Help identify forced air burner behavior

[Selkirk]

Hi all,

I recently had time to get back to my forge. My ribbon burner was cracking and was always touchy so I went to a straight pipe with a flare. I am seeing some odd behavior I would like to understand and am hoping the forums expert eyes can help.

this is when forge is first lit. I would like to close off the front opening more but when I do, the flame dies.
https://photos.app.goo.gl/4dw4SYqCp2GYnkqJ6

 

this is when I start to turn up the gas and air. Not sure what is happening here… lots of yellow flame but all burning in secondary flame??

https://photos.app.goo.gl/yRVrkohjmPKuTkbe7

 

The next two are taken as forge warms up and air and gas are slowly increased.

https://photos.app.goo.gl/LcfZzboWH2xjtTWA7
 

https://photos.app.goo.gl/2famm9GPUncGaCFp9

 

am I too gas rich at startup? Why can’t I use a firebrick to partly close off entrance without flame going out? At what point is it safe to start increasing propane injection to increase heat and minimize oxidation of steel?

thanks all.

 

 

[Latticino]

Your blower looks like one that is optimized for flowrate rather than pressure, so it may not be capable of putting out the required air pressure at the outlet of the burner. Not an uncommon problem for those who select a squirrel cage blower. 

Flame doesn't look that bad to me in the final video.  I can't tell from the photos or videos whether you are inducing secondary air at the burner nozzle entry to the forge or not.  This secondary air would be very subject to variations in backpressure from the closing of the forge doors.  If you gradually close the doors, does the flame turn more yellow before it goes out?  That would be an indication.

If the blower doesn't develop enough pressure to provide the correct mixture a the temperatures you are looking for (BTU output from the air/gas mix) you will either need to get a better on (axial rather than squirrel cage) or drastically reduce the mixing tube line length and remove elbows.  Is the air valve open all the way during high fire?

Note: it is fairly common to have to keep a burner at a lower fire when the forge warms up.  This has to do with the flame front speed vs the air/gas mixture flowrate at the outlet.  The issue with blocking the door and having the flame go out is different.  In my blown forge I can just about completely shut the door and make do with a 1/4" gap all the way around.  However I use an axial centrifugal blower optimized for more pressure and it has a 1/2 HP motor.

[Mikey98118]

The flame looks pretty good to me too. The fluttering orange secondary flame looks like the product of oxidized debris being blown down into the primary flame. Give that time to clear itself up, before doing anything further. Congratulations on a nice forge build

[Frosty]

It looks and sounds a LITTLE bit rich when you light it but not much. Rather than closing the doorway consider Mike's door baffles instead. They allow exhaust gasses to pass reasonably freely but the brick baffle absorbs flame energy and radiates it back into the forge.

In general I agree with Latticino, a centrifigul blower and straighten the supply pipe so the mix can flow easier will supply more air fuel to the forge per second and it'll get hotter.

Frosty The Lucky.

[FlatLiner]

It looks like you are necking a 2" pipe down to a 1" pipe. In my experience when you neck down a blown burner so drastically you have a hard time getting a stable flame because it likes to blow off the end of your burner. Try shortening your air pipe, you only need it half as long as what you have. Cut your 90⁰ bends down to two, you don't need any more than two. If your necking down a 2" pipe only neck it down to 1 3/4" or 1 1/2" or leave it un necked down. It it's a 1 1/2" pipe only neck it down to 1 1/4"  or leave it un necked down.

[Selkirk]

Thank you for all the valuable feedback gentlemen.

Latticino - I am not introducing any secondary air. It's all strictly through the blower. Although I've read quite a bit in these forums about blower pressure vs CFM I have to admit I'm still not clear on the details. I used this blower simply because it was free. I regulate air with a gate valve and have never been able to fully open it. When I start the forge I open less than 10%. I've always taken this as a sign that I have adequate pressure since I seem to get enough air. Please let me know if I'm missing something with that conclusion. I'll have to observe more carefully the conditions in which the flame goes out after partly closing the opening as I don't recall a color change. 

Mikey - I was able to create the yellowish flame when I increased the gas with my needle valve. This was shortly after startup. I definitely saw that yellow color right after I coated my wool with Matrikote! I was quite alarmed but it went away after I had run it for a short period of time.

Frosty - I did a search for door baffles but I'm not sure what you are referring to. Any elaboration would be appreciated. My forge is not that hot or fuel efficient so any tips here are appreciated.

