- Here are some thoughts and considerations regarding building a home foundry furnace. The first is the lining. To save fuel use a hybrid lining in your furnace instead of a solid refractory. A hybrid lining has a hard inner shell, perhaps an inch thick, with a Kaowool outer layer, and may or may not have a third layer of hard refractory or steel. You would be way ahead in money and time in the long term to use 2 of Kaowool in your system. Every time you do a melt with a solid refractory furnace you will be putting a huge number of BTUs into that lining just to get it up to temperature, which costs lots of time and fuel to reach operating temperature, not to mention the heat that will be radiated from it during the melt, all of which has to be continuously replaced. With fuel prices what they are today, and steadily rising, a hybrid furnace shell will save you lots of money in the long term.
- One possibility to make things more efficient, yet still retain a hard lining, is to simply wrap the outside of your solid refractory furnace in 2 of Kaowool, and wrap the Kaowool in a protective refractory cloth, or even chicken wire. That wont save on the initial warm-up, but it will save on lost heat during the run. There are two ways to do a melt quickly, but only one is efficient. You can use a huge input of fuel to overcome large thermal losses in a poorly insulated system, or you can use a much smaller quantity of fuel if you have a high efficiency well insulated system. I have always found it far better to build the most fuel efficient system possible right from the start. That will reduce melt times and overall cost per melt, and it will be much more satisfying to use.
- Also use ITC-100 on the interior surfaces of your furnace, but not on the crucible. ITC-100 will raise the operating temperature of your system significantly. You should also have your crucible lifted up off the floor of the chamber on some kind of refractory plinth, and if it allowed the hot gasses to circulate under the pot, that would be even better, but the circulation underneath is not a requirement. Use a bond breaker between the pot and the plinth so they dont sinter together during the heat, graphite powder works well.
- I recommend you use a 4 diameter exhaust hole in the top lid. A 3 may work OK, but the Rex burners are naturally aspirated atmospheric burners and depend on having almost zero back-pressure to operate well. This will tie in to what I will suggest next.
- Instead of mounting the burner up against, or in, the furnace wall, it would be a far better plan to to use a 2 - 3 (yes, feet) length of pipe, a manifold of sorts, that the burner tube (without nozzle) can slide into about an inch at one end. Cast a flare in the wall of the chamber when you cast your furnace lining so you can do away with a nozzle all together, and insert the other end of the pipe into it. It is OK if the flare in the refractory is larger than the nozzle flare that comes on the burner, which it may have to be in order to allow the pipe to fit in it. You will have a step where the burner tube enters the larger diameter pipe, and that is good. This pipe has two functions. It gets your burner away from the furnace, protecting it from the heat, and it will also provide a secondary emergency dump vent if you have a crucible break and dump a charge of aluminum into your furnace. Somewhere in the middle portion of the pipe you need to cut out a section that is perhaps 4 to 5 inches long and 2 wide. It will be on the bottom of the pipe when in use, and have a piece of paper, not cardboard, taped around the outside. If liquid metal ever pours down your manifold pipe, that paper instantly burns away and dumps the metal out the bottom of the pipe on to the ground. Obviously you dont ever want to use the furnace on top of a concrete slab or wet surface.
- I should add something about using a manifold. You need to be aware of two things that can happen. When you light the system you may get a burn back into the manifold which will make a strange noise. If you do, you need to try cranking up the pressure to push the burn out into the chamber, or shut it off and light it again using more gas pressure initially to prevent the flame from backing into the manifold. The other is, you will probably get a pop when shutting down. This is caused by the flame burning back into the manifold and exploding the gasses in there. It is harmless. Do remove your burner from the system right after you make your pour and are done with it.
- When you use a manifold system with naturally aspirated burners it is really important to pay close attention to back-pressure, because you are introducing back pressure right from the start. It will help if your manifold pipe is high quality and smooth and clean inside. Also, you want to cast your entry flare into the chamber at a sharp angle so the flame will be injected sideways into the chamber, and isnt impinging directly on your crucible. It is bad for the crucible to have an intensely hot spot on its side, and it is bad for burner performance to blast into a wall. You want to create a cyclone rotation in the gasses so they circulate around the crucible as they move upwards and out the top. That way they are exposed to the crucible for the maximum amount of time, allowing the maximum transfer of heat energy.
- Remember back pressure? Leave plenty of clearance between the crucible wall and the chamber wall for the free flow of the hot gasses around and upward.. 3/4 to 1 separation between crucible and the chamber wall should be about right. If it is too big you lose thermal efficiency and too small and you get back-pressure and burner problems.
- The 1 burner may do the job fine, and possibly a 3/4, but I am a person who doesnt enjoy waiting forever for a melt to conclude. Besides, the longer a melt takes, the more you pay for lost BTUs that radiate out the sides of the furnace. The bigger the burner the faster you will get the metal melted and the pour done. This is one time the girls are right, bigger is better. If I were building a medium to large system, I would use the 1-1/4 burner. A forty pound charge is a lot of metal, and it requires a huge quantity of heat energy to convert it to liquid. The more heat you can deliver, the better the system will perform for you, and the more you will enjoy working with it. Those girls got it right. :-)
- Make sure your propane tank is well away from your furnace, and use an industrial quality hose and hardware...no hardware store BBQ junk. You life could depend on it. It would be a very good plan to keep a length of 1-1/2 pipe handy to run the propane hose through to protect it from possible metal spills or other damage. An lot of guys totally space out when designing their propane supply system, and ignore the importance of keeping it well away from hot metal. Also, you want a ball valve on the end of the propane hose where your burner is for instant access if something goes wrong. It also makes lighting the system easier. You will need to use a large propane tank, or keep your smaller tanks in a large tub of water during the melt. Otherwise the propane will cool down and your pressure will drop off to the point that your system will slowly shut down on its own. A 500 to 1000 gallon propane tank would be a good size for running large home foundry operations. I have two 1000 gallon tanks to supply my needs, but my requirements are also dictated by my emergency power generator, which uses 5 gallons per hour under full load, and the fact I am heating my shop building and the house with the same propane too.
- Ron Reil
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