Every cleanroom filters air, but there are two ways of going about it. Which one you choose affects everything from cost to how much control you have over temperature and humidity.
In this article, we’ll explain the difference between one-pass and recirculating cleanrooms, as well as how to figure out which one you need.
How Does a One-Pass Cleanroom Work?
A one-pass cleanroom (also called a single pass) only filters air once and then exits, either through exhaust grills in the walls or, in the case of softwall cleanrooms, under the curtain walls and into the surrounding space.
The basic point to understand about one-pass cleanrooms is that there is no return path and no recirculation. The air comes in, gets filtered once, and leaves.
For this reason, one-pass systems are simpler to design, take up less space, and cost less.
How Does a Recirculating Cleanroom Work?
A recirculating cleanroom works a little differently.
Air is pushed by the HEPA fan filter units into the room, flows toward the floor, and then gets pulled back through low-wall returns into the plenum. From there, it passes through the filters again.
In other words, the air moves in a continuous loop, getting cleaner with each cycle.
The cleanroom also has its own HVAC system built into this loop. It conditions the air between cycles, handling cooling, heating, humidification, or dehumidification as needed. This is what gives recirculating cleanrooms control over temperature and humidity that one pass systems don’t have.
As you might expect, recirculating systems cost more to build (and operate), but they are much more efficient in reducing particle counts, and they also help extend the life of HEPA filters, since the filters are processing cleaner air, as opposed to the air from outside.
Which One Do You Need?
The decision usually comes down to how much environmental control your process requires.
As we spelled out already, if you need to control temperature or humidity directly within the space, then you’ll need a recirculating cleanroom.
If you’re operating in a GMP-regulated industry like pharmaceutical manufacturing, biotech, or compounding, recirculating is almost always required. The same goes for any application where particle counts need to be as low as possible or where the ISO classification is strict.
One-pass cleanrooms work well in other cases, like for basic assembly, packaging, or industrial manufacturing. They’re simpler, cheaper, and perfectly adequate under the right circumstances.
If you’re not sure which type fits your situation, the answer usually becomes clear once you define your ISO classification, your process requirements, and whether or not you need to control temperature and humidity.
