Homemade furnace

[ramses]

You should build an injection style waste oil burner. See here.

[velocity3x]

Would this work?
http://www.harborfreight.com/propane-torch-91033.html

[Technician1002]

That torch is designed to use excess air that is heated by the flame. Those can be identified by the short burner that has a burning gas jet in the center and extra air added outside the jet. If you control the ratio of the air and gas mix, and fit the head in your furnace, you may be able to exclude enough excess air to function properly. You will want the tip of the torch to connect to the outer shell of the furnace, but not extend into the inside cavity where the radiant heat will melt it.

A hotter fame can be generated with a smaller diameter torch head with a blower to increase the velocity of the gas air jet. This increases the mix rate resulting in a shorter hotter flame. As mentioned earlier, a fixed turbine blade set to swirl the air will increase the rate of combustion, reduce flame length and raise the temperature where you want it, at the bottom.
Study the photo below. It is a typical oil furnace burner.

Note the distance between the blower and the tip. This provides even flow to the tip. The fuel nozzle is in the center recessed. This permits ignition of the fuel jet before it is highly mixed with air. The next part is important. The air stream goes through a fixed fan blade to swirl the air and rapidly mix the air with the burning fuel. the hot part of the fire is off the tip. For proper combustion and not wasting power heating extra air, the blower has a damper to control the amount of air. This is adjustable so it can be adjusted for different capacity nozzles.

Too little air results in a flame with an undefined tip and some smoke. Air is increased until the flame has well defined tips. Excess air results in a smaller flame that looks about the same. A furnace repairman would close the damper until the flame tips are not distinct and then open it to where the tips just become distinct. This is the transition between an incomplete burn and complete burn of the fuel. You can adjust your furnace the same way. Remember, the furnace should not have any other place for air to enter except the burner.

[Alster370]

Had a go with the 10 minute ghetto furnace today, bad winds though 30mph+. Its a terracotta plantpot, maybe about 1L and its being fired with charcoal.(not sure what type but not lumpwood). The temperature was easliy enough, but the amount of heat was not, so nxt time il get a much larger pot . This isn't the actual furnace either, just a quick test so I can get use to what im dealing with and such.

[Technician1002]

The air feed looks too small. Your heat output is controlled by the amount of oxygen supplied.

[Fnord]

Yeah, You should try to make the air inlet big enough to take a good portion of the flow from the hair drier if possible. You won't need more than maybe a 1" port for this thing though. If you want an idea of what you'll need when scaling up, I had a piece of 2" thinwall conduit feeding a foundry made from a cut-up 20lb propane tank and it worked just fine. I did a couple casts with 10 pounds of aluminum all at once.

[Technician1002]

Upon close examination of the photo, it looks like you are attempting to blow the coals near the center on the bottom. I wondered why you had a hot spot in the middle.

Due to radiant heat, for melting metal, the air generally is blowing in at the perimiter and along the perimiter so the crucible is heated from the outside in instead of just on the bottom. To reduce radiated heat, a lid is used with a smallish vent hole in the center. This way the light very hot gas swirls along the perimiter and only the cooler denser spent gas makes it's way to the center and out the vent. This swirl reduces the amount of hot gas that escapes out the top from heat rising and lets it give up more heat to the load before being allowed to escape. An open furnace sheds huge amounts of heat from radiant heat and rising hot gas allowing cold gas to enter, mix and dilute the furnace. Use a lid.

This photo shows a typical small foundry furnace with the lid moved aside.

[Mr.Tallahassee]

I used regular bricks for a furnace to melt aluminum. I have a 500000BTU Propane torch that works wonders for melting aluminum and tempering steel. It costs $100 new. It is a GOSS AP14. If I had refractory cement I might have been able to melt steel as well.

[Alster370]

I know it wasn't brilliant, but hey it was just a quick test. Ive got a nice empty 5L pot also in the garden that il probably use as another prototype, But just as an outer shell for the furnace. Mainly because its not terracotta, (just clay) and its cracked already. Il make a large 1" bore hole in the side for the air feed and fill it with firecrete to make a 1" wall. That should leave me with about 6-7" diameter for the coals and crucible. I will obtain a small bag of that lumpwood charcoal too, and il see about borrowing a friends leaf blower for the air supply. Il also see about a lid for it too, which will help in terms of heat, just like having more air and coals will.

[Heimo]

and il see about borrowing a friends leaf blower for the air supply. Il also see about a lid for it too, which will help in terms of heat, just like having more air and coals will.

just remember too much air can actually blow heat out of the furnace and cool it down too, so rig some kind of air dump if you use the leaf blower so you can control the amount of air going into the furnace...

[Crna Legija]

I use a vacuum cleaner with a blow function because you can adjust the power for heating/melting.