# Propane furnace optimization



## Noxx (Feb 20, 2012)

Hey,

I did use my propane furnace yesterday for the first time since it was build two years ago 8) 

I melted 900 grams of karat gold but it took 30 mins. I ran it at 15 psi.

Few questions:

-How long does it normally takes to melt 1kg of karat gold for a commercial furnace ?
-How big should the hole in the lid be compared to the ID of the furnace ?
-Does the burner flame orientation has a big impact on heating efficiency ?
-How can I improve overall efficiency ?

I know that my furnace is well insulated since after a 30 min run, outside walls were only at 60*C. Or maybe I can try to crank the PSI a bit more ?

Thanks a lot!


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## Geo (Feb 20, 2012)

you may add oxygen to the mix for the first part of the melt, but be careful as I've heard of this melting the refractory vestment.


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## rusty (Feb 20, 2012)

Noxx said:


> Hey,
> 
> I did use my propane furnace yesterday for the first time since it was build two years ago 8)
> 
> ...



Sounds about right to me, first firing always takes longer, each succeeding melt will go much faster now that your furnace refractory is up to temperature.

Next time your firing, have a seconded melt ready, you will be pleasantly surprised how quick it melts.


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## Smack (Feb 20, 2012)

Your fine Noxx, hes right it takes quite some time to heat up that much metal to a couple thousand degrees.


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## elfixx (Feb 20, 2012)

In fact it is quite long to melt anything in a propane furnace if it ain't already hot, wait till you try large amout of fine gold powder, it can take up to 45min.
You'll enjoy a induction furnace for that matter when you get one, melt almost anything in under 10min from cold crucible.


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## Noxx (Feb 21, 2012)

I know, I've been wanting an induction furnace way before I built this.

I'm not ready to spend $15k-$25k on a quality one.


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## elfixx (Feb 21, 2012)

You can get one just like mine from china for 3000$


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## Noxx (Feb 21, 2012)

Yes, and it broke the next week...


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## rusty (Feb 21, 2012)

Noxx said:


> Yes, and it broke the next week...



Yea I'm having similar problems with my plasma cutter, when you try to get parts the Chinese go into dumb mode.


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## elfixx (Feb 21, 2012)

That's for shure, quality goes with the price. But still it work pretty right and they replaced the part free of charge.


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## MysticColby (Feb 21, 2012)

I haven't melted gold before, but I have melted pure silver (slightly lower melting point), and I have insights:
- 900 g in 30 min is about right. furnaces take longer than melting dishes simply because there is a larger area to heat (inside the furnace + outside of crucible + metal). But, they do hold heat longer. 2nd, 3rd, 4th melts I'd expect to be ready in 10 min each.
- psi doesn't mean much without knowing the propane orifice diameter (hole the propane comes out of). Personally, I use a 1/2" Shorty Burner 
http://www.hybridburners.com/products.html 
which has a 14-30 tip (0.030" diameter hole). For preheating, I set it to 5 psi. For melting; 10-15 psi. smaller tip would need more psi for same BTU (I would guess that smaller tip would also produce faster moving air, maybe less efficient).
- When making my furnaces, I like to make the burner inlet just barely bigger than the burner nozzle (flame seems to come out that end if there's too much room, which wastes heat). For the exhaust, I make the hole about 1-1/2 times as big as the inlet. (if inlet is 1", outlet would be about 1-1/2"). smaller than inlet might suffocate the flame from high pressure inside resisting more air/gas coming in, too large would be inefficient.
- it is highly recommended to position the burner so it enters the furnace perpendicular to the crucible, glances the side of the crucible, and swirls around it as it goes up toward the outlet. I designed my first furnace so the flame was directly on the crucible. It worked, but really only one side of the crucible was ever red hot. It did seem to take longer to melt than how I do it now. The reason to swirl the flame around the crucible is a couple things: even heating (prevents crucible cracking). cold spot in center of flame. IR heat is heats quicker than heat transfer from air to solid (the flame heats the walls of the furnace then the walls radiate heat onto the crucible is quicker than hot air from flame heating crucible, as there is more surface area on the furnace walls than the crucible - this effect is why furnaces take longer the first melt).
- efficiency can be improved with a couple things: more insulating furnace (though, since yours is still cool on outside, it's already well insulated). quicker melts (less time to lose heat. but, again, wouldn't really help your furnace). ITC-100 (it's a refractory paint that you put on inside walls of furnace - reflects the IR heat onto the crucible better than other materials. Supposed to decrease heating time by like 30%. I use it, but haven't not used it with this design, so don't have anything to compare it to).

