Shop Build Ventilation

[MC Hammer]

So I'm in the planning stages for my shop addition for my forge.  I' m thinking through things before I jump into building.  I've read a ton on here regarding ventilation.  My shop will be 12 x 20 and I'm using a 2 burner Diamondback Blacksmith forge connected to a 100 pound propane take that will be located outside the building.  My plans are to build a cement block base (suitable to add a coal forge someday if I go that direction) with a metal top.  My thinking is a large hood directly over the gas forge and connected to an insulated stove pipe chimney exiting out the roof straight above.  I also plan to install a turbine cap at the top of the chimney like the one pictured below.

My chimney would extend a bit higher than the one in the picture so I can be clear of snowfall in the winter here in NY.  One end will have 6 foot double doors while the other end of the shop will have a steel regular sized door.  I'll also have a window that opens on the prevailing wind side.  I'm thinking this will be plenty of ventilation in the summer, but I'm wondering if a cracked door and the turbine at the top of the chimney will be enough for the winter months.  I'm open to suggestions on this plan and any corrections from those with more experience.  I'm obviously going to have a CO detector in my work area and one at the back of the shop to warn me well before I have trouble.

[Latticino]

Need to plan for air inlet to makeup the air you are trying to exhaust as well.  I have a similar 10" turbine at the roof peak of my 2 car size forge structure and need to crack the door open or open a window to have it be effective.  Use of a hood over your gas forge won't hurt either, and you can calculate the stack effect to determine how much it will draw (CFM) then figure from that what the optimal size hood will be at the nominal 100 FPM capture velocity.  Left my Industrial Vent text at work or would check for recommended size, but at minimum I would suggest that you keep the hood in as close proximity to the forge as possible and transition to the spiral duct riser at no more than a 30 degree (60 degree included) angle if possible to maximize the effect of the hood.

I err on the side of caution in my shop (and add much needed forced ventilation for summer forging) by including a fairly large sidewall exhaust fan that really moves the air out of the shop.

[ThomasPowers]

I would suggest Boxing in the forge with the hood, except that that model has the burners on top and so I'm trying to figure a way to collect the hot exhaust WITHOUT any of it getting sucked into the burners and rerun---tricky

[MC Hammer]

Latticino - I was thinking about a straight shot up from the hood to the pipe going to the roof.  I know a side draft works well for a coal forge, but my thought is that the turbine attachment will create the draft to pull up the gas forge nasties.  I also think Thomas brings up a good point with the burners sucking in the fumes as there's no better way to get sick fast than for it to multiply the effect by recycling bad air over and over.  When thinking about this I wonder if it's not possible to think about Thomas's point like we do a car engine.  What if a fresh air intake was built to draw in air from outside the forge directly to the burners like the function of an air cleaner for a car engine?  I've never heard of this, but if it were constructed of steel pipe up near the forge it would seem to work.  When I owned a tepee years ago, I ran a steel pipe underground to the camp fire inside so that it could draw in fresh air and burn better.  In theory it wasn't needed as the updraft from the fire should draw in fresh air under the bottom of the tepee.

Latticino - how do you feel about me installing a high vent and a low vent in the shop for fresh air during the winter?  Maybe it combined with keeping the shop doors / window cracked open would be enough in the winter.  I'd like something I can open and close as needed.  I plan on insulating the shop well, so it will be pretty tight due to having the space heated to about 45 degrees when not in use during the winter.  I'll have to seal it up pretty good to keep the critters out during the winter.  I think the turbine on top will make it pretty tough to keep up with replacement air in the winter, but I think it will do a decent job of pulling the CO and combustion gases out which is half of the equation.  April - November should not be an issue as I should be able to have the double doors open and the back door on the opposite end open along with the window on the prevailing wind side. 

 

[Marc1]

About ventilation and flue in general ... I always wondered why is it that almost no one uses fan forced flue. THere are some good fans with the motor outside of the box and you can get away with 6" instead of having to use 12". 

