Forge blower idea: good or bad?

Drew Riley

Drew Riley

Knifemaker / Craftsman / Service Provider
Joined
Oct 17, 2007
Messages
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I’m thinking of building my first forge, and was toying with the idea of a ribbon burner design.

I see that it uses a blower, and was thinking a 4” duct booster might be a cost effective option? Something like this:
VenTech VT DF-4 DF4 Duct Fan, 100 CFM, 4" https://www.amazon.com/dp/B00F6BL11U/ref=cm_sw_r_cp_api_Mm01AbCVP9KGR

I thought as well that a simple stovepipe style damper might be used to adjust airflow?

All in, I imagine something like this could be put together pretty cheap.... but now I need the more experienced smiths to tell me why this is a terrible idea. ;)
 
kuraki said:
I don't think that will provide the necessary pressure. I have one of these from Kayne and Sons. It's a great value at $100. https://www.blacksmithsdepot.com/products/forge-fan-fuel/blowers/gas-forge-blower-2.html
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You may be right. I wander what the pressure is on one of those things? Likely not overwhelming.

That said, I don’t plan on a comically large forge chamber or anything, but probably better to have way too much and throttle it down, vs not enough or just enough.

My other option, of course, is just going with a basic Venturi style burner, but I was under the impression that RBs are a lot more efficient. Maybe for a smallish forge, it doesn’t matter much? Still reading up on things....
 
Well. Take this for what it's worth. I've built 3 forges and own an Atlas mini. I've made 3 venturi burners and still use 2, one little one for my parkerizing tank and one big one for my heat treat drum(which is soon to be replaced by a blown burner and PID control). Javand talked me out of building a ribbon burner but I'll never build another venturi forge for forging steel. It's just not worth it, for anything but something the size of an Atlas mini in my opinion. Forced air is just infinitely easier to tune to the exact conditions you want because you don't have to depend on Bernoulli to get the right air/gas mixture. You can have exactly as much air as you want regardless of gas velocity. In fact, I'm so positive that it's worth a $100 blower to not dick around with venturi burners, if you buy that blower and decide you don't want it, I'll buy it from you and either keep it on the shelf as a spare or build another horizontal forge.

Pressure is more important than CFM because you have to overcome the forge atmospheric pressure. Those duct fans create almost no pressure, you'd be far better off with an induction blower for a wood stove if you want to do it on the real cheap, they're at least doing something similar to what a forge blower needs to do. The duct fan is just trying to circulate air in a static pressure (or near static) pressure atmosphere. Using your shop vac as a blower would work better than that duct fan as well.
 
kuraki said:
Well. Take this for what it's worth. I've built 3 forges and own an Atlas mini. I've made 3 venturi burners and still use 2, one little one for my parkerizing tank and one big one for my heat treat drum(which is soon to be replaced by a blown burner and PID control). Javand talked me out of building a ribbon burner but I'll never build another venturi forge for forging steel. It's just not worth it, for anything but something the size of an Atlas mini in my opinion. Forced air is just infinitely easier to tune to the exact conditions you want because you don't have to depend on Bernoulli to get the right air/gas mixture. You can have exactly as much air as you want regardless of gas velocity. In fact, I'm so positive that it's worth a $100 blower to not dick around with venturi burners, if you buy that blower and decide you don't want it, I'll buy it from you and either keep it on the shelf as a spare or build another horizontal forge.

Pressure is more important than CFM because you have to overcome the forge atmospheric pressure. Those duct fans create almost no pressure, you'd be far better off with an induction blower for a wood stove if you want to do it on the real cheap, they're at least doing something similar to what a forge blower needs to do. The duct fan is just trying to circulate air in a static pressure (or near static) pressure atmosphere. Using your shop vac as a blower would work better than that duct fan as well.
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I appreciate and value your input.
What kind of forced air forge did you build, in lieu of a ribbon burner? What was the deciding factor?
 
I built a vertical forge since my main intent was making damascus. Being vertical, with the work above the jet of flame, heating is plenty even. The additional forge I may or would like to build is a horizontal forge with a larger opening for odd shaped work like axes, or work that I don't want to or can't weld a rebar stud too and so it needs a floor to sit on. There I'm still considering a ribbon.

The forced air burner on the vertical is incredibly simple. It's simply a "T" with the gas coming in from a needle valve and the air from the blower coming from 90 degrees to it. Then a reducer that terminates inside the forge. That's it. No wire size drills, no bell mouths, nothing.
 
So I would advise strongly against using anything other than a centrifugal blower for any sort of forge. Pressure is part of the reason, but by no means all of it. The bigger reason is how the fan deals with airflow restriction. The load on a centrifugal blower is essentially the same regardless of restrictions to airflow. Back pressure does not cause strain on the blower (instead it just circulates the same air). Furthermore, for all centrifugals I am aware of, even if you restrict inlet airflow, you will not damage the blower.

