What is "Source Strength"?

New ISO 14644-4 Cleanrooms and associated controlled environments, Part 4 "Design, construction and start-up" was published end of 2022 with several new updates. One of these new elements, source strength is still the hottest topic. All cleanroom industries get used to designing cleanrooms based on "air change rates" to keep them "clean" however, source strength is now offering a more scientific way; more than just a table of wide numbers like "20 to 200".

What does "Source Strength" mean?

As per standards' definition, source strength is the number of airborne particles or other airborne contaminants considered emitted per time unit expressed as a rate. This "source" can be a person, equipment, or an object and each rate should be indicated with a specific particle size.

"The particle generation from sources of contamination is expressed as the number of particles dispersed per second. The cumulative number of particles larger or equal to a specific size between 0,1 μm and 5 μm and/or macroparticles larger than 5 μm is considered"

the contribution from machinery is small compared with that from people

Why table-based "Air Change Rate" was not the best?

The air change rate is simply the time taken for the air within the cleanroom to be replaced with fresh air, expressed as the number of times this happens every hour (ACH). This is one of the best measures for contamination control. However, ACH alone may not be enough since there are several contributors such as total contamination load within the cleanroom (human+machine) removal efficiency that depends on a cleanroom design and initial cleanliness target for that particular cleanroom. Plus, several different guides/technical documents give quite a wide range, like 10x more/less of ACH values which makes things even more complicated.

The effectiveness of the air system in removing airborne particles depends on the ventilation system's effectiveness. This is influenced by the type of airflow between the supply air terminals and exhausts, as well as disturbance of the airflow patterns by, for example, obstruction or thermal effects. ACH as a "guidance value" may not represent all these variables.

How to Calculate Source Strength

A reasonable estimate of the total source strength in a cleanroom can be obtained if the airborne particle concentrations during operational conditions and the supply air volume flow rate are known. The calculation was given as;

S =Q·C·ε

where;

Q is the supply air volume flow rate to the cleanroom (m3·s?1);

S is the rate of particle emission in cleanroom air (source strength) (number·s?1);

C is the particle concentration limit in the cleanroom (number·m?3);

ε is ventilation effectiveness (dimensionless).

The ventilation effectiveness, ε, includes the correctional factors of the “actual mixing” condition and the effectiveness of various airflow patterns. For an existing cleanroom, ventilation effectiveness can be obtained as air change effectiveness (ACE) or contaminant removal effectiveness (CRE). The selection of the index depends on the application and the data that is available or can be acquired.

The value of ACE is calculated as:

ACE=(Recovery time measured at 1point) / (Room air changes)

The CRE ventilation efficiency is calculated as follows:

CRE= (Airborne particle concentration at exhaust) / (Average airborne particle concentration in the room)

Conclusion; There is no magical number!

Keep in mind that there is no "magical" number for the source strength value. The best way to obtain this is to get it from the operation, consider using a k factor and always challenge this data with the final results. Your "initial" air change value will provide numerous data for a "go/no go" decision. Remember that "more is better" is an old saying and we have ISO14644-16 "Energy Efficiency in Cleanrooms". Optimal air change values will provide us with the best removal efficiency & cleanup period while avoiding spending more energy and increasing our carbon footprint.

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