[Bldg-sim] VAV fan energy

[James V. Dirkes II, PE]

Dear Per Sahlin,
 
The simple fact is that fans which unload (those with variable inlet vanes ,
VSDs, etc.) do so in a way that is fairly represented by a curve.  The
generic curves found in most energy analysis programs are reasonable
representations of the unloading characteristics.  It is a good idea to have
a library of typical curves if your energy analysis program of choice
doesn't have such a library.
 
For a new building where your only purpose is to compare a "baseline" to
your specific design, it's reasonable to use a generic curve.  This is
because the actual operating conditions are not known for new construction;
you know only the designer's best calculation, not the "as built"
conditions.
 
For an existing building where you want to establish a model that represents
the actual conditions as closely as possible, it's probably worth developing
specific, custom curve coefficients for the installed fan(s).
 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

 


  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Per Sahlin
Sent: Wednesday, April 29, 2009 10:46 AM
To: bldg-sim at lists.onebuilding.org; EnergyPlus_Support at yahoogroups.com
Subject: [Bldg-sim] VAV fan energy



Hi all,

 

In E+, ASHRAE standard 90 (App. G)  and in some other places, fan power  for
VAV systems with variable speed drives is modeled as a given function of
part load ratio. I think the theory behind this originally comes from BLAST.
This must be based on assumptions for the shape of the fan efficiency curve
and on the pressure characteristic and control of the VAV system. Does
anyone have access to the original theory on this? Alternatively, can anyone
explain why and when the "single curve for all fans and systems" approach
works?

 

Best regards,

 

Per Sahlin

 

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