Camvacs and cfm's

[Jervisekken]

CFM, Cubic feet pr minute (or a metric equivalent), is the number that tells you how much air your DC is transporting. Every machine in our shops has a required or recommended cfm, for example a 14-16 in bandsaw requires 400 cfm, a drill press and router table the same, and a 15 in thickness planer 600 cfm.

Some large vacuum cleaners are said by some to be able to be connected to a central collection system with ducting and all. I think even some manufacturers sell ducting with their vacuums.

Typically these machines deliver around 200 cfm. However, and very important, that is with "no load". Static pressure are lost in the ducting. Elbows and wyes and every foot of duct and hose contributes to this loss, so effective cfm can be considerably lower than the no-load-CFM. At least it is for the collectors that I have seen fan performance curves for. I haven't seen such curves for the vacuum extractors so can not tell for sure.

So, in theory the vacuums should not work in a ducted system, and indeed several people have reported here that they have given up this route. Still some claim they do work, even with planers and thicknessers.

Is anyone able to enlighten us on this topic?

 

[jumps]

I think you have answered your own question

"Elbows and wyes and every foot of duct and hose contributes to this loss,"

the chances of like for like comparative observations is very very low

 

[ondablade]

Some of those cfm numbers could even be a bit low Jervis, there are credible sources that suggest more like 800cfm plus is needed at the hood on a planer or a saw to create a large enough low pressure volume around the blade to get decent collection. The OSHA standard is less demanding and specifies lower numbers, but fine dust collection suffers in this situation. i.e. chips are transported just fine, but fine dust will escape.

It's an interesting topic this one. The above sorts of numbers need around 150mm dia ducting and 4hp + using typical radial centrifugal impeller type fans on large garage type systems with permanent ducting, and connected to one machine at a time. Shortening the duct runs right down by placing the dust system beside your machine makes it possible to still do a decent job with less HP, maybe down to around 2 or a bit more. (Bill Pentz's dust collection pages is the info source I'm thinking of)

The large duct size is a bit of a pain, and people from time to time suggest using higher velocity and pressure fan types to get away with smaller ducting. It's hard to see though how this can deliver similar airflow without more power though - losses.

Others argue for using lower cfms. This can be done too, and will enable a reduction in HP (but the duct size needs to be reduced with the reduced cfm to ensure there is still enough velocity to transport chips up e.g. a vertical ducting run.

The problem with lower cfm is that it must place a much greater emphasis on efficient machine guarding/hooding if large scale escape of dust is to be avoided. Which I think is why it's mostly on smaller stuff like routers and the like with small and very carefully designed hoods that you see lower volume vacuum type units used - they just don't move enough air to generate the required inwards velocity at the air entries to give decent collection on larger hoods.

I'm not too sure what the story is on some of the recent higher pressure 2kW approx vacuum type bin mounted systems using 4 in hose is. They may be a middle ground that still shifts enough air provided the ducting runs are kept very short.

Don't forget that if you want to get out very fine dust that you also need a fine dust capable (removes dust to well below 1 micron in size) cyclone and HEPA 15 or so filter set - or some alternative like exhausting outside (which is problematical in winter due to heat loss). Coarse bag filters will just recirculate the fines back into the shop.

There's some prior threads about you might like to look at...

 

[richard.selwyn]

There is an American called Pentz who seems pretty clued up on all this.
Here is a link to his site:
http://billpentz.com/woodworking/cyclone/index.cfm
I don't know if he can be of any help
Richard