HVAC Flex for muffler question

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NickLazz

New member
Reading posts and Bill's site I have found that the 8" HVAC Flex duct is an effective way to reduce noise, in the form of a muffler. With that said, the information says it needs to be 'long'... what is 'long'? I have mounted my cyclone about 12' off the floor, with the filters stacked it leaves about 5' between the exhaust transition and the filters. I will still need to fabricate some sort of clean out, so will say 48" of flex be considered 'long' enough?

Also, if any of you have used this type of muffler, is an 8" round with flange attached on both ends sufficient for attachment? I will be using the standard nylon zip ties used for HVAC. I have the tool to secure them very tight so I am not thinking a band clamp will be necessary.

Thanks,

Nick
 
HVAC Length

HVAC Length

Hello again, Nick,

I seem to recall this was a discussion item a year or so ago, and the "answer" was it should be 8 ft or so. Though it would seem to me that what's acceptable to some might not be to others, hence your result may vary with your desired result.

Might pay to do a search on HVAC.

Regards,
DWD

Edit: My mistake. I realized the figure I quoted was actually from Pentz' site, in the "G" section under "Ducting" (the "Noise Control" tab in the index):
I did a little testing on the various forums of mufflers. On my cyclone with its biggest blower, a 15.5” diameter impeller powered by a 5 hp motor, my galvanized metal cyclone measured 96 db with the blower just going directly into the filter through metal duct. Trading the metal duct from the blower to the filters for PVC dropped it to 92 dB. Adding my homemade muffler in place of that duct dropped it to 88 dB. Using 8’ of insulated HVAC duct in place of the muffler dropped it to 87 dB. Using insulated HVAC duct and my own homemade muffler it went down to 82 dB.
Click to expand...

Quote from Pentz' site.

DWD
 
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Thank you

Thank you

I appreciate the feedback. I will try what I have I suppose. Do you know if the HVAC flex has to be in a straight line? In other words does it have to be vertical or can I put it in an 'S' to make up the 8'?

Thanks again,

Nick
 
Nick,

As its on the pressure side of the blower, should be relatively unaffected by bends. Of course, the "within reason" qualifier applies, don't twist your flex into knots.

Think there's an example in the gallery, for the CV1400. Yes, RickG in WV, here. Look at some of the other photos Rick's provided as well. Above the view in the photo I referenced, the flex makes a 90, continues over to his ceiling mounted filters.

Regards
DWD
 
Very good, No knots!
Maybe I will try the 4' section first as it will look cleaner, but then test it with a longer piece and see if there is any noticeable difference with noise. It may be a little while but I will try and post what I learn if any are interested.

Thanks for all your help.

Nick
 
Sounds like a plan! Which reminds me of the best line in Hunt for Red October and delivered by the admiral being played by Fred Thompson: "Son, the Russians don't take a dump without a plan!"

Will be interested in your findings.

Regards,
DWD
 
Well, I ended up using about 19' of 8" HVAC flex for my filters. For the most part it runs horizontal. I can only compare it to the original test of the machine, however I am very pleased with the noise level, which is way under my expectations.
I did not experiment with different lengths, I just went with the 'more is better' idea and used as much as I could.

Not very scientific, but it works great.

Nick
 
Flex Muffler

Flex Muffler

Nicklazz,

Now that you have the long flex duct installed, where is the remaining noise coming from and what does it sound like. Is it mainly a low tone now or like normal fan noise, only not as loud? Is it coming from the cyclone, the flex duct or the filter? I am designing a muffler and the answers will help me determine the design. I only planned to use about 4 or 5 feet of the flex duct.
 
Bill said:
Nicklazz,

Now that you have the long flex duct installed, where is the remaining noise coming from and what does it sound like. Is it mainly a low tone now or like normal fan noise, only not as loud? Is it coming from the cyclone, the flex duct or the filter? I am designing a muffler and the answers will help me determine the design. I only planned to use about 4 or 5 feet of the flex duct.
Click to expand...

Well, I guess I would say it is just a low hum. I haven't noticed any noise from the flex or filter. Everything I read though said about 8 feet was minimum. Some have used parallel runs to accomplish this, I just had the room for the long run. If you haven't seen my pictures, you should probably check them out, my CV is mounted 12+' off the ground. Since the blower housing and motor are at that height, I imagine that also has some impact on noise.
Anyway, one thing I did not mention is: when I mounted my motor housing bracket I used stall mat material between it and the wall. The stuff is used for horse stalls and is a heavy, dense ruber-like material. Very easy to find and it is sold by the foot. Good luck!
 
Flex Muffler

Flex Muffler

So you can't tell where the "low hum" is coming from? The low hum is probably blade-pass frequency, which for a 3450 rpm motor and a 6 blade fan is about 345 Hz. That might be low enough to be hard to detect the origin, for the same reason that it doesn't matter where in the room you put your sub-woofer. I suspected that the duct would have done a pretty good job on the higher frequencies, but not so good on the low. A tuned (345 Hz) resonator upstream of the muffler might help. I plan to try one on mine when I get more time this winter to finally set it up. Not out of necessity, just for fun. Another thing that might help is a shell (sound barrier) around the flex to reflect the low frequencies back into the duct. It could be a section of metal duct pipe. You really have to just experiment to see if these ideas work. We have used hydraulic and acoustic resonators at work that theoretically should have helped reduce noise and vibration, but often they don't. Sometimes the do.
 
Stall Mat?

Stall Mat?

I wouldn't think the "stall mat" would help much unless you had isolation material under the bolt heads. If you bolt down solid through a mat, you still have a solid mechanical path for vibration through the bolts.
 
Bill said:
I wouldn't think the "stall mat" would help much unless you had isolation material under the bolt heads. If you bolt down solid through a mat, you still have a solid mechanical path for vibration through the bolts.
Click to expand...

Maybe it did, maybe it didn't. But my gut tells me it was a heckuva lot better than attaching to straight to the drywall.
All I know Bill, is that my CV is not a nuisance. To my ears, I don't hear anything coming from my filter or flex assembly.

From some critics claims of this machine being too loud, well, mine isn't. The only noise complaint I have (complaint is a little strong) is the blower or motor squeaks a little when it is shutdown.
 
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One last item I never really answered in your first post: the noise seems to be from the cyclone itself. To me, it only appears to be fan / motor noise. In other words, there is no other noise I can detect that I wouldn't expect from a 5hp motor attached to a blower. And to tell you the truth, I doubt there is any way to dampen that even if I needed to, aside from enclosing it.

I think it is just the air turbulence from the fins themselves that cause the majority of it. Just like a prop plane...you don't hear the engine, you're hearing the propeller.
 
Thanks JJ! I wasn't really worried about it, but glad to know it's normal. I appreciate you taking the time.

Nick
 
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