6 & 1/2 Cardinal Benchmarks Of Cleanroom Design/Build
Practice
by
H. Kent Craig
©1999
In one way or another, I've been occasionally involved
in cleanroom engineering, design, and construction for most
of my career. The cost per square foot of cleanroom
construction has dropped exponentially over the past
decades, while the ease of construction by has gone up.
It's where almost anyone with a basic understanding of HVAC
principles and some manufacturers' catalogs can design and
build a rudimentary cleanspace. But, designing a cleanroom
that is simultaneously functional, economical, flexible,
and practical remains as much of an art as a science.
Below are the handful of benchmarks which I believe any
cleanroom designer should work from, doing that delicate
balancing act on the highwire between initial and long-term
cost, between "bunnysuiter" comfort and total site
operating expense.
1: Minimize right-angles as much as possible
Any right angle in the cleanroom, even if outside of the
main airflow patterns, creates eddy currents, and those
eddys screw up the efficiency of the overall airflow
design. Any right angle, even one underside as opposed to
topside of a cabinet ledge or similar, creates microeddys
at minimum, which can enable temporary suspension and
concentration of airborne contaminants.
While it's virtually impossible to eliminate all right
angles from the airflow rivers in a given cleanroom, being
conscious of obvious ones at least in the main channels of
supply and return and trying to mitigate their negative
effects will lead to a tighter and more consistent level of
classification.
2: Concerning rawside capacity, sometimes too
much is not enough
There's a fine line between designing "adequate" capacity
for present and near-term future needs and spending more
money initially for extra capacity that might not ever be
used. Every time you add another 10,000 btu's to the
conditioned raw supply air capacity, every time you spec' a
humidifier that has an additional row of steamjets that you
really don't need initially, every time you increase the
pipe size from the chiller to the main air handler just so
you'll have extra capacity should another slave air handler
be needed whenever, the initial costs obviously tick
upwards.
But a year or two down the road, when the owner replaces an
item of equipment with a similar but different one that
sheds a much higher thermal gain inside the cleanspace and
all of a sudden the temp and RH factors start ping-ponging
all over the place, who's he going to blame? Look in the
mirror for the answer.
I've always allowed a 10% over-capacity on the
cooling side, at minimum, except where unusual
circumstances dictated otherwise. And more than once, I've
pushed the chilled water or cooled air supply envelope to
past 25% of what was needed initially, because the owner
indicated that various pieces of equipment might be
shuttled in and out of the cleanspace at any given moment's
notice, recertification be damned.
3: Don't reinvent the box
For those outside the business who have a rudimentary at
best concept of cleanrooms, most of the time they think of
a cleanroom as a cleanbox, nice and square. While often if
not usually true, that's not always the case. Especially
when retrofitted into an existing space, cleanrooms often
end up with all sorts of the nooks and crannies and small
offset spaces that serve no function other than to add
aggravating right angles and create dead areas can't be
used for anything practical but which never the less must
be within the cleanflow.
Not meaning to state the obvious, common sense and
real-world experience tells you that the most flexible and
usable space is the one closest to a nice box shape.
Especially when not crammed into a corner of an existing
space somewhere, which creates two deadwalls that the dirty
ends of production equipment can't be stuck through, a
boring box perimeter creates the most flexible and usable
of all potential shapes.
Are U-shaped, "snake", and reverse-S designs more practical
for certain types of production environments? Yes, for
certain, specific types. But for the majority of
needs of typical cleanspace application, the monotonous,
slight unimpressive boxy square or rectangle will often
prove the most economical to build, use, and maintain.
4: Human beings are walking, talking microfilth
factories
If human being weren't constantly shedding dead skin cells
and loose hairs and dead dermal mites and other assorted
microparasites like the cartoon character "Pigpen", leaving
a cloud of microfilth behind wherever they saunter to and
from, then cleanrooms would be a hell of a lot cheaper to
design and build if not altogether become unnecessary.
You can't do anything about enforcing anti-contamination
measures once the job is finished and you're no longer on
premises, but you can design the facility to help ensure
that the bunnysuiters don't add the potential problems.
Make the gowning area large enough, and put enough benches
in it, to where changing up isn't a royal pain. If in the
budget, add enough employee lockers so that sharing isn't
necessary. Extend the tackmat at the vestibule door at to
least 18" on either side of it, where possible. Add another
tackmat on the opposite side of the vestibule door.
Consider adding a delayed action or other timer with
infrared sensor to the air shower area, so that employees
are airwashed sufficiently enough before entering the
cleanspace and don't try for a quick duck-through just
because they're impatient. Make sure the gowning area
maintains positive pressure on ingress from the outside.
5: Classification costs too much to be a mere
status symbol
No, I'm not asking you to talk a potential client out of
the correct classification they need to accomplish the
mission objectives as outlined in their initial RFP or
Scope Of Work. I'm not asking you to undersell or downplay
the need for a correct level of contamination control for a
given process or production paradigm. I am asking you to be
follow the Golden Rule, to be a true professional, and to
offer your potential client all the facts regarding
the cost/benefit ratio of one level of classification over
another.
If your client needs a Class 100 room, but wants most of
the interior area to be Class 10, then as long as they know
the price leverage for said, then it's their call, it's
their money. But if they only need a Class 1,000 and you
try to sell them a Class 10 room because "all your
competitors have similarly classed rooms for their
processes", then that's borderline unethical to me.
Most of the time, your clients will be sufficiently
knowledegable enough, or have the people who are, to where
overselling isn't possible. But, I have run across enough
clients who really have no clue, who don't have in-house
personnel who have said clues, and who look to vendors and
contractors for most of their guidance and help in figuring
exactly what they do need and how and when they need it. A
true professional gives the client exactly what they need,
even when they are beyond hopeless in groping around for
expressions of what they want.
6: Every cleanroom should be designed for one
specific process, not cookie-cuttered from a set of canned
specifications
It may or may not seem obvious to a layman that cleanroom
designed for semiconductor manufacturing would be designed
and built different from one constructed for, say, small
batch pharmaceutical drug research and production.
Sometimes, I think it's not always obvious to some
cleanroom designers, either.
When I see cleanrooms for pharmaceutical uses that have
low-shed but potentially high porosity surfaces on cabinets
and such, it makes me wonder. When air filter media for the
above aren't made from stainless stell, it makes me wonder.
And when one sees such nonsense as a lab refrigerator where
live specimens and culture media are kept directly
beside a laminar flow hood (yes, I've seen that!), it
really makes you want to scratch your head in puzzlement,
along the lines of "what the hell were they thinking?"
Even if you have years of experience within the cleanroom
industry, you can't possibly know every detail about every
potential client's processes within their cleanroom, and
how that should affect your design. This is where direct
communication with those who'll be living inside your
cleanspace during their shifts becomes so important. You're
the expert on low contamination ways, not ways of making
whatever they make inside the space you build. They won't
think you any more or less ignorant than they already think
you are if you ask tons of dull, repetitive questions about
the critical path involved in making the product in
question.
And they'll be ever so grateful when you ask them for their
direct input into your initial design. Ask them for a
descending-order wishlist, what they'd like to see or not
see inside their new cleanspace, and why. Even if you
incorporate just one out of a hundred suggestions they
make, they'll feel they were given the respect due them as
fellow cleanroom professionals, and you'll end up with a
better design and a happier client.
6 & 1/2
: In the end, the only thing that matters to a
client is the ROI provided by your expertise
'Nuff said.
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