Flatliner - At startup it definitely seems that air speed is too fast to keep the flame. It was my understanding that a big diameter reduction like this was intentional and somewhat of a safety mechanism to prevent the flame from propagating up the tube? How much difference in diameter is really needed?

A few of you mentioned reducing the number of 90-degree bends. I was making due with the materials I on-hand. It's difficult for me to get the cuts I need and have them threaded. Also, I was counting on the angles helping with the mixing. If I am getting adequate air flow does this really matter?

The first time I fired the forge I had kept the air low even after it had warmed up because I kept losing the flame. I was assuming it was excess airflow. I noticed that my flare and even a bit of the burner pipe started getting cherry red and turned off the forge. I think this could be a sign of combustion in the tube? On my next fire I increased air and fuel as the forge warmed and did not see this occur. Is it possible that the flame going out when I block the forge opening is because I have to turn down the air so much at startup that my flame is going out due to lack of oxygen? And on my first startup this also resulted in combustion inside the flare? If this is the case then perhaps if I block the port after the forge has warmed up I might not have any issue?

Thanks again for your insights!

Selkirk

 

[FlatLiner]

My forced air burner I necked down from 1 1/2" to 1 1/4" if I neck it down any further then it has a hard time holding a stable flame until the forge is up to temperature. I've never had an issue with the flame crawling back up the burner on a forced air burner, with the positive air pressure I would think that it would be nearly impossible but others might know more than me about that.

[FlatLiner]

Here is my set up. I use a furnace exhaust fan that you see in one of the pics. I had it necked down to 1" when I was using a hair dryer and I was able to forge weld with it like that, but as soon as I got the bigger fan I had to neck up the burner to get a stable flame.

[Latticino]

No, if you don't need to fully open the gate valve for the air during full fire your pressure is fine.  I have no problem with necking down the mixer outlet before introducing into the forge, however there are consequences.  With a  smaller outlet the air/gas mixture will be flowing faster for the same quantity flowrate (BTU output directly related to that flowrate).  Faster flow means a different flame front location relative to the output for each forge interior temperature.  Too fast and it will lift off the front of the tube and blow out (you also may get a "faster", longer flame which can be a problem for certain forge configurations).  Too slow and it will burn back into the mixing tube and heat it up (or the air/gas mixture won't flow enough to cool the mixing tube from the radiant heat from the forge, if done once the forge is up to temperature).  Remember that flame speed is also related to forge interior temperature, so it needs to be different for each, a balancing act.

If your forge is only losing combustion under low fire when warming up and you try to block the doors, that is likely a product of not having enough pressure to push the products of combustion out of the forge interior (ever try to heat the interior of a can with a propane torch?).  You are choking off the flame.  Simple answer is to not close the doors until your forge gets up to temperature and you increase the air/gas to more usable levels.  You will probably be able to block off the door part way.

Door baffles are just doors that stand a little way off the door opening (parallel, but 1/2" to 1" away from the face).  This addresses radiant losses, but allows relatively free flow of combustion products.

 

[Frosty]

You can also read more about thermal baffles (forge doors) in Burners 101. 

Also the faster the flame velocity the less time it stays IN the forge to work for you. 

Frosty The Lucky.

[Mikey98118]

Don't let that word "baffle" worry you; it only denotes the fact that an obstructing object forces a change in exhaust flow direction. You can place a little wall of bricks to create a baffle in front of, and an inch or two from the exhaust opening, and be perfectly happy with it, while you take your time scheming about building a fancy baffle door on the forge; you may never get around to doing that

[Irondragon Forge ClayWorks]

Here is our version of adjustable baffles (doors) on our 20 pound tank forge. They can be opened all the way to put larger stock in and shut all the way but still allow for some exhaust to exit the forge on both ends. I really need to coat the insides of the bricks with Plistix which I have, but just haven't gotten around to it.

[Mikey98118]

Yup; that works too. Really, a baffle can be as simple or complex as you like.

[Selkirk]

Awesome. Thanks for everyone’s input. I think I’ll I’ve got a lot of small tweaks here I’ve picked up here that I believe will help considerably.

I actually have a “baffle” on the back of the forge for my rear exit for larger stock. Just an insulating brick with some Matrikote painted on its face. I need a more elegant solution in the front but I’m glad to know that I’m on the right track.

The flame velocity/ keeping the flame in the forge longer is resonating as well. I think this makes increasing that pipe diameter a top priority.

