Thread: Dust Extraction CFM Loss

Originally Posted by snapper1

G'day
I am planning a ducted system for working in my double garage
Say I make it with 3 pickup points in different places
Now I wish to minimize the losses of CFM due to length of ducting
I am only intending to use 1 pickup point at a time

Will it make a difference where I install the shut off valve in each line if at any time 1 is in use and 2 shut off ?
It would be more user friendly in my case if the valves were located at pick up point rather than at dust collector end where ducting branches out

It won't make that much difference cfm wise but what it will do is tend to fill the open branches with sawdust.
If once branch (A) is not used for a while and it is attached and open to an much used one (B) then A will tend to block up.
If the blast gates are located close to the junction then the volume of sawdust that can build up between the junction and blast gate is small and easily cleared when the gate is opened.
If the blast gates are located well away from the junction then the volume of sawdust that can build uo between the junction and blast gate is higher and may not be easily cleared when the gate is opened.

i.e. with blast gates at A1 and A2 the most sawdust that can build up is that in the yellow brown colour.
With the gates at B1 and B2 the most is the yellow brown Plus the rest of the ducting back to the blast gate



There is a way to prevent this happening but it involves directing the branch upwards and then back down again. This is OK if you have oodles of OOMPH in your system otherwise you have just added a 180º turn which will add unnecessary resistance to flow.

The idea behind the location of gates is to reduce the number of unused open junctions/branches so that air and dust has a straight line flow direct to the DC.

Thanks Bob for the detailed and prompt reply

I will definitely install shutoff valves near the machine ends
and if sawdust build up is an issue I may then install valves at the junction ends OR should I do this at the start ?

I should explain my reason for asking is that I will have to run as much of the ducting as possible through the ceiling space .
I do however have reasonable access to there via a fold down ladder

Garry

Originally Posted by snapper1

I should explain my reason for asking is that I will have to run as much of the ducting as possible through the ceiling space .
I do however have reasonable access to there via a fold down ladder

Garry

OK I understand - in that case the connection to a machine Via a screw cap "Y" as follows is worth considering.

The machine is joined to the angled part of the Y and a screw cap is used as an inspection port. If necessary the screw cap cap be removed and any sawdust in the vertical can be removed or loosened so the DC can pull it through.

A hole can be cut and a transparent window can be glued into the screw cap if you want to get fancy.

Thanks again Bob ; I will also leave the pipe and fittings in those areas unglued until system has been in use for a while
so that changes can be made if necessary

Originally Posted by snapper1

Thanks again Bob ; I will also leave the pipe and fittings in those areas unglued until system has been in use for a while
so that changes can be made if necessary

I wouldn't glue them - just use gaffer tape.

I have a setup similar to what bob has presented ... With the left most gate to my thicknesser (producing oodles of chips) and the right gate to my SCMS. Blast gates at the machine head.

I have got into the habit of only opening/closing gates when the DC is running. On one occasion, I had just dressed a bunch of stock on the thicknesser, and knew I was done so closed the gate. As I knew I was about to use the scms, I opened its gate without the DC on, and dumped a half of bucket of chips into my SCMS enclosure. Cough cough ... Bugger !

Moral is, in my experience, you would need to be machining a lot of material to cause a clog, and an occasional ... what shall we call it ... backwash ? ... Of each gate could avoid a fair amount of cost in PVC fittings.

I also used grey duct tape to seal joints rather than PVC cement so change is pretty easy.