Vacuum Pump Filtration

[WayTooLate]

We have a wide belt sander with a vacuum hold down under the belt. 
Needless to say, the vacuum sucks a significant portion of the dust fines being generated. 
It has a large filter bag attached, but it needs a separator in the system. 

The vacuum has a 5" duct (120mm).  Velocity is good: 4500fpm when fully covered, to ~6500fpm when open.    The specs on the vacuum rate it at >800cfm which seems about right. 

Question #1 is: Is there an optimal diameter for the separator? 
Since it is a sander, it only has fines, no chips. 
I am planning on at least a 2x Top Hat with a bell mouth port. 

Due to the configuration of the machinery, it is quite practical to place the separator after the blower. 
So, Question #2: Are the other considerations for having a 'positive pressure separator vs. the conventional negative pressure designs? 

I'll look forward to any suggestions the 'collective wisdom' can provide.

Thanks!
Jim

[alan m]

is this running through the normal dc system or a high pressure system

[WayTooLate]

Alan -
The wide belt of the sander is a vacuum hold-down.  It can securely hold large sheets of thin veneer very flat for sanding between finish coats.  The perforated belt travels over a perforated bed with a sealed box underneath.  The box is ducted to the intake of a blower capable of several inches of suction.  The 5"output of the blower currently goes to a large filter bag. 

My plan is to place the separator after the blower because of the difficulties in re-routing the ductwork and modifying the machine.  I'll still keep the bag after the separator to trap the 'very fines'. 

My assumption is that the separation will still happen in a positive or negative pressure.  I just don't know if the diameter is better increased or decreased from the common sizes that people are building since I am running a larger airflow than common DC units. 

Thanks for the inquiry, I'll look forward to any recommendations you may have.
- Jim

[alan m]

if i was doing this myself (which i havnt)
i would go a fairly large diameter
double or triple height.
narrow slot . its only fine dust
i would put a section below the baffle to keep the seperator well away from the settled dust


but the most important thing to do first is to catch as much dust as possible before it ends up in this system.

could you use some kind of low density board to close off the vac table but still be able to get suction through it

[WayTooLate]

Alan -
Thanks for the suggestions - all good ideas that will help. 
Unfortunately, the last suggestion is the only one I cannot feasibly implement...  It is a precision sander (designed to take off 1/2 the last coat of sealer/finish you applied) so the abrasive wear on a porous MDF sheet would always have to be compensated for during use. 

The machine is designed to create dust, so that is to be expected.  However, anything I can trap in the separator will be much easier to dispose of than from the bag! 

One question I have though is about the diameter.  Theoretically, the larger the diameter, the slower the rotation.  When I think of a tetherball (do kids still play that?) as the ball winds the rope around the pole, the linear velocity of the ball does not change, but the angular velocity increases.  The total energy (less gravity and air resistance) remains the same, but the 'effect' on the ball appears more dramatic. 

What allows for greater separation?  Does the lowered velocity of the air permit the fines to drop out?  Or does the rapid spinning of the air drive the fines to the outside wall? 

Is that the design philosophy of traditional tapered cyclones?  The larger diameter at the top allows the particulates to drop out and the narrowing diameter keeps them 'spun' against the sides?  I will look forward to the discussion and knowledge that will ensue!

Thanks!
Jim

[retired2]

WayTooLate,

Here is a good read for you.  You will not find the answer you are hoping for, but you will find a very good discussion about the difficulties of separating fines - and cyclones are not the answer.

http://www.jpthien.com/smf/index.php?topic=1167.0

I'm sure with the search function, you can find other discussions on this subject, but not likely any solutions.

[retired2]

WayTooLate,

I forgot to tell you to pay particular attention to my comment about bellmouth outlets and fines bypass.  The comment is in the thread I linked in my previous post.