Fuel consumption

[Donnie]

How long will a 20lb. propane bottle run a small forge. By small, I mean something that would be no larger than to heat 3/8" stock or smaller. I've only used coal/coke. Any input would be greatly appreciated.

[ThomasPowers]

How small a small forge?  Just to heat the end of a 3/8" bar is a very different forge than a forge to heat 3/8" steel bent into a helix.

 

Anyway my small forge can run at non-welding heat for about 12 hours.  It HUGELY depends on how good a burner you have.  I have a burner from Stephen Gensheimer that radically reduced gas consumption in my teaching forge compared to my previous burner.

[jmccustomknives]

I'll run 20lb in my Diamondback Bladesmiths forge.  It will last 2 hours or longer depending on what I'm doing.

[jcornell]

3/4" burner, coffee can forge - 8-10 hours at non-welding heat.  Probably 5-6 hours at welding heat.

[timgunn1962]

I've just weighed my cylinder before and after a day of playing with a new burner in my mini-forge.

 

The forge chamber is just over 2 1/2"  diameter and about 12" long, so around coffee-can forge sized. I weighed the cylinder at 80lb before and 74 lb after, for 6 lb Propane used (possibly 7, as I was using a cheap luggage scale) over 9 hours of continuous running at 10 PSI (measured on a cheap gauge).

 

I can get temperatures from 830 degC/ 1526 degF to 1245 degC/2273 degF at 10 PSI just by adjusting the choke. The burner uses a 1/2" industrial Venturi Gas mixer made by Amal.

 

I was trying to get the full range of useful temperatures from Heat-Treat soak temperature at just under 800 degC/1472 degF to welding at around 1300 degC/2372 degF, or maybe a little higher, without  having to adjust the regulator. I couldn't do it.

 

Welding temperature needs more pressure; 60 PSI will get over-range on my type K thermocouple, which tops out at 1370 degC/ 2498 degF, though I've not tried welding in this forge because it is lined with 2" of Ceramic Fiber and flux will kill it.

 

A 20 lb cylinder could be expected to last over 24 hours forging on a similar setup.

 

Gas consumption varies with the square root of the pressure, so 6 times the pressure would use almost 2 1/2 times as much gas, which means 20 lb should last 10 hours plus at welding temperature.

 

 

I've used a similar burner in a scaled-down version of a Don Fogg drum HT forge, made from an 18" length of 10" pipe and that will hold HT soak temperatures and go to forging temperatures by adjusting both the choke and the pressure, but it will not reach welding temperatures, even at 60 PSI.

 

[Donnie]

Thanks for the input. I have considered a small gas forge, but have never used one. I wondered about the cost to operate one. A very small one would do all I am able to do anymore. I don't even forge anything larger than 3/8" stock. I still enjoy forging small knives, hooks, and other small items. I've only used coal and coke.

[Hefty]

I just searched for this topic and I'm resurrecting/hijacking this thread because I'm really not sure if my forge is as efficient as it should be or not.

I built my forge about two years ago out of an old 9kg (20lb) gas bottle (propane tank). 2 inch ceramic wool walls with castable refractory inner surface and heated by 2 x 3/4" T burners with (from memory) 0.23" mig tips.

I'm really happy with it in terms of heat times and I've been able to do 1 or 2 small forge welds with it. I haven't tried many in there yet. Basically I'm really happy with it while I'm using it, but I kept track over the course of one tank of gas recently and, at 12 PSI (kept it at that pressure for general forging to control one variable) I got 6 hours of burner time.

Does this sound appropriate? or too short an amount of time? I KNOW: not all forges are created equal, accuracy of burner construction, alignment of orifices, alignment of mixing tubes in refractory shell, elevation about sea level and many other variables can have an effect on this rate of consumption; BUT I'm just after a kind-of ball park "Yes, that's about right." or "No, that's way off. you should look for issues with your construction that might be causing poor efficiency."

One anomaly I've noticed is that the front burner appears a little richer, and the back one a little hotter. The photos below show the forge after about 5ish minutes. You can see the front flame but not the rear one, but the area around the rear one is clearly hotter. I wasn't sure if this is purely because the front opening is larger (although I did have a cast refractory door in front of it until I took the photo) or that the burner itself is just not tuned as well as the back one.

Also there is almost no visible dragon's breath at this stage (basically invisible in this photo but you can just see it IRL), but I get more and it is more visibly yellow/orange after about 15 minutes (full heat, I guess).

