rthibeau

Exhaust Fan instead of Chimney

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Can't get this idea out of my head.  Was thinking of an exhaust fan in the wall, just above the coal forge firepot, to vent all instead of the traditional chimney arrangement.  Unorthodox maybe, but it should work just fine ..... wouldn't it ?  I'm going to be building a new shop and that thought popped up on me.  Seems it would suck the smoke and fumes out just like a conventional side draft setup.   .... (waiting to hear Frosty's input ) ....;)

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I saw something like that in an old 'Anvil's Ring'. Guy set a hood made from the end of a 275 g oil tank above the forge, with an exhaust vent cut into the wall. He then rigged the fan in the vent, but put it on the end of a shaft, so the motor was mounted  a few feet back into the shop, thru the hood, so the fan motor wasn't exposed to the dust and smoke. Wired it up, and away he went. Seemed pretty effective. I just went and did the Hofi style side sucker, but I considered that exhaust fan idea. Either way you avoid roof penetration.

Steve

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Yes it works fine provided it's a suitably sized extractor, ducting and hood over the forge. 

Doubt a domestic kitchen extractor would work. 

You can just about make out such a setup in this image:

PHYbqE4.jpg

The ducting takes it across the shop to a wall vent on the other side where two other hoods and extractors join. 

I can possibly get more info/pics on Saturday when I'm next down there. 

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Take a look at a Tjernlund draft inducer.  That's exactly what they're built for, albiet in a wood stove arrangement. I got one I was thinking of using, but with a super sucker hood and 14' of 10" pipe there was no need.

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I am not sure the Tjernlund draft inducer would work with green smoke from a bituminous coal fire. Not for very long anyway until the soot built up.

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Tjernlund's materials say that they're for both wood and coal stoves. I can't imagine that the soot from coal smoke would be more of a build-up problem than creosote from a wood fire.

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Greetings RT,

         Consider the point of discharge.  You will find it to be very dirty and will stain anything in its path.. Eventually everything goes up .  In my small studio I designed sliding roof vents for ventilation in addition to a side draft at the forge.. Also consider the noise of a large fan induced draft..  I like quiet..  Just this ol boys 2c

Forge on and make beautiful things 

Jim

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I had a similar situation/need.  I have a press-formed aluminum roof that i will NOT put holes in so the need for an alternative vent system prompted me to design a powered vent. 

I used an old walk behind leaf blower housing and impeller for the "power". I am driving it with a 1/4hp 1740rpm 110v motor.  pillow block bearings and shaft for the impeller, 1.5" drive x 6" driven pulley gives it 435 shaft rpm which is about perfect.  The housing has a 6" inlet from the forge hood (B-vent through the ceiling), and 4" outlet which I connected to a B-vent and exited through the gable with a thimble (I even made a little platform for the birds that WILL nest above it).  Mounted a switch on the wall next to the forge to turn it on/off.   The blower assy sits in the attic space right above the forge, so nice and quiet without any insulation but even quieter with !

There is enough fresh air being drawn in with the smoke to keep the temp down but the housing and impeller are thick enough to handle it. I am planning on splitting the input to use above my welding table. will need to fab some metal slide valves similar to those used in a woodworking dust collection.

  Todd

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Howdy folks! First time posting here.  I am currently trying to get a permanent set up for my forge and was taken back by the price of chimney pipe.  I was thinking of using a duct fan and found this thread and wanted to ask here so I didn't need to make a new thread. 

what I'm thinking is, having a good sized hood over the forge, with 8" chimney pipe L over to a horizontal 8" duct fan, then L out a window.

What kind of CFM are we talking about as being enough for something like this to work?  I have my eye on an 8" duct fan with 420 CFM.  Would that work? 

Thanks!  Ben

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I need to buy a vehicle to carry my stuff; how big a vehicle do I need to buy?  Hard to give good answers without knowing all the details isn't it?

Can you tell us what type of forge you will be using?

What fuel you will be using?

Your general  location?

Your general set up---remember if you do not use an approved method and your shop is attached to your house it may negate your home owner's insurance if any thing bad happens... My shop is a steel building a distance from the house in a rural area and so *much* looser local orninances!

When I put a chimney in for my moderately sized coal forge the ductwork cost me  under US$10----of course I didn't have to use triple wall pipe and pay for a roof penetration. (10' of 10" diameter spiral seamed duct pipe from the restore---I over spent as I could have gotten it for free when they remodeled the building next to my endocrinologist's office and had a construction dumpster full of the stuff...)

