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Propane vs. Natural Gas forges


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Ladies and gents -

I am building a gas forge this winter.  Planning to use Michael Porter's basic propane cylinder design.  Because it is already piped to my property, I would like to use natural gas.  This would eliminate the need for bottles, and for keeping them changed.

Is it possible to use natural gas instead of propane with a simple, naturally aspirated forge?  Are there any cons to doing so?  Do you know of any guides or FAQs about doing so? 

Any advice would be greatly appreciated.

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The very low presure of the natural gas plumed to your house requires a large orifice and forced air. I have read that naturaly asperated burners can be fired with a high presure tap (the meter is regulated after, not before the meter) now high presure is a relitive turm...

natural gas isn't as energy dense so takes more gas to achive the same effect so a larger orifice, and some what less air (in relation to the ratio needed for propane) 

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It is possible, but not at all easy.

The problem you are up against is gas pressure: household NG is usually delivered at a pressure measured in inches of water column. Here in the UK, it's normally 8" WC, about 0.3 PSI.

In a NA burner, the gas velocity through the gas jet is what causes the air to be drawn in, and the gas velocity is dependent on the gas pressure. Most NA Propane forges seem to use 0-30 PSI regulators, giving up to 100 times the pressure likely to be available from your NG line (I actually use a 0-60 PSI Propane regulator).

The energy density and air:fuel ratio differences that Charles refers to pale into insignificance against the pressure problem.

If you have a high-pressure NG feed (usually only for large industrial users), a NA forge is not particularly difficult. Finding good instructions for a self-build is likely to be harder though, since most use Propane.

If you really want to use NG, a blown burner can work well, even on low-pressure NG.

I'm not sure what your rules and regs will require. Here, portable Propane-fuelled equipment has fewer restrictions on what can and cannot be done than fixed NG equipment, so the legal and insurance issues tend to rule out NG forges for the DIYer. Fully-approved NG packages are available of course, but carry a pretty hefty price tag, so tend to be used mainly in industrial and educational applications.

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The commercial forges used in schools, Johnson, were natural gas.  They used a blower.  Blower burners tend to be much simpler as the blower provides the mixing power and so orifices are simpler in design as they just dump the fuel into the air stream.  If you are plumbed for NG you probably have access to electricity---go for it!

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I have a "low pressure" natural gas forced air forge in my shop which works quite well.  I am using an industrial burner with high quality mixer, ventauri chamber for expansion, high pressure blower, and zero pressure regulator.  I've built several pieces of equipment with this kind of burner assembly, and it works just fine, but might not be the easiest system to configure (I got mine for free from a glass studio that was being upgraded - everything was in the dumpster!).  Forced air systems are a lot easier to use as far as dealing with backpressure of forge configurations as well as varying the proportion of the gas air mixture to change the forge atmosphere.  There are safety considerations though, and your local codes may require either a gas shutoff interlock with UV sensors for flame proofing and/or airflow sensors to ensure the blower is running.  Without these I would not leave a blown system unattended.

I have seen sketches for home built NG burners with forced air.  They are essentially the same as propane/forced air units with larger gas orifices and gas feed connections.  

As Tim states it is not really feasible to use low pressure natural gas in a naturally aspirated burner for a forge.  The pressure isn't sufficient to create the velocity needed to induce the right amount of air.  Blown burners don't need this velocity, but you will be surprised at the gas pipe diameter needed to move the quantities of gas required for forging in a reasonable size enclosure. 

These also work great with ribbon or Giberson style burner heads which give you a much quieter and more distributed flame.

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Latticino's advice was very good; he came closest to pointing you in the exact right direction, not that the others didn't do well too. The bottom line is that you want to head for the richest sources for NG burner information. If I were in your shoes, Ward Burner Systems web site is where I'd start; their  free articles will give you the best information on setting up an NG system that will pass muster with your local Fire Marshal. Second best source for information is Giberson's book on home built glass blowing equipment. These were my two favorite information sources back in 1999, when I was doing research to decide what way I wanted to jump with my own heating equipment decisions. I basically chose NA burners only after working in a metal arts shop just to get aqainted with their equipment, and ended up deciding that I definitely wanted to control the dragon's breath problem better than a fan-blown NG burner system would allow. Take a little more time doing the background research and then get back to us with your impressions, yes?

