Shop Air Exhaust (for cooling)

[Bill Adamsen]

Apologies in advance if this is not the right place to ask this question.

My shop is a small barn ... inside dimensions 22.5' X 34.5' ... open format with a gable rood and an average height of almost 17-1/2 feet. Volume is about 13,500 cubic feet. It has a recirculating air dust collection system and many operable windows and doors. The high gable windows are not operable. When I built the barn it just didn't seem like I'd ever climb a ladder to open them.

The barn is designed for a cupola - in fact the base is already installed - I'm finally getting around to designing the actual cupola. I have two 12 gauge circuits built into the base since I was planning on adding lights and an exhaust fan. So that's actually the question. How do I calculate the number of CFM for the exhaust fan - which will blow air out the gable ends of the cupola roof? The inside of the cupola measures 31" square, and the intention was to place the exhaust fan in the cupola, above the windows facing up, to force air out through vents on the gable ends of the "cross-gable" roof. I am designing the cupola to require service (paint, etc.) only once every ten years ... and to bring the crane in to pull it off, do the service, and replace. Just in case the fan doesn't last that long, I want to be able to get it out through the inside of the cupola. So the question is ... how do I spec that fan? How many CFM are required, and are there properties other than CFM I should be considering? Are there any "simple" insulating systems detailed anywhere which I could borrow ideas from to be able to close this off when desired?

There are some additional photos of the building on my thread Clarification on Dust Collector Design (http://www.sawmillcreek.org/showthread.php?213720-Clarification-on-Dust-Collector-Design)

If this is too far outside the criteria for a workshop topic, I can delete.


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[Jim Andrew]

I assume what you are talking about is installing a whole house fan in the ceiling? Saw one in the Menards add this week, a 24". I installed one in the house when I built it, think it was a 30", built a box around it in the attic so it didn't interfere with the insulation, and put a 3/4" plywood lid with insulation on it with hinges so I could close it during hot and cold weather. It is really nice during moderate weather.

[Brian Tymchak]

Not a HVAC pro, not even a rank amateur, but since you've not received much response, I'll throw in what I've heard. I think the rule of thumb is to size attic ventilation to exchange the air ~10x per hour. Given your volume, I think that comes out to 2250 cfm. Hopefully a real pro will jump in here to better answer your question.

BTW, I think it's a fine topic for this forum.. We need a break now and then from all the dust collection threads that get started here.. :D

[Michael W. Clark]

I think Brian is on the right track. Best I remember 6 changes /hr is typical for general industrial hvac. I think the other criteria is what fan will physically fit and you want keep the velocity low enough in the cupola to avoid excessive noise.

[Bill Adamsen]

Thanks Gents. I think that was what I was looking for - how to size (CFM), and other attributes such as noise (mentioned), and probably corrosion resistance as well. That CFM seems to match up to 14" to 16" blades in the 2 amp range - and at that small size should be serviceable even in the constrained volume of a cupola.

As I research fans I'm learning a bit about motor and bearing types, other materials (stainless, galvanized, plastics) that would impact longevity, the CFM and how they're impacted by resistance, and also noise as measured in sones. I can see that it could be easy to make a choice one might regret! Thanks again for helping me think about this.

[Dan Schrecongost]

The net free area of the gable vents may limit the volume of air you can exhaust, so it's worth considering that as well as the fan capacity. There are both belt-driven and direct drive fans; the direct drive fans have no belt to break. Also, consider two-speed fans. Sometimes the low speed moves enough air and is more quiet. It's nice to to have the flexibility of low speed. I have a 24" 2-speed fan from Grainger that has a pull chain to operate and change speeds.

[glenn bradley]

Good stuff here. There is also wisdom in a in/out approach. One fan blowing air out of the area and another blowing in. This is generally done above the area to be cooled and removes the heated air that has risen to that space.

[Art Mann]

BTW, I think it's a fine topic for this forum.. We need a break now and then from all the dust collection threads that get started here.. :D

I agree with you on dust collection threads but exhaust fans do have implications for dust control. I installed a 3600 cfm exhaust fan in my shop that has about 6700 cubic feet of space. I did it mainly for cooling purposes but the speed at which that fan can remove both dust particles and lacquer residue is amazing. I can move the entire volume of shop air once every 2 minutes. That is faster and more effective than any air filter you could buy. I think it is a big bonus for dust collection.