Replies

1. 
   Harryd | May 12, 2005 03:17am | #1

   Bob,

   I remember seeing something like that. It makes no sense at all. Air flow is established by the blower characteristics and the suction line and hood geometry. Adding a venturi is simply another flow-reducing element.

   Consideter a venturi cross section shaped like two V's pointing into a small connecting duct.  Air passing through the duct has to be moving faster than at the open end of the V because the cross section area is less.  The air pressure is less in the duct than in the approach because of this velocity increase, and if a small bleed hole with a line  attached to a fuel bowl is placed in the side of the duct, you have a primitive carburator.

   A way of looking at the cause of the lower pressure is that more air molecules in the duct are headed out and fewer headed toward the side than in the approach to the duct, and fewer  molecular impacts means lower pressure.

   Hood design is complex-about all I remember is the pickup point had to be optimally placed to exceed the "pickup velocity" of the material in question, and then it didn't work well. I remember a multi-million dollar packaging machine with retrofit four inch flex ductwork duct-taped all over it.

2. 
   frenchy | May 12, 2005 04:31am | #2

   Bob J

        Sorry to disagree with someone but shape matters a great deal..  those square corners in your box are dead zones in your collection system,   They cause all sorts of turbulance issues.  Turbulance causes particules to drop.    For a really effective collection try to think like a raindrop..  they are round and fatter at their leading edge and gradually taper off at their trailing edge....

    PS race cars are lousy examples because they are trying to kep the race car on the ground at speeds it wants to fly.. look instead towards airplanes  round and fat at the front tapering to thin at the back.

       

3. 
   mikegagne | May 12, 2005 08:17am | #3

   its important to remember what a venturi is and does. The venturi effect is created by air being drawn through a smaller opening into a larger vessel. The effect is enhanced by a tube shape being the smaller opening (think of a carburetor if you can remember those). Making this work on your shaper would only take some ingenuity and is quite possible. I suggest you google "venturi". Aloha, Mike

1. 
   Harryd | May 12, 2005 08:43pm | #4

   The "venturi effect" is the increase in velocity and the resultant decreas in pressure of a gas passing through a restriction in an otherwise straight line. A well-designed venturi will have a lower pressure drop across it that a poorly designed one ( a greater pressure recover on the downstream side), but any venturi placed in a line will reduce the air flow through the line.

   Venturis are used to create a little vacuum from a lot of compressed air.  There are also venturi devices using water to produce a vacuum.  A common air flow meter uses a u-tube manometer connected before the venturi and to the vena contracta (minimum pressure zone) in the venturi: the pressure differential is proportional to the square of the flow. But venturis are not devices to increase air flow.

   With the same air handling system, a small pickup point  has a higher inlet velocity than a large one.  That's trivial.  But it also has entry losses, and a flat plate with a tube attached has significantly  lower entry losses than a tube stub. Entry losses in a dust collection are measured in feet of line the size of the entry, and 100 feet equavlent entry loss is not unusual. The critical parameter is the air velocity at the dust source, and that must be high enough to blow the dust into the opening.

   A shop dust collector has a marginal blower - it can't pull half the suction it takes to slurp a shake - and its design is to pick lightweight dust and smaller flakes and collect then in a container. All in all, they do the job quite well as designed and built by a reputable manufactor.  "Tweeking" may be enjoyable, but it is  not necessarily enhancing.

   HarryD

1. 
   bobJackson | May 13, 2005 12:44am | #5

   Thank you all for your replys. I am unsure what I will do now. I suppose I will play around and try to make a hood tear drop shaped.Maybe forced air heating floor vents could be modified to this use. Thanks again  Bob

1. 
   lwj2 | May 13, 2005 08:20pm | #6

   Bob:If you're going to go for specially sized or modified sheet metal work, you might check the phone book for sheet metal shops and talk to them.They may be able to come up with what you want for a favourable price, or suggest alternative designs based on their experience.Good luck, let us know what you come up with, eh?Leon Jester

