Differential Pressures and Duct Design 101...

Posted by Jake on January 15, 2007, 7:35 pm

Not that I'm planning on designing any industrial dust collector
ducts... but I am curious how the design work is done.

Any book suggestions on that? Is there a "ASHRAE Duct Design for
Dummies" book around?

Here's the thing... I'm installing some instrumentation on several mid
'80's design dust collectors for a steel mill. They have 'wet type'
collectors made by an outfit out of Michigan named Schnieble, and
they're in pretty good shape....

I'm putting pressure transmitters in various places and not liking the
feedback I'm getting. The DP is not what it should be. I've talked to
the engineers from Schnieble, and while they support these units they're
no longer manufacturing them... modern technology triumphs again.

The manufacturer says all is (physically) as it should be... but then I
got to looking at the original prints... circa 1980... and they show
something called an 'evase' on the fan discharge. Now, I Googled this...
and now have some understanding what it's supposed to do... but it isn't
ON the units... I looked... twice. I still don't know how to pronounce
it, though (=;.

Anyhow, the original fan design calls for 14.3" wc @ 48,000 CFM. I'm not
measuring anywhere close to either number and I'm just curious as to why...

The customer will be happy if the DP is in range... but running 'out of
design' has me curious... and curious minds WANT to know... )-.

Jake

Posted by on January 15, 2007, 8:52 pm

Jake wrote:

>Not that I'm planning on designing any industrial dust collector
>ducts... but I am curious how the design work is done.
>Any book suggestions on that? Is there a "ASHRAE Duct Design for
>Dummies" book around?
>Here's the thing... I'm installing some instrumentation on several mid
>'80's design dust collectors for a steel mill. They have 'wet type'
>collectors made by an outfit out of Michigan named Schnieble, and
>they're in pretty good shape....
>I'm putting pressure transmitters in various places and not liking the
>feedback I'm getting. The DP is not what it should be. I've talked to
>the engineers from Schnieble, and while they support these units they're
>no longer manufacturing them... modern technology triumphs again.
>The manufacturer says all is (physically) as it should be... but then I
>got to looking at the original prints... circa 1980... and they show
>something called an 'evase' on the fan discharge. Now, I Googled this...
>and now have some understanding what it's supposed to do... but it isn't
>ON the units... I looked... twice. I still don't know how to pronounce
>it, though (=;.
>Anyhow, the original fan design calls for 14.3" wc @ 48,000 CFM. I'm not
>measuring anywhere close to either number and I'm just curious as to why...
>The customer will be happy if the DP is in range... but running 'out of
>design' has me curious... and curious minds WANT to know... )-.
>Jake

Jake,
SMACNA sets the standards as far as duct Contruction (sizing, guage of
material, stiffners, etc). ASHRAE sets the standard for quantifying
how much air to be moved for varying applications, filtration etc.

So, to design duct system, you'll need SMACNA's Duct Construction
Standards.

http://www.smacna.org/bookstore/index.cfm?fuseaction=search_results&topicID=4&cfid=4671513&cftoken=75048866

Do you know for a fact if your system ever achieved the design airlow
& static?

Perhaps these guys can help you design & install the evase your system
is calling for...

http://www.sdscorp.com.au/evase.html

as usual, if any of this is useful info, my paypal still works.

LOL j/k

Posted by -zero on January 16, 2007, 8:26 am

<snip>
> The manufacturer says all is (physically) as it should be... but then I
> got to looking at the original prints... circa 1980... and they show
> something called an 'evase' on the fan discharge. Now, I Googled this...
> and now have some understanding what it's supposed to do... but it isn't
> ON the units... I looked... twice. I still don't know how to pronounce it,
> though (=;.

Sounds like a major contributor to your discharge woe's. If it's not done
close to correct, it'll turn in upon itself, (redirecting the energy)similar
to a whirlpool in a white-water rapids. *(If someone has a more better
analogy, please chime in.)

This seams to cover evase 101.
http://www.aireng.com.au/Downloads/pdf/Diffuser%20&%20Outlet%20Perf.pdf

> Anyhow, the original fan design calls for 14.3" wc @ 48,000 CFM. I'm not
> measuring anywhere close to either number and I'm just curious as to
> why...

What you are getting now vs. what you will get, should you have the
opportunity to
remove the first discharge spool piece, should help determine your inlet
design
application worthiness since most all fan manufacturers will also show their
fans
discharge specs into free-air as well.

Also, a true RPM off the fan pulley (opposed to measuring and amp draw
alone)
narrows the variable of the fans true performance at any given time or after
any alterations or adjustments.

And, for all other fan things;
http://www1.eere.energy.gov/industry/bestpractices/pdfs/fan_sourcebook.pdf

-zero

Posted by =?iso-8859-15?Q?Tekkie=AE?= on January 16, 2007, 7:44 pm

-zero posted for all of us...

> This seems to cover evase 101.
> http://www.aireng.com.au/Downloads/pdf/Diffuser%20&%20Outlet%20Perf.pdf
>

snippy do dah

> And, for all other fan things;
> http://www1.eere.energy.gov/industry/bestpractices/pdfs/fan_sourcebook.pdf
>

It's not rocket science - anybody can do this... As one can see *NOT*
--
Tekkie Don't bother to thank me, I do this as a public service.

Posted by Bob_Loblaw on January 17, 2007, 12:48 am

> It's not rocket science - anybody can do this... As one can see
> *NOT*

Ya...'zactly!...what he said...it ain't rocket surgery!.... :-)

--
Respectfully, Bob

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