely better/easier, and shields you from most of the furnace heat.

I use two layers of cardboard beneath the crucible, and have never had a crucible stick using this method.
If you spill a significant amount of slag over the side of a crucible and it runs down to the plinth, chances are your crucible will stick to the plinth.
Caution should be used with skimming slag. The slag also is highly corrosive to the exterior of the crucible.

I think you could easily melt cast iron in your furnace (and I think you already do).
The cast refractory linings you have would make melting cast iron easy with low maintenance.
I have a buddy who has melted large amounts of cast iron (perhaps 1,000 pounds) in a furnace about the size of yours, with a very similar design.

Melting and pouring cast iron is surprisingly easy if................
1. You tune your burner correctly. This was probably my biggest problem; learning how to correctly tune an oil burner for iron.
2. Your furnace refractory is rated for iron temperatures.
3. You use a Morgan Salamander Super clay graphite or equivalent crucible that is iron (ferrous-metal) rated.
4. You use heat shields on your skimmer handle, and pouring shank handle.
5. If you start with a small melt, create a molten pool, and then add relatively small pieces of (very dry) scrap to that pool, pushing the scrap completely under the slag that is on top of the melt. This was the step that I really did not understand, and one of the reasons it took me so long to figure out how to cast gray iron. Without doing this, the slag can turn into a solid hard mass on top of the melt, and actually insulate the melt and cool it. This method also greatly reduces the amount of slag that is created, and leaves a minimal amount to be skimmed at pour time.
6. Use a slight amount of 75% ferrosilicon in the melt, and pour as soon as the ferro is stirred into the melt.
The ferro keeps the iron from developing hard spots, and makes the iron very machinable.
7. Pour iron as fast as possible after removing the crucible from the furnace, and within about 30 seconds, else you may get cold metal.
Molten iron drops below pour temperature very quickly after the crucible has been removed from the furnace.
I add the ferrosilicon while the crucible is still in the furnace, and with the burner running, with the furnace lid open (yes, this is a very hot thing to do, but it works).
8. Leave the iron castings in the sand mold overnight, and allow them to cool very slowly.

Like you, I have learned so much over time.
There is really only one way to learn how to cast metal, and that is to get in there and build a furnace, and see what works (using all of the appropriate safety precautions of course).
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[100model]

The fuel consumption rate is very high. The flame temp is too low.

To melt 14 kgs of cast iron the furnace burns 8 kgs of waste motor oil and 8.8 kgs of propane. Both fuels take about the same time to melt 50 - 60 minutes.

 

[GreenTwin]

I have seen discussions about different fuel types, and so I dug up this information online, which seems to be accurate.

I think if you do the math on 100model's data in the post above, you will see that it does not matter if you are inputting oil BTU's or propane BTU's (or any fuel type).
What is important is that you input approximately the same BTU's into the furnace regardless of the fuel type, along with sufficient combustion air to fully burn that fuel inside the furnace.

Oil Burner Approximate Values:

1 gal/hr = 138,500 Btu/hr (40.59 kW)
2 gal/hr = 277,000 Btu/hr (81.18 kW)
3 gal/hr = 415,500 Btu/hr (121.77 kW)
4 gal/hr = 554,000 Btu/hr (162.36 kW)

5 gal/hr = 692,500 Btu/hr (202.96 kW)
6 gal/hr = 831,000 Btu/hr (243.55 kW)
7 gal/hr = 969,500 Btu/hr (284.14 kW)
8 gal/hr = 1,108,000 Btu/hr (324.73 kW)

9 gal/hr = 1,246,500 Btu/hr (365.32 kW)
10 gal/hr = 1,385,000 Btu/hr (405.92 kW)
11 gal/hr = 1,523,500 Btu/hr (446.51 kW)
12 gal/hr = 1,662,000 Btu/hr (487.10 kW)

Common Flame Temperatures

Acetylene with Air 4532 F
Acetelene with pure Oxygen 6296 F
Natural Gas with Air 3562 F
Propane with Air 3596 - 3623 F
Propane with pure Oxygen 4579 - 5110 F
Wood 3596 F
Kerosene 3810 F
Light Fuel Oil 3820 F
Medium Fuel Oil 3815 F
Heavy Fuel Oil 3817 F
Coal 3950 F approx.

