Basic Documentation

down according to instantaneous changes in loads.
The other is called 2-Position Constant Volume
(CV2) because it selects single high flow rate when
the lab is in use and a low rate when it is
unoccupied.
The design team needs to choose which approach
best suits the facility and the way people work with
it. Spaces that stand unoccupied for days or weeks
at a time are well suited for a CV2 system. So are
spaces that are used on a very regular schedule. In
such systems, the energy savings pile up in a
predictable way. In contrast, some research labs are
in use for all but a few scattered hours in a week. In
these spaces it takes a VAV system to significantly
cut energy use.
In rooms with chemical fume hoods, a VAV system
adjusts the airflow rate dynamically to match the
opening of the sash. This keeps the face velocity in
the range selected by the health and safety officers.
With a CV2 system, the same reasoning leads to an
interlock that keeps the hood at the high flow setting
while the sash is open, and reduces it when the
hood is closed AND the room is unoccupied.
Some designers and facility operators have been
disappointed to find energy conservation efforts
thwarted by laboratory users who leave fume hood
sashes open. Some respond with designs that cut
the flow on hood while the sash is open. This
approach sets up conflict between two important
goals: energy conservation and laboratory safety.
The Green approach keeps energy efficiency and
indoor environmental quality pulling in the same
direction. The Green approach engages the lab
users, teaching them to use the facility in a way that
meets sustainability goals.
Table 1. Approaches to Laboratory Ventilation.
Airflow
Normalized
Energy Use
Comparative
Energy Use
Constant flow 100 %
Dynamic flow
(VAV or CV2)
40% to 70%
Dynamic flow
with sash
management
30% to 50%
Siemens Industry, Inc. Page 3 of 14
Document No. 149-488
Fume Hood Sash Management
A fume hood is a safety appliance. Its purpose is to
protect workers from the air borne effluent released
by their work. The hood protects them when the
sash is open; however, it is more effective when the
sash is closed. Lab safety authorities are unanimous
on this issue: proper procedure is to close the sash
whenever possible.
Whether the airflow control concept is VAV or CV2,
closing sashes has a huge effect on energy
consumption. Labs21 writers compare the energy
load of a single constant volume fume hood to that
of three typical houses. Dynamic flow controls cut
that by 40% to 70% if the users close the sash. This
easily exceeds the savings available by using low
velocity hoods. By an even wider margin, it exceeds
the savings available from a temporary velocity
reduction when a user steps away from the hood.
Nothing cuts the flow like closing the sash!
This is important common ground between safety
managers and energy managers. Both sides need
fume hood users to close sashes. Sometimes that
means changing behavior; motivating the staff to
develop and maintain practices that make the facility
actually run Green. Put in that context, it’s easy to
convince users of the need, but it takes diligence
and a comprehensive program to make it real day
after day.
That’s why the Labs21 team includes Sash
Management in the Lab Ventilation pre-requisite for
the Energy and Atmosphere credits. Leading
institutions have broken ground in this area. Harvard
School of Public Health
4
and Duke University
5
are
among those reporting success. They recognized
that changing behavior is fundamentally a human
issue, not entirely susceptible to a technical fix. But
we can apply technology as one element in the
solution. To manage performance, you need to
measure it. A Building Automation System (BAS)
can deliver the necessary data and get it to the right
person, whether that’s the lab manager, or the
safety officer or a sustainability manager. BAS can
report which hoods are open more than a selected
duration each day, or any hood that’s left open
continuously for longer than an hour or other
indicators of proper and improper practice. At Duke
University, laboratory BAS data was processed to
calculate a fume hood airflow management ratio
4. Jaclyn Emig, "Shut the Sash: Behavior Change Programs in
Labs at Harvard" (Labs21 Conference, San Antonio, TX,
October 17-19, 2006).
5. U.S. Department of Energy, Laboratories for the 21st
Century: Best Practice Guide, "Metrics and Bench Marks for
Energy Efficiency in Laboratories", October 2007.