Gas forge build advice

[Gerhard Gerber]

I have almost everything I need to for the forge build.

For the body of the forge my only option till now was the tank from an old geyser, didn't like the idea because of all the insulation that needs to be cleaned off, and I have no idea what shape the tank is in.

Somehow I prefer something that's not round, just got a call back from a local shop that will be able to assist with the sheet metal work.

 

I'm leaning towards a rectangular box, depth equal to the width of the kao wool I have, and one of the top corners 45'ed lengthwise where the ribbon burner will mount.

I've seen several like that on videos, and with my welding and fabrication skills I would prefer not dealing with round surfaces.

 

One video I watched covering a forge build (round forge) they put in the kao wool, then a pipe of slightly smaller diameter and the refractory cement was poured into the gap and allowed to dry.
Not sure how to manage that with a square forge, but I want to find out if this is the only correct way, or can you plaster the koa wool by hand basically?

[Alan Longmire]

4 hours ago, Gerhard Gerber said:

or can you plaster the koa wool by hand basically?

 

Depends on what your refractory cement is.  Castable, you cast it.  Rammable, you ram up like a sand mold, you might be able to do that by hand, but a liner/form is still helpful.  Trowelable like Satanite, you can slap on by hand.    I don't know what's available to you, though.

[Jerrod Miller]

2 hours ago, Alan Longmire said:

Castable, you cast it.  Rammable, you ram up like a sand mold, you might be able to do that by hand, but a liner/form is still helpful.

I do not recommend this at all, because it is not an area where I would consider myself an expert on, but I have thinned down castable and rammable refractories to be applied via trowel (gloved hand).  This is with foundry specific material, not with those generally offered for forge building.  But if you have spare material and wish to experiment, it may work out.  

[Gerhard Gerber]

Don't mean to harp on about it, but TIA, life is tough in the 3rd World 

I have a temperature rating and that's about it, nothing on the container, no idea what the brand/manufacturer is 

Thank you @Jerrod Miller, I believe the fact I have 25 litres lends itself to some experimentation.

[Gerhard Gerber]

I've found somebody that can build the steel part for me, ends supplied loose so I can weld them in after insulation is done.
Hope to buy plumbing this weekend, been doing the usual research, one thing I can't figure out is where to bring in the gas.

On the ribbon burner forge I've seen with my own eyes the gas feed was higher up, closer to the burner.  Will confirm my recollection, but I believe feed was still through a MIG nozzle.
One complete build video I've watched the gas feed is right above the valve that controls air flow, which is basically attached to the fan.  This would mean most of the up pipe to the burner is a gas/air mix, ready to go boom.....?
I'm getting the needed tubing and elbows, valve for the air and needle valve for the gas, so there will be a ball and needle valve on the gas feed.
I believe feeding the gas without a nozzle would be much easier for me to accomplish, but it needs to work first and foremost.

[Dan Hertzson]

In my experience small (say MIG nozzles at 0.28 - 0.35) nozzle/orifices for the gas supply are not required or recommended for most forced air burner designs.  The orifices are typically included in naturally aspirated (NA) burners (Ventauri) where the high velocity gas exiting the nozzle is used to induce the combustion air into the mixing chamber.  Depending on the type of  gas being burned and the acceptable pressure loss for that gas you typically can get away with a connection somewhere between 1/8" and 1/2" in diameter for a forced air burner (larger connection for lower pressure gas.  There is a significant difference between the required port for residential pressure natural gas at 1/4 psi and propane at 5-30 psi).  Note: in addition to the isolation ball valve for the gas source, I strongly recommend use of an adjustable regulator.  I'm not a big fan of needle valves and would only use them if you need to run multiple burners off a single regulated source or your gas regulator is so remote from your forge that is is difficult to access to adjust the gas/air ratio while observing the flame.

 

The desired length of mixing tube (from where the gas and air are joined out to the burner outlet) will vary depending on your overall system configuration.  As may be anticipated from the name, the purpose is to get the two well mixed so you achieve good combustion at the burner outlet.   You are correct that the mixture in the tube will be potentially "explosive", which is why it is important not to let this tube get too hot and maintain the correct flowrate of gas air mixture to avoid preignition and "pop-back".

 

There are a number of different strategies to reduce this length (most of which add friction to the system which may have an effect on your blower selection) including:

1. Adding sharp elbows in the mixing tube routing to induce some turbulence
2. Adding some form of perforated pressure plate or swirl inducer prior to the burner outlet (to some extent the ribbon burner outlets accomplish this, but not always very effectively)
3. Putting a 90 degree bend in the gas entry to the mixing tube so the gas is injected pointing in opposition to the flow of air 

Note: using a forced air burner is different than a NA burner.  There are recommended procedures for tuning, lighting, shutting off, and turning down from high to low fire that need to be followed.  They aren't too difficult to work out, and I actually find it easier to tune my low pressure natural gas forced air system than a NA propane burner, but I've been using these for a while.

