Looking for feedback on forced air burners I built. Should I add flow straighteners?

[Dr.X]

Hi folks.

I decided to convert a cheap Amazon dual burner Venturi forge I own to forced air. Did some internet research, but didn't do any calculations. Kind of winged it.

It works pretty well, as you can see, but I'm sure it can be improved upon. Feedback/suggestions would be appreciated.

I used pipe from Home Depot and decided not to add flow-straightening sink strainers at the business end of the burners. Seems to be OK without them.

A few questions: 

-should I stick to the leaner blue flame as seen at the beginning of the video, or the redder, richer flame shown near the end?

-the burners glow red after about 10-15 minutes with the richer flame. I imagine this is not great for the longevity of the unit, but is it safe?

The forge will be used infrequently to forge small knives by me. I dabble in many things...

Many thanks.

Nick

 

 

 

[Frosty]

Welcome aboard Nick, glad to have you.  Yeah, your burners have an issue alright. It's normal fort the outlet end to turn orange making the last nipple a wear item. They WILL burn up eventually. However, they should NOT be that hot that far up and certainly not the burner mount sleave!

I think when you turned the air down the flame started burning in the burner tube. A little bit is an annoyance but yours has a problem. They are getting the forge hotter though so that's good.

How far into the forge are the burner outlets? (nozzle end) Ideally the ends of the burners should just penetrate the shell and extend maybe 1/4" into the refractory liner. 

If they're not too deep you should put flame holders on them. You can buy high temp stainless screen which will last a lot longer than floor drain grates. The term is "flame holder" not flow straightener, not a bad guess though makes intuitive sense.  Running the fuel air flow through a screen breaks up the single stream into maybe 100 small flames depending. The smaller the flame the less distance it goes before combustion is complete and the flame stops. What it does for your situation is the flame can not burn back past the screen so your burners will stay cooler.

Flame holders and pull the burners back if necessary and if there is very much space between the burner tubes and mounting sleaves block the space with ceramic refractory blanket. Naturally aspirated burners like used to be there sometimes need the extra air for complete combustion and entraining it through the gap between burner tube and sleave serves. You have bun burners now and they do NOT need extra air you can adjust it precisely so the gap is a bad thing. Couple that with how close the burner nozzles are to the floor and flame will be forced back out through those gaps. I'd block them as close to the end of the sleaves as possible.

If you can get a pic of the roof inside the forge of the burner ports it'll help evaluate things.

I also think you could probably double the height of your forge and get good heat, maybe better.

Everything I said above are first impressions and could be wrong or need tweaking.

Frosty The Lucky.

[Dr.X]

Whoa. Thanks for the warm welcome Frosty! Your posts provided a lot of guidance when I was researching how to build this.

RE: the second paragraph in your reply above. What I did was to increase the propane a tiny bit via a needle valve - I did not decrease the air (which I suppose accomplishes the same goal to some extent). But this should not cause the flame to burn in the burner tube if it didn't previously, right?

To answer your questions: the sleeves are on the long-ish side, so the end of the nozzle sits between 1/2 to 3/4 inch above the refractory. The nozzles actually end outside of the shell. Kinda hard to see in this pic but it's as good as my borescope will do:

Certainly explains why the sleeves got so hot!

So, #1 job (after getting and installing flame holders) will be to get the nozzles into the shell sitting a tiny bit into the refractory, as you suggested.

Increasing the height of the forge would require me to get or build a new one, which I'm not prepared to do quite yet.

BTW, there is virtually no space between the nozzles and the sleeves. I had to sand the black iron reducers quite a bit to get them to fit into the sleeves.

What do you think about the color of the flame? Should I aim for the pure blue like at the beginning of the video, or the richer slightly redder version?

Thank you again.

Nick

[Goods]

I’m not a gas forge guru, but I’m guessing that the bell reducers you’re using as nozzles are too large of diameter change from the mixing tubes. This will cause the flame to settle in at the change in diameter, heating the bell significantly. Try using pipe coupler, or thread protectors (Frosty’s favorite), or just form you nozzle in the refractory of the forge. (You do have the ceramic blanket covered with a refactory right?)

Frosty, Mikey, what are your thoughts.

Keep it fun,

David

[Frosty]

Yes, adjusting either fuel or air does the same thing for tuning the flame. I was judging by flame shape more than color but I like a SLIGHTLY rich flame so it prevents scaling in the forge. 