The only kind of naturally aspirated burner that possibly could work with the very low pressure natural gas normally piped to homes might be a Vortex burner, but since I haven't had time to research that possibility yet, I can't recommend trying to build one for the purpose. Also, since more flame at less velocity will produce the least amount of dragon's breath in a fan-blown system, I would be inclined to build a ceramic based ribbon burner to heat the forge and use something like a Fisher burner to supply it with a gas/air mixture; just a thought.

That same Michael Porter

 

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  • 1 year later...

Obviously this is a long-dead thread but I'm new to the subject, and this had the most informative thread I've seen so far, so I'm tagging on here.  I spent some time on the Ward Burner Systems site...you're right, if you're looking for a blown burner, they have the goods.  I haven't yet decided whether to go with one of their setups or build my own; I'm an HVAC guy by trade and everything they use comes right from an HVAC supplier, so DIY is totally doable.  What I don't know is how to calculate the amount of heat required.  There are HVAC calculations for determining how many BTUh's the equipment needs to supply given the desired internal temp vs the exterior ambient temp vs the heat loss of the structure, but I'm not sure how they'd be applied to forging, or if they even could be applied.  If we use forge welding temps as the target for the internal temp, then we're talking 2300-2500F.  The R-factor (insulation) of the forge wall construction could obviously be calculated, but there will be a ton of loss through the open ports where the working piece is inserted.  The internal cubic space also figures in, obviously.

I saw a mention of Giberson's book on glass blowing furnaces and I'll look into that, but I expect that those temps will be significantly below what we need for steel.  Does anyone have a source for calculations of this sort?

Thanks in advance

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Calculations may be helpful in getting you in the ballpark, but as you mentioned there are a lot of confounding variables which will make it difficult to come up with anything precise. Rather than go through that headache I'd recommend using the general rule of thumb of one 3/4 inch burner per 350 cubic inches or a single one inch burner per 700 cubic inches of forge interior.  Of course forge shape and burner orientation have impacts as well.  One of the threads on this site regarding this topic mentions at least 450 BTU's per cubic inch for forge welding if you want to run your calculations, but that was specifically for a certain style of 3/4 inch naturally aspirated burner.  I would err on the side of too much heat though.  You can always turn a burner down if it gives you more heat than you need.

This discussion may be helpful to you:

https://www.iforgeiron.com/topic/8785-i-couple-of-gas-forge-questions/

 

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1 hour ago, ClevelandGuy said:

 If we use forge welding temps as the target for the internal temp, then we're talking 2300-2500F.  The R-factor (insulation) of the forge wall construction could obviously be calculated, but there will be a ton of loss through the open ports where the working piece is inserted.  The internal cubic space also figures in, obviously.

I saw a mention of Giberson's book on glass blowing furnaces and I'll look into that, but I expect that those temps will be significantly below what we need for steel.  Does anyone have a source for calculations of this sort?

Dudley Giberson's book is still very helpful, and glassblowing temperatures definitely get up to the 2400 deg F range (needed for melting glass from batch).  Another good source of info is a somewhat more exhaustive book on glass equipment design and DIY style construction by Henry Halem.  Both of these have great info on forge burner design, and I believe that Dudley still sells his multiport burner, and has even developed one for a small gas forge.  Here is a link to his site, which is crammed with useful info: http://www.joppaglass.com/

Calculations are certainly helpful, but far from straightforward.  As I see it the internal volume isn't as big a factor as the forge surface area (which is, of course, related to the volume, and why the typical rules of thumb are useful).  That surface area is what directly influences  both the conductive loss (that you insulate against) as well as the convective loss (as the forge heat gets released to the environment).

Also as you correctly state the ports that are used for combustion product exhaust/product doors have a significant effect.  These not only in the loss of heat due to the hot exhaust gasses (which is why Michael and Frosty often mention that a slower flame speed is more desirable, all other things being equal), but also due to radiation losses. 

