HEPA filters are composed of a mat of randomly arranged fibres. The fibres are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. Key factors affecting function are fibre diameter, filter thickness, and face velocity. The air space between HEPA filter fibres is much greater than 0.3 μm. The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect. Unlike membrane filters at this pore size, where particles as wide as the largest opening or distance between fibres cannot pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles.:
HEPA filters, as defined by the DOE standard adopted by most American industries, remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter. The filter's minimal resistance to airflow, or pressure drop, is usually specified around 300 Pa at its nominal flow rate.
Lastly, it is important to note that HEPA filters are designed to arrest very fine particles effectively, but they do not filter out gasses and odor molecules. Circumstances requiring filtration of volatile organic compounds, chemical vapors, cigarette, pet, and/or flatulence odors call for the use of an activated carbon (charcoal) filter instead of or in addition to a HEPA filter.
ULPA is an acronym for "Ultra Low Penetration Air (filter)" An ULPA filter can remove from the air at least 99.999% of dust, pollen, mold, bacteria and any airborne particles with a size of 100 nanometres (0.1 µm) or larger.
Class 100 – less than 100 particles per cubic foot and 300-480 air changes per hour
Class 1000 – less then 1000 particles per cubic foot and 180 air changes per hour
Class 10000 – less then 10,000 particles per cubic foot 60 air changes per hour
Class 100000 – less than 100,000 particles per cubic foot 20 air changes per hour
Airlock is composed of two doors that are electrically interlocked in such a way that the two cannot be opened simultaneously. Airlocks are used in a variety of situations but in essence they are there to control the unwanted passage from one area to another.
Airlocks may be used to control the entry of personnel to a secure area such as a cleanroom, where dust or small particles may be a problem. Similarly if a constant temperature must be maintained then an airlock can be invaluable in reducing temperature fluctuations when doors are opened.
Sometimes gowning rooms are used as air locks without the interlocked doors. The purpose remains the same – prevent particulates or non controlled air to enter the cleanroom. However the gowning room relies on procedure or light indicators to make sure.
In laminar air flow the air tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of air In laminar flow, the motion of the particles of the air is very orderly with all particles moving in straight lines.
Well designed cleanrooms use ceiling mounted HEPA fan filter units and low wall air returns to achieve laminar air flow from ceiling to floor. This means air borne particulates are driven downward away from critical process areas and out of cleanroom instead of moving around cleanroom in turbulent air flow. This type of cleanroom is more effective in removing particules.
Workers typically wear non particulating cleanroom gowns and booties when working in cleanroom. They are either laundered by cleanroom laundry service or disposable. The level of gowning depends upon class of cleanroom and processes being performed. If facial hair is present than a beard/mustache guard is also worn. In lower class cleanroom (class 100k) sometimes just lab jacket, hair net and booties are worn.
Hard wall cleanroom give advantages of softwall cleanrooms because
- Most common type of cleanroom
- Cleanable surfaces
- Can control temperature and humidity
- Can maintain positive air pressure
- Can have gowning rooms, air lock
- More durable
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