Dust Control systems that we all (hopefully) use, provide high volume airflow and have low suction power. Our shop vacs, provide high suction but very low airflow for use with small power tools. I am not versed enough in the engineering to analyze this. I do know that water pump systems we change the impeller design to provide more pressure and lift capabilities and multiple impellers in many cases. But that is fluid dynamics, non compressable material, and the physics are quite different.
Why are our DC systems designed the way they are, and is it possible to provide more suction power and high airflow? Is it possible to design a system that does both, and if so why don’t we have it?
AZMO pondering the suction needs of life….
Replies
You can raise the static pressure by using a series of impellers. You can also run a large fan: check out the grain blowers at W. W. Grainger. Expensive, but it does what you are after.
-Jerry
Jerry, I could not find a grain blower in Grainger, but I know what one is. Do you have a link to one?
Morgan <!----><!----><!---->
-----------_o
---------_'-,>
-------(*)/ (*) http://www.EarthArtLandscape.com
Morgan,
Just go to the Grainger site and look at blowers: high pressure. Sorry about the "grain blower" appellation: I'm showing my age. (:>
The car wash my wife goes used to have an old Spencer pipe organ blower running their central vac system. It may still be there going strong.
As woodworkers, we are not wedded to "off the shelf" dust collection: its just easy, ready to go, and cheap. I'm running both a DC and a central vac system in my shop. Between the two, I stay clean! (But not cool: Richmond, Va was trying to set a muggy weather record this afternoon.... AAUG!!!)
Best!
-Jerry
A dust collector is designed to take airborne particles and keep them airborne over long distances (relatively), the shop vac is designed to lift and remove static particles.
Two very different dynamics.
At least this is how I understand it.
Mike
Mike that makes some sense, but can one system be designed to do both?
I have 1.5HP Delta with a bag, works great on the big machines. But if you reduce the hose size down, and connect to a small orbital sander, the air flow stops from the resistance of small holes and openings. That is also an airborne material, not static. As the restriction gets smaller, we need a higer negative pressure to move the air through the small opening. Does that makes sense? <!----><!----><!---->
-----------_o
---------_'-,>
-------(*)/ (*) http://www.EarthArtLandscape.com
"I have 1.5HP Delta with a bag, works great on the big machines. But if you reduce the hose size down, and connect to a small orbital sander, the air flow stops from the resistance of small holes and openings. "
Now you are getting into the design of air pumps. And the design criteria is at odds for vacuum vs flow - that which generates high vacuum, does not generate high volume. You could effectively get both at a home woodworkers requirements using one of the commercial dust collectors - the ones you see mounted on the outside of manufacturing plants. But a dust collector the size of a small garage would be, expensive, loud, unsightly, and probably a zoning violation.
The other way is to use a two stage impeller. Think of it like a turbo charger. But, the electronic controls to match the two air flows dynamically would again be expensive.
The cost of a shop vacuum and a dust collector is just too low to be competed with - given todays technology - someday we may have the best of both in one unit, but not economically feasible today.
My ideal dust collection would be a shop vac similar to a fein or festool and a cyclone dust collector. Not to be forgotten is an air cleaner, something that has the capacity to cycle the shop air 5 or 6 times an hour and on a timer so I can leave it running for a bit when I leave the shop.
Oh, as to why the dust collector seems to stop moving air when restricted - its the balance between incoming and outgoing airflow. When restricted the air would rather just spin in the vane than move through it.
Mike
Mike & AZMO,
Now that's odd about restrictions reducing the suction.
I have the Delta 50-760 1½ HP DC and a 4" to 2¼" reducer connector on my Rouseau router table fence. When I connect the DC to the fence coupler it sucks everything into the DC. As a matter of fact, very little escapes down through the opening in the tabletop around the cutter.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Bob,
We are not talking a single small restriction, but rather a lot of restriction.
There is a range of pressure for inflow and outflow that outside of the design parameters, the air movement through the fan drops. What happens is that the air in the fan does not move through and simply goes around and around - like a hamster in an exercise wheel.
Ducted fans work within a given range of pressure differentials and become less and less efficient as you move away from those parameters. A bell shaped curve, where suction is y, restriction is x, and exhaust is held constant.
Mike
Mike,
OK, I'm not sure I understand.
For example, I have a 8' length of 4" diameter dc hose. On one end is the impeller sucking air through this 8' length. On the far end away from the impeller is a reducer that goes from 4" to 2¼". In this case wouldn't the flow of air increase as it gets closer to the 2¼" opening? Unless the end where the impeller resides is plugged.
