Burners 101

[Alphafarrier]

Another mikey burner question.  I am building some box forges.  I have decided on top mount burners. (I did some tests with burners mounted on the sidewalk.  Although that works very well, it will not be practical as these forges will need to be portable).

 

The insulated space is about 2.5” tall (and 10” deep and 12” wide, because I know some will ask).   The forge takes quite a while to get up to temperature.  I feel like the 2.5” is not enough, and is causing back pressure.  Is there a rough rule of thumb on how much vertical space a 3/4” mikey burner would need in this type of scenario to burn efficiently?  

 

 

[671jungle]

Hello Alpha. are you saying your inner space is only 2 1/2” tall? If so then yes, that is probably what is causing inefficiency and too much back pressure. Also, you said “burners”.Do you mean you are putting more than 1 burner in this space? I didn’t do the math but that seems like overkill and probably contributes to the issues. 
On mounting: the reason burners are mounted on the side or bottom is to increase the hangtime of the flame inside the forge and combust as much fuel as possible creating more heat inside the equipment before the flame leaves as dragonsbreath. One way to help remedy this is to mount the burners on a tangent or a diagonal and by rounding the corners inside the forge. You can always take the burners out for transport if they are an issue. 
Apologies, I have not been on this thread in a bit I may have missed some other important details to your build. You also said “forges”. So more than one forge. One burner per forge should be ok but I still think the work area is too short. Try backing the burner out of the forge until the flame is stable and back pressure is minimized. 

Why such a flat forge design? With this design, maybe multiple smaller burners across the top will distribute more heat over that area.
 

[ThomasPowers]

Sounds like he wants to build some shoeing forges; what with a user name like Alphaferrier.

[Frosty]

A MIkey burner is a pretty high velocity design so aligning it perpendicular to an opposing wall is a problem. It will cause back pressure and inhibit good combustion and output. If you increase it to 3" high the forge will have 360 cu'in but the back pressure will be reduced. 

Two 3/4" burners is WAY too much gun, NA burners need to have a free path and a second burner in that close a space will cause a high pressure area between them. Think of running the exhaust from two V8 engines into one muffler. 

I think you'll have much better results using two 1/2" Mikey burners aligned at a SLIGHT back angle. I'm only talking a couple few degrees to help break up what amounts to a "bubble" of high pressure on the floor.

The rule of thumb regarding how much burner to use goes like this. "One well tuned, 3/4" naturally aspirated burner will bring 300-350 cu/in to welding temperature." You can extrapolate the rule of thumb to any size burner or furnace. The important dimension is the area of the burner "throat" this is the narrowest part of the mixing tube where it meets the intake structure. A Bell reducer on a Reil type linear burner OR the slotted air intakes of a Mikey or the T fitting on one of mine. 

The AREA of the throat's cross section is THE number and burner output matches. For instance a 1" dia burner has 2x the area of a 3/4" burner and will put out 2x the BTUs per second. 

So, TWO ea. 1/2" burners have the same output as ONE ea. 3/4" burner. The advantage is being able to spread them out to distribute the heat more evenly which makes a long, narrow forge functional. 

Make sense?

Frosty The Lucky.

[Alphafarrier]

Hey guys, thanks for the help.  671jungle, these are farrier forges, and a rectangle makes a lot of sense because for the most part we are putting flat shoes in.  On top of that, it allows the use of board instead of soft fiber insulation, which for a production run is far less labor involved in setting up, applying, and slow curing coatings for said fiber blanket insulation.

This forge design will actually allow for 2.5"-4.0" of vertical space inside, depending what combination of insulation/hard facing is used.  I thought i'd start with less vertical height to see what I could do in terms of maximizing efficiency.  Obviously all the parts need to fit together, so I will have to sacrifice a small amount of efficiency to gain some by having burners that actually function.  Having the full height get's over 350 cubic inches, hence the need for two 3/4" burners.  The turndown range is great, so they can just tick over while on the road doing farrier work, but in a competition setting where every second counts, I would like to be at the absolute maximum I can be in terms of burner capacity.  This may still be too much burner, but we'll see, I'll enlarge the inside and then we'll see what's what!

The burners are set towards the back of the forge, and aimed at a 15 degree angle to just behind the center of the floor in the middle of the chamber. 

 

And I know you guys LOVE pictures, so here are a few for you

[Mikey98118]

I agree everything Frosty stated, and will add that his "T" burner would be my first choice in a box forge; not a Mikey burner.

[Alphafarrier]

The Frosty T would be my first choice as well.  One of the challenges with building farrier forges is the many requirements.  One of them is space, and while I may yet go with the T burner if I can't get good enough performance out of the Mikey, I would love to be able to make them work, even if it means a small performance or efficiency loss.  The mikey burners have some non-performance points in their favor: they are much shorter, and they are easier to mass produce.

[Frosty]

Are your burners shorter Mike? 

