Question about two gas forges

Tom Lewis

Tom Lewis

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Feb 24, 2000
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A number of years ago I bought a vertical blown forge from Uncle Al. It had 1" layer of kawool covered with satinite and ITC100. I used it for a number of years to make Damascus until it needed a new layer of kawool.

While I was re-doing Uncle Al's I bought another vertical blown forge. I won't say where I got it. It also had a 1" layer of kawool covered in satinite and ITC100. A couple of years ago I got a pyrometer and was surprised to find that the second forge would not get hotter than 2,240 degrees F. I tried more gas, less gas, etc. The hottest it got was at 3 lbs. If I raised the poundage even a little the temp. went down. If I lowered the poundage, the temp went down. I had the blower wide open. Yesterday I used it to make some san mai and it took the forge 45 minutes to get up to 2,000 degrees.

Meanwhile I had re-done Uncle Al's forge. I put 2" of kawool and covered that with satinite and ITC 100. I decided to try out Uncle Al's forge. In less than ten minutes the forge had reached 2,450 degrees and was still climbing. I used the same pyrometer in both forges.

I can't understand why the forges didn't work pretty much the same. I don't think 2" of kawool would make that much difference. What's wrong with the 2,240 forge?
 
Blowers are the same size? Gas metered by both regulator and needle valve? Same number and size of openings?

2" makes a difference but I don't think that's why the other takes so long to heat up and acts funny. My home made one works about like your uncle al.
 
If the forge was blown, and raising the gas volume made the temp go down it would be because you've got way too high a fuel ratio, you should have been getting green flame, and a lot of it out the door. This would mean you don't have enough air for the volume of gas. Bear in mind, pressure isn't really the deciding factor with blown forges, venturis need the pressure to pull air, but blown burners, it's regulated by the blower itself.

Now, many blowers used in blown forges are barely sufficient for the task. I highly recommend, and only use the Blacksmith's Depot (Kayne and Sons) small or medium sized blowers for this purpose (the large one is WAY overkill, and even with my large ribbon burner, the medium sized blower is gate valved down to 20% or so). Disregard, and ignore any information about CFM of the blowers, it's completely (well, functionally) irrelevant in this use, what matters is air pressure in an obstructed path, i.e. the diameter of your pipe it's hooked up to. CFM ratings are typically vented to atmosphere, i.e. no obstructions, and most of those little surplus blowers, can't handle much pressure.


Do you have gate valves between the blower and the forge body? If not, you're running your blower balls out, it's simply underpowered, and potentially dying. Also, those outside dampers on the intake of the blowers wears them out prematurely. I remove mine, and put a sliding gate valve after the blower, and before the gas input, which has a needle valve right before it comes into the burner tube.


I can reach 3000 (or whatever) degrees at any PSI I care to run at, using the above configuration, although I typically run at about 5 psi, with as I said, both the gate and the needle valve adjusted at far less than 50% capacity.



Although yes, you should have a minimum of 2" of ceramic fiber, 1" vs 2" would potentially make hundreds of degrees of efficiency, however, what I said above is as pertinent or more.

If I had one of mine running at 3-4 psi full blast and walked away for 30 minutes, I'd imagine I'd melt the liner, although in all honesty, it wouldn't stay lit without some damper on the blower. These cast aluminum blowers from K&S are 10x more powerful than any little sheet metal surplus blowers when it comes to pressure and functional CFM in our setups.
 
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Thanks for the answers. javand, these pictures are of the blower on the wimpy forge. I have been running the blower wide open. Do you think this blower is underpowered?
 
No that blower should be plenty powerful, it's the earlier version of the same ones that K&S sells now. Is it worn out? Is there an obstruction in the pipe somewhere potentially?


They do wear out over the years, especially if you run it dampered down too low for too long, and/or turn the forge off and the blower at the same time. Heat from the forge backs up into the blower and destroys the bearings and motor.
 
javand said:
If I had one of mine running at 3-4 psi full blast and walked away for 30 minutes, I'd imagine I'd melt the liner, although in all honesty, it wouldn't stay lit without some damper on the blower. These cast aluminum blowers from K&S are 10x more powerful than any little sheet metal surplus blowers when it comes to pressure and functional CFM in our setups.
Click to expand...