So, is 9kg or 20lbs of gas in 6 hours way off in efficiency, or in an appropriate range?

Any advice/opinions are greatly appreciated.

Cheers,

Jono.

 

[Buzzkill]

I don't run 2 burners, so I don't have a direct comparison.  However, when using a single burner with a 3/4" mixing tube I get 10+ hours from a BBQ size propane tank for general forging.  My forge is a little smaller than yours though.  When forge welding that time drops significantly.  Generally speaking, a .023 mig tip would be appropriate for a 1/2" mixing tube.  For a 3/4" mixing tube I'd be  running .030 or .035, depending on a few different factors.

All that to say you're probably in the ballpark for a 2 burner setup in my opinion.  If someone else who is running about the same size forge with 2 burners chimes in you should definitely give their opinion more weight.

[Frosty]

Morning (here) Jono. You must be using a 0,035" mig tip or you'd have to be running much higher psi and choking the air ports to get those decent looking flames. two, 3/4" burners running @ 12psi sounds like about 6hrs. worth on a 20lb tank. Give or take a little of course.

I get a little more but I only run mine at about 15psi when I'm welding, the rest of the time I run around 8-10psi, depending on the project.

I find that shooting for a happy medium like your 12psi. isn't really happy for either general forging or welding. Turning the regulator up or down only takes seconds and the forge will catch up in a minute or two. Changing pressure is the ONLY reason to have a gage on a hobby forge, it saves time. For general stuff I throttle the flame by ear. It only takes a little practice to get good at hearing what temp you've set the pressure to. 

Next time you change between welding and forging try to do it by the burner's roar, THEN look at the gage. It won't take long and you'll only glance at it to verify for tricky temps.

Frosty The Lucky.

[Mikey98118]

To add to what Buzzkill and Frosty told you:

(1) You have twice the amount of 3/4" burners needed to turn your forge yellow hot; not orange like you show.

(2) The photo seems to indicate heavily oxidizing flames, which is what I would expect from such a small gas orifice in a 3/4" "T" burner. Follow Frosty's advice, so that your burners will produce nuetral flames; then you will be anle to follow Buzzkill's advice; resulting in fuel reduction, along with a whole lot less scale on your parts

[Hefty]

On 3/23/2024 at 12:52 AM, Buzzkill said:

For a 3/4" mixing tube I'd be  running .030 or .035, depending on a few different factors.

Buzzkill, you're right, I just went and checked and I was wrong: I'm running 0.9mm or 0.035" tips.

Frosty, I picked 12 because I have sometimes had trouble with sputtering at slightly lower pressures. I think most of this was from icing up so I have since put my gas bottle in a water bath which has helped so maybe I'll aim for 10psi until I learn to tune by ear.

Mikey, I haven't experienced much scaling in the forge. They behave close to what I would expect a neutral flame would do (based on my limited knowledge) but I agree that they look a little oxidising. I'm very confused by this though, because my MIG tips are still longer than halfway across the air intakes of the T in both burners so I would have expected them to induce less air and be much richer than they are. This is another reason for me posting in this thread.

I should reiterate that this photo is not at full heat for this pressure. It had only been heating for about 5ish minutes. It did get to a bright orange that was more like the colour of the hot spot in this photo (though not a full yellow throughout) when the temp stabilised at this pressure.

 

[Frosty]

12 psi is a reasonable pressure, on my 4 burner forge I had to match burners so I could run two at the same psi and get close to the same flame intensity. I don't turn them down much below 11 or so or back pressure starts effecting them. Unless I'm mistaken your intake ports are larger than I make mine so they will induce more air and to balance the mix you'd probably have to move the gas net closer to the throat than I do. It's to be expected.

I used the same T-inducers to drive NARB and they perform far better, they're stable from stop to stop on my 0-30psi regulator. The problem running NARB at low pressure is the heat build up in the burner head starts pre-igniting the mix inside the burner. Running it a higher psi than about 8-9 gives you close to 6 hrs. work time before overheating. 

I haven't tried improving NARB, I don't spend much time at the anvil anymore and certainly not 6hrs at a stretch.

Frosty The Lucky.

[Florida Man Metals]

This might not be the right place for this but I was curious how many of you are actually using a back flow preventer?

