Advice for my ribbon burner forge design

[Gustav]

Hi everyone!

I've recently started designing a new forge I'm going to build out of a 13.5 kg propane tank. It will only be used for knife forging, so it does not have to be big.

I'm writing this basically to get your opinion on what dimensions to change, or if complete aspects of the design need to be changed. I also have tons of questions.

Here's where I am at now:

 

 

These dimensions are a mixture of what I could find online and of what may be suitable for me. For instance, I found some 50x50mm square pipe laying around, so it would be nice to use it for the ribbon burner if it isn't a stupid idea to have so "small". This also means that I can't properly fit a 2" pipe to the burner, but 1.5" should work as well?

Here are some questions regarding the dimensions:

1. The inner diameter of the forge (145 mm) feels fine to me, since I will not forge anything that requires a larger volume. However, is it better to change it to improve things like circulation?

2. The thinnest parts of the refractory (15 mm and 20 mm) is something I made up on the go. What is the recommended minimum thickness?

3. I plan on cutting the tank into three pieces and casting them separately, but what about not cutting off the back and casting it as a whole? Anyone who have tried that?

4. I currently plan on having 2 layers of ceramic wool in the middle section. Is this overkill? Only having one lets me have thicker refractory.

5. Some people cast room for firebricks, some don't. Is there really a big need for this, since the forge will be cast in refractory either way? If people use their forges daily, I could understand, but I will definitely not use it that much. For some reason, I wouldn't even mind having a circular inside of the forge instead of flat to enable better circulation (OCD reasons lol).

 

Ribbon burner:

6. I've tried to grasp a general rule on [number of holes] * [hole diameter] with respect to [intake pipe area]. If I go for 1.5" or 2" pipe, what's the recommended amount of holes if they are each 8 mm in diameter? I've seen one youtuber "fill up" the pipe with straws to see how many are needed. Is this a good rule of thumb?

7. The spacing between the holes (15 mm center to center) may be too tight, what's the minimum recommended distance (hole to hole, hole to wall)?

 

Gas piping:

8. Does the gas system have to be designed differently if the burner pipe is either 1.5" or 2" pipe (obviously not talking about the final adapter)? Or will 2" pipe work fine for the entirety of the piping? What about the propane piping, should it be 1/2", 1/4"?

9. Would a 29 mbar regulator from the propane tank suffice?

10. I have a 230V blower that I've used in my coal forge, and I'd like to use it in this forge to avoid buying a new one. There is no "cubic feet per minute" rating of the sort, but I recorded a video trying to show the airflow. Do you think it's powerful enough?

11. The end of the propane piping that "sticks into" the "air pipe"; what should it look like? No MIG-tips as I understand it?

 

As of now, I am struggling to find suitable refractory and ceramic wool. If anyone in Sweden reads this and knows a good supplier, please let me know! I've seen someone use Höganäs Low-Cement but I can't find it anywhere.

Also, I have yet to design the front and back "porches" to the openings, a way to fasten the burner, legs to the forge etc.

There are more questions that will arise but I'll leave it here for now.

Thoughts?

 

[Buzzkill]

I have a few thoughts, but most are my opinion.

That seems like a lot of burner for a small forge.   Most people want to make a forge that is too large.  Your design is not.  You say you want to make knives, but forging a large kukri style knife needs a different forge size/shape than a puukko. 

Personally I prefer the "D" shape forge so I have a flat floor where the forge interior is at its widest.  Mine is nearly 9" across at that point and is right for me.  You may not need as much space.

If you are using a quality high alumina refractory it does not need to be that thick.  I think of the refractory as armor to protect the insulation from damage.  The thicker the refractory, the more heat is absorbed.  That translates into more time required to bring the forge up to temperature.  For everything but the floor I'd probably stay in the  6 to 8 mm range. 

As far as casting a recessed area on the floor for a brick or kiln shelf, for me this is determined by whether you intend to forge weld.  If you plan to make pattern welded billets and use flux then I do recommend casting in such a way that you can use a replaceable floor slab.  Over time flux tends to make a mess of your floor and it's nice to be able to replace that portion without having to refurbish the entire forge.

Keep in mind that the forge interior will deteriorate with use.  The combination of thermal shock, mechanical damage, and some chemical attack from burning fuel/gas and flux takes a toll.  Nothing I've seen is impervious to those conditions.  You will get cracks in the refractory.  Coatings such as Plistex, Matrikote, or ITC will flake off or be rubbed off and have to be patched or replaced periodically.

