Noise suppression in ductwork? Minimum segment length?

Posted by Steve on October 25, 2009, 8:55 am

> Bennett Price wrote:

>> Perhaps this site (and links there) may help
>> http://www.engineeringtoolbox.com/noise-reduction-silencers-d_81.html

> Thank you.
> Your response is characteristic of what the web should be.
> It appears that you:
> 1) Actually read the question...UNDERSTOOD the question.
> 2) Thought about the question before you started typing.
> 3) Provided information DIRECTLY RELATED TO SOLVING MY PROBLEM
> and with sufficient context to be believable/useful.
> 3) Presented in a lucid manner.
> 4) Refrained from making unfounded assumptions and aspersions.
> 5) Refrained from name-calling, bickering and other inciting statements.
> If only a few of the newsgroup denizens followed your example,
> the web would be a lot better place.
> I believe your input will lead to the solution to my problem.
> Thanks, again,
> mike

First off, you are *NOT* a interweb hall monitor, second, you were told
repeatedly what your problem is, and what its going to take to correct it.
You got your system for "free", and you got what you paid for.... now your
whining about it because you have buyers remorse. Your being an EE doesn't
mean that your a certified master technician any more than parking a pinto
in your garage makes it a mercedes.
BTW... my first clue??

" I had a new Coleman 99.5% gas furnace installed.

The return duct is a 25' 16" diameter metal straight shot from the
hallway to the furnace in the attic. Air flow is about 900 linear
feet/minute. That's over twice the airflow for my 37 year-old Lennox."

Coleman is also the cheapest piece of crap on the market, right next to
Armstrong and Goodman. They don't even put Coleman in trailers anymore,
because it won't hold up

Get over yourself, get the ductwork issues corrected and go away.

Posted by on October 25, 2009, 12:12 pm

wrote:

>> Bennett Price wrote:

>>> Perhaps this site (and links there) may help
>>> http://www.engineeringtoolbox.com/noise-reduction-silencers-d_81.html

>> Thank you.
>> Your response is characteristic of what the web should be.
>> It appears that you:
>> 1) Actually read the question...UNDERSTOOD the question.
>> 2) Thought about the question before you started typing.
>> 3) Provided information DIRECTLY RELATED TO SOLVING MY PROBLEM
>> and with sufficient context to be believable/useful.
>> 3) Presented in a lucid manner.
>> 4) Refrained from making unfounded assumptions and aspersions.
>> 5) Refrained from name-calling, bickering and other inciting statements.
>> If only a few of the newsgroup denizens followed your example,
>> the web would be a lot better place.
>> I believe your input will lead to the solution to my problem.
>> Thanks, again,
>> mike

>First off, you are *NOT* a interweb hall monitor, second, you were told
>repeatedly what your problem is, and what its going to take to correct it.
>You got your system for "free", and you got what you paid for.... now your
>whining about it because you have buyers remorse. Your being an EE doesn't
>mean that your a certified master technician any more than parking a pinto
>in your garage makes it a mercedes.

No, but it does explain some things ....

>BTW... my first clue??
>" I had a new Coleman 99.5% gas furnace installed.
>The return duct is a 25' 16" diameter metal straight shot from the
>hallway to the furnace in the attic. Air flow is about 900 linear
>feet/minute. That's over twice the airflow for my 37 year-old Lennox."

You're think that an EE would realize velocity <> airflow

VOLUME.

>Coleman is also the cheapest piece of crap on the market, right next to
>Armstrong and Goodman. They don't even put Coleman in trailers anymore,
>because it won't hold up
>Get over yourself, get the ductwork issues corrected and go away.

--
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Posted by spamme0 on October 25, 2009, 7:05 pm

> You're think that an EE would realize velocity <> airflow
> VOLUME.
>
>

I admit that my skills in fluid dynamics are weak.

I'd been working with the assumption that I could get a
reasonable first-order approximation to airflow by multiplying
the average velocity across the duct times the area of the duct.
Given that the duct size didn't change, the relative volumes
should be the same as the relative velocities...to first order
for small changes. I assumed I could ignore the small changes in
air density due to the small change in pressure within the duct.

Please disclose the additional math required to determine the
airflow volume with sufficient accuracy to solve the noise problem
described.

Thank you for your willingness to educate.
mike

Posted by Steve on October 25, 2009, 8:00 pm

>> You're think that an EE would realize velocity <> airflow
>> VOLUME.

> I admit that my skills in fluid dynamics are weak.
> I'd been working with the assumption that I could get a
> reasonable first-order approximation to airflow by multiplying
> the average velocity across the duct times the area of the duct.
> Given that the duct size didn't change, the relative volumes
> should be the same as the relative velocities...to first order
> for small changes. I assumed I could ignore the small changes in
> air density due to the small change in pressure within the duct.

pressures are measures in .01 inches of water column.
900 fpm is twice what you should have for return air.
airflow should be 400cfm per ton of air conditioning.

> Please disclose the additional math required to determine the
> airflow volume with sufficient accuracy to solve the noise problem
> described.

Your the EE..... you figure it out.

> Thank you for your willingness to educate.
> mike

Posted by on October 25, 2009, 9:53 pm

wrote:

>> You're think that an EE would realize velocity <> airflow
>> VOLUME.
>>
>>

>I admit that my skills in fluid dynamics are weak.
>I'd been working with the assumption that I could get a
>reasonable first-order approximation to airflow by multiplying
>the average velocity across the duct times the area of the duct.
>Given that the duct size didn't change, the relative volumes
>should be the same as the relative velocities...to first order
>for small changes. I assumed I could ignore the small changes in
>air density due to the small change in pressure within the duct.
>Please disclose the additional math required to determine the
>airflow volume with sufficient accuracy to solve the noise problem
>described.

Please educate yourself on HVAC duct design fundamentals. Try
buying something called Manual D from acca.org or
http://www.manuald.com/

I am not going to type the book into this newsgroup for you.

>Thank you for your willingness to educate.
>mike

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Noise suppression in ductwork? Minimum segment length? - Page 3