Cleanroom Air Flow Principles
01 November 2019

Cleanroom Air Flow Principles

Cleanrooms 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 using an 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, such as microprocessors, also requires a 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 based upon the purity of the air that is contained in the facility. There are two standards typically used for this determination:

  • ISO 14644-1
  • US Federal Standard 209E

The ISO standard assigns a class ranging from ISO 1 to ISO 9, with the higher classes permitting more particulate contamination per unit volume of air. A cleanroom graded ISO 1 is permitted to contain no more than 10 particles of size greater than per 0.1 microns per cubic meter of air. A cleanroom graded ISO 2 increases this limit to 100 or fewer particles of size greater than per 0.1 microns per cubic meter of air. In addition, limits for concentrations of particles of 0.2 microns and 0.3 microns are added. Typically, as the ISO class number increases, the stated limits of particle concentration step up by a factor of 10 with each higher ISO class, and larger particle size contaminants are added to the standard limits.

The US Federal Standards limits are expressed as Class numbers of 1, 10, 100, 1000, etc., with the lower class number representing a cleaner facility. Class 1 cleanrooms are permitted to have one or fewer particles per of size 0.5 microns per cubic foot of air. Class 10 cleanrooms have 10 or fewer particles of size 0.5 microns per cubic foot of air. Ascending class grades raise this limit at the 0.5-micron size by a factor of 10 for each step. There is an equivalency between the ISO and the US Federal Standards classes.

Because people often work in cleanrooms, they are required to follow dress and behavior guidelines to limit the number of particles they may bring into a cleanroom or particles they might 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 Flow Filter and Filtration Systems
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 number 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-unidirectional 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.