Atmospheric or Forced air

[ciladog]

With all this talk about blown and atmospheric forges recently I decided that I would start a new thread instead of high jacking someone else’s thread. I don’t mean to offend anyone but reading some of these threads is like mental masturbation, lots of information but no conclusions. Maybe I included.

Okay, so the topic is which is better, more efficient, easy to make, and more controllable. We already know that an atmospheric forge is most likely less expensive to construct so let’s not go there. And please no engineering data, coefficients of refractory materials, or how big to make your orifice. My orifice is pretty small like a closet. That information is all over this site.

What I would like is your opinion and experience and why you feel the way you do. Support it with pictures of your forges if you like and what you went through to make it work. I would have a hard time believing that some of you old-timers (with respect) made the perfect forge the first time around.

It could be a good resource for someone who wants to build a forge for the first time, or second, or third on deciding which type to build.

So I will start it off and if you are so inclined you can chine in, OK.

I started in blacksmithing with a coal forge. It served me well and I still use it a lot. But when I built my new smithy, I had propane piped in from a large outside tank. That was an ordeal to get approvals in that, in my state, the maximum pressure of propane inside an industrial building is 10 psi and my shop is on my residential property. Eventually it worked out but I had only the 10 psi to work with.

I went on evilBay and bought an atmospheric three burner propane knife forge from a guy who seemed to have it all together and sold lots of forges. Someone here on IforgeIron just sold one of his forges to someone. You figure it out.

Well, the forge came; I hooked it up, fired it up and put a piece of steel in it for a fagot weld. And I waited, and waited, and waited. It never got up to welding heat. So I email the guy and tell him. He asks, “What altitude are you at?” “Three hundred and eighty feet”, I said. So he tells me that he has never had this problem before. Oh really! I’m the first guy to have this problem? He told me to get a propane bottle and crank it up to 20 psi. Well I had a propane bottle and did exactly that and I waited, and waited, and waited and it still never got up to heat. Back in the box and I returned the forge.

Still wanting a propane forge, I started looking at all sorts of designs. I decided that I would make my own. I took a course at Peter’s Valley and they had propane forges that would melt steel if you weren’t careful. So I decided to build one modeled after those. It was a combination of aspiration and forced air.

The body of the forge was pretty straight forward. Getting the burners to heat such a large area was not. In all my trials and errors, the problem was air volume. Squirrel cage blowers just couldn’t do it. I had to go to a pressure blower.

This is the forge as it turned out. It is large but I pack it with firebrick when I don’t need a large forge. It evolved into a digital controlled forge that will reach 2300F in about 30 minutes.


I control the gas air mixture very precisely using a manometer. It measures the air manifold pressure. It is very simple to make and easy to use.

My vote is forced air all the way.

[looper567]

That's a great looking forge!
Atmospheric forges can offer excellent performance as well, you need to be careful what you buy though. There are alot of people only too willing to separate you from your cash in exchange for junk. The problem is especially worse for those without any real knowledge of what makes a gas forge tick. I've seen so many bogus claims and outright lies by ebay sellers that nothing surprises me anymore. I stopped selling on ebay completely for about a year because our products got confused with some of that garbage, and didn't want the fallout tainting our forges...
I personally prefer an atmospheric forge but there are advantages and disadvantages on both sides of the debate. Our "hobby" forge we offer from time to time is shown below. At 7PSI (.040 gas orifices) we are at 2204 degrees in about 5 minutes. Atmospheric forges can perform great, if they are built properly.

[ciladog]

That's a great looking forge!
Atmospheric forges can offer excellent performance as well, you need to be careful what you buy though. There are alot of people only too willing to separate you from your cash in exchange for junk. The problem is especially worse for those without any real knowledge of what makes a gas forge tick. I've seen so many bogus claims and outright lies by ebay sellers that nothing surprises me anymore. I stopped selling on ebay completely for about a year because our products got confused with some of that garbage, and didn't want the fallout tainting our forges...
I personally prefer an atmospheric forge but there are advantages and disadvantages on both sides of the debate. Our "hobby" forge we offer from time to time is shown below. At 7PSI (.040 gas orifices) we are at 2204 degrees in about 5 minutes. Atmospheric forges can perform great, if they are built properly.

