Gas Forge "Efficiency"

[Nakedanvil - Grant Sarver]

In a direct-fired forge with a +2000F exhaust temperature, the insulating value of the walls is a pretty small component of "efficiency". Sure, in a totally enclosed electric oven, the biggest loss is through the walls. Hold your hand two inches from the shell of your forge. Now hold your hand two inches from the front opening. Where is the greatest loss of heat? The exhaust temperature has to be pretty close to the forge temperature or how could colder gases be coming out from a hotter chamber? 90% of the heat is going out the front door! How much of the remaining 10% are you willing to fight for?

Too many people get hung up on "thermal efficiency". The only "efficiency" I care about is how many dollars of fuel does it take to make a given dollar value of product. Once you have an adequate amount of insulation (say 2") adding more will gain you very little. Far more "economy" can be gained by having a smaller forge for the smaller work.

There are areas where you can gain efficiency. Good mixing helps. I always inject my gas into the intake of my blower. No, I don't have any more problem with "flashback" than any other design. Only if I try to turn the fire down unreasonably low. Multi-port burners or concentric ring burners have a much shorter flame front. They're like a bunch of small burners. Combustion is complete in a much shorter distance.

In industry, they understand that it works best to use the flame to heat the walls and let the incandescent walls heat the work. Turn on your forge from cold and try to heat a piece in the flame. Doesn't work very well, does it? In fact, after the forge is fully up to speed, you can turn it off, put in a small piece and heat it up with no fire at all! Why does it work so much better after the forge is warmed up? RADIANT energy from the walls! Some texts claim as much as 80% of the heat transfer is due to radiation. It's also much better for welding because the flame is incomplete combustion and contains hot oxygen, unburned fuel and all sorts of nasties.

After exhaust gases, radiation is the next biggest loss. Keep openings as small as possible. If it dazzles your eyes, that's radiant energy escaping.

There is very little difference in efficiency between insulating refractories and hard fire brick. Brick just has a much higher "thermal mass". It takes longer to heat up, but all that stored energy keeps the temperature up when you put a big piece in the forge. Brick maintains a much more stable, constant temperature for the same reason. While bricks and hard castables don't insulate that well, it still takes a long time for a BTU to travel from the hot side to the cold side. Iv'e seen brick forges that were still dull red inside the next morning! Now that makes a nice "turn off the forge, brick it up and leave over night" anneal.

Other than the above, the only way to gain significant efficiency is by using the exhaust to heat the incoming air. Preheating also can yield significantly higher forge temperatures.

Edited September 6, 2009 by nakedanvil

[Chris P]

I thought this was a very interesting, especially since I'm considering trying to build a gas forge. Thank you.

[HWooldridge]

Grant, do you think there is any advantage to a round interior vs. a box? I have always made box forges (because I'm a square kind of guy) but several people have been in my shop and said "Oh, you should build a round forge, they are much hotter for the BTU's spent."

[Nakedanvil - Grant Sarver]

I lean toward the round with the burner coming in tangent, but even square seems to work the same if the burner is placed to induce a swirl. I like the longest path the flame can take before striking the work. In fact, I built some very effective forges that had the burner in the floor, in an "alcove" off to one side pointing up.

[JNewman]

What do you think of the big Johnson forges Grant? I have one out in the shed that I picked up at an auction for $40 acouple of years ago.
I am currently using a forge I built using Ralph Sprouls sidewinder design but slightly modified. I do like this forge in that I can make it small to save fuel and get it hotter, or I can lift the roof and move the brick out and make it up to about 24"x14"x12". What I don't like about it is that the flame blows straight down on top of the work and that it could be a little hotter. I have been thinking of picking up a blower from the Kaynes at Quadstate and changing the forge to blown burners. Possibly changing them to ribbon burners, tipping them or putting some sort of splitter in the outlet.
I was contemplating moving things around in the shop the other day and my dad said to me why don't you replace the gas forge with the one in the shed. I am not sure that the Johnson will give me the versatility of the current forge but I could be wrong. Unfortunately I dont have room to install both.

[Nakedanvil - Grant Sarver]

The Johnson's seem to use a lot of gas, but that's not a problem if you feed them plenty of steel. We often ran 20 - 30 parts at a time in them, they're great for that. Throw away the lid and make one out of pleated fiber. The factory lid directs the flame straight out at ya. Just a loose lid sitting on bricks and no more opening than you need. You keep it back so the fire comes out nearly straight up. Using loose brick you can build quite a large chamber for that odd piece too.

[arftist]

Grant, would you mind expanding a little on the improvements to the Johnson forge? I have one and the lid is on it's very last legs. Like you said, it uses a lot of gas, but is great for "many irons in the fire". I think I understand what you are saying, but what do you mean by pleated fibre and what supports said fibre? Thankyou.

