Dust Collector airflow dynamics question

  • Advertise with us

« back to Focus on the Workspace forum

Forum topic by steveinaz posted 12-22-2019 09:16 PM 416 views 0 times favorited 4 replies Add to Favorites Watch
View steveinaz's profile


50 posts in 2189 days

12-22-2019 09:16 PM

I wanted to hook up a jobsite saw to my dust collector. I put a 4” to 2 1/4” reducer right on the dust collector. I ended up with a lot less suction. I’m guessing that there is some airflow dynamics reason for this, but I’m totall clueless. Is there a solution? Thanks in advance.

-- Steve in AZ

4 replies so far

View woodman71's profile


193 posts in 4338 days

#1 posted 12-22-2019 09:25 PM

You should not reduce at collector only at tool . You should have 4inch pipe at the tool and reduce to fit the too

View steveinaz's profile


50 posts in 2189 days

#2 posted 12-22-2019 10:24 PM

I’ll give that a try. Thanks woodman71

-- Steve in AZ

View Madmark2's profile (online now)


2320 posts in 1602 days

#3 posted 12-23-2019 03:04 AM

4” dia is 12.5 sq in while 2-1/2” dia is just under 4 sq in or about 1/3 the raw area and over time, volume. Couple that with the higher resistance of the smaller hose and a significant loss of suction is to be expected. Remember too that the smaller diameter makes the system more prone to clogging.


-- The hump with the stump and the pump!

View AndyJ1s's profile


485 posts in 769 days

#4 posted 12-24-2019 10:53 PM

Also consider that air resistance increases with the cube of the air speed. So if air needs to move three times as fast to make up the same volume (e.g. CFM), it needs 27 times the force (suction) to move it (not likely gonna happen…), so you won’t get as much CFM through the smaller hose, especially if is it a significant portion of the total hose length.

There is also a minimum air speed at which particles will stay suspended (depending on particle size and mass). So increasing duct diameter, past the point where the air speed drops below the carrying speed, is not a solution either.

Generally, unless runs are very long, it does not pay to increase duct size past the size of the inlet to the extractor.

As stated before, if you have to reduce the duct size to fit the tool (dust producer) then do so as near to the tool as possible.


-- Andy - Arlington TX

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics