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Technology Compared

Air purifiers and air cleaners use a number of mechanisms to achieve their purpose. The following is a guide to the main methods of air cleaning and purification. Often the manufacturers will dream up new terminology to describe their particular machines function, however, leaving aside the new names, the basic mechanisms remain as follows

Mechanical filtration

Electrostatic filtration (type A)

Electrostatic filtration (type B)

Carbon filtration

Ultra Violet Light

Incinerators

Ozone

Mechanical filtration

Mechanical filters are made of a matrix of numerous fibres. Consider the different filter effects at the scale of an individual fibre. Air is passed through a filtration media. The dust particles are trapped by either;

Impingement is where large particles collide with a fibre whilst the air is diverted round the fibre and are retained.

Interception is where smaller particles move with the air flow but are attracted to the fibre with a stronger force than carries the particle in the air stream.

Diffusion is where the particles are so small that they follow irregular path like gas molecules (Brownian motion) and randomly collide with fibres and are retained.

Straining occurs when a particle is too large to pass between the gaps in the fibre matrix.

In most air cleaner filters treating domestic air all of these filtration effects are happening at once, giving an overall particle removal efficiency. Air cleaners with mechanical filters always need a fan to move the air through the filter. The efficiency of an air cleaner with mechanical filters depends on the quality of the filter, the air velocity through the filter and the state of the filter (how laden with dust)

Pros Cons

As mechanical filters become more laden with dust as they are used, the more efficient they become.

These type of air cleaners can be very straight forward and very robust, with easily available, cheap and fitted spare parts

You can in most cases get a filter of a efficiency to catch and hold the particles that are a problem

These actually remove the particles from the air, rather than change them. There for they reduce the dust settlement on surfaces rather than increase the dust settlement

Easy to maintain

As mechanical filters become more laden with dust as they are used, the lower the airflow becomes (i.e. pressure resistance of filter increases)

Because they have to over come the pressure resistance of a filter, fans in these types of air cleaners tend to be more powerful, therefore generate a higher sound level that would otherwise be the case

Watch out for air cleaners in plastic moulded bodies and components. They will not be as robust as ones made of wood or steel, although should be fine for domestic applications.

Electrostatic filtration (type A)

Electrostatic filters are sometimes referred to as electrostatic precipitators. They comprise of a 'spiked ioniser' which charges dust particles and a bank of collector plates which are oppositely charged to attract the dust particles. The Trion air cleaners listed on this site work by this principle.

Pros

Cons

The collector plates are easily removed and cleaned, therefore there is not a requirement for ongoing replacement filters.
They should be less noisy that air cleaners with mechanical filters as the fan does not have to work so hard to pull the air through the machine

Without special suppression circuitry they can be prone to make a crackling noise particularly when the collector plates are dirty. Trion units have special suppression circuitry to prevent this.

Electrostatic filtration (type B)

These are basically mechanical filters with a twist. The fibres are synthetic and they therefore can hold an electrostatic charge, which means that the interception effect is vastly increased. Some electrostatic cleaners use a glass fibre media between charged meshes. The charges meshes apply a static charge to the dust particles and these then are more efficiently trapped by the glass fibres.

Pros

Cons

The filtration efficiency of a mechanical filter can be matched with less media, therefore slightly smaller aircleaners.
The filtration efficiency of a mechanical filter can be matched with a more open structure of media, therefore less fan power required to overcome the media and less noise

The media will over the life of the filter discharge as the fibres become covered in trapped dust. Theoretically this matting of dust increased the surface area of the fibres and decreases the porosity of the media which means that the electrostatic efficiency loss is offset by an increase in efficiency by mechanical filtration.

Carbon filtration

Carbon is deployed in a filter in an ‘activated’ state. This means that the pore structure inside the carbon has been massively enhanced to open up all the pores and to increase the internal surface area of the carbon. Larger gas molecules tend to be attracted to carbon surface in the pores, by what are known as Van der Walls forces. Activated carbon is usually in the form of small granules which are used to make the filter either as a granular biscuit or sandwiched between layers of other filter material. As the air passes through the filter, the contaminant molecules are attracted and held (adsorbed) by the carbon.

