Replies

1. 
   Don01 | Jul 09, 2010 05:03am | #1

   You will likely not find anything tuned to 4" ducting because it drops the rated capacity down so far. Most experts now agree that you need 800 CFM @ 4000 FPM at the source for good dust collection. You cannot achieve this target with 4" ducting.

   Go to Bill Pentz' site and download the static calculator spreadsheet.  Besides the calculator there is some other decent information which may help you.

    http://billpentz.com/woodworking/cyclone/testsnfaqs.cfm 

   There are fan curve tables at Cincinnati Fan, but a lot tougher to work with.

   Don

2. 
   Tinkerer3 | Jul 09, 2010 04:16pm | #2

   I'm not a professional with hydraulics at all but it seems you might have the wrong idea of how air movements act.  If you  are moving air at 500 CFM  with a given blower in a 6 in. duct in an average shop and replace that duct with a 3 in. duct your flow would be considerable less that 500 CFM.  This is because of the added friction.  You will notice less strain on the fan motor because it has less air to move.  If you have two three inch branches from your fan and you close one of them you will notice the same effect.  If you close both branches the fan will run free-er yet because you have essentially no air to move.  Note in this condition though that it is possible to burn the fan motor up if it is cooled by the same air that normally flows through its pipes.

3. 
   Don01 | Jul 12, 2010 10:31am | #3

   Blower Math

   The primary limiting factor is probably $$$$ and theoretical model vs. practical application.

   I am sure that if the solution were feasible, someone would have done it by now. As there is no such solution you would have to fund the development.

   You must have entered some fitting / accessory items to get the 148" WC value. That is the amount of static pressure your blower needs to overcome before it moves 1 cfm of dust.

   I don't know how the chart works, but for some reason increasing the velocity from 4000 fpm to 10000 fpm pushes the SP value for fittings and pipe way way up. I suppose that it is possible that the velocity of a 4" airstream creates so much turbulence that the model (desired values) collapses.

   A nice mental pursuit, but if you want to get down to making and collection sawdust, go with what is known. There is enough black art in the world of dust collection.

   Don 

4. 
   Tinkerer3 | Jul 12, 2010 11:11pm | #4

   I was hoping a professional engineer would poke his head in here to give us specifics and a better understanding of the situation but in his absense, here goes a try.  One thing we have to understand is that it is the difference between the air pressure on the outside driving the air through the duct work and the pressure on the inside (after the fan).  I think the outside air pressure at sea level is something like sixteen pounds / square inch (psi).  No matter how big a fan motor you have, you cannot attain a differential of more than that 16 psi. or the flow of air that would result through the duct at that air pressure differential.  I'm using your figures here because I just don't know the specifics but I see the possibility that if a 1.5 HP motor cannot "pull" a given amount of air through the 4 inch duct, it is quite possible that a 500 HP could not pull that given amount through either.

   Possible a poor analagy but if you take a gallon jug full of water and tip it upside down.  Will the jug empty out any faster if the water falls a mile or a foot?  The answer is no.  It will empty only as fast as the water is replaced with air that bubbles back into the jug to replace the water.

5. 
   davcefai | Jul 13, 2010 03:01pm | #5

   There's more to it than you may think. This post will try to expand on the (very valid) posts above.

   Horse Power is the amount of work done. In a DC work is done to actually move the air AND to overcome the (substantial) frictional and turbulence losses in the pipework.

   The faster the air tries to move the greater these losses are and I think they increase faster than the flowrate does. So when you decrease the pipe diameter to increase speed - you don't! Also. the longer the pipe, the greater are the losses.

   It gets worse. Pumps have a "pump curve". This is a graph of volume (or mass) moved plotted against the pressure difference involved. These curves can be "flat" (ish) or very peaky. There is always an optimum where the pump is most effective. Once you move out of the sweet spot the performance drops dramatically.

   So when you throttle your DC by narrowing the pipe the motor is going waste more horsepower overcoming the losses. The flow rate drops, vacuum increases, the motor slows down and you're way off the sweet spot.

   I am not an engineer but have spent all my working life in process. Pump selection is a key part of designing a liquid process. When it comes to moving gases it's a lot worse. You always need a specialist. If you add an extra bend to a duct you may find yourself needing to move up a size in the fan.

   In your post you asked

   "In order to move 500 CFM at 6in WC how much HP is required?"

   You need to know how long the pipe is and its diameter.

   "For the same HP can I move 1000 CFM at 3in WC?"

   Only for a very short length of pipe.

   Sorry for the length of this post. If you made it this far, thanks for reading. Now go and study Bill Pentz's web site. You won't like the answers but they're right.