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Duct Construction and Uses
Discussion in fabricating and use of products that move air including spiral pipe, duct board and typical sheet metal work.
       


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  #21  
Old 11-25-2009, 11:23 PM
tinmetal tinmetal is offline
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single vane only good up to med pressure because ..on high pressure is goin to rattle or vibrate that why you need to put a double to make it stronger...and the vane plate need to be stichs weld...nut and bolts to secure the vane plate is not acceptable on smacna standard even you squesh the end of the bolt to secure the nut ...
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  #22  
Old 02-07-2013, 12:31 PM
basheroftin basheroftin is offline
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Quote:
Originally Posted by Rob View Post
I don't see any advantage in using double (aerofoil) vanes over single vanes. Anyone?
You're absolutely correct, here's the proof from the SMACNA guide;

B. TURNING VANES
1. Single vs Double Thickness
Duct fitting loss coefficient tables for elbows with turning
vanes have been in earlier editions of the
SMACNA HVAC Systems Duct Design manual and
the ASHRAE Fundamentals Handbook (American
Society of Heating, Refrigeration, Air Conditioning
Engineers) since 1977 SMACNA research on duct
fitting turning vanes still indicates that using double
thickness turning vanes instead of single thickness
vanes, increases the pressure loss of elbows (see
new data in Chapter 14, Table 14-10H).
Single thickness vanes have a maximum length of 36
in. (914 mm) as outlined on page 2-5 of the 1985
Edition of the SMACNA "HVAC Duct Construction
Standards." Turning vanes over 36 inches (914 mm)
are used in a double thickness configuration to keep
their curved shape with the higher airstream velocities
found in some HVAC system ductwork and to
prevent vibration or fluttering. They are not more aerodynamic
than single-blade vanes as originally
thought, as the loss coefficients in Table 14-10H indicate.
Of course, there often are higher losses caused by
the shape of short, single thickness vanes because
of the distortion created by some turning vane rails
(runners). But, multiple, single thickness turning vane
sections with vanes 36 inches (914 mm) long or less
can be installed in large elbows instead of using double
thickness vanes.
2. Trailing Edges
Trailing edges shown on single thickness vanes, design
numbers 1 and 3 in Figure 3-8 of ASHRAE 1989
Fundamentals Handbook Chapter 32 also have become
an industry problem. SMACNA research has
shown that unless these turning vanes are made and
installed perfectly, trailing edged vanes, when made
with average workmanship, actually have a higher
loss than vanes without them. And when the vanes
are accidentally installed with the airflow reversed,
much higher losses develop.
Because of this research, the SMACNA Duct Design
Committee has recommended that turning
vanes with trailing edges be eliminated from fitting
loss coefficient tables and duct construction
manuals when manuals are revised. They
have been eliminated from Table 14-10H in this
manual.
3. Vanes Missing
For many years contractors, often with the system
designer's approval, have eliminated every other turning
vane from the vane runners installed in rectangular
mitred duct elbows. Some contractors even believed
that they would lower the pressure loss of the
elbow by doing this. But they were wrong! This practice
more than doubles elbow pressure losses, and
definitely is not recommended.
Figure 5-13 is a chart developed from SMACNAsponsored
research performed by ETL Laboratories
in Cortland New York. ETL tested single thickness
turning vanes with a radius of 41/2 in. (114 mm). The
distance between vanes was varied from 3 in. to 61/2
in. (75 mm to 165 mm) in increments of 1/4 in. (6mm)
using embossed rail runners. Airflow velocities varied
from 1,000 to 2,500 fpm (5 to 12.5 m/s) in the 24-in.
x 24-in. (600 mm x 600 mm) test elbow. The loss
coefficient of 0.18 for the standard spacing of 31/4 in.
(82 mm) may be compared with the loss coefficient
of 0.46 at a 61/2 in. (165 mm) spacing (every other
vane missing). The pressure loss of the elbow with
missing turning vanes was over 21/2 times the pressure
loss of a properly fabricated elbow containing
all of the vanes.
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  #23  
Old 01-15-2015, 01:16 PM
cactassdupree cactassdupree is offline
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Quote:
Originally Posted by energystar View Post
I was told by an engineer that a radius throat with a square heal is the way to go. It's better than a sq/sq with no vanes
This guy didn't know what he/she was talking about. Or, you remembered it backwards. No offence but think about it. You don't have to be an engineer to see the Sq heel radius throat would send the air right into a dead end street. I had a Journeyman say that he saw Duct made out of clear plastic. Then they would send smoke through it. He said you can see what all the fittings did to the airflow. dupree
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  #24  
Old 01-15-2015, 11:19 PM
b.c.tinbasher b.c.tinbasher is offline
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Default smoke in the duct

