Dust Collection Question.

  • Advertise with us

« back to Focus on the Workspace forum

Forum topic by Bobmedic posted 02-27-2013 07:04 AM 6569 views 0 times favorited 16 replies Add to Favorites Watch
View Bobmedic's profile


379 posts in 2826 days

02-27-2013 07:04 AM

Topic tags/keywords: question

This may seem like a stupid question but here it goes anyway. Does adding a chip separator increase suction in a dust collection system? I know dust collectors work off of an airflow concept rather than vacuum but adding a chip separator essentially creates a vacuum right? Maybe I’m wrong but I would like to hear why.

16 replies so far

View crank49's profile


4032 posts in 2995 days

#1 posted 02-27-2013 09:21 AM

You can’t say a dust collector doesn’t work on vacuum. It does.
And no static devise added to or taken away from the air stream in the ductwork generates vacuum. The fan generates the vacuum.

Simply put, a dust collector system is a fan sucking air from a dust source and either pulling it through, or blowing it into, a filter of some sort. Some systems do both, they pull dust laden air through a cyclone separator and then blow it into a bag or cartridge filter. these are called Two stage collectors.

But in either type system, the fan is the component that creates the vacuum that moves the air.

The movement of air is measured by volume (CFM cubic feet per minute) and velocity (FPS feet per second).
You can have a small amount of air moving very fast or a large volume moving slowly, or with a large amount of horsepower you can do both large volume and high velocity.

A shop vac is an example of a collector that moves a small volume of air but can move it at very high velocity. It takes high vacuum pressure to generate the high velocity and that is what shop vacs do best.. These are good collectors for capturing dust that is moving very fast, like chips coming off a router or a small contained area like a Random orbital sander. These tools may have small 1 1/4” hose connections so it takes a lot of vacuum pressure to suck the air through those small hoses. Many shop vacs pull around 100 CFM but can generate a vacuum pressure of 60” to 90 ” of water. That measurement referrs to how high the machine could lift water in a pipe.

A single stage bag or cartridge type dust collector typical of the ones used in many home shops will move a much larger volume of air than a shop vacuum, but it much lower pressure and thus at lower velocity. A 2 hp collector might be able to pull 800 to 1000 CFM (10X more than a vac) but only generate 10” of vacuum presure.

So, finally we get to the original question ”does a chip separator increase suction?” NO. It reduces suction. Every time you change the direction or speed of air flowing through a system it requires energy. A bunch of crooked pipes and hoses increase the resistance of air to flow and thus take away some of the velocity and pressure you had available. A chip separator does the same thing. BUT, the advantage of a chip sepatator is that it removes the chips and some of the dust from the air stream and that allows the system to run longer before the filter bag or cartridge gets clogged up. It is usually easier to empty the container of a separator than the bag on the collector.

View b2rtch's profile


4861 posts in 3073 days

#2 posted 02-27-2013 11:36 AM

Not to strat a fight, please.
But in either type system, the fan is the component that creates the vacuum that moves the air.
The fan causes the air to move, creating air movement which in turn create air displacement, as when you take air from one place and push it somewhere else, the place where it was now has a lower quantity of air in a same volume of space causing a lower pressure and the volume where the fan forced the air now has a higher volume of air in the same space, creating a higher pressure.
In a Dust collection system , the “blower” ( which in reality is a sucker) forces the air around it out, the air forced out need to be replaced , the ambient pressure in your shop try to compensate the new air pressure inbalance between the air pressure in the shop and the air pressure in the DC pipes and it force the air in the room inside the pipes trough every opening possible. creating air flow.
In chip separator two things happen: a sudden pressure drop caused by the incoming air entering a much larger volume ( the volume inside the trash can compare with the volume inside the pipe) and also if properly designed, it creates a cyclonic flow, where the air spin around creating a centrifugal force which forces the heavier parts of dust in the air flow outside and down into a trash can.
This created a significant pressure drop in the system but this also it the best system we have to separate the chips from the dust.
The lighter parts then go through the blower outside , through a filtering bag or a cartridge filter.