You aren't kidding. I'm using the cheaper 2" one they sell and until I stuck a probe in there I didn't realize how much I was over heating.

This may be a stupid question but are both forges hooked up to their own propane tank?
 
Thanks again for the answers.
The X is when the switch is on. The fan was new when I got the forge.
Both forges run off the same 100 lb propane tank but not at the same time.
I have always run the fan wide open and it still seems to put out plenty of air. I have been turning the fan and gas off at the same time. But as far as I can tell the fan still works. But maybe it is worn out and I don't know it. I wonder how I can test the fan to see if it is working as it should.
If the forge won't handle more than three lbs of gas maybe the fan is the problem.
I appreciate your interest and help.
 
Tom Lewis said:
Thanks again for the answers.
The X is when the switch is on. The fan was new when I got the forge.
Both forges run off the same 100 lb propane tank but not at the same time.
I have always run the fan wide open and it still seems to put out plenty of air. I have been turning the fan and gas off at the same time. But as far as I can tell the fan still works. But maybe it is worn out and I don't know it. I wonder how I can test the fan to see if it is working as it should.
If the forge won't handle more than three lbs of gas maybe the fan is the problem.
I appreciate your interest and help.
Click to expand...


Worth checking the regulator also, when you crank it above 3 psi with the gas full open, do you get a huge 2 foot long column of dragons breath that's green? If you're over gassing, it should be apparent.

You could pull the fan, run it outside of the tube and check the pressure in the open.

To be clear, the difference between 1 and 2 inches of insulation is significant, but you should still be able to "overcome" it, with that blower and enough gas, it'll be inefficient, but yeah, you shouldn't be maxed out.
 
kuraki said:
You aren't kidding. I'm using the cheaper 2" one they sell and until I stuck a probe in there I didn't realize how much I was over heating.

This may be a stupid question but are both forges hooked up to their own propane tank?
Click to expand...


I didn't realize how hot I usually ran until I started using a probe also.. After that, just by watching the color, it's pretty clear that most people are welding at 2400+, and simply don't realize it.
 
I think I have been welding at 2150 degrees. I haven't noticed any green flame coming out of the forge. If I use it again, I will look for that.
 
2200-2250F is the welding temp for eutectoid steel. 1% carbon and higher can be welded 100F lower than that. The higher the carbon the lower the weld temp. On the other end of the carbon spectrum, pure iron has to be almost at the melting point to weld.
 
Tom Lewis said:
I think I have been welding at 2150 degrees. I haven't noticed any green flame coming out of the forge. If I use it again, I will look for that.
Click to expand...


Well that green flame will happen when you crank the gas up to the point that it starts getting colder, and should make longer dragons breath, as the fuel is combusting outside the forge, due to lack of air in the mixture. This will diagnose not having sufficient air. If this isn't the case, let us know.
 
I find many people who just raise/lower the blower volume/pressure, or raise/lower the gas pressure. It is a blend of the two that makes the forge hotter or cooler. Getting them out of balance will cause effects like more gas causing a lower temperature.

On a blown burner, the first diagnostics are:
Is air volume sufficient - is choke or manifold valve correctly set and the blower the right type. Is blower volume in CFM sufficient. It takes more air than you might think when running a welding force. Those hair blower burners won't cut it. A good welding setup has 150CFM or more. A big welding forge needs at least 250CFM.

Is gas flow sufficient - inlet pipe size, loops kinks or blockages in hose, or obstruction in inlet pipe (wasps and spiders love to nest in those 1/4" pipes). Needle valves are also common blockage places. It doesn't hurt to remove the valve and clean the seat if you are having problems. Make sure the needle valve is as large (or slightly larger) as the inlet pipe on a blown burner - normally that would be a 1/4" or 3/8" valve for 1/4" pipe.