Without going out and measuring I believe a 3/4" T is a bit over 8" not counting the thread protector. Maybe 9 1/2" including fittings?

The burner alignment in the photo is reversed of what I was thinking. In that orientation the flame is going to hit the floor, deflect and blow straight out the opening. Leaned the other direction and moved forward the flame will hit in approximately the same spot but be flowing backwards into the forge, up the rear wall and over the roof before exiting the forge. That's what, 3x or 4 x the hang time in the forge?

Sure the burner intakes are closer to the opening but the door deflects exhaust gasses away from the top of the forge. 

Frosty The Lucky.

[Alphafarrier]

Frosty: maybe I had some bad info.  I thought I read somewhere that your mixing tubes were like 1.5 times a mikey or something like that.  I'll rethink my options

That's a good thought.  I had hoped I would get some of that effect at the front end of the forge with the door closed, but you are correct, the exhaust draft does seem to pull everything out the side before that happens.  I may have to go out tonight and flip the lid tonight, mount it backwards, and see what happens!

If I do that, do you think there will be any problems with a high pressure area in the back of the forge creating back pressure?  What I mean is, will it be hard to vent the exhaust because new incoming gas is going the opposite direction?  (New gas is travelling towards the back of the forge, while exhaust gas needs to travel towards the front of the forge to go out the exhaust vents).

[Frosty]

Yeah, you see that kind of junk on the web all the time. The T was intended to be effective and require the minimum shop tools and skills. It follows the same basic ratios as most NA inducers. I posted a set of illustrated directions here some time ago. 

 The back aimed burner flames will impinge the floor and be deflected along the floor towards the back where it will be deflected up along the back wall and again forwards on the roof and out. I could be wrong of course and it might not flow so nicely but it WILL flow in a close approximation of that pattern. The fluidics is pretty easy to predict. The only obstruction will be horse shoes if you stack them instead of lay them on the floor. 

If you give it a try PLEASE keep us in the loop, pictures of it running before and after the change would be good. Please.

Frosty The Lucky.

[Mikey98118]

2 hours ago, Frosty said:

Are your burners shorter Mike? 

I expect that they are pretty close to the same overall length; can I make them shorter than yours if a work real hard too? Why would I care?

If, for what ever reason he wishes to use a Mikey burner, the smart fix is for him to stick them in a side wall near the top, so that it can pass over the work, shorten the height of the forge, and increase its width. It isn't always a question of of "either/ =or" sometimes the bottom line is How we go about having our druthers

And about trying to help others to achieve theirs, even if it "ain't the way we would do it."

[671jungle]

16 hours ago, Frosty said:

close approximation of that pattern. The fluidics is pretty easy to predict.

I hope this link is ok to post. It is pretty fun to play with and is a great way to get an idea of of how our home-built forges will handle the torches we put in them. 

https://physics.weber.edu/schroeder/fluids/

16 hours ago, Mikey98118 said:

And about trying to help others to achieve theirs, even if it "ain't the way we would do it."

I wish the education system had more Teachers with your style.

[Frosty]

That's a neat modeling site but I don't know if there's a way to aim the flow, I couldn't find one but I only looked for a while. It might be a good tool to play with flow patterns in a device say burner tube. Unfortunately I haven't figured out how to model a flow pattern of a burner in a forge chamber. If you run across one PLEASE let up know. 

I've bookmarked the site as is. Thanks for the link.

Frosty The Lucky.

[671jungle]

I could not find a way to aim or add another source either. I did however go down a rabbit hole of researching other simulation sites. Very pretty to look at but not much more on customizing environments on the free online versions.

[Mikey98118]

The site makes a good start. Mostly, imagination is the fastest tool to advance knowledge in flow dynamics, while smoke tests is the surest tool; especially for newbies. Frosty has some knwoledge about it, if we can coax him to go into his methods one more time, for the experimenters among us?

[Another FrankenBurner]

The first time I did a smoke test, I immediately learned that I had it all wrong.  It flowed completely differently then I had thought.

For the forge chamber flow patterns, I laid a sheet of copper across the floor of a cold forge and let it heat.  As the plate heated, different portions of the flow went brilliant green.  The whole process took only a minute but I had a good idea of where it went after it left the burner.

[Frosty]

Yeah, I've discovered most sites offering things like this one are teasers for the "pro" versions and I can't afford a fancy dinner if I cook it myself. 

Yeah, I did some smoke experiments years ago and they mostly verified some of the things the written stuff I was gifted with said. Largely that the jet has to be as close to dead center down the mixing tube as possible. I wasn't prepared to try forming clear plastic tubing and experiment with different "venturi" structures. 

I just used some clear plastic tubing and glued a mock up of a T. It was nothing fancy nor maybe very accurate, it  mostly got me in the ball park so I wouldn't wast a bunch of time looking for the ball park with steel pipe.

Food coloring in water should work as well but you'd have to compensate for the incompressible nature of water. 

