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New shop flue - Sanity check required


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Hi all.

 

Now moving into a new shop and unlike the old one, this one has an opening I can "in theory" run a passive flue through (In the past I had to work with a fabricobbled centrifugal extractor fan, which worked VERY poorly).

For reference, this is for a small, bottom blast coal forge (forge pot about 10" diameter at top).

Theres a window behind where the forge is going with an opening leaf with an apeture of about 10.5". Going through the roof is NOT an option (rented shop, strict requirements).

I'd like to install an overtop hood (I cannot afford the extra 12" space behind the fire for a side draft flue) which will start as a 12" hood (8"-10" above fire pot), reduce to a 10" pipe and go up a small way. It will need a 90deg bend to go through the window and a horizontal run of 1'-2' to clear the eaves of the building. Then a 90deg elbow and up for another 4.5' - 5'. Clearing 4' above the flat roof with a cowl.

Unfortunately the window openeing is too small to get through at a 45deg angle, and the eaves of the building extend enough to get in the way of that too.

 

Should this work? Unfortunately I have neither the time nor money to get it wrong. If I can't be sure this will work, I'll need to resort to powered extraction, which im pretty certain will work but I'd like to avoid for it's own complications.

Any help/advice would be amazing.

 

Thanks a lot,

Danny

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For your situation, looks pretty good. If you have a problem, make your vertical run taller. A 12" flue pipe adds to the draw as well, but a 10 will do. In the US we can get two sections of put it together yourself 6" and put them together to get a 12" flue. 

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Think carefully about how you are going to support this (including from wind loads).  Also keep the duct at least 18" from any combustible structural elements.

You may have issues with the fumes spilling past the 12" opening.  That is one of the advantages of the side draft hood.  A smaller opening can be set very close to the fire, so capture is very effective.  

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Can you go with a side draft, kind of like what Joey van der Steeg has in his latest video, straight out of the lower portion of the window? Maybe a smaller diameter for the horizontal than the vertical section. If I understand right, the opening of the side draft should have lower cross sectional area than the vent pipe, for higher velocity at the entrance. I’m not sure if that would apply to the full horizontal run though. Maybe Mikey or  Latticino would have more input there. (I think one of them maybe both have HVAC backgrounds.)

 

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10 hours ago, Goods said:

If I understand right, the opening of the side draft should have lower cross sectional area than the vent pipe, for higher velocity at the entrance.

Yes, that's right. The constriction at the opening creates an area of higher velocity and lower pressure inside the hood, which pulls the combustion gasses out of the fire and up the flue.

The original Super Sucker design has a 10" square opening and is supposed to be used with a 12" flue; that makes the flue cross-section about 13% bigger than the opening's. My own helium tank super sucker runs on a 10" round flue and originally had an 8" square opening which I've since choked down even more. Take a look at that hood's build page (at THIS LINK) for more details, including how quite a lot of the flue is running horizontally without issues.

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Yes, I have a HVAC Engineering background, and yes Steve, Goods and JHCC are giving good advise (as usual).  Note that after the reduced size entry opening the flue duct should be full size for the entire length to attempt to minimize duct friction which will reduce your flow velocity.  Also:

  • The height of the stack is the driver for the exhaust.  Within practical limits the taller the better
  • Try to minimize the number of elbows in the line.  If you must have them use 45 degree elbows if possible, or at worst long sweep elbows with smooth interiors (pleated elbows of flexible material are the worst option)
  • Check your building code for details on penetration of structure and required height above adjacent roof pitches as well as termination proximity to operable doors and windows.
  • Make sure you have adequate make up air for your exhaust.  If the air can't enter your forge the combustion gasses can't be exhausted.
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Another thought occurs to me: you said that the window has "an aperture of about 10.5". If the aperture is 10.5" square, could you use a 10" square flue to fill up the full space instead of the 10" round flue pipe you were planning to use? That would take the cross section of the flue from about 78.5 square inches to 100 square inches, an increase of almost a third.

21 hours ago, Drunken Dwarf said:

fabricobbled

Love this.

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Thanks for all the replies. A lot of great information here.