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 7/31/2008 2:18 pm ET by KiddervilleAcres
Edited 7/31/2008 2:19 pm ET by KiddervilleAcres
Another aspect to consider is that the DC impeller is not in a sealed enclosure. An airfoil impeller in a sealed enclosure will act as an air pump. This is what a vacuum cleaner is. The size of the impeller will determine the amount of airflow and the fact that it is sealed means that there is no leakage around the impeller. Therefore, as long as it spins, it is moving air. The down side to this is that if it does stop do to high static pressure (duct gets plugged up), the motor would burn out without additional circuitry to protect it. If enough hp is given, it will just collapse the ductwork or self destruct (sometning's gotta give) A Fein Turbo III has an airfloil and produces very high pressures (99") but it has a fan to keep the motor cool. The leakage around the impeller in a DC allows space for the dust and wood chips to pass through. In a DC with all the blast gates closed, the impeller still turns but is not moving any air through the system so it is doing very little work. The amount of pressure it can produce, usually less than 11", is not enough to make 26ga duct collapse.
David
Bob,
the airflow (in a system as small as a personal shop) is limited by the total opening.
You could, theoretically, seal a room, attach a hose out of one wall, and another from the opposite wall, and there would be no change in flow. What gets pulled out at one side is replaced from the other. It is called equilibrium.
So, total inlet size is all that counts. and you HAVE to include all the leaks as well, TOTAL inlet size.
Practically, having the restrictor at the source size allows for keeping particles aloft on their journey. In the example above, the particles would fall into the room (look ma!, I created a cyclone type or two stage DC!). If you put the restriction on the end closest to the fan, then you would have particles accumulating in the hose as the flow slowed and the flow was no longer sufficient to keep the particle suspended.
Keep in mind that there is both velocity and volume. For a given output you can trade one for the other - BUT the idea is to keep the particles suspended until you wish them to drop (in the collector bag).
So the restrictor gives a localized increase in velocity, but the total flow is slightly reduced. You feel more suction, but the amount of air per unit time is decreased.
Kind of the water hose vs fire hose thing. if you connect a fire hose to a water hose, you would not expect any decrease in flow, BUT if you connect a fire hose to a water hose, you will shoot the water out farther, but not as much water per minute
Mike
Edited 7/31/2008 5:07 pm ET by mbholden
& David,
OK, I understand, I think.
My response was directed to AZMO and his example citing lack of performance when reducing the size of the inlet on his 1½ HP Delta (I think I have the same one).
Maybe I misread what he was saying but when I connect mine to my Rousseau router table fence (4" to 2¼" reducing connector) I see no appreciable reduction in performance. In fact I think it enhances performance as it works great! The suction at the opening in the fence (2¼" opening) is such that almost no debris falls through the table.
Another question that comes to mind is: Given a 4" duct, How does the length of a run of ductwork effect the performance of the dust collector? If a run is completely sealed along its entire length why would I see a reduction in performance? Is there a point where it drops off dramatically or is the reduction fairly consistent along the length of a run?
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 8/1/2008 9:41 am ET by KiddervilleAcres
Bob -
Regarding a long run.
Two effects, one is friction from the sides slows down the flow and creates restriction - this is why the emphasis on smooth tubes vs the wire hose; two at some point there is so much volume in the tubes that the dust collectors efficiency is no longer capable of making a difference and then the fan spins the air in it and nothing is sucked in. Remember the sealed room in the middle of the tube run? If the room is large enough (like the tube being really long) then there will be a drop in room pressure and minimal flow at the inlet side. (it has gotten to the point that it takes less energy to draw a vacuum then to move the air)
Mike
Bob,
My issue was only related to dado cuts on a router table. I have a side suction, 2 1/4 in that does great. If you have a router bit that is 1/2 size and use a router plate that has a 5/8" hole for it, it leaves no room for air flow around the bit. IN my case, I did not have an enclosure around the motor and all the debris goes out and down the dado slot, spewing all over the floor.
New change is to enclose the motor, hook up a 4" hose and use 1" or larger router plate. My Jessem has 4 throats can be interchanged, so I upsized and now the dado cut is debris free!
In trying to understand the DC effect, you should not really think about fluid dynamics, or use those analogies. Fluids are non compressible, so a pressure on one end results in pressure on the other of equal amount. Air won't do that in the same manner. If you follow Nascar, they use restrictor plates to reduce the air flow into the engine and the Horse Power. Simple rule, is if you are collecting from a reduced flow area, then you should also open a gate part way somewhere else to provide at least 400CFM of airflow to keep the dust suspended and moving to the collector bag, just like Mike describes.
If you have not read Bill Penz website on DC, you should, it is amazing. I can only read 4 or 5 pages at time, it is so relentless! THE best reference out there.
Morgan <!----><!----><!---->
-----------_o
---------_'-,>
-------(*)/ (*) http://www.EarthArtLandscape.com
Gee, I thought the post was going to be about the Dems & Repubs.
Work Safe, Count to 10 when your done for the day !!
Bruce S.
It is kind of related, they both make noise and suck!
................................................
You can't wait for inspiration. You have to go after it with a club. Jack London
This forum post is now archived. Commenting has been disabled