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Homemade furnace

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Homemade furnace

Unread postAuthor: Alster370 » Sun Sep 11, 2011 3:44 am

Ive been having trouble with my home-made furnace project, as I know that my blowtorch cant supply enough heat from its flame to power the furnace. So I came up with an idea to supply compressed air to the gas mixture before it enters the furnace, to increase the heat from the flame. The only problem I have is where the airline should be placed. Would someone help me out?

Heres a simple diagram below of a top down view:

Image

thanks.
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Unread postAuthor: inonickname » Sun Sep 11, 2011 4:13 am

The blowtorch is already making an optimal mix, more air is just going to extinguish it and cause waste. You need more power. i.e. a coal forge or larger gas bottle
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Unread postAuthor: Alster370 » Sun Sep 11, 2011 4:19 am

ahh i see. Could I modify it in any way, or make a new burner head for the torch to increase the heat?
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Unread postAuthor: Heimo » Sun Sep 11, 2011 4:30 am

your blowtorch cannot create enough flow to heat a furnace, and if you removed propane from its tank at a rate that would work for a furnace, the tank would ice up in minutes causing the pressure to drop to a point that you won't be able to run your furnace, you need at least a 9Kg tank to run a furnace from...
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Unread postAuthor: Alster370 » Sun Sep 11, 2011 4:35 am

oh no this isn't a particularly large furnace, its quite small. about 10" OD 6" ID with a crucible that can hold about 500ml of aluminium. :D Ive just been looking at this that's all, and wondering if my blowtorch could supply enough gas flow for a burner about 2/3 of its size.

http://www.backyardmetalcasting.com/oliverburner1.html
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Unread postAuthor: inonickname » Sun Sep 11, 2011 5:05 am

You need even more gasflow for melting metals as opposed to heating/forging. And no, you definitely need a larger gas supply.
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Unread postAuthor: Heimo » Sun Sep 11, 2011 6:31 am

also I would like to add that if you added compressed air to your naturally aspirated burner (blowtorch) you would created a highly oxidizing environment in your furnace that eats through your crucible in no time at all, and you create a hell of a lot of extra dross. foundry furnaces really need their own dedicated burner with their own dedicated fuel supply. If your blowtorch is your only option, try running your furnace of charcoal instead, you will need a blower to supply air for combustion but it is a easy way to start.(use lump charcoal not briquettes)

my second furnace was run mostly on charcoal it was a 7" ID furnace
Image

my current oil fired furnace is 10" bore and I sometimes run it off charcoal as well, as you can see in this picture (one of my favorite pictures :D )

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Unread postAuthor: Crna Legija » Sun Sep 11, 2011 6:51 am

At work we have a pretty nice furnace, only 4.5 metric tonnes of melting capacity per hour aluminium and zinc.
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Pouring aluminium into the crucible, 250l at a time lol.
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Unread postAuthor: Alster370 » Sun Sep 11, 2011 7:13 am

I might go with charcoal, easier to work with in my opinion and will still do the job required. Il be using mine mostly to make cheap round bar stock for my lathe which im getting in a week (seig c0). All the tutorials for small charcoal furnaces say that hair dryer motors can provide enough air?

ps, nice furnace btw :D
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Unread postAuthor: Fnord » Sun Sep 11, 2011 9:46 am

Hair driers work but you'll end up melting it somehow. It's kind of inevitable.

Try to find a squirrel cage blower if you can. The one I used has taken far more abuse than I care to mention and still works just fine.
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This is not mine. Mine isn't very recognizable due to the rust/dents/burn marks from being set on fire a dozen times.
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Unread postAuthor: D_Hall » Sun Sep 11, 2011 9:53 am

Heimo wrote:your blowtorch cannot create enough flow to heat a furnace, and if you removed propane from its tank at a rate that would work for a furnace, the tank would ice up in minutes causing the pressure to drop to a point that you won't be able to run your furnace, you need at least a 9Kg tank to run a furnace from...


I've seen this happen. I've also seen a successful work around.... A bucket of water. As in...