I have attended a couple duct design courses put on by Smacna, and they show two videos showing air flow behaviour through straight duct and various fittings, vanes etc.
In one they used smoke, the other bubbles, both demonstrate very clearly the effect of square throats & heels, turning vanes (can't remember if they showed single AND double vanes) It was amazing how much 'straighter' the air was in a turning vane elbow. The air coming out of the vanes was literally in straight lines that would merge into a straight column a few duct diameters down from the vanes. Another thing some guys used to like to do was to put a 'scoop' in the duct to draw air into a take off. The videos demonstrated that this practice really only creates a lot of turbulence and noise and does little good overall. A properly designed takeoff works very well, puts quieter air through the takeoff, and does not contribute to turbulence in the main duct downstream of the takeoff.
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  #25  
Old 01-16-2015, 09:23 AM
Bradleyc1982 Bradleyc1982 is offline
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so in conclusion whats the best?? square throat and heel with vanes or radius throat and heel?? anyone got a picture of a single and double vaned elbow??
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  #26  
Old 01-16-2015, 09:57 AM
b.c.tinbasher b.c.tinbasher is offline
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Default The best elbow design

My opinion, FWIW is that the square throat & heel elbow with properly installed turning vanes is the best choice of design for the least friction loss and turbulence in a duct system. The vanes can maintain true laminar flow of the air going into them and coming out especially whereas the radius throat and heel design allows more air/higher pressure along the heel and down stream, while a negative pressure can develop along the throat.
I have seen this in action; On one installation we did, a 5 ton Liebert Challenger (vertical unit) went into a very narrow and short UPS room (high heat load) there was not much room for duct and to deal with the 2,000 cfm we needed a minimum number of grilles, all along one side of the duct of course, no space to put them anywhere else. This forced us to install one of the takeoffs too close to the elbow at the throat, I kid you not; air was going into one end of that grille on the throat side and out of the same grille on the other end (the grilles were wide like about 30" and narrow in height around 8" or 10")
Always been partial to double turning vanes, but only in the larger duct sizes, like 24 x 24 and greater. Seat of the pants guesstimate here, but the double vanes seem to take up too much space in smaller duct elbows.
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  #27  
Old 01-16-2015, 10:53 AM
Bradleyc1982 Bradleyc1982 is offline
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Quote:
Originally Posted by b.c.tinbasher View Post
My opinion, FWIW is that the square throat & heel elbow with properly installed turning vanes is the best choice of design for the least friction loss and turbulence in a duct system. The vanes can maintain true laminar flow of the air going into them and coming out especially whereas the radius throat and heel design allows more air/higher pressure along the heel and down stream, while a negative pressure can develop along the throat.
I have seen this in action; On one installation we did, a 5 ton Liebert Challenger (vertical unit) went into a very narrow and short UPS room (high heat load) there was not much room for duct and to deal with the 2,000 cfm we needed a minimum number of grilles, all along one side of the duct of course, no space to put them anywhere else. This forced us to install one of the takeoffs too close to the elbow at the throat, I kid you not; air was going into one end of that grille on the throat side and out of the same grille on the other end (the grilles were wide like about 30" and narrow in height around 8" or 10")
Always been partial to double turning vanes, but only in the larger duct sizes, like 24 x 24 and greater. Seat of the pants guesstimate here, but the double vanes seem to take up too much space in smaller duct elbows.
So why not square heel round throat??would that be better if there was enough space for it?? Also what about round heel/throat AND turning vanes? Would that be better still?? I would to love to watch the SMACNA bubble DVD but have never been able to find it anywhere??
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  #28  
Old 01-17-2015, 12:52 AM
b.c.tinbasher b.c.tinbasher is offline
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Default No square heels

I'd say that any square heel elbow without vanes will be causing a lot of turbulence in the duct, whatever design of throat it has.
We have built a few large radiused heel and throat elbows with turning vanes, it's a good way to go when you have high velocity air. I think they provide better laminar flow than a 'square' elbow with vanes, especially with the radiused heel.
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