I haven't seen anyone mention this vital safety device. I have always used one installed on my propane supply line.

It is designed to keep a flame from traveling down your gas line and reaching your tank. Boom....

And yes i have a real regulator attached to my propane tank.

I'm rather new to this forum and this might have been answered already. If so please excuse. I don't really run a forge. I run a propane burner in my furnace to refine precious metals and cast base alloys. I'm talking fairly high temperatures. Hot enough to refine paladium from catalytic converters.

[Frosty]

The reason flash back / back flow preventers aren't seen on propane forges is simply because they are fuel air torches, there is NO high pressure oxidizer in the circuit. 

Without an oxidizer NO fire CAN flow up the hose let alone into the tank. NO oxygen NO FIRE. Yes?

What you describe above can NOT happen. 

What can happen is a cut or burned supply hose and a flame monster flowing BADNESS breath in your shop. This won't happen with modern propane tanks because they have internal safety valves to prevent high gas flow. Makes it hard to use new tanks, you have to turn them on gradually or they shut off. 

If you have an oxy fuel torch backfires used to be a danger, drag a cutting torch on the steel you're cutting and you could get a backfire going and literally melt the tip if you don't shut it right off. This is possible because the oxygen is at a higher pressure than the fuel so a blockage of just the wrong type and oxygen could be forced back into the fuel hose potentially turning it into a detcord analog.

We heard lots of stories about acetylene  tanks exploding. I started hearing them in my first metal shop class in 66 but have you EVER seen pics? And to be honest I've never seen a pic or talked to someone (I'd believe) who saw a fuel hose explode. One instructor said he'd seen one do a fast burn for a couple but that's it.

That said, I'll be the first to admit I do NOT know even a fraction of what's to know. If you find examples of propane or acetylene tanks exploding do to a back fire I'll be on the "PREVENT THIS" bandwagon immediately.

Frosty The Lucky.

 

 

[Florida Man Metals]

Thank you for the detailed explanation. Makes sense to me (the layman) in theory. However the sound the burner makes when I shut it off makes it seem possible.

I have no understanding of the physics necessary to build a proper burner. I felt it was a safer option to buy a pre-built burner with a proper regulator from a company than risk a homemade one. 

The manufacturer recommended a flash suppressor. So I followed their advice.

I guess better safe than sorry. Another case of the manufacturer trying to cover their *ss?

 

[Frosty]

You're welcome, it's my pleasure. 

Often burners will pop or sputter when you shut them off, this is normal. What is happening in a naturally aspirated burner is they are tuned to produce a neutral or SLIGHTLY rich flame in operation. The primary pressure, propane jet, is inducing combustion air be causing a low pressure zone around the expansion cone, just like a truck driving by.

When you contain the jet in the mixing tube a couple things happen, as Bernoulli states a fluid flowing over a curved surface causes a low pressure boundary layer between the flow and the curved surface. The most common demonstration is a piece of paper in your fingers and blowing over the curved top, the paper lifts because the pressure below is greater than above. An airplane wing is an every day example. 

Back to the propane jet in the mixing tube, It begins expanding as soon as it leaves the jet it begins expanding taking up a larger volume but not slowing much so the pressure falls and combustion air is induced to enter the intake ports to fill in. The jet is also inside a tube which is a curved surface which causes more pressure loss. 

Okay, all that said, the fuel air mix is flowing down the tube driven by the fuel jet and all's well. I assume you turn your burner(s) off with the tank valve on the propane tank. Tank psi is roughly 200 at room temp from the valve to the regulator, it's reduced to your operating psi over typically 6' of propane hose a couple fittings to the jet. It takes a little time for the propane in the circuit to bleed off. While it is going down the velocity of the flame will be dropping at one point the rate of propagation or flame front velocity exceeds the mix's velocity so the flame travels back up the mixing tube and she sputters or POPS and goes out.

Make sense? 

I've never cared for torches popping when I shut them off which is my least favorite thing about owning an oxy propane torch, the rosebud sounds like a rifle, a loud one. The easy way to eliminate well . . . minimize sputtering or popping on shut down is to install a 1/4 turn propane valve on the output side of your regulator. The circuit still needs to bleed down but only from the regulator to the jet, the reg doesn't bleed down and more importantly it takes a few seconds to close the tank valve. 

propane bleed down goes from a few seconds to maybe 1/2 a second and she's off.

Frosty The Lucky.