Since you are planning to use forced air on your burner it doesn't require a high level of precision.  I didn't see any glaring problems with the design.  Feeding it with a 1.5" pipe shouldn't be a problem.  However, you do want an elbow and/or a change in pipe diameter between the point where the fuel is introduced into the air flow and the burner.  This is to aid in mixing the air and the fuel gas. Propane and air don't naturally mix well, so creating turbulence before the fuel/air mixture reaches the burner is a good thing.

For a blown burner the fuel pressure is not nearly as important as volume.  If your regulator allows enough throughput it doesn't matter that much if it's at a low pressure.   However, you shouldn't use a highly restrictive orifice (like a MIG tip) to inject the fuel into the air stream if you have low pressure. The only real value (in my opinion) of knowing the pressure is to note at what pressure the forge maintains the heat you desire.  That is still variable depending on what you do with the air.  You need to be able to control the flow of air and fuel independently in order to achieve the desired atmosphere (oxidizing, neutral, or reducing) over a range of temperatures.

While it's a good idea to have a specific plan with precise dimensions for assessing the quantity of materials needed, when it comes to functionality that level of precision is not needed, nor is it likely to be attained.

Generally speaking it looks like you are on the right path.  Without knowing the largest knife you plan to forge and it's shape I can't say whether your design is appropriate.  If you plan to do any forge welding you may also find your designed chamber a little restrictive for placing a stack in when starting a billet.

Hope that helps.

 

[Mikey98118]

I agree with Buzzkill. Also, while larger is generally not better, a little larger to ensure enough room to do your work is a good idea. Furthermore, I believe that ribbon burners heat larger equipment interiors better than highly restricted ones, so a little more interior room is offset by that tendency too.

Finally, the inner shape of your forge need not be determined by its exterior. Some guys are building "D" shaped forges in cylindrical shells; just a thought.

[Frosty]

Gustav, your post showed up on my computer again! 

I'm afraid you're looking for what most of us wanted when we started tinkering with multiple orifice burners, ribbons in this case. What I and many others have discovered is a lack of basic rules of thumb for building them, without using advanced math. Sooooo, I experimented by replacing the "burner block" with 2" x 2" lumber, drilled with holes the same dia. as the crayons I used for cores when casting the final burner block.

Confusing? The terminology for the parts we on Iforge have come to use goes like this. Your 2" sq. tubing makes the "Plenum" where the incoming air can equalize in flow and pressure over "all" the nozzlettes. The "nozzlettes" means little nozzles and are the holes through the cast refractory burner block. 

The inlet is the coupling through the plenum it connects to the piping leading from your mixing tube and blower and propane jet. 

In my experiments I got poor results with the different diffusers, some very much like what you drew. The curved and perforated strip of steel in the plenum to disperse and help equalize the flow from the inlet to all the nozzlettes. It didn't work worth spit for me and I tried some pretty elaborate diffusers.

By not working I mean the length of flames from the mock up burner blocks were very uneven usually longest in the center and diminishing to close to the ends where they got longer again. That was the most common flame pattern I got using a diffuser.

My most elaborate diffuser was a second 2" x 2" lengths of square tubing stacked and welded together. The tube the burner block was to be cast onto was of course opened along one side. I found that using a reciprocating saw and leaving a little lip around the opening allowed the refractory to bond much better. I then drilled a lot of small holes between the two pieces of tubing. They were much smaller than the crayons I used for the nozzelettes. Finally I stuffed the top tube with steel wool.

It didn't work so well, flames were still long in the center and ends and short between, it looked sort of like a wave.

So I tried putting the inlet in the side of the plenum so the air fuel mix impacts on the far wall of the sq. tube (plenum) and the flames evened out considerably. No, they're not completely even but they're close enough I have a forge chamber with darned even temperatures end to end.

I haven't built one in quite a while and probably won't again, arthritis in my thumbs makes blacksmithing unattractive anymore. It's pain that is easily avoided but were I to make another one I have some 2" x 3" rectangular tubing I have left over from a job and would see how a larger plenum would effect the flames. That experiment would have the plenum 3" tall and still 2" wide giving the plenum a much larger volume for the pressure and flow to equalize.

For the hard refractory it has to be a calcite bonded refractory and NOT a refractory "Cement or mortar." Cements and mortars are for sticking bricks and tiles together in a high temperature environment, they are NOT intended to be in direct contact with a propane flame. Propane flames like are produced by a burner such as we use are VERY chemically active and tend to dissolve refractory mortars and cements because they're made to be sticky, not chemically inert in a high temperature environment.

