Rich vs Lean vs Neutral

[xa.logan]

I've just gotten a new forge and I'm trying to figure out all of this information on propane forges. I keep reading about rich mixes, lean mixes or neutral mixes but I can't figure out what they actually are, why they are good or bad, and how to tell (when the forge is running) what your mixture is. Can someone enlighten me?

[Latticino]

This refers to the proportion of air and gas in the burner firing.  If I am remembering correctly, rich is typically a reducing mixture, neutral is a "perfect" stoichiometric mixture and lean is a gas-poor mixture. 

With a NA propane burner, setting these is a bit more challenging, IMHO, than with a blown (gun style) burner, particularly if you don't have some kind of shroud for the air inlet.  Usually it is easiest to determine your mixture by looking at external cues.  Once the forge is up to a stable temperature, if you have a couple inches of yellow flame shooting out of the exhaust port (or door) you are running slightly rich (which IMHO is a good setting for limiting scale formation inside the forge).  If you have no dragon's breath you are probably either lean or neutral.  A bit harder to determine which, but you can do it experimentally.  As you turn up the gas pressure at the regulator for your NA burner, from a minimum setting where you are just maintaining a good stable flame at the burner tip (for each setting of the air inlet shroud), the forge should start to get hotter.  Just before you start to get flame showing out of the exhaust port (check in a darkened room) you should be pretty close to neutral. 

This is most likely the ideal setting for maximum energy efficiency, but is tougher to maintain while limiting scale and avoiding inducing extra air which can cool the forge interior.  Most burners are not ideally constructed and do not fully mix all the air and gas in their mixing chamber, so some extra induced air can be introduced into the forge chamber.  With a borderline setting, like the neutral one, you are always in danger of straying over to lean when forge conditions change (i.e. opening or closing forge doors, introducing stock that obscures the burner outlet...).  That is one of the reasons why I typically run slightly rich. 

Note that this is  a pretty gross simplification, and you will need to be more creative if you have a shroud for the air inlet on your burner, but the general concept should be sound.  In my experience it isn't all that critical, as I usually run slightly reducing most of the time, but YMMV.

[Mikey98118]

 

A rich flame means fuel rich; a lean flame means its fuel poor; in other words, it has too much air; a neutral flame has a correct balance between fuel and air to burn completely-- supposedly. In the real world that may be true, or decidedly not so.

That's what happens when you ask a leading question, and by the time you understand the answer you'll be looking back; way back. So lets try an oversimple answer instead, so you can get along while a lot of time goes by and that knowledge percolate down somewhere deep enough to do you some actual good.

Just make a hard fast oxidizing flame, and then back off of it until you aren't oxidizing heck out of your work (sign by the formation of massive scale). After all, what you need to do is tune your burner to get the desired result; all the rest comes along by and by...

[Latticino]

1 hour ago, Mikey98118 said:

That's what happens when you ask a leading question, and by the time you understand the answer you'll be looking back; way back. So lets try an oversimple answer instead, so you can get along while a lot of time goes by and that knowledge percolate down somewhere deep enough to do you some actual good.

Darn, I thought I was giving a simple answer...  Oh well, I do get carried away sometimes I guess 

Thanks for clarifying.

[Frosty]

I wouldn't have made that simple an answer but it's good enough to work. . . IF the user knows a little bit. A simple test is to shine up a little strip of steel when the forge is hot toss it in and watch it. A lean fire will start causing scale almost immediately, the shine will go away not just change colors as it heats. A neutral fire will stay shiny as it heats until it's glowing too brightly to see. A rich flame will actually clean a rusty piece up but shiny staying shiny is good enough.

With practice you can hear the air fuel ratio and literally tune by ear. I've never used one of Mike's burners so I'd have to fiddle a little before I knew her song.

If you're seeing yellow in the dragon's breath it's a LOT rich, just a tinge of orange is a little rich and good for most cases. Forges vary though some need their burners runnning richer than others to keep the overall forge atmosphere how you need/want it.

Not as simple as Mike's answer but not my usual long winded ramble. Pretty good huh?

Frosty The Lucky.

 

[xa.logan]

Believe it or not, each answer read alone would have been hard to determine what I was actually wanting to know (which is really just general working knowledge of this subject). But reading all of those together was a really good and full reply. 

 

So next question- 

With scaling a piece of metal, can I just use a piece of Home Depot A1? 

When I'm looking at the flame, do I want something that is entirely blue? Or just a tinge of yellow on the outside? What does a lean flame look like?

Also, double post but the edit button isn't workin on my phone. 

Thanks for for the quick replies! I'm pretty new to all things forging and I've been lurking this community for a few weeks. I'm glad I found this. Thanks for the replies!

[Frosty]

Not knowing what kind of forge and what kind of burners it has makes it darned hard to give answers.

Forget the scale test technique I threw that in as an alternative test and perhaps a good final tuning meter.