None of these things are true about a fan style blower, even if you could get your static pressure high enough at the flow rate you are looking for. When sizing a blower, you have two options. You can do the BTU math based on forge volume and insulation, or you can figure 100 CFM centrifugal for a small to medium sized forge. You go with 100ish CFM (no back pressure) because you will want a bit of spare and will want about 30CFM with all the flow restrictions your blower setup will create.

For a larger forge you will need to go up from there. I use a 650ish CFM bounce house blower on one (large) forge because they are rated for continuous operation, can take the pressure very well, don't mind flow restriction, and I often overbuild things. Plus I like to be able to get to forge welding temps as quick as possible. In fact, I don't run the thing full out even when bringing to temp because the heat that it puts out is INSANE at full blast. My rough math said that it spits out several million BTU at full grunt.
 
I have a horizontal and vertical forge and currently building a ribbon burner. Being that I rarely use flux any more I want a welding forge with a floor. I also need one that’s much longer then what my vertical gives me. But if your just starting out and not planning on doing a lot or any welding then just build a horazontal blown forge.
 
I think I do have a bounce house blower out in the tool shed now that I think of it. I may put the ribbon burner on hold and just do a more basic horizontal blower to start out.
 
Horizontal forge is my preference for most tasks. Made from a tube with an angled blower is kinda nice for knife making (or more specifically heat treating). I tend to prefer a burner coming in from the top straight down into a rectangular cross section forge for general smithing, as it allows a more localized heat (still not as nice as coal for heat concentration). That being said, the tube design is much easier and cheaper to make and line, especially if you got some welding equipment. A layer or two of inswool and some rigidizer, a floor of castable (or I am fond of bubble alumina), and you are off to the races.

Some pointers for the blower. About a foot of steel pipe extending from the exterior forge wall before you transition over to PVC. Use a PVC gate valve for your air gate if you want to go cheap and easy. Use high temp, semi rigid aluminum ducting for the air supply as the lighter duty stuff like dryer vent line will get holes in it from flying slag, flux, etc. And if reasonable, if you forge in an enclosed shop, try to get the blower positioned outside/far away from your anvil. They are LOUD. Obviously, wear hearing protection while forging either way (which I totally, always do. I am never lazy about it. I promise.).
 
Bounce House blower is HUGE for a knifeforge. We just use hairdryers to blow gassers.
Buy several at thriftstore for a few dollars. With daily use, Some will last a couple years, others a few months. A wrap of duct tape seals it into the blower tube. Its not rocket science.
knife to a gunfight said:
I think I do have a bounce house blower out in the tool shed now that I think of it. I may put the ribbon burner on hold and just do a more basic horizontal blower to start out.
Click to expand...
 
Lieblad said:
Bounce House blower is HUGE for a knifeforge. We just use hairdryers to blow gassers.
Buy several at thriftstore for a few dollars. With daily use, Some will last a couple years, others a few months. A wrap of duct tape seals it into the blower tube. Its not rocket science.
Click to expand...
I use it on a large general use forge. Yeah, it is big, but then again it doesn't hurt to go oversize if he has one laying around.
 
ive used a bounce house blower for 8-9 years. I asked Cashen what he used and he said "a bounce house blower" i regulate the air flow with a gate valve.
 
That Blacksmith Depot blower is a good unit. It can be wired to a fan/light dimmer control and made variable flow. The cheap HF router speed control will also slow it down. They also have a DC version that controls well.

I have built a small blown burner with a hairdryer, too. It will run a small forge OK.
However, it does not compare to a blown burner with a proper air source on a medium to large forge.

Lastly, the VERY BEST way to control the air flow from a blower is to reduce the air flow ....NOT the blower speed. Putting a gate valve or blast gate ( the type used for shop vacs is fine) just after the blower will allow controlled flow with no drop in pressure. The blower will make more noise, but will not be harmed at all even if closed of fully.

I saw a neat setup where the person had a small line tapped off the manifold just after the blower and before the gate valve. This line went to a pipe at the front of the forge angled so that it blew across the dragons breath diverting it away from the operator. This line was always at full pressure, regardless of how much the gate valve was closed.
 
That's a neat idea. I singed my beard a couple times this winter since my window is right up at face level.
 
My blower has a built in damper on the intake. I drop the speed with a varac becaus it’s crazy loud and WAY WAY over powered for my use. I drop the speed with the varac and fine control the air with the damper. It’s amazing how much better a forge will run when you have high pressure. Knowing what I know now I would not hesitate to buy a proper blower from blacksmith depot. I would not even mess with a hair dryer. Yes thy work but once you switch to a high pressure blower you will be shocked.
 
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