Edited March 2 by Dan Hertzson

[Gerhard Gerber]

Thank you @Dan Hertzson
I do have an adjustable regulator.

Gas fittings I'm hoping to do in 10mm, same as the existing ball valve I have, and one of the two sizes of tap I own  

I'm planning at least one 90 degree bend, actually need a 45 degree as well, but I don't think I've ever seen a 45 degree elbow, will find out on Saturday.
Uncle Neels who is advising me was adamant about an angle iron flare to spread the gas inside the burner, I'll have another long talk with him before I start the build.

[Dan Hertzson]

Don't know what an "angle iron flare" is, but suspect it is a baffle plate inside the ribbon burner head?  It does make sense to configure your burner so the air/gas mixture is properly spread out to as many of the multiple ports as possible.  Again there are a number of ways to approach this, but I will refrain from going off on a tangent regarding multiport burner design.

 

45 degree elbows are standard pipe fittings.  Any decent plumbing supply shop should have them in any size or type that I can imagine you would need.

 

Careful regarding your use of taps.  Machine screw taps and plumbing fitting taps are different and not interchangeable.  The latter are tapered.

[Gerhard Gerber]

I got the 45's at the second shop, long & short same as the 90 degree elbows to give me some flexibility. Got everything I need except the needle valve and an easy way to mount the fan.

 

Quote

Don't know what an "angle iron flare" is, but suspect it is a baffle plate inside the ribbon burner head? 

Correct.
The air tap is just a big ball valve meant for water same as the pipe, irrigation type stuff (50mm)

On the gas sit I have some copper tubing and the ball valve tap meant for gas, wanted to describe how it connects but I ran out of English 
Found an adjustable regulator with connections to two bottles that I plan on getting as well, quick disconnect between that and the forge (which I also still have to buy)

I'm a bit worried about the welding from the plumbing to the burner, buddy pointed out the elbows are cast iron and not easy to weld.
Another issue is the square tubing part of the burner, I was advised to get 80x80mm, 3mm wall thickness, but closest I can find is 75x75mm (2.95x2.95") which looks a bit narrow.

Edited March 6 by Gerhard Gerber

[Dan Hertzson]

Hopefully you are not assembling your burner with all welded fittings.  Typical mixing chambers are in the 1 1/2" to 2" NPT size, which can pretty easily be assembled with threaded fittings.  I've never seen cast iron welded pipe fittings.  Usually any welded pipe fittings are steel.  Are you using cast iron sanitary plumbing fittings?  Those will either have a socket end for a hub and spigot joint or plain end for a no-hub style fitting (similar to the Fernco ones below).

 

Be careful with your selection of a quick connect.  The ones that are rated for gas service are a lot harder to source and more expensive than standard quick connect compressed air fittings.

 

Ball valves can work for metering airflow, but they are not a great choice (very non-linear operation).  Butterfly or gate valves will work better, but may be harder to source or more expensive.

 

I mount my blower either on the floor or to a metal stand that supports the rest of the burner assembly.  Depending on the configuration of the blower outlet, one way to nicely tie it into the burner assembly is using a Fernco style fitting (see below).  This gives you a flexible connection between two dissimilar types of systems (duct and pipe) that don't normally interface.

 

[Gerhard Gerber]

@Dan Hertzson updated my previous post with some pics, I believe the pipes & elbows are meant for irrigation.

[Gerhard Gerber]

This project has stalled a bit, mostly because I'm unsure about the measurements for the forge body.

I considered C(ardboard)AD but I don't have big enough pieces 
Made a scale drawing and I think I have my answer, but I'm unsure about a critical point - I should have enough kao wool to put a double layer in, is that advisable?

 

My dimension are 400x400mm (15,7"), top and one side is 250mm (11.8") with a 45deg section connecting then, this is where the burner fits.  Close as I can determine from my "scale" drawing the 45deg part will be 140mm(5,5") that needs to accept the burner which is 75mm (3") wide.

As far as I can estimate from my drawing the 45deg section (with double layers) should be almost the same width as the burner.

 

Inside dimensions ignoring the 45deg bit should be 240x240mm (9,4"x9,4"), 600mm (23.6") deep.  Does this sound okay?

