[Equest-users] Fan energy fundamentals

[Nick Caton]

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

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick
Caton
Sent: Thursday, September 29, 2011 10:32 AM
To: Equest-users at lists.onebuilding.org
Subject: [Equest-users] Fan energy fundamentals

 

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 reduce
the supply airflow rate, 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.

SENIOR ENGINEER

 

Smith & Boucher Engineers

25501 west valley parkway, suite 200

olathe, ks 66061

direct 913.344.0036

fax 913.345.0617

www.smithboucher.com 

 

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