How does the air actually move inside a cleanroom?
It’s a question worth asking, as the answer affects everything from the cleanroom’s ISO classification to how much it costs to build.
In this article, we’ll cover what unidirectional airflow is, how it differs from non-unidirectional airflow, and why it matters for cleanroom design.
What Is Unidirectional Airflow?
Unidirectional airflow is just what it sounds like, that is, air that moves in one direction. Generally speaking, this airflow moves in smooth parallel layers and at a steady speed.
In the vast majority of cleanrooms, this means straight down from the ceiling to the floor.
HEPA or ULPA filters mounted in the ceiling push filtered air down into the room. That air moves over the most vulnerable areas without mixing with the surrounding room air, almost like a curtain, and exits through low-wall returns or floor, from which it again recirculates through filters.
The whole purpose of unidirectional airflow is to carry particles away from important areas before they get a chance to settle. Because the air flows in parallel streams, there are no circular movements, and therefore nowhere for particles to linger.
For this to work, the ceiling needs a lot of filter coverage, which is one of the main reasons cleanrooms with this type of airflow often cost more (both to build and operate).
You’ll also hear unidirectional airflow sometimes called “laminar flow.” The terms are typically used interchangeably, but they are not technically the same thing. Laminar airflow is a physics term describing perfectly smooth, parallel layers of fluid movement.
How Is Non-Unidirectional Airflow Different?
Non-unidirectional airflow, sometimes called turbulent flow, is just the opposite.
Air still enters through ceiling-mounted FFUs and passes through HEPA filters, but instead of flowing smoothly in one direction, it naturally mixes with the air already in the room.
It is not a special technique, per se, but a default.
All it is is a less-controlled airflow pattern, and it works well enough for a lot of applications. Non-unidirectional airflow is the standard for ISO 6 through ISO 8 cleanrooms, where the particle count limits aren’t as strict. It also requires less ceiling coverage, and therefore costs less, and is simpler to design.
When Is Unidirectional Airflow Required?
Unidirectional airflow is used for ISO 1 through ISO 5 cleanrooms. At these classifications, particle counts are so low that dilution alone probably isn’t going to achieve cleanroom objectives.
Some of the most common applications include pharmaceutical manufacturing, semiconductor and electronics production, biotech, and medical device manufacturing.
Of course, you don’t always need unidirectional airflow to cover the entire room. In many cleanrooms, only specific zones use it.
For example, a laminar flow hood or workbench may use unidirectional airflow over a critical work area within an otherwise ISO 7 cleanroom.
This is known as mixed airflow, and it’s a common way to get the benefits of unidirectional flow where it counts without the cost of building the entire room to that standard.
What This All Means for Cleanroom Design
If your cleanroom requires unidirectional airflow, that will shape how it’s designed and built.
A unidirectional airflow cleanroom needs a ceiling that can support 80% or more filter coverage, which is much more extensive than your run-of-the-mill non-unidirectional room, which might only need 15 to 25 percent. The ceiling grid, the structural support, and the HVAC system all need to be built accordingly.
In some cases, the floor has to be designed differently as well. Raised perforated floors allow air to exit downward and return to the plenum below, which helps maintain the parallel airflow pattern. Not every unidirectional cleanroom uses a raised floor, but it’s more common in stricter classifications.
Where you place your equipment and how people move matters too because each can disrupt the airflow pattern. The fewer obstacles in the path of the air, the better.
All of this, ideally, needs to be planned from the start. Converting a non-unidirectional cleanroom to unidirectional later is a huge undertaking, and in many cases it may be easier to build a new room than to retrofit an existing one.
If you’re unsure of what type of airflow your cleanroom needs, your ISO classification and process requirements will usually point you in the right direction.