 

I recently watch a video by Valhalla Ironworks, "Why your gas forge sucks". Very insightful, and my main take-away was those small uninsulated parts of the forge are your worst enemy.  

I can't think of an easy way to insulate the front and back, so my idea was to cut the openings the same size as the interior cavity.  Doors front & back, proper firebricks at the back and fireplace bricks at the front. 
Other option would be smaller doors and using my non-existent gynaecological skills to fit insulation after the front and back are welded in place.

[Gerhard Gerber]

Got the forge body, looks like my pen & paper calculations were close 

[Gerhard Gerber]

It's been a battle getting a needle valve for the gas, one local company can order, maybe, waiting to see if they reply.

One of the local makers with a working ribbon burner forge has been giving me advice, he uses the regulator to adjust the gas feed, no needle valve.
Considering my previous failure with the venturi burner build I'm a bit paranoid, I'd like to give myself the best chance for success, but it looks like the needle valve I can write on my stomach and wipe off with my shirt  

His setup has already changed my plans a bit, he has a T-piece between the pipe going to the forge and the downpipe to the blower, gas feeds into the other end of the T.

I'll see how far I get this weekend, but I might be able to combine a work trip to the neighbouring town with a long weekend in his shop, he has a lathe, 50+ years of experience and always willing to help, and I think I should take him up on his offer.
 

[Dan Hertzson]

On 3/2/2023 at 9:30 AM, Dan Hertzson said:

I'm not a big fan of needle valves and would only use them if you need to run multiple burners off a single regulated source or your gas regulator is so remote from your forge that is is difficult to access to adjust the gas/air ratio while observing the flame.

 

Repeating myself, but see above for my attitude regarding needle valves.  Basically an adjustable regulator is a better metering valve for gas IMHO.  Typically more expensive that a needle valve, but should give you more reliable flowrate control.

[timgunn]

There are two different approaches to achieving the same objective: progressive and repeatable control over the amount of gas being fed to the burner. You can either use a fixed gas jet (restriction) and vary the pressure fed to its upstream side, or you can feed a fixed pressure to the upstream side off a variable restriction (needle valve in this case). Both systems work. If you really want to make things awkward, you even can combine the two systems. 

 

Naturally-Aspirated burners tend to use fixed gas jets because they need the gas speed out of the jet to entrain air. Blown burners don't need to use the speed (I "think" it's actually the momentum of the gas that is conserved during an inelastic collision with the air, but it's a long time since I studied any Physics). Blown burners don't need the gas speed so can use much bigger gas "jets": so big that the term "jet" is barely applicable. 

 

If the needle valve is unavailable, you can copy what your mate has done. I'm guessing he has a fixed jet in the tee, pointing along the horizontal tube. It might be a drilled hole. It might be a mig tip. It might be something else. Whatever it is, it will have an area that means there is a measurable pressure upstream that is the output pressure of the regulator. He can vary the regulator pressure to vary the amount of gas flowing through the jet. Note that the flow through the jet will vary as the square root of the pressure difference across the jet. The pressure change on the downstream side of the jet will be small enough to ignore, so you only really need to consider the regulator pressure. If we say that maximum gas flow is at 32 PSI (I've picked easy numbers here), Half maximum gas flow will be at one-quarter pressure (flow varies as the square root of the pressure): 8 PSI. One-quarter-maximum gas flow will be at 1/4 x 1/4 = 1/16 maximum gas pressure: 2 PSI. 

 

As far as I can tell, Propane flow becomes "choked" somewhere around 35 PSI (-ish): the gas speed through the jet reaches the speed of sound and the pressure:flow relationship changes. NA burners tend to be used with regulators that give up to 30 PSI, 2 bar. 

 

If your mate's forge works well and you can copy it, it would be daft not to. If your forge is a wildly different size to his, I'd pick a gas "jet" size that would suit a Naturally Aspirated burner/forge of similar size to yours and otherwise copy his system.

 

 

[Jerrod Miller]

Tim, or anyone else that knows, please feel free to correct me if I'm wrong here, but isn't a regulator needle valve a good option for NA burners to control the atmosphere?  Sometimes you want a higher pressure (gas speed) and lower volume, sometimes you want a lower pressure (gas speed) and higher volume, since the gas speed affects the amount of air being sucked in.  This assumes the needle valve is near the burner and the distance and area/volume of the nozzle doesn't appreciably reduce the speed of the gas after the needle valve.  

[vlegski]

I'd follow Timgunns advice of copying ur friends. U know it works. 

The 45 degree diffuser is needed on top fed ribbon burners to ensure fuel distribution. I made my Frosty style Rb with a side fuel  entrance and it doesnt need a 45. I don't have a automatic fuel shut that some recommend as the when power goes out it acts like a Naturally aspirated burner albeit smaller flames.  Never had the flame go out. 

