New shop flue - Sanity check required

[Drunken Dwarf]

7 hours ago, Frosty said:

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.

Unfortunately, I can't go straight out. I've seen Hofi's setups, they look amazing but the forge is considerably lower than the sil of the window and only the top section of the window can be opened/removed.

7 hours ago, Frosty said:

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!

I'm definately going to be installing brand new CO2 detectors in the new workshop before any forging starts. That's not optional in my view

 

 

... I've been back to the shop today and taken measurements, so now for the first time, my pictures are TO SCALE! The opening part of the window is 10" tall but actually 17" wide, so I can go straight up with it rather than tapering in as in my last upload. Still fairly unsure about opening size though.

[ThomasPowers]

CO detectors not CO2!   Takes a heck of a lot more CO2 to cause problems; while CO will build up in your body.

[Drunken Dwarf]

Lol, I meant CO, carbon dioxide I'm not overly fussed about , I'll just get a plant 

[Charles R. Stevens]

That is looking mighty blacksmithy right there! 
I would start with an opening 80% of the flue area. You can always make the opening larger. Say to 85 or 87%. 
a wad of paper in the flue opening will jump start the draw and cut down on start up smoke. 
with a good flue you don’t have to get crazy with ventilation, but don’t neglect it either. 

[Irondragon Forge ClayWorks]

I was going to say the opening should be smaller than the flue size, about 50-75% of the flue is what I kinda remember from other posts. Your last drawing looks good to me. These at ESSA are 50% and they will suck the stuff out of your shirt pockets.

[Latticino]

On 9/2/2022 at 4:57 PM, Drunken Dwarf said:

why a side draft is beneficial over a top hood

As JHCC noted, a high speed inlet very close to the source of the fumes will capture the adjacent fumes.  The larger size hood above the source of hot fumes will both capture the fumes and the environmental air from the sides of the hood.  If the fumes happen to be rising at a higher velocity than at the capture zone at the face of the hood, they can "hit" that lower velocity section and create turbulence which can lead to the fumes spilling out past the hood.  If you look at it from a fluid dynamics standpoint, there is a characteristic flue velocity that will be created by the buoyancy of the lighter hot gasses inside the flue.  The hotter the gasses, the potentially higher the velocity (Note: as previously mentioned in other threads, a typical capture velocity for the face of an overhead hood is on the order of 100 ft/min).  If you induce significant room air into the flue, you cool those gasses by mixing.  The volume flowrate is directly related to this "chimney effect".  If the flue stack geometry is the same for an overhead hood and a sidedraft hood the hotter gasses from the latter will pull in more of the fumes you want to extract, and the proximity ensures that.  It is not at all uncommon to see the fire at the top of your coal pile to get sucked horizontally by the induced draft.

[JHCC]

16 hours ago, Drunken Dwarf said:

my pictures are TO SCALE!

That looks very nice!

14 hours ago, Charles R. Stevens said:

I would start with an opening 80% of the flue area

Agreed. An opening of 11.5" square would be a good starting point.

 

[Charles R. Stevens]

If the ventilation guru suggests 50% I would start there. You can always cut it larger. Then again one can always make a slide door and make it adjustable. 

[Drunken Dwarf]

Thanks for all the help, I'm ordering the steel tonight I think. Would 19 gauge (1mm) be thick enough? Assuming I can weld that with my little stick welder without blowing through (1.5mm it's the thinnest I've welded so far).

[Charles R. Stevens]

1mm is probably overkill, but it keeps with the blacksmith look. Bent flanges and rivets are good if you melt stuff.

[Drunken Dwarf]

So, after various problems with suppliers, my sheet finally arrived. On with the work! 

1mm sheet is surprisingly hard work to bend without a proper jig:p

[Charles R. Stevens]

Don’t forget to rivet a little shelf on it to hold a bit of soap stone, hoods make great chalk boards 

[Frosty]

Good reminder Charles, I like flat or semi-gloss black paint on hoods for just that reason. I found a magnetic soap stone holder at a yard sale years ago but haven't seen one anywhere since. So, a shelf / chalk tray it is.

Frosty The Lucky.