Fan energy fundamentals

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Hi everyone! A recent discussion with a mechanical mentor brought up a question I'd like to share with the list... If anyone has a partial or complete response I think we'd all benefit and appreciate it if you'd share your thoughts! For a given duct distribution system, you've got a certain static drop. At full flow, your supply fan also has to deal with a certain velocity pressure drop due to friction with the duct distribution. If you lower the volume of your supply airstream, you also lower the velocity and therefore the velocity pressure drops for the given duct distribution. The net result is system supply fan has a lower total pressure drop to overcome, and so should operate more efficiently. Does anyone know: Whether we choose "variable speed" fan control, use the FAN-EIR-FPLR curve generated by the wizards, or use the VSD curve defined in 90.1 appendix G... are we simultaneously accounting for the efficiency gains due to lowered velocity pressure drop in addition to the drop in fan energy from lower RPMs? If this is not accounted for inside/outside the fan curves, is there a direct way to define that or logically work it into the curves? If it helps anyone answer, I can illustrate from recent checks that the wizard-generated FAN-EIR-FPLR curve has a "Nike Swoosh" shape with an inflection around the 20% mark. This shape best matches the (limited) data I've seen for "real world" VSD input measurements. Here's a screenshot: The following illustrates the 90.1-2007 VAV curve we're made to use for baseline systems 5-8 (I haven't checked to see if this still applies to 2010). It does not have the same "inflection point" as zero is approached. I think it's based on the fan law power equation or something similar that doesn't account for motor/VSD losses: The help files illustrate a different, third shape for "variable speed" control different from both of the above... Seeing these side-by-side raises another question for me regarding 90.1-2007 intent/practice... not specific to eQuest... but I've already spent too much of my time today writing this out! Another day =)... Thanks in advance for anyone joining in the discussion! ~Nick NICK CATON, P.E. 

Nick-Caton's picture
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Bill Bishop suggested a good clarification that I want to take on. I've
edited my language below (highlighted) to clarify the question I'm
trying to pose.


Nick-Caton's picture
Joined: 2011-09-30
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Nick -

We're in the midst of asking the same questions about the Pump-Power-fFlow equations.

I imagine in both types of flow networks there may some fixed pressure drops that won't be reduced:

1) Min static pressure across flow control devices (VAV boxes, water control valves)

2) Static pressure in critical branches that may not be at reduced flow, even though the overall flow network is at reduced flow

3) In staged plants, static pressure across individual constant-volume plant components (eg in our case, chiller 1 of 6 always has 12 psig, regardless of whether it is the only chiller on or not)

With this mixture of fixed and variable pressure drops, I imagine multiple different power-versus-flow curves for the same building, depending on the physical location of the prime movers and the intervening pinch points / critical flow networks. 20000 cfm out of a peak 30000 cfm fan probably looks different in terms of power depending whether the majority is headed to west zones from an east mechanical room 400 ft away or headed to east zones just a few feet away.

Probably the most accurate method would be explicitly defining the flow network and calculating these pressure drops dynamically (if the modeler had the time to do that explicitly, or software cool enough (BIM?) to do it automatically). Anyone familiar enough with Energy Plus to know if that's a feature?

I guess no answers here, just more questions ...

Aaron Dahlstrom , PE, LEED(r) AP

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As for fans I suggest two resources:
Advanced Variable Air Volume VAV System Design Guide by Energy Design Resources has some good discussion and suggested fan curves.

Reid Hart of PECI did a presentation to BSUG on September 16, 2009 on fan curves and static pressure reset. BSUG posted their items, including this one, to their yahoo group at BSUG has morphed into BESF and has a new home at

Paul Riemer, PE, LEED AP BD+C

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