ASHRAE Journal - ACMA Supplement - Fall 2013 - 19

rectangular equivalents (often requiring 25% less metal by
weight) and demand less fan energy.
Smart deSign uSing total preSSure, avoid and
account for SyStem effectS

AMCA recommends that ducted fans be selected based on the
total pressure (static pressure + velocity pressure) and required
airflow. System designers often estimate the pressure drop in
ducted HVAC systems using well-established tools and techniques.
The resulting pressure drop represents the total energy per unit
volume of airflow, and this should be matched to the fan total
pressure. However, common practice is to match the pressure
drop of the system to the fan static pressure. This is incorrect
because it ignores energy available in the fan velocity pressure
that can be utilized by the system to meet the pressure drop
requirement. This common design error is usually more than offset
by system effects that are not accounted for in the system design
process. Uneven air distribution at the fan inlet or outlet and the
effect of fan-to-duct transition fittings are examples, as are fitting
transitions placed too close together downstream. Most commercial
building duct systems exhibit more pressure drop than estimated
in the design because of these system effects.
When fans are selected on the basis of fan static pressure to meet
a total pressure requirement, the resulting selection will likely

consume more power than necessary to meet flow requirements.
System effects (unaccounted pressure drop) generally offset this
oversight. But good design practice requires proper estimation of
duct pressure losses, including system effects, and fan selection
based on fan total pressure. High-performance air systems need
to be well designed to take advantage of energy saving benefits.
reduce refrigerant riSk

Minimize refrigerant volumes. Refrigerant is expensive, and
prone to escape. Refrigerant leak detectors are expensive. The
last few decades have forced CFCs and now most HCFCs off
the market, rendering obsolete some equipment that used them.
The future of refrigerants is uncertain—change in the refrigerants we use is certain. Keeping refrigerant in the equipment
room and reducing your exposure to change is smart. Packaged
rooftops and chillers minimize this risk.
Systems using expansive refrigerant piping must also be considered against the refrigerant concentration limits described in
ASHRAE Standard 15. Since the entire building’s refrigerant
charge can escape into the smallest room served, the design
professional must calculate this worst-case refrigerant concentration level and compare it to the limits described in ASHRAE
Standard 15. Multi-evaporator systems increase the number of
potential refrigerant leak points, and the concentration levels

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ASHRAE Journal - ACMA Supplement - Fall 2013

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