More Forced Air Ribbon Burner Questions

[Chris C]

I've read everything I can find, and have done tons of searches and not found my answers or I wouldn't be asking these questions.

I'm a total newbie.  Trying to design my first forge.  From all I've read, I've decided a FARB is the direction I want to go, for my purposes.  I'm trying to get all this "forced air" stuff to make sense in my feeble brain.  It would seem to me that the total area of the holes in the burner should equal the area of the incoming pipe through which the forced air and gas travel.  My forge will end up with a 6"x6"x12" interior.  I've not welded it up yet, but my burner tube is 2.75"x3"x10.5".  Incoming pipe is 2".  The area of a 2" (mine is actually 2.080") is 3.397946614".  I think I prefer smaller than crayon diameter holes so I'm planning on using plastic straws.  The ones I presently have on hand to measure (maybe not what I'll end up using) measure 9/64th".  Their area is .06212622191".  So if my calculations are correct, that would mean I need 54 straws.  That's going to be difficult to cast around.

So, a couple of questions:  First, is it important, with forced air, to match the area of the exit holes to the incoming pipe as I've shown?  Second, with the proposed 432 cubic inch interior of my forge, is the 10.5" burner too large?  Oh, and another thing..................since the forge cavity is square would introducing the burner from the side.........toward the top of the cavity, be more conducive to even heat than coming directly down from the top of the forge, or does it make any difference?

[D.Rotblatt]

My 2 cents.  I've got one FARB and one NARB.  The forced air is pretty much as Wayne describes.  The NARB is my own modification; an 8 x 2.5" using 122 holes @ 1/8" each.  

On 5/26/2019 at 10:45 AM, Chris C The Curious said:

It would seem to me that the total area of the holes in the burner should equal the area of the incoming pipe through which the forced air and gas travel.

This doesn't really apply.  The smaller the holes, the more the friction, so equal area with more smaller holes means less air then you expect.  Besides, you are making a forced air unit, so you can push as much air as the fan will allow.  I would make your burner with as many holes as possible, you can always plug them up.  Better more holes, and plug them if you need to then not enough and have to recast.  Example: I have a NARB with 122 1/8" holes. The injector is a standard 3/4" riel burner with a 1" nozzle, giving it an area of .79".  The 122 x .125" nozzles on the ribbon burner head it runs have an area of 1.46", about twice as large.  

If you want to test your idea, put together all the metal parts and use a piece of wood with holes drilled in it for test burns.  You can't run it more then 15 seconds or so, but you will be able to see if the number vs size vs CFM works well.  Really, as long as you can push enough air through the holes you'll be fine.  That's the advantage of a FARB.  I like the idea of using smaller holes.  1/8" has worked very well for me allowing me to run the burner at very low pressure and up to welding.  Yours are .14 which is close.  

On 5/26/2019 at 10:45 AM, Chris C The Curious said:

I've not welded it up yet, but my burner tube is 2.75"x3"x10.5".  Incoming pipe is 2".

I assume you mean plenum when you say "burner tube".  Square pipe?  The length Ive made mine is 1.5 to 2" on either side shorter then the inside length of the forge.  So 10.5 is a little long, but it would work.  9-10" would be the max length.  Others may disagree.

 

On 5/26/2019 at 10:45 AM, Chris C The Curious said:

Incoming pipe is 2". 

2" pipe is great.  The bigger the better.

 

On 5/26/2019 at 10:45 AM, Chris C The Curious said:

since the forge cavity is square would introducing the burner from the side.........toward the top of the cavity

From the top at a slight angle is what many do.  With smaller holes like you have combustion completes closer to the burner face.  My NARB has the burner pointing straight down, seems to work well for me, nice even heat.  My FARB, which has larger nozzle holes, has the burner coming in horizontally, but it's a round forge so the circles the interior.   Bottom line, both work.  With a ribbon burner you'll get an even flame the length of the forge.

On 5/26/2019 at 10:45 AM, Chris C The Curious said:

The ones I presently have on hand to measure (maybe not what I'll end up using) measure 9/64th".

At about .11 inches, the flame cannot burn up the hole and you will not get backfire.  There's some discussion of this in the NARB section.  I've found this to be true at .125" which is what I have on mine.  You are talking about .14", so it should still work well.  

Dan R

[D.Rotblatt]

On 5/27/2019 at 12:43 AM, D.Rotblatt said:

The NARB is my own modification; an 8 x 2.5" using 122 holes @ 1/8" each.  

I just reread my own post and that potentially comes off wrong, sounds like I invented the NARB. Wayne Coe first made the ribbon burner and printed instructions which were copied by many for years. Frosty modified Wayne’s ribbon burner to run with a naturally aspirated burner instead of forced air. I modified it further by reducing the hole size and increasing the number. Just a lucky addition to some great work before me.