The New Home for the Sheet Metal Industry Forums

Go Back   Sheetmetal Talk > Sheet Metal Forums > Duct Construction and Uses

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

Share This Forum!  


Thread Tools Display Modes
Old 09-05-2004, 01:57 PM
jessea1015 jessea1015 is offline
Junior Member
Join Date: Jun 2003
Location: Columbia, MO
Posts: 7
Thanks: 0
Thanked 0 Times in 0 Posts
Default air through elbows

We have a chart at my shop that tells how many feet different kinds of fittings are worth. It says an elbow with a square heel and throat has the same amount of air loss as an elbow w/ a radius heel and square throat.Is this true? Excuse me if my lingo is not correct.
Reply With Quote
Old 09-09-2004, 10:08 AM
CadMan CadMan is offline
Join Date: Sep 2004
Location: Texas
Posts: 30
Thanks: 1
Thanked 4 Times in 3 Posts

I read thru the SMACNA Duct Design book and all I found was that a radius throat is about 70% better airflow than a standard 90. Couldn't find anything about the heel though. I was interested in where you got this chart and where I might get one.
Reply With Quote
Old 09-09-2004, 03:36 PM
tazman tazman is offline
Junior Member
Join Date: Aug 2004
Location: The Netherlands
Posts: 4
Thanks: 0
Thanked 0 Times in 0 Posts

Radius is always better then square, when it is about the air resistance.
We ALWAYS use elbows with a radius. Except for the inner radius, when there is too less space we use square.

I have a simple mind and see it simple. Air likes a quiet easy ride, and not a ductsystem like rollercoaster!
Also watch out for noises!!! I never here about sound in here. There are some golden rules.
One of them is sharp edges, they can cause some noise.
Reply With Quote
Old 10-20-2004, 03:52 PM
bordontn bordontn is offline
Professional Member
Join Date: Sep 2003
Location: McEwen, TN.
Posts: 212
Thanks: 0
Thanked 2 Times in 1 Post

Be careful of those square throats on a radius elbow....I'm posting part of a chart which shows the resistance in feet of duct..
the itinerant tinker a.k.a. bordontn
Attached Images
File Type: jpg elbows.jpg (20.5 KB, 330 views)
Reply With Quote
Old 10-27-2004, 06:26 PM
Gate Gate is offline
Join Date: May 2004
Location: Ohio, USA
Posts: 51
Thanks: 0
Thanked 1 Time in 1 Post

A vaned square throat/heel ell is better for airflow than a radius heel with square throat. If you vane this configuration you need to run full vanes evenly spaced...

- Gate
Reply With Quote
Old 03-05-2008, 10:25 AM
energystar energystar is offline
Junior Member
Join Date: Mar 2008
Posts: 5
Thanks: 0
Thanked 0 Times in 0 Posts

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
Reply With Quote
Old 01-15-2015, 02:16 PM
cactassdupree cactassdupree is offline
Professional Member
Join Date: Aug 2008
Location: Sunny Florida
Posts: 602
Thanks: 161
Thanked 59 Times in 50 Posts
Default engineers

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
Reply With Quote
Old 01-16-2015, 12:19 AM
b.c.tinbasher b.c.tinbasher is offline
Join Date: Mar 2011
Location: Central British Columbia
Posts: 76
Thanks: 12
Thanked 9 Times in 6 Posts
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.
Reply With Quote
Old 03-05-2008, 04:36 PM
Bud's Avatar
Bud Bud is offline
Join Date: May 2003
Location: Waukesha
Posts: 2,102
Thanks: 416
Thanked 125 Times in 88 Posts

Better then vanes? I don't think so. The radius throat and heel is great way to fab elbows, and you really want to learn more about the radius to width ratio for maximum air flow or air flow with the least resistance. If you can't use a radius throat, you should probably consider vanes to help the air flow around the corner.
Reply With Quote
Old 03-05-2008, 08:53 PM
legacy legacy is offline
Join Date: Oct 2006
Posts: 62
Thanks: 1
Thanked 1 Time in 1 Post

We once had a guest speak at a union/management committee day, he was representative from the ITI (body that governs all SMWIA apprenticeship programs). ...Anyway, he was retiring from the ITI and was moving from CA to TN and was having a house built. On one of his final trips to check on construction, he noticed that the HVAC constractor had used all srare throat/ radius heel 90's. He made the take them out and replace them with full (1-1/2X) radius 90's where they would fit and sqaur with vanes where they did not.

I have made sq throat/radius heel for people that have walked into my shop looking for duct, but have never made that fitting for one of my jobs.

Just like a point to point (no miter) offset, if you measure the distance from the inside of a square throat to the mid point of the radius heel of a sq/rad 90, its got to choked down by at leats 10%-15% I would think.
Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


All times are GMT -6. The time now is 11:36 AM.

Powered by vBulletin®
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.