Air/gas balance - Start with a neutral and normal size flame. As you increase the gas, increase the air .... retaining the neutral flame. If you reach a point where the flame changes, you will either need to raise the gas pressure ( or open the needle valve more if it isn't at max), or increase air volume (open choke or turn up a variable blower). Note that more fan speed is not necessarily more air. A proper blower has sufficient static pressure to deliver at low and high volume. Some cheaper squirrel cage fan type blowers don't work on larger forges or at higher volume. A true toroidal forge blower is really worth the money if you are welding. They also come in DC, and occasionally in 3 phase, so they can be speed controlled. A light dimmer switch can work on most 120VAVC 1Ph blowers with OK results.

Burner efficiency - Things like adding a 3X4" mixing chamber to the burner tube, making a slight flare in the end of the burner tube, changing worn out burner tubes, and eliminating 90° bends in the manifold and assuring smooth walled piping for then air/gas flow. Turbulence is a cause in inefficiency, and turbulence increases as you up the air and gas flow. There should be no hash angles from the point the gas enters the air stream in a proper burner. Even the ends of pipe fittings can cause turbulence. Something as simple as inner chamfering of the piping can increase efficiency by over 10%.
A smoother forge chamber wall and a slightly bell/funnel shaped entry port through the refractory will also lower turbulence and increase efficiency.

Burner size - it is sort of obvious, but if you are straining your burner to run at higher temps, then a larger burner ( and probably a larger blower) will change that appreciably. Blown burners are far better in 1.5" pipe than 1" pipe. I find 1.25" pipe a good blend for regular forging as well as welding in a normal size forge.

Forge port size - This is almost never discussed, and you often read/hear advice that is absolutely wrong.
The more you block the ports the less efficient a forge gets. It needs to exhaust the combustion gasses (H2O, CO2, CO, and some unburned hydrocarbons) to allow new gas and air to burn. People block off the rear port completely, and close up the front port (occasionally completely) and can't figure why the forge temp went down, not up. There are complex calculations and charts concerning input volume of air and gas VS forge efficiency, but suffice to say that a good welding forge will need plenty of exhaust port. I like a 2X3" front port and the same or slightly smaller in the back. On vertical forges, the two ports should be identical.
I will make a broad generalization here and say that it is hard to have too much port area ( within reason) but easy to restrict it too much. The only place that closing down the rear port is advantageous is when doing HT in a forge. If the flame can be lowered enough to hold at 1550F ( or whatever) then blocking the rear port, and even partially blocking the front port, can help create a more evenly soaked chamber. But there is a fraction of the volume in exhaust gasses at that temp than in welding.

Forge Design - Obviously, all forge can't do all tasks. Two forges, one for general work (and HT if you don't have an oven) and one for welding is a wise step in equipment building. If you make them both at the same time it will save on materials and labor time. If I had to give size recommendations, I would say to make you general use forge smaller rather than larger, and the welding forge larger over smaller. More efficient refractory ( 2" vs 1") higher temp rating ( Hi-Z), and properly applied, cured, and coated lining will make a big difference. Having a chamber size suited to the work is also something to figure before you start getting materials.
A general use forge with a 6" round chamber is really big for knives and swords.
A welding forge with a 6" chamber is too small.
A good rule of thumb for those getting started is a 4" general use chamber by about 12" length, and a 10" welding chamber about 12-16" high.
Ports on a forging forge are good around 2X3" to 3X3" front and 2X2" to 2X3" rear.
Ports on welding forges need to be sufficient for stacked billets, so 3X3" to 3X4" is a good start.
Obviously, the type objects you forge will change these, so plan ahead when building instead of modifying or re-building.
 
Thanks, kuraki, javand and Stacy for the answers and information. I have had gas forges for a number of years, but have learned a lot from what you have posted. I am now using Uncle Al's forge which works great. Maybe sometime in the future I will hook up the "Wimpy" forge and try giving it more air. From what you have told me, I think it's problem is not enough air. It could be the fan overheated, wore out, etc.
Thanks again, I plan to make a copy of this thread and keep it for future reference.
 
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