HEPA air filters play a vital role in maintaining indoor air quality (IAQ) by reducing the fine airborne particles that can harbour bacteria and viruses – critical in many hospital settings, locations including but not limited to cancer wards (immune suppressed wards) and operating theatres. They also play a role in the containment of sub-micron particles from exhaust systems, where particles may be toxic or harbour airborne bacterial and viruses.
HEPA filters are designed to capture small particles that can be measured to a quantifiable level. Ratings are usually based on a particle size of 0.3 micron airborne particles at an efficiency rate of 99.95% or greater.
HEPA air filters are classified according to their retention (efficiency) at the given MPPS (most penetrating particle size), typically 0.3 micron:
The location or primary function of different hospital areas and departments determine the type of HEPA filters that will be used.
HEPA filters are typically used to supply clean air to a range of hospital areas. Terminal HEPA filters systems are generally preferred for supply air applications within hospitals, as the clean air is supplied directly to the room or space served.
|Terminal HEPA filters in immune suppressed areas:
||Terminal HEPA filters and Ultra Clean Ventilation systems (UCVs) in:
||Terminal HEPA filters in clean spaces:
HEPA filters are also used to contain contaminates from being discharged to outside or other spaces within the hospital through exhaust HEPA filtration systems, which may be located within:
Mechanical sealing utilises gaskets of varying forms, mounted on the perimeter frame of the HEPA, which act as a sealing mechanism between the HEPA filter and the housing mating surface. The surfaces must be smooth and flat, in some instances; considerable compressive pressure is required to ensure a good seal. Mechanical sealing is a proven method. It can however be quite time consuming to install and remove (especially if there are any mating surface issues).
As mentioned, flatness of the mating surfaces is critical to sealing, as any significant twisting or deformation along the surface length can result in gaps that the gasket is not capable of sealing. It is possible to over tighten, and twist and fracture the HEPA filter itself. Similarly, gaskets, when over compressed can either “cold flow” or deform permanently, resulting in gaps and thus air leaks.
The development of inert, soft gel polymer materials and reasonably priced, extrusion based HEPA frames, resulted in the development of gel-knife edge seal type HEPA filters. The gel is a soft, semi set material which is non-viscous and has the consistency off set jelly. The “knife edge” of the housing embeds into the soft gel sealing material within a channel around the HEPA Filter. This is considered a more mechanically forgiving and versatile sealing method, as this gel-knife edge seal will tolerate minor installation issues (such as uneven surfaces) and helps to limit the possibility of air bypass sometimes associated with gasket seal applications when they are disturbed.
In Australia, the use of gel seal HEPA filters has become the “default” choice over the last decade. Soft polyurethane gels may be used (for silicone sensitive applications); however, recent trends show the general chemical and environmental stability of Silicone gels are delivering the most popular solutions for HEPA applications.
Misguided attempts to retro-fit mechanical seal to gel-knife edge seal (or the reverse) to existing housings of one type are generally cost and time ineffective. If there is an application specific reason for using one particular sealing type, that needs to be determined on installation – or if change is needed, replace the entire housing and filter style.
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 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.
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.
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:
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, (HEPA’s 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.
In areas where high levels of diesel fumes are present, very high humidity or high probability of mould activity, it is common to shorten the above change out periods significantly to assure high IAQ levels. Your HEPA tester can be a good person to speak to about the state of your HEPA filters, when regular NATA testing is done.
In new buildings, premature installation and or inadequate initial duct (and facility) cleaning may dramatically shorten filter life, as residues from the building process, plaster dust, building dust and concrete dust, put a massive initial load on filter systems. In older buildings, equipment failure upstream of the filters may load filters with chemical or physical residue that can instantly virtually destroy a filter set. So it is wise to keep in mind downstream filters when major repair works are done to in-duct equipment.
The initial balancing and setup of air flows done at building commissioning, will need to be reconsidered as the building ages (often during the annual filter testing), or with changing usages or other building changes. Subtle, “real world” effects as buildings age can lead to sub standard or excessive air flows, incorrect room pressure regimes, both of which partially defeat the original design intent of the HEPA filter and room.
The annual retest of HEPA filters by NATA accredited testing agents is necessary to validate performance. These independently certified Technicians will, during certification testing, expose the HEPA media and housing to a challenge agent. A regulated and calibrated amount of chemical “mist” is pumped into the downstream side of the filter, and the filter and housing are scanned for leakage. To perform testing, adequate access needs to be available to physically view and scan the HEPA face. Access to, or a connection point upstream of the filter is needed to introduce the challenge aerosol.
This routine maintenance and regular check-ups of HVAC components and HEPA filters enable early detection of faulty air control systems. It also helps maximize functionality and prolong service life.
HEPA filter systems perform a vital role in Hospital IAQ. They are very rugged, reliable and effective devices. The selection, care and maintenance of the important components follows very clear rules – and you should see years of reliable, high performance operations, if the above mentioned recommendations are followed.
Written by Airepure Australia and published in Healthcare Facilities Journal Vol 40, No 1, March 2017.