[looper567]

Well said. Gas forges are not the most "efficient" appliances for sure.
The key has always been to minimize unused space and restrict the openings as much as possible without causing too much back pressure. I have found though that lining material is also a critical factor. So I disagree on the Dense vs. Insulating Refractories. I have seen many dense lined forges which take forever to come up to temperature, while those lined with insulating refractories are ready to work in only a few minutes. Modern lightweight ceramics begin radiating back right away, and with only a dense floor with a small cross-section, your chamber is up to heat and ready to work in no time. I have taken forges lined with dense brick or dense castables, which had a hard time getting material to bright orange, relined them with modern lightweight refractories, and had them at welding heat using lower gas pressures. No changes to anything other than the liner... I've seen it many times. Not only greater temperatures from the insulating refractories, but the chambers are up to heat MUCH MUCH faster, and with lower gas pressures.
With all things equal, 2 forges with the same liner material, and same burner configuration, the one with the properly tuned burner and smaller end openings will yield better performance. But to say there is very little difference in a dense liner and a modern insulating liner is, in my experience, incorrect. I have observed, time and time again, exactly the opposite...
The Insulating refractories we use also allow us to maintain very consistent temperatures in the forge. This has been not only observed, but measured with our digital meters and thermocouples. I have identical forges in the shop right now, one with a dense liner and one with an insulating liner, and the performance is nowhere near close. Even switching the same burners between the forges, the dense lined forge can not come close to the potential of the insulated forge. The insulated forge does cool off quicker, but if I need to anneal, I can always bury a blade in vermiculite...

[Frosty]

I tend to agree with Grant on most everything but the dense vs insulating refractories.

An insulated forge will keep the heat in the chamber longer than an uninsulated forge.

Conserving heat in the chamber may not be your first priority though. In the example of having up to 30 irons in the fire at once, rapid cycling of the parts is the goal so the greater thermal mass (specific heat) and conductivity of a heavy liner is more efficient. Rapidly heating the forge itself and getting one or two pieces hot isn't particularly important.

A light insulating refractory will heat faster, get hotter and take less fuel to keep at heat. But it doesn't have the specific heat and conductivity to transfer that heat to the stock simply because it isn't the thermal battery heavier refractories are.

For example when I put 12" of cold 1" sq bar in my variable volume forge with light fire brick walls and lid you could watch it go from high yellow to high orange is a matter of seconds.

Like so many things no tool, machine or piece of equipment works best for everything.

John:

If Ralph's forge isn't performing up to expectations shoot him a message and ask him to help you trouble shoot it. My bet is the burners need a little fine tuning a pair of 1" sidearm burners is more than enough juice to bring that volume to welding temp.

Frosty

[JNewman]

My forge will get up to a welding heat, but when I start sticking 3" round or 40 1" round bars in it, it does take a long time to get up to temperature. When I make a really small chamber or block off the opening too much the forge really pumps out the CO. These are the reasons I am thinking of changing to blown burners.
I think I will try blown burners in that forge first because that will be easier than building a new lid for the Johnson.

[JNewman]

Here is the the gas forge I currently use. The top moves up and down using the trailer jack on the side. Bricks are piled around the sides to adjust the size and openings.

[Nakedanvil - Grant Sarver]

They make one that's about 2 feet long and one about 3 feet long.

Arftist: Not a very good picture, sorry. The pleated modules are available from refractory suppliers, usually 8 X 8 or 12 X 12, four inches or six thick. It's just refractory mat folded zig-zag fashion. They put an attachment in the back. Simple frame with end plates. You want to compress the modules down so they are held in tight.

JNewman: Make a new top for the Johnson, already. For your work you'll be happy with it. You'll adjust it the same way. Use the 12 X 12 modules then you'll be able to turn it into a really big chamber on occasion.

[looper567]

Here is the the gas forge I currently use. The top moves up and down using the trailer jack on the side. Bricks are piled around the sides to adjust the size and openings.

That's a very nice looking forge, I love the design!

Nakedanvil,
I just realized you're the KA75 guy, we had a KA75 in the last shop I worked at, 7 or so years ago... before I started my own shop. It was a fantastic striking hammer and a great companion to our self contained air hammer. Thanks for creating such a useful product.

[Frosty]

John:

Yeah, that's Ralph's design alright. However it looks like you've used plumbing elbows to make the bends. Ralph used weld fittings for smooth flow internally, that may be part of your problem. Also the first tube is overly long, it should be between 8-9 x the diameter of the throat. If you make it much more that 10x you need to start increasing the bore diameter to counter skin friction.

Too long a tube and plumbing fittings are probably what's causing poorer induction than you'd like.

What may help and I say "may" because I've never had particularly good luck turning corners with plumbing parts. Other's have no trouble so I know it'll work, I just haven't played with it enough.

