CW or CCW?

[guy48065]

Dead link.

[guy48065]

ahh...simple fix:
https://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/fan_sourcebook.pdf

[guy48065]

You can dismiss what I say, but to disregard what industry experts are saying makes no sense. 

I am NOT dismissing what you have written (or quoted), I'm just trying to get a better understanding.  In your posts you have expressed concern that your counter-rotating top hat may be hurting performance.  As I have said the straightener may have helped you--but is it a universal improvement?
You incorporated this straightener in your counter-rotating top hat and wrote about it 1 1/2 years ago.  Surely someone has tried this on a normal-rotating system to see if it is an improvement?

As for the DOE link...well...
On page 13 "Airflow Control Devices":

Flow control devices
include inlet dampers on the box, inlet vanes at the
inlet to the fan, and outlet dampers at the outlet of
the fan. Inlet box dampers are usually parallel
blade dampers. Inlet vanes adjust fan output in two
principal ways: by creating a swirl in the airflow
that affects the way in which the air hits the fan
blades, or by throttling the air altogether, which
restricts the amount of air entering the fan. The
inlet vanes and dampers must be designed for
proper fan rotation and are to be installed in such
a way that these inlet vanes and dampers open in
the same direction as the fan rotation. The prerotation
or swirl of the air helps reduce the brake
horsepower of the fan. If the inlet dampers on the
inlet box are located too far away from the inlet of
the fan, the effect of pre-rotation may be lost or
reduced, and horsepower savings may be negligible.

They seem to be saying that the swirl at the centrifugal fan hub lessens the load on the motor.  They go on to state that a reverse rotation increases motor load as it "unwinds" the airstream.  Is it reasonable to assume that zero spin falls in the middle between helping and hurting the fan rotation?

[alan m]

thats seems to be contradicting each other

[retired2]

Here is what the DOE document says on page 39 about rotation in the same direction as the blower.

"A pre-rotational swirl in the airflow rotates in the
same direction as a fan impeller.  This phenomenon
reduces the load on the fan and shifts its performance
curve down and to the left."

The last thing you want to do is move the fan performance curve down and to the left.  That is lowering SP and CFM's.  So the previous statement says exactly what Cincinnati Fan is saying in their Engineering Manual.  The article then goes on to say:

"If possible, the fan should be configured
so that there is enough distance from the closest
bend for the airflow to straighten out. Because space
constraints often do not allow ideal configuration,
an airflow straightener, such as turning vanes, also
shown in Figure 2-16, can improve fan performance."

Now, I don't know how it can be any clearer than that!  With regard to counter-rotation the article states:

"A counter-rotating swirl rotates in the opposite
direction of an impeller. This swirl creates an
additional load on the impeller. Although it tends
to shift a fan's performance curve upwards, a
counter-rotating swirl is an inefficient method of
increasing fan pressure."

Again, that agrees with Cincinnati Fan's comment that counter rotation increases the hp requirements substantially.  So, if you want to increase your SP and CFM's, go with a counter-rotation pre-spin - but you're motor might not last as long.

Frankly this whole discussion has become a little tedious, and I don't have any more time to waste on it.   I'm not sure why anyone would be so worried about an air straightener when their plumbing has 1xD bends and 90 degree tee's.  The line losses from these fittings far outweigh anything that an air-straightener can or cannot do for you.

Regards

[guy48065]

The article snips do seem contradictory.  I've read the whole document and believe the section I quoted refers to the impeller inlet while the paragraph retired2 quoted is about the housing inlet.

[Schreck]

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. 

[guy48065]

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?

The issue is completely moot in a system where the separator is remote from the fan.

[alan m]

im no expert.
the only info i have to go one is retired2 s build adn the 2 articals posted about it.
all 3 claim that an air straightener improves cfm.
i dont know if you are wrong or not but you are right about there being a lot of extra friction added to the pipe due to the added surface area.


i dont think that it is moot if the fan and seperater are farther appart.
if there are gains to be gotton with them close coupled then in sure there are some gains still when they are farther apart.



schrek.


thats inrteresting
that seems to show that varying the rotation  can be used to fine tune the motor  speed and current draw.

[Schreck]

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?

[retired2]

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?

Been there, done that......with an ampmeter and an anemometer.  Test results are posted in the thread describing my build, and an air straightener absolutely improves air flow in a close-coupled design!

Since my build imparts a counter-flow air rotation, the question has been raised whether or not an air straightener would provide similar results when the pre-rotation is in the same direction as the fan.  Well, if you accept what industry experts say about pre-rotation, i.e. CFM's are compromised more by a pre-rotation in the direction of the fan than by a counter-rotation, then it holds that an air straightener would benefit a pre-rotation in the direction of the fan more than one that is counter-flow.

My air straightener consists of multiple plastic tubes, simply for convenience of installation and testing.  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.  If I thought otherwise, I would have replaced the plastic tubes with crossed vanes long ago.

[phil (admin)]

It is likely that a multi-vaned straightener would reduce frictional losses, but I think any further gains would be very small.

I agree, I bet any difference would be nearly impossible to even measure.

[Schreck]

[/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.....

[alan m]

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.....

there could be something to be gained by thet.
it would be hard to acomplish for the average joe.
im thinking something like an air ramp(like what bill pentz has inside his cyclone) but only going maybe 1/4" of the way around.

i better get building my 8" before there are any more things added to the list

[retired2]

Schreck,

I did understand the intent of your response.  I just wasn't sure if you had seen my test data which validates what has been stated by industry experts.  And I was merely pointing out that with what we know about counter-rotation that data can be used to infer what the results would be for the opposite rotation.

I stopped posting in this thread when it became clear guy48065 was more interested in arguing for argument's sake than he was in accepting the facts, regardless of the evidence or the source.