Feature34

Filtration+Separation January/February 2013

Practical filtration:

Improving indoor air quality

Probably the largest number of air filters is found in the systems controlling the quality of the air in living accommodation (domestic, commercial and institutional) and in working spaces, especially the growing number of clean room installations for critical assembly processes. These are now being supplemented by the vent filters controlling discharges from working spaces in which potentially hazardous atmospheres are used.

Grouped under the term ‘air conditioning’ or HVAC, these filters are the basic means whereby improvement is achieved in the indoor air quality. It is now believed that ‘sick building syndrome’, a condition that

There are many factors in selecting the correct filter to improve indoor

air quality. Filtration+Separation highlights a few aspects to take into

consideration.

Air conditioning is widely used in improve indoor air quality.

causes people to feel sick inside a certain building only to recover on leaving it, is an air conditioning problem, likely to be cured by better filtration.

Common air contaminants come is a wide range of different sizes, from heavy industrial dust ranging from around 100-10,000 microns, down to gas molecules at a fraction of a micron. Filters designed for the treatment of air fall broadly into three categories.

Primary filters

Primary filters are designed to trap the majority of larger airborne dust particles of

5-10 microns in size, and have high dust-holding capacity. These are usually of the dry panel type or roll filters, capable of working with relatively high airflow velocities.

Second-stage filters

Second-stage filters have finer media for trapping and retaining finer particles passed by the primary filter, such as particles of 5 microns diameter and smaller. These smaller particles (0.5-5 microns) are the most damaging as regards staining of décor in buildings and harmful effects on machinery, for example. These filters may be of the unit or panel, pocket or bag type, with extended depth of filtration. Maximum air velocities are generally low, of the order of 0.12 m/s or less.

Ultra-fine filters

Ultra-fine or final stage filters yield very high efficiencies – 99.95% or better – even with sub-micrometre particles. The main types in this sector are the high efficiency particulate air (HEPA) and ultra low penetration air (ULPA) filters. These employ a high density medium built up from synthetic spun fibres with a sub-micrometre diameter and made in the form of a closely pleated pack. Air velocity in this case is limited to about 0.03 m/s.

Electrostatic precipitators also come into this last category because of their capacity for ultra-fine dust filtration, although they can operate with much higher air velocities.

A single-stage system using a low or medium efficiency filter has the lowest capital cost, but will not filter very fine contaminants. A multi-stage system will filter virtually all atmospheric pollution but at higher capital investment. The filter cost itself will be low, because the cheaper primary filters prolong the life of, and protect, the more expensive filters.

FISE0113_feature IAQ 34 06-02-2013 12:51:04

Feature 35

Filtration+Separation January/February 2013

However, attention must be paid to the consequences of installing inadequate air handling filters, which will allow fine particles to enter the air handling system. Over a period of time, airborne contaminants will build up in the system to create a potential hazard. While the correct filter selection is important, proper system maintenance is equally important. Clogged filters impede airflow and damaged filters contribute to a loss of efficiency, ultimately damaging the air handling and distribution equipment, allowing dust and pollutants to circulate. A multi-stage filter system will guarantee clean air, so long as it is kept in good order.

Air filters and other types of air cleaning equipment can be grouped together as follows:

cartridge filters – mainly used as engine • intake filters and filters for compressed air systems.pad filters – disposable elements made from • flat pads of thick fibrous material mounted in frames or panels.panel filters – unit filters (including pads) • of various media forms, which may be disposable (for example, by using synthetic fibre or spun glass media), washable (by using polyurethane foam or a similar material), cleanable (for example, woven metal wire mesh cleaned by immersion in an oil bath) and non-combustible.viscous panel filters – employing screens or • media wetted with oil, or dry fibre coated with an adhesive gel.roll filters – which are basically panel filters • with the filter medium automatically fed through the panel frame from a clean roll on one side to a dusty roll on the other.rotating viscous panel filters – in the form • of a continuous curtain loop of metal slats or something like it, automatically rotated across the frame, and passing through an

oil bath (the oil acting both as a viscous impingement collector and as a cleaning agent. bag or pocket filters – normally arranged • as a group across the panel frame and extending through the dividing wall, to provide high efficiency filtration with high dust retention.particulate air filters (HEPA and ULPA) • for final stage filtering.electrostatic precipitators – which may • be of the dry type (agglomerators), or have the plates periodically cleaned by water washing (in the dry type, the dust is collected in filter bags or a separate downstream filter).louvres – an aerodynamic type of separator, • which also has a capacity for collecting liquid mist particles.separators – various types of equipment, • working on aerodynamic principles, andscrubbers – wet or dry separators using • liquid sprays or packed beds of granular solids.

Classification

Air filters are classified according to their filtration efficiency, when measured under defined standard conditions. There is no single international standard for such classification, but there are a few national and regional standards that have been brought together by CEN (Comité Européen des Normalisations) and EUROVENT (European Committee of Air Handling & Refrigerating Equipment Manufacturers), as well as in the United States by ASHRAE (American Society of Heating, Refrigerating and Air-conditioning Engineers).

The classifications are considered to be a proven method for designing air filtration

media according to average synthetic dust weight arrestance and average dust spot efficiency, although it should be stated that the whole test situation is complicated by not only the diverse test procedure but the diversity of particulate materials specified for them.

Glass fibre media used to be favoured because they were produced with a graduated matrix to provide very high dust retention capacities. However, due to the possibility of element migration (the more likely with chopped strands than with continuous filaments), the trend is now towards the use of non-breaking, non-shedding synthetic organic fibres. Synthetic air filter media are progressively structured, high in density and performance, nonwovens, made from synthetic fibres, usually thermally bonded, or resin bonded, with an adhesive coating in full depth on each individual fibre.

Synthetic fibre air filter media can be used in most atmospheric air or recirculated air filtration applications, and are particularly suited for the ventilation of offices, factories and public buildings, for air handling in hospitals, computer rooms, schools, airports, pharmaceutical and food processing plants, clean rooms and laboratories.

Air filter selection

Air filter units should be selected on the basis of the following items:

level of filtration needed,• filter classification requirements,• cost-effectiveness,• stable collection efficiency, and• dust storage capability at a low pressure • differential level.

Particular points to be borne in mind during the selection process include:

the air flow should be as uniform as • possible across the face of the filter,pre-filters should be used with high • efficiency fibres to give longer service life,if the system draws in air from an external • wall, then weather louvres and bird screens should be fitted at the intakes,a differential pressure drop gauge should be • fitted across the system to determine when a filter should be serviced,sufficient access should be provided for • servicing the filters,filters should not be used beyond their • specifications,the recommended final resistance should • not be exceeded,electrostatic air cleaners should not be • installed where free moisture can affect them,the selection of air filters on the basis of • lowest cost is not a reliable option in any system design, future needs should be borne in mind, andsystem requirements should be fully • discussed with equipment manufacturers.•

Synthetic fibre air filter media are employed for the ventilation of offices.

FISE0113_feature IAQ 35 06-02-2013 12:51:07