# Down Draft



## mdhyams (Oct 2, 2012)

I am new to the dust collection systems. I built a 2'x4' down draft box out of 2x4's, plywood, and a peg board top. I am trying to find out what would be best for the bottom connection: 

2" opening that connects to a 4" hose
or 
4" opening that connects to a 4" hose.


Thanks


----------



## woodnthings (Jan 24, 2009)

*none of the above...*

If you can, run the largest hose your dust collector can manage.
The larger the hose the better the fine dust will stay in suspension.
But lacking a 6" hose inlet, use the 4" all the way.:yes:


----------



## Dave Paine (May 30, 2012)

+1 on using a bigger hose. The Woodworking Videos forum had a recent post on down draft tables.

The person made a table about your size with a 2 1/2in hose and found he was not getting sufficient airflow.

He then built a second table as part of a mobile cart and this time used a 4in hose.


----------



## mdhyams (Oct 2, 2012)

*draft*



Dave Paine said:


> +1 on using a bigger hose. The Woodworking Videos forum had a recent post on down draft tables.
> 
> The person made a table about your size with a 2 1/2in hose and found he was not getting sufficient airflow.
> 
> He then built a second table as part of a mobile cart and this time used a 4in hose.


 
How about putting 2 - 4" lines every 16" in the box. I was told by a guy today that he built something similar and it made a world of difference putting the extra line. He said he has his down draft as an extension on his table saw. He runs his down draft anytime he is cutting.


----------



## Dave Paine (May 30, 2012)

mdhyams said:


> How about putting 2 - 4" lines every 16" in the box. I was told by a guy today that he built something similar and it made a world of difference putting the extra line. He said he has his down draft as an extension on his table saw. He runs his down draft anytime he is cutting.


This can be done, but how many extra lines will be trial and error.

The challenge is the more lines into the box, the more fittings you need to join the lines into the single main line which goes to the dust collector.

Fittings have a much larger pressure drop than straight pipe.

The short radius 90 deg elbows have high pressure drop. The typical "Y" has high pressure drop. Sad, but it is very difficult to find the numbers.

The more pressure drop, the lower the flow. So in theory you could add too many fittings, have too much pressure drop and have less resulting airflow through the box.

If you add enough fittings you will eventually feel the reduction in airflow.


----------

