Blower Design Questions

B

blabbott

New member
I previously built a DIY Pentz cyclone, and am now adding the blower (rather suck on it with my 1.5hp PennState DC). I bought a 16" impeller from Clearvue and a 5HP Leeson. Looking at the 16" design on Bill's site, I noticed that it is overdesigned for a 6" duct system. The design he shows was designed for Alan Beinlein with 8" ducts and a 10" diameter equivalent outlet. I am looking into redisigning for 6" ducts and an 8" diameter equivalent outlet. Bill gives almost enough info to do this but I am still somewhat confused by the effect of the overlap of the fan into the outlet. Bill says overlap should be less than 50% but doesn't give much explanation.
1. Can anyone explain the effect of the overlap? Specifically, as overlap increases and decreases, what is the effect on noise and blower efficiency?
2. For the Clearvue 16" design what is the dimension for:
a. outlet width and height?
b. overlap.
c. Does Clearvue uses a gore point gap of 1.6" as recommended by Bill.
3. I have seen the threads on the stick for noise reduction across the outlet from the gore point; Is there any effect on whether this is rounded over or square corners? Any new info on the improvement in noise with this?
Thanks, Bob
 
As much as everyone experiments here I would have thought someone would have been able to answer some of these questions by now or at least point you in the right direction to get some answers. I have been wondering the same thing also. Would my blower have been more efficent if my overlap was 4" instead of 3.5"? How much does the size of the outlet on the blower housing realy matter?
 
Hi Guys,

I saw the post but have not had the time to think about it to give some kind of reasonable answer. I will try to see if dad has time to look at this.

Matt
 
Hi guys,
The first thing I want to say is we aren't experts in this.
Blower discharge height on our blower is 5 1/2". The width is 7" and we have about a 4" rise in the spiral in 360 degrees. The gore point clearance is about 1.4 - - I think. I'd actually have to measure it to be sure. We have experimented with several different things with little differences in the results of Maximum SP or maximum CFM.
If you look at a squirrel cage blower you will see that they use a very high rise on the spiral – almost none of the blower wheel protrudes into the discharge. I think this high rise in the spiral will yield higher CFM at low static pressures but the CFM would drop off more sharply as the static pressure increases. Furnace blowers are designed to move a lot of CFM at almost no SP which is why they are using 20x20 ducts or 24 x 24” ducts.
Moving air through a 6” pipe automatically increases the SP beyond what a squirrel cage blower can produce. So, several things change. Giving a higher rise to the spiral might yield a little more CFM at our lowest pressures (with no pipe attached) but it would cause the CFM to drop off faster as the SP increases. But, all of this is relatively minimal and we have NOT done any extensive testing of different designs. I can tell you that they are ALL loud. Spinning a 15" or 16” impeller at 3450 creates a roar and different blower shapes have very little effect on that roar.
We have built a few blowers with closer gore points. The siren effect started happening at about ¾” clearance. As the distance increased the siren sound went away. Increasing this point to 2” had no reduction in the noise level over 1 .25 clearance. Rounding the gore point also didn’t help. There is a pressure wave created as the blade passes this point. At 3450 RPM with 6 blades, you end up with 3450 x 6 blades passing that point every minute. Divide that by 60 and you get 345 cycles per second or 345 Hz. That is an annoying pure tone that the second gore point cancels out.
Last but not least, I have done Google searches for blower design help and have found almost no information relative to the specific items we are talking about which is the spiral rise, gore point shape or clearance etc. Cincinnati fan and New York blower have websites with a lot of different fan shapes and good CFM and SP charts but they don’t tell you how to build a “quiet” blower that moves 1500 CFM through a 6” pipe.

Ed
 
Hi Ed,
Thanks for responding. This has provided some great information and clears a number of questions up. However I do have a couple of additional questions.
1) With a blower height of 5 1/2 " this leaves just 1.25" total for fan clearance (.5" on top and .75" on the bottom?). Have you reduced the clearance on the bottom consistent with a recent thread http://www.clearvuecyclones.com/Bullentin/showthread.php?t=1067 suggesting the noise was reduced by reducing the bottom clearance? I wasn't aware that you confirmed this. Sounds like good news!
2) You didn't mention the overlap, but calculating from the 4" spiral over 360 deg, 7" outlet width, 16" fan and 1.4" gore space I get only ~2" overlap. This (2"/7" = 28.6%) seems small to me, given your discussion of loss in drop in CFM under high SP load with a very small overlap in a squirrel cage blower. I would have assumed more like a 3" overlap (3"/7" = 42.9%) which gives a 3" spiral over 360 deg. What am I missing here, or did I misinterpret your numbers? Have you done any experimentation with overlap that you could share?

