Handbook of air conditioning and refrigeration / Shan K

30.14 CHAPTER THIRTY
30.9 CLEAN-ROOM SYSTEMS
System Description
Airflow
A clean-room or clean space central air conditioning system, or simply a clean-room system (CRS)
or clean space system, has a central plant and water systems to supply hot and chilled water to the
AHUs, uses HEPA and ULPA filters with prefilters and water coils in the AHU to remove air contaminants,
and conditions the air. The conditioned air is supplied to the conditioned space through
ducts, terminals, and air distribution devices in order to provide an indoor environment strictly controlled
with the required cleanliness, temperature, relative humidity, airflow pattern, pressurization,
and noise.
A clean-room system is shown in Fig. 30.1 and has the following characteristics:
● Because of the complexity of the system configuration and the higher requirements in the control
of indoor environments, a clean-room system is a custom-built central system with AHUs and water-cooling
and heating coils to condition the supply air.
● A clean-room system is required to provide airflow of specific velocity to reduce lateral air contamination.
Therefore, a clean-room system is a constant-volume system.
● A clean-room system can be either a single-zone system or a multizone system. In a multizone
clean-room system, the zone reheating coil is used to compensate for the variation in zone sensible
load to maintain a nearly constant preset zone temperature.
● A clean-room system always has a separate makeup air unit (MAU) to condition the outdoor air,
and a recirculating air unit (RAU) recirculates the space air, filters it, cools it, and pressurizes the
mixture of outdoor and recirculating air. Such an arrangement minimizes the cross-contamination
of airstreams as well as consolidates the filtration of the outdoor air.
Clean rooms and clean spaces are widely used in semiconductor, pharmaceutical, aerospace, and
health care industries and facilities.
The volume flow rate of the cleaned and conditioned air supplied to the clean room depends on the
desirable air velocity that must be provided in the working area of the clean room. As discussed in
Sec. 20.17, the supply volume flow rate , in cfm (L/s), can be calculated from Eq. (20.73) as
V˙ s
V˙
s � Arvr (30.4)
According to ASHRAE Handbook 1999, HVAC Applications, U.S. Federal Standard 209E does not
specify velocity requirements. The 90 fpm (0.45 m/s) figure is still widely used in clean rooms.
Current research suggests lower velocity may be possible if the required cleanliness levels can be
maintained.
Proper airflow pattern is essential to predict the paths of the airstreams as well as to prevent contaminants
from being deposited on critical surfaces in the working area. In clean rooms, there are
two types of airflow pattern: unidirectional airflow, as shown in Fig. 30.1a, and nonunidirectional
airflow. In a unidirectional airflow pattern, airstreams flow through the working area of the clean
room in a single-pass, single direction of parallel airstreams. The unidirectional airflow can be subdivided
into vertical unidirectional airflow and horizontal unidirectional airflow.
When the ceiling of a clean room is fully covered by HEPA or ULPA filters, the downward airflow
produces a unidirectional flow of ultraclean air that covers the working area of the clean room.
Baylie and Schultz (1994) reported for clean rooms with ceiling partly covered by HEPA or ULPA
filters, a porous membrane that is added beneath the HEPA or ULPA filters forms a small plenum
which equalizes the pressure across the face of the ceiling and minimizes the turbulence created by
the larger grid required to support the HEPA filters and lighting fixtures. When a membrane ceiling