need advice for homade propane furnace

I've put in an embarrassing number of cash and hours trying to build my own furnace. At this point it's an obsession and if anyone here can help I'd be
so very appreciative. I'm quite sure I'll be leaving out critical details but having blown my whole weekend on a new design that was yet another failure I fear a full description might end up in a rant! Let me know any specific info you'd need.
I have a high pressure manual regulator rigged to a burner based on this design...
http://www.backyardmetalcasting.com/oliverburner1.html
I've used every combination of upwind/downwind and pipe size combo's you can imagine. And read many web sites, many. The part that feeds the furnace (end of the pipe) is 3/4" metal blackpipe angled offcenter, etc. The outlet hole in the lid is about 1.5" across. The issue is that as soon as I place the lid on, the flame either goes out or comes out the top creating a very dangerous gas buildup in the furnace so that when I remove the lid, POOF major dangerous flamage.
Can you not cover a propane furnace? If I rig the same burner config to a 3/4 to 1 flare it works great and in the open oven just fine.....but I can't get this thing anywhere near hot enough to melt silver and (LOL) have burned up almost a whole tank of propane with failed designs.
Any help is appreciated.
=B
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Ben wrote:

Bigger outlet hole in the lid.
Cheers Trevor Jones
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[snip]
Some pics may help, I just removed the burner, put the flare on a temporarily rigged it to a metal stand just to see how hot I could go. It couldn't get hot enough to melt sterling but was right there (orange glow it gets right before it gives it up). Can't get any hotter with the open flame which is why I built the furnace.
Here's the burner rig:
http://www.spiderinfo.com/BurnerRig01.jpg
And here's the furnace:
http://www.spiderinfo.com/Furnace01.jpg
Here's the open flame at max temp with a couple sterling buttons to guage , as you can see it's *almost* there.....
http://www.spiderinfo.com/MaxTemp.jpg
Trevor, I see you replied bigger hole, how bigger is bigger??
=B
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Ben wrote:

Bigger so it does not choke off your fire. You can try just setting the lid on crooked and adjusting the exit size a bit to see what works.
Cheers Trevor Jones
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That sounds like a good idea. YOu also might try forcing air, with a blower, down your air intake to see how much that helps, which I suspect it will. If it does, try something bigger than your 1" to 3/4 inch reducer as your venturi. Just removing the 1" pipe might help. a 1/16 "orifice" seems a little large to me too.
jk
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Hi Trevor, I don't think that's the issue. As you can see here I have a fairly low flame and if I even cover it half way the flame rises above the furnace.
http://www.spiderinfo.com/Flame01.jpg
http://www.spiderinfo.com/Flame02.jpg
This isn't the first time. What I was thinking is this was due to not enough air coming in from the burner so I modified it to a 1.5 inch reducer, still same thing. Also tried the upwind method with 6, 3/8" holes exactly as it states at backyard and same effect but with an added issue of unburned gas coming out the back topmost air intake hole.
Stuck man, any help? Even tried a blower on the back end but as expected, blew the flame out.
=B
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Ben wrote:

Do a bit of reading about the Mongo and mini-mongo burners. Rupert Wenig is a friend of mine. He melts cast iron on a naturally aspirated burner.
Your flame looks way to fuel rich. Balancing the amount of fuel to the amount of air, in a naturally aspirated burner, is critical. Better to have a small hole and drive the fuel through at a higher pressure, then choke off the air as required, IMO, but balanced is best. For a given size pipe burner, there is going to be a optimum size gas orifice, and that one is HUGE compared to the ones I have seen , made or used. Try working up from a number 60 drill bit, one drill bit at a time.
A naturally aspirated burner is going to be a bit more sensitive to back pressure from too small a exit as well.
A simple forced air set-up with a choke plate on the inlet is a easy way out. Simply add fuel, then adjust the air until there is just a hint of flame coming out of the top. More fire than that means too much unburnt fuel and a cold fire. Those flames look like a chafing dish burner. OK for heating the lunch line, but not really very hot in the scheme of things.
Cheers Trevor Jones
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Ben wrote:

You can regulate the blower air with a blast gate or possibly (depending on the motor) a router speed control.
If you have gas exiting the side air holes, move the orifice further into the burner until it stops.
--

Ron Thompson
On the Beautiful Florida Space Coast, right beside the Kennedy Space
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Ben wrote:

I have a propane furnace setup that is based on a set of plans that was offered in Home Shop Machinist a number of years ago. It's uses a forced air burner that's always worked very dependably and is a bit different from the design that you describe. If you'd like to see the burner design, or whole set of plans, send me your email address offline.
--
Gary Brady
Austin, TX
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Ben wrote:

I built a furnace one time long ago using just a five gallon metal bucket lined with firebrick and morter with two ( I think) half inch x 1 inch tees and a pipe mounted in two holes on each side on the bottom of the burner. I used an old vacuum cleaner to blow air in each of the one inch pipes coming out of the tee with the gas from a hose connected to the stove going into each of the half inch fittings of the tee. A stub from the other side of the 1 inch opening of the tee went into the burner bucket. The gas pressure out of the stove fitting,( I took the burner off the orifice right after the valve and shoved a hose on the stub. That furnace would melt aluminum and would almost melt brass. I bet if I opened the orifice up on the stove it would have melted it. I was only 14 at the time and didn't know any better.
John
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Your probably not getting enough air in at the burner. First thing I noticed looking at the design page vs your pictures is that you seem to be missing the air intake holes after the propane.
I would use a longer tube and drill holes/mill slots. Many of the designs out there use a sliding choke to partially cover the holes depending on the setup (pressure from the propane).
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wrote:

