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Duct cleaning - controlled environments
A cleanroom is a room whose defining feature is the cleanliness of its air - and it achieves that almost entirely through its ventilation. Filtered air, high air-change rates and controlled pressure work together to keep airborne particles down to a specified level. Here is how cleanroom ventilation works, the standards that classify it, and why the ductwork behind it has to be kept clean.
The short answer
A cleanroom is a controlled environment where the concentration of airborne particles is kept to a specified low level - and the ventilation is what achieves it. Three things work together. HEPA filtration cleans the supply air to a very high standard, removing almost all particles (HEPA filters retain at least 99.95% of the most penetrating particle size), so the air entering the room is extremely clean. High air-change rates - from tens of air changes per hour in a less strict cleanroom to hundreds in a very clean one - continuously flush the room with filtered air, diluting and removing the particles generated inside by people and processes. And a pressure cascade keeps the cleanroom at a higher pressure than the surrounding areas, so air flows out of it rather than in, keeping unfiltered, dirtier air from entering. Cleanrooms are classified by the international standard ISO 14644, which defines classes by airborne particle concentration - a lower ISO class number being cleaner. So in a cleanroom, the ventilation is the cleanliness - which is why its ductwork and filters have to be kept scrupulously clean, since any contamination in them would undermine the very thing the room exists to provide.
The ventilation is the cleanroom
What defines a cleanroom is the cleanliness of its air - the concentration of airborne particles is kept to a specified, very low level, far cleaner than ordinary indoor air. This matters in industries where tiny particles cause problems: pharmaceutical manufacturing, electronics and semiconductors, medical devices, and others, where contamination by airborne particles must be prevented. So a cleanroom is a room built to keep its air within a specified particle limit, for processes that need that cleanliness. And crucially, it achieves this almost entirely through its ventilation - the way it handles air is what makes it a cleanroom.
This is the key thing to understand about cleanrooms: the ventilation is not just a service to the room, it is the essence of the room. A cleanroom is clean because its ventilation continuously supplies extremely clean air, flushes the room to remove particles, and controls the pressure to keep contamination out. Take away or degrade the ventilation, and the cleanroom is no longer clean - it is just a room. So the ventilation system is the cleanroom's defining, essential feature, doing the work that the room exists for. This is quite different from an ordinary building, where the ventilation is a comfort and health service; in a cleanroom, the ventilation is the whole point, delivering the air cleanliness that is the room's purpose. Understanding that the ventilation is the cleanliness is the foundation for understanding how a cleanroom works and why its ventilation, ductwork included, is so critical.
HEPA filtration and air changes
Two of the three things that make a cleanroom work are the filtration and the air-change rate. The filtration cleans the supply air to a very high standard before it enters the room: cleanrooms use HEPA filters (high-efficiency particulate air filters), which remove almost all particles from the air - a HEPA filter retains at least 99.95% of particles at the most penetrating size, and even cleaner rooms use ULPA filters that go further still. So the air entering a cleanroom, having passed through HEPA filtration, is extremely clean, with almost all particles removed. This filtered supply air is the source of the room's cleanliness - clean air brought in through the filters.
The air-change rate is the second element: a cleanroom is supplied with a high volume of this filtered air, giving many air changes per hour, which continuously flushes the room. This matters because particles are generated inside the room too - by the people, the equipment and the processes - and the continuous flow of clean air dilutes and removes them, carrying them away before they can build up. The air-change rate needed depends on how clean the room must be: a less strict cleanroom might have tens of air changes per hour, while a very clean one has hundreds - the cleaner the room, the more air changes needed to keep the particle level down. So the filtration provides clean air, and the high air-change rate uses that clean air to continuously flush the room, together keeping the airborne particles to the specified level. These are the core mechanisms by which the ventilation delivers the cleanliness.
Pressure cascade and standards
The third element that makes a cleanroom work is the pressure cascade - keeping the cleanroom at a higher air pressure than the surrounding, less clean areas. Because air flows from higher to lower pressure, holding the cleanroom positive relative to its surroundings means air flows out of the cleanroom through any gaps, rather than dirtier air flowing in. This keeps unfiltered, contaminated air from the surrounding areas from entering and contaminating the cleanroom - a crucial protection, since the cleanroom's cleanliness would be undermined if outside air could leak in. The pressure cascade, often stepped across a series of rooms of increasing cleanliness, keeps each cleaner space protected from the dirtier ones around it.
Cleanrooms are classified by international standards, principally ISO 14644, which defines cleanroom classes by the concentration of airborne particles allowed at specified particle sizes. The classes run so that a lower ISO class number is cleaner - an ISO 5 cleanroom is cleaner than an ISO 7, which is cleaner than an ISO 8 - each class specifying how many particles of given sizes are permitted per volume of air, and how the class is verified by measurement. So the standard gives a precise, measurable definition of how clean a cleanroom is, and a cleanroom is designed and operated to achieve its required class. The class required depends on the process - a semiconductor fabrication needing a far cleaner class than, say, some medical device assembly. So ISO 14644 is the framework that classifies cleanroom cleanliness, against which a cleanroom's ventilation is designed and its performance measured. The filtration, air changes and pressure cascade are engineered to achieve the required ISO class.