21 hours ago, Latticino said:

Think carefully about how you are going to support this (including from wind loads) 

I was thinking perhaps a coupld of lengths of 1" pipe, one either side of the flue and concreted into the ground outside 1' - 2' deep.

 

21 hours ago, Latticino said:

You may have issues with the fumes spilling past the 12" opening

I can increase the hood if need be. A 12" hood is just what's currently on it.

 

21 hours ago, Steve Sells said:

example 4 inch by 20 inch will work also taking up less floor space

So, would this be 20" tall, or wide? I get how side drafts work, with the pressure and drawing through horizontally, I don't think I really understand why a side draft is beneficial over a top hood. Doesn't the side draft box thing also have to have the flue attached to the top, which would require it being 10" deep for a 10" flue to attach, or have I misunderstood entirely?

 

18 hours ago, Goods said:

Can you go with a side draft, kind of like what Joey van der Steeg has in his latest video, straight out of the lower portion of the window?

Unfortunately not, the lower part of the window is a fixed pane, only the top 10" opens.

 

4 hours ago, JHCC said:

could you use a 10" square flue to fill up the full space instead of the 10" round flue pipe you were planning to use?

Yes! I could absolutely use square ducting to increase the cross sectional area.

 

6 hours ago, Latticino said:

Make sure you have adequate make up air for your exhaust.  If the air can't enter your forge the combustion gasses can't be exhausted.

It's only a couple of feet from an external door, which will be open at all times when forge is running.

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What one is building is essentially a zero clearance fire place so none of what your doing is crazy or particularly difficult. 
As suggested the smoke didn’t care if it’s a square or round flue (or any other shape with in reason)

how big is your forge hearth and we’re is the fire pot situated in the footprint? A 30” hearth with a standard fire pot would have plenty of room for a side draft hood. 

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39 minutes ago, Charles R. Stevens said:

how big is your forge hearth and we’re is the fire pot situated in the footprint? A 30” hearth with a standard fire pot would have plenty of room for a side draft hood.

I've attached a drawing of my hearth, 36" x 24" with a 12"x12" corner cut off on each side. The fire pot is centered on the middle-rear 12"x12" section (about 8"-10" diameter). I don't think there's space behind it within the footprint of the hearth, couple of inches at most. And on either side the hood in combination with the wall would prevent me working on long bars as I cannot post through the fire.

And still I can't see why a side draft hood is so preferable. The question was about design of an overhead hooded flue.

 

1 hour ago, Irondragon ForgeClay Works said:

It should be easy enough to remove the fixed window pane and make a sheet metal surround for the side draft box to extend through

After going back to the shop today, my original drawing may be slightly inacurate in that the forge is lower than the sill of the window. I could raise it, but removing the fixed pane would not be allowed in my tennancy, even if I kept it to reinstall.20220903_000657.thumb.jpg.d2a8603e63bd97df3c34df833806dafb.jpg

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I was talking about 4 inches deep and 20 wide.  Above the fire, out of the way, it transitions to a 10 or 12 inch round.  The point is you wont have to give up 12 inches of space between the fire pot deck and the wall to have a side draft.

 

A side draft sucks the gasses into the pipe, where as a hood attempts (poorly) to collect it as it rises, along with massive amounts of room air

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So, if I'm getting the idea right, a 4" deep, 20" wide box (Theres about 2" between the fire pot and edge of hearth, but I can probably live with loosing 2" of floorspace). Above the opening it's get deeper and thinner (increasing depth to 10" and reducing width to 10") to then transition into flue. How big would the opening be? I'm guessing pretty wide otherwise there's no reason for it to be 20" wide, although still less area than 100"sq.

I get that the side draft actively sucks and is very effective at pulling in the smoke, I think whar I dont understand is why an overhead hood doesn't suck. I'd have thought with heat below heating the air and lots of vertical, the stack would've created a similar amount of suction. I think that's where my lack of understanding stems.

Also, stupid question. feel free to call me a fool. There's an open door providing lots of new airflow. The window opening is above the fire and theres another similar one the other side of the room (10' away). Since smoke rises cause it's hot. Would a powered 10" extractor fan in both these windows not also keep my shop smoke and carbon monoxide free?

