Blown burner problems

[Marius A Bacher]

Hi, I just got my new vertical forge up and running.

I've run into a problem with the burner, It's a blown burner with 2" dia mixing chamber and necked down to 1" at forge, the blower is a hairdryer controlled by a dimmer and a ball valve.

I get a good air/fuel mixture at good heat and with small flames out the doors, but after 30min to an hour of forging there's a loud bang and the air/fuel mixture starts to burn inside the 2" pipe.

 

Have any of you had this problem and/or know how to fix it.

 

Here's a picture of the setup.

 

Any helpful suggestions would be very appreciated

 

Thanks!

[KPeacock]

Without seeing/hearing it myself, I'd guess that your pipe into the forge is getting too hot and eventually gets hot enough to autoignite the fuel and air beore it reaches the forge body. A quick way to tst and see is to wrap an old shop rag around the pipe near the forge body. Soak it with water and then start the forge. Keep the rag wet and it will keep the pipe below the temp needed to auto ignite propane. If that "fixes" it, then at least you know what the problem is. After that its a matter of insulating it properly so you don't have to keep pouring water on the thing.

[Marius A Bacher]

Thanks a lot.

 

How would you insulate it, I thought about using ceramicwool but won't that just let it get hot faster?

Maybe I could make a heat sink or something?

[Geoff Keyes]

How deep into the combustion chamber does the pipe extend? It looks like you are heating the last bit of your burner quite a bit, from the discolorasation of the pipe.

 

Try shortening the nozzle so that it doesn't extend into the combustion chamber, and pack the tube that it goes into with some scraps of Kaowool to prevent blowback.

 

Let us know how it goes,

 

Geoff

[Marius A Bacher]

Thanks Geoff.

 

You're right, I had the 1" pipe to far into the forge at first, that's why it's discolored, It was actually inside the forge chamber.

I pulled it out so the end is in the middle of the 3 layers of 1" Superwool lining. Should I pull it out further and have the end inside the pipe on the forge?

[KPeacock]

well, before you go and change the geometry of it, I'd try the wet rag thing first to see if thats the problem. My burner nozzle just barely extends into the body of the forge. I have my burner held in place with three bolts contacting the burner tube. there is not too much heat transfer to the burner tube with ounly three points of contact. The tube does still get hot to the touch after a few hours of forging. I rarely run the forge for more than 4 hours at a time and have not experienced the problem you're describing.

[Brian Madigan]

Does the explosion blow the blower right off the top? It does look like your mixing chamber is backing up and igniting. Maybe try moving the mixing chamber further away from the burner.

Does the burner end in a flare or straight pipe?

 

I'm not an expert on blown burners.

[Marius A Bacher]

Yeah I'll try the wet rag first and see if heat is the problem.

 

No, it doesn't blow the blower off it just starts burning inside the mixing chamber.

I have no flare on it, it's just the 1" iron pipe.

 

Thanks guys. I'll try your ideas tomorrow and report back.

I hope it's a quick fix, I really need to get a couple of knifes and maybe a sword forged before winter sets in here, so that I'll have something to work with through the snowy months.

[Sam Salvati]

Marius, I think maybe the hair dryer is not making enough pressure.

[Marius A Bacher]

Maybe Sam.

The hairdryer can give more than enough air in the mixture and make the fire really oxidising though.

I'm a bit confused about the whole pressure vs. volume thing.

It might be a blower size/fan speed issue, I really don't know.

 

I'll get back to you guys tomorrow with the results from the wet rag test and we'll take it from there.

 

Thanks.

[Doug Lester]

Are you having burning gas blow out of the forge around the burning pipe? I had that problem with the first forge that I built. It had a venturi burner. I just wedged some ceramic fiber between the outer wall of the burner tube and the inner wall of the mounting tube. That kept the burner tube from getting red hot.

 

Doug Lester

[Marius A Bacher]

I got some time at the forge and tried wrapping the burner pipe with a wet rag, it worked, I did not get any blow back at all.

 

Here's a pic of the burner with the rag on at heat.

 

I also pulled the burner a little further out of the forge so it extends just past the outer shell.

As you can see it generated a lot of steam so the burner tube is getting hot.

 

Doug, I haven't seen any flames coming out of the mounting tube. I can fire it up again later today and take a closer look.

 

I could try wrapping the burner tube with Superwool, but I don't think it will fit inside the mount.

The mount is a 40mm pipe with a wall thickness of about 3mm and the burner is 1" so there's not much space there.

 

Any other suggestions?

 

Thanks

[Mythbuster]

Marius, your problem is that the air gas mix is to slow, it must be faster than the flame speed of your combustible mix, not to ignite inside the tube. I think Sam Salvati is right, you need more pressure. You can try two things, reduce the diameter of the burner tube, this will speed up the flow if your hair dryer is building up enough pressure. But I´m afraid it won´t and there is a good change that this measure will stall your flow.

2nd try to get a better blower (suitable for heavy duty and continous operation).

 

MythBuster

[nakedanvil]

I like to use a burner tip kinda like a cutting torch tip. Usually have about six 3/8" holes. I like to use a really solid chunk of SS the same as the OD of the pipe and 2" long and weld this to the end of the pipe. Mount it so it is a little back of flush with the inside of the forge. Some people use concentric rings of pipe. This increases the velocity and usually prevents flash-back.

[nakedanvil]

OK I took some pictures. Yes, the burner is in the floor in this one. In an alcove off to one side. Yes again, it is two forges side-by-side. These forges have probably 300 hours running on them, sometimes at welding heat. The floor is a cast refractory and the shell is a from a vacuum mold I did many years ago.

Edited October 21, 2009 by nakedanvil

[Marius A Bacher]

Thanks!

 

I think I'll try to make a burner tip first because it's the quickest thing to test.

If that doesn't work I'll look for a more powerful blower, I predict I'll have to make one though.

 

Thanks again to all of you guys!

I'll report back with the results.

 

btw. Nice forge. How big is that burner tube?

[Matt Bower]

That's a very interesting forge design, Grant. What made you decide to bring the burner in from the bottom?

[Ken Burns]

I had same problem with mine when I tried to lower temp to heat treat in 'forging' forge - flame front would burn back into burner and start 'poping'. I made a 'bushing' that I can jam in burner outlet that reduces burner outlet area about half - speeds up air flow. No more problem.

ksb

[Beau Erwin]

Here's a relatively inexpensive solution

https://www.surpluscenter.com/item.asp?UID=2009102122591081&item=16-1353&catname=electric

 

I bought one a while back but I thought mine had a thin guage sheet metal body.

 

I too think the problem is not enough pressure from the air. The hair dryer first has to fill the 2" pipe and then push it into the 1", and it just doesn't put out the volume to to fill the 2" fast enough, the one above would have no trouble at all.

 

That one I listed puts out 100CFM of air where i know the hair dryer isn't coming close to that, so it'd be able to keep up easy, and might even help keep the pipe going into the forge cooler, as mine is arranged pretty much the very same way and I don't think it gets all that hot.

[Marius A Bacher]

I made a burner tip from a washer with more holes drilled in it and it seams to have done the trick.

I forged a couple of hours today without the wet rag on and had no blow back.

 

Thank you Edgar. I'll probably buy a blower like that too, I'm looking for one here in Norway, but if I can't find one I know where to get it.

 

Thanks guys!

[nakedanvil]

Matt, Re: Floor mounting. I like to use the the flame to heat the walls of the forge and let the radiant heat from the walls heat the steel. I also don't like the flame hitting the work until combustion is complete. Floor mounting gave the longest path and satisfied both of the above desires.