I’ll post an update once I’m able to get it updated. Hopefully with some great results to show!

[Mikey98118]

While flame velocity can vary between burner designs, A certain amount of flame velocity is present in any burner that achieves complete combustion in its primary flame. Frosty' "T" burners create slower flames than my burner design, yet can produce such primary flames. The main goal is to produce complete combustion.

Once that goal is achieved, we can take time to wrestle with the consequences of that combustion; which mainly is a high speed exhaust, that is rushing those very hot gases rapidly out of the nearest exhaust opening

Exhaust gases begin to slow the instant they leave the flame envelope, which is the actual reason that aiming a burner so that the exhaust swirls around the inside of heating equipment is so valuable. Adding a baffle wall just outside of the exhaust opening also helps slow exhaust speed a little more.

But the biggest help in increasing hang-time for exhaust gases comes from the fact that, SMALLER FLAMES SLOW DOWN MUCH FASTER THAN LARGE FLAMES.

Multi=-hole flame nozzles, like ribbon burners, make very small flames.

[Latticino]

Very well put.  

[Selkirk]

I’ve got the 1.5” pipe installed but the flare from 1.5” to 2” at the end of the burner is not fitting. Rather than cutting at the forge to fit it, I am considering shaping the flare out of the ceramic blanked I need to pack around it.

is this a safe strategy? For a forced air burner does the shape of the flare matter? Bell? Cone? Straight step?

Thanks!

Selkirk

 

[Latticino]

All the forges and furnaces I've made had flares or flame retention outlets shaped from cast refractory until my most recent one (which has a commercial SS flame retention outlet that I picked up out of the trash).  I usually defaulted to the 12 degree angle cone for mine, starting at the forge skin and progressing to the forge interior.  My take on it is that you want the air/gas mixture to be as large as possible without going overly turbulent.  Larger because that slows down the mixture velocity (slower, shorter flame).  I like some sort of transition between the forge interior and the mixing tube diameter as it makes it a bit easier to tune the burner (locate the flame front outside of the mixing tube).

Each to their own though.  Many forges operate perfectly well without a flare.

[Selkirk]

Ok, just got to test it last night. I increased the pipe diameter to 1.5 and hand shaped a slight flare in the ceramic blanket with rigidizer. It Definitely seems more stable at startup. Once hot, I had no issue closing off the front door a bit. 

Thanks all!

Selkirk

 

 

[Mikey98118]

Your flame was a good one, so far as completing combustion is concerned; it was already making plentiful heat in your forge. If you had a single burner flame in that forge, I would conclude it is fine and dandy. But, as a ribbon burner, it lacks definition, for all the individual primary flames are coalescing into one very large secondary flame.  The whole point of ribbon burners is to make many small short primary flames; do these create more heat? NO!!! What those flames do is to conserve that heat much better than the very much larger flames of single flame burners

So, certain things will not become clear by looking at your photos, because they are all about fuel management over time. Is your forge good enough as is? I think so. Can you do better? Yes, you can. Whether or not you go forward is all up to you...and your wallet

[Selkirk]

My wallet needs a break for a bit 

but this forge is just a straight pipe burn. My ribbon burner block started cracking so I swapped it out for a straight pipe. The change was why I was asking for help troubleshooting the burner.

My ribbon burner never worked amazingly anyway, so I didn’t feel it a big loss. I definitely need a redo on my forge. Like you said, it works, but it can be better.

I think I’ll end up with 2 forges. One larger oval shaped forge with a wide but short opening for my large pieces and a small NA forge for my smaller projects. Probably a brick pile.

I loved the design I saw in frosty’s post earlier with the beginner forge. That would be perfect for small hooks and nails and the like. 

It was a great exercise though. I learned a lot!

Selkirk

[Mikey98118]

Good luck. I think you will go far.

[Frosty]

I agree, you're flexible enough to build what you'll want eventually. You can always come back to a ribbon if you like down the road.

Frosty The Lucky. 

[Mikey98118]

2 hours ago, Frosty said:

You can always come back to a ribbon if you like down the road.

I'm thinking we will all fall down that rabbit hole...eventually

[Frosty]

See, there's the problem right there, Mike. Walking down rabbit holes eyes wide open is a LOT more fun than falling down them. I like to take friends or unsuspecting bystanders along. That's REALLY fun.  

Frosty The Lucky.