Why the ductwork must be clean
Because the ventilation is the cleanroom, the ductwork and the whole air-handling system have to be kept scrupulously clean - any contamination in them would undermine the very cleanliness the room exists to provide. The supply air passes through the ductwork on its way to the room, so if the ductwork is contaminated - carrying dust, particles or microbial growth - it can add contamination to the air after the filtration, or before the final filters, compromising the cleanliness of the air delivered to the room. In a cleanroom, where the whole point is exceptionally clean air, contamination introduced by dirty ductwork is a serious problem, potentially defeating the room's purpose.
So cleanroom ductwork and air-handling systems are maintained to a high standard of cleanliness, and cleaned as needed to keep them from becoming a source of contamination - a specialised, careful task, given the standards involved and the need not to introduce contamination during the cleaning itself. This is a more demanding cleanliness requirement than ordinary ventilation ductwork, because the consequence of contamination is so much greater: in an ordinary building, some dust in the ducts is a hygiene and air-quality matter; in a cleanroom, it can undermine the controlled environment entirely. So the ductwork behind a cleanroom's ventilation is a critical part of maintaining the cleanroom, kept clean to protect the air cleanliness that defines the room. This is the duct-cleaning dimension of cleanrooms: the ventilation being the cleanliness means the ductwork's cleanliness is essential, and keeping it clean is part of keeping the cleanroom a cleanroom. The air is only as clean as the system that delivers it.
The takeaway
A cleanroom is a controlled environment where the concentration of airborne particles is kept to a specified low level - and the ventilation is what achieves it, making the ventilation the essence of the room rather than just a service. Three things work together: HEPA filtration cleans the supply air to a very high standard, removing almost all particles; high air-change rates - from tens to hundreds per hour depending on how clean the room must be - continuously flush the room with filtered air, removing the particles generated inside; and a pressure cascade keeps the cleanroom at a higher pressure than its surroundings, so air flows out rather than dirtier air flowing in. Cleanrooms are classified by ISO 14644, with a lower class number being cleaner, against which the ventilation is designed and verified.
Because the ventilation is the cleanliness, the ductwork and air-handling system have to be kept scrupulously clean - any contamination in them would undermine the very thing the room exists to provide, potentially defeating its purpose. So cleanroom ductwork is maintained to a high cleanliness standard, a more demanding requirement than ordinary ventilation because the consequence of contamination is so much greater. The air is only as clean as the system that delivers it. So cleanroom ventilation is clean air by design - filtered, flushed and pressure-protected to a measured standard - delivered by ductwork that itself has to be kept clean. Keeping that system clean is part of keeping the cleanroom a cleanroom, which is why the ductwork behind it matters so much.
Questions
A controlled environment where the concentration of airborne particles is kept to a specified low level, far cleaner than ordinary indoor air - used in industries where tiny particles cause problems, like pharmaceutical manufacturing, electronics and medical devices. It achieves its cleanliness almost entirely through its ventilation, which is the essence of the room rather than just a service to it.
Through three things working together: HEPA filtration cleans the supply air, removing almost all particles; high air-change rates continuously flush the room with that filtered air, removing the particles generated inside by people and processes; and a pressure cascade keeps the room at a higher pressure than its surroundings, so air flows out rather than dirtier air flowing in. Together these keep the airborne particles to the specified level.
High-efficiency particulate air filters, used in cleanrooms to clean the supply air to a very high standard - a HEPA filter retains at least 99.95% of particles at the most penetrating size, so the air entering the room is extremely clean. Even cleaner rooms use ULPA filters that go further still. The HEPA-filtered supply air is the source of the cleanroom's cleanliness.
By the international standard ISO 14644, which defines cleanroom classes by the concentration of airborne particles allowed at specified sizes. A lower ISO class number is cleaner - an ISO 5 cleanroom is cleaner than an ISO 7, which is cleaner than an ISO 8. The class required depends on the process, and the cleanroom's ventilation is designed and its performance measured against the required class.
Keeping the cleanroom at a higher air pressure than the surrounding, less clean areas, so air flows out of it rather than dirtier air flowing in. Since air flows from higher to lower pressure, this keeps unfiltered, contaminated air from entering and undermining the cleanroom's cleanliness. It is often stepped across a series of rooms of increasing cleanliness, each protected from the dirtier ones around it.
Because the ventilation is the cleanroom's cleanliness, so contamination in the ductwork would undermine it. The supply air passes through the ductwork, so if it is contaminated - carrying dust, particles or growth - it can add contamination to the air, compromising the cleanliness the room exists to provide. So cleanroom ductwork is maintained to a high cleanliness standard, a more demanding requirement than ordinary ventilation because the consequence of contamination is so much greater.
In a cleanroom, the air is only as clean as the ductwork that delivers it - contamination in the system undermines the controlled environment. Clean ductwork is essential wherever air cleanliness matters.
