Dust collection

[Jonathan Spool]

I don't want to beat this to death, but I am still trying to get my head around why CFM doesn't equal CFM when comparing my Jet Cannister to my Festool CT33 (which I did purchase based upon recommendations from this site!). The CT33 definitly does a better job.

CFM is CFM, as long as the hose size is constant which will determine the speed, not volume of the air passing through. However, in the past discusions, what was missing was the vacuum b eing created by the two different systems.

I suspect that the difference between a shop dust collection system being necked down to 1.5", and the Festool or Fein system with a 1.5" hose, is not the CFM, which can be similar with each other, but the amount of vacuum being pulled, which really comes into play when you are sending massive clumps of chips through from a router, or similar.

I'm too cheap to spend $80 to get a vacuum cleaner test guage to do the test myself, so does anyone here happen to know how much vacuum is actually being created by the two different types of systems?
Jonathan

[Jason Beam]

Static Pressure

That's the key.

Necking down a 4" dust collector to a 1.5" hose will undoubtedly kill your CFM. If that thing will pull 400cfm through 4", it'll be down to like 75-100cfm at 1.5.

Exactly the opposite is true, by the way, with a shop vac.

Necking up a 1.5 port to 4" will reduce the static pressure so much that 300cfm at 1.5" will likely be more like 30-40cfm at 4".

I'm pulling numbers outta my butt to illustrate the dramatic impact of changing the diameter of the port can have on the flow. But the concepts are all about Static Pressure.

The impellers on dust collectors are meant to move a LOT of air, not necessarily very fast, just a lot of it. It does this by taking big gulps of air (the space between the vanes on the impeller). It takes a surprising amount of power to move a large volume of air.

The impellers on vacuums are teeny and spin really really fast. They're meant to move a little bit of air really really quickly. It takes less power to move a little air, and they make up for the teeny nibbles by spinning that impeller real fast.

These two designs respond differently to resistence (static pressure). Taking a whole bunch of little nibbles really fast can handle quite a bit of static pressure - since over a given time span, the reduction of airflow caused by the resistence is spread over those little tiny nibbles - if you divide up a 100cfm loss over 500 nibbles, it's way better than say over 50 gulps ...

You could make a dust collector that spins a shop vac-type impeller, but it would have to spin so fast (tens of thousands of rpms) in order to keep the air speed up inside of a large enough port. Fine dust pulled from the air needs volumes of air movement. Coarse dust on the floor needs fast speed.

Likewise, you could make a shop vac using a dust collector-style impeller, but it wouldn't work on 120v because the motor would have to be pretty good sized.

Overcoming static pressure in a shop vac is done by speeding up the impeller - take more bites. On a dust collector it means keeping the impeller from slowing down - that takes bigger motors.



It's really hard to describe, but that's how my head likes to think of it.

[Philip Rodriquez]

A simple shop-vac is designed to move small amounts of air under high static pressure (lots of vacuum pressure). Doing so generates a low CFM (Cubic Feet per Minute)

On the other hand, a simple DC unit is designed to move large amounts of air. Doing so generates much higher CFM, but low static pressure.

A Festool CT33, for example, would be a far better choice for running a sander or small hand tool because small dust collection ports restrict airflow – therefore, those tools require higher vacuum pressure to move air.
On the other hand, a DC unit would do a much better job if it were hooked to a 15" planer. What do you think the CT33 would do?

[Rod Sheridan]

Jonathan, CFM does equal CFM.

If you had two systems that had a flow rate of 100 CFM through a 1.5" hose, you would have no way of knowing which was connected to a cyclone and which was connected to a vacuum cleaner.

The difference is that a vacuum cleaner is designed to have low air flow at high vacuum values, and a cyclone is designed to have high air flow at low vacuum values.

For portable tools such as sanders, you need portability, so a small hose is required. Nobody wants a random orbital sander with a 5" hose.

If you want to flow 100 CFM down a 10 foot length of corrugated 1.5" hose, you need a lot of vacuum to overcome the frictional losses in the small hose. (Try breathing through a straw).

The small amount of vacuum produced by a cyclone however can have very high airflow because the frictional losses in a smooth piece of 6" pipe are very low. (Try breathing through a card board toilet paper roll).

Your lungs are like a cyclone, they're designed to have high airflow with low restrictions, so you need a large diameter pipe (toilet paper roll).

For home machines, you may need airflow values ranging from 400 to 900 CFM. If you had a 1.5" hose and needed 400 CFM it would result in an air speed of 32,600 feet per minute. The frictional losses would be enormous.

Conversely for 400CFM in a 4" pipe, the airspeed is aproximately 4,600 feet per minute. Frictional losses are reasonable.

Regards, Rod.

[Chris Tsutsui]

Let me see if I got this straight...

If you took a Jet DC with 1.5" hose and a Festool CT33 with 1.5" hose. Hoses were equal and lets just say both of them sucked the same exact CFM. There is one easy way to distinguish which one is which:

Well if I were to BLOCK the hose with my hand completely. You would feel static pressure sucking your hand. The Festool CT33 is designed to have a high static pressure for that 1.5" hose, thus it's going to be more difficult to remove your hand from that hose. The Jet DC is designed for far more CFM and not so much work like a vacuum. Thus, it is not going to have that much static pressure with a 1.5" hose. There is a big impellar in the DC compared to the smaller one in the Festool. Just because the JET DC has more CFM capability, doesn't mean it can suck with a higher vacuum.

Another example comes to mind...

Lets say you took a vacuum compressor pump that has 1 horsepower. Well it's not moving that much CFM but it will be moving air with an incredible amount of force that can fill up a tire with 50 PSI. Lets say it only moves 10 CFM.

Then you built a computer fan on a jig that has it sucking air through a small tube. Lets say it's also 10 CFM. There is no way a computer fan can fill up a tire with 50PSI, but it can blow 10 CFM just like the vacuum compressor pump.

Anyways, where the Festool CT33 will be humbled is when you hook it up to a large machine with a 4" or 5" chip port that spews out a large amount of dust. You need more CFM to catch all that dust and debris. You want all the air in the vicinity of that large machine to get sucked into the large port. The Festool is not designed to compete with larger DC's and Cyclones in that respect.

[Jonathan Spool]

Thanks for some good explanations, especially in regard to static vacuum.
Since I have both systems I cn sleep at night! I am relatively new to using Festool equipment and am beginning to appreciate the benefits of working with less dust when using hand tools (my definition of hand tools is any power tool that fits in my hand!).