Friday, May 13, 2016

Cleanroom Air Flow Principles

are facilities designed for conducting research or manufacturing products that require extremely clean environments. Typically, cleanrooms employ a broad range of techniques to prevent air particles, bacteria, and other contaminants from entering the workspace, often by means of employee dress code and washing, pass-thru lockers and chambers, and intensive detail to cleaning. However, one of the major forces keeping a cleanroom particle free is the air filter system. Cleanrooms employ many different types of filters, including HEPA filters and ULPA filters, but there are two standard air flow patterns that are consistently used: laminar flow and turbulent flow.

Cleanroom Basics 
Cleanrooms are necessary for various kinds of scientific research that require particle- and bacteria-free environments. For example, when scientists grow cultures, it is important to reduce the introduction of other bacteria so that results will not be compromised. Manufacturing various kinds of products like microprocessors also requires particle-free environment, because even a human hair contacting the small chips of a microprocessor can inhibit or destroy functionality.
Cleanrooms are either hard- or soft-walled. A hard wall cleanroom is a permanent structure or part of a larger permanent structure, while a soft wall cleanroom can be transported or augmented depending on requirements, and primarily exists within a larger, permanent structure. Modular, soft wall cleanrooms are needed for medical emergencies or when smaller runs of environment-sensitive materials are produced within a larger facility.
Cleanrooms are graded depending on how clean the air in the facility is. There are two standards used for this determination: the ISO and United States federal standards. ISO grades are numbered sequentially, advancing from 1. A cleanroom graded ISO 1 contains ten or fewer particles per 0.1 micrometer cubed area. A cleanroom graded ISO 2 contains 100 or fewer particles per 0.1 micrometer cubed area. The rest of the series feature the amount of particles rising by a factor of 10 per level. US federal standards are numbered 10, 100, 1000, etc., with the lower class number representing a cleaner facility. Class 1 cleanrooms have one or fewer particles per 0.5 micrometer cubed area. Class 10 cleanrooms have 10 or fewer particles per 0.5 micrometer cubed area. Ascending class grades rise by a factor of 10.
Because people often work in cleanrooms, they are required to follow dress and behavior guidelines to limit the amount of particles they will bring into a cleanroom or particles they will shed while working in the environment. Workers must change from street clothes into specially designed outfits, often with full hood coverings, gloves, and breathing masks. Workers must also enter through an air shower to eliminate remaining particles on the cleanroom suit, and then pass items into the cleanroom through a small chamber that prevents outside air from entering the clean environment.
Cleanroom Air Filtration
Cleanrooms employ air filtration to limit the particles in the environment air. Typically, this is through the use of either a highly efficient particulate air (HEPA) or ultra low particulate air (ULPA) filter. These filters can remove roughly 99.9 percent of all microparticles in room air by applying either laminar air flow or turbulent air flow techniques to the environment air.
Laminar air flow refers to air that flows in a straight, unimpeded path. Unidirectional flow is maintained in cleanrooms through the use of laminar air flow hoods that direct air jets downward in a straight path, as well as cleanroom architecture that ensures turbulence is lessened. Laminar air flow utilizes HEPA filters to filter and clean all air entering the environment. Laminar filters are often composed of stainless steel or other non-shed materials to ensure the amount of particles that enter the facility remains low. These filters usually compose roughly 80 percent of the ceiling space. Cleanrooms employing laminar air flow are typically referred to as Unidirectional Airflow Cleanrooms.
Non-unindirectional airflow cleanrooms utilize turbulent airflow systems to clean particulate air and maintain a clean environment. While laminar air flow filters are often a component of turbulent airflow systems, they are not the only systems employed. The entire enclosure is designed to use laminar flow and random, non-specific velocity filters to keep the air particle-free. Turbulent airflow can cause particle movement that can be difficult to separate from the rest of the air, but non-unidirectional airflow systems count on this random movement to move particles from the air through the filter.

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