 

20220903_121902.thumb.jpg.92d258b0d2395fb94e2bf84003b66ff6.jpg

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I dont think you really need 20 wide honestly. 20 was not an exact measurement, 14 or 16 may be better.  the HVAC guys here would have a better idea.

MY numbers were just an example.  FYI My forge is huge, 30 x 60 inch deck, 2 pallets of brick and 10 or so flue tiles to make it

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Steve’s forge is a thing of beauty. Last I saw it he has an adapter tuyere to make long fires for heat treating sword blades and a 3’ hunk of rail on the table with built in measuring marks and a small divot for blade straitening. 
he has a fine small work space for making pattern welded blades of all kinds. Packaged in a space that can’t be more than 8x12

My forge is also 30x60 but unlike Steve’s it’s a cart. 
 

you might consider raising the transition so to make it easer to get large bulky grills and scrolls in the fire. You can actually go square right threw the window opening. That would make the transition easier and then simply go round up the other side past the roof. 
 

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4 hours ago, Charles R. Stevens said:

you might consider raising the transition so to make it easer to get large bulky grills and scrolls in the fire. You can actually go square right threw the window opening. That would make the transition easier and then simply go round up the other side past the roof. 

Do you mean a 4" depth all the way up, then just out the window? (Sketched below if I understand right)... You guys are going to be the end of this sketchbook :P

I'm thinking I might want to transition to round after it's come through the window, upping to 12" round for the external flue so I'm not reducing the cross-sectional area at all (actually increasing it). That way I can weld the internal stuff from mild as it's not open to the elements and use cheaper galv flue outside as I'm not worried about it getting hot and off-gassing zinc when it's outside the shop.

My main reason to want to go round is that presumably it would suffer a lot less from wind forces. I don't reallt want to give the wind a flat surface to push against.

 

20220903_223753.jpg

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On 9/3/2022 at 7:33 AM, Drunken Dwarf said:

I get that the side draft actively sucks and is very effective at pulling in the smoke, I think whar I dont understand is why an overhead hood doesn't suck. I'd have thought with heat below heating the air and lots of vertical, the stack would've created a similar amount of suction. I think that's where my lack of understanding stems.

It’s a matter of efficiency. Basically, the side draft creates a lot of high-speed suction right next to the fire; the overhead hood passively catches whatever air and smoke rises up from the heat of the fire. In other words, the side draft creates additional suction that augments the suction from the stack; the overhead hood has the suction from the stack only. 

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8 hours ago, Charles R. Stevens said:

square would be easier to fabricate. 12” elbows are expensive 

I was thinking more of a fabricated square curve for the transition, rather than a 12" elbow. As attached (you know it's getting serious when the CAD comes out).

Any advice on an appropriate size of opening?

 

1962016502_SideDraftForge.thumb.jpg.5b1b5a469295b5aa6eb921fe3e8d6fdf.jpg

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It's looking better. I'd put it out the window as close to the forge as possible, Uri Hofi uses an excellent side draft on his school forges. They're a square steel duct from the fire about where you show the opening straight out the wall, then connects to a round vertical stove pipe and up above the minimum distance to the highest point of the roof. His side drafts draw like a inhaling dragon.

Yes, your extractor fans, will inhibit your side draft from drawing smoke but you shouldn't have smoke or harmful fumes escape from the forge once your stack starts drawing so the extractor fans shouldn't be needed. That said be sure to keep an eye on your carbon monoxide CO detectors until you KNOW FOR SURE Co is not escaping into your breathable air! Another good smoke detector is ask someone who hasn't spent time in your shop to walk in and sniff the air.

Human noses are pretty effective detecting stinky stuff like sulfur smoke but not if you've been breathing it for a while. TV commercials over here have come up with a term I like, "Nose Blind." Ever notice a gym locker room doesn't smell so bad after you work out? Nose blind. 

I forgot, a good way to start the draw, if necessary is to burn a crumpled sheet of news paper in the hood opening when you light your fire. Once it starts drawing the fire provides all the heated fumes it needs.

It's looking good.

Frosty The Lucky.

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