Thread: Not quite dust extraction......rathermore hot air extraction....

Originally Posted by FenceFurniture

. . . . .While the extractor is running it will take the very hottest air and then the air under that will obviously rise up to take the place of the previous air, etc etc for as long as I run the extractor (probably ~5 minutes)

Am I right in thinking that most of the dust will go along for the ride with the air that it is already in? And that therefore much of the dust will be sucked out and dust from the cooler layers will rise up to replace the previous, and also be then sucked out?

Sounds logical to me, but invisible dust seems to do some counter-intuitive things.....

If you run the DC for 5 minutes with a 4" duct (~400 cfm) that will remove ~ 400 x 5 - 2000 CF.

If your shed is 9 ft high x 15 ft long x 10 ft wide thats 1350 CF so running your DC for 5 minutes will give you ~1.5 room air changes.
This time is probably not enough to clean the shed

To fully vent a room really requires ~10 room air changes because the fresh air coming in mixes with air already in the shed and some of this fresh air will get vented out of the shed before some of the existing dirty air.
10 room air changes will required running your DC for ~35 minutes.
The long time required is a problem with using 4" ducting and a smallish.

Contrast that with a DC that can fully use 6" ducting (~1250 CFM) and its more that 3 times quicker and the time required will be under 10 minutes for that size shed.
This is why my time delay switch operates on the 0 to 12 minute range.

Yes, I see. Actually by 5 mins I meant for getting rid of hot air (and that would be variable with the ambient temp). Understood that dust removal would be longer.

How's my logic on the dust movement?

Originally Posted by FenceFurniture

Yes, I see. Actually by 5 mins I meant for getting rid of hot air (and that would be variable with the ambient temp). Understood that dust removal would be longer.

How's my logic on the dust movement?

Correct.

You bewdy! Thanks very much for all your help Bob.

Shortly I'll be posting about the ducting upgrade from 40mm to 50mm pipe for the Festool vac system which has made a huge improvement. This allowed me to install a 36mm hose on the drop saw (which is significantly better than Festool's recommendation of 27mm). However, there is some dust left behind, and no doubt plenty of scatterable invisible dust.

A little while ago I did a trial with the 5" hose from the big DE sitting just behind the back of the cut (along with the CT Vac as normal) and voila - no dust. One would have to think that a goodly amount of the invisible went along with it. So tomorrow I'll jimmy up a jig to hold the big hose in place. The hose will have to stand away from the saw a little way to stop it fouling the movements of the saw, so I'll use a very short ducted rig and plug the hose into the top of that.

The next phase is shown here:
https://www.woodworkforums.com/f200/ducted-system-shop-vac-mk-ii-184635

Never seen any sawdust in Brett's shed!!!!

It's always as clear as a hospital operating theater...........

I'd be trying to keep the heat in at Katoomba .... at 1000mtrs .......

and just the dust out...oh, you already trying to do that............

In a previous life I had an unlined 6M X 9 M tin shed. It got hotter than Hades in summer because the roof radiated heat into the shed. The quick and cheap fix was to put a series of sprinklers along the ridge line that were run occasionally to cool the roof. This made a big difference.

My roof was not lined, making it worse than yours. However, stopping the iron from getting too hot may be one component of a solution for you. The little black sprinklers are invisible unless you really look for them.

Have fun!

John

Good idea John, and one I have considered - just have to fix the leaks then