* Put an extension hose on the torch head so that you've got (say) a 3' hose between the tank and the torch.

* Fill a 5 gallon bucket with water.

* Place tank in bucket and put something (anything) on top of the tank to keep it submerged.

Basically, the water provides a heat source and efficient heat transmission to the tank and it's contents to prevent icing.



edit: OK, I wasn't being 100% honest. I haven't seen the above scenario. What I *HAVE* seen is a 200 gallon propane tank freezing up due to pulling out too much propane too quickly. THAT problem was solved by aiming a firehose at it. It worked beautifully. If THAT worked, the bucket should work too.
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Unread postAuthor: Fnord » Sun Sep 11, 2011 10:04 am

It's a common practice among the pulsejet community to drop a 20lb propane tank in a styrofoam box full of warm water in order to maintain enough gas flow.


Though if you're going for propane you can probably liquid fuel it with diesel cheaper... or maybe free if you have some fast food places nearby that'll give you cooking oil.
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Unread postAuthor: Heimo » Sun Sep 11, 2011 10:32 am

Fnord wrote:Though if you're going for propane you can probably liquid fuel it with diesel cheaper... or maybe free if you have some fast food places nearby that'll give you cooking oil.


waste oil is really the way to go with home foundries, but the only drawback is you need to make your furnace from at least 3000F refractory or you will melt your furnace eventually... also the burners are a bit more complicated, the simple designs almost always requires you preheat the furnace with propane or charcoal, then you get the siphon nozzle burners that work with compressed air and then there is the burners that work like the domestic heaters with a nozzle and a oil pump along with a blower for air...
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Unread postAuthor: Technician1002 » Sun Sep 11, 2011 11:04 am

A solution to the fuel delivery rate problem is to use a proper vaporizer. For this you need a forklift propane tank that delivers liquid, a heat exchanger, and an expansion chamber to prevent liquid spray from going downstream.

The components are in order

A forklift propane tank
A flow control valve. This is needed to drop the pressure from the tank and generate flash gas.
The vaporizer can be either water heated or fire heated such as used on hot air balloons.
The expansion chamber is simply a smallish tank or large pipe to catch any liquid mist and finish vaporizing it before it exits as gas.

The rest of the burner needs to be your nozzle to direct the gas into the furnace. To prevent overheating in this area, it is common to use a gas jet instead of a gas spray. This carries the over rich gas stream inside the air stream. Shortly before the air fuel mix enters the furnace, a twisting vane can be used to spin the flow causing fuel air mixing right as it enters the furnace. This causes the maximum heat to occur at the bottom of the crucible on the first rotation around the furnace. The hot gas then spirals up spreading the heat to the crucible. This design is common in small commercial furnaces. The air feed will need either a variable speed blower or dampers on it to control the amount of air to match the combustion needs of the gas. Too much air results in an oxidizing flame and with the extra bulk of air being heated, results in a cooler fire. Air is restricted to reach the highest temperature possible. To little air results in incomplete combustion in the furnace and a larger fire from the top of the furnace.

Some furnaces have two or more burners. The one I learned on had two burners. They both ran off a common blower. It used natural gas and worked fine for aluminum and cast iron. It has a 4L crucible so it was decent size.

It is amazing the difference in radiated heat between pouring Aluminum or Iron. Iron will toast your toes from 8 feet away. It is impossible to hand pour without a firesuit.
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Unread postAuthor: Alster370 » Sun Sep 11, 2011 11:21 am

Well thanks for the advice so far, im going to try a simple charcoal setup in a terracotta pot before getting the materials for the real thing. I've read that it can withstand high temperatures (600C+) because it is a type of ceramic?
Ive also been busy making a new burner head for my torch, at first it didn't work too well because of the lack of gas flow, but then I adjusted the reducing nut to allow more gas flow. Now it shoots a bright blue flame at least 3 times bigger, so I might try that tomorrow aswell. On another note my zippo arrives tommorrow, so no more lighting torches and such off the gas hob! :roll:
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