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Welcome aboard Ben, glad to have you. If you'll put your general location in the header you  might be surprised how many of the Iforge gang live within visiting distance. Thomas in his endearing round about way of using metaphor to make points is saying we can't give you a meaningful answer without knowing more. You need to present some basic information such as: Building type, residence of not, forge size, etc. 

Typically solid fuel forges don't generate high stack temperatures so you need a little larger than for a stove and single wall will work. You have to check local code though, it might. Some places don't care what kind of fire is in the appliance it MUST have a triple wall stack. Different places, different rules.

Frosty The Lucky.

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Also, you lose draw with any angles in the pipe especially 90 degree bends. Can you go through the roof or definately not? Or does it just seem convenient to direct it out a window?

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Sorry for not providing more details, I assumed there was a typical CFM requirement for this kind of thing.   I currently am using a coal forge I built for being mobile, but I am wanting to build a brick hearth in the building I am going to be making into my permanent smithy.  Nothing enormous, so moderate sized coal forge.  The building itself is between 8 and 10 foot square. I live in the country, so no regulations.  It is not attached to the main house.   it's a concrete walled building, an old well house and it's about 6' at the top of the walls and 10' at the peak.    I'm thinking a 2' sq. hood to 8" single wall chimney pipe. 

52 minutes ago, Daswulf said:

Also, you lose draw with any angles in the pipe especially 90 degree bends. Can you go through the roof or definately not? Or does it just seem convenient to direct it out a window?

The reason I am wanting to go out the window is because it's there lol, I wasn't really wanting to cut a hole in the roof.  I don't have any experience with that type of stuff.  Also I didn't know if the duct fan being directly above the firepot would lower it's life span.  If I went straight up there would only be about 2 or 3 feet worth of pipe before it was through the roof.

 

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Straight up is the best rout. If the forge is close enough to the window that you can do slight curves you may be ok. Also there are other things to consider for good draw. 10"-12" single walled pipe will work well. I was a bit scared to cut into my roof at first but mine (10" sw pipe straight up and 4' above the roof within 6-8' from the highest point) has worked just fine for me. 

Give Glenns link a read through and definately know how you want your shop layout but it's nice not needing an electric helper fan if you can help it.  

 

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Yeah, I know that a straight up chimney would be best, but with a duct fan, can it overcome the loss of efficiency due to the bends in the pipe?  and what about the direct heat exposure on the motor if the pipe was straight up and out?  I could go for a bigger duct fan, I just had my eye on one in 8" duct that had 420 CFM and was wanting to know if that would be sufficient, if not I can try to find a bigger one with more CFM

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Is the motor for that duct fan inside the duct?  If so that is NOT a good idea.  Heat and smoke/soot will kill it rather quickly. (I didn't have access to IFI 25 years ago when I tried it...)

Cast concrete or concrete block?  If concrete block you could build a super sucker chimney and have it exit the building through 1 removed block.

Also look at how Hofi did his side exit chimneys for his blacksmithing school

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Direct heat will definately wear on a motor. If you look at an above post they used a blower external from the motor and a bit further up from the heat source. 

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3 hours ago, Irondragon Forge & Clay said:

Depending on the height and location of your window, you might be able to use the side draft chimney. Don't have to go through the roof and they draw like crazy with no electricity required.

 

That looks like a really good chimney to go with, it isn't as tall as I thought it would have to be.  My biggest issue with traditional chimney is the price of pipe though, but with it only being 10' tall I may be able to afford that.

3 hours ago, ThomasPowers said:

Is the motor for that duct fan inside the duct?  If so that is NOT a good idea.

On the model I was looking at, yes the motor is inside the duct.  I thought it might.  The walls are poured concrete.

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I have a side draft poking horizontally thru the wall with the stack outside. The horizontal box is 13" square by 4' long, of 10 ga mild steel. I had a local guy shear the sheet and bend it for me, then I welded it together. The stack is 12" dia single wall galv duct- standard duct is probably 20- 22 ga or so. It is braced on the outside with some angle, and I put a basic turbine whirly- gig on the top. Works a treat for me and no roof penetrations. Natural convection with some help from the turbine (unpowered except by wind) gives me plenty of draft.

Steve

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