Duct Turning Vanes - Single vs Airfoil Type ??
Dennis Myer
type turning vanes. An application our firm designed called for airfoil
type turning vanes in the specifications but single thickness vanes were
installed. My manager (who designed the system) asked me to find some
information to verify that airfoil vanes would have worked better - but
what I have found in SMACNA and ASHRAE is that airfoil turning vanes
produce a higher total pressure loss across an elbow than single thickness
vanes.
If this is indeed the case - then why in the world would anyone ever want
to use double thickness (airfoil) turning vanes?? Obviously they must be
more expense.
More expensive + higher pressure loss = makes no sense.
Can anybody explain??
Dennis Myer
HVAC Engineer
PS - an old Carrier design manual states the equivalent length of an elbow
with airfoil vanes to be less than the equivalent length of an elbow with
single thickness vanes - so what is correct Carrier, SMACNA, ASHRAE, ...
????
rto...@removethis.oanet.com
wrote:
>I am wondering if anyone has any information on the advantages of airfoil
>type turning vanes. An application our firm designed called for airfoil
>type turning vanes in the specifications but single thickness vanes were
>installed. My manager (who designed the system) asked me to find some
>information to verify that airfoil vanes would have worked better - but
>what I have found in SMACNA and ASHRAE is that airfoil turning vanes
>produce a higher total pressure loss across an elbow than single thickness
>vanes.
>
>If this is indeed the case - then why in the world would anyone ever want
>to use double thickness (airfoil) turning vanes?? Obviously they must be
>more expense.
>
They obviously did not understand the action and purpose of turning
vanes. One certainly does not want "lift" from the vanes, just a
simple change of direction with the least surface area to generate
friction. Often one vane is better than two for just this reason.
>
>More expensive + higher pressure loss = makes no sense.
>
>Can anybody explain??
>
>Dennis Myer
>HVAC Engineer
>
>PS - an old Carrier design manual states the equivalent length of an elbow
>with airfoil vanes to be less than the equivalent length of an elbow with
>single thickness vanes - so what is correct Carrier, SMACNA, ASHRAE, ...
>
The Carrier manual is correct. Because of the reduced turbulence with
turning vanes, the effective length is less. However, this needs
qualifying depending on the number of vanes used. Too many will
increase the effective length. It depends on the size and shape of
the duct.
john williamson
I am wondering if anyone has any information on the advantages of
airfoil type turning vanes. An application our firm designed called for
airfoil type turning vanes in the specifications but single thickness
vanes were installed. My manager (who designed the system) asked me to
find some information to verify that airfoil vanes would have worked
better - but what I have found in SMACNA and ASHRAE is that airfoil
turning vanes produce a higher total pressure loss across an elbow than
single thickness vanes.
If this is indeed the case - then why in the world would anyone ever
want to use double thickness (airfoil) turning vanes?? Obviously they
must be more expense.
More expensive + higher pressure loss = makes no sense.
Can anybody explain??
Dennis Myer
HVAC Engineer
PS - an old Carrier design manual states the equivalent length of an
elbow with airfoil vanes to be less than the equivalent length of an
elbow with single thickness vanes - so what is correct Carrier, SMACNA,
ASHRAE, ...
????
++++++++++++++++++++
Dennis
I can't give you anything except some experience that I've had working
with them years ago. At one time it was the thing to draw square 90
degree turns and put airfoil vanes in them, may still be for I've been
out of it for awhile. The problem was that all the ells had them
whether the duct was 10 x 6 or 48 x 24. My experience is that it looks
good on paper but try making a 10 x 6 square 90 and put a couple of
those large airfoil vanes in and you will soon realize that something is
wrong here.
My belief is the air foil vanes may be good for higher velocities and
larger ducts. I would not put them in any smaller elbows than
approximately 36 x 18 and velocities less than 1500 FPM. Anything
else, if it has to be a square turn, I prefer the single turned vanes.
Like you said, less pressure drop and less cost.
Actually I prefer a nice radius turn elbow over square ones by far.
John W
(Nothing is impossible for the person that dosen't have to do it)
paul milligan
>I am wondering if anyone has any information on the advantages of airfoil
>type turning vanes. An application our firm designed called for airfoil
>type turning vanes in the specifications but single thickness vanes were
>installed. My manager (who designed the system) asked me to find some
>information to verify that airfoil vanes would have worked better -
First, what was spec'd should have been put in. Whoever made
that design change is now responsible for it, IE, the contractor who
decided it would be a good way to save a few bucks and would never be
seen / noticed is liable. This is a very common item for contractors
to think they can scrimp and get away with it.
Second, a month ago I was on a job where the company I work
for tried the same BS. They were back within two weeks picking pieces
of single-thickness vanes out of the ductwork, and trying to figure
out how to get the new airfoils into the ductwork without having to
rip everything apart.
>what I have found in SMACNA and ASHRAE is that airfoil turning vanes
>produce a higher total pressure loss across an elbow than single thickness
>vanes.
>
>If this is indeed the case - then why in the world would anyone ever want
>to use double thickness (airfoil) turning vanes?? Obviously they must be
>more expense.
And thicker, thus the added drop. But, hugely stronger and
more durable.
Paul
>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~
pjm@(remove this part )pobox.com
My WWW site is at http://www.pobox.com/~pjm, featuring free HVAC software.
The Sci.Engr.Heat-Vent-AC and Alt.HVAC FAQ is at http://www.elitesoft.com/web/sci.hvac/
john williamson
First, what was spec'd should have
been put in. Whoever made that design change is now responsible for it,
IE, the contractor who decided it would be a good way to save a few
bucks and would never be seen / noticed is liable. This is a very common
item for contractors to think they can scrimp and get away with it.
Second, a month ago I was on a job
where the company I work for tried the same BS. They were back within
two weeks picking pieces of single-thickness vanes out of the ductwork,
and trying to figure out how to get the new airfoils into the ductwork
without having to rip everything apart.
And thicker, thus the added drop.
But, hugely stronger and more durable.
Paul
~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~>~~
++++++++++++++++++++=
Regardless what our thoughts are on the merits of the airfoil vanes,
Paul is indeed right about if they were spec'd, they should have been
installed.
IMO, if it is deemed by the Engineers to be detrimential to the design
or operation, they should be changed. If they are not deemed
detrimential to the design or operation, then perhaps a cost adjustment
is in order, or sometimes I've seen situations like this used to fund
"change orders" that came about during the job.
John W
(Nothing is impossible for the person that doesn't have to do it)
2ric...@nettaxi.com
increases. For residential work the
cheaper vanes are fine.
RAC
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