Thread: More questions for Bob

Originally Posted by mannum3

I am asking a theory question.

If I placed a fan that moved 1000 cf/m into a "cone" 8" dia doubling in dia every 3', so that at 12' the "cone" would be 10'8" dia.

Ignoring any losses due to friction, what would the air speed be at each end of the cone?

Would the heavier chips fall, leaving the finer dust to float away?

Cheers Barry

You can work this out for yourself.
covert 8" into ft, then find the area of the circle in ft and divide that into the 1000 cfm
The bigger chips will start falling out of the dust stream at ~3000 fpm, the small chips at around half that and the fine dust would just float away.

BP uses 4000 FPM as the drop out figure for saw dust but he's added a safety margin to make sure it doesn't happen
8" ducting is generaly too large to use for only 1000 CFM

Thank you Bob,

More theory/rule of thumb....all figures are in feet....square fluming

Point Dia Size Sq/f Speed f/m Volume Cf/m
a 0.5 0.25 4000 1000
b 1 1 1000 1000
c 2 4 250 1000
d 4 16 62.5 1000
e 8 64 15.65 1000

What would be a reasonable distance between point a and point e ?

Cheers Barry

Point ......Dia .........Size Sq/f......... Speed f/m ..........Volume Cf/m
a........... 0.5,,,,,,......,0.25 ,,,,,,,,,,,....,4000 ......................1000
b........... 1............... 1.................... 1000...................... 1000
c ...........2 ...............4 ......................250 ......................1000
d ...........4............. 16 ........................62.5 ...................1000
e........... 8............. 64 ........................15.65 ..................1000

a bit clearer

Originally Posted by mannum3

Thank you Bob,

More theory/rule of thumb....all figures are in feet....square fluming

Point Dia Size Sq/f Speed f/m Volume Cf/m
a 0.5 0.25 4000 1000
b 1 1 1000 1000
c 2 4 250 1000
d 4 16 62.5 1000
e 8 64 15.65 1000

What would be a reasonable distance between point a and point e ?

Cheers Barry

In theory to get air flowing evenly through a duct of diameter D to flow evenly through a duct of diameter 2D requires about 10D of duct distance will be needed for the transition.

So between every step above you have to go 10D
a) - b) 10 x 0.5
b) - c) 10 x 1
c)- d) 10 x 2
d) - e) 10 x 4
Adding all this up will come out to a ridiculous distance. (7.5 x 10) = 75ft!!!!
notice it's not an even transition either but a sort of inverse trumpet path.
In theory a 95% degree of evenness can be achieved with by using a 5X multiplier but that is still a long way.

However Friction of the air with itself dominates the airflow at these air speeds which means
1) the air at a) won't be flowing evenly inside the duct anyway and unless the air starts our even it will not magically develop an even flow
2) if the duct ends at e) the friction of an open duct will extend well inside the pipe and rapidly slow and disperse the air well before it gets to the end of the transition.

I'm not sure what this is for - are you trying to see at what distance the dust will drop out of the air stream?

Thank you for your prompt reply, even before my correction to the format.

I needed to get the theory clear in my mind before "designing" a prototype.

I imagine most dust will fall before 10 to 12 feet.

Thanks again Bob, you are a gold mine of information

Cheers Barry

Originally Posted by mannum3

Thank you for your prompt reply, even before my correction to the format.

I needed to get the theory clear in my mind before "designing" a prototype.

I imagine most dust will fall before 10 to 12 feet.

It depends what you mean by most of the dust?

Most of the "mass" or weight of the dust will indeed do this - you can see this by watching the sawdust coming off a thicknesser or a lathe that are not connected to a DC

However, depending on the woodworking activity a considerable fraction of the dust particles are so small that they cannot be seen and will stay suspended in the air for hours even when the air is moving very slowly,l so path length is more or less irrelevant for these. Some of these will eventually be seen when they settle out of the air.

And remember, it's the fine stuff that has the health/allergy risk associated with it.

It sounds like you are trying to design a filter based on length and diameter of air ducting. In that case I wouldn't bother to use a duct. Just shooting the dust into a large volume air will do the same thing. It will work well for chips but not for finer dust. Depending on the volume of dust produced and you will end up with a dust plume that could be 10's to 100m long.

All good stuff.

I was considering building an ever increasing sized structure that grows around and up. ( an upturned snail shell shape)

I have ideas of how to completely containing the fine dust, but first I need to separate the coarser material.

Cheers Barry

Originally Posted by mannum3

All good stuff.

I was considering building an ever increasing sized structure that grows around and up. ( an upturned snail shell shape)

I have ideas of how to completely containing the fine dust, but first I need to separate the coarser material.

Cheers Barry

I thought that was the case but if you have the space the easiest way to do that is just to use a really BIG chip collector.
Most chip collectors are way too small and hence generate significant back pressure to flow
By BIG I mean around 10 times bigger than the ducting so if 6" ducting is used then a 60" diameter drum (rainwater tank) should work.
Put the sawdust into the top side at an angle like a cyclone and use a 12" exit in the middle of the top.
This effectively translates into an extra right angle bend worth of resistance which is much less than a small cyclone or small chip collector.

Of course the best way to do this is just to use a cyclone and that will tidy up most of the fine dust as well