Do this instead. In a dim light standing next to the forge so you can look across the opening adjust the fuel air mix till the dragon's breath has a FAINT tinge of orange and you're there. If your work develops scale IN THE FIRE choke it a little more, make it a LITTLE more rich.

Hot steel WILL scale up as soon as it hits ambient air so don't adjust for what happens after you take it out of the fire. O-K-A-Y?

Hopefully these burners have a choke be it a sleeve or a plate to adjust the intake air. There are factors beyond the control of the manufacturers such as altitude, humidity and such which effect tuning so they make burners that draw too much air and put a choke plate on it so you can adjust the ratio without grinding on parts and such. To adjust my burners you have to grind the mig tip til it's drawing the correct combustion air.

Frosty The Lucky.

 

[Latticino]

19 hours ago, Frosty said:

If you're seeing yellow in the dragon's breath it's a LOT rich, just a tinge of orange is a little rich and good for most cases. Forges vary though some need their burners runnning richer than others to keep the overall forge atmosphere how you need/want it.

It has been a while since I've studied the dragon's breath coming out of my forge, so I could be off on color.  Of course it may be different in mine as I burn Natural gas, not propane.  What I really intended to say was just a small visible lick of flame, not specifically a certain color.

[Mikey98118]

Latticino,

It was a simple answer; all your answers are simple and straightforward. I enjoy listening to all your posts. Unfortunately, that doesn't mean that someone who feels like he's drowning is going to hear a word of it. That's why I reduced it down to the barest essentials. Of course he will want to hear the simple straight forward truth...eventually

[Mikey98118]

"When I'm looking at the flame, do I want something that is entirely blue? Or just a tinge of yellow on the outside? What does a lean flame look like?"

First, you need to remember that there are at least two different flames going on in a gas forge; the flame being input by the burner, and the possible output flame leaving the forge via the exhaust opening. When guys discuss terms like dragon's breath it is the exhaust flame they are speaking of, which is a very different animal than the incoming flames from a burner. Not that both flames aren't equally important, but they need to be treated separately for clarity.

So, if we are speaking about the burner flame, straight blue from a total primary combustion envelope is desirable, but many older burner designs have a white inner flame ahead of a blue flame, followed by a darker larger and less substantial appearing flame of  "secondary combustion"; BUT by that I refer to the combustion of by products of the primary combustion, which is something of a fiction in this case, for the white inner flame IS actually is the primary flame envelope in this case, and the blue flame is the secondary flame envelope here, so that what is normally considered as the secondary flame envelope is actually the third envelope. How to resolve this; don't go there buy or build a good enough burner to see now white in the flame, and then tune it up well enough to have none.

The next question tends to be "how dark a blue?" Different fuels give off different hues, and lean flames are always darker blue than neutral flames in any given fuel. In fact I made one burner that could be run so lean that the flame turned purple from the amount of red that could be included in it. On the other hand, any slightest tinge of green in the flame is an unmistakable sign that it is fuel rich; such a flame will be pumping out dangerous amounts of carbon monoxide.

You can also get thin yellow and red streaks in a perfectly tuned burner's flame, due to breakdown products of oxidation from some alloys of stainless steel. Flame nozzles #304 stainless can put on quite a show that way; it's harmless.

Normally, methane gives off the darkest blue of all available fuel gases, yet a compressed air/natural gas torch I used at one school for several months put out the lightest blue I've ever seen in a high speed air/gas flame.

Air/butane flames from so called "blue flame" torches are darker blue than from air/propane flames, yet butane pocket lighters started out being set up to make soft yellow flames.

Which brings us back to the fact that understanding all you "know" on the subject will take time to settle in. It is good that you want to know these things, but don't try to hard to know it all at once; it just doesn't work that way.

[Mikey98118]

But isn't the exhaust flame just the tale end of the burner's flame alter all? Yes, it can be just that in a forge that is loping along, but in a forge turned up into yellow to white heat ranges...NO. In fact the goal is no output flame at all; just clear super-heated flue gases. If you have a forge and burner capable of this kind of performance everything else about the exhaust changes too.

With the average forge a small amount of blue exhaust flame is considered normal, but in our example of a really hot forge, if you keep turning up the input flame beyond the forge's ability to completely burn it, you still won't get blue exhaust flames; it will spew yellow white exhaust flames that will complete combustion within a few short inches, instead of massive amounts of carbon monoxide.

What changed? The forge itself is changing the combustion equation  by super-heating the byproducts of the primary combustion envelope before it comes into contact with secondary air.

How is this possible, since immediately after combustion, flame temperatures naturally decline? Radiant energy input from the incandescent surfaces on the forge is being bounced back and forth through the by products.

If the forge is orange hot you could consider their losses to be multiplying faster than energy is being added. In a white hot forge losses are being subtracted while radiant energy is multiplying their gains.