Standard propane torch 2,012 F

Air = 21% oxygen

Furnace Efficiencies

Crucible (gas) 7-19%
Cupola 40-50%
Dirct Arc 35-45%
Induction 50-70%

Heat Value of Materials

#2 Diesel 138,500 BTU/gal
Kerosene 135,000 BTU/gal
Natural Gas 100,000 BTU/therm
Propane 92,500 BTU/gal
Sawdust (green) 10,000,000 BTU/ton
Sawdust (dry) 18,000,000 BTU/ton
Electric 1.0 kWh = 3,412.14 Btu

Miscellaneous

1KWH = 1 kilowatt operated for a 1 hour period

1 BTU = 0.000292071 kWh

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[GreenTwin]

Checking the math, #2 diesel (assuming it is roughly approximate to waste oil) produces 138,500 BTU's/gal.

For waste oil or diesel:

8.0 kg of waste oil is 17.63 lbs.
Waste oil or diesel is about 7.0 lbs per gallon.
17.63 / 7 = 2.52 gallons of fuel to melt 14 kg (32 lbs) of iron, using 100model's data from above.

138,500 BTU/gal x 2.52 gallons = 349,020 BTU's to melt 14 kg (32 lbs) of iron using waste oil.

This is almost identical to the fuel flow rate I use for diesel, and my furnace is similar in size to 100model's furnace.
I have talked with others who also have approximately the same furnace size, and the same number keeps coming up, which is about 2.5 or 2.6 gal/hr to melt iron.
Everyone I have talked to has reached this fuel flow number independently (all using approximately the same furnace size).

For propane:

8.8 kg of propane is 19.4 lbs.

One gallon of propane weighs about 4.11 lbs.
(a full 20 lb propane cylinder contains about 4.7 gallons of propane, per the internet; check me on this)

19.4 lbs / 4.11 gal/lb = 4.72 gal of propane to melt 14 kg (32 lbs) of iron using propane.

92,500 BTU/gal x 4.72 gal = 436,600 BTU's to melt 14 kb (32 lbs) of iron using propane.

Ratio:

The ratio of waste oil to propane gallons used should equal the ratio of BTU values between waste oil and propane.

4.72 gal propane / 2.52 gal waste oil = 1.873 ratio

#2 Diesel 138,500 BTU/gal
Propane 92,500 BTU/gal

138,500 / 92,500 = 1.49 ratio

I can't quite prove the correlation between BTU/gal of propane and BTU/gal of waste oil, but you get the idea, ie: a fuel with a higher BTU/gal value will require less total fuel to melt a given amount of iron than a fuel with a lesser BTU/gal value.

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[GreenTwin]

From what I have read in various white papers, the trick to achieving the hottest temperature inside of a furnace is to find the optimum fuel droplet size that produces the largest surface area of the hottest part of the burning droplet.

Perhaps you have seen the candle experiments where the interior of the candle flame is actually cool, and the hottest part of the candle flame is on the outside of the flame envelope.

Its the same way with fuel droplets.
For maximum heat, maximize the area of the hottest part of the flame envelope around each fuel droplet.
What this means is that a finer fuel droplet size does not necessarily burner hotter than a more coarse droplet.

Pulsing the fuel pressure is reported to save a significant amount of fuel oil when used to fire a furnace, but I have not tried this yet, and not sure how I would produce the pulses.
The pulsing is helping to maximize the hottest part of the droplet flame envelope.

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[GreenTwin]

Some have questioned the data above that shows that a fuel fired crucible furnace generally as a 7-19% efficiency, which is incredibly wasteful, but this is a realistic value I think.

I call an oil-fired furnace "a poor man's induction furnace".
But an oil fired furnace can produce about 400 kW (with dual burners), and it would be very difficult to draw this much energy out of a typical residential electrical service.