Frosty The Lucky.

[Alphafarrier]

When you are all doing your smoke tests, what do you use to make smoke?

[Alphafarrier]

Also, Frosty, just a note: this forge has side ports, and would be operated with the door closed.  Will that change the comments you made?

[Mikey98118]

On 6/30/2020 at 11:06 AM, 671jungle said:

I wish the education system had more Teachers with your style.

The last time I had teachers, I was a fifty year old with an attitude; they were both older women, whom I have nothing but utter respect for.

[Frosty]

On 6/30/2020 at 6:27 PM, Alphafarrier said:

When you are all doing your smoke tests, what do you use to make smoke?

I tried a number of things but an incense stick provided a nice smooth stream of smoke that let me track what was going on in the various parts of the inducer. I only smoke tested jet ejectors and mostly testing how jet position and alignments effected flow and induction. 

19 hours ago, Alphafarrier said:

Also, Frosty, just a note: this forge has side ports, and would be operated with the door closed.  Will that change the comments you made?

Interesting, good question. I suppose if the only time you open the door is to place or remove shoes which way the burners tilt wouldn't matter at all, they might as well aim straight down. Exhaust will be through the side ports and as long as they're balanced with burner output to maximize flame hang time against back pressure it should all be good. 

As for burner placement and portability issues. Why not make the burners just drop into the sleeve mounts? I'm brainstorm thinking something like a key in a keyhole. Say 3 pins on the burner and 3 slots in the sleeve, The pins slip into the slots and a turn of a few degrees puts them in a pinch and holds steady. To remove you twist the other direction and give it a LITTLE shake to break the pinch. The burner just slides out of the sleeve and goes in a garage sale metal tool box. 

Then you could place the burners to maximize effect, horizontal, aligned lengthwise makes for a screaming hot forge. One of the guys in our club is getting excellent results from updraft burners.

Hmmm?

Frosty The Lucky.

[Alphafarrier]

I'll give the incense sticks a try.

As to removable burners, the problem is that for Farriers, usually you can get away with a little bit of vertical space, but horizontal space is at a premium.  Forges need to be slid out to the back of a pickup truck bed, or are mounted on van doors or swing-out arms.  A horizontally mounted burner is just not realistic.  As to removable, again, it's just not realistic.  Farriers need to set up and tear down a work station several times a day, and messing around with a hot burner, having to store it somewhere safe, and then trying to get it back in again, all while keeping in mind that whatever mechanical system to hold the burners in place will be constantly expanding, contracting, and corroding with heat, it's just too many things to go wrong.  

I will mess around with proportions and do some smoke tests when I have time, and will keep you all in the loop.

One question I have is also with regards to burner location on a top mount.  Right now, I have the burners mounted in such a way that the two burners divide the lateral outside dimension of the forge into thirds.  I was thinking I should change this so that the forges would laterally divide the inside insulated space into thirds, as viewed from the front (I can draw a diagram if this isn't making sense).  Basically, from side to side, there would be equal space from the inside left sidewall of the forge to the middle of the first burner, the middle of the first burner to the middle of the second burner, and from the middle of the second burner to the inside right sidewall.  

Right now the burners are further apart.  Do you think putting them closer together would cause that high-pressure spot you were talking about in between the burners?  

[Frosty]

16 hours ago, Alphafarrier said:

Right now the burners are further apart.  Do you think putting them closer together would cause that high-pressure spot you were talking about in between the burners?  

No, dividing the volume in thirds should be fine. I try to divide forge volume evenly with burner placement. Just do NOT make a dozen before you test one! 

As to making burners easily removable. Burners don't get very hot in use except at the output end, if you remove it and heck dip it in water you won't be handling hot anything. Use uh . . . . DRATS it slips my mind but I think it's 316 stainless for the pins and tune the burners so you don't need secondary air around the mounting sleeve and you don't have to worry about corrosion or distortion to speak of.

It's just a thought, I can't help it, I see ideas.

Frosty The Lucky.

[Alphafarrier]

Alright, I’ll give that a go, and we’ll see how it works.  
 

Not being able to help seeing ideas is not a bad thing!  Heck, even if only 25% of them work (and I’m sure your batting average is probably a lot higher), the world is still a much better place!  I, for one, am learning a lot.

One more thing  

Exhaust port size.  Is there a rough rule of thumb for a given burner size?  I know adjustable is ideal, but I don’t have that freedom in this application.  If I had to pick one side of the spectrum, let’s say I was more concerned about less back pressure and less concerned about efficiency? 
 

[Frosty]

25%?  !

If my average was 0.025% I'd own my own space station.

Look into Mikey's door baffles rather than exhaust ports. You have to balance flame hang time against back pressure but you can waste a whole bunch of potential forge time worrying about another point of efficiency.  Efficiency isn't as important as effectiveness and YES there's a difference. 

Frosty The Lucky.