CW or CCW?

[guy48065]

It's been stated that inlet angle & swarf flow direction doesn't matter.  I imagine that's true for a separator fed with hoses but what about when the baffle is mounted right on the fan housing?  Seems to me you would want the swirl to run the same direction as the impeller since it's so close it may be "coupled" to the air mass.

OR--is counter-rotation desirable in case it might reduce the amount of fines that get pulled in by the impeller?

Anybody ever play with this choice?

[DarthVader]

You want to keep the rotation going the same way when the impeller housing is directly on top of the baffle. switching directions is not advisable when the distance from the impeller inlet to the baffle outlet is minimal. the incoming air is still spiraling from the rotation of the impeller so reversing the direction of the baffle would cause turbulence upon entering the baffle...

my .02 at least

[alan m]

the golden rule with dust colection (and seperation) is reduce turbulance.

turbulance kills cfm  and allows bypass of seperaters.
in a top hat design the benifits are PROBABLY smaller that on a built in baffel design
i would definetly want the air spining the same way in a built in design.

retired2 (member here) i think did some reserh into air straighteners. he (?) found benifits in having one

[retired2]

Below is a statement I've posted several times.  It comes from a Cincinnati Fan Engineering Manual regarding duct inlet spin.  As you can see, spin in either direct is undesirable, and it was very evident in my close-coupled top hat build.  When I added an air straightener, the air flow improved and noise level dropped due to the decrease in turbulence. 

Duct Inlet Spin

"A major cause of reduced fan performance is an inlet duct connection that produces a spin or pre-rotation of the air entering the fan inlet.  Inlet spin in the same direction of the fan wheel will reduce air volume and pressure ratings.  Inlet spin in the opposite direction of the fan rotation will substantially increase the motor horsepower requirements.  An ideal inlet condition is one which allows the air to enter the fan axially and evenly without spin in either direction."

[guy48065]

retired2 (member here) i think did some reserh into air straighteners. he (?) found benifits in having one

I'd like to read more about it but a search of "air straightener" turned up nothing.


Maybe I'm missing something...Why did Cincinnati Fan use an elbow on the intake of their 2-stage DCs, like everyone else?  Or were they referring to avoiding bends and elbows ahead of the inlet?

[guy48065]

I'm also confused whether that quote is referring to the system inlet (where I thought spin was desirable) or fan inlet (and how would you control that?).
And in context is that quote about dust collectors where you are trying to separate material from air or ventilation? 

[retired2]

I'm also confused whether that quote is referring to the system inlet (where I thought spin was desirable) or fan inlet (and how would you control that?).
And in context is that quote about dust collectors where you are trying to separate material from air or ventilation?

I'll give you my opinion.  Cincinnati is talking about the negative effect of air entering the inlet of a centrifigal fan with a radial spin.  Spin in either direction effects the efficiency of the fan's impeller.  I do not think it matters what application you are talking about, dust or ventilation, there is still an adverse effect.  I would even go so far as to speculate that radial spin of a fluid would adversely effect the performance of a centrifigal pump.

When I was building and testing my Thien separator, I initially had it connected to my blower with about 6 ft of flex hose.  I did not take any FPM/CFM measurements in that configuration because I wanted to eliminate the flex hose since it causes 3X the line loss of smooth pipe.  However, when I close-coupled the separator to the inlet port of the DC (about 8"), the DC became much louder and when I put my hand on the housing, I could feel the increased turbulance.  That is when I added an air straightener in the hopes of getting rid of the objectionable noise level.  What I discovered after taking measurements with an anemometer, not only did the noise decrease, but the CFM's increased when the air straightener was added.

So, my experience suggests several things.  The Thien separator imparts a radial spin on the air exiting the outlet pipe.  That spin when close-coupled to a fan inlet will cause turbulence inside the fan that increases noise levels and decreases the performance of the fan.  Since my system is a counter flow arrangement, my comments only address that configuration.  However, I have no reason to dispute Cincinnati's comments that radial flow in either direction has a negative effect on on the fan's performance.

[guy48065]

That seems counter-intuitive to me.  If the air entering the fan  spins in the same direction as the fan shouldn't there be less turbulence?  What is an air straightener?  If it's just a pipe then the air is de-coupled from the dirty air spinning around in the chamber, but is still spinning--inside the pipe--because it's coupled to the impeller.  Unless it's one of those bundles of plastic straws I've seen used to straighten a fan output.  In which case when used on the inlet I would think there would be some sort of shear effect going on where the "straightened" air hits the spinning impeller.  That can't be good for CFM or noise.

It's hard to wrap my mind around these effects.  I didn't have any fluid dynamics classes in college.

In any event many of these options aren't easily available to me.  My system is in a small closet and the pipes drop in from ceiling height straight down into the lid of my 2-stage DC.  Not much "wiggle room" there to change flow directions or greatly increase DC inlet height.

