vacuum hood design

[weatherbird]

I am working on designing a vacuum hood for cleaning sand, coal ash, and brick chips. A dust collector system is the preferred way of cleaning, but I have to design a hood to create enough vacuum to pick up the particles as it sweeps over an uneven surface. I have to have at least a 5/8" opening to collect the particles, but the hood has to be close to 60" wide, and thus creating enough suction on one end, but not enough on the far end, since the inlet for the system has to be on one side or the other. Any suggestions as to how to design the hood to get even vacuum across over the entire hood?

 

[IRstuff]

Vacuum doesn't sound like what you really want; after all, if that were the case a sealed glove box would give you "uniform vacuum."

However, being an ME, you should recognize that it's the pressure differential that drives the air flow; hence, the "vacuum" cannot possibly be uniform within the hood.

I can't tell from your posting what kind of sand, but most sands require pretty manly airflows to get them to move, which might be counterproductive to whatever the user is actually doing. I'm thinking along the lines of 40 mph wind?

Perhaps you might consider a grill as a worksurface, i.e., something that's mostly holes that are small enough to prevent your objects from falling through, but will allow your dust and sand to fall through and get sucked away.

TTFN

FAQ731-376

 

[unclesyd]

I haven't seen a vacuum hood as you describe. All hoods that I've seen that handle the materials you stated have been of the downdraft variety.
You might want to look a bead blasting cabinet design.


The standard fume hood in a laboratory creates a draft across the working surface by using duct fans and normally a room with positive pressure to assist the draft. There is essentially no vacuum in these hoods only a draft. Anytime that would create a vacuum to accommodate your materials would be complex system.

 

[weatherbird]

I just have to remove these particles from the surface. The idea was to build a hood that would suck the particles off the surface. I can do so with a moderate hood design and a hi-vac system. But it is about 1/4 the cost to use a dust collector, if I can just find a way to get the particles up off the surface for the dust collection system to carry them away. I have to do this from above the surface, as there is no way to get under the surface, or to modify the surface. Prehaps instead of using one big hood, I could go to several smaller hoods to keep the "picking power" uniform.

 

[IRstuff]

If you're so constrained, you should configure it like the Jenn-Air stove tops and run ducts directly to specific spots on the worksurface and only work in those specific spots.

TTFN

FAQ731-376

 

[weatherbird]

Thanks for the help.

 

[scastillo]

if I can just find a way to get the particles up off the surface for the dust collection system to carry them away

Think street sweeper. They use large roller brushes to kick the dirt up and into the suction mechanism.

 

[unclesyd]

You might be able to use one of the standard laboratory hood designs where there is both a top mount exhaust and more importantly in your case a bench top level adjustable exhaust in combination with one of the products produced by Vortec or Exair. In your case you could have the majority of the air flow exit at bench top level. There could be some type collector immediately after the entrance slot to collect the dust. The air operated devices could act as a sweep to direct the dust toward the slot.

Look at all their products as there are several possibilities like the air amplifier to exhaust the dust.

http://www.exair.com/Cultures/en-

US/Primary+Navigation/Products/Air+Nozzles/

http://www.vortec.com/curtain_transvectors.php

 

[rhodie]

Coupled with unclesyd's comments:

Have you looked into a "push-pull" system; or rather a system that "blows" compressed air from a source across the work surface "into" a vacuum system tuned to pick up the same volume?

 

[berkshire]

weatherbird
You might look at a tapered tube design with a narrow slot in the side of the tube. Make the area of the tube equal to the sum of the area of the length of the slot.
You could put several of these across the pickup area if one by itself will not work. You would need a face velocity of at least 4000 fpm.
B.E.

 

[weatherbird]

The air knife idea didn't work. The tapered tube may end up being my best option. However, to equal out the areas requires the slot to be much to small to pick up the particles required. If anyone has any other ideas to either aid the previous ideas, or a completely new idea, please let me know. Thanks

 

[IRstuff]

The only way you could come close is to make the front opening of the hood be less than 6" tall. Even then, you're looking at something that can move 3000 cfm across a 3 ft frontage

Your only plausible option is to have a re-positionable duct that you can move around within the hood that places its inlet near your work piece. If the duct were something like 3" diameter, it would be much more manageable.

TTFN

FAQ731-376

 

[ventguy]

As a hood designer I need some information.

1.What "capture velocity" do you you think you need to pick up this dust and move it into your hood? 100 feet/min?
500 feet/min?

2. How far away is the hood slot or nozzle from the most remote dust particle that you want to collect? One foot? Two feet? 3 inches?

If you can answer these questions, I can tell you the flow you need, assuming the hood is 5 feet long as you say.

You can achieve even distribution across the hood face with a properly designed hood plenum. The Industrial Ventilation Manual, published by the American Conference of Government Industrial Hygienists (ACGIH) describes how to do it.



Peter Ott

http://www.utieng.com

 

[asishonline2000]

i hope you are not designing for any industrial use.If so then it is very coplex process.If not then try with cyclone seperator.This type of seperator is for sure seperates dust and carbon particles.Only you have to quantify the flow measurements.This is some sorts of gravity seperator.

 

[rebelbrill5]

A downdraft table or a backdraft hood might be better options. Perforated metal or a screen that passes the debris that you want to collect and stops the material you're trying to clean would cover the suction side of both these devices.

The downdraft table can be used as a working surface, or it can be placed under a section of conveyor that allows the debris to fall thru under the action of gravity plus the airflow of the table, depending upon your process and production volume.

 

[KENAT]

As suggested above by scastillo , anyway you can brush the items to pick the dust up off the surface.

If not street sweeper then how about the attatchment on your vacuum/hoover at home with the bristles?



KENAT, probably the least qualified checker you'll ever meet...