The essential functions of air filters are to improve indoor air quality and to protect downstream equipment and facility components. Within hospital environments, air quality has a more direct impact on patients and hospital staff.
This article seeks to cover air filtration and its practical application in the hospital environment.
The two main streams of filtration employed by air filters are particulate and gaseous filtration.
Particulate filtration typically uses a randomly arranged mesh of fibers to capture small particles via the methods of impaction, interception and diffusion. In a simplistic way, particulate filtration relies on the probability that as a particle passes through these fibres in the direction of bulk airflow, they will either impact onto a fibre (A) or come close to and be drawn onto a fibre through interception (B). Smaller particles that are closer in size and mass to gaseous molecules tend to move radially to the direction of bulk airflow, resulting in a longer residence time and more random pathway through the fibres; and therefore greater opportunity to be captured – diffusion (C).
Figure 1: Particle filtration mechanisms – impaction, interception & diffusion
The particle size of 0.3μm (commonly referred to as MPPS- most penetrating particle size) is typically selected as the test point for rating filtration efficiency in particulate air filters. This is because particles above and below this size (0.3μm) are generally easier to capture.
Gaseous filtration methods commonly used by air filters are adsorption or chemisorption. Activated carbon media, which consist of micro-pores within larger granules, use the adsorption method. Small gas molecules find their way inside these pores where various forces adsorb the gas molecules to the media and hold them there, therefore removing them from the airstream (like a sponge adsorbing water). Chemical infused media such as potassium permanganate use the chemisorption method –whereby small gas molecules find their way inside these pores and are molecularly changed by chemical reaction when they contact the media. This chemical reaction permanently converts odourous/toxic gas molecules into a harmless salts and gases.
Figure 2: Activated Carbon Adsorption of Gas Molecules
Whilst chemical infused media can be targeted to react with specific contaminants, activated carbon media is not selective with the gas molecules it adsorbs – it will adsorb whatever contaminant that it is exposed to, including water vapour. If activated carbon media or chemically infused media are subject to particle contamination, the outer surface of the granule becomes coated, creating a seal. This seal results in the ineffective adsorption of gas molecules due to the unused pores inside the granule being inaccessible. Therefore, it is best practice to ensure that particle pre-filters are used before activated carbon filters to ensure optimal performance.
There are several aspects of air filter design that impact on filtration performance. The important parameters include:
|Air filtration media|
|Material / Type||Different media types can have various effects on efficiency. Synthetic filter media are resistant to moisture build up and ideal for preventing the proliferation of mould and mildew. Glass fibre is typically a finer filter media, more tolerant of chemicals and high temperatures, but can be quite fragile.|
|Thickness / Density||Thicker or denser filter media have higher filtration efficiency and higher pressure drop. Deeper, graduated filter medias can hold more contaminants, than thin media grades.|
|Fibre Quality / Size||Quality air media lasts longer and does not shred fibres in the duct. Fibre size can have various effects on efficiency.|
|Surface Area||Higher media surface area equates to a lower pressure drop and higher contaminant holding capacity. Pleated air filtration media has a greater surface area and contaminant holding capacity than flat air filtration media. The higher the number of pleats, the larger the surface area it can contain. Ultimately, the higher surface area maximises the filtration and contaminant holding capability and reduces energy consumption.|
|Frame (Metal, Plastic, Cardboard)||Strong, durable filter frames should withstand the force of the air stream and support the filter media. A stable housing module helps to achieve a longer filter service life.|
|Media Bonding / Frame Seal||Air filter media should be safely and tightly bonded to the filter frame to prevent air and contaminant bypass (where contaminants can escape through the small gaps on the sides of the filters).|
|Resistance||Some filters are required to perform under high heat or constant high humidity – and these filters have frames, media and glue specially designed to withstand these conditions. Within hospitals, exposure to cleaners, decontaminating agents and UV can cause a breakdown of some types of filter media.|
|Airflow||Filters are generally designed for a maximum velocity of 2.5m/s, however filters can be designed to withstand higher velocities, and in some cases specialised filters will have a maximum velocity below 2.5m/s. Maximum suggested velocities for HEPA filters would be 0.5m/s in a terminal application, 1.0m/s in an exhaust application and 2.5m/s for an in-line application – however at 2.5m/s the air pressure across the filter would be extremely high and increase energy usage.|
|Geometry||The geometry of the filter also impacts performance. Typically, an even laminar air flowacross the filter face will result in a lower pressure drop, reducing energy consumption. Adding excessive pleats can reduce free airflow and increase pressure.|
|Filter Capacity||Factors affecting the contaminant holding capacities of a filter include filter construction, type of contaminants/dust, temperature and humidity. Filter performance may vary between different filters or manufacturers, and different locations.|
|Filter Lifetime||Static pressure is a recognised measurement to indicate appropriate air filter change out times. A common rule of thumb for when to change a filter is 2-2.5 times the original pressure drop of the filter. It is however recommended to consider the energy used (costs) compared to the filter change-out cost.|
|Filter Access||Suitable access is required for the removal, replacement and testing of air filters. This is particularly important when locating filters in ceiling spaces (FFU’s/inline housings) and wall cavities (low level HEPA modules).|
Accepted filter performance rating systems are EN779:2012 (G1 to F9), EN1822:2009 (E10 to U17), ASHRAE 52.2 (MERV 1 to 16) and ISO16890 (ISO coarse to ISO ePM1).
