Flexibility in design of permanent, modular, and mobile cleanrooms is second to rigorous standards of cleanliness class
Perhaps the popular view of a cleanroom is as a stark, cold antechamber where brave biocontainment specialists don slightly trimmed versions of spacesuits and battle novel pathogens threatening to escape clandestine BSL-4 facilities. In the movies, and sometimes in real life, that may be true. However, cleanrooms pervade research, testing, and industrial facilities, with prominent roles in pharmaceutical and semiconductor manufacturing, food safety, immunosensitive research and clinical care, and mobile or temporary applications that require pop-up and tear-down capabilities. Consequently, they run the gamut from moderately restrictive additions within permissive facilities, to isolated and sterile environments that function like giant, single-purpose biosafety cabinets.
Although the precise mission of a cleanroom may vary depending on the materials, protocols, and personnel inside, its basic function is static and clearly defined. A cleanroom controls the concentration of airborne particles, including dust, aerosols, and microbes, and must therefore be designed to minimize their introduction, retention, and generation while also maintaining temperature, humidity, and pressure commensurate with rigorous standards. Cleanroom standards are defined by ISO 14644-1, and they guide design, testing, and certification from the most restrictive class (ISO 1) to the least (ISO 9, equivalent to normal room air). The ISO cleanliness scale, like an inverse of the Richter scale, indicates a shift of an order of magnitude for every unit change, such that an ISO 3 room (equivalent to the former designation Federal Standard 209-E Class 1) allows 8.3 one micron-sized particles per square meter, while an ISO 4 room (209 Class 10) allows 83.
Design strategies to clean air of particles incorporate airflow inside the room as a first principle, and the desired cleanliness class of the room itself will dictate the geographical positioning of many structural and functional elements. This includes fans and filtration systems that can either be subsumed within, or placed externally to, ceilings; and floor, wall, and seam/junction designs that facilitate the requisite airflow current pattern and face velocity. Depending on positioning of these elements, airflow can be conventional and turbulent, semi-laminar, or unidirectional in vertical or horizontal paths. Airborne particles are reduced by dilution and filtration, with frequent outward ventilation of unclean air and recirculation through a series of increasingly restrictive filters, ending with HEPA or ULPA filters. HEPA and ULPA filters themselves employ differential mechanisms to stop large and small particles sequentially through straining/sieving, impaction/ impingement, interception based on van der Waal’s forces, diffusion via Brownian motion, and finally, electrostatic attraction for the smallest particles, down to the size of some virions. The placement of less restrictive household filters antecedent to HEPA and ULPA prolongs filter lifespan and mitigates the need for disassembly and maintenance.
The net effect is to not only clean the air, but also exchange it more frequently and move it at greater speed than is achieved with normal building HVAC systems. For instance, while a normal unrestricted room may recirculate air 10 times per hour, an ISO 7 cleanroom will often undergo up to 60 air changes, and ISO 2-5 rooms between 240-600 changes. High-velocity HEPA filters can move air up to 500 feet per minute, depending on cleanliness class requirements, which for higher classes can require consistent velocities of over 100 fpm, with complete ceiling filtration coverage. These considerations take precedence over, for instance, overhead lighting, which must be adequate, but should not compete for space with the filtration required to maintain class.
Cleanroom class is additionally maintained by positive pressure in relation to outer spaces, usually with a differential of greater than 5 Pascals, so that air is flowing outward, even when doors are briefly open. Therefore, for complex facilities with several competing regulatory requirements, there is often an onion skin design to cleanrooms, with a central room at the most stringent class, and progressively more permissive ones radiating outward.
Doors, walls, windows, and floors all need to incorporate the desired ISO standard into their design, such that particulates are minimized and cleanliness class is maintained. Consequently, airlocks primarily function to maintain the appropriate pressure differential between the cleanroom and external rooms that adhere to different ISO ratings. The doors allowing entry should avoid rubber foam or brush seals, in favor of neoprene or similar materials that will not flake or degrade over time. There are arguments for maintaining air-tight seals to block two-way particle travel, versus maintaining small gaps under doors to stabilize and cheapen the maintenance of pressure differentials. Regardless, there should be safeguard mechanisms in place against having two doors open at once, or opening a door while a cleanroom is in active use.
Walls function analogous to the skin of the whole structure, and are thus composed of smooth, seamless materials resistant to flaking, fissure, or opening via bolts or rivets, or other surface imperfections where microbes can colonize. Additionally, where there are seams or imperfections, assiduous attention should be paid to proper application of silicone or polyurethane sealant. A typical cleanroom wall can be composed of coated steel or aluminum, or even gypsum paneling as long as it is epoxy-sealed to avoid exfoliation. Latex or acrylic paints should be avoided, although walls are often painted white with epoxy, polyurethane, or baked enamel, because when it gets dirty, one can easily see it needs to be cleaned. Cleanroom walls, especially in modular designs meant to be quickly erected and disassembled, can be made of lap seam-sealed polyurethane; its transparency facilitates observation from without, allowing certification and quality control measures that might otherwise disrupt airflow and cleanliness. In hard-walled cleanrooms, windows can serve this function, but should avoid features normally common to windows like sloped sills that complicate cleaning and certification, and increase cost with each individual window. Flooring should be designed to coordinate with whatever airflow current is associated with desired cleanliness class, and is often raised, with channels and vents to facilitate air changes. Floors are often composed of heat-sealed vinyl or epoxy over a concrete base. For modular, soft-walled rooms, flooring is more commonly contiguous with polyurethane walls.
Finally, cleanroom plans should be clearly communicated to all associated contractors, so that elements like electrical and security installations are properly concealed without disrupting seals and airflow patterns.
In research and manufacturing, at permanent or temporary sites, there is a wealth of options for cleanroom design, which may facilitate their unique functions. However, function is subservient to standards, no matter the design.
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