ASHRAE Journal - August 2023 - 48
TECHNICAL FEATURE
levels. The resulting particle
concentration at a critical
plane shows nonuniform
distribution with local zones
of high concentration levels.
Though increasing the ach
levels enhances dilution and
reduces the overall particle
concentration, it neither
ensures the required level of
cleanliness in the critical zone
of a cleanroom nor creates
well-mixed uniform concentration
in the space. Rather
than the ach, the layout of
supply diffusers plays a prominent
role in determining the
effectiveness of contaminant
removal. Short-circuiting of
the supply air without sweeping
the sources of contaminant
and recirculating airflow
patterns promotes formation
of high concentration zones.
Placement of supply diffusers
over the sources of contaminants
(operators) improves the contaminant removal
and reduces the particle concentration without increasing
the air change rates.
(a)
(a)
These analyses indicate that the cleanroom contamination
control criterion based on a single value of ISO
class or a single value of an average particle concentration
based on the well-mixed assumption cannot
address the nonuniform particle concentration and
the required level of cleanliness in the critical zone of a
cleanroom. CFD analyses can help optimize the design
and operating conditions of a cleanroom to achieve a
higher level of cleanliness with lower air change rates.
Acknowledgments
The author is thankful to Thierry Marchal, Ansys, for
providing the software support for this study.
References
1. 2019 ASHRAE Handbook-HVAC Applications, Chap., 19, " Clean
Spaces. "
2. ASHRAE. 2017. ASHRAE Design Guide for Cleanrooms: Fundamentals,
Systems, and Performance. Atlanta: ASHRAE.
48
ASHRAE JOURNAL ashrae.o rg
A U G UST 2023
FIGURE 8 Distribution of particle concentration at 3 ft (0.9 m) elevation in a cleanroom for the two layouts of the supply
diffusers.
Average: 4,381 (#/ft3)
Average: 3,546 (#/ft3)
(b)
Particles (#/ft3)
>10,000.0
9,000.0
8,000.0
7,000.0
6,000.0
5,000.0
4,000.0
3,000.0
2,000.0
1,000.0
0.0
FIGURE 9 Spread Index indicating a portion the cleanroom that does not meet the ISO 7 cleanliness level for the two layouts
of the supply diffusers.
Spread Index 10.7%
Spread Index 1%
(b)
3. IEST. 2015. " Recommended Practice (RP-CC012.3): Considerations
in Cleanroom Design. " Institute of Environmental Sciences
and Technology.
4. Tschudi, W., E. Mills, T. Xu, P. Rumsey. 2005. " Measuring and
managing cleanroom energy use. " HPAC Engineering (12):29-35.
5. ISO. 2015. " ISO 14644-1:2015, Cleanrooms and Associated Controlled
Environments-Part 1: Classification of Air Cleanliness by Particle
Concentration. " International Organization for Standardization.
6. ISO. 2015. " ISO 14644-2:2015, Cleanrooms and Associated
Controlled Environments-Part 2, Monitoring to Provide Evidence of
Cleanroom Performance Related to Air Cleanliness by Particle Concentration. "
International Organization for Standardization.
7. ISO. 2005. " ISO 14664-3:2005, Cleanrooms and Associated Controlled
Environments-Part 3: Test Methods. " International Organization
for Standardization.
8. Khankari, K. 2021. " Analysis of spread of airborne contaminants
and risk of infection. " ASHRAE Journal (7).
9. Khankari, K. 2019. " Dynamics of unidirectional airflow. " ASHRAE
Journal (6).
10. Khankari, K. 2017. " Air change rate analysis for a minienvironment
cleanroom. " ASHRAE Journal (8).
11. Khankari, K. 2018. " CFD analysis of hospital operating room ventilation
system, part 1: analysis of air change rates. " ASHRAE Journal
60(5).
12. Khankari, K. 2018. " CFD analysis of hospital operating room
ventilation systems, part 2: analysis of HVAC configurations. "
ASHRAE Journal 60(6).
https://ashrae.org/
ASHRAE Journal - August 2023
Table of Contents for the Digital Edition of ASHRAE Journal - August 2023
ASHRAE Journal - August 2023 - Intro
ASHRAE Journal - August 2023 - Cover1
ASHRAE Journal - August 2023 - Cover2
ASHRAE Journal - August 2023 - 1
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