[retired2]

That seems counter-intuitive to me.  If the air entering the fan  spins in the same direction as the fan shouldn't there be less turbulence?  What is an air straightener?  If it's just a pipe then the air is de-coupled from the dirty air spinning around in the chamber, but is still spinning--inside the pipe--because it's coupled to the impeller.  Unless it's one of those bundles of plastic straws I've seen used to straighten a fan output.  In which case when used on the inlet I would think there would be some sort of shear effect going on where the "straightened" air hits the spinning impeller.  That can't be good for CFM or noise.

It's hard to wrap my mind around these effects.  I didn't have any fluid dynamics classes in college.

In any event many of these options aren't easily available to me.  My system is in a small closet and the pipes drop in from ceiling height straight down into the lid of my 2-stage DC.  Not much "wiggle room" there to change flow directions or greatly increase DC inlet height.

You probably won't find air straighteners in a catalogue, or even at your local HVAC supply house.  You will probably have to come up with your own design.  My intent was to simply slip four straight vanes into my pipe and attach them somehow, i.e. rivets, weld, or maybe epoxy.  In the short term I've kept the "straws" that I used to test the concept simply because it works just fine.  My straws are about 1-1/4" in diameter.  To see photos, look for my build.  It has scrolled to about page 10 now. 

To answer your question about spin in the same direction as the impeller, I am guessing it reduces efficiency because it causes "cavitation".

[guy48065]

My straws are about 1-1/4" in diameter.  To see photos, look for my build.  It has scrolled to about page 10 now.

To save anyone else the aggravation of searching back 18 months with no useful Search tools:
http://www.jpthien.com/smf/index.php?topic=563.0

OK so your Straightener is a bundle of "straws" similar to what I've seen the Mythbusters use in trying to get decent laminar flow from their cobbled wind tunnel.

[retired2]

My straws are about 1-1/4" in diameter.  To see photos, look for my build.  It has scrolled to about page 10 now.

To save anyone else the aggravation of searching back 18 months with no useful Search tools:
http://www.jpthien.com/smf/index.php?topic=563.0

OK so your Straightener is a bundle of "straws" similar to what I've seen the Mythbusters use in trying to get decent laminar flow from their cobbled wind tunnel.

Did I say it wasn't a bundle of "straws"?  Regardless what you call it, it is an effective air straightener that has improved the performance of my DC.

[alan m]

retired2 s data (not independant or certified but still valid) shows a marked increase in cfm  with the straightener (and bell mouth)
for his 4.5" pipe he found a 47 cfm increase from using the air straightener on his bell mouth pipe. thats 12% increase.
there is also a noticable reduction in noise.
im not sure why its helping  but does it really matter. no but i would like to know so we might be able to optimise it better.

this bundle of straws seems to work.
did you find any info on the most effiecient ratio or no of pipes  to the area of the pipe . is there a study on this similar to the wonderfull  one you found on bell mouth fittings

im was thinking of putting a simple x across the duct going the full length of the duct.

[retired2]

this bundle of straws seems to work.
did you find any info on the most effiecient ratio or no of pipes  to the area of the pipe . is there a study on this similar to the wonderfull  one you found on bell mouth fittings

im was thinking of putting a simple x across the duct going the full length of the duct.

Alan,
I didn't spend much time looking, but I did not find much, if any, information online for air straighteners.  However, my gut tells me you will find very little difference in benefit between a bundle of tubes or several straight vanes positioned radially inside the pipe.  Length might be a different story.  I'm sure there is a point where the air straightener is too short to be of any benefit, and likewise a point beyond which adding additional length provides no additional benefit.

[guy48065]

The thing that's nagging me about the straws is a hunch that in retired2's situation it's a band-aid that improves the undesirable counter-rotation of his tophat. 
"Cavitation" was mentioned.  If anything it seems to me that introducing an air mass to the spinning impeller in a straight line would cause shear--or cavitation.  I have no doubt this did improve performance and noise vs. presenting the impeller with an air mass spinning in the wrong direction.  Has anyone else tried this trick--on a separator that spins the air the same direction as the fan?

[retired2]

The thing that's nagging me about the straws is a hunch that in retired2's situation it's a band-aid that improves the undesirable counter-rotation of his tophat. 
"Cavitation" was mentioned.  If anything it seems to me that introducing an air mass to the spinning impeller in a straight line would cause shear--or cavitation.  I have no doubt this did improve performance and noise vs. presenting the impeller with an air mass spinning in the wrong direction.  Has anyone else tried this trick--on a separator that spins the air the same direction as the fan?

You can dismiss what I say, but to disregard what industry experts are saying makes no sense.  Here is another source for you.  It is a link to the US Department of Energy's sourcebook for industry titled "Improving Fan System Performance".  In it they say the same thing as Cincinnati Fan, i.e. pre-rotation in either direction has undesirable effects.  They discuss vanes and other methods to counter this undesirable rotation.

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

I'm sure if you spend even a little time researching this subject, you will find more sources confirming these statements.