Air filters for hospitals and healthcare facilities

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 fibres 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 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, a more random pathway through the fibres and therefore a greater opportunity to be captured (C).

The particle size of 0.3μm – commonly referred to as the most penetrating particle size or MPPS – is typically selected as the test point for rating filtration efficiency in particulate air filters. This is because particles above and below this size are generally easier to capture [1].

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.

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 or toxic gas molecules into harmless salts and gases.

Material / Type

Different media types can have various effects on efficiency. Synthetic filter media are resistant to moisture build up and are ideal for preventing the proliferation of mould and mildew. Glass fibre is typically a finer filter media, more tolerant of chemicals and high temperatures.

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 shed 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 Material

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. Contaminant bypass is when contaminants escape through small gaps on the sides of the filter.

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 cleaning agents, 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 flow across 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.

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 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, gel or knife edge methods.

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.

HVAC 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.

HEPA filters – Annual NATA testing and validation

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.

Gas-phase / carbon filter testing

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.

• All filters have a finite capacity and benefit from staged filter sequencing i.e. pre filters followed by higher efficiency final filters - allowing inexpensive filters to be sacrificed to protect more expensive and difficult to replace final filters (HEPA/HEGA).
• Allow space for filters and filter banks, as they are the biggest consumers of energy.
• Consider the initial and final pressure drops when selecting filters. Plans should be sized off the loaded filter pressures.
• Subject to filter selections, changing out one stage of a higher rated filter more often (quarterly) may be better from an energy usage perspective than changing out two stages (quarterly and yearly).