I hope this help.

-- Bert

View jmos's profile


839 posts in 2394 days

#3 posted 02-27-2013 12:38 PM

I think this has been well covered; only one thing to add – Crank is correct that the chip separator will reduce your suction by adding resistance. However, if you look at the suction over time as you collect dust, the suction will drop off more slowly than it would have without the separator. This is because the separator is removing dust that would have loaded up the filter and cut the suction. If the curves cross, the separator system could average higher suction than the same system without the separator, but not always.

I had a PM single stage DC and did a trashcan separator/Wynn filter modification. It did a noticeably better job trapping fine dust, but the overall suction took a big hit as both changes reduced air flow. It did a better job trapping what it sucked up, but it sucked up a lot less. I ended up going the ClearVue route.

Bert – Blower or sucker would depend on which side you were standing! :-)

-- John

View Fred Hargis's profile

Fred Hargis

4999 posts in 2518 days

#4 posted 02-27-2013 12:42 PM

The info above is really well stated. I only add that cyclones will normally have larger motors and impellers to move the same amount of air that a single stage DC will have. This is due to the cyclone adding drag to the system (reducing air flow). Some of that is made up by cyclones having more efficient impellers, since they don’t have chunks of wood hitting them…but all in all the separator will add drag, not reduce it.

-- Our village hasn't lost it's idiot, he was elected to congress.

View b2rtch's profile


4861 posts in 3073 days

#5 posted 02-27-2013 01:27 PM

Blower or sucker would depend on which side you were standing!
Correct but in our case we are more interested by the sucker side than the blower side of the device.

I am always glad to show what can be done with an inexpensive HF Dust collector.
I had a Thien separator in the trash can, a “cyclone inducer” lid on the trash can and a wok under the filter.
It worked very well, I now have sold it.

In the last Fine Woodworking magazine issue they ran some test , they found out that for a small system,a Thien separator does as good a job or better than a cyclone would do at a fraction of the price .

-- Bert

View Bobmedic's profile


379 posts in 2826 days

#6 posted 02-27-2013 03:37 PM

Thanks everybody for the good information. I understand the concept better now. Bert, from your picture the is almost what I was thinking of doing. With the exception of the bag and filter I am thinking of venting to the outside. Did you find a significant advantage to your set up verses the stock set up? Enough to justify the cost of the upgrade?

View Bobmedic's profile


379 posts in 2826 days

#7 posted 02-27-2013 03:47 PM

From what I am hearing the only way to increase suction is to increase airspeed. Since the speed of the motor is fixed, could I accomplish this with a belt/pulley system like on a variable speed bandsaw? Place a small pulley on the fan shaft and a larger one on the motor shaft.

View Bluepine38's profile


3379 posts in 3110 days

#8 posted 02-27-2013 04:00 PM

If you increase the speed of the fan, you place more stress on the motor and the motor is usually matched
to the fans present requirement, so you would have to add more power to the motor to run the fan or the
motor would overheat. Some fans operate best at a set rpm, whether your fan would operate better at a
higher rpm is not a given, although in an ideal world it should.

-- As ever, Gus-the 79 yr young apprentice carpenter

View b2rtch's profile


4861 posts in 3073 days

#9 posted 02-27-2013 04:07 PM

I am hearing the only way to increase suction is to increase airspeed
Suction is function of air speed and air volume.
If you have very high spreed ( FT/mn)but little volume (SCFM) it will not work.
This s why I upgraded my system from a 2HP to a 5HP, to increase not so much speed than volume.
If you want to capture fine dust( the most dangerous for your lungs) you need high volume.
High volume with low speed will not work either in reality you need both at the same time.
I do not see how you could do this with a band saw.
My best advice for small shop, is to buy the Harbor freight dust collector, around $150.00 on sale and with the 20% off coupon and to modify it, even before you put it together, the way I modified mine,

