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HEPA filter & UVC? Or a choice of one or the other?

HEPA filter & UVC? Or a choice of one or the other?
28/05/2020

HEPA filter & UVC. A combination or a choice? The Covid-19 peak appears to have passed and whilst there may be subsequent outbreaks we are encouraged to return to work and to reopen the economy. This has prompted employers across the country to investigate measures to make the work places safer. The purity of the air circulating around building ventilation systems and delivered into occupied spaces is now understood to be an important aspect of workplaces safety. Here we take a closer look at the choice between retrofitting HEPA filter and UVC to building ventilation systems, or to use one of the other of the technologies to ensure that air supplied into occupied spaces is safer.

Understanding Existing Filtration Capability In choosing between HEPA filter & UVC in the quest to make buildings safer from Covid-19 it is worth firstly understanding how effective how effective the existing filters in a system are against the virus.

Mechanical ventilation systems that process the air supplied to, and extracted from, occupied spaces have typically been designed and installed with at the very least some filtration to protect the heating and cooling coils and the fan from dust accumulation. Often the filters will be a combination of a panel type prefilter, perhaps G3 or G4 efficiency followed by a F7 efficiency filter.

The size of the virus Covid-19 is widely accepted as being 0.12 microns, and looking at the graph you will notice that this size is in keeping with the larger size viruses (red dots) and that with F7 filters you could reasonably expect to remove around 25%. Worth also noting that even with F7 filters you can expect some relatively high filtration efficiencies against larger micro organisms, particularly fungal spores.

Better efficiency filtration in air handling unit will go further than this and offer in the case of F9 filters, capture efficiency of over 95% of PM2.5. This goes a huge way to protecting building occupants from lung damaging dust, airborne spores, and bacteria. Viruses however are of a much smaller scale and typically less than 0.2 microns. Although fine filters such as F9 grade will catch some particulates of this size, the efficiency of fine filters of this type are not tested and rated against this scale of challenge.

HEPA minipleat HEPA filter pleated media

The HEPA Filtration Solution HEPA filters are tested against what is known as the MPPS (the most penetrating particle size). MPPS is generally accepted as being 0.3 microns and is the most difficult size to catch. The efficiency of HEPA filters at catching the MPPS depends on grade. With H13 being the lowest grade of HEPA having 99.95% or better efficiency up to H14 grade having 99.995% efficiency. These efficiencies are tested and easily demonstrable.

So if we know that the new bank of HEPA filters fitted in the AHU will catch 99.95% of the most penetrating particles, i.e. the MPPS (0.3 micron) we can be certain that the efficiency of catching Covid-19 (0.12 micron) will be higher because the efficiency at particle sizes smaller than the MPPS are greater.

The filtration performance of HEPAs is not in doubt as the standards to which they are manufactured and tested are rigorous. However ensuring that the installation of the HEPA filters in the air handling unit has the seal integrity to ensure that the performance suggested by filter class is maintained, is far more problematical. The problem areas are inevitably the seal between the HEPA filter and its holding frame, the seal between one holding frame and adjacent holding frames, and the seals between the outer perimeter of the holding frames and the skins of the AHU. Ensuring these seals are air tight and that there are no holes to allow 0.12 micron size particles to slip through is a challenge not to be underestimated.

Achieving the HEPA filter and holding frame is not the only installation issue. The pressure drop that a HEPA filter stage in the air handling unit will add to the overall system will be considerable and in order to retain the same airflow through the ducting system to the occupied spaces, it is certain that the fan will need to be run at a higher speed or it will require modification (replacement pulleys and belts) or changing for one that can accommodate the additional pressure from the HEPA filters.

Once the fans have been altered or changed to suit this additional 600 Pascals of pressure (recommended change pressure for most HEPAs), the true cost of this increased level of filtration becomes apparent by the big increase in the running costs due to the increase in energy used by the fan.

uvc induct fitting UVC in duct fitting

The UVC Solution UVC does not catch micro organisms like Covid-19 virus like HEPAs do. But it can, if properly designed, deactivate micro organisms to efficiency levels comparable with the capturing efficiency of HEPA filters. But the problem is proving it. Whilst it is easy to measure particles in an air stream and test HEPAs, the same luxury of testing is not economically available to UVC. Instead for UVC the proof of efficiency is in the maths.

It is widely accepted that the quantity of UVC energy required to achieve a certain percentile deactivation of micro organisms is known. Tables containing UVC dosage for different micro organisms are always being refined and updated as new ones are identified. This information together with information about UVC intensity from lamps and the parabolic reflectors in which they are held forms the base information to a mathematical modeling program.

For each application we add the site specific parameters including duct size, airflow, temperature and the specific target organism (which these days is typically covid-19). An in duct UVC product selection is made and the % efficiency of deactivation on 1st pass and subsequent passes is calculated. The maths is complex and even takes into consideration the material of the ducting as the different reflectivity of different materials has a quantifiable effect of on the efficiency of the UVC fixture in the application.

In duct UVC fixtures are normally installed down stream of the air handling unit, so they work in combination with the existing fine filters that are in the air handling unit. The effect is that the limited removal efficiency of the F7, F8, or F9 filters is complimented by the very high deactivation efficiency of the UVC array.

For more information on  HEPA filter & UVC? Or a choice of one or the other?  talk to  Puravent

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