I'll post pictures of my furnaces when I get home. I'm proud of them ^_^


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## Noxx (Feb 21, 2012)

MysticColby said:


> I haven't melted gold before, but I have melted pure silver (slightly lower melting point), and I have insights:
> - 900 g in 30 min is about right. furnaces take longer than melting dishes simply because there is a larger area to heat (inside the furnace + outside of crucible + metal). But, they do hold heat longer. 2nd, 3rd, 4th melts I'd expect to be ready in 10 min each.
> - psi doesn't mean much without knowing the propane orifice diameter (hole the propane comes out of). Personally, I use a 1/2" Shorty Burner
> http://www.hybridburners.com/products.html
> ...



Thanks a lot!

I've hear of ITC-100. It's costly but I think that it works...

My furnace is around 7'' ID and the exhaust hole is 3-5 times bigger than the burner... That might be the problem. Too much heat escaping...


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## Barren Realms 007 (Feb 21, 2012)




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## Barren Realms 007 (Feb 21, 2012)




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## MysticColby (Feb 22, 2012)

ooo, you got a pretty furnace too! just a little bigger than mine.
I can't really figure out how the inlet gets from the torch to the chamber. Is there an inner tube that goes half way around the base before coming out inside?
Is there a reason it's in 3 pieces instead of 2? just ease of casting?
yeah, I'd say that lid has too big an outlet.
Another thing comes to mind: I've read that the furnace should be about 1.2 times the size (both diameter and height) of the crucible. It's just most efficient that way - smaller size crucibles will still work, but require more than a proportional difference in heat. Basically, if that size crucible takes 30 minutes to heat, a crucible 4 times the capacity might take maybe 35 minutes.
You can try covering part of the outlet with bricks to see how much difference it makes.

Here's my furnace  made of K-23 fire bricks, held together by friction and those metal bands. Took a while to cut and shape them, but I like the results! the whole thing (base and lid) weighs about 10 pounds. I'm planning a bigger one for my aluminum castings.

Discolorations inside are from melting silver chloride with insufficient sodium carbonate. It also boiled over and got borax on the inside  Doesn't affect anything, just unsightly. The slice of brick in the bottom has saved me sooooo many times. at these temperatures, graphite will bond to refractory - kinda like solder. It glues the crucible to the floor of the furnace. instead, I have it glue to that slice of brick which picks up like it wasn't even there. After there isn't molten metal inside the crucible, I can knock away the little brick.
Yes, that brick is cracked. has been since the first firing. Hasn't gotten any bigger, though, so I'm not worrying. I wish there was a super glue equivalent for refractory bricks


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## MysticColby (Feb 22, 2012)

And this is my mini forge  good for "small" melts below about 1,000 g silver. Takes longer to melt than it's big brother, but probably uses far less propane (it runs off a burnz-o-matic), and the outside is scalding hot by the time it's molten. But it never gets too hot that it can't be picked up with gloves.
This is the second one I've made, just this last weekend. The lid of the first one formed a big crack - maybe this will fair better (it's K-26 instead of K-23). I also thought I'd try making one out of dense fire brick. I'll see how that one works as well


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## rusty (Feb 22, 2012)

MysticColby said:


> And this is my mini forge  good for "small" melts below about 1,000 g silver. Takes longer to melt than it's big brother, but probably uses far less propane (it runs off a burnz-o-matic), and the outside is scalding hot by the time it's molten. But it never gets too hot that it can't be picked up with gloves.
> This is the second one I've made, just this last weekend. The lid of the first one formed a big crack - maybe this will fair better (it's K-26 instead of K-23). I also thought I'd try making one out of dense fire brick. I'll see how that one works as well




Job well done..


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## Noxx (Feb 22, 2012)

Thanks a lot for the pictures !