Based on the successful removal of airborne particles by size, these ratings provide a relative measure of filter effectiveness; whereby lower rated filters remove larger sized particles and higher rated filters remove smaller sized particles. Higher rated filters will remove more airborne particles; however this is almost always at the cost of energy and more frequent filter changes. Similarly, lower rated filters will impact air quality, duct cleanliness, coil/heat exchanger performance, and in some cases safety.
Inspection of your current filters will reveal their rating, and you should replace these filters with (at minimum) a comparable rated filter. Over time, it is worth consulting with a knowledgeable and trustworthy filter manufacturer, to see if higher performance filters can result in higher IAQ and lower energy costs. Often by making a change to a modestly more expensive filter that provides lower pressure drops and larger dust holding capacities, you can actually reduce total costs of your clean air equation.
There are many hospital design standards and guidelines – nearly all states within Australian and New Zealand have their own. Some standards and guidelines vary, but generally they all follow the same basis and often refer to each other. Some areas also refer to international guidelines (eg. operating theatres – HTM03 / DIN9016, commercial buildings –Greenstar / Nabers / Wells).
All hospital design standards and guidelines are open to interpretation and must consider the actual requirements of the area, and ultimately, a risk based design element is required.
In any given state, both 1668.2 and state guidelines may apply. AS 1668.2 are documented in legislation via BCA, and its guidance should be adhered to in all cases.
Metal Framed Flat Panel Filter (20-25mm deep G4)
Flat panel filters are the most basic type of air filter, whereby a panel of flat surface air filtration media is enclosed within a metal frame. Commonly used in hospital applications as a pre-filter for terminal mounted exhaust HEPA housings where there is limited space for pre-filtration (such as isolation room ensuite areas), they serve to protect the HEPA filter and prevent large particles from entering the system.
Cardboard Pleat Filter (100mm deep G4-F7)
Cardboard disposable filters are constructed from pleated air filtration media bonded to a cardboard frame (typically moisture resistant beverage grade cardboard). Diagonal cardboard supports across the filter face are included for rigidity and durability. Typically used in hospital AHU’s and applications as a pre-filter for inline containment exhaust filtration systems where there is sufficient space for higher grade, high capacity pre-filters, which protect high capacity HEPA filters.
Glass Fibre Pleated Panel Filter (100mm deep F8-F9)
Glass fibre pleated panel filters are constructed from fine, paper thin micro glass fibre media, formed into packs of closely spaced pleats and bonded to a cardboard frame (typically moisture resistant beverage grade cardboard). Suited to variable air volume systems or turbulent flow conditions, they typically provide high pressures for a given airflow. Commonly used in hospital applications as a final filter within FCU’s, where space is a premium due to size and media limitations.
Multi Pocket Bag Filter (380mm deep F8/F9)
Multi pocket bag filters are constructed from a six / eight pocket (full size) or three / four pocket (half size) deep bed bags, constructed from electrostatically charged filter media bonded to a metal header frame, and generally have excellent contaminant holding capacities and reasonable pressure resistance. They are bulky, and take a significant duct length to house them. Commonly used in hospital AHU’s.
Rigid/Hybrid Media Filter (100mm deep F8/F9)
Rigid, formed hybrid media filters are constructed from self-supporting, layered filter media that uniquely incorporates pre filtration, medium and fine filtration layers, bonded to a metal frame. These filters have excellent contaminant holding capacities and reasonable pressure resistance– particularly for high efficiency grades. Within hospital applications, these filter are selected to replace glass fibre pleated panel filters as a final filter within restricted space locations (bed bays or location specific FCU’s).
Constructed from finely pleated air filtration media with high surface area properties for high efficiency filtration, HEPA filters are typically manufactured from the highest quality of materials under strict quality control conditions, and are factory certified to ensure filtration performance to EN 1822:2009 standards.
HEPA filters are typically sealed into their housings via mechanical or gel/knife edge methods. Refer to “Effective HEPA Design and Operation for Hospital Environments” for further details.
Specialty separator style and dimple-pleat/separator-less style HEPA filters are factory tested to meet the requirements of IEST RP-CC001.3 for Type A, B, C, D, E or F filters (industrial grade, nuclear grade, laminar flow grade, bio/hazard grade, VLSI, ULPA or pharmaceutical grade); and can be certified to UL586 or UL900 standards.
Mini-Pleat HEPA (H14)
Mini-pleat HEPA filters are constructed from fine, paper thin, micro glass fibre media, formed into packs of closely spaced pleats that are separated by a continuous thermoplastic cord and bonded to a metal frame. Typically used within terminal supply or exhaust HEPA housing within hospital applications.
