ASHRAE Journal - March 2020 - 53

TECHNICAL FEATURE

air across its face.
Because the HEPA ﬁlters used in cleanrooms have high
air pressure drops across the faces, it is more economical
to provide low pressure levels in the above-ceiling plenum and allow the fan ﬁlter unit blower to provide the
extra static pressure needed to overcome the HEPA ﬁlter.
This style of cleanroom design is still used today in largescale cleanroom or pharmaceutical applications; however, the passing of USP 797, Pharmaceutical Compounding-
Sterile Preparations, which was published in September
2018, necessitated the creation of cleanrooms not best
served by this design.
USP 797 forced many existing hospitals and pharmaceutical compounding operations to retroﬁt their existing
facilities by mandating all pharmaceutical compounding
be done in a minimum ISO class 8 environment. While
large drug compounding facilities already complied
with this standard, small hospitals and pharmacies were
forced to upgrade their compounding rooms. Terminal
units ducted to fan ﬁlter units allowed these spaces to
come into compliance. Branch ducts were added to the

RETHINK AIR CONTROL.

existing ductwork, and the new air volume ﬂow was to be
controlled via the damper inherently equipped in a terminal unit. Because the terminal unit is within medium
pressure ductwork, initial control schemes allowed a
terminal unit to manage the air volume ﬂow, and the fan
ﬁlter unit to handle the extra static pressure needed to
pass the air across the ﬁlter face. This design allowed for
code compliance with small changes on the main airhandling unit, and with minimal cost or room structural
changes. These compounding facility terminal units rely
on a constant air volume (CAV) control scheme (Figure 1),
as opposed to a variable air volume (VAV) control scheme
responsive to a thermostatic signal (Figure 2).
The most recent innovation in cleanroom design
came with the introduction of electronically commutated (EC) fan ﬁlter unit motors, compared to the older
fan ﬁlter units that used a permanent split capacitor
(PSC) style motor to overcome the ﬁlter pressure drop.
These PSC motors did not compensate for ﬁlter loading, nor was it easy to balance or monitor the units.
EC motors offer smaller sizes, better controllability

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MARCH 2020

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ASHRAE Journal - March 2020

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