Stopping the nozzles inside the mounting sleave is the overheating problem, you've basically turned the sleave into a step nozzle/flame holder and are heating the sleave with flame contact. Ideally you want the flame burning ON the leading tip of the nozzle. Inside it even a little causes the sleave to heat up which lowers the rate of propagation meaning the mixture's velocity doesn't overcome the flame front and the flame moves farther up the burner. What we'd call a "back fire" in a oxy fuel welding torch. 

I'm not a fan of using bell reducers for the nozzle flare but they will work if the burner is adjusted correctly. I use thread protectors myself but we can work with what you have. The first thing I'd try is replacing the final nipple from the elbow by at least 1 1/2" maybe 2" to extend the end of the flare to just outside the mounting sleave. The sleaves should NOT be inside the shell more than just to the edge of the refractory if that far.

The refractory wool doesn't appear rigidized or covered with a hard refractory, this is a serious health hazard especially in the burner ports. You REALLY should address this issue and it'll make a big difference in the flame shape's quality. Think of it like having the inside of your water nozzle fuzzy. Picture the stream of water, Hmmm?

Those are your main issues right now, pull the sleave back if possible and most definitely push the burner flare to JUST outside the sleave.

Forget the flame holders that is NOT your problem, they would in fact make it worse with the configuration as it sits now.

I suggested them because the video showed that the flame was burning back and a flame holder is a method or preventing back burning. However with a little more information I see it's the mounting configuration that forces the back burn. So, forget the screen flame holders. Yes?

Maybe later but not now.

Frosty The Lucky.

[Dr.X]

7 minutes ago, Goods said:

I’m not a gas forge guru, but I’m guessing that the bell reducers you’re using as nozzles are too large of diameter change from the mixing tubes. This will cause the flame to settle in at the change in diameter, heating the bell significantly. Try using pipe coupler, or thread protectors (Frosty’s favorite), or just form you nozzle in the refractory of the forge. (You do have the ceramic blanket covered with a refactory right?)

Frosty, Mikey, what are your thoughts.

Keep it fun,

David

Whoa. That's an intriguing suggestion. I thought the fact that it was forced air should "keep things moving" rather than cause a ton of mixing to occur in the nozzle itself, but I see what you're saying. 

Am currently going from 3/4" to 1.25". I could go from 1" to 1.25 inch. 

To keep costs lower I could replace the Tee to a 3/4 --> 1 inch one and convert everything downstream from it to 1 inch. OR, if I could find reducing elbows that go from 3/4 to 1 inch that would be even better. I already have one 1in-->1.25 in reducer.

What do you think?

 

[Dr.X]

Sorry Frosty - we cross-posted.

To make sure I understand - is the drawing below correct?

Clarification: the end/bottom of the sleeve is flush with or 1mm above the shell. This is a 'store' bought, not made by me forge.

And the refractory blanket is coated. I had to cut away a little at the entrance of nozzle and didn't re-coat it. I will rectify that immediately.

 

[Goods]

Don’t run you mixing tube larger. That will lower the velocity, and increase the chances of the flame front moving into the mixing tube. The flame front has a fixed velocity, if you lower the fuel/air velocity, you could run into problems. (Even if it works, that will reduce your turn down capacity, you may not want to run it at full heat all the time.) Better to reduce the bell size, but I will definitely defer to Frosty!

Keep it fun,

David 

[Frosty]

I think you missed my point, I wasn't clear enough. Mixing is NOT an issue the flow slamming into the T causes more than enough turbulence to mix the propane and air well enough but then turning a hard 90 in the elbow ends mixing as even a possibility. 

What flares do in basic physics type terms is increases the channel (pipe ID) by the square of the radius increase.  double the diameter and the carrying capacity increases by x4. Yes? So, with the flow volume filling a greater volume it MUST slow down and the way yours is set up it increases ID twice and both abruptly. This slowed your flow velocity to below the rate of propagation FORCING the flame to burn back until it reached a point when the flow from the blower was greater than the flame front velocity.

Make sense so far?

Okay the next orange screen of burner death. The overheating burner nozzle and sleave raises the temp of the flow and the hotter it is the FASTER chemicals react, meaning the basic rate of the chemical reaction, combustion MUST increase forcing it further up the burner nozzle, and potentially back into the mixing tubes. 