Electrical services often have a demand charge, and so if you fire up your 400 kW induction furnace once a year, you pay for 400 kW per month whether you use the furnace every month or not. The utility company charges you for the capacity they have to guarantee you, regardless of whether you actually use that much capacity.

400 kW at 240 volts, single phase is 1,667 amperes.
400 kw at 208 volt, 3-phase is 1,111 amperes.

Buying an induction furnace is one thing (I have seen this recently), and finding enough power to operate it is an entirely different thing.
Do the math before you buy an induction furnace.

And if the induction furnace inverter does not come with input line reactors, then you can put so many harmonics onto the power company system that you can actually damage their equipment, and they are never happy when you burn up their equipment.

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[Richard Hed]

Many use a beer keg as a shell for a furnace, and that is the material I used.
I ended up expanding several beer kegs, and my final exterior shell ended up being the diameter of a 55 gallon drum.
If I had to do over, I would have just purchases a stainless steel 55 gallon drum, since welding stainless steel with a nickle rod is a bit tricky.

My recommendation is to select a crucible first, and then let that dictate the overall size of your furnace.
Usually a 2"-3" clearance is required around the crubible for iron melts, and sometimes you can get away with a little less, especially for aluminum.

The burner tube enters the furnace on a tangent, off to one side of the crucible, so as to prevent any direct impingment of flame onto the crucible.
The burner tube entry into the furnace is called the tuyere (I think pronounced tweer), and the tuyere should be at the bottom of the crucible.
The crucible should sit up on a plinth made of dense high temperature refractor that is about the size of the bottom of the crucibl.
The plinth should elevate the crucible up above the bottom of the furnace at least several inches, and to allow the centerline of the burner tube to be located at the bottom of the crucible.

Here is the layout I used for my furnace, which can use anywhere from a #10 to a #20 (possible a #30) crucible.
If you are going to use a crucible smaller than a #10, then the entire furnace can be scaled down proportionally.

Most folks turn a flange out at the top of the hot face, but since I used a layer of rigid insulating fire bricks behing my hot face, I omitted the flange.
I flange at the top of the hot face is probably the best idea though, and if I had to do over, I would probably add that feature.

A less expensive furnace that would perform as well as the one below would use three layers of 1"ceramic blanket around the hot face, in lieu of the insulating fire bricks and two 1" layers of ceramic blanket that I used.
.

Ha! Just as you say. I am trying to find out the physical sizes of some crucibles, as I intend to build a furnace around the crucible, rather than first build the furnace then select a crucible. But the vendors, when asked what the physical dimensions, said, "they are in the description". Well, no they aren't. What is there is the number of lbs/kgs which is NOT the dimensions of the crucible.

 

[Jasonb]

Not looking at the right suppliers

One of our suppliers to the hobby use lists the size, don't forget to allow for the size of teh open tongs
https://www.artisanfoundry.co.uk/product_info.php?cPath=26&products_id=115

 

[dazz]

...
The thing to remember is that the furnace interior is pressurized by the combustion air blower, and so any crack or opening will generally leak hot gasses, which can cut through things like exterior shells like a hot knife.
The fewer openings you can have in a furnace, the fewer hot gas leaks you will have.
...

This is the first comment I have seen on cracking and air bleed in a home foundry. I have been speculating that hot liner cracking would lead to problematic hot air bleeds. It is a problem on full sized furnaces.

I have been considering controlling (throttling) burner air flow by diverting surplus air into the cavity between the hot liner and the exterior sheet metal shell. Ideally this would raise the air pressure slightly above the furnace pressure. Air would then leak into the furnace. I expect this would not be enough to cool the hot liner, but it would reduce the damaging effects hot air bleed leakage. I suspect the added injection of fresh heated oxygen rich air through cracks would slightly increase furnace combustion temperature.

The way I was thinking of implementing this feature is to drill a series of holes in the burner tube along a helix. I would then have a value tube that was a sliding fit over(or inside) the burner tube. If the holes are located between the hot liner and the shell, any uncovered holes will dump pressurised air into the cavity. Sliding the value tube in/out would throttle the burner air flow and the cavity air flow.