8 CLEAN APPLICATIONS
means exhausting the entire internal air volume of
a typical laboratory into the atmosphere every 10
minutes. The Northwestern University traditionally
prescribed air exchange rate for laboratories is 6. 2
per hour. The currently designed air exchange rate
for a vivarium is 15 per hour. Since spill incidents
and unsafe releases occur only rarely, laboratories
are over-ventilated about 99 percent of the time. In
the rare case of a spill or release, no extra clearance
ventilation is provided.
New technologies implemented
In 2010, NU installed its first ventilation optimization system supplied by Massachusetts-based
Aircuity Inc. to monitor air quality related to use of
a heat wheel — an energy recovery heat exchanger positioned within the supply and exhaust air
streams of the air-handling system.
In 2011, NU committed more than $40 million
as a part of the Northwestern Energy Retrofit Fund
(NERF) to pay for gas and energy conservation
projects across its two campuses. This commitment
to energy efficiency has decreased the amount of
electricity purchased from the grid by almost five
percent, even as the square footage of occupied
space has increased.
Then, in 2012, a large national energy services
company implemented an energy conservation
project for the Chicago campus research buildings, and a demand control optimization system
was implemented on various lab floor remodels.
In 2013, the NU Facilities Department began
coordinating installations of the system at several
Evanston campus science buildings, completing the
installations in 2015 and 2016.
The ventilation optimization system monitors
laboratory air quality parameters of total volatile
organic compounds (TVOC) and particulates.
Other laboratory and building control systems
monitor occupancy via ceiling mounted occupancy
sensors. The information collected dynamically
controls ventilation rates. The intelligent system
adjusts the volume of laboratory air exhausted and
respectively supplied within a specified range —
usually between 2 to 12 air changes per hour. This
dynamically controlled system provides the least
amount of ventilation in an unoccupied lab ( 2 air
changes). It can provide about twice the traditional
ventilation rate in a detected release event ( 12 air
As of spring 2017, the Aircuity system had been
installed in 130 laboratory rooms on the Evanston
campus and 370 laboratory rooms on the Chicago
campus. Each system includes a “sensor suite,”
built to accept a variety of sensors for multipoint
sampling of the indoor environmental parameters.
The sensor suite analyzes each air packet and sends
smart signals to optimize ventilation. A total of 34
sensor suites were installed, with 2 more planned
for a vivarium on the Evanston campus.
System offers transparent communication
Transparent communication with the laboratory
users is important, since most of the sensors and
control elements are hidden from view. Additional
touch screens were installed in laboratories to
provide feedback on current ventilation settings.
Researcher adherence to operational limits is considered to be more important in laboratories where
the air change rates can be below 4 ACH.
Capabilities and limitations of optimization
The optimization system’s sensor suite capabilities
to measure carbon dioxide (CO2) and dewpoint are
most interesting in non-laboratory applications.
CO2 and dew point sensors are in use in some
non-laboratory and vivarium settings.
For laboratory applications, the optical particle counter counts small particles (PM2.5) in one
range: 0.3 to 2. 5 microns (μm). For vivarium applications, particles can be counted in two ranges:
0.3 to 0.5μm and 0. 5-2.5μm. This feature can be
useful to verify HEPA filter integrity on the supply
side and efficiencies in clean space operations. The
installed system commands the lab control system
to dynamically increase the laboratory air changes
above a particle count of 500,000 per cubic foot of
air. This is accomplished by opening control valves
in the ventilation ductwork of the affected space
and may also affect the variable-frequency drive
(VFD) controls of the ventilation fans. The system
calls for the highest air change rate at and above
5,000,000 particles per cubic foot.
For laboratories, the most useful capability is
the sensing of TVOC, either using a metal oxide
semiconductor (MOS) or a photoionization detector (PID) calibrated to isobutylene. The PID can
be calibrated to both ammonia and isobutylene.
Once a sample air packet reaches the sensor suite,
the response time is 30 seconds. The accuracy of
the PID is ± 0.2 parts per million (ppm) or 2. 5
percent of reading (whichever is greater). The resolution of the PID is 0.025ppm. The drift stability
of the PID is ± 2ppm/6 months 5ppm isobutylene. The maximum TVOC range is 100ppm for
the MOS and 20ppm for the PID. According to
the optimization system vendor, the MOS sensor
provides complementary detection capabilities, as
it will detect parameters (including but not limited