Another part of the equation is that all things being ideal, the time it takes to heat your stock can have a great effect on the usability of a forge.  That time is not just proportional to the temperature inside the chamber, but also to the thermal mass of that same chamber (which is one of the reasons that many industrial forges have a thick, heavy refractory layer instead of the light insulation a hobbiest may favor).  A production shop can afford to trade the slower initial forge warm up time against improved product throughput.  Try heating up a 2" piece of bar stock for a hammer head and you can see it suck the heat right out of a thin shelled gas forge.

The only downside to having too much burner for your forge is the potential for not having enough turn-down capability to run the burner at the correct settings for the chamber (unless it is just so large in comparison that the flame can't develop at all inside the chamber).  As I see it, if the burner can't turn down far enough the flame speed will be so great that the forge won't operate efficiently.

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Once again Latticino has a good handle on matters.

I would add that a combination of multi-port burner head, and exterior baffle wall to lower radiant heat loss at the exhaust port will bring a natural gas forge right up alongside a top flight propane forge, so go for it. Just build the forge carefully, and grin for years, because you don't have to bother with fuel cylinders.

Since you will need a fan blown burner for natural gas, a ribbon burner is no extra work for you. One of the guys here installed a ribbon burner in a five gallon forge already, so look that up in the old threads.

Since you are only building a forge, rather than a glass day tank, you probably won't be required to add the usual safety shut off system. Do talk to you local Fire Marshal's representative; do not try to make an end run around safety regulations.

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I would expect them to be a natural pick for you, as they can magnify the advantages of fan-blown systems, while ending the one thing about them that is hard to live with; dragon's breath. I stay mostly mum on the subject in an effort to keep from ending up with too much on my plate; not from a lack of belief in them. That won't prevent me from pointing out their advantages. I expect them to become the number one choice of most smiths in coming years; yet, I also maintain that there is no such thing as a best burner or best forge for everyone:)

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Wow!  I'm amazed at the quick and informative responses to such an old thread.

In response to Buzzkill's statements about "3/4 burner", I'm not sure what's being referred to by this dimension.  Is this the diameter of the burner outlet itself or the feeder tube?  Or is this even relevant when discussing blown burners?  The subsequent statement of 450 BTUh per inch of cubic space inside the forge seems usable, although I don't understand the statement about it being related to a certain size or style of burner.  Required heat would be a target number, regardless of how you get the heat in there, similar to the way we calculate the heat loss for your home...if you need 60,000 BTUh to keep your house at 70F on a 15F day, it doesn't matter whether you create that 60,000 BTUh with a furnace, a fireplace, a heat pump, or by leaving your oven open and running the broiler.  What I was basically looking for is a required amount of BTUh inside the forge to get the steel to the required temperature, so if that 450 BTUh number is valid, I think we may be there.  I can back out of that equation by next figuring out what equipment will put that amount of heat into the forge, and then determine whether my gas meter/gas supply has the ability to give me what I'll need.

Latticino, I'll definitely look into those books/links you recommended.  Your statement about the surface area being more important than the internal volume is a bit confusing, since they're basically linked together.  I suppose one could make a really oddly shaped forge, but for most of us, a cube or cylinder will be the way to go.  Your point about thicker and more insulating wall construction is taken.  One thing I'm curious about...a lot of videos I see show the flame impinging almost directly onto the metal.  I'm wondering how much of the heat being put into the metal occurs from the flame, itself, and how much from the ambient temp inside the forge?  In other words, are we baking the metal or roasting it, or is this even a relevant question as long as the metal gets hot?  Holding an oxy-acetylene torch against a piece of metal will heat it up one heck of a lot faster than trying to heat the metal by placing it into a small container with the same torch.

The ribbon burners are interesting.  I see the advantages of spreading the flame over a larger area to keep noise down and provide more even heating, although this harkens back to my previous paragraph about whether the flame should strike the metal or just put heat into the cubic space of the forge, as the wider burner will have less actual flame coming out than, say, a single port burner.

Here's a drawing of one of the Ward Burner setups:
http://www.wardburner.com/images/MB_Operating_Instructions.pdf
If I built/purchased such a thing, would I be able to feed a ribbon burner directly from it, or would I have to stick with their burner design?  Maybe that's a question for Ward..
 