I don't know if it is available to you but it is sold internationally so it might be. I use "Kast-o-lite 30 li." a 3,000f, water setting, insulating, high alumina castable refractory. I've found about 3/8" thick is more than enough and a final kiln wash or plistex 900 provides a final bit of armor against forge welding fluxes based on Borax. Molten borax is VERY caustic and dissolves silica based masonry like water through sugar cubes. 

The above is just a general outline of my experiences, yours will differ of course but we'll be happy to help.

Frosty The Lucky.

[Buzzkill]

Frosty, it appears as though he's planning to use a blower on his burner, and unless I'm mistaken you were giving advice based on your experience with a naturally aspirated ribbon burner.   I'd hate to confuse the guy.

[Frosty]

Yes, I'm acutely aware of the difference between naturally aspirated and gun burners. Nothing I said is any different for a gun / blower driven ribbon burner. 

He already has the basic idea for trying to diffuse the fuel air flow for a more even distribution across the nozzlettes. My experience is his idea doesn't work with the inlet aimed directly into them. Mounting the inlet on the side of the plenum worked far better than anything else I tried.

I know I got into a ramble, hopefully anything he doesn't understand or is confusing, he'll just ask and I'll do better.

Frosty The Lucky.

[Buzzkill]

Fair enough. I have limited experience with blown burners, but I don't recall having difficulty getting fairly even flames using a rear mounted inlet and a diffuser.  However, I never tried a side mounted inlet on a blown burner, so I can't personally compare the two designs.

[Frosty]

Did you build yours or using a commercial ribbon? If you're getting even flames from a home built I'd be very interested in your design.

Frosty The Lucky.

[Buzzkill]

I built it.  However, I didn't like the noise of the blower (I had a bouncy house blower) and being tied to power, so I didn't use it for long.  I went back to a standard T burner until you introduced the NARB to the forum.  I've tinkered with a number of variations of NARBs since then, but I don't foresee ever going back to a blown burner or a single port burner.

I was trying to figure out how many years ago it was that I used a blown burner based on that progression.  It had to be at least 7 years ago now, and I can't tell you the exact dimensions of the design.  If I still have notes from that project I'm not sure where I put them.  I did (and do) a lot of tinkering with burners, but I have a tendency to toss records of the things I didn't feel turned out well.  If I don't do that then I have piles of papers with "bad" designs and over time I forget which ones are worthwhile.

[Frosty]

Oh MY that's way more blower than I'd put on a burner I didn't want to use as an engine! I started picking up mattress inflator blowers when I stumbled onto a Coleman Inflatall. It'll blow up an air mattress in about 20 seconds. Then I started seeing them at yard/garage/etc. sales for a couple bucks so I picked up a few. Most were 120v. then I started seeing 12vdc. 

If I ever wanted to make a gun burner I'd use one of those but holy MOLY would it be a beast of a burner! I thought of a couple ways to determine what volume of air a 3/4" T burner put out by inflating a known volume for time, say a large plastic laundry or leaf bag.Bbut was never interested enough to give it a try.

The fellow in the club who hosts meetings has a propane burner that is a blower with a propane connection. The propane is introduced into the blower fan, the blower is adjustable and a regulator takes care of the propane. I'd really like having one but evidently they aren't being made anymore. You have to turn it way down to use in a 20lb. propane tank forge and it handles 50lb. bronze melts effortlessly turned to less tan 1/2 blow.

I would not mind using that little gun burner but. . . I'll be sticking to the T burner I guess.

Frosty The Lucky.

[Mikey98118]

1. The inner diameter of the forge (145 mm) feels fine to me, since I will not forge anything that requires a larger volume. However, is it better to change it to improve things like circulation?

I think you will find that your ribbon burner is simply too large for the space you plan to use. You would do better to increase the space 25%, or reduce your burner that much.

2. The thinnest parts of the refractory (15 mm and 20 mm) is something I made up on the go. What is the recommended minimum thickness?

Fifteen millimeters should do just fine.

3. I plan on cutting the tank into three pieces and casting them separately, but what about not cutting off the back and casting it as a whole? Anyone who have tried that?

What about not cutting anything, and simply casting each area, one at a time; then whetting the surface of the cast areas, before casting the next sections against them.

4. I currently plan on having 2 layers of ceramic wool in the middle section. Is this overkill? Only having one lets me have thicker refractory.

Thicker refractory is okay, only if that refractory is a high alumina type, with something included in it to create air voids. That something can be as basic as Perlite, or high tech bubble alumina.

