J. Phil Thien's Projects

Quote from: retired2 on March 10, 2013, 06:13:13 PM
  If an air straightener is used, it makes no difference which rotation you choose, both should perform equally.  So, do what I did, use the rotation that suits you approach piping best, but be sure to use an air straightener.

Here is an interesting paper that seems to confirm this.  Look at test case #4 on page 9.  The diagram indicates the rotation in the cyclone outlet is opposite that of the cyclone.  With straighteners, it does not matter.
http://www.banksengineering.com/Fan%20Troubleshooting%20ED100.pdf

Quote from: guy48065 on March 06, 2013, 09:03:42 PM
Evidently nobody else believes in the idea enough to try it.

Or maybe they are testing as we write and simply haven't posted yet!

Well, I am very new to the forum, and I haven't read all of the threads.  But I did read your "5" rectangular inlet..." thread and found it very illuminating.  Measuring velocity pressure was a good move when you found the amp readings to be so close together.  I am curious about your test process: what did you use for an anemometer, was there a single reading taken in the center of the duct and was it fixed at the same location so it could not move in between tests?

[/quote]
Since my build imparts a counter-flow air rotation, .....

My air straightener consists of multiple plastic tubes,..... This may not be the optimal design to minimize frictional losses, but in spite of that the test results show a very worthwhile improvement.  It is likely that a multi-vaned straightener would reduce frictional losses, but I think any further gains would be very small.
[/quote]

Retired2, I agree with you.  Any further increase in your case would be minimal.  With a counter-rotation such as yours, any kind of straightener is very beneficial.  But you have a different situation than the one I was discussing.

The question I was addressing was guy48065's contention in reply#22 that close-coupled top hats and DC blowers SHOULD have a pre-rotation in the same direction as the impeller.  They will, but I don't think it is beneficial. 

I think pre-rotation in the same direction as the impeller reduces fan capacity, as you have stated repeatedly with supporting documents (thank you!). 
I also think that the insertion of a straightener in this situation would improve overall performance of the DC.  I think the additional friction is minor compared to the increase in fan capacity that would be gained by not having any pre-rotation. 

My suggestion to insert a straightener and measure the change in amps was directed at guy48065 or anyone with a close-coupled top hat and DC  that are oriented with the same rotation (I didn't think you were tuned in to this thread any longer!)

I'm liking the thought of a bell-shaped top hat outlet with radial straighteners, perhaps with the leading edges curved to ease the transition from swirling to straight airflows.....

Quote from: guy48065 on March 05, 2013, 08:22:51 AM
In a 2-stage DC where the fan sits on top of a chamber (drum, bag or Thien top hat) the air in the chamber should be spinning in the same direction as the fan rotation.  IMO any use of vanes or straws at the fan inlet causes frictional losses greater than any gain you might get from achieving a straight laminar flow.  The distance is too short.  Am I wrong in thinking this?

In a 2 stage DC where the top hat is constructed to rotate air in the same direction as the DC fan that is mounted directly above it, the air in the chamber WILL be spinning in the same direction as the fan impeller, unless measures are taken to prevent this.  Should it be?  I suppose the question is, will the energy lost by stopping the pre-rotation in the short duct between the top hat and the inlet of the fan be more or less significant than the reduction in fan capacity that results from NOT stopping the pre-rotation of air at the fan inlet. 

It depends on the design of the device that stops the pre-rotation.  Less surface area is better, I would think.  Since we want to stop the rotation of air in a round duct, we only need to insert surfaces that are radial; concentric surfaces provide no benefit, but add surface area and therefore increase friction losses.  It may be enough to insert sheet metal in 2 planes into a short length of round duct.  Picture an X placed over an O.  A duct that is only 1 or 2 diameters in length with such an insert may be all you need. 

Measure DC motor amps with and without the straightener in place.  Higher amps will mean more CFM.  More CFM is good for a dust collection system, right?

It was common in the HVAC field to use an assembly of inlet guide vanes to induce a pre-rotation in the air flow immediately prior to entering the fan inlet.  This pre-rotation in effect reduced the effectiveness of the fan's rotation, resulting in reduced discharge air volume and reduced fan power draw.  These inlet guide vanes are usually adjustable (i.e., their angle can be varied to vary the amount of pre-rotation and hence the volume of air delivered by the fan) and are modulated in order to maintain a static pressure setpoint at a location downstream in the supply air ductwork.  They may also be used as balancing devices, in which case they are adjusted shortly after installation to tweak the capacity of the fan, after which they operate at the same angle.  Below are a pair of images of inlet guide vanes mounted onto fan inlets.

(It is now standard practice to vary the fan output by varying the speed of the fan and motor through a variable frequency drive, but prior to VFD's, motors ran at constant speed and inlet guide vanes were used to control fan output.)

Fan output (in cfm) is highest when there is no pre-rotation of the air entering the fan inlet.   

Retired2's bundle of straws arrests an undesireable pre-rotation that is counter to that of the fan impeller. 
Air that is leaving the separator and entering the fan inlet spinning in the same direction as the fan impeller will reduce the net capacity (in cfm) of the fan. 

The discussion of elbows and turning vanes is something else altogether, pertaining to the duct design on the suction side of the fan.