Whenever ambient air is taken into a building, it needs to be cleaned. All major pollutants needs to be considered and sufficient cleaning technologies to be chosen:
- Particulate matter:
- Mechanical particulate filters;
- Electrostatic precipitator;
- Molecular (chemical / active carbon) filters where sorbent media is selected based on the gases that need to be removed;
- Wet scrubber (water soluble gases).
- Microbiological contamination:
- Mechanical particulate filters – especially HEPA (remove);
- Electrostatic precipitator (remove & kill);
- UV-light (kill);
- Gold plasma technology (kill);
- Photo-hydro-ionizer (PHI) (kill).
- Corona discharge ozone systems (in non-occupied spaces only).
In India, the ventilation system is nearly always one of the reasons for poor indoor air quality and therefore improvements in ventilation system operation are also part of the solution. Indoor air quality cannot be improved in India without proper ambient air particulate filtration and positive pressurization of spaces. In the areas where ambient air gas concentrations are high, also molecular filtration is required.
Removal of microbiological contamination needs to be studied case by case. There shall be a balance between a sterile environment and the protection of immunity in its occupants.
Regular ventilation system in India (both in old and new buildings) has only one layer of particulate filters. Filters are typically washable coarse filters, which after few times of washing starts to loose their filtration efficiency. This solution mainly filters the dust that is bigger than 30 µm in size.
To remove fine and ultra-fine particulates from the air, at least three layers of particulate filtration is required – pre-filter, coarse filter and fine filter. HEPA filter is not typically needed in commercial buildings or homes unless occupants already have serious respiratory symptoms. Electrostatic precipitator (ESP) can be used instead of coarse and fine-filters.
Electrostatic precipitator (ESP) is an air filtration unit that removes dust, fine & ultra-fine particulates, smoke, droplets and microbiological contamination (like bacteria and viruses) from the air flow using electrical forces. In the ionization unit a high voltage is applied to the discharge wires to form an electrical field between the wires and the collecting plates. Then, the particulates are given an electrical charge. The electrically charged collecting plates in the center of the air flow are collecting the charged particulates onto the plate surfaces.
Pressure loss of the electrostatic precipitator is very low and stays nearly constant either the unit is clean or dirty. The electrostatic precipitator, which is made from steel, has same life time as the air handling unit. The maintenance of the electrostatic precipitators is cost effective and no waste is generated. Both the ionization unit and collector plates only need to be washed regularly (time depends on the pre-filtration efficiency and the amount of particulates in the air). The payback time of ESP installation in TFA-unit is typically 2-3 years as fan energy and filter replacement savings.
The operation voltage needs to be optimized to ensure the high particulate filtration efficiency as well as minimizing the ozone generation in the ionization unit. Active carbon or honeycomb type chemical (KMnO4) filter is recommended to be used after the electrostatic precipitator to collect the small amount of ozone generated in the unit and to remove the ambient air gases.
Filtration efficiency of these two filtering solutions in terms of particulate matter is about the same. However, electrostatic precipitator both kills and removes microbiological contamination from the air.
Another major difference is the pressure loss of filtration. With mechanical filters the pressure loss difference between the clean and the dirty filter is big, where in the electrostatic precipitator the impact is much smaller.
- Pre-filter (MERV 8 / EU 4): 50 – 150 Pa;
- Coarse filter (MERV 9 / EU 5): 70 – 350 Pa;
- Fine filter (MERV 15 / EU 9): 100 – 350 Pa;
- HEPA filter: 250 – 500 Pa;
- Electrostatic precipitator: 10 – 50 Pa.
If building does not have mechanical ventilation and it can not be retrofitted, room air purifiers are used for air cleaning. This solution does not provide the same air quality as it is not able to pressurize spaces and therefore infiltration of polluted ambient air continues. Same filtration principles can be used in room air purifiers as described above.
Filtration efficiency of filter package does not alone ensure clean indoor air but there are other factors that impacts the result too like tightness of building and pressurization. In India, correctly selected room air purifiers with sufficient air flow rate provide about 50-70% cleaner room air (particulate pollution) than the outdoor air, and well-balanced mechanical ventilation system or ambient air purifiers provide 80-95% cleaner room air.