|
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
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
|