Using HEPA filters to bring about a clean room environment

Clean rooms are highly controlled environments where the air quality is monitored to ensure the extreme standards of cleanliness levels required for the manufacture of pharmaceutical, electronic and healthcare goods are met. These stringent standards usually require high fresh air rates, extensive filtering, temperature and humidity control

Clean rooms are designed to different class specifications (Class 1/ISO3 – Class 100,000/ISO 8) depending on the sensitivity levels of the production process. The higher the class in cleanroom the more stringent the demands on air quality become and therefore the need for improved air filtration increases.

EPA/HEPA/ULPA filtration

To ensure that a clean room is capable of reaching the desired cleanliness levels it is extremely important that the supply air into the clean room is filtered using EPA/HEPA/ULPA filters that conform and are certified to EN1822:2009. EN1822:2009 is the standard that governs high efficiency air filters.

This standard not only sets out the efficiency standards that are demanded from these air filters but it also sets out the testing methods required to classify these filters into their categories. EPA, HEPA and ULPA filters are the names given to the different filter groups for high efficiency filters.

The filter class description are:

EPA 10 – 12: Efficiency Particulate Air Filters

HEPA 13 – 14: High Efficiency Particulate Air Filters

ULPA 15 – 17: Ultra Low Penetration Air Filters

Mark-3 Testing methods for HEPA filters

Testing to EN 1822 for filter grades H14 and greater requires the filter face to be scanned, this is done using a probe that can be moved over the entire surface of the filter. The results from moving the probe (scanning) give values for the measurement of the local collection efficiencies.

These local efficiencies can be used to calculate the overall efficiency of the filter or the leak rate of a specific area of the filter. The overall efficiency calculation is often termed ‘global efficiency'', while the individual results are termed the ‘local value''.

Tests are performed on new filters at specified nominal volumetric air flow. Filters of H14 or above must be scanned with a particle counter probe designed for this purpose. An oil thread test can be utilized on filters of H13 and H14 classification – this is method that is not as sensitive as the scan test and does not result in a report (Camfil test all clean room panel type filters H14 and greater using the scan test method)

As part of EN1822:2009 a test certificate must be produced with all High efficiency H and U grade filters.

Filter testing includes the following measurement:

Pressure drop and airflow rate

Overall collection efficiency at most penetrating particle size (MPPS)

Local collection efficiencies at MPPS

HEPA filter lifetime

In clean rooms it is important that the cleanliness of the room is maintained to ensure there is no disruption to the manufacturing process and it is for this reason that it is important to understand the lifetime of your HEPA filter. Just like other air filters the lifetime of a HEPA filter depends on the operating conditions of the HVAC system.

The general recommendation is that, at minimum, the filter be replaced when the pressure drop reaches two times the initial pressure drop (assuming normal design airflows) however most HEPA filters can reach a maximum of 450PA before needing replacement. Factors that may influence filter service life include: volume of outdoor air filtered, volume of recirculated air filtered, aerosol challenge exposure frequency and duration, and the filter efficiency of pre-filtration. The use of low grade or synthetic bag filters (as a prefilter) can lead to the shortened lifetime of your high efficiency HEPA filter.