Separator Style HEPA (H14/U15 or specialty)
Separator style HEPA filters are constructed from fine, paper thin, micro glass fibre media, formed into packs of closely spaced pleats that are separated by corrugated aluminum separators and bonded to a metal frame. Within hospital applications, these filters are used within in-line HEPA housings, AHU mounted HEPA housings or high efficiency exhaust BIBO (bag/in-bag/out) airborne containment systems.
V-form Mini Pleat HEPA (F9-H10, 95% DOP & H14)
V-form mini-pleat HEPA filters are constructed from fine, paper thin, micro glass fibre media, formed into mini-pleat media packs that are arranged into a V-bed configuration and bonded to a box style metal frame that provides strength and durability for high velocity applications. F9-H10 and 95% DOP rated V-form mini-pleat HEPA filters are typically used as a final filter within critical care areas, or as a final pre-filter for HEPA filtered areas. H14 rated V-form mini-pleat HEPA filters are typically used in high volume duct mounted HEPA applications (within a supply AHU or exhaust plenum).
Dimple Pleat, Separator-less HEPA (H14/U15 – specialty)
Dimple pleat, separator-less HEPA filters are constructed from a self-supporting and self-separating layered filter media bonded to a metal or wooden frame. These filters are typically used in high efficiency exhaust BIBO (bag/in-bag/out) airborne containment systems or nuclear medicine / cyclotron areas, due to their relatively low pressure and high volume characteristics. Subject to the application, the filter media packs and frames can be incinerated if required.
Activated Carbon Filters
Activated carbon filters, which use activated carbon filter media are typically available in flat panel style, cardboard pleat style and V-form mini-pleat style. Some filter types use hybrid particulate/activated carbon media for combined particle and gas filtration. Commonly used to remove VOC’s and other inorganic gases that are generated from indoor and outdoor sources to improve IAQ. They are particularly useful when outdoor air intakes are near roads, loading docks or helicopter pads.
Figure 12: Activated carbon V-form mini-pleat filter
High Mass Carbon / Chemical Infused Media Filters
High mass carbon or chemical infused media filtration is available through carbon / chemical media pellets installed into housings / canisters, and through filters constructed from composite structure, extruded monolithic blocks of carbon / chemical media inserted, supported by wire mesh within a metal frame. Typically used where bulk or high percentage gas/odour removal is required, these filters are commonly used within animal laboratory areas and helipad areas to remove odours and jet/diesel fumes.
HEGA (High Efficiency Gas Adsorption) Filters
HEGA filters are manufactured from the highest quality of materials under strict quality control conditions as custom targeted high mass carbon media installed into V-bank style housings, and are factory tested to meet the requirements of IES RP-CC-008- 84, “Recommended Practice for Gas Phase Adsorber Cells”. HEGA filters are commonly used in conjunction with HEPA filters in high efficiency exhaust BIBO (bag/in-bag/out) airborne containment systems or radioactive dispensing / cyclotron areas. They are typically type tested and media is batch tested. Where required, field testing to confirm performance can be undertaken. The primary thing to consider is the handling of these filters – they are bulky and extremely heavy due to their construction and carbon mass.
Figure 14: HEGA carbon adsorber filters
Dust, temperature, humidity and other contaminations are all factors affecting the contaminant holding capacity of a filter – which again may vary between different filters or manufacturers. Actual historic data will be your best guide regarding filter replacement frequency. Location specific calculations should be undertaken to facilitate the most economical change out point (change-out cost Vs energy usage).
In well-designed systems, static pressure is a recognised measurement to indicate appropriate air filter change out times. The appropriate static change-out value for filters change significantly depending on the airflow rate, type of filter, grade of filter, hours of use per day and the contaminant concentration of the air being filtered. A common rule of thumb for when to change a filter is 2-2.5 times the original pressure drop of the filter. In general, it’s always better to change filters early – rather than late.
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/aerosol. A regulated and calibrated amount of “smoke/mist” is pumped into the upstream side of the filter, and the filter and housing are scanned on the downstream side 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.
It is more difficult to monitor the life of gas phase filters, as the pressure drop over a gas phase filter does not increase over the life of the filter. Common methods for checking life include; media samples sent to a test lab to determine remaining life, observation of increasing levels of VOC’s from VOC sensors located indoors, or replacement after a set period of time based on manufacturers recommendations.
Written by Kristian Kirwin (B.ENG Mechanical) and Shannon Roger (B.Ed) and published in Healthcare Facilities Vol 41, No 3, September 2018, p53-61 (IHEA/IFHE Conference Edition).
 M. K. Owen and D. S. Densor, “Airborne particle sizes and sources found in indoor air.” Atmospheric Environment., vol. Part A., no. General Topics, 26(12), pp. 2149-2162., 1992
 Airepure Australia, “Effective HEPA design and operation for hospital environments.” Healthcare Facilities Journal Vol 40, No 1, p 29-32, March 2017.