As to jet size I'm using the .35 mig tip that came with my NA  burner from High speed tool and refractory.,  took his burner, added a drop pipe with ball valve and blower like urs, tied in my ribbon burner, works fine so far.

 

[timgunn]

12 hours ago, Jerrod Miller said:

Tim, or anyone else that knows, please feel free to correct me if I'm wrong here, but isn't a regulator needle valve a good option for NA burners to control the atmosphere?  Sometimes you want a higher pressure (gas speed) and lower volume, sometimes you want a lower pressure (gas speed) and higher volume, since the gas speed affects the amount of air being sucked in.  This assumes the needle valve is near the burner and the distance and area/volume of the nozzle doesn't appreciably reduce the speed of the gas after the needle valve.  

 

It seems entirely logical, but NA burners don't tend to work like that.

 

Over a pretty wide range of gas pressures*, the Physics means that the fuel:air mixture, and therefore the flame temperature, remains more-or-less constant. Effectively, adjusting the gas pressure at the upstream side of the jet simply adjusts the amount of mixture being fed to the forge and, with the needle valve between the regulator and the gas jet, it is just another thing that can reduce the gas pressure at the jet.

 

If you want to vary the fuel:air mixture ratio, and thereby the flame temperature, you need some way of varying the relationship between the amount of gas flowing through the jet and the amount of air entrained. It would be great if there was a simple way of varying the jet area, but I've never seen anything that'll A/ work, B/ be affordable and C/ survive outside a loboratory environment. As a result, we tend to mess with the air side instead of the gas side and the control is usually achieved with an adjustable choke. Chokes range from fairly rudimentary sliding plates and sliding sleeves through to the screw-adjustment arrangements used on the British Amal atmospheric injectors and Australian Gameco burners. The latter can be adjusted to hold a temperature within a couple of degrees for hours, with any variation being down to ambient temperature changes. 

 

With a choked NA burner, initial tuning is usually done by changing jets until a temperature at, or just above, the highest working temperature expected to prove useful is achieved with the choke fully open. The temperature can then be reduced, and the forge atmosphere made more reducing, by closing down the choke.

 

I made a very poor video to illustrate this to someone during an online discussion some years ago. Apologies for the dire quality and complete lack of entertainment value, but it might help to explain it.

 

 

* <<1 PSI to at least 30 PSI is the range I've mainly played with, though it still seems to hold true up to 60 PSI. Things get a bit noisy for my liking above about 35 PSI, so I tend not to go there much.

[Doug Lester]

What do you mean by noisy?  If it sound like a jet engine revving up for takeoff then I would say that it is basically the nature of the beast.  If it is popping and sputtering then I would look to plumbing problems, however I would expect that when the pressure is too low which allows combustion in the burner tube.

 

Doug

[Gerhard Gerber]

Thank you everybody, especially @timgunn, you've put my mind at ease.

I got a T-piece and end cap this weekend, mig nozzle will be mounted into that.
Learned there is such things as NPT and BSP threads, so I hope to visit the uncle in Walvis Bay somewhere in the next 3 weeks. 

[timgunn]

Just the jet engine noise. No popping or sputtering; as You say, that usually tends to be more of an issue at low pressures, particularly at higher forge temperatures. I tend to run in the 2-30(ish) PSI range to keep between the risk of burning back down the burner tube and being too loud to concentrate. Others will have different ranges, depending on their equipment and tolerance of noise.

 

NPT is an American 60-degree thread form. BSP is a 55-degree Whitworth thread form. BSP certainly has tapered and parallel forms: BSPT and BSPP. The pipe od is the same for BSP and NPT and some, but only some, of the sizes have the same thread pitch: 1/2 and 3/4 from memory. At very low pressures, like those seen on burners downstream of the gas jet, these will usually screw together and seal with an anaerobic pipe seal. Most of the world uses BSP, with the US being the main exception. 

[Gerhard Gerber]

4 hours ago, timgunn said:

NPT is an American 60-degree thread form. BSP is a 55-degree Whitworth thread form. BSP certainly has tapered and parallel forms: BSPT and BSPP. The pipe od is the same for BSP and NPT and some, but only some, of the sizes have the same thread pitch: 1/2 and 3/4 from memory. At very low pressures, like those seen on burners downstream of the gas jet, these will usually screw together and seal with an anaerobic pipe seal. Most of the world uses BSP, with the US being the main exception. 

