FSE contains six classes, while the ISO classification system adds two cleaner standards and one dirtier standard see chart below. The "cleanest" cleanroom in FSE is referred to as Class 1; the "dirtiest" cleanroom is a class , ISO cleanroom classifications are rated according to how much particulate of specific sizes exist per cubic meter see second chart.
The "cleanest" cleanroom is a class 1 and the "dirtiest" a class 9. By law, Federal Standard E can be superseded by new international standards. It is expected that E will be used in some industries over the next five years, but that eventually it will be replaced internationally by ISO In cleanrooms, particulate concentration changes over time — from the construction and installation of equipment to its operational status.
ISO delineates three cleanroom classification standards: as-built, at-rest and operational. As instruments and equipment are introduced and particulates rise, an "as-built" cleanroom becomes an "at-rest" cleanroom.
When people are added to the matrix, particulate levels rise still further in the "operational" cleanroom. ISO describes the type and frequency of testing required to conform to certain standards.
The following tables indicate mandatory and optional tests. Many are in the final voting stage and can be legally used in the trade see chart.
A critical factor in cleanroom design is controlling air-change per hour ACH , also known as the air-change rate, or ACR. This refers to the number of times each hour that filtered outside air replaces the existing volume in a building or chamber.
In a normal home, an air-conditioner changes room air 0. In a cleanroom, depending on classification and usage, air change occurs anywhere from 10 to more than times an hour.
To meet optimal standards, ACR must be painstakingly measured and controlled. And there is some controversy. In an appendix to its ISO cleanliness standard, the International Standards Organization addressed applications for microelectronic facilities only.
ISO classes 6 to 8; Federal Standards 1,, 10, and , The appendix contained no ACR standards for pharmaceutical, healthcare or biotech applications, which may require higher ACR regulations.
According to current research, case studies and experiments, using an ACR range rather than one set standard is a better guideline for cleanliness classification. This is true because the optimal ACR varies from cleanroom to cleanroom, depending on factors such as internal equipment, staffing and operational purpose. Everything depends on the level of outside contaminants trying to enter the facility versus the level of contaminants being generated on the inside.
The breadth of these ranges reflects how dramatically people and processes affect cleanliness. Low-end figures within each contamination class generally indicate air velocity and air change requirements for an as-built or at-rest facility — where no people are present and no contaminating processes under way. When there are people and processes producing contaminants, more air changes are required to maintain optimal cleanliness standards.
For instance, some manufacturers insist on as many as air changes per hour to meet Class 10 standards. Determining the appropriate number of air changes for a particular application requires careful evaluation of factors such as the number of personnel, effectiveness of garbing protocol, frequency of access, and cleanliness of process equipment.
So when these older standards are applied, the resulting ACR is often too high. In fact, some studies have found that reducing the ACR and its attendant air turbulence can result in a cleaner atmosphere. The study measured air change rates in several ISO Class-5 cleanrooms and came to the conclusion that there is "no consistent design strategy for air change rate, even for cleanrooms of the same cleanliness classification. ACR rates have critical design implications, especially when considering desired cleanliness, fan size and lower energy costs.
In short, a lower ACR often resulted in cleaner air. The study focused on Class-5 cleanrooms, concluding that an ACR range of from to air changes per hour is standard, but that "actual operating ACRs ranged from 90 to Finally, the study concluded that rarely does a Class-5 facility require an ACR of more than The study also found that the "[b]est practice for ACRs is to design new facilities at the lower end of the recommended ACR range," with variable speed drives VSDs built in so that air flow adjustments can be made under actual operating conditions.
Designers and operators need evidence from others who have tried similar strategies in order to address the perceived risks of lowering air change rates. In summary, current research and thinking on air change rates indicate that some existing standards are too high and can be lowered while still meeting all ACR criteria.
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