Energy Efficiency and Air Handling Units
TThe air handling unit (AHU) is a complicated device involving
many interrelated design selections that affect its energy efficiency.
The overall efficiency of an AHU is determined by the energy it
takes to move a given quantity of air against the system. Three
major energy choices for the AHU design must be considered..
|Reducing AHU pressure drop
The AHU pressure drop can be reduced by:
Lowering face velocity
Lowering face velocities necessitates larger area
coils and filter elements and bigger AHU housings
than conventionally used. These larger items increase
the AHU's cost, but a resulting decrease in face velocity
from 500 fpm to 400 fpm usually makes up for the increased
cost over the life of the AHU. Face velocity reductions
from 500 fpm to 400 fpm can have a simple payback
of about three years. A comparable decrease in face
velocity at filters can have a simple payback of one
year or less. [Brown, 1990]
Considering filter loading and VFDs
Variable system resistance, primarily resulting from
filter loading, can be efficiently corrected for with
a VFD on the fan. AHU pre-filter pressure drop will
typically vary from 0.5 in. H2O (125 Pa) when "clean"
to 1.7 in H2O (423 Pa) when "dirty." The
system design point is usually sized for the "dirty"
condition. However, on average, about 90 percent of
the design head and energy is actually required. The
energy saved by a VFD will pay for the drive in two
to five years, depending upon system capacity. [Brown,
Adding bypass dampers
Some AHU components are only utilized during a portion
of the year; therefore, bypass dampers should be used
to reduce parasitic energy losses when these components
are not needed. [Brown, 1990]
|Ductwork System Effect
The heart of the HVAC system is the fan. Improper fan
design or installation can cause serious degradation
of performance because of "system effects."
System effect factors for connections between fan and
duct system are presented in "Fans and Systems"
(AMCA, 1973), the ASHRAE Fundamentals Handbook [ASHRAE,
1997], and the DFDB [ASHRAE, 1994]. System effects cause
changes in a fan's aerodynamic characteristics that
result in an entirely new fan performance curve. They
appear when inlet and outlet connections cause non-uniform
inlet and outlet flow and swirl at the fan inlet.
An engineer designing ductwork should provide uniform
straight flow conditions at the fan inlet and outlet.
An elbow at a fan's inlet causes turbulence and uneven
flow into the fan opening. An elbow at a fan's discharge
causes a non-uniform backpressure. An engineer can
only estimate how much pressure will be lost as a
result of system effects by reviewing the air distribution
System effects are rarely measured during system
start-up procedures. They become evident only when
not enough air is delivered to the building. Start-up
personnel must then speed up the fan by changing pulleys
or even increasing the fan's motor size in order to
get design air-flow rates.
System effects can be reduced or largely avoided
by a few easy precautions for centrifugal fans, as
With a free inlet fan (no inlet duct), a minimum
of one half the fan wheel diameter, D, clearance should
be maintained from the inlet to the wall or structure.
An elbow entrance to the fan should be at least two
times the inlet diameter distance from the flex connector
at the fan, C=0.15 (remember, the fan entrance is
a high-velocity duct).
A 2D cone to the inlet flex (vibration isolator) should
be used to decrease friction losses (C=0.10).
If this 2D inlet cone is attached to a rectangular,
vertical duct, its dimensions should be two times
the fan's inlet diameter by one half the fan's inlet
diameter and extend one diameter past the center line
of the fan's inlet (Figure 1).
A duct expansion from the fan discharge should be
at a 10° angle up on a top horizontal discharge
fan (Figure 2). Down angles and increases in the width
of the duct are less efficient.
A straight duct for a distance of three to six duct
diameters from the fan discharge should be used in
order to develop a full dynamic head. Branching and
turning sooner causes system effect losses.
|Incorporating the most efficient fan "system"
The fan "system" includes the following:
Centrifugal fan efficiencies range from 50 percent
to about 70 percent depending upon blade configuration
and vane-axial fan efficiencies range from 80 percent
to over 90 percent. However, the energy engineer must
keep in mind that all fans add heat to the air stream.
