what happens to CFM when dust extractor is hooked up to 2 1/2" port or 2 1/2" hose?

[Michael Burnside]

I watched a video yesterday showing that Straight pvc had little to no loss on an extractor/shop vac velocity. Where as a single 10' piece of corrugated hose even held straight cut 1/3 of the velocity. Hopefully this info helps

Oneida sells their SD rated hose. It's a bit more expensive but is nearly smooth on the inside. It is still flexible but will resist compression and is, as expected, not as bendy as what you'd get at Rockler or Woodcraft. Where I don't have Nordfab runs, I use this stuff, it's great.

[Cory filder]

And this is why islands form at the mouth of fast flowing rivers - where the velocity drops because the channel is wider.

i am a bit blow away by this. never would habe thought about it.

The CFM for dust extractors (i.e. ~150 CFM) is rated at the turbine itself with no hose attached. As soon as you put on a 1-1/2" or a 2" (i.e. 50mm) hose, your actual CFM will be significantly lower. That being said, the primary reasons for getting a dust extractor is the higher suction performance and the ability to adjust or turn down the suction. Also, some dust extractors can be quieter than a normal shopvac. Smaller tools like random orbit sander will actually perform better with less suction.

If you want to use this for a bandsaw and strip sander, then a larger shopvac with the 2-1/2" will have much better collection over any normal dust extractor. However, the shopvac is a one speed high speed solution that just won't work as well with the random orbit sander (or any other smaller tool that you need to reduce suction).

now this i dont quite understand. why do i have to reduce the speed? (what is the Consequence of higher suction)

Oneida sells their SD rated hose. It's a bit more expensive but is nearly smooth on the inside. It is still flexible but will resist compression and is, as expected, not as bendy as what you'd get at Rockler or Woodcraft. Where I don't have Nordfab runs, I use this stuff, it's great.

if the speed does not decrease in a smaller hose as mentioned before, technically hooking up a 1 1/4" hose (at 150cfm) should perform just as well as a standard shop vac 2 1/2" which are also typically around 150 cfm. do i care about the inlet size? no, not unless i am using a jointer or thickness planer. it is not often my brain hurts on this sort of thing.

[Aaron Inami]

On random orbit sanders, a high amount of suction will pull the sander down onto the material too hard. It creates almost an air-tight seal and you don't have as much airflow to pull the debris away through the holes in the sander. Also, the sander does not flow across the wood as easily/smoothly and does not sand as well in general.

[Jim Becker]

To clarify even further at least with a mental image, with sanders, they start to get "sticky" as the abrasive gets finer which makes them harder to move around and reduces collection. It's typical to reduce the "speed" of an extractor to help combat that...and you will still get good extraction with a quality setup.

[Aaron Inami]

if the speed does not decrease in a smaller hose as mentioned before, technically hooking up a 1 1/4" hose (at 150cfm) should perform just as well as a standard shop vac 2 1/2" which are also typically around 150 cfm. do i care about the inlet size? no, not unless i am using a jointer or thickness planer. it is not often my brain hurts on this sort of thing.

Okay, first thing. CFM is not airspeed. CFM is "cubic feet per minute" which describe how much air is being moved from one place to another. The CFM on the 1-1/4" hose is never going to be as high as a 2-1/2" hose because of the actual volume/space inside the hose. If the turbine is advertised as 150 CFM, then the 2-1/2" hose may get you something like 120-130 CFM. A 1-1/4" hose will yield something like 30-40 CFM.

Airspeed is described as "velocity". The smaller 1-1/4" hose will likely have a bit less velocity than a 2-1/2" hose just because of the resistance of a smaller hose and all the internal hose ridges.

Once again, it depends on how much air you need to move. If I'm using a random orbit sander, I will use a smaller Festool 27/32 hose because it weighs less and is more flexible. The 27/32 hose is a 32mm diameter at the vac which then gradually tapers down to 27mm at the tool end. Even with this, I still need to turn down the suction on my extractor for best random orbit sander performance. A heavier/larger hose on a random orbit sander will tend to cause the sander to tilt back and it becomes harder to keep the sandpaper flat on the material.