I'm a bit lost on this topic, my local advisor told me I'll need to get different thread cut on the mig tip.
Big advantage of this setup is I don't need to worry about a lot of the gas plumbing I'd planned, none of which I've seen readily available at the shops I've been to.

[Gerhard Gerber]

On 4/22/2023 at 4:06 AM, vlegski said:

I'd follow Timgunns advice of copying ur friends. U know it works. 

The 45 degree diffuser is needed on top fed ribbon burners to ensure fuel distribution. I made my Frosty style Rb with a side fuel  entrance and it doesnt need a 45. I don't have a automatic fuel shut that some recommend as the when power goes out it acts like a Naturally aspirated burner albeit smaller flames.  Never had the flame go out. 

As to jet size I'm using the .35 mig tip that came with my NA  burner from High speed tool and refractory.,  took his burner, added a drop pipe with ball valve and blower like urs, tied in my ribbon burner, works fine so far.

 

Please excuse me, going to stick with metric for now.....
I was advised to get 80x80mm square tubing with 3mm wall thickness for the burner, closest I could get locally is 74x75x3mm, so I'm worried the burner will be a bit narrow.
Also don't quite understand how to do a deflector to get the flame to the far ends of the burner without creating a dead spot in the middle.......not to mention how fiddly it will be welding something in there.
The side feed idea interests me because I've been contemplating using rectangle tubing that's 75mm on the short side, more head room.
Considering earlier advice about keeping the feed pipe cool I'm slightly worried about the flame hitting the steel side of the burner instead of the refractory face.
Or have I got this completely wrong and the flame is (supposed to be) only outside the burner? 

[timgunn]

The flame is supposed to be on the outside of the burner and on the inside of the forge chamber.

 

Basically, you have a fuel/air mixing section, which can be a Venturi in a Naturally Aspirated burner or "just" a gas feed into the pipe carrying the air in a blown burner, usually followed by a length of pipe in which the flow is turbulent and the air:fuel mixture becomes homogeneous, followed by a plenum where the mixture slows down and converts its Velocity to Pressure. Then the mixture flows through the holes in the ribbon burner block and burns in the forge. 

 

With a single-port burner, the mixture speed through the burner tube needs to be higher than the flame speed through the mixture. If the flame front gets faster than the mixture, the flame will travel back down the burner tube. 

 

The flame speed depends on a number of things, one of which is temperature (others include the mixture ratio and the pressure). In some cases, you might find that you start a Naturally Aspirated single-port burner in a forge at a certain pressure and everything is fine, then, as the temperature increases, the flame will start popping back down the burner tube because the higher temperature means the flame front is moving that way through the mixture faster than the mixture is flowing towards the forge. The usual quick-and-easy workaround is to turn up the pressure significantly.

 

In a multiport burner, each of the holes is effectively a little single-port burner. Mixture speed through each of the ports needs to be higher than the flame speed.

 

This is probably where the deflector comes in: in many designs, the feed is into the middle of the plenum and there will still be some velocity component remaining. The mixture speed through the middle burner ports will be higher than that through the outer ports, so there is a higher risk that the outer burner holes will get popping back. Putting a deflector in the plenum can overcome this. The deflector only needs to be perhaps two-thirds of the plenum length and can be held in by drilling the plenum and then plug-welding the deflector in place. 

 

Metal multiport burners are quite common in relatively low-temperature applications: boilers, BBQs, and similar. These work because the (radiative) heat flow into the front face of the burner is low enough that the back (plenum) face does not reach the ignition temperature of the gas mixture. If it does reach ignition temperature, it will ignite the mixture in the plenum.

 

Heat transfer is complex and the Math can be pretty daunting. Oversimplifying somewhat, Radiative heat transfer varies as the fourth power of the temperature. The radiative heatflow from a chamber into the front face of a multiport burner will be "around" 16 times as great from 2000-degrees as it will be from 1000-degrees. A metal multiport burner that copes fine with the radiated heat input from a 700 degF BBQ would be seeing "around" 160 times the heat input if used in a 2500 degF forge. Ignition in the closed plenum would certainly result (aka an explosion).

 

A ribbon forge burner is "just" a multiport burner that is designed and built to cope with a high temperature chamber radiating a lot of heat into the front face. With only the mixture to cool the rear face, a metal multiport burner would quickly reach ignition temperature. We therefore use a big thick lump of an insulating refractory material as the working face of the burner in order to dramatically reduce the heat flow to the plenum face. 

 

I don't know whether that is helpful, or if it just adds to any confusion. If the former, great. If the latter, I apologize. If there's something that needs clarifying and it seems like I might be able to do so without further muddying the waters, please just ask.