HVAC design engineers face many choices throughout the
planning process
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equipment, a principal consumer of energy. This article will discuss options for
improving energy efficiency when designing an HVAC system and selecting a fan.
Fan power consumption is directly proportional to system flow rate and
static pressure. For theoretically perfect efficiency, the minimum power
required to move air against resistance is defined as:
AHP = (Q × P) ÷
6,356
where:
AHP = air horsepower
Q = volumetric flow rate (cubic
feet per minute)
P = pressure (inches of water gauge) or resistance
Flow rates are predetermined based on space type and occupancy. Although
local codes determine minimum requirements for HVAC systems, ASHRAE
Handbook--HVAC Applications1 provides general design criteria for various
commercial and public buildings. These criteria include air movement, room
circulation
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important, room circulation, in air changes per hour, typically determines
airflow requirements. Because flow rate is driven by design criteria, a design
engineer's primary means of reducing energy use is to minimize the static
pressure needed to move air through a system.
Air Movement and Control
Association (AMCA) International defines system pressure loss as "the sum of the
static-pressure losses due to friction
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the system discharge, and the static-pressure differences between the entry and
discharge openings of an air system. The static pressure a fan must overcome is
dependent on many variables, only some of which the design engineer can control.
The location of equipment often is determined by the architect and, therefore,
limits the engineer's options.
Duct configuration and fittings used to
connect components are large contributors to static pressure. Other sources of
system pressure loss are balancing and control dampers
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coils, filters, and other components in an air stream. Given that velocity
pressure is proportional to the square of velocity
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proportional to velocity squared. This makes size an important factor, as
cross-sectional area dictates fluid velocity. For example, reducing air velocity
throughout a system by 10 percent would result in a 20-percent reduction in
system static pressure. With air power proportional to pressure, this would
equate to a 20-percent reduction in energy consumption.
In addition to
accounting for all static-pressure loss in a system and achieving required room
airflow, a design engineer must adhere to Section 6.5.3 of ANSIASHRAEIESNA
Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential
Buildings
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consume per cubic foot per minute of airflow the fan generates. With flow
requirements defined, a design engineer must limit system static pressure to
meet this power limitation.
A major contributor to energy consumption
that often is ignored is system effect. AMCA International defines system effect
as "a decrease in fan performance capability, observed as a pressure loss, which
results from the effect of fan-inlet restrictionsobstructions, fan-outlet
restrictions, or other conditions influencing the performance of the fan when it
is installed in a system."2 System effect is a reduction in a fan's ability to
generate pressure and can be looked at as an additional system pressure loss. It
can lead to underperformance
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System effect can be described by its impact on a fan curve. Fan curves
are produced from laboratory testing, with fans configured for ideal
installations. Testing is performed in accordance with ANSIAMCA Standard
210-07ANSIASHRAE Standard 51-2007, Laboratory Methods of Testing Fans for
Certified Aerodynamic Performance Rating. A fan curve displays performance for a
constant speed (revolutions per minute) in terms of static pressure vs.
volumetric flow rate. In Figure 1, the intersection of the system-resistance
curve and the pressure curve is the operating point of the fan. To move along
the system curve to alter fan performance, one must increase or decrease fan
speed accordingly. Operating power is where the power curve intersects with a
vertical line running through the operating point.
The ideal inlet and
outlet conditions under which fans are tested rarely are seen in the field. As a
result
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flow rate, static pressure, or both. In Figure 2, a suitable fan was selected,
but system effect was ignored. The blue lines represent how the fan would
perform under AMCA International test conditions. Actual measured performance is
indicated by the red dot. The red line shows how the fan would perform in an
AMCA International air test when operating at design speed, but takes into
account system effect. The only way to achieve the desired performance is to
speed the fan to the rate designated by the green line. The consequence is a
higher operating speed
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levels.
Another key to reducing power consumption is fan selection.
Generally, propeller or tube-axial fans are more efficient for relatively low
static pressures, while centrifugal-type fans are used for relatively high
static pressures. Too often, fan selection is based solely on first cost. The
consequence is that a relatively small-diameter, high-speed fan is used. A small
fan operating at a high speed generally requires more operating power and
produces more noise than a large fan operating at a low speed.
Most of
the energy lost in a
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