Here's a video of the melt & pour in question:

[youtube]http://www.youtube.com/watch?v=JXxo8g939ss[/youtube]


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## Geo (Feb 22, 2012)

hey Noxx, the link doesnt work for me.

ill type it out as it appears to me with spaces so the link will be broken after the (.)

[youtube]http://youtu. be/zJEZNk4emgk[/youtube]


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## Noxx (Feb 23, 2012)

Fixed.


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## Smack (Feb 23, 2012)

I have an extra steel table I would give you if you were not so far away.


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## MysticColby (Feb 23, 2012)

neat pour. REALLY hot. based on the glow, I'd estimate about 1200 C (2200 F) (orange yellow glow, visible in sunlight).
I probably pour colder than that (about 1000 C - dim orange glow), but that's probably why my bars don't look as pretty. I should work on that....


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## MysticColby (Feb 23, 2012)

Where did you get those lift out tongs? I've been looking for some (for a larger #10 crucible), but they all seem to be about $300. I nearly decided to make my own.


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## Noxx (Feb 24, 2012)

They are homemade 

Fairly easy to make if you know how to weld. I had to reinforce from the original design because they flex too much when lifting a heavy crucible.


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## MysticColby (Feb 24, 2012)

I DO know how to weld (just bought a MIG a couple weeks ago)
I'll make my own, then. I was planning on something like this:





and this:




From your video, I see it's possible to just have the first one, but I feel I'd get better control with the second.


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## Noxx (Feb 24, 2012)

I'll post pictures of my design later today or tomorrow.


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## goldsilverpro (Feb 24, 2012)

I know this is heresy, but I always used just charging tongs (designed to put metal bars into the crucible) to handle the crucible in and out of the furnace and to pour. The ones I had were either 36" or 42". I was told they were safe on #16 crucibles or less and that's what I used them on. I never had a mishap with them in 30 years.


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## Westerngs (Feb 24, 2012)

I use about the same as your charging tongs except that mine are curved to grip the crucible better. Haven't had a problem in over 20 years. They work well for stuff up to 150 t.o.

Anything over that I use a larger furnace with a yoke for the crucible.

Noxx... Does your flame come in at an angle to the crucible? I could not tell from the video. Furnaces seem to heat better if the flame comes in at an angle say about 45 degrees and swirls around the crucible. It seems to heat more evenly.


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## MysticColby (Feb 27, 2012)

So far, I've only gripped the edge of crucibles with tongs. But so far, I have only used small ~1 kg crucibles or large cast iron. I felt nervous about lifting a 10 pound crucible with 10 pounds of molten aluminum in it by it'd edge, which is why I wanted those vertical lift-out tongs for when I replace the cast iron crucible with a graphite one. Graphite is easier to break than cast iron...

My tongs are half-done so far  bent them into a basic shape, twisted, and made them scissor-like. just need to bend the part that actually touches the crucible, then weld them on. I'll post pictures when it's all done and not looking like an amateur made them (they're the first steel-working things I've made)


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## ericrm (Mar 11, 2012)

Noxx, do you know the propane consumption of your furnace?(combien de gaz sa boit a l'heure)


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## greatgems (Mar 23, 2012)

would this design with acetoleen work better and get hotter i need a good 1600c to melt copper fast


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## Harold_V (Mar 24, 2012)

greatgems said:


> would this design with acetoleen work better and get hotter i need a good 1600c to melt copper fast


You mean acetylene, I assume. 
If you make that your choice, you'll spend more money on fuel than the copper is worth. 

What you need is most likely not greater heat, but more btu's. It's not the same thing. Copper can be melted perfectly well with propane or natural gas. I did it time and again with natural gas, and it's done that way routinely by foundries that are still melting with gas fired furnaces. 

Copper is not friendly to melt and pour. I expect you're going to discover that fact. 

Harold


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## Barren Realms 007 (Mar 24, 2012)

greatgems said:


> would this design with acetoleen work better and get hotter i need a good 1600c to melt copper fast



The furnace I posted picture of in this tread should do what you want to do. The crucible pictured will melt about 700g of silver powder in roughy 8-10 minutes once the furnace is up to full temp. I generally melt one charge remove and add a 2nd charge and then melt that and then pour in the mold. When I am melting silver my regulator is only turned on about 60-75% so you could get more BTU's out of it if nescessary, if you need more BTU's you can look at safely injecting oxygen to bring the temps up. The furnace is desighned to hold a #6 crucible that will hold about 12 lbs of copper.


