A more practical and reliable approach is to
provide bypass air to the exhaust stack, downstream of the primary fan, via a secondary
fan, not connected to the building’s exhaust
plenum. A schematic of this system is shown in
Figure 1. As the primary fan is turned down,
the bypass fan is ramped up to maintain a
constant exit velocity and the required dilution
performance. Most lab buildings are designed
with exhaust plenum pressures in the range of
- 4 to - 6 in. of static pressure (-1,000 to -1,500
Pa). A typical bypass damper allows outside
air into the plenum, which then is exhausted
through the fan against that pressure. By supplying bypass air directly to the exhaust stack,
downstream of the primary fan, the bypass
fan is not required to work against the negative
pressure of the plenum, reducing the power
required to supply the bypass air by about 90%.
The reduction in total power, as the primary
fan is turned down, is therefore only slightly
less than the reduction in primary fan power.
By simply slaving the bypass fan to the primary
exhaust fan, the volume flow from the stack
can be maintained, and the primary exhaust
fan can be turned down to any level appropri-
ate for the building, without fear of reducing
dispersion from the stack. No monitoring of
wind conditions with
anemometers is required
for this method of fan
control. At least one fan
manufacturer has rec-
ognized this saving and
provides a specialized
fan for the purpose.
Energy modeling of
typical labs with fume
hoods shows that about
35% of the fan energy
can be saved, resulting
in as much as 12%
reduction in total build-
ing energy consump-
tion in a building with
a large number of fume
hoods. An energy com-
parison of this method
to the active control
method shows that the
savings are nearly identical, but the cost and
therefore payback period of the powered ple-
num bypass method is much lower, approach-
ing one year for many large systems. This pay-
back relationship is shown in Figure 2.
Joel Good has been consulting on building perfor-
mance for nearly 10 years and is a project consultant
at RWDI who regularly contributes to design teams
by participating in design charettes and providing
technical design guidance. firstname.lastname@example.org.
Glenn Schuyler joined RWSI in 1981 and has been
a principal since 1984. email@example.com.
LaboratoryDesign|NOV|DEC 2013 31
Figure 2: Payback comparison
The 2014 I2SL Annual Conference remains the country’s premier educational and networking opportunity
for those who design, build, own, and operate sustainable laboratories and other high-tech buildings.
Learn more at: www.i2sl.org/conference/2014/index.html.
2014 I2SL Annual Conference
The 2014 I2SL Annual Conference provides a unique opportunity for
your organization to meet with high-tech research facility owners and
managers. The Caribe Royale in Orlando, Florida, is the host hotel for
the conference, allowing us to have our meeting space and lodging in the
same location. In addition, the Caribe Royale is conveniently located near
Disney World and provides complimentary shuttle service, making it easy
to plan a vacation before or after the conference.
Here’s How to Get Involved
Exhibitor registration is open, so secure your booth space today. The
Technology and Services Fair will be held in a ballroom, so exhibitors
will not need to purchase carpet and can enjoy a more intimate
atmosphere. Exhibitors can also openly market and showcase products or
services to conference attendees by presenting a 20-minute Technology
Demonstration. Sign up on the Tech Fair registration form or contact
firstname.lastname@example.org to learn more.
Become a conference sponsor to draw the attention of hundreds of
organizations expected to be represented at the 2014 I2SL Annual
Conference. Sponsorship starts at just $500.
Answer the Call for Presenters. Attendees interested in offering a
Technical Session presentation can learn more when the Call for
Presenters opens in January 2014. Look for more information on
suggested topics and deadlines on the conference Web page.
Get Involved with I2SL in the Sunshine State