to use. Reducing a 100 feet per minute (fpm) fume hood, with an 18-in
sash opening by 2-ft, users reduce the air volume by 300 cfm and save
$1,178 per year. 4 Sharing hood use, especially for teaching, is one method
to eliminate a hood; ( 2.5-ft per person or 5-ft wide minimum with 6-ft
recommended) and staggering use times is another strategy. Teaching
labs also have the opportunity to shut off constant volume hoods when
contents have been removed and placed back into the dispensing hood.
Ductless fume hoods that recirculate lab air and use filters to capture
fumes, vapors and particulates are one alternative to reduce air volume.
Many of these are used for chemical usage that is known to be captured
by the filter system. There is also new ductless filter technology available that allows the simultaneous handling of solvents, acids and bases
with the same filter and is not as limited by predictable processes.
High-performance fume hoods are designed to operate at lower face
velocities of 60 fpm or less, (compared to 100 fpm for standard hoods).
Manufacturers employ a variety of techniques including motorized baffles, special airfoils, induced airflows and currents to improve containment at lower face velocities. These are usually deeper and more expensive than standard hoods due to the additional containment techniques.
But lower face velocity equals lower cfm and energy savings. A 6-ft fume
hood with a face velocity of 100 fpm and an 18-in sash height can cost
$3,043 per year to operate, while a face velocity of 60 fpm would have an
operational cost of $1,826, saving $1,217 per hood annually.1
Airflow reductions can be achieved with variable air volume (VAV)
and two-position mechanical systems. They reduce airflow to the hood
based on the position of the sash, while constant volume systems draw
the same amount of air whether the sash is open or closed. VAV systems
have finer modulations that can save the most energy, but have high initial costs. The more hoods there are, the more this system will pay back.
VAV systems on fume hoods can reduce cfm by 63% per year, compared
to a constant volume system according to Victor Neuman in a 2007
Labs21 presentation. If the total number of hoods does not financially
substantiate a VAV system, two-position systems should be considered.
Two-position systems are less expensive to install and still provide an
air volume reduction when the sash is closed. The first position is set for
the designed airflow when the sash is open, and the second position is
set to exhaust at a minimum flow rate when the sash is closed. To control vapor concentrations inside fume hoods ANSI/AIHA Z9.5 notes 150
to 375 hood ACH have been used when attempting to save energy. 6
Auto sash operators close the fume hood sash with occupancy sensors that
detect when the user is not at the hood after a set period of time. Combined
with a VAV or a two-position mechanical system that reduces airflow, significant energy savings can be had. Mott Manufacturing states a 6-ft hood with a
face velocity of 100 fpm could save $1,294 annually with a closer and VAV system. Sash closers are not inexpensive and add to the overall cost of the hood.
Some facilities will establish fume hood sash management plans with signage
and staff training that instruct on proper sash management in lieu of closers
or in facilities with existing hoods that do not have closers.
Maintaining a holistic view of fume hoods and the part they play in
the air management system of a building is the key to reducing their
energy impact. The fume hood selection, its options and arrangement
within the lab can be optimized; and when working together will result
in significant energy savings for the building.
1. Calculations made using Lawrence Berkeley National Lab Fume
Hood Energy Model Calculator with Boston as the location:
Ronald Blanchard, AIA, LEED, AP BD+C, is an associate and Andrea
Love, AIA, LEED, AP BD+C, is a building scientist, both with Payette,
LaboratoryDesign|SEP|OCT 2013 19
Kansas City, MO | 800.821.5525
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