Manufacturers have become much more sophisticated in their knowledge and understanding of AMC and its effects in the cleanroom. They have a better general understanding of where AMC control should be applied and why, and as their knowledge of AMC-related problems have increased, so too have their expectations for an AMC control system.

We’ve previously discussed airborne molecular contamination (AMC) in terms of its classifications, monitoring, and control considerations and technologies, as well as AMC filter types and applications. This final installment will focus on specifying an AMC control system and final considerations for AMC control.

SPECIFYING AN AMC CONTROL SYSTEM
Manufacturers have become much more sophisticated in their knowledge and understanding of AMC and its effects in the cleanroom. They have a better general understanding of where AMC control should be applied and why, and as their knowledge of AMC-related problems have increased, so too have their expectations for anAMC control system.

In fact, some manufacturers’ concerns about the proper selection of a control system have become so acute that it is reflected in their control specifications. One manufacturer may call for a minimum of a 90% removal of target contaminants, while another will set AMC control limits at 1 ppb or less. Still another may require that the system must last a minimum of one year between filter changeouts. As strict as they may seem individually, thereare manufacturers that require all three of the above criteria to be met.

Compounding the situation, some manufacturers insist on trying to find a “one filter fits all” solution for AMC control, demanding that a single filter should meet all of their control criteria for all contaminants of concern. However, chlorine requires one type of filter, ammonia another, and organic compounds still another. The type of filters/systems used for toxic gases wouldnot be the same as odor control.

“Are you looking for high efficiency or long service life?” “Do you require absolute control of one contaminant or relative control of a group of contaminants?” These are only some of the questions that an AMC control system designer must consider, even though his customer may not understand the implications of not takingall of this into consideration.

Specification by Removal Efficiency
The removal efficiency of an AMC control system can be considered the fraction of a single contaminant or group of contaminants that are removed eitherby physical or chemical means.

Many manufacturers are able to provide test data for their systems that show removal efficiency over time. However, this testing has been performed almost exclusively under accelerated conditions using highcontaminant challenge concentrations that can be up to three to four orders of magnitude higher than what would be expected under actual use conditions.