That's a nice looking forge. Thanks for the post.

[Dave Budd]

I'm still new to gas forges myself, only having played with them over the last four months or so. I decided from the outset that i wanted to go atmospheric and not blown. Partly that was to keep construction more simple, but it was mostly to degate the need for electricity. My workshop is off-grid (necessity not choice)so if I can run a forge without power then it saves the batteries for other things

slightly daunted by the idea of building a burner and not liking the idea of things that can explode if you get them wrong, I ended up buying a burner and building a body. I got one of theseburners from a kiln supplier. Not cheap but it works really well. I had been running it at just under 2000F for forging and was on about 6 psi and doing fine. That was on body that had an internal space of 10" diameter and a foot long, lined with a couple of layers of fibre

I've just made a welding fore that is smaller (7x9" inside) and lined with castable. I don't know how hot it was when I tried it the other day (my thermocouple only goes to 1250C) but at about 11psi I was happily welding up 1lb (1 1/4" square and about 5" long) lumps of damascus .

So for me, so far, I'm in favour of atmospheric. fewer things to control and it's plug'n'play as far as changing a burner between bodies if experimenting like I am. It may be that during the winter the cold and low pressure weather might cause more issues, but I won't know for a few months.

[HWHII]

I have two atmospheric forges and built both, A two and three burner. I have run both over ten years. I am at 2500 ft elev. I do know people at higer elevation and who have problems with atmospheric forges and they inject compressed air into them to compansated for the altitude. When I first started out I built a Hans Peot style forge which is a blown type and could not get a even heat in it. I ended up putting it in a iron in the hat.

[maddog]

I dont see that one has to be loyal to one design at the expense of the other. They both have their place. I use a forced air burner for my main forge (which is pretty small) and atmospherics for various heating jobs and temporary forges.

Blown burners give much greater control over the air propane ratio. They can also run forges where there is quite a bit of back pressure. The blowers need not be expensive if you shop the surplus merchants or simply scrounge which is what I do.

They are considerably more cumbersome, requiring electricity and several feet of 1" or greater plumbing for the air.

Amospheric burners are very compact. The parts are cheap. You can move them around, set them up in different parts of the shop or run them in the back yard with little hassle. All they need is the propane hose. If you are a farrier and need a portable rig to take out into the field, the choice is a no brainer. I think they are a totaly neat design and I love making them and fooling with them.

It's true there are a lot fewer parts in making an atmospheric burner, they are by no means simpler to make. The design and construction is critical requiring some precision and a good understanding of the operating principles. Blown burners are much more forgiving to build and operate. They also have a more limited operating range because air intake and fuel delivery are coupled.

For this reason I would steer a newbie away from atmospheric burners as a first effort. I see so many posts from people getting started who have made an atmospheric burner and either dont know how to tune it or have made some critical mistake in the design. Blacksmiths tend to be the kind of people who look at something and are convinced they can design and build it better even if they never made one before. I admire that attitude but in this case it can lead to problems. That said, a lot of people have made an atmospheric burner for their first forge and had success with little frustration. This was my own experience.

Other than that, they are both great. They both have their strengths and weaknesses and what you choose to use in your main forge is just a matter of preference.

[ThomasPowers]

I run both types at 4600' and have been asked to test a new burner design which I plan to do at 4600, 6600 and 10,000 feet.

BTW I do not know that blown burners cost more than atmospheric.
I do know that they seem to be easier to build and have a much wider range---I can run my blown forge from "warm" to melting steel---at 4600' altitude

Sure is nice not to have to have electricity though!