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Okay, so I took the most conservative number I saw in the discussion Buzzkill referenced, which was 540 BTUh per cubic inch of internal space for the forge and drew up an imaginary forge that might offer something like the working space I'd like to have.  I imagined it as a 55-gallon drum on its side, filled with 3" of the appropriate insulation/fireproofing, so that the resulting internal cylinder came out to 18" diameter x 35" in length.  See attached pdf.

Google says the internal volume of a cylinder is (Radius x Radius x Height x Pi), so that means 9 x 9 x 35 x 3.14159.  Which makes the internal volume ~ 8900 cubic inches.  Which comes out to almost FIVE MILLION BTUH!!!

Since I don't have access to a Saturn V engine, I'm stumped here.  I realize that this imaginary spitballed forge is quite a bit larger than the knifemaker coffee can forges I see people making, but something doesn't seem right here...
 

forge.pdf

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I just meant that the 450 BTUH/ cubic inch is a rule of thumb and that there are a lot of factors in addition to the interior volume that need to be considered.  Good place to start though.   The interior volume and exterior surface area are linked, but not directly, as the forge configuration and even more the overall thickness of the insulation also play a factor.  I agree it is not a huge issue, probably just being overly pedantic.

As far as flame directly impinging on the stock, I'm of two minds.  There are times when it is a good idea to have a "hot spot" on your stock, but in my experience I have found it more useful to have general radiant heat inside the forge rather than spot heating.  Again, a lot depends on how you actually use your forge.  I tend to put my stock inside and walk away, knowing that I'm looking for a nice full soaking heat that heats not just the surface across from the burner, but all the way through to the interior of the stock so it moves easily under the hammer.  I believe that keeping the stock directly under the flame risks accelerated scaling at that point as well as well as decarb, but I could be wrong.  I will admit that when I needed to reweld a wrought iron/5160 2 bar billet that had experienced too aggressive cross peening and split, that I did make an effort  to put that section directly across from my burner outlet, and I think that may have helped, so go figure.

The Ward system looks lake a good one, and I have used similar ones in the past for glass equipment.  I currently use a SS flame retention nozzle like the Ward, but plan on upgrading to a ribbon burner when I get a free moment.  Based on the scale of the piping for the gas line it looks to me like this one is for natural gas, if you order from them make sure that you specify the fuel choice.  As far as feeding a ribbon burner, it is all based on whether there is enough external static pressure available from the blower supplied.

No reason to make a 18" diameter x 3' long forge, unless you are doing very large industrial forging.  Your forge design is way too large.  What are you planning on making and how many tons is your press or power hammer?

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1 hour ago, ClevelandGuy said:

n response to Buzzkill's statements about "3/4 burner", I'm not sure what's being referred to by this dimension.  Is this the diameter of the burner outlet itself or the feeder tube?  Or is this even relevant when discussing blown burners?

That refers to the internal diameter of the final tube (excluding flares) before it enters the forge.  It is relevant somewhat to blown burner discussions, but maybe not as crucial as for naturally aspirated discussions. Take it to the extremes if it helps to understand.  Just in terms of heat output (we'll ignore max temp at this point) you have to deliver enough fuel and air mixture to achieve that.  If the inlet to your forge was only a quarter inch, then to get enough fuel and air into the forge that stream would have to be moving really fast (i.e. at high pressure).  On the other hand if  the inlet to a small forge was six inches in diameter then you'd either worry about the flame burning back into the mixing tube or you'd be way overpowered for the forge, and most likely not have enough space for the flame to even form properly.   So, what we're trying to balance here is getting enough fuel/air into the forge and the speed at which it enters the forge.  A slow, hot flame is our best friend, which is why a lot of people like ribbon burners which produce that effect.   I have used 3/4 inch diameter final tubes for blown burners, but I would not go smaller than that, and bigger is most likely better within reason.  The forge I used it in only had about 240 cubic inches of volume, but even there I probably could have been ok at a 1 or 1.25 inch diameter burner.

As to your other point.  In general I'd say you are correct.  However, not all burners reach the same max temperature due to a number of variables.  So, while 2 burners side by side may be using the same total amount of fuel/air and therefore putting out the same total BTUh, that does not mean identical temperatures will be reached inside identical forges. Again, take the extremes.  If you used enough butane cigarette lighters side by side do you think you could reach forge welding temperatures even if your total output was 540 BTUh or higher per cubic inch?