If I'm using a router, I might use a 36/32 hose (starts at 36mm, then tapers down to 32mm at the end) or even a straight 36mm hose because I want as much air pull power as possible to draw the routed debris away. It depends on the router and the dust port connection. The full 36mm hose is larger, heavier and stiffer. Sometimes I want just a tiny bit more flexibility when handling the router and also how durable the dust port plastic is on the router. (and yes, I understand that routers sometimes have a smaller 1" or so inlet, but a bigger hose will have less resistance and the end result will still be better).

If I would be using a bandsaw or belt sander, you want as much air movement as possible, so a full 36mm or even a 50mm is recommended. These types of tools actually work better with traditional dust collectors and big 4" or larger hoses because you need massive air movement, but smaller bench top tools rarely have a port larger than 2-1/2" and are usually smaller than that.

[Cory filder]

Okay, first thing. CFM is not airspeed. CFM is "cubic feet per minute" which describe how much air is being moved from one place to another. The CFM on the 1-1/4" hose is never going to be as high as a 2-1/2" hose because of the actual volume/space inside the hose. If the turbine is advertised as 150 CFM, then the 2-1/2" hose may get you something like 120-130 CFM. A 1-1/4" hose will yield something like 30-40 CFM.

Airspeed is described as "velocity". The smaller 1-1/4" hose will likely have a bit less velocity than a 2-1/2" hose just because of the resistance of a smaller hose and all the internal hose ridges.

Once again, it depends on how much air you need to move. If I'm using a random orbit sander, I will use a smaller Festool 27/32 hose because it weighs less and is more flexible. The 27/32 hose is a 32mm diameter at the vac which then gradually tapers down to 27mm at the tool end. Even with this, I still need to turn down the suction on my extractor for best random orbit sander performance. A heavier/larger hose on a random orbit sander will tend to cause the sander to tilt back and it becomes harder to keep the sandpaper flat on the material.

If I'm using a router, I might use a 36/32 hose (starts at 36mm, then tapers down to 32mm at the end) or even a straight 36mm hose because I want as much air pull power as possible to draw the routed debris away. It depends on the router and the dust port connection. The full 36mm hose is larger, heavier and stiffer. Sometimes I want just a tiny bit more flexibility when handling the router and also how durable the dust port plastic is on the router. (and yes, I understand that routers sometimes have a smaller 1" or so inlet, but a bigger hose will have less resistance and the end result will still be better).

If I would be using a bandsaw or belt sander, you want as much air movement as possible, so a full 36mm or even a 50mm is recommended. These types of tools actually work better with traditional dust collectors and big 4" or larger hoses because you need massive air movement, but smaller bench top tools rarely have a port larger than 2-1/2" and are usually smaller than that.

if the hose size changes, the CFM should remain marginally the same, not air speed. airspeed is what should change.

good idea on hose change sizes depending on application.

the case i would make for a larger hose most the time, is lesspressure loss. wouldn't the speed increase if at the end of the line (attachment to sander or router or whateber) an adapter is needed anyway? if this is true, wouldt adapters be better and using the large hose always?

[Aaron Inami]

if the hose size changes, the CFM should remain marginally the same, not air speed. airspeed is what should change.

Sorry, but you still have it backwards. Think of CFM as gallons per minute. While the larger 2-1/2" hose will allow you to send through 120 gallons per minute of air, you are never going to be able to push that many gallons per minute through the smaller 1-1/4" hose. The speed of air through these hoses will be somewhat similar, but I can see the smaller hose have more resistance and a bit slower air speed.

the case i would make for a larger hose most the time, is lesspressure loss. wouldn't the speed increase if at the end of the line (attachment to sander or router or whateber) an adapter is needed anyway? if this is true, wouldt adapters be better and using the large hose always?

When talking about dust extractors, a larger hose will generally yield more power because there is less resistance in the hose. This is always better for more suction, more air, etc. However, the larger hoses are much heavier and stiffer with less flexibility. You may not want a large heavy hose trying to hang onto a small trim router with a plastic port facing upwards. You definitely don't want a big huge hose attached to a random orbit sander (and you still have to turn down the suction).

As far as speed, it really depends. The small port at the tool end will basically choke the entire system and prevent higher CFM and this will essentially limit speed. The extractor turbine has to fight against this, so you will likely not get the velocity you're looking for. That being said, we are basically splitting hairs at this point. In the end, just use the biggest hose that is "usable" for the tool you are operating.