Thanks for the great work you guys are doing. Your 2nd gore point is another great addition. Also thanks for the info about the Cincinnati Fan and New York Blower sites. I will check into them. I also haven't found much practical technical info on blower design.
Bob
 
blabbott said:
Hi Ed,
Thanks for responding. This has provided some great information and clears a number of questions up. However I do have a couple of additional questions.
1) With a blower height of 5 1/2 " this leaves just 1.25" total for fan clearance (.5" on top and .75" on the bottom?). Have you reduced the clearance on the bottom consistent with a recent thread http://www.clearvuecyclones.com/Bullentin/showthread.php?t=1067 suggesting the noise was reduced by reducing the bottom clearance? I wasn't aware that you confirmed this. Sounds like good news!
2) You didn't mention the overlap, but calculating from the 4" spiral over 360 deg, 7" outlet width, 16" fan and 1.4" gore space I get only ~2" overlap. This (2"/7" = 28.6%) seems small to me, given your discussion of loss in drop in CFM under high SP load with a very small overlap in a squirrel cage blower. I would have assumed more like a 3" overlap (3"/7" = 42.9%) which gives a 3" spiral over 360 deg. What am I missing here, or did I misinterpret your numbers? Have you done any experimentation with overlap that you could share?

Thanks for the great work you guys are doing. Your 2nd gore point is another great addition. Also thanks for the info about the Cincinnati Fan and New York Blower sites. I will check into them. I also haven't found much practical technical info on blower design.
Bob
Click to expand...

Bump! Anyone have any ideas or theories on this?
 
2nd Gore Point

2nd Gore Point

Ed told me about he 2nd gore point modification and then I read it here. It looks like the 2nd gore point is added 1/4 wavelength (of 345 Hz) from the original gore point. My guess is that the out-of-phase pulsations cancel each other to some degree. I think it is pure luck that the positions 1/4 wavelength apart (9.5") also happen to be somewhat aligned with 2 of the blade tips at the same time. So before I install this, I am wondering if the 2nd gore point is in the best location. The current suggested location is right where I would guess it would have the most detrimental affect on the flow. If it were closer to the blade tip, maybe a similar radius to the other gore point, it might produce even stronger pulsations and cancel even better and also reduce flow less. It might not be quite as close to 1/4 wavelength, but it might be good enough. This is just a theory and I might be all wet. I don't know how much science went into the current suggested design. Any thoughts?
 
2nd Gore Point

2nd Gore Point

After thinking about it more, I am wondering why this even works. It seems like, if it is wavelength related noise cancelling, it should be 1/2 wave, not 1/4 wave. If that is correct, it would work better to put the gore point by the opposite blade (180 degrees) from the original gore point, which would be close to 1/2 wavelength. This should cancel the discharge noise even more, but it might make the noise radiated from the blower surfaces worse. I'll try it.
 
2nd Gore Point

2nd Gore Point

Bill,
I've had about the same thoughts. Two different blades pass the gore points at about the same time. If they did not one might expect the second harmonic to be strengthened. The effective spacing of the gore points depends on the wavefront / wave direction. If you think the wave travels out the exit/ output then you would need to look at the gore point spacing projected onto the wavefront. I could make little sense of that view. I'm not at all sure the sound exits primarily from the output. I think the plastic sides might make good baffles with the sound passing right thru them. I suspect that the fan enclosure acts like a resonator with the fan pulsing the pressure up as a blade passes the original gore point and pulsing the pressure down at the second gore point. Interesting problem.
bababrown
 
2nd Gore Point

2nd Gore Point

Well, I put a gore point at 180 degrees and it got louder by about 4 dBA. So much for that idea. Tomorrow I will try the suggested 2nd point and see what happens.
 
2nd Gore Point

2nd Gore Point

Today I put a 2nd gore point in at the suggested location. It reduced the level to about 86 dBA with an open discharge port (nothing hooked up to outlet elbow). I haven't looked at an FFT yet to see how it affected the tonals.
 
Fan Enclosure as Resonator

Fan Enclosure as Resonator

Richard,

When I was blathering on a few years ago on this subject, most of it nonsense, I, too, wondered about the plastic wrap of the fan surround being a noise source, like a drum. Several of the others doing their build at that time tried various things, Dynamat, wrapping the housing with various other noise absorbing materials. Bottom line was there was little improvement. So I gathered then that the noise wasn't being transmitted and/or amplified by the PTEG wrap of the blower housing.

My recollection of the discussions from back then is that the greatest results then were accomplished by doing something with the blower outlet, such as adding 8 ft or so of insulated HVAC flex duct.

I haven't tried any of the schemes, so others' experiences trump my perhaps fuzzy recollections of past discussions.

Regards,
DWD
 
Best blower design?

Best blower design?

This seems to be an old thread and I was wondering if anyone has found an "ultimate blower design" using the 16" impeller with 5 hp motor? Anyone drawn up a dxf or sketchup design? Here's my welded steel version (I'm using a plasma cutter and welder to put mine together!

:)
 

Attachments

  • welded blower assembly.pdf
    33.6 KB · Views: 0
You must log in or register to reply here.
Back
Top