Pros

Cons

Carbon filters can be very good at removing smells, airborne chemicals like ozone, volatile organic compounds and various oxides of sulphur and nitrogen

Carbon will prefer to adsorb heavier molecules to light ones. Where the carbon is coming to the end of its life as a filter, light molecules can be displaced by larger ones (preferential adsorption)
The smaller the carbon filter the lower the filter life will be. The cheaper the filter/air cleaner the smaller the carbon quantity
The efficiency at removing particles will depend in part on the efficiency of the substrate materials supporting the carbon
Often carbon filters in domestic air cleaners have so little carbon that they are more or less useless, and are included as a gimmick feature.

Ultra Violet Light. (Sometimes known as photo plasma or light plasma)

UV light is very good at inhibiting growth of many micro organisms and fungi. Moreover sufficient doses of UV light (intensity x exposure) at the right wavelength can break down mould, germs, viruses, bacteria and spores in the air.
In UV based air cleaners, air is moved (usually with a fan) through a UV chamber that comprises of UV lamps, and reflective inner surfaces to the chamber. The intensity and time within the chamber will determine the effectiveness of the air cleaner. It is in this chamber that the ‘photo plasma’ is created by the UV. The plasma has the effect of removing electrons from atoms (electron deficient atoms). This creates negative electrons, neutral atoms, and negative ions. These atoms collide with each other and form hydroxyl radicals trace ozone, and oxygen with no unpaired electrons (singlets). This removes many impurities from air by changing their molecular structure. The plasma chamber effectively breaks the contaminants to water, oxygen and carbon.

Pros

Cons

Very effective within its remit of reducing airborne biological contaminants (allergens, viruses, bacteria and smells) and chemical contaminants (cleaning chemicals, odour masks, benzene and other VOCs),
Very little pressure drop there for low fan power requirement.
Very quiet
Can be used in systems in combination with filters or in ductwork
Some devices bases on this technology are designed to produce a low concentration of ozone. Ensure that the concentrations does not exceed 0.04 ppm (parts per million)
The photo plasma has the effect of imparting charge to dust particles and this makes dust drop our of the air to the floor. This is not as an effective way of air cleaning as a filter based air cleaner

Dirt can build as a layer on a UV bulb and reduce in intensity of UV emitted, which will have a marked effect of lowering the efficiency.
The bulbs are very sensitive to be touched and have to be handled very carefully. This means that cleaning can only be done with a soft cloth and some alcohol. They do require replacement from time to time.
Obviously there is a power requirement to the UV lamp as well as the fan
Watch out for strange smells being produced by some units which could be some of the materials of construction reacting to UV, or the creation of Ozone. The smells on the first start up usually go away as the appliance settles down.
Beware of filter based cleaners that have UV lamp added on as an afterthought. Typically the UV aspect of the machine is useless and will not have the intensity and exposure to air that it needs to be effective. More gimmick than air purification.

Incinerators

Whilst the use of industrial scale incinerators is widespread for destroying air borne contaminants in factory process air streams, its use is rare in domestic air cleaners. The air enters the unit at the base and rises through heated ceramic honeycomb matrix. The hotter the air the more complete the destruction of allergens. Air cools before passing out of the top of the unit.

Pros

Cons

Quiet as there is no fan
Given sufficient time would significantly reduce odours, dust, viruses, bacteria and spores in a small and perfectly sealed room.

Typically these units are so small and have such a minute airflow that they take a very long time to treat the air in a room. As soon as a door or a window is opened the effect is lost. Ideal for life sealed from the outside world. Not suited to normal life
They rely on convection of air through a small heated element, and do not have a fan. They never state an air flow. If they did it would be miniscule compared to proper air cleaners
Availability of spare parts and costs

Ozone

With ozone based air cleaners the unit will generate ozone with is a molecule comprising of 3 oxygen atoms. Ozone is a colourless gas which when it comes into contact with contaminants like mould and mildew, pollen, odours, or other airborne bacteria, one of those atoms combines with the molecules making up the pollutants rendering them inactive by oxidation, resulting in more stable version of oxygen, O2.

Pros

Cons

Effective and quiet
Effective at odour reduction

The smell of ozone is quite distinctive (like an old laser printer) Watch out for using systems that generate more than 0.04 ppm of ozone, as these are likely to smell and may be likely to produce ozone concentrations above recommended exposure limits. A unit which does smell may be located in a room that is smaller than it was designed for. Too much ozone is actually quite bad for you.

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