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## MysticColby (Mar 25, 2012)

I had a request for plans for that mini forge pictured earlier, figured I'd share with everyone:

Total cost of everything consumed is about $4 for the fire brick. Initial cost includes about $30 for tools.
Things to get:
one insulating fire brick (9" x 4.5" x 2.5" or 9" x 4.5" x 3")
3 inch carbide hole saw (normal hole saw will work, but won't last as long. looks something like: http://ecx.images-amazon.com/images/I/21Nk%2BQugqxL._SL500_AA300_.jpg)
1 to 2 inch spade bit (looks something like: http://benrroberts.files.wordpress.com/2010/10/spade-bit.jpg)
5/8 inch spade bit
carbide hack saw (normal hack saw will work, but won't last as long)
Doing this on a drill press is best and easiest but a power drill should work alright

1) take out the center drill in the hole saw. Most can be removed by unscrewing, but may require to be cut off.
2) cut off pointy part of the 1 to 2 inch spade bit. Use a hack saw or dremel. Afterward, it should look close to have a square end.
3) cut fire brick exactly in half. use a tape measure and sharpie to mark a line, then use carbide hack saw to cut it. Doesn't have to be perfect, but as close as possible gives most insulation.
4) drill circle hole in both halves of the brick. put hole saw in drill press, set it to stop when it is 3/4 inch from the bench. Align hole saw in center of one of the faces of one of the brick halves, drill down so that it forms the outline of a hole that leaves 3/4 inch intact on bottom. Do the same to the 2nd brick half. (if bricks are 2.5 inches, the hole will be 1.75 inch deep and final chamber will be 3.5 inch tall. if bricks are 3 inches, the hole can be shallower to allow for thicker floor which increases insulation)
5) remove inner part of hole. put 1 to 2 inch spade bit in drill press, set it to stop when it is 3/4 inch from the bench. Drill out inside part of the holes you just drilled. works best if you drill out very center, move it half an inch, drill out that part, and so on. Careful not to drill outside of the hole. Do this for both halves.
6) drill hole for torch and exhaust. put 5/8 inch spade bit in drill press. align to center of one of the brick halves, drill a hole. The hole should be on the side for the second half, off center so the flame swirls around the crucible. It doesn't require perfect placement, but if that's what you want: 1-1/16 inch from bottom and 1-1/16 inch from side.
7) Place both halves together so 3 inch holes are facing each other. final dimensions should be 3 inch diameter cavity, 3.5 to 4.5 inch tall, 3/4 inch thick walls at thinnest. Best to place on top of another insulating fire brick (when in use) as most of the heat will be lost to the table (also to avoid catching table on fire).
8) The end.

I recommend editing the sizes of the drills to match what you have (my torch has a 5/8 inch diameter, so that's the size hole I wanted). If you have a smaller crucible, make it a smaller hole.
Drilling into brick results in a bit bigger hole than the size it should be.
It just occurred to me that you might be able to get a 3 inch spade bit and just use that (instead of the 3 inch hole saw).
I read somewhere that the size of the furnace should be 1.2 times the size of your crucible for maximum efficiency. (2.5 inch diameter, 2.75 inch tall crucible = 3 inch diameter, 3.3 inch tall furnace)
The neat thing about this furnace is that when it's opened, you can directly grab the crucible by the side with appropriate tongs. This means you can potentially melt more in the crucible as you won't have to reach tongs inside to grasp the crucible wall. Just be aware: larger volume = takes longer to melt = less efficient due to heat lost through walls.
Do not touch the outside with your bare hands while it's on (you can touch it, at least for a short time, if you're wearing welding gloves)
The largest crucible that will fit in this furnace is a size B0000 (3-1/16 tall, 2-5/8 diameter, 811 g silver max) or A1 (2-1/2 tall, 2-1/2 diameter, 1,234 g silver max) (http://www.budgetcastingsupply.com/Crucibles.php)


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