ASHRAE Journal - June 2024 - 54

TECHNICAL FEATURE
In the case of Person 3 (Figure 7c) who is located closer
to the exhaust grille near the door, the impact of DCV
is realized relatively sooner than the other two persons.
Increased airflow rate increases the dilution, and
increased ach enhances the depletion of concentration
levels in front of Person 3. However, during the first 13
minutes, the cumulative exposure levels remain almost
identical in both cases. The impact of DCV on the
cumulative exposure is realized only after 12 minutes-
six minutes after the initiation of DCV.
It indicates that the impact of DCV on cumulative
exposure is not realized immediately after initiation.
Several design and operational factors that impact the
airflow patterns would affect the effectiveness of DCV.
Simply increasing the airflow rates (and the resulting
ach) would not immediately reduce the exposure. The
impact of enhanced ach would be realized during the
decay phase after the release of vapors stops. DCV can
certainly help in quicker recovery of the laboratory
environment.
Follow-up analyses are required to evaluate
the impact of several other parameters. As stated
before, increasing ach does not alter the flow path of
contaminants. The HVAC layout, that is, the number
and locations of supply diffusers and exhaust grilles,
can impact the airflow patterns, which may in turn
affect the performance of DCV. Similarly, the locations
of sensors in the breathing zone instead of in the
exhaust duct may impact exposure levels. System delay
in activating DCV and the relative benefit of operating
the lab below 6 ach and increasing the supply airflow
rate beyond 12 ach need to be evaluated.
Summary and Conclusions
This CFD study evaluated the impact of demandcontrol
ventilation on the concentration levels and
the resulting exposure (dose) of occupants during
the accidental spill scenario in a laboratory. A threedimensional
transient CFD model was developed
to study the transient dispersion of contaminants
in a laboratory space under a controlled release of a
contaminant. Newly developed metrics of Spread Index
(SI)TC and Purge Time (PT)TC were used to evaluate the
effectiveness of the DCV system.
These analyses indicate that the demand-control
ventilation systems can reduce the time required
for recovering the laboratory environment after
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ASHRAE JOURNAL ashrae.org J U N E 2024
an accidental spill of chemicals. This is achieved
by increasing the ach levels after the onset of the
spill, which enhances the rate of removal (purge
time) of contaminants. Increased airflow rate due
to increase in the ach also enhances the dilution of
contaminants. The Spread Index (SI)BZ that indicates
the extent of the spread of contaminants in the
breathing zone reduced by 37% due to the DCV.
The Purge Time (PT)TC-the time required for the
ventilation system to bring the concentration levels
below a certain acceptable threshold value-reduced
by almost seven minutes.
However, the concentration levels in front of
the occupants and the resulting exposure showed
significant spatial variations. The analyses indicate
that exposure levels will depend on the location of
the occupant. Interestingly, the DCV does not show an
immediate impact in reducing the occupant exposure.
On the contrary, the exposure levels either increased or
remained unchanged for a long time after the onset of
DCV. The benefit of DCV in reducing occupant exposure
can be realized mostly during the decay phase after the
release of chemical vapors (source) stops.
Several parameters that can impact the effectiveness
of demand-control ventilation need further evaluation.
These include the layout of supply diffusers and
exhausts, the system lag in starting the DCV system
and the locations of sensors. CFD can be a valuable
tool in analyzing and optimizing demand-control
ventilation systems.
References
1. 2019 ASHRAE Handbook-HVAC Applications, Chap. 17,
" Laboratories. "
2. Sharp, G.P. 2008. " Dynamic variation of laboratory air change
rates: a new approach to saving energy and enhancing safety. " ALN
Magazine (11/12).
3. ANSI/ASSP Z9.5-2022, " Laboratory Ventilation, " American
Society of Safety Professionals.
4. Sharp, G.P. 2010 " Demand-based control of lab air change rates. "
ASHRAE Journal 52(2):30 - 41.
5. Klein, R., C. King, A. Kosior. 2009. " Laboratory air quality
and room ventilation rates. " Journal of Chemical Health & Safety
16(5):36 - 42.
6. Khankari, K. 2016. " Analysis of Contaminant Flow Path and
Laboratory Ventilation Effectiveness. " ASHRAE Conference Paper.
7. Khankari, K. 2018. " CFD analysis of hospital operating room
ventilation system part 1: analysis of air change rates. " ASHRAE
Journal 60(5).
8. Khankari, K. 2018. " CFD analysis of hospital operating room
ventilation systems part 2: analysis of HVAC configurations. "
ASHRAE Journal 60(6).
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