cooling coils are utilized within the cleanroom recirculation air system to “trim” the temperature to meet
the tight tolerances. There are three viable options
for recirculation air systems, each with its own
advantages and disadvantages. These options are
recirculation air handlers, VLF fan towers, and filter
fan units (Table 2).
Recirculation air handling units (RAHUs) are
usually a lower cost option for a recirculation air
system. They are typically located on a fan deck
or perimeter of a cleanroom, which provides
maintenance accessibility from non-clean space.
Disadvantages of RAHUs include high noise levels — rarely below 70 dB(A) and higher energy
usage. These large units also take up critical facility
space, whether at the fan deck level or in an adjacent
mechanical room, and fan deck RAHUs can raise the
height of a building.
Vertical Laminar Flow (VLF) fan towers rely on
laminar airflow to control particulate contamination through the vertical profile of the cleanroom.
Laminar airflow refers to air moving at the same
speed and in the same direction, with little or no
cross-over air streams that can randomly deposit particles. Usually located on the perimeter of
a cleanroom, VLF fan units fit in the same area
required for sensible coils and a return chase, which
allows for maintenance accessibility either from
the subfloor beneath the cleanroom or from an
adjacent chase. VLF fan units are usually a lower
cost option for large, open ballroom cleanrooms
with filter coverage greater than 50 percent. Used in
conjunction with variable frequency drives (VFDs)
equipped with large premium efficiency motors,
they have lower energy usage. While installing
small, mini-VLF fan towers of around 12,000 cfm is
relatively easy, large VLFs greater than 75,000 cfm
are difficult to install. Like RAHUs, these systems
can be noisy, with sound levels of 65 dB(A). VLF
fan units also require a more costly and time-inten-sive gel ceiling grid and a positive pressure plenum
can lead to particles or gel leaking into cleanroom
Filter fan units are located on top of the cleanroom ceiling grid in the plenum space, so these
systems don’t take up facility floor space and offer
maintenance access from below or above. These units
offer quiet operation, with noise levels as low as 55
dB(A) or less easy to achieve. Units using the latest
DC motor models offer high reliability, with mean
time between failures approaching 10 years, and have
very low energy usage. They have a negative pressure
plenum that forces any leaks up into the plenum not
into the cleanroom and a gasketed ceiling grid can be
used. Filter fan units are easy to install and very cost
competitive when coverage is less than 50 percent,
but typically a high quantity of units is required for
greater coverage or for large cleanrooms.
Cleanroom construction process
Building out a cleanroom and adjacent environments
is a multi-stage construction process and particular
protocols must be maintained at each stage of the
construction to insure the integrity of the cleanroom.
The stages are: Stage 0 — Regular Construction;
Stage 1 — Clean Construction; Stage 2 — Pre
Cleanroom Construction; and, finally, Stage 3 —
Active Cleanroom. The following are the important
steps in each stage.
The regular construction phase is what must
contractors are used to, but in the context of a
cleanroom special attention should be paid to
housekeeping to set the tone for cleanliness. At this
stage there are no material restrictions and standard personal protective equipment — such as hard
hats, safety glasses, and vests — should be used.
Take care to protect building materials as well.
Construction workers should apply standard good
housekeeping practices and complete daily and
All drywall work should be complete, if possible,
before the first stage of actual clean construction.
During this phase, conduct daily and hourly cleanup using vacuums and start using a daily cleaning
service. There should be no unnecessary trash or
debris in the cleanroom area, and you should use
Cleanroom Class rating Area (SF) Height (ft.) Volume (CF) Filter velocity (fpm-low) Filter velocity (fpm-high) Air flow (cfm-low) Air flow (cfm-high) Design fir change (low) Design air change Filter coverage (%)
ISO 8.0 (Class 100,000) 10,000 10 100,000 60.0 80 48,000 64,000 29 38 8%
ISO 7.0 (Class 10,000) 10,000 10 100,000 60.0 80 72,000 96,000 43 63 12%
ISO 6.0 (Class 1,000) 10,000 10 100,000 60.0 90 150,000 225,000 90 135 25%
ISO 5.0 (Class 100T) 10,000 10 100,000 72.5 100 290,000 400,000 174 240 40%
ISO 4.0 (Class 10) 10,000 10 100,000 72.5 100 652,500 900,000 392 540 90%
ISO 3.0 (Class 1) 10,000 10 100,000 72.5 100 725,000 1,000,000 435 600 100%
Table 2: Cleanroom recirculation air design analysis.