Recently, someone asked me if it made sense to add turning vanes to a radiused elbow. That’s one of those questions where there is no exactly a right answer that applies for every situation. The impact of the turning vanes has a lot to do with both the geometry of the fitting and the velocity of the air moving through it. To gain some insight into this, lets consider a 12” square elbow with a 12” centerline radius (i.e. the centerline radius is equal to the duct width) like the one pictured below.
At 1,500 fpm, the loss through such an elbow without vanes is about 0.03 in.w.c.; virtually nothing. If we add one turning vane, the loss drops to .01 in.w.c.; with three vanes, we end up with a calculated loss of 0.00 in.w.c. The parameters associated with these configurations are summarized in the table below.
To me, these results indicate that in a low velocity application, all three elbows are pretty close to the same and its probably not worth the added cost for the vanes. But, if you double the airflow, the losses are much more significant. Now, the unvaned elbow has a loss of 0.12 inches w.c. while the elbow with three vanes calculates out to 0.01 in.w.c. In other words the elbow with out vanes has a measurable loss while the elbow with vanes is nearly inconsequential. Here is a table summarizing the parameters associated with the elbow operating at a higher velocity.
To me, these results indicate that the vanes may be worth something in a high velocity application.
If you take the three vane elbow at a high velocity and start to change the geometry of the elbow, the loss starts to increase. For instance, if you reduce the center line radius from 12” to 8”, the loss goes up to 0.02 in.w.c. all other things being equal. If you start to flatten out the elbow, the same thing happens; a 7” x 21” elbow has a loss of 0.5 in.w.c. all other things being equal (those are funny duct dimensions, but I did it to illustrate the effect while holding the velocity relatively constant). Here is a table summarizing the parameters
associated with the elbow operating at a higher velocity.
So, my point is that you have to look at the vane decision in the context of the application; the turning vanes will be more important and worth more in high velocity applications and in applications where turn radius are tight and aspect ratios are high (thin, flat ducts).
In the next post, I’ll continue this discussion and look at what happens when you increase the size of the duct.
Senior Engineer – Facility Dynamics Engineering