ASHRAE Journal - June 2024 - 47

TECHNICAL FEATURE
of contaminants in lab space
and remove the contaminants
quickly to bring the lab
environment to an acceptable
level of concentration,
especially in the breathing
zone (BZ) of occupants. Under
such conditions, it makes it
necessary to increase the ach
levels beyond the normal
operating values.
FIGURE 1 Schematic diagram of CFD model of a laboratory showing persons' locations and the breathing zone.
B
Exhaust Grilles
Breathing Zone
A
Supply
Four-Way
Diffusers
Spill
Role of Air Changes Per Hour
Often the role of ach is not properly understood by
the designers. It is defi ned as a ratio of the total airfl ow
rate into the space to the volume of the space. However,
only the airfl ow rate component of the ach determines
the extent of dilution in the space, whereas the volume
component of the ach is required only when the rate of
purge (removal) is critical, as in the case of an accidental
spill scenario. Therefore, under most operating
conditions when the dilution of contaminants is critical,
simply the airfl ow rate is an appropriate metric for the
airfl ow specifi cation rather than the ach. However,
under an accidental spill scenario when both the
dilution as well as quick removal of contaminants are
important, the ach metric is appropriate. Traditionally,
the supply airfl ow rates for laboratory spaces are always
specifi ed in terms of ach irrespective of the purpose of
ventilation, which then yields higher values of ach.
Thermal and indoor environmental conditions in labs
are dynamic, which precludes the need for operating the
laboratories constantly under high ach. The demandcontrol
ventilation (DCV) involves varying the ach
by monitoring the indoor conditions in the labs and
modulating the supply airfl ow rates accordingly. Such
DCV systems often monitor the concentrations in the
exhaust/return ducts with a suite of sensors and provide
feedback to the building's control system to ramp up
or ramp down the air handler fan speed to adjust the
supply airfl ow rates. For example, under an accidental
spill scenario, the ach can be increased from the normal
operating fl ow rates to enhance the dilution and allow
for quick removal of contaminants. Whereas under
normal operating conditions, labs can be operated at
lower supply airfl ow rates than specifi ed values to meet
the thermal and dilution demands of the space.
Airfl ow Patterns Matter
The distribution of supply air into the space and not
just the supply fl ow rate determines the effectiveness of
dilution and removal of contaminants. Air is the primary
carrier of heat, moisture and volatile contaminants
in laboratory spaces. Therefore, the air distribution
determines the resulting air velocities and the fl ow path
of airborne contaminants, which, in turn, determines
the distribution of temperature and concentration levels
of contaminants in the space. Since the contaminant
concentration and the resulting exposure levels are
spatially varied, it cannot be represented by a single
number for the entire space.
The understanding of the nature of airfl ow patterns
and the fl ow path of contaminants play a critical
role in estimating the chemical exposure levels
of workers. The dispersion of contaminants and
their fl ow path can depend on several interrelated
factors including the location and type of supply
diffusers; supply airfl ow rates and associated diffuser
throw; supply air temperature; size and locations
of exhausts/returns; locations and strengths of
various heat sources in a room; fl oor layout including
the location and size of fume hoods; location of
workbenches and any other obstructions to airfl ow;
and, importantly, on the strength, location and
source of contaminants. Therefore, the airfl ow
distribution can play a critical role in determining the
effectiveness of DCV systems.
This study with the help of CFD simulations
investigates the effectiveness of DCV for an accidental
spill scenario in a laboratory. This study evaluates the
transient dispersion of contaminants in a laboratory
space under a controlled release of a contaminant. Two
measures, namely, Spread Index (SI)TC and Purge Time
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