Fan and Duct Interactions

I recently received an e-mail from a blog visitor asking my opinion about a system they were working on. They had read one of my previous posts where I discussed the impact turning vanes and general fitting geometry can have on elbow pressure losses and were faced with a situation where they had to “shoe horn” an air handling unit into limited space and connect it to a number of ducts, 2 on the supply side and one on the return side. As a result, there were going to be a number of elbows located immediately on the fan discharge and they were concerned about the impact this might have on the system performance.Their concern about the impact of the fittings on fan performance is well founded. That said, the amount of concern is very much related to the geometry of the fitting and its relationship to the fan as can be seen from the following slide.

(This slide is from a class I developed for PECI and the Pacific Energy Center on Pump and Fans. For those who are interested in more information, I have loaded the portion of the PowerPoint that this slide came from into my Google account and published it there.)

What all of this means is that to really understand the impact of a fitting on fan performance, we need to look at the fittings in the context of the fan connection. For instance, if the fan in the AHU in question is a plug or plenum fan or for that matter a SWSI (Single Width, Single Inlet) or DWDI (Double Width, Double Inlet) arrangement discharging into a plenum and the ducts and fittings in question connect to the plenum rather than directly to the fan, then the duct to plenum connection becomes critical.

If, in contrast, the fan is a SWSI or DWDI arrangement and the ducts connect directly to the fan discharge, then the distance between the fan discharge and the first fitting, the configuration of the fitting relative to the fan arrangement, and the fitting geometry itself become important.

The other thing that will come into play in applications where space is at a premium is that you may end up with  several closely coupled fittings; i.e. fittings with little or no distance between them. That means the distorted velocity profile leaving the first fitting becomes the entering velocity profile for the second.

Since fitting loss coefficients assume a uniform velocity profile entering the fitting, the distortion means that the loss through the applied fitting will be different from what the loss coefficient would predict. AMCA did some research on this and for two closely space fittings, this interaction can nearly double the loss you would anticipate if you just based your projections on the fitting loss coefficients.

There is a graph of this relationship and a discussion about it in the Energy Design Resources design brief titled Design Details starting on page 21.

So, bottom lines;

  • Like so many things in HVAC, the answer to the question “will the fittings on the discharge of the AHU impact its performance” is “it depends”.
  • The configuration of the fan relative to the fittings on its discharge as well as the configuration of the fittings can be critical.
  • Irrespective of the fan/fitting issues, closely coupled fittings will have losses that are higher than anticipated based on normal loss coefficients.

Depending on how deep you want to dig into this, there are a number of resources out there that you may find useful.

  • The ASHRAE Handbook of Fundamentals discusses both system effect and duct fitting losses in Chapter 21. The tables and coefficients contained in the chapter will allow you to project the losses that might exist for just about any arrangement you can dream up. Before I obtained a copy of the ASHRAE duct fitting data base (next bullet), I used these coefficients in a spreadsheet(both the paper kind and the electronic kind;  they’ve been around for a while) to estimate duct system static requirements.
  • The ASHRAE Duct Fitting Data Base allows you to enter different fitting geometries into a table and calculates the losses based on the loss coefficients developed by ASHRAE over the years, which are published in the Handbook of Fundamentals. This streamlines the more labor intensive approach I allude to in the first bullet.
  • AMCA publishes a number of guidelines, four of which are particularly relevant to your situation; specifically AMCA Publication 200 – Air Systems, AMCA Publication 201 – Fans and Systems, AMCA Publication 202 – Troubleshooting, and AMCA Publication 203 – Field Performance Measurement of Fan Systems. Publication 201 is the most relevant to the current discussion followed by publication 200. But I refer to all of these publications frequently and you can by them as a set in the AMCA Fan Application Manual.
  • The Trane Fan Manual is also a great resource, especially for the cost and is probably the first resource I obtained years ago on the topic when Phil Sutherlin, one of my mentors handed me a copy. It’s still just as relevant as it was then and can be purchased for a modest cost on the Trane website.
  • United McGill has a number of handy tools on their website and also publishes a duct system design guide which you can obtain for free by registering on their site.

So there you have it, perhaps a bit more than you cared to know about fan and duct interactions.  But then again, maybe not since fans are one of the workhorses of the HVAC industry.  Understanding how the interact with the systems they serve has the potential to save energy in just about every building we touch.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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