ASHRAE Journal - ACMA Supplement - Fall 2013 - 18
abandons the “70s” design approach which used smaller, low
efficiency fans with dampers controlled from pressure sensors
two-thirds of the way down the duct.
Lower suppLy air temperature
Today, building sensible loads are being reduced by LED lighting and more efficient heat-producing office equipment. As the
sensible heat ratio changes, the remaining latent cooling loads
favor lower supply temperatures. Generally, colder supply
temperatures save energy, because fans that run continuously
use less energy while cooling system compressors use more,
but only at full load conditions where operating time is short.
Further, chillers and DX refrigeration systems have increased
their efficiency so significantly since the 1970s that the optimized balance between air and refrigeration energy is no longer
55 F. Instead, it ranges between 45 to 52 F depending on the
efficiency of the refrigeration choice. Supply air temperature
reset is also a common control strategy in today’s more efficient
systems. Supply temperature rises as
cooling demand diminishes when outside
air temperatures drop. This minimizes
the compressor energy penalty except at
full load and reduces reheat risk on the
perimeter. Finally, system first cost and
sound levels drop as ducts are right sized
for lower supply air volumes.
Good design practice
requires proper
estimation of duct
pressure losses,
including system
effects, and fan
selection based on
fan total pressure.
spiraL ducts
Air systems have their challenges as
well—most notably, system leakage. If
10% of the delivered airflow fails to exit
the diffuser, isn’t that a 10% loss of
mechanical and air movement energy?
Spiral round and oval duct systems
designed using the static regain method
with AMCA certified low-leak duct and
fitting sealing systems provide exceptionally tight duct performance with leakage
rates of below 1%. Specifying low leak
VAV boxes, downstream ducts, and air
handlers or rooftop units resolves the
system leakage challenge. Total system
leakage below 3% is easily achieved.
Concerns about round ducts not fitting in
confined ceiling spaces can be dispelled
with longer dedicated runs (early splitting
of duct runs), use of multiple rounds, and
round oval shapes when necessary. Smartly
designed high-velocity spiral duct systems
that exploit static regain cost less than
www.info.hotims.com/44636-11
18
Fa l l 2 0 1 3
a m c a I n t e r nat I o na l
inmotion
w w w. a m c a . o r g
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ASHRAE Journal - ACMA Supplement - Fall 2013
Table of Contents for the Digital Edition of ASHRAE Journal - ACMA Supplement - Fall 2013
Contents
ASHRAE Journal - ACMA Supplement - Fall 2013 - Cover1
ASHRAE Journal - ACMA Supplement - Fall 2013 - Cover2
ASHRAE Journal - ACMA Supplement - Fall 2013 - Contents
ASHRAE Journal - ACMA Supplement - Fall 2013 - 2
ASHRAE Journal - ACMA Supplement - Fall 2013 - 3
ASHRAE Journal - ACMA Supplement - Fall 2013 - 4
ASHRAE Journal - ACMA Supplement - Fall 2013 - 5
ASHRAE Journal - ACMA Supplement - Fall 2013 - 6
ASHRAE Journal - ACMA Supplement - Fall 2013 - 7
ASHRAE Journal - ACMA Supplement - Fall 2013 - 8
ASHRAE Journal - ACMA Supplement - Fall 2013 - 9
ASHRAE Journal - ACMA Supplement - Fall 2013 - 10
ASHRAE Journal - ACMA Supplement - Fall 2013 - 11
ASHRAE Journal - ACMA Supplement - Fall 2013 - 12
ASHRAE Journal - ACMA Supplement - Fall 2013 - 13
ASHRAE Journal - ACMA Supplement - Fall 2013 - 14
ASHRAE Journal - ACMA Supplement - Fall 2013 - 15
ASHRAE Journal - ACMA Supplement - Fall 2013 - 16
ASHRAE Journal - ACMA Supplement - Fall 2013 - 17
ASHRAE Journal - ACMA Supplement - Fall 2013 - 18
ASHRAE Journal - ACMA Supplement - Fall 2013 - 19
ASHRAE Journal - ACMA Supplement - Fall 2013 - 20
ASHRAE Journal - ACMA Supplement - Fall 2013 - 21
ASHRAE Journal - ACMA Supplement - Fall 2013 - 22
ASHRAE Journal - ACMA Supplement - Fall 2013 - 23
ASHRAE Journal - ACMA Supplement - Fall 2013 - 24
ASHRAE Journal - ACMA Supplement - Fall 2013 - 25
ASHRAE Journal - ACMA Supplement - Fall 2013 - 26
ASHRAE Journal - ACMA Supplement - Fall 2013 - 27
ASHRAE Journal - ACMA Supplement - Fall 2013 - 28
ASHRAE Journal - ACMA Supplement - Fall 2013 - Cover3
ASHRAE Journal - ACMA Supplement - Fall 2013 - Cover4
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