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15 minutes ago, Latticino said:

I just meant that the 450 BTUH/ cubic inch is a rule of thumb and that there are a lot of factors in addition to the interior volume that need to be considered.  Good place to start though.   The interior volume and exterior surface area are linked, but not directly, as the forge configuration and even more the overall thickness of the insulation also play a factor.  I agree it is not a huge issue, probably just being overly pedantic.

As far as flame directly impinging on the stock, I'm of two minds.  There are times when it is a good idea to have a "hot spot" on your stock, but in my experience I have found it more useful to have general radiant heat inside the forge rather than spot heating.  Again, a lot depends on how you actually use your forge.  I tend to put my stock inside and walk away, knowing that I'm looking for a nice full soaking heat that heats not just the surface across from the burner, but all the way through to the interior of the stock so it moves easily under the hammer.  I believe that keeping the stock directly under the flame risks accelerated scaling at that point as well as well as decarb, but I could be wrong.  I will admit that when I needed to reweld a wrought iron/5160 2 bar billet that had experienced too aggressive cross peening and split, that I did make an effort  to put that section directly across from my burner outlet, and I think that may have helped, so go figure.

The Ward system looks lake a good one, and I have used similar ones in the past for glass equipment.  I currently use a SS flame retention nozzle like the Ward, but plan on upgrading to a ribbon burner when I get a free moment.  Based on the scale of the piping for the gas line it looks to me like this one is for natural gas, if you order from them make sure that you specify the fuel choice.  As far as feeding a ribbon burner, it is all based on whether there is enough external static pressure available from the blower supplied.

No reason to make a 18" diameter x 3' long forge, unless you are doing very large industrial forging.  Your forge design is way too large.  What are you planning on making and how many tons is your press or power hammer?

I agree on the flame impingement issues.  I would think that 'baking' the steel would work best if you're attempting to, say, heat a billet piece.  Like ThomasPowers said, if I just want a small piece of the structure heated, a torch could to that.  Or a coal forge, for that matter.  I've always thought that one of the points of a gas forge was the ability to heat the ENTIRE piece without having to build a bonfire of coal 2-ft tall.

Which brings us to the size of the forge discussion.  While I agree that I'd never have a piece of metal 18" x 35" long that needs to be heated in its entirety, my concern was more about the ability to get the part I DO need in there, if the piece isn't shaped like a bar.  In this regard, knifemakers have the advantage of those of us who prefer to do more artistic stuff, as EVERYTHING they work with is a small thin rectangle.  But for instance, how would you heat a larger scroll, say something that came out to 15" in diameter, in a smaller gas forge?  You might only need a small part of the scroll to be hot, but how to get that part into the heat without a larger internal space?

 

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1 minute ago, ClevelandGuy said:

 But for instance, how would you heat a larger scroll, say something that came out to 15" in diameter, in a smaller gas forge?  You might only need a small part of the scroll to be hot, but how to get that part into the heat without a larger internal space?

Look at clamshell or brick pile designs.   The brick pile will let you form just about any shape forge that you need, but the down side is the insulating bricks are somewhat fragile and have to be replaced frequently.

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3 minutes ago, Buzzkill said:

Look at clamshell or brick pile designs.   The brick pile will let you form just about any shape forge that you need, but the down side is the insulating bricks are somewhat fragile and have to be replaced frequently.

This makes sense, although I suppose the point is that you build the forge around the small spot you need to have heated, thus removing the requirement for a large internal forge volume.  Trouble is, in order to work the piece, you'd have to at least partially disassemble the forge to get it back out...and then put it back and close up the forge.  You could leave some kind of space to move the piece in/out, but then you're letting the heat escape and will have to counteract that.

Understand that I've got no experience working with gas forges, so I'm envisioning these possible scenarios, not talking from experience.  I do find that I often have trouble even working with coal forges at times, when the piece is of a certain shape.  With a standard firepot coal forge, the hottest part of the fire is actually slightly down below the lip of the firepot.  If I'm trying to heat the middle of a piece that's long and flat, it's often difficult to get it into the hottest part of the fire.  It's this type of thing I'm trying to address with the gas forge design.

 

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