REDUCING PRESSURE DROP:
GO WITH THE FLOW
The third highest level of the pyramid has mid-range cost and
delivers good energy savings. It involves reducing “pressure drop”—
power wasted due to high static pressure as air moves through filters
and other ventilation system components. All things equal, higher
static pressure means the lab HVAC system must work harder than in
a system where design static pressure is lower.
Key points of low-pressure-drop design include:
• Use low-pressure-drop air-handling units (AHUs). Upsizing
the cross section of the AHU will reduce face velocity and pressure
drop across filters, cooling coils and so on. (In addition, there can
be vast differences in pressure drop associated with filter design.)
A traditional design might call for air moving through the AHU at
500 fpm; a low-pressure-drop design might be 400 fpm or lower.
Admittedly, bigger AHUs involve net incremental costs, includ-
ing a bigger sheet metal box, larger coils and filters and potential
tradeoffs in mechanical room space. However, motors and variable
frequency drives can be smaller, and sound attenuators and mist
eliminators can often be omitted altogether. The result of these
choices: Simple and reliable energy savings over the life of the AHU,
which can never be overridden through control sequences.
• Size ducts and pipes for low pressure drop. This strategy works
hand-in-hand with the AHU decisions made above, resulting in an
efficient system. In general, designing for 400 fpm is a no-brainer,
and further reduction (down to 350 fpm, for instance) typically has a
good payback as well. In some cases, utility incentives are available to
help cover the initial costs of a low-pressure-drop system.
INCORPORATING ENERGY RECOVERY:
HOT/HUMID CLIMATE CHOICES
Regardless of climate, most lab facilities can benefit from incorporating energy-recovery systems. However, the most appropriate
choice of equipment may vary depending on whether a building is
in a hot/humid, cold or temperate zone. Building layout is another
key factor, since some recovery technologies require side-by-side
exhaust and supply airstreams.
Air-to-air energy recovery is now required by the International
Energy Conservation Code (IECC) for many projects, with rules
based on climate zone, building ventilation requirements and outside
air percentage. Because labs tend to use 100% outside air, energy
recovery is a code requirement in many cases, rather than a “nice to
have” sustainability option.
Below are the available equipment choices, followed by a discus-
sion of performance patterns in hot and humid climates:
• Enthalpy and desiccant wheels. These exchangers have the high-
est effective energy recovery, but can’t be used for hazardous exhaust
(for instance, fume hoods), due to the potential for cross-contamina-
tion. They require adjacent streams of supply and exhaust air.
• Heat pipes. Effective energy recovery and ease of maintenance—heat pipes have no moving parts—are key benefits of these
systems. They also require less space than heat wheels.
• Plate heat exhangers. Like heat pipes, these systems boast effective heat recovery and minimal maintenance. They are large and
require adjacent airflows.
• Pumped run-around systems. Though somewhat less effective at
recovery, pumped run-around systems have an important benefit: The
airstreams can be far apart. Glycol (or, in some cases, a refrigerant)
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