Anyway, if you shorten the primary tube to the 8-9 x throat dia. and increase the tube dia when you make the bend. AND use a street elbow to minimize sharp edges and corners. You can buy street "L"s that increase by 1/4" easily enough if you have a good plumbing supplier. This should help increase the induction and make for a more robust flow. The stronger the flow the less it'll be effected by changes in back pressure, external breezes, etc.

Ralph and I brainstormed these things a few years ago and until a year ago I didn't build one. He's made some good improvements in his design since then.

Below is version variable volume forge I built. The main differences being I use 4 ea. 3/4" burners for more even heat and more economy as I can shut it down smaller. I used the scissor jack because I have a couple scissor jacks laying around and didn't have to buy a trailer jack.

The burners are of course the "T" burners I make rather than Sidearms.

Frosty

[Nakedanvil - Grant Sarver]

Has anyone tried this bio-soluble ceramic fiber?

"SAFE" Non-Ceramic Fiber Blanket 1" 8# dens. SW607 HT - eBay (item 140328938749 end time Sep-21-09 07:05:02 PDT)

[Frosty]

No, I've never seen it, probably because it's a British company. Still, I wouldn't mind using a comparable ceramic blanket without having to worry about particles in my lungs.

Did you check out the web site? I read parts but my connection is in the tank again so I didn't read a lot. Sounds good though.

Frosty

[JNewman]

Frosty, the pipe sweeps are weld on ones I got from Ralph when I visited him in his shop. The length of the pipes is longer than 9xdia (11.5") but I am pretty sure that it is the length in Ralphs plans. As I said, I have welded out of the forge but I want a little more heat for heavier stock and high volume parts. I may be wrong but I think I can get more heat out of some blown burners.
I may put that Johnson in but it means some major rearanging because of the size of it and the fact that it will not be portable like the current forge. I will probably have to get rid of my flypress and maybe replace my big post vice with a large machinist vice mounted on my platen table. It takes up the spot I was going to put my new belt sander and my buffing wheel in. It may involve a larger gas meter as well. Although having it hooked up to natural gas will elminate the running out of propane in the middle of a big job. The advantage of the permanent location is it is next to my windows, so it would be easy to add a nice big powered exhaust hood.

[JNewman]

When you mention putting the lid to the back Grant do you mean so that the front of the trench is open with nothing above it? Quite a few of the jobs I have been doing are short bars 3-4" long, do they get put down in the trench?

[Frosty]

What you're talking about is BTU output more than efficiency. This is an instance where having more thermal mass would be beneficial unless you're just cycling pieces fast enough the forge liner doesn't have time to recoup.

A gun burner can indeed make more heat per diameter because you can force more air through it with the blower. More air and you can feed it more fuel.

How big is your Johnson? (FORGE!) I have a #122a and it isn't so large it'd seriously crowd me nor keep me from making it portable. I don't use it, have never fired it since getting it. didn't really WANT it in the first place but it was part of a package deal.

It does have an impressive amount of hard firebrick so once it got to heat it'd be able to heat large steel pretty fast without dropping it's own temp much. Still, it has a darned good sized blower on it and a reputation as being a fuel hog. Lots of fuel, lots of BTU's just what you're looking for eh?

Frosty

Frosty, the pipe sweeps are weld on ones I got from Ralph when I visited him in his shop. The length of the pipes is longer than 9xdia (11.5") but I am pretty sure that it is the length in Ralphs plans. As I said, I have welded out of the forge but I want a little more heat for heavier stock and high volume parts. I may be wrong but I think I can get more heat out of some blown burners.
I may put that Johnson in but it means some major rearanging because of the size of it and the fact that it will not be portable like the current forge. I will probably have to get rid of my flypress and maybe replace my big post vice with a large machinist vice mounted on my platen table. It takes up the spot I was going to put my new belt sander and my buffing wheel in. It may involve a larger gas meter as well. Although having it hooked up to natural gas will elminate the running out of propane in the middle of a big job. The advantage of the permanent location is it is next to my windows, so it would be easy to add a nice big powered exhaust hood.

[JNewman]

I have the 133 johnson. If I get a chance this week I am going to borrow my neighbors forklift and move things around in my yard and bring in the forge so I can check things out.

[Frosty]

I was wrong, I have a 133a and it is a bear to move around. I had to take it off a little utility trailer it was stored on a couple weeks ago, I needed the trailer not the forge, and it was a real PITA. I finally used a length of heavy wall rec tubing bridged across a couple ladders and a come along. That just got it off the trailer and standing on the ground.

I would've gotten rid of it long ago if I weren't so Scott.

