In the previous post, we were looking at how the number of turning vanes in an elbow impacted its performance and discovered that there are a lot of variables to consider. One variable that we didn't change (other than to experiment with aspect ratio) was the size of the duct. In other words, we looked at a duct with a relatively low flow capacity. The following table illustrates what happens if you take the same fitting shape and scale it up to handle a lot more air. As you can see, a poor fitting that was of little consequence in a small duct could have a significant loss in the large duct. This is because small ducts take a lot more perimeter to surround the volume they contain. Since most of the friction associated with moving air through a straight duct is related to the interaction of the air with the duct wall at the perimeter, then a small duct must operate at a lower velocity than a larger duct of the same aspect ratio if the friction rate is to be held constant. As a result the change in carrying capacity associated with changing a duct size is not directly proportional to the change in duct cross section.
Lower velocities imply lower velocity pressures, so the impact of a poor fitting in a small duct will be much less significant than if that same fitting were installed in a larger duct operating at the same friction rate.
Now, consider for a minute how duct designs are conveyed to the field in our industry for the purpose of bidding work. The drawings are always "to scale". So, on a two line duct drawing that illustrated a 12" x 12" three vane version of the elbow geometry we have been discussing, the elbow would be 1/2" wide when drawn on a 1/2" to the foot scale plan detailing a mechanical room. If that same elbow were drawn on 1/8" to the foot scale floor plan showing the distribution system, it would only be 1/8" wide. At that size, it would be very difficult to convey the details of the vane geometry that are critical to controlling the losses through it. If the drawings were one line drawings, it would be impossible. In fact, on a one line drawing, both the 60" elbow and the 12" elbow illustrated in the table would look the same. With out further guidance, a tradesperson fabricating the duct could easily decide to build both elbows with out turning vanes, especially if they are under pressure to complete the job quickly and bring it in under the budget established by the competitive bid that won their company the work. But, the large elbow with no vanes could impose an unacceptable loss on the system it served if the designer had anticipated that it would be fabricated with them. This could be a really big deal if there were a number of such elbows. In contrast, the impact of the smaller elbow fabricated with out vanes may not be significant enough to show up as a performance problem even if the designer had intended that vanes be provided in all locations.
The bottom line is that the losses through a fitting are very dependent on a wide array of variables, many of which will impact its cost. All of this can seem a quite overwhelming at first, especially if you are under time and budget pressures; how can you analyze each elbow and make the best decision? The fact is that you probably can’t,. But, by asking questions like the one that started this string of posts and gaining an insight into the critical factors that are associated with the losses through a fitting, you will gain the ability to make good judgment calls with out the analysis.
Plus, with computers and software, there are some neat tools to help you make a very informed decision in a timely fashion. The numbers I used for most of the tables in the preceding post were generated using the ASHRAE duct fitting database software in a matter of a few key strokes; I spent more time writing about and printing the results than I did doing the math. In my opinion, a tool like the ASHRAE duct fitting data base is well worth the investment because it allows you or your design team to make very good decisions with very little effort. And as you and your team use it, your "gut" learns and soon, you can make good decisions on the fly with out the software. This is a skill that is very handy out in the field if you are trying to decide if a fitting needs to be re-done on a construction walk-through or a retro-commissioning/troubleshooting project.
Clicking here will link you to the point on the ASHRAE website where you can find out more about the tool. The ASHRAE handbooks have similar information in tabular form; very useful but a bit more time consuming. United McGill publishes loss coefficients for their products and you can download their literature from their web site. If you register with them for their electronic newsletter, I believe you will receive a free copy of their duct design CD, which is very good and probably worth what they would charge for it even if you couldn’t get it for free.
As you may have gathered from this discussion, duct fittings need to be considered in the context of the system they serve. AMCA International "publishes and distributes standards, references, and application manuals for specifiers, engineers, and others with an interest in air systems to use in the selection, evaluation, and troubleshooting of air system components" (the quote is from their web site; the emphasis is mine). For example, AMCA publishes data on the impact of fittings on fan performance when the fittings are installed in the immediate vicinity of the fan inlet or outlet. The following link will take you to their website. I have found their publications 200 - 203 to be particularly valuable and well worth the investment. Their biannual publication "In Motion" can also be downloaded from the site for no cost and contains informative articles and a listing of AMCA members.