To vent outside is good as long as you do not have neighbors, neighbors will not like the noise nor the dust.
If you do not vent outside then the cartridge filter greatly increases the air flow/suction compare with the original bag more important it also protects your lungs,wood dust is carcinogen

Suction is the results of many factors, all important: dust collector Horse power, diameter and smoothest of the lines/hoses/pipes, restrictions ( such as fittings), length of the lines and so on. All factors must be taken into consideration. If you are just starting and you use only one tool at the time , the HF dust collector is the way to go , if your shop grows you will over grow it after a while but to start, it is a great DC.
Many lumberjocks use it and are very satisfied with it.

-- Bert

View Bobmedic's profile


379 posts in 2826 days

#10 posted 02-27-2013 04:19 PM

My bandsaw reference was only to illustrate what I was talking about with the pulley system to increase fan speed. I have the same HF dust collector that you do but it is currently in the stock configuration. I have neighbors but I don’t like them very much lol just kidding. Where I plan to vent outside shouldn’t bother anyone. What is the name and size of your canister filter Bert? I really like what you have done with that configuration. Bluepine, I see what you are saying about adding stress to the motor, I’m just thinking out loud and trying to find a way to get max performance.

View b2rtch's profile


4861 posts in 3073 days

#11 posted 02-27-2013 04:31 PM

I also cut in window in the trash can wall, to see when it get full.
I used a clear plastic 2liters pop soda bottle for “glass”.

-- Bert

View rum's profile


148 posts in 2610 days

#12 posted 02-27-2013 11:17 PM

I have pretty much exactly the same setup as Bert shows. I believe that it didn’t hurt airflow from the stock setup as much as you might initially think because changing the outlet from a curved pipe to a straight pipe (and in my case from Flex to smooth, a chunk of 5” stovepipe works perfectly as a coupler there FYI) reduces the loss from the flex/pipe curve. This was fairly significant when I did some ad-hoc testing without the separator in place yet.

Not relevant to you (since venting outside++), but with the separator and bags/filters I think you might get close to break even in the long run because you’re dumping less junk into the filter so you have less loss there, but I’m not positive on that.

The thein discussion board has some really great data on it. There is one thread here:
That shows that the separator imposes a pretty big hit up front – around 40% – which is pretty brutal when you look at the baseline. He’s using a Delta 50-760 which (I believe) has an 11.5” impeller, I measured my HF at 9.75” so I’m reasonably confident that the initial suction is worse.. In the end is works.. but I think Bert has the right idea with the Clearvue ;D

View b2rtch's profile


4861 posts in 3073 days

#13 posted 02-27-2013 11:30 PM

The people on the Thien forum have way way too much time on their hands.
They need to get a real life.

-- Bert

View woodbutcherbynight's profile


4838 posts in 2433 days

#14 posted 02-28-2013 12:35 AM

b2rtch ==== AMEN

-- Live to tell the stories, they sound better that way.

View Shawn Masterson's profile

Shawn Masterson

1325 posts in 1973 days

#15 posted 02-28-2013 01:07 AM

#1 with outside venting is a bad choice for you if you heat your shop. removes to much heat, and if you heat with wood it will cause a negative pressure in the shop and draw through the shop(smoking out the shop). #2 you have to watch you amp draw when you start opening up air flow. if you take a blower and remove all restrictions it will speed up to its max and my draw to many amps causing it to burn up. the 3HP cyclone I built draws 21 amps at it’s most open, but with the restriction of the cyclone, and filters it draws right at 16 which is just barley in it’s limits. #3 when it comes to overdriving the impeller, you could speed it up to 4000 rpm safely. that’s only 400 rpm over factory. I thought the same thing double the speed with a jack shaft, but most impellers are poorly made and even worse balancing. Imagine a 12” steel impeller poorly balanced spinning at 6900 rpm (that’s double). then through a small chunk of wood at it. sounds like shrapnel to me.
go to bill Pentz’s website it has all the facts you need.

showing 1 through 15 of 16 replies

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