Gun burners don't need any flare at all. Flares purpose on NA (Naturally Aspirated) burners like you replaced is to reduce back pressure up the mixing tube and increase the ease of combustion air induction. It also helps hold the flame at the end of the flare.

Gun (blown) burners do not have an induction issue air is forced into them making flares superfluous. You see them in how to videos, magazines, etc. as an artefact of the more is better mentality. The ONLY benefit a flare CAN have in a gun burner is as a flame holder. And a flame holder is EXACTLY the opposite of what your burners need at this time.

Make sense?

Yes, the sketch is a definite improvement and worth a try. It WILL entrain additional air through the gap between flare and sleave so you'll need to adjust the fuel accordingly. Additional air entrained between sleave and flare will help cool them for an additional benefit. 

Well said David. I wonder how many of us are typing at the same time?

Frosty The Lucky.

[Dr.X]

Now I get it Frosty. Thank you (all) for taking the time to help out a strange dude on the internet.

I didn't know everything you guys just explained previously, but even if I had, a flare would have been necessary unless I went 1.25 inch from one end to the other. Which would increase the costs exponentially.

I imagine I would also need a much bigger air source to keep the velocity up enough with pipes that big. 

Hopefully I can make it work with what I have. I'll get to work on it.

[Frosty]

You're more than welcome, it's my sincere pleasure. Talking to strange dudes on the internet is part of the reason I do this. 

OR you could tack weld a large washer shape (NOTHING PLATED IN THE FORGE!!!) that also fits the ID of the sleave, buy a longer nipple and forget about a flare at all. That is NOT a suggestion at this point it's just another possibility, just like making the forge larger. Something to put in the maybe idea notes for a later day. 

The downside being a higher flame velocity which means the flame would spend less time in the forge transferring energy to the liner to re-radiate as IR radiation and heat your work.

Frosty The Lucky.

[Dr.X]

Done. 

Now I have to let the Matriokote dry and I'll fire her up (probably tomorrow if I get time). Will let you know the results.

[Dr.X]

Gents,

While I'm waiting for the paint to dry, I was reflecting on your posts.

Based on David's comment, I wonder if it makes sense for me to cut the flares down.  (BTW, I noticed/remembered yesterday that I'd removed the threads inside the bells, making the cavity even larger!)

A is what it looks like today. B and C are options.

Always compromises:

-I worry about the mechanical stability of B and C. The sleeve walls are thin, and the cap screws loosen every time I fire up the forge.

-the space between the sleeves and the reducers will get larger in B and larger still in C. Frosty, you said this might keep them cooler, though I wonder if it will also allow heat and possibly flame back up.

What are your thoughts?

PS. What about using the stainless sink strainer flame holders? 

[Frosty]

Were I working hands on with your set up, I'd replace the bell reducers with thread protectors and use longer screws to hold them in place. 

No, I didn't say trimming the thread protectors would keep things cooler though it's a reasonable inference seeing as more cool air can circulate around them and the sleave. 

So some flame comes back up through the mounts? They'll run WAY cooler anyway. 

There are other things to try but I'm not going to mention any until you've tried the first ones and played with them a little. 

You're in the same position everybody is when starting a new craft, you're looking for the ideal . . . thingy without knowing what works, doesn't, what you want as opposed to what you need. Everybody has to figure it out, nobody's born knowing any of this stuff. Heck I just make a lot of it up as I go.

If you start changing multiple things you will NEVER know what worked, didn't or why. Change ONE thing at a time, fire it up, evaluate the results and write what you did and what happened. If it didn't work do NOT try to fix it! put it bac the way it was and try another something. Rinse, repeat until you get there. OR you figure out enough of how these things work to make it work.

Frosty The Lucy.

[Dr.X]

Frosty, you're brilliant.

The new set up after 10 minutes or so:

  compared to before:

[Frosty]

Aw Pshaw, I've just been messing with these things longer than most and made more mistakes than I can count.

It's looking much better, keep tinkering though. Done any welding with it?

Frosty The Lucky.

[Dr.X]

I've never tried to make Damascus. Really I just did it because I like to tinker with things and wanted to use less propane.

Honestly, though, what a huge difference by just advancing the burners a cm or so. Amazing.

[Frosty]

You don't have to make damascus to make a forge weld, you know.

Frosty The Lucky.