The Furnace Thread

I see a lot of people new to the metal casting hobby or perhaps considering upgrades to their present capabilities asking questions about foundry furnaces. I thought it might be useful to start a thread about different styles/types of furnaces and the Pros and Cons of each approach. This is not intended to be a how-to or build thread but moreover discuss the merits and drawbacks of various furnace types. If you happen to have links to representative builds, that would be good to include. There’s also nomenclature. Furnace, Kiln, Oven. What’s the difference? It’s really just how they’re referred to in the various fields of use.

Furnaces differ in:

• The materials and mechanics of construction – The outer support shell (if they have one), and refractory liners.
• Type of design and how you access the contents: Crucible, contact, tilting, lift-out, bail-out, lift-off, cupola, reverb.
• The fuel or source of heat – wood, charcoal, coal, coke, diesel, kerosene, waste motor oil, waste vegetable oil, propane, natural gas, electric resistance, electric induction, electric arc….to name a few.
• Intended use – Metal Foundry furnace (crucible or other), Investment or shell burn-out and curing oven.

I’ll start this thread with some examples. Everyone else feel free to add but try to keep it focused on the merits of different types of furnaces and maybe some links to builds or other examples of the breed. If the thread progresses well, I'll update the Pros & Cons with your inputs.

I have a 10" & 14" Bore lift off furnaces. They can be resistive electric or fuel fired.


Best,
Kelly

 

Conventional Lift Out Crucible Furnace.

There are many subtle variations on this design. Most have a Tuyeres that enter the cylinder tangentially near the base of the crucible.



Myfordboy build thread

http://www.alloyavenue.com/vb/showthread.php?10898-Myfordboy-s-new-oil-fired-furnace-build&highlight=furnace+build

Pros:

1. Tried, true, proven design that has stood the test of time on both a commercial and hobby level.
2. Typically most rugged and durable depending on choice of materials can be suitable for most any metal duty including iron.
3. They can potentially be used as bulk scrappers with adequate shape to base, drain hole, and metal contact rated refractory.

Cons:

1. Depends on size but can be very heavy and difficult to move and store if not on wheels.
2. Can vary in cost depending upon choice of materials but generally speaking best value for those that want a durable furnace that will see frequent use and have long life with low maintenance.

 

Lift Off Body Furnaces:

My Build Thread:
http://www.alloyavenue.com/vb/showthread.php?12107-Kelly-s-Furnace-Build-Log

Pros:

1. Probably the biggest advantage over a conventional lift-out furnace is having full access to the crucible so it can be snatched and poured in one step with an open ring shank as opposed to having to lift the crucible out of the furnace with tongs and placing it in a pouring shank. This reduces handling of the hot crucible full of molten metal and minimizes heat loss between the furnace and pour. It can also have the side benefit of accommodating bigger crucibles for a given bore size since you don’t have allocate space to accommodate tongs to grip around the crucible.
   

Cons:

1. They can be more difficult build, particularly for furnaces of heavy construction. Higher temperature metals will often require high refractory materials which are often heavy. These can require a fairly robust lifting mechanism. My furnace build would be a possible example of such.
   

There is a style of lift off that is very simple and easy to build. There are many examples if you search Raku Kiln or Top Hat Kiln. They’re basically just a light support structure to which you attach refractory wool. The structure can be wire or a can or barrel.

Pros:

1. Very scalable in size. They can be the size of beverage can or big enough to walk in. They don’t have to be round. They can be any shape.
2. Very inexpensive and easy to build.
3. Very light, easy to handle, move and store. You can hang the furnace body from the rafters of your garage and the base can just be loose insulating fire bricks that you stack and store.
4. Versatile. You can use it as a foundry furnace and a burn out kiln.

Cons:

1. They won’t be as durable as a furnace built with insulating or dense castable refractories but surface treatments such as colloidal silica rigidizer or application of a thin Satanite hot face can dramatically improve durability
2. They’ll be more limited in temperature. Certainly good enough for aluminum and also bronzes with the best refractory wools.
3. They will do best with Propone, Natural Gas, or Electric heat. Oil and diesel burners will likely be too hot unless sized and tuned accordingly.
4. Most refractory wool fibers pose carcinogenic health risks if breathed. Some are not but those tend to be somewhat lower refractory. A gas fired furnace can cause such to be air born. Where a mask in vicinity of the operating furnace or the fibers can be easily stabilized with rigidizer (colloidial silica).

Here are some examples:



Here are some examples of lift offs that have more robust lifting mechanisms.




Pros:

1. Since the lifting mechanism is more robust, these furnaces can be much more ruggedly constructed and suitable for most any fuel and metal duty.

Cons:

1. Added complexity in build and increased cost.

 

Hinging Barrel Vertical Orientation.



Pros:

1. Provides nearly full access to crucible.
2. Hinging is reasonably simple to build and post mount.
3. Can potential use light weight wire frame and wool construction or durable castable refractories.
   

Cons

1. Some added complexity in construction.
2. Potentially more difficult to have flame sealing barrier at opening surfaces.
3. More difficult to have removable lid so entire furnace must be opened to access the crucible causing loss of heat. Charging crucible and skimming must be done through vent hole.

 

Hinging Barrel Horizontal Orientation.



Pros:

1. Convenient height and easy access to contents.
2. More horizontal space for burnout of low profile objects.
3. May be more advantageous for direct melt/metal contact.

Cons:

1. Slightly more difficult to position plinth and shelving, and/or crucibles.

 

Tilt Back Body



Picture courtesy of forum member Junkyard

Pros:

1. Nearly full access to crucible so open ring shank is possible for one step crucible snatch and pour.
2. Simple hinging and allows use of high refractory materials.
3. Can be positioned at any height for ease of crucible lifting and the also accessed by overhead lifting mechanisms.

Cons:

1. High radiant heat when open. Best to stand to the side and not directly in front of the open barrel.
2. The tilting half can be very heavy depending upon size materials of construction. The one pictured has a pneumatic cylinder to actuate the top barrel. It could potentially be counterweight spring assisted as well.
3. For home foundry furnaces, there's a potential tipping hazard without appropriate base and mounting provisions.

 

Barrel Bulk Scrapper

Tobho Mott's Bulk water Bath Scrapper

http://forums.thehomefoundry.org/index.php?threads/tobhos-water-bath-bulk-scrapper-for-aluminum.20/



Pros:

1. Can accommodate large scrap such as automotive rims and resizes into nuggets easily managed in all size crucibles.
2. Ease of construction, an open ended 55gal drum, and grating
3. Can use virtually any fuel including fire wood.

Cons:

1. Barrels are degraded consumed….but barrels are plentiful.
2. Use more commonly limited to lower temp metals such as aluminum

 

Tilting Contact Furnace

Master53Yodas Tilting bulk scrapper



Pros:

1. Reduces large scrap to liquid for processing in holding furnace and on to ingot
2. Can have high melt capacity
3. No crucible metal contact rated refractory accommodates all shapes of feed stock

Cons

1. Somewhat higher build complexity
2. Requires high burner capacity