HEPA filters and housings for hospital environments

HEPA modules and housings

Supply applications

Inline/ induct HEPA housings are typically used where clean air is required but it is not highly critical (i.e. grade D, with possible scope to achieve grade C). HEPA filters used in this supply application are typically large in size and have a high airflow capacity. This allows for fewer filters to be used to achieve the required airflow, and often the final contaminant loaded pressure is high.

What must be considered is that the duct between the inline HEPAs and the terminal may generate (or seep in particles) oxides of metals, mould etc, and these contaminants can enter the airstream after the filters — hence the popularity and effectiveness of terminal HEPA filters, which assure a supply of clean air direct to the room/area required.

In many cases the “disposable” HEPA filter that supplies air to a given room will be housed in a simple “fixed” HEPA module, and these units will be sized to achieve an effective airflow to the room at approved face velocity rates that deliver the specified values of IAQ.

Generally, these plenum boxes will have top air inlet spigots, and sometimes have options for inspection plates and damper adjustment units for air flow balancing. The fascia sections of these units are precision devices, to afford a flat, square mounting for the HEPA filter. Materials of construction are typically extruded aluminum facias with a sheet metal rear plenum with options for protective mesh covers which serve to diffuse air flow from the HEPA and protect the delicate HEPA filter from in house cleaning processes.

Disposable HEPA modules/filters are not recommended for any application requiring testing. If a filter fails, it cannot be repaired and must be replaced. This is expensive and time consuming ceiling works are required. The clean zone can be exposed to possible contaminated ceiling/service spaces.

There are filter modules similar to disposable units that have replaceable filters, and this is the minimum recommended selection.

Proprietary manufactured terminal HEPA housings are specially manufactured units of varying construction (subject to the requirements of the application) and typically manufactured from sheet metal and or extrusion, with a stainless or powder coated finish. There is a correlation between product quality, performance and service life — and that is where true value lies. Subject to manufacturer, customisation can be undertaken for specific applications, or finish requirements (i.e. non-magnetic materials for MRI room applications).

In more specific circumstances such as operating theatres, HEPA filters are often arranged in purpose built, multi-filter housings to promote superior laminar air flow control and simplified installation. Refinements such as accommodation of various pendants and lighting systems are further enhancements.

Exhaust applications

Accessibility to the upstream air side of the HEPA filter for initial and annual testing by a NATA accredited testing agent is a vital consideration for all HEPA exhaust filtration applications. This is applicable for all scenarios — whether the HEPA filters are located in terminal housings or inline within the duct.

Terminally mounted HEPA containment housings allow contaminants to be contained within the affected area (i.e. isolation room or cytotoxic area). As the ductwork is protected by the HEPA filter, there is a reduced risk of a potential contagion or contaminant spreading in the ceiling service spaces.

As space within hospitals is at a premium, there is often minimal space at the terminal to provide suitable levels of pre filtration to extend the life of the HEPA filter (unless this is factored in within the design stage). Alternatively, inline HEPA containment exhaust filtration Bag-In/Bag-Out (BIBO) systems are used. This type of system, being much more stringent usually includes bubble tight isolation dampers, decontamination/fumigation ports, a remote scan arrangement for testing and BIBO arrangements for filter change out.

HEPA “life” aspects

To be operated in a cost effective manner, pre-filtration should be considered to protect and extend the life of the HEPA filters. HEPA filters require adequate protection from general dust, lint and contamination by inexpensive, disposable filters to stop premature loading and replacement. In general, the higher the rating of the pre-filters, the longer the protected HEPA will last.

On a supply side application, a typical filter arrangement may be as follows:

1. Pre-filter (G4 pleated filter) — to seize larger particles, such as dust, lint, etc.
2. Intermediate filter (F8 multi-pocket bag filter) — to intercept small to medium sized particles, such as some bacteria, mould, etc.
3. Final filter (HEPA filter) — to capture the fine, elusive particles at 0.3 microns, such as viruses.

Without the sacrificial pre-filters, the HEPA filters are likely to clog rapidly, resulting in a service life of months instead of three to five years.

When good practices are followed, useful, energy efficient, high performance particle filtration should be achieved for 3 to 4 years of operation of the HEPA filter. The upstream pre-filters will be changed at far more frequent intervals, of 3 to 9 months. The pre-filters are “sacrificed” to capture small to large particles, and will generally be changed out to maintain energy efficiency and assure no particle “break-through” occurs due to overpressure that has affected the structural integrity of the pre filter.

The concept of “saving” money by infrequent pre-filter changes is a false economy. The cost of the increased energy consumed by an overly dirty pre-filter quickly outweighs the cost of replacing the relatively cheap pre-filter. Additionally the fan may not have the capacity to be able to move the required amount of air to or from the space if the pre-filter is excessively dirty.

Generally all these filters, (HEPAs included) will not look “dirty” when they are due to be changed. In general, when the filter is clearly discoloured, it is an indication that the filter has been in far too long and is consuming excessive energy.

The most effective way to monitor filters is to measure the pressure drop across them, either with a local magnehelic gauge or a gauge wired to the BMS. The rate in which the pressure drop across the filter increases can be used to provide an indication as to how quickly they will need to be replaced and when to order them. Other factors are involved in determining this, including the seasons, adjacent construction works, and HVAC plant loads. Suppliers are happy to help calculate an effective change out period when given an indication of the rate of pressure drop increase for a given system.

This is an excerpt from the published technical article entitled “Effective HEPA Design & Operation for Hospital Environments”, written by Kristian Kirwin (B.ENG Mechanical) and Shannon Roger (B.Ed) for Airepure Australia.

For detailed information regarding HEPA filters for supply and exhaust filtration applications, the differences between mechanical or gel-knife edge frame sealing approaches, practices to maintain HEPA filter life and the annual NATA testing and validation of HEPA filters, please refer to https://www.airepure.com.au/effective-hepa-design-operation-for-hospital-environments/.