Aspirating smoke detection may be a straight forward concept but the balance between early detection and false alarms depends on getting the details right. Peter Fox guides us through the main points.
One of the prime factors in detecting fire is undoubtedly time. The earlier a fire can be detected, the less damage will be caused, a point clearly illustrated by the well-known fire development graph. However, it is important that the focus on speed of detection does not overlook other factors. In particular, those considering fire protection systems that offer ‘high sensitivity’ levels should always be mindful of the potential for unwanted alarms. When you consider that in the UK alone, unwanted alarms are estimated to waste some GB pound 105million in fire service resources, coupled with the fact that frequent unwanted alarms create complacency on the part of a building’s occupants, it is clearly evident that getting the balance right between early detection and a ‘true’ alarm is vitally important.
Aspirating smoke detection, also widely known as air sampling, is a method of detection where a sample of air is drawn from the protected area through a simple arrangement of sampling pipes to a laser detection chamber. The performance of such systems is generally not comparable to conventional point detection, which is considered suitable for most ‘standard’ applications, where cost and basic compliance to regulations are the main drivers. In applications requiring an earlier degree of detection, or those in which point detection is just not suitable (for example in areas with high levels of dirt, RF interference, high humidity etc.), aspirating smoke detection (ASD) often offers an effective solution. The seven main criteria by which it scores highly are:
– Very early warning is required
– High air flow present
– Hostile environment (very cold, very hot, humid, dusty or subject to strong RF fields)
– Detection is to be covert (for example in heritage buildings)
– Where building height will cause smoke stratification
– Where access for detector maintenance is impractical
– Where ‘co-incidence’ detection is required for automatic extinguishing system release
Early warning
It is important to recognise, though, that while several of the laser based aspirating smoke detection systems can operate at sensitivity levels sufficient to detect ‘incipient’ fire, there are also systems available which employ conventional ‘point’ type detection in an aspirating enclosure. These offer advantages in some fire detection situations – most notably in high airflow situations – but they lack sensitivity to detect fire at a very early stage.
Some high sensitivity aspirating smoke detectors feature artificial intelligence, which automatically maintains appropriate performance by making small and continuous ‘tracking’ adjustments to system sensitivity. This allows the variation of alarm thresholds in case of environmental change, or indeed, according to the degree of cleanliness of the detector’s air filtration system.
Electronic data processing environments are one of the prime applications in which ASD technology is normally employed. Such areas pose a number of unusual problems to the fire engineer, particularly the need to detect even the tiniest ‘incipient’ fires. In an era in which computers are fundamental to so many businesses, data processing areas have effectively become the hub around which many organisations operate – banks and telecommunication facilities being obvious examples. Without this critical hub, such organisations cannot function. These environments typically employ ‘close control’ air conditioning and recirculation systems to remove the heat generated by such equipment. But these air conditioning systems also prevent relatively cool ‘smoke’ from small incipient fire situations from being able to enter point type detectors.
Furthermore, combustion products generated by a fire in an environment containing substantial quantities of synthetic materials, such as PVC, can be very harmful to delicate electronic equipment. Very early incipient fire detection in this type of environment can also remove the need for costly and potentially damaging discharge of fire extinguishing gas.
Flexibility
It was recognised many years ago that the relatively cool smoke produced by the early stage of electrically initiated fires travels on the air currents generated by the air conditioning system, rather than being able to rise