[mike-hr]

There's been plenty of input on this subject in past threads. I'll let that rest, and bring up the subject of,,,, Recovery Rate! I run a blower type ribbon burner for production, coal forge for art work. I do a lot of millwright stuff for a local concrete guy. He called one time, he was pouring a retaining wall in 3 hours, and there was no J-bolts in town, for the spec the plans called for. The steel yard had 24 inch studs that would make the spec, if bent correctly. we did 75 J-bolts in 1-1/2 hrs , 3/4 stock, he made it to the pour easy. The advantage to blown vs. atmospheric,in my opinion, is the ability to crank the mother up to eleven,adjust to a neutral flame, and get the job done.

[ThomasPowers]

Mike next time I re-gauge a regulator I'm going to have it set up for 1-11! Just don't take up the drums as there is too high of a turn over rate!

Good Point too. If you stick a piece of 2.5" sq stock in your forge it sucks a heck of a lot more heat out than a piece of 1/4" sq.

When I teach the beginners with their 1/4" pieces use the aspirated forge and the advanced class with their larger stock use the blown forge.

[Frosty]

Most of this has been covered several times already but what the hey.

There are two basic differences between Naturally Aspirated (NA) and Gun burners. A gun burner is easier to build and tune but you're tied to electricity so it's less portable. A NA burner requires a fairly high degree of precision to make but once tuned it's self adjusting, best of all you're not tied to electricity.

Having supplied air a gun burner is easy to tune simply by adjusting the gas flow by eye. They're also a lot better for multiple burners as they're insensitive to back pressure or breezes. They work just fine. Running a ribbon burner almost cries out for a gun but a NA may do it, I haven't given it a try so I can't say.

NA burners on the other hand are more susceptible to back pressure and breezes over the air intakes.

Which is superior depends on a few variables that have little to do with the burners themselves. For one, heat is measured on two scales for what we're talking about. #1 is absolute temperature and #2 is BTUs per hr. Absolute temp of a propane and air flame is right around 3,200f but that isn't going to do you much good if your burner's only putting out 2,000 BTUs/hr unless you have lots of time.

BTUs/hr determines a couple aspects of your operation, #1 forge chamber size and #2 recovery or heating time.

I'm sure most of you have seen my Variable Volume Forge but I'll post another pic just to annoy someone. This pic was taken during construction and testing, it's a lot dirtier, warped and singed now and maybe one day I'll get a pic of it up to temp.

Anyway, I run about 8-9 psi into 300-350 cu/in chambers. If I crank it up to 11psi it tends to melt steel if I don't keep my eye on it.

Outside I have a Johnson Appliance #133 1/2 natural gas forge I've never lit. I've used them and wouldn't've taken this one if I'd had a choice. However, it's a big honkin gun burner forge and they are pretty well known for not getting to welding heat. This forge is what I based the one Each 3/4" burner per 350 cu/in ratio ron Reil and I decided on when we were brainstorming these things. I'm pretty sure the JA 133 would weld fine if I converted it to propane but I just don't need a HUGE forge, I can adjust my VV to whatever I need.

Okay, part of your original questions are NO, there just aren't any absolutes to this unless you want to buy commercially produced burners and I'm not talking about something made from plumbing parts you buy from someone making forges.

Making, tuning and using gas burners is just like anything else about blacksmithing in it's a judgement call and you MUST be able to adapt, either the tool or yourself to make them do what you need.

Oh yeah, a NA burner meters the air intake depending on the propane pressure so if you make it and tune it properly it doesn't matter what altitude you're using it at unless you're so high there isn't ANY oxy. Higher altitude simply means less pressure of the same ratios of gasses until you hit really high elevations. Thinner air means the primary (propane) requires less energy to draw the air. Basically the higher you go the more air the same gas psi draws and the air fuel ratio remains the same.

Both Ron Reil and Mike Porter have run their burners unmodified at elevations ranging from near sea level to well over 6,000'. By unmodified I mean no adjustment of the air fuel ratio. As far as I know my NA burners work the same from my elevation of 150' to near 4,500', the highest I know one's been used.

Use what works for you. Buy it, make it, whatever, it doesn't matter so long as it does what you need.

Frosty the Lucky.