Exhaust fan for existing vertical steel chimney through flat roof

[Humphreys Hammer]

My existiing coal forge set up includes a hood that is 45x34 at the bottom, 56" high and 34x45 at the top.  The 10" steel chimney extends 48" above the hood where it enters the ceiling/flat roof. The chimney extends approximately 12 feet above the flat roof.   The distance from the fire to the bottom of the roof is approximately 9 feet.  

I want to add an exhaust fan. The chimney draws reasonably well but would like to minimize smoke/soot that doesn't get drawn out with this passive hood and chimney.   Received a proposal for a fan rated to 650 degrees, but price is $7,000 without delivery and installation. Proposal suggested at least 12 inch chimney and bigger hood. These costs are much more than I can pay.

Any suggestions or leads to a less expensive solution greatly appreciated. Shop is in Boston.

Thanks

 

[Irondragon Forge ClayWorks]

Welcome from the Ozark mountains.

I had a similar situation as yours. The hood didn't draw strong enough. I tried an exhaust fan, no real help. My solution was to bring the chimney opening closer to the fire. It draws just fine now. Been working for about 20 years. Pictures were taken for another thread. Might put in some pictures for reference.

This was after the hood burned out some so I riveted on an extension, draws better now.

I can't control the wind, all I can do is adjust my sails.

[Latticino]

My recommendation is similar.  Ditch the overhead hood, or use an insert like Iron Dragon did, and substitute a super-sucker type side draft.  These draw surprisingly well if you have adequate rise in your vent stack.  Note that code requirements for roof penetrations are not always easy to achieve.  Also, allowing an air path to makeup the exhaust you are removing is also critical

If you do go with an accessory exhaust fan, you should size it for approximately 100 ft/min velocity at the inlet face of your hood.  It is a little hard to understand from your description, but I assume the inlet area is 45 x 34/144= 10.6 SF.  Therefore you should select a fan capable of, say, around 1,060 CFM at a nominal 0.75 inches of static pressure (both values required for selection).  The motor ideally will be out of the airstream, and the fan rated for high heat, which can get expensive.  You might get away with a used upblast, rooftop, commercial kitchen exhaust fan, but then you will have to correctly install a roof curb.  A 1/2 HP Greenheck CUE-120A will fit the bill, but will cost around $1,400 just for the fan, and is only good to 400 deg F.  With a large hood like you have, you should entrain enough shop air to cool the stack down quite a bit under normal operating conditions.

[Humphreys Hammer]

First, I am super grateful to you for your response.  I have looked at the Super Sucker drawing on Anvil Fire and can see that

this promises to be a much cheaper solution than an exhaust fan.  Has to be the better than what I have now.   I hope I am not imposing too much on you by asking a few more questions.

1. If I understand correctly the supersucker concept and drawing, I would have to move the coal forge on which the supersucker is to be mounted  so that the supersucker's top opening for the chimney would be directly under the opening/flue of existing vertical chimney.  Am I correct?

2. Having so moved the coal forge and fire pot, the supersucker's intake opening would sit  on the 1' thick wall of the coal forge, which is 12" inches from the near edge of the firepot?  Or should the intake opening be closer to the firepot edge?  If so, what is optimal distance from the firepot's edge to the intake opening?

3. If I replace the 10" diameter chimney with at least 12" from the top of the supersucer and through the roof, would it detract from performance to connect the new 12" to the existing 10" chimney above the roof line? Or would it better to dump the 10" inch and install 12" for the entire chimney?  In any event, what is the optimal height of chimney measured from the surface of the flat roof?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Latticino]

Very respectful response.  I appreciate that.

In reply to your questions:

1. While the flue does not have to go straight up, it isn't a bad idea to route it that way.  If not, use (2) 45 degree elbows to offset the duct, not (2) 90 degree elbows.
2. The super-sucker style hood should be mounted with the opening as close to your forge's fireball as practical (how close depends on the typical size of your solid fuel pile, but I would go no more than a couple of inches at most).  Properly used they will suck the fire horizontally right up the flue.  Typically this means that your new hood is mounted right onto the end of the forge, a couple of inches from the fire pot.  Don't be afraid to rest the bottom of the hood right on your forge.  That's how most I've seen have been installed.
3. A 10" diameter duct should be fine for a super sucker.  If possible, the section connected to the new hood should be black steel, not galvanized duct.  If you have a large coal fire it can get pretty hot.  Going up to 12" won't hurt, but I doubt it will be required.  The code requirement for a flat roof is a minimum of 36" above the roof surface.  The higher you go, the better the draft on your hood.  The limit is the stability of your stack in wind, but you can always guy it off to the roof if you want to really go high.

Just be careful regarding the roof penetration itself.  Typically these need to have a certain clearance to the structure, and depending on how your roof is constructed you may need to include a ventilated thimble.  For example, the code clearance for single wall duct penetrating a wood frame roof is 18" between the duct and any combustible material.  You can buy special preinsulated duct that is rated for much less clearance, but that gets expensive.

[JHCC]

Super suckers are awesome. I had one I made from a helium tank, and it did really, really well, even though the flue was mostly horizontal and had four 90°bends in it.

(But follow Latticino’s advice anyway.)

[Humphreys Hammer]

Many many thanks to both of you.  Sounds like I can build and install this using the existing 10" chimney.  Will probably tear down the exiting hood. That will enable me to get easy access to the existing flue to connect the supersucker to it. 

Landlord is replacing; the flat roof rubber membrane in September so I need to get this done soon. Will keep you posted and thanks so much again.

 

[JHCC]

Here’s a video of a test fire with my super sucker; this was with a 10” flue.