Thread: Exhaust fan questions (from US)

Originally Posted by mwells

Question 1. Is the 8" fan good enough?

It depends on the real CFM. An 8" fan should be able to deliver inside the CFM range you are after.

Question 2: Does it make sense to have a damper inside the shop as well? I was interested to find that Vents publishes fan curves for their butterfly dampers, also: KOM U 200

In these cases the damper just seems like a crude way of regulating the fan speed. Given "more is better" for wood dust unless factors like noise are an issue then I would not bother with speed control.

Question 3: They recommend installing 1m of duct before and after the fan. I'm not clear if that's for air movement, noise, or safety. If I do that, then the inlet will be 1/3 of the way into my shop. Has anybody had trouble just using the fan's inlet without extra ducting?

Probably helps with air movement but it will not help to get a one way flow across a shop.

Originally Posted by BobL

It depends on the real CFM. An 8" fan should be able to deliver inside the CFM range you are after.

I guess one of my questions is what is a good target air exchange rate. I have seen 8" centrifugal fans marketed for hydroponics that say they move 740 CFM, which seems very unlikely. (VIVOSUN 8 Inch 740 CFM Inline Duct Fan) I'm not sure when we quote 20 air exchanges an hour if that's real air exchanges, or accounts for manufacturer's inflated CFM and louvers.

Originally Posted by BobL

In these cases the damper just seems like a crude way of regulating the fan speed. Given "more is better" for wood dust unless factors like noise are an issue then I would not bother with speed control.

I should have been more clear. I mean a backdraft damper to keep air from entering the workshop when the fan is off. The wall vent has a thin metal damper, held in place by a spring. Does it make sense to have another backdraft damper right before or after the fan? I know bathroom fans often have two.

I really appreciate the fast, knowledgeable responses on this forum!

Mark

Originally Posted by mwells

I guess one of my questions is what is a good target air exchange rate. I have seen 8" centrifugal fans marketed for hydroponics that say they move 740 CFM, which seems very unlikely. (VIVOSUN 8 Inch 740 CFM Inline Duct Fan) I'm not sure when we quote 20 air exchanges an hour if that's real air exchanges, or accounts for manufacturer's inflated CFM and louvers.

Using one or even a few fans never results in even one real room air change. To achieve real room air exchange requires a wall of fans at one end of a shop and an opposite wall of air entry ways. The air flow path between air entry ways and the fans should be unobstructed as possible and nothing inside the room (like a motor fan) should upset the air flow.

It depends on the shape of the workshop but most squarish workshops and rooms with just a few exhaust fans on one wall typically achieve a "1/2 dilution" model.
This model says when a whole rooms worth of air has gone into and out of the room only about half the air exiting the room is air that contains dust, the rest is fresh air that has just come into the room.

This means each room air exchange at best only 1/2 the amount of suspended air dust i removed.
The next whole room air exchange removes 1/2 of the remaining dust.

So the sequence is 50%, 75%, 87.5%, 93.75%, 96.875% etc
So some dust always remains and is why 20 real room air changes pr hour is a target but if all you can get is 10 real room changes that is still very good.

NB the above assumes that no more dust is being produced in the meantime and the shop is not crowded or divided by large cupboards, shelving or timber racks etc which can create dead air spaces that will be difficult for one or a few ventilation fans to clear air from behind.

I should have been more clear. I mean a backdraft damper to keep air from entering the workshop when the fan is off. The wall vent has a thin metal damper, held in place by a spring. Does it make sense to have another backdraft damper right before or after the fan? I know bathroom fans often have two.

Backdraft dampers and louvres will usually just reduce flow. These should be removed and replaced with some sort of manual damper that can be removed or placed well away from teh air flow.

That all makes perfect sense. I was trying to do an exhaust fan as a interim step before a cyclone, but especially since I have so many windows and doors in the shop (see picture in first post on this thread), I think it makes more sense to just use the open windows, doors, and standard box fans for now. Then I can focus on getting a cyclone setup. The cyclone can always be used as an exhaust fan. These little fans just optimized for less noise and energy usage.

Thanks Bob, you really helped clarify my thinking.

Mark