The air stream temperature typically rises between
three degrees F and six degrees F for inefficient
fan types. Also, actual installed efficiency is typically
worse than the manufacturer's rating data indicates,
dropping the fan's efficiency by 10 to 30 percent
in some cases.
Direct drive fan/motor combinations are the most efficient
at 100 percent. Standard V-belt drives are about 93
percent efficient when first installed but drop over
time. Synchronous belt systems offer an energy-efficiency
increase to about 96 percent for the life of the belt.
Premium efficiency motors that have full-load efficiencies
of greater than 94 percent depending upon horsepower
are preferable. It is also important to consider the
part-load motor efficiency and its operation with
a VFD in VAV systems.
Sound Power Requirements
Acoustical attenuators, used to reduce a fan's sound,
increase the pressure drop of the AHU. Therefore,
engineers should consider the disadvantages of high-efficiency
but noisy fan that will require acoustical attenuation
devices. These devices can negate the energy savings
from face velocity reductions.
Evaporative cooling is an alternative to conventional
compressor-type cooling systems. Direct and indirect
evaporation systems or a combination can supplant or
displace conventional cooling systems. Evaporative cooling
systems have been used to condition facilities since
before refrigerated cooling was invented. There are
two types of evaporative cooling techniques: indirect
and direct. The indirect approach removes sensible energy
only from the air stream with an exchanger that has
the air stream flow on one side of a plate and water
on the other. The direct approach removes energy from
the air stream adiabatically by adding moisture to the
air stream. These approaches can be combined to provide
cooling and humidification that displaces energy from
a conventional compressor-type HVAC system. Annual energy
savings of more than 15 percent for a one-stage, direct
system and over 38 percent for a two-stage indirect/direct
system can be realized when compared to a conventional
HVAC system. Brown (1993) says, "The use of direct
evaporative cooling to humidify increases the opportunity
to recover heat by 36 percent and reduce energy consumption
during the heating season." One- to two-year paybacks
are possible. The justification of evaporative cooling
should not be based solely on the ability to replace
mechanical cooling but should be considered as an opportunity
to displace mechanical cooling/humidification energy.
[Brown, 1992; Brown, 1993]
Evaporative cooling should always be considered for
all make-up air systems, regardless of geographic
region, as noted in the descriptions of the approaches
Indirect evaporative cooling is applicable when there
is a large process equipment-cooling load, the HVAC
system operates on a continuous basis, and a direct
evaporative cooling opportunity exists. [Brown, 1993]
Direct evaporative cooling can be justified on the
basis of eliminating the need for a steam humidification
system, enhancing economizer operation, reducing humidification
energy used in conjunction with heat recovery, and
controlling the temperature of a space independent
of controlling its humidity. Direct evaporative cooling
for both make-up air and recirculation-type systems
should always be considered when the outdoor wet-bulb
temperature is 53°F (11.7°C) or less for 3,500
or more hours per year. Direct evaporative cooling
of air conditioning condensers, which are normally
simply air-cooled, can reduce electrical energy and
demand consumption by 20 to 40 percent. [Knebel, 1997]
[Brown, 1990; Brown, 1993]
Typically, a two-stage evaporative cooling system
comprises a direct evaporative cooler downstream of
an indirect evaporative cooler. The two coolers can
be combined in a myriad of arrangements to increase
When mixing of the facility's exhaust air with make-up
fresh air is permitted, economizers can save energy
in both cooling and heating modes.
Brown (1993) points out that placement of the fan
upstream from the cooling coil and evaporative cooler
has two energy advantages. In cooling mode, fan heat
enhances operation of the cooling coil, and, in heating
mode, the same fan heat is used to provide humidification
energy and to displace preheat energy input. [Brown,