Frosty

[Michael Kellough]

In a direct-fired forge with a +2000F exhaust temperature, the insulating value of the walls is a pretty small component of "efficiency". Sure, in a totally enclosed electric oven, the biggest loss is through the walls. Hold your hand two inches from the shell of your forge. Now hold your hand two inches from the front opening. Where is the greatest loss of heat? The exhaust temperature has to be pretty close to the forge temperature or how could colder gases be coming out from a hotter chamber? 90% of the heat is going out the front door! How much of the remaining 10% are you willing to fight for?

Too many people get hung up on "thermal efficiency". The only "efficiency" I care about is how many dollars of fuel does it take to make a given dollar value of product. Once you have an adequate amount of insulation (say 2") adding more will gain you very little. Far more "economy" can be gained by having a smaller forge for the smaller work.

There are areas where you can gain efficiency. Good mixing helps. I always inject my gas into the intake of my blower. No, I don't have any more problem with "flashback" than any other design. Only if I try to turn the fire down unreasonably low. Multi-port burners or concentric ring burners have a much shorter flame front. They're like a bunch of small burners. Combustion is complete in a much shorter distance.

In industry, they understand that it works best to use the flame to heat the walls and let the incandescent walls heat the work. Turn on your forge from cold and try to heat a piece in the flame. Doesn't work very well, does it? In fact, after the forge is fully up to speed, you can turn it off, put in a small piece and heat it up with no fire at all! Why does it work so much better after the forge is warmed up? RADIANT energy from the walls! Some texts claim as much as 80% of the heat transfer is due to radiation. It's also much better for welding because the flame is incomplete combustion and contains hot oxygen, unburned fuel and all sorts of nasties.

After exhaust gases, radiation is the next biggest loss. Keep openings as small as possible. If it dazzles your eyes, that's radiant energy escaping.

There is very little difference in efficiency between insulating refractories and hard fire brick. Brick just has a much higher "thermal mass". It takes longer to heat up, but all that stored energy keeps the temperature up when you put a big piece in the forge. Brick maintains a much more stable, constant temperature for the same reason. While bricks and hard castables don't insulate that well, it still takes a long time for a BTU to travel from the hot side to the cold side. Iv'e seen brick forges that were still dull red inside the next morning! Now that makes a nice "turn off the forge, brick it up and leave over night" anneal.

Other than the above, the only way to gain significant efficiency is by using the exhaust to heat the incoming air. Preheating also can yield significantly higher forge temperatures.

Is this practical? Have people done this?

Thanks for a very interesting post Grant!

[Frosty]

Yes they have Michael, Rob Gunter's recuperative forge made while he was at Sandia labs is probably the best known and I think you can still get plans for it.

There are other recuperative schemes as well including the recuperative wall furnace. This is a situation where there is a gap between an inner liner and an outer insulating liner. The hot gasses in the forge are encouraged to exit the main chamber into the gap before being exhausted from the furnace all together.

A couple years ago while doing one of my semi-regular web surfs for new burner info I ran across a British firm who were getting much beter performance by preheating the propane instead of the intake air. There were charts and diagrams showing how they got their results and what those were. I was very intrigued but haven't gotten around to rigging a propane preheater.

One of these days I'm just going to have to do some serious experimenting, there are all these tantalizing ideas out there I'd just love to mix and match.

Frosty

[Michael Kellough]

Yes they have Michael, Rob Gunter's recuperative forge made while he was at Sandia labs is probably the best known and I think you can still get plans for it.

There are other recuperative schemes as well including the recuperative wall furnace. This is a situation where there is a gap between an inner liner and an outer insulating liner. The hot gasses in the forge are encouraged to exit the main chamber into the gap before being exhausted from the furnace all together.

A couple years ago while doing one of my semi-regular web surfs for new burner info I ran across a British firm who were getting much beter performance by preheating the propane instead of the intake air. There were charts and diagrams showing how they got their results and what those were. I was very intrigued but haven't gotten around to rigging a propane preheater.

One of these days I'm just going to have to do some serious experimenting, there are all these tantalizing ideas out there I'd just love to mix and match.

Frosty

This sounds very interesting and may be fairly easy with a smaller forge. But, I guess efficiency isn't so important in smaller scale equipment. As Grant said, the main way to improve efficiency is to scale the forge to the job.

Pre-heating the propane and I suppose making it more volatile makes more sense than heating the intake air and reducing it's oxygen density. But it sounds a little scary.

[Frosty]

Exactly right on the propane it increases it's volatility, makes it behave more like a proper vapor than finely divided droplets.

Preheating the intake air is mostly to recup some of the energy necessary to heat the nitrogen.

You want to hold the intake air under 1,000f to prevent pre-ignition. Even then it's not particularly dangerous. I mean seriously, we're operating home made open flame propane torches in an enclosed chamber anyway. What's the risk of a little pre-ignition, the thing does have a shutoff valve eh?

Hey wait! You said a little scary.

Well . . . . Yeah!

Frosty