5. Some people cast room for firebricks, some don't. Is there really a big need for this, since the forge will be cast in refractory either way? If people use their forges daily, I could understand, but I will definitely not use it that much. For some reason, I wouldn't even mind having a circular inside of the forge instead of flat to enable better circulation (OCD reasons lol).

Forget the firebricks, and use a high alumina kiln shelf, from a pottery supply store, instead. The shelf can simply lay on top of the floor, and extend outside the front and back openings, to serve as the porch you want. When covered in hardened fluxed, it can be removed, so the flux can be removed with a wire wheel.

    As to avoiding a flat floor area, to improve circulation, you are over thinking the problem. Circulation does just fine in loads of forges with flat floors.

[Steve Sells]

What does that post have to do with the question about ribbon burners?

[Gustav]

Thanks for the answers!

I'll definitely reconsider the size of the burner. I might go for 175 mm or 150 mm in length instead. And yes, it is indeed going to be a forced air burner. And regarding the refractory; Kast-o-lite is unfortunately not available in Sweden, but it is good to know some key words to look for (high alumnia, calcite).

If any mod/admin sees this, please remove THIS post we are in right now. This post is actually a duplicate of the original post. It appeared when I tried to edit the OG post, so let's continue this conversation over there:

 

[Frosty]

Just a kid with delusions of impressing the adults by stating things he doesn't understand like he's an expert. 

7 hours ago, happy days said:

Quit a few home made gas forges have been known to of  blow their make up , due to restricting to much air flow.

I'm just wondering what kind of make up forges should wear and why.

[Gustav]

Mikey:
1. I'll change length of the burner to either 150 mm or 175 mm!
3. I'm not sure I can picture the process of casting without cutting anything. I'll probably end up cutting it in 2-3 pieces.
4. I'll look for high alumnia refractory! I saw one with it as "main binding component", and it was rated for 1300 °C (2372 °F). However, I feel safer with 1400 or 1500 °C (2552 or 2732 F).

 

[Mikey98118]

10 hours ago, Gustav said:

I'm not sure I can picture the process of casting without cutting anything. I'll probably end up cutting it in 2-3 pieces.

So long as you wet the existing surface of the first  cast section, before casting each following section, they will all adhere to one another; this allows you to simply move the outer shell into different positions, and cast complex shapes, section by section.

[Gustav]

@Mikey98118

Alright, I'll give it some more thought!

Just a small update on the progress:

The propane tank has been emptied, filled with water, and drained (each with 24 h between). I've cut away the bottom "ring" but have yet to slice the main tank up in any way.

Next up: paint stripper and cleanup

[Steve Sells]

ok lets get back on topic,  I have had about enough of this good bye Happy days

[1forgeur]

Thank you

[Frosty]

I apologize to the forum for engaging him. I was hoping he might come along, we've had a few other mouthy kids turn into pretty good members. Instead I fed the troll. My bad.

Frosty The Lucky.

[Gustav]

Some people just have to spread their misery to others.

 

Regarding the forge, I have found some Superwool online that I'll be able to use. There are some refractories I've found that can handle the temperatures, but I'm not sure about the contents. For instance, we have Höganäs Cast CC 50 (I hope your browser can translate if you want to take a look). It's contents are e.g.:

48% Al2O3, 43% SiO2, 6.2% CaO, 1,3% Fe2O3

Anything of this which is too much or too little? I don't believe this one in particular is the best for forges, but it's one I found at least. They make other types with "high alumina" but I couldn't find them available anywhere.

This company lists lots of different types of cement/refractory with their contents. Roughly speaking, what should I aim for in terms of the percentages of Al2O3, SiO2, CaO, Fe2O3, etc? If I know this, I could know if some random bag of refractory is suitable.

[Mikey98118]

2 hours ago, Gustav said:

48% Al2O3, 43% SiO2, 6.2% CaO, 1,3% Fe2O3

That might as well be clay; the silicon content is way too high, and the alumina content is way to low!

[Gustav]

Alright, the search continues.

[Gustav]

Found Superwool (25x610x7320mm) which is rated for 1200C (2192F) continuous, 1300C (2372F) max. Is this enough? It will won't be exposed to direct flames, instead deep inside of the refractory. The size is more than I need but I'll sell what I don't use up.

[Buzzkill]

Yes, that will be fine as long as it isn't directly exposed to flames - which it shouldn't be.  Once  it's coated/covered with refractory it should be fine for everything through forge welding temperatures.