Thanks Marc, as I mentioned in orig post I have several configs. The one show is a downwind setup where the air is pulled from the back. I also have the exact upwind setup with 6, 3/8" holes as well as another larger downwind setup using a 1.5 inch to 1 inch intake. *All* fail when capped.
It appears my problem isn't air but fuel. I couldn't find a drill bit smaller than 1/16" so that's what size hole I have for fuel jet. Their saying too big unless I use forced air. Using forced air is a possibility but I'm going for a naturally aspirated config so am going to attempt to use a "vamp-clamp" to pierce a smaller hole in brass today. I know I'm shooting for about 3/4 millimeter vs what I have now which is about 1.5 millimeters.
Hey if you know of a common place to get a #57 or #60 drill bit please let me know.
Thanks for the reply.
=B
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Ben wrote:

Virtually and good hardware store or even Home Depot ,Lowes and Harbor Freight have sets with numbered bits. Or go online and order from any number of places.
http://www.mscdirect.com/ http://www.grainger.com/ http://www.mcmaster.com/ http://www.micromark.com/ (#60 is a BIG bit for them!) http://www.wttool.com/
--
Steve W.
Near Cooperstown, New York
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Home Depot (at least here) doesn't carry smaller than 1/16" but I stopped by Ace today and found #56 thu #60. Alas, still not enough heat.
Here's the rig at full throttle. Notice I have to step down from 1.25" to 3/4" using two reducers. I can't find anything that will go to 3/4" except 1" (eventually required for my furnace intake).
http://www.spiderinfo.com/NewConfig01.jpg
I have a #60 hole and the flame burns MUCH better. But not hot enough. When setup over the crucible I still can't get to melting temp. If I can't get it hot enough to melt 5 grams open and directly on the crucible then I know the furnace rig won't work. I did try it anyway and no where near hot enough.
What if anything could I do without welding or spending more money to get this sucker hotter?
= Ben
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Ben wrote:

Got any soft firebricks? Ceramic wool insulation? Use them to insulate and close up the space you have inside your furnace.
What is the refractory you used? If it was a heavyweight solid refractory, it's gonna take a while to get up to temp. You won't melt anything in it until the furnace is up to temperature. Lightweight castable has better characteristics than the heavy, and is not overly expensive at $35 or so a bag. Refractory in the phone book yellow pages.
When a jeweller melt precious metals he does a couple things. One is to limit the amount of other materials he has to get to heat. Another is to pound the heat into the metal and crucible at a rate that allows for rapid heating and melting. Usually this is with a rosebud tip on a Oxy-Acet torch. If you want to melt your silver with that flame (which looks plenty hot enough, as I have used a Bernzomatic propane torch to melt silver in the past) you will have to close up the amount of material you are to heat. Firebricks (lightweight ones, like pumice) are good, ceramic better, but pricey. A small slice of light firebrick under the crucible (keeps the heat from being drawn down to the cold floor) and a surrounding pile of stacked bricks with the burner stuck into the side of the pile, should do.
If nothing else, put a plinth of refractory under the crucible to allow the heat to flow around it, rather than aiming the burner at your crucible.
The flame looks OK to me. Flares on the burner end are a pretty OK thing for use on a forge, but should not be required on a furnace. In the furnace, with the lid on, is where the way it burns counts. Tune the flame in there.
Did you read up on the mini-mongo on the Riell forge page?
Cheers Trevor Jones
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Ben wrote:

Oh yeah. If you want to melt a small amount of silver, put it on a slab of charcoal, or just on a slab of wood, and pound the heat to it. You will get a mess of flame and smoke untill the charcoal forms, but the heat will go into the metal rather than into the crucible or refractory. Try it with a Bernzomatic. I used to melt silver coins this way.
Cheers Trevor Jones
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wrote in message Thanks Marc, as I mentioned

I have some inswool, will line with that and giver a shot.
Yes, I read much of the site. The designs for Reil, EZ , T-Rex and Mongo are all about what I have (latest mod tonight). The flame is clean but as soon as I hook it to the furnace, no good. So I'm wondering if it's the 90 degree turn throwing things off. The only diff is the 90 degree and lack of flare.
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Normally aspirated burners are really fussy about air flow, they do not take kindly to any back pressure of any kind. Your right angle turn using a short radius fitting adds huge back pressure. ditto for your lid with small hole.
Ben wrote:

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Thanks Roy that must be it. I will remove the 90 and shoot it straight in.
As a side note I GOT IT with the burner over the kiln!! Finally!!
Ron's suggestion to get the crucible off the refractory did the trick. Didn't realize it was sucking the heat out like that. Basically lined the kiln with inswool, pointed the burner right up the middle and boo-ya we got all sorts of melted silver. Was able to easily reduce a full pound of sterling to casting grain in no time at all.
Although not exactly like the others I've seen, my burner now works wonderfully with a #60 dead drilled dead center through a cap fitted to 1/8" pipe I rigged to run the middle and end at the sweet spot 1.25 reducer. So stoked!!!
Thanks to the many replies here and the web (Reil, EZ and T-Rex sites). You guys are awesome thanks again.
=B
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Ben wrote: ...

...
An alternative to drilling a small hole is using a mig-welding contact tip as a nozzle. Common mig wire diameters include .030, .035, .045 etc, [vs. #57 0.043" or #60 0.040"] with the hole diameter slightly larger than the wire diameter, of course. I think the tips I used have a 5x1 mm threaded shank. If you don't have metric taps, that's close enough to 3/16"-24 to turn in a few turns. Any welding store will have a variety of these contact tips at a variety of prices. See pictures at eg http://www.tptools.com/dg/85_Welding-Parts-and-Supplies.html
--
-jiw

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