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Forge Design and Specific Blower Requirements

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I'm looking at building a forge for a combination of general purpose work and for tempering long items such as swords. I've decided on using an electric blower for the air supply, and already know to use radial/centrifugal type blowers. I've done a bit of searching and have come across information stating to use a blower with a flow rate of about 150-400 cfm, and suitable for anywhere from 1.5 to 6 inches of static pressure. My first question is: Is this information accurate?

To design the forge for general purpose work, would it be easiest to design a forge with long firepot with multiple tuyere "branches" going to separate grates in the bottom of the firepot, or perhaps make one long grate with an air gate just underneath that could slide out to adjust the amount and size of the air blast? Perhaps there are better ideas than this that I couldn't find? With the multiple branches, an air gate on each branch could control the size of the blast, but I'm afraid of uneven heating causing problems for hardening.

The final problem is how much air flow will all this require. Going from what was stated before, for a typical forge an average of 250 cfm could be a basic reference point. With 4 tuyere "branches" or the equivalent thereof, the needed cfm would be more around 1000. Does this sound correct, and if there is any information I missed or anyone has a better idea please let me know!

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Your basic cfm/pressure data is generally accurate. However, a typical electric Buffalo or Champion blower installed on a single 11x14 firepot is overkill; that same blower will run two forges with no problem.

You don't need 1000 cfm. Some years ago, I built a forge to heat treat anvil faces. It had a 28" long, 3/4" wide slit running the length of the hearth. I used a Champion 400 hand blower on this forge and it worked well.

Based on your description, I would build a sliding gate to cover a slit and control the length of the fire. A 400 cfm blower making 2" SP should be plenty to make the type of fire you are after.

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About how much cfm do you suppose it would need then to HT a piece about 3-4 feet long, the main reason I ask is because I plan on ordering a "good" blower rather than getting the wrong thing and trying to make something work that isn't well suited to the situation. Hence reaching out to the community. :) I figure something that can handle that can do double duty for regular work with an air gate to help keep things under control.

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Well, based on what I was able to do with my slit forge, I'd expect you can get a 4' long fire to HT temps with a 400-500 cfm blower. With that said, I think most sword makers (and I hope one will chime in) typically use a shorter, hotter fire and move the blade forward and back until proper temps are reached. From your posts, it sounds like you are aiming to lay the blade in the fire and leave it alone but I'm not sure a solid fuel pile will reach consistent temps all along its length. In other words, you still may get hot and cold spots.

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I had seen a guy with a gas forge doing that, but I'm not sure if it would be a good idea or not with a coal/coke forge due to disturbing the fire. I came across one video where the guy had a trough the size of the sword and he did move it some but he wasn't cycling a 36 inch pice of steel through 8 inches of fire ( http://www.youtube.com/watch?v=OvmGvAM93bg&feature=related ). I can see why you wouldn't just leave it though as you would possibly burn off the tip or worse. And HWooldridge, I like the idea of the sliding gate, possibly making something like the trough from the video with the gate at the bottom and a blower able to handle the whole thing. (I hope one chimes in too to hear their opinion on the matter.)

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Of course the size and number of air holes in the tuyere and the type and size of the coal you use will make a different too.

In general it's a good idea to build a separate forge for heat treating as it's done so seldomly compared to regular forging. If you try to build 1 forge for both tasks it's like commuting 50 miles to work everyday in a dump truck just because you need a dumptruck twice a year.

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