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Forced Air Forge (NG) - Pipe Size, how small can I go?

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I'm going to again echo Latticino on the ball valve issue, but if you did have it working properly with a different air supply then I understand why you're focusing there.  For burners it's not just about the volume of air that a fan will move; it's the volume of air that the fan will move at a certain pressure.  As you know when the fuel/air mixture ignites that creates expanding gases, which will move in all directions unless contained.  Your burner tube only allows them to move into the forge or back towards the blower.  Regardless of the CFM capability of the air source, if it does not provide enough pressure to overcome the pressure created by the ignition of the fuel/air mix then the flame will travel back towards the blower - at least a little bit until an equilibrium is reached. A continuing pattern of the flame burning back into the burner tube towards the blower while the conditions are right and then moving back towards the forge could possibly create what you're observing and hearing even if the fan speed and output remained relatively stable. There's more to it than simply the volume and speed of the air relative to the speed of the flame front.  The burner in the forge is also likely to experience more back pressure than it would in open air, so that's another factor for the blower to overcome. Anything at the burner port entrance to the forge that impedes the progress of the flame into the forge can increase that back pressure even more.  It's even possible that if you have very low pressure NG available, the pressure in the burner temporarily exceeds the pressure of the fuel gas and therefore stalls the fuel input until the pressure drops below that point.  If that were to happen repeatedly and rapidly the effect could be pulses of fuel rather than a steady stream being introduced into your burner.

It kind of reminds me of the swingfire heaters we used to warm up the coolant in the HMMV's when it was -40 degrees or so outside.  As I understand it they had no internal moving parts, but they sounded like an engine when they were running.  I believe they were designed to use pulses of fuel/air which created that sound.  I think you may be getting roughly that same effect.

At some point here I'm just offering a SWAG.  Blown burners are typically considered to be very easy to construct and operate due to the ability to easily control both the fuel and air input independently.  For me at least it worked as expected pretty much immediately.  I'd still be using a blown burner if I had a permanent setup, but because I forge outdoors I don't like running extension cords and/or long air hoses from buildings to my forge.  The two things I keep coming back to with your setup is the valve you are using on the fuel side and possibly the PC fan on the air side.

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On 4/7/2017 at 9:12 AM, Latticino said:

MHO you are still missing a key step that has been suggested by several parties:  install an appropriate valve that can meter your gas supply adequately.

Apologies if it appears I have ignored this advice. I haven't - I have been looking into it quite a bit, trying to see if I can find a good and reasonably priced option.

I assumed, perhaps incorrectly, that the recommendation to go a needle or globe valve was to help me adjust the flow more easily and not because it would actually meter or regulate the gas flow any better (once set) than the ball valve. I've assumed it I actually want any improvement in regulation, I'd need an actual regulator. Do I have this wrong?

All the fuel gas needle and gate valves I can find are in the $30+ range. There are $7 Water & Air rated stop valves at my local homedepot, but I've assumed it would be unwise to use them? There are cheap valves on ebay that claim to be gas rated, but I half think the "air" rated HomeDepot valve might be more trustworthy than something random and unbranded on ebay.

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On 4/7/2017 at 1:26 PM, Buzzkill said:

Regardless of the CFM capability of the air source, if it does not provide enough pressure to overcome the pressure created by the ignition of the fuel/air mix then the flame will travel back towards the blower - at least a little bit until an equilibrium is reached.

Yes - that's what I was getting at with the momentum thing. I'm picturing that pressure wave going backwards to the PC fan. I figure for lots of reasons (momentum, flex, etc..) that PC fan will be less capable of keeping the air pressure steady than a bigger blower. Plus, I figure that the more pressurized air (e.g., in the supply duct) the more the pressure drop would be diluted.

Kinda like a pulse jet effect.

Anyway, half the time when I think about this it makes sense, and the other half of the time it seems crazy.

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Ok, so I got a chance to run the thing again yesterday. Now that I have a better idea what I'm looking for, it was easier to recognize what was going on. Here is how it's behaving:

State 1:

  • Hot forge
  • Gas set low.
  • I set the fan at 100%

Result: I can get a good roaring burn with no visible dragon's breath (to me, in dim-ish daylight).

