IAQ Series： Fresh air intake filter

Outdoor air has various pollutants and it could badly affect IAQ and health of occupants if it is not properly filtered before entering into a building.

Outdoor air contains various pollutants especially in busy commercial zones. It contains high levels of particulate matter (PM), NOx, SOx, pollen, etc. If it is not properly filtered before entering into the air-conditioned space, the indoor air quality (IAQ) would be impaired and badly affect health of the occupants.

Recently, we revealed during the visits to our clients’ premises that different arrangements on fresh air intake filters are adopted.

They include:
1. Metal mesh pre-filter plus MERV 13 pocket filter
2. Metal mesh pre-filter plus MERV 11 pocket filter
3. Metal mesh filter only
4. No filter at all

Fresh air is directed to the air handling unit (AHU) where it mixes with air returned from air-conditioned space and then filtered by air filters of AHU before passing through the heat exchange coils. In case of fan coil unit (FCU), fresh air duct is very often tapped into the return air plenum at down stream of air filter (usually metal mesh filter) hence the fresh air will not be filtered at FCU.

The filtration efficiency of air filters installed in series

To understand the effectiveness of different arrangements of fresh air filter and AHU filters, we need to know the resultant filtration efficiencies of air filters installed in series, which is determined by the following formula.

where η1,2,3…..t are the filtration efficiencies of air filters installed in series. Based on the formula, we can obtain the resultant filtration efficiencies of different combinations of air filters of PAU and AHU. It can be noted that, if the grading of fresh air filter is below MERV 13, the filtration efficiency at 0.3-1 micron would be below 75% and 1-3 micron below 98%. It is also noted that metal mesh pre-filter has no notifiable effect on the resultant filtration efficiencies.

It has been proven that particulate matters of 2.5 micron or below would be harmful to people health, Feature 5 of WELL Air – ‘Air Filtration’ requires MERV 13 (or higher) media filter are used in the ventilation system to filter outdoor air.

Particular concerns on fan coil system

In FCU system, it is even worser cause fresh air tapping into the return air plenum would not be filtered before it is discharged into air-conditioning space.

In addition, if the fresh air intake point is at low level, it is prone to draw in polluted air containing harmful gases from car emissions, bad odor, VOCs, etc. In that case, molecular filter has gas adsorption properties need to be considered.

Proven case at a shopping mall

Recently, we have replaced the ‘metal mesh pre-filters plus pocket filters’ of a PAU at a shopping mall with our Mini-pleated molecular filters. The results are improved IAQ, reduced maintenance costs and increased air flow rate due to lower pressure drop of the mini-pleated filter. (ps. the pressure drop of pocket filter of MERV 13 or higher often exceeds 100pa while the pressure drop of MERV 13 mini-pleated filter is only 45-50pa). Meanwhile, the frequency of filter change could be reduced cause the rate of increase in pressure drop of pocket filter during the course of usage would be steeper than that of mini-pleated filter with larger surface area of filter media and different structure.

Change of ‘metal mesh pre-filter + pocket filter’ with mini-pleated molecular filter

NanoFIL molecular filter also has antibacterial/antiviral properties that can exterminate 99.99% of bacteria commonly found in air (SGS) and over 99% COVID-19 Omicron Variant viruses (VRS, UK). It can reduce risks of pandemic and biological contamination that could give rise to odor problems.

To protect health of employees, tenants and occupants, save energy and operating costs, it is advisable to recommend the building/facility management practitioners to examine the current situation of fresh air intake filters of the ventilation systems under their jurisdiction. It is particularly essential for fan coil system where the fresh air might not be filtered or only filtered by coarse filter before it is discharged into air-conditioned space.

Challenge the status quo! Call us if you want to save energy, reduce maintenance cost and improve IAQ. Nanofil can help you!

Email: info@nanofil.com.hk WhatsApp: (+852) 6273 0573