2. 
   forestgirl | May 14, 2005 03:32am | #7

   Harry, this phrase "it can't pull half the suction it takes to slurp a shake" reminded me of a puzzlement I've had for several days now.  Two or three times, I've read someone's comment about the "vacuum" of a dust collector (at this pipe size, or that wye joint, or whatever) not being sufficient.  Is "vacuum" the appropriate word to use there, or is there something else that's more precise.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)Another proud member of the  "I Rocked With ToolDoc Club" .... :>) 

1. 
   MarkRD | May 14, 2005 06:04pm | #8

   FG,
   This is taking me way back to my college Fluid Mechanics class - which I did poorly in...Vacuum is "any pressure that is less than atmospheric pressure"Perhaps the term is "Suction" is what you want.A vacuum creates suction.Your blower motor creates a vacuum. Air trys to fill the vacuum and "runs to the hole" carrying the dust with it.The size of the vaccuum effects the speed (velocity) at which the air approaches the hole. Draw a larger vacuum, get faster air movement. The size and nature of the pipe leading to the source of the vacuum also effects the velocity of the air.Mark

   Measure it with a micrometer, mark it with chalk, cut it with an ax.

   View Image

1. 
   forestgirl | May 14, 2005 06:50pm | #9

   Thanks, Mark.  My one and only college physics class did nasty things to my grade average!  Made organic chem look downright easy.

   Seems like air speed is a focus in the equation for people who get real technical about this stuff.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)Another proud member of the  "I Rocked With ToolDoc Club" .... :>) 

2. 
   mikegagne | May 16, 2005 05:55am | #11

   suction is what we see in the action of a vacuum. More correctly atmosheric pressure rushes in to fill (balance) a vacuum. "Suction" is that measurement we make intrinsically by placing our hands/fingers over a vacuum. It is only specific to a perception rather than a measurement. Our view of how air moves is complicated by what we feel as wind as if it was moving in a straight line when it mostly moves as swirling mass under atmosphereic pressure. Its all relative to where you stand or maybe how smoke looks when moving with air. Is this confusing enough? Aloha, Mike

2. 
   Harryd | May 14, 2005 09:29pm | #10

   Physicists get red in the face when people talk about suction.  To rigidly mathematical minds it is all about pressure gradients and some other specialized terms that, thankfully, unuse and time have erased.

   If you hook a clear tube to the source of vacuum, put the open end into a container of water and measure the hight the vacuum pulls the water, this is the pressure in inches of water (or centimeters--but enough confusion with English units) below atmospheric. A strong set of cheek muscles can create a draw of more that 24 inches water column.  A milkshake - supersized, maybe- would have a straw about 10 inches.  A shop duct collection system produces about eight inches water column differential pressure at shutoff (no flow.) Each blower has a design performance curve that is differential pressure plotted against air volume at a stated barometric pressure and temperature (or "standard conditions" of about 75* F and 1000 Millibar).  The curves I have seen show about six inches at design air flow. This is the differential pressure measured from the intake to the discharge.  A dirty filter bag may use three or four of those inches on the pressure side, leaving little on the suction side.

   Because air has mass it has momentum.  From essentially still air in the workshop, air is accelerated io conveying line velocity.  This produces the entry pressure loss. A turn in the line changes the direction of the air movement and this requires work on the air and a creates a pressure drop resulting in a loss of volumetric capacity as seen on the performance curve. When air enters an open-end tube the air comes from a spherical volume around the opening, meaning a part of the air takes a hair-pin turn around the opening, which for that bit of air is twice the momentum change (and twice the pressure loss) as an intake in a flat plate.  A summation of all little air bits shows,, I believe, something like 30% greater loss overall with a stub tube compared to a flat plate, and contoured corners on the entry port have a slight positive effect.

   The chemical plant where I worked most of my chemist days in process design and support twenty years ago rolled in money.  Line operators earned about 60-70 K annually with OT and shift pay. It was cheaper to pour money into the plant equipment than to have a couple hundred people idle. A niusance dust pickup system in one area had a 250 HP multistage turbine pulling about 400cfm at about 60-80 inches water column. The "dust" it picked up was worth $3.50 per pound. No woodworking shop could afford such a system.  But the dust collectors in woodworking shops make a huge difference in air quality, and usually a reputable vendor will have plenty of information to set up a decent system.

   HarryD