State 2:

  • From State 1, I increase the gas:
  • As the gas quantity increases, the flame goes from roaring, to sputtering, to (I believe) burning in the mixing tube.


  • The mixing tube gets hot.
  • The burner is very quiet (but what roar there is has become higher pitched in keeping with the idea of burning occurring in the tube).
  • Lots of orange flames come out of the forge openings.

When this is occurring, if I blow into the back of the PC fan then the flame goes roaring again but only while I am blowing into the fan.

State 3:

  • While in State 2
  • Back down the fuel again.


  • The burner continues to act as if it is in State 2.
  • If I blow into the back of the fan, it will toggle back into State 1 and stabilize there.

So this is the state that confuses me. I don't know why the flame position is changing the richness of the burn. I can guess that the flamefront position is affecting the backpressure felt by the fan. I guess if combustion is happening in the tube, there would be a marked increase in backpressure and thus less air from the fan? Perhaps when I have a roaring burn in the forge I'm actually getting some air induction?

I figure that States 1 and 2 could be explained purely by richness changes - but that State 3 cannot. So in combination, I feel like I've confirmed that the air/gas velocity is not sufficient. 

Determining this was my immediate goal, since I do have the option of upsizing the fan.

Side Note: I believe I've confirmed that burner position doesn't seem to matter (I've upgraded my burner mount so I could move the burner around better).

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  • 2 weeks later...

Exciting updates:

I tried a few new configurations:

  1. 3/4" burner, fan attached to the "drop" of the T, natural gas attached to the "run" of the T. Air-rated stop valve added inline with shutoff ball-valve.
  2. 1/2" burner, gas via the "run" and air via the "drop"
  3. 1/2" burner: 3/4" to 1/2" reducing T, 1/2" mixing tube approx 4" long, air into the 3/4" run, and NG via stop-valve through the "drop" of the T.


All 3 are successful!

In all 3 cases I could turn the burner up high enough to make it stutter a bit, but it never burned in the mixing tube.

Having the NG feed in through the run (rather than through the drop) definitely increased the airflow. I assumed it would increase the velocity by about 10% but I think it did more than that. Configuration #2 would feed gas/air fast enough that I'd get a cold spot on the impact wall, whereas #3 did not have a cold spot.

This is configuration #3:

Configuration #3 is running pretty well at the top of it's potential, as far as I can tell, in this video, and seems to stutter just a bit but remained cool. This configuration was sensitive to the backpressure created by the firebrick doors, so I had make sure I didn't restrict the airflow too much.

In this configuration I was able to successfully weld this steel. I say successfully, in the sense that 1/2 the weld worked and the other 1/2 failed and I think that's my fault. This is my second ever organized attempt at forge welding, so lots of potential error on my part.


It's not conclusive, but it seems to me that the stop valve is making the difference (and for that: Thanks everyone! I frankly never would have bothered without everyone suggesting this).

I don't understand why it is helping (and would happily take an explanation if anyone has one).

I assumed/thought that for any arbitrary position I put the stop valve in, there would have been an operationally equivalent position on the ball valve. Yet, all 3 configurations above worked successfully with the stop valve. Technically, I've not done a fair comparison (none of the 3 configs above were tested without the ball valve - I'll try that on another date).

Without the stop valve, there were occasions where I smelled unburned NG escaping the forge. So the only guess I have is that I had too much NG all along and the extra pressure drop caused by the stop valve is helping..?

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Not familiar with the term "stop valve", but what you installed does seem to be a globe valve, which covers what I had originally suggested to allow metering of your gas supply.  My best guess is that the correct gas/air mixture is required for stable flame at various forge running temperatures and geometric configurations, and this valve allows you to adjust same.  Keep the ball valve for a quick safety shutoff, but use the globe valve for metering.  Ball valves suck at metering gases as they are pretty much full flow until closed quite far then have a very small adjustment range before they are completely closed.

Glad it is working.

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  • 1 month later...

For anyone interested, this is the 3/4" version running with the PC fan at full (the last batch of videos were the 1/2" version). I don't think I can run the PC fan less than about 70-80% and still maintain the necessary output velocity. I believe the 3/4" works better.


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