HEPA and ULPA filter systems

High-efficiency particulate air (HEPA) filters are the final barrier between a cleanroom's air handling system and the room. Ultra-low penetration air (ULPA) filters go a step further for the most demanding applications. A filter is only as good as its installation, though. A tiny leak in a gasket, frame or filter medium can let unfiltered air bypass an otherwise excellent filter.

This guide explains how HEPA and ULPA filters are rated, what to look for in housings and seals, and how installed filters are tested and maintained.

How high-efficiency filters capture particles

HEPA filters are made of pleated glass fibre or synthetic media. They do not act like a sieve. Particles are captured by three main mechanisms: interception, impaction (larger particles that cannot follow the airflow around fibres) and diffusion (very small particles moving randomly into fibres). Each mechanism works best at a different particle size, which produces a size at which the filter is least efficient, called the most penetrating particle size (MPPS). For typical HEPA media at normal velocities, MPPS is generally between about 0.1 and 0.3 um.

This explains why HEPA filters are very effective against both larger particles and very small ones, and why ratings are tied to the hardest particles to capture.

Rating systems

Several systems are used, and specifications should name one clearly.

EN 1822 and ISO 29463

The European standard EN 1822 classifies filters by efficiency at the MPPS. ISO 29463 uses a closely related method with its own class designations.

H13 and H14 filters also have local efficiency limits tested by scanning, so a filter cannot pass on overall average with a localized weak spot.

IEST recommended practice

In North America, IEST-RP-CC001 defines filter types by test method and efficiency. The traditional HEPA definition is at least 99.97 percent efficiency at 0.3 um. ULPA filters are defined at efficiencies of 99.999 percent or better at smaller particle sizes, typically 0.1 to 0.2 um. IEST types also specify whether each filter is scan tested at the factory.

What about MERV?

ASHRAE Standard 52.2 MERV ratings apply to general ventilation filters and stop at MERV 16. HEPA and ULPA filters are outside the MERV scale. The guide to filtration: MERV ratings, HEPA and pressure drop explains the relationship.

Choosing filter efficiency

Most ISO 5 to ISO 8 cleanrooms and pharmaceutical facilities use H13 or H14 (or IEST HEPA equivalents). ULPA filters are mainly used in semiconductor and some electronics cleanrooms at ISO 4 and cleaner. Choosing ULPA where HEPA would do adds cost and pressure drop without improving room cleanliness, because room particle levels are usually dominated by internal sources, not by the tiny penetration through HEPA filters.

Prefiltration

High-efficiency filters are expensive to replace and hard to access, so they should be protected by upstream prefilters in the air handler. A typical arrangement is a MERV 8 prefilter followed by a MERV 13 to 15 secondary filter, with the HEPA or ULPA filter at the terminal. Good prefiltration can extend final filter life considerably.

Housings and seals

Terminal housings

Ceiling-mounted terminal filter housings connect the supply duct to the room. The alternative, filters built into individual fan-powered modules, is compared in fan filter units vs ducted HEPA systems. Features to look for:

• Room-side replaceable filters, so filters can be changed and tested from inside the room without entering the ceiling space.
• Test ports for injecting challenge aerosol upstream and sampling upstream concentration.
• Diffuser faces or perforated screens that protect the filter and distribute air.
• Airtight construction with sealed seams and collars.

Seal types

Gel seals are generally more forgiving of installation than gaskets, but gel must be kept clean and undamaged.

Velocity and pressure drop

HEPA filters are usually selected for face velocities of about 0.45 m/s (90 fpm) or lower. Initial pressure drop for a typical deep-pleat HEPA filter is commonly in the range of about 125 to 250 Pa (0.5 to 1.0 in. w.c.) at rated airflow, depending on media and depth. Filters are typically replaced when pressure drop reaches a set limit, often around twice the initial value, when they fail a leak test that cannot be repaired, or when damaged.

Lower velocity across the filter reduces pressure drop and fan energy. Selecting larger or deeper filters is one of the simplest energy efficiency measures in cleanroom design.

Installed filter leak testing

Every installed HEPA and ULPA filter should be leak tested after installation and periodically, as part of the wider program described in cleanroom certification and testing. ISO 14644-3 and IEST-RP-CC034 describe the methods.

Photometer method

1. Introduce a challenge aerosol, usually a polyalphaolefin (PAO) or similar oil, upstream of the filter.
2. Measure upstream concentration with an aerosol photometer to confirm it is adequate and uniform.
3. Scan the entire downstream face of the filter and its perimeter seal with the photometer probe, moving slowly in overlapping strokes close to the face.
4. Treat a reading at or above the acceptance threshold as a leak. A common criterion for HEPA filters is 0.01 percent of upstream concentration.
5. Repair small media leaks with an approved sealant within limits on repaired area, or replace the filter; reseat or replace leaking gaskets.

Discrete particle counter method

For ULPA filters, or where a lower challenge concentration is preferred, a discrete particle counter can be used to scan for leaks. This method is more sensitive and allows a lower aerosol concentration, which reduces filter loading by the challenge.

Biological safety cabinets are field-certified to their own standards, including NSF/ANSI 49 and, in Canada, CSA Z316.3, using similar principles.

Replacement and handling

• Handle filters by the frame only and inspect them for damage on arrival.
• Do not install filters until construction dust is under control.
• Record serial numbers, locations, installation dates and test results.
• Trend pressure drop and plan replacement before it reaches the limit.
• For filters in containment exhaust, use bag-in/bag-out housings and decontamination procedures so personnel are not exposed during changes.

Key takeaways

• HEPA and ULPA filters are rated by efficiency at the most penetrating particle size under EN 1822, ISO 29463 or IEST methods.
• H13 or H14 HEPA filters suit most pharmaceutical and ISO 5 to 8 cleanrooms; ULPA is mainly needed for ISO 4 and cleaner.
• Housings, seals and installation quality matter as much as filter media.
• Installed filters must be scan tested for leaks after installation and periodically, typically with a 0.01 percent photometer criterion.
• Lower filter face velocity reduces pressure drop and fan energy.