Contamination Control Requires Controlling The Total Cleanroom Environment
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A cleanroom provides a controlled, isolated environment for handling contamination-sensitive substances or for protecting the exterior environment from dangerous substances in the controlled area. The density of sub-micron and larger airborne particle contamination inside a cleanroom is kept within tightly controlled limits by forcing clean, filtered air into the cleanroom.
Controlling particulate contamination is a process, not an event. Contaminants are generated continuously by people, processes, facilities and equipment and must be continuously removed from the cleanroom. The allowable level of contamination is dictated by specifications for the operations being performed.
Contamination control requires controlling the total cleanroom environment. Air flow rate and direction, pressurization, temperature, humidity and specialized filtration all need to be tightly maintained to remove particles from the cleanroom. Particles sources must be eliminated whenever possible.
This creates an internal air pressure greater than ambient pressure, which reduces particulate intrusion around doors, and minor un-sealed areas in walls, ceilings, etc. Wall venting near floor level exhausts cleanroom air with the particles it carries into the external cleanroom environment and promotes laminar flow of incoming air. The ability to remove particles and thus create a particulate-controlled environment inside the cleanroom is measured by the number of air changes per hour; the more clean air, the cleaner the cleanroom. Basically, a positive pressure cleanroom protects the internal cleanroom environment.
In a given cleanroom, cleanliness can generally be increased by simply adding more Fan-filter units (FFUs), and thus filtered air, in the correct locations. Positive pressure can be built with hard walls made of solid plastic or steel, or soft walls made from flexible plastic strips and curtains. In hardwall cleanrooms, temperature and humidity can be controlled to provide human comfort, protect processes and prevent static charges by adding air conditioners, humidifiers and dehumidifiers.
Negative pressure cleanrooms are designed to keep contaminants from entering the external cleanroom environment. A common design uses exhausting FFUs, or air handlers, and high-efficiency particulate air (HEPA) filters to remove internal air. The cleanroom's in-room processes contaminate the air, that is then exhausted to a safe location. This exhaust system creates a negative pressure inside the cleanroom that draws external air into the cleanroom either directly or through filters. A common application for this design is a powder filling operation. A negative pressure cleanroom, then, protects the external cleanroom environment but not necessarily the internal area.
Positive/negative pressure cleanrooms, a more complex design, typically use FFUs to inject clean air into the cleanroom and exhaust contaminated cleanroom air into a buffer area. An air-handling system then removes slightly more air from the buffer area than is supplied by the FFUs, thus creating a negative pressure with respect to both the cleanroom and the external cleanroom environment. This negative pressure prevents cleanroom contaminates from entering the external cleanroom environment and also prevents external particulates from entering the cleanroom. A positive/negative pressure cleanroom, then, protects both the cleanroom and external environments.