As far as those whirly birds. Do they actually do anything? 

[Latticino]

57 minutes ago, MC Hammer said:

Thomas brings up a good point

This is a good point, and one that I had not considered as I work with a blown burner setup that is not prone to this type of effect.  Actually the bulk of the combustion exhaust gasses exit the doors of your forge, so capture hoods placed just over those openings will be effective in drawing off the fumes while still allowing the natural convection from the heat rising off the skin of your forge to draw "clean" air to the NA burners for proper operation.  Based on my image of the anticipated fluid flow around the forge I would expect that an overall capture hood would be unlikely to re-entrain the combustion byproducts, but better safe than sorry.  Unfortunately there is no simple way to duct clean air into the inlet of a NA burner.  They are typically carefully tuned to maintain the correct proportion of air/gas mixture and adding resistance via inlet ducting can throw off the design.  

48 minutes ago, Marc1 said:

I always wondered why is it that almost no one uses fan forced flue

As stated, I don't hood my gas forge, but do use a relatively large sidewall fan to provide mechanical forced air ventilation.  In my experience, very few gas forges are hooded.  In the case of solid fuel forges I typically don't recommend putting anything into the flue (like the impeller for a forced draft fan, or even worse the motor) as that can be a location for solid combustion byproducts to settle and unbalance the system.  It is also a potential failure point, which a system that just relies on barometric pressure does not have.  In industrial applications these combustion exhaust fans are sometimes used for complex large boiler installations where multiple boilers share a common main flue, or the chimney effect is not sufficient to maintain proper operation due to required flue duct routing.  Invariably these fans are set at the termination of the stack and have complex pressure dependent controls to ensure correct operation.

1 hour ago, MC Hammer said:

how do you feel about me installing a high vent and a low vent in the shop for fresh air during the winter

This is good practice for any room that has equipment that requires air for proper combustion.  The International code requires this and you can check for the recommended size based on the BTUH rating of the burner in your forge (or you can use a single, larger high wall vent opening).  Personally I compromise by using doors and windows...

1 hour ago, Marc1 said:

As far as those whirly birds. Do they actually do anythin

Yes the wind activated turbine vents are effective, but not necessarily better than a true chimney design for a hood hooked to a solid fuel forge.  They are really designed to provide good "forced" ventilation for a cold attic, and do work significantly better than a conventional ridge vent for that.  I will be removing mine when I finally get to install my coal forge and use that roof opening for my side draft hood.

[MC Hammer]

All good information everyone.  If I'm picking up what everyone is laying down there seems to be two schools of thought:

1)  The technically right way is to exhaust the combustion byproducts figuring the proper amount of fresh air to replace the volume of air you are displacing through the hood/chimney/ turbine AND replace the oxygen used during combustion by the forge.  This would be akin to having an OSHEA certified engineer come out and do the proper calculations to recommend fresh air replacement options, etc.  This is the safest route.

2)  The way most guys operate is to roughly figure this out by opening windows / doors settling on the side of more fresh air is best in conjunction with a working hood /turbine.  No real calculations are done, but safety is still front and center.  This option seems to do everything a smith can to get the combustion byproducts out and let all the fresh air possible into the work area while smartly placing CO detectors as an early warning system.

Honestly, I'm leaning toward #2 and that's why I'm putting double doors that can be opened on one end, a back door on the other end, and a window on one wall.  In the winter, I think I'll choose the days I work wisely so that as many openings as possible can be cracked / opened without the forge being too cold to work in.  I think I'll pick the warmer days.  Warmer weather can allow the doors to be completely opened.  Does that sound like a wise choice or am I off target?

[ThomasPowers]

You can tune your ventilation system with a good CO detector.  

Living where excess heat is usually the shop problem, 10' walls and open gables and 10'x10' roll up doors seem to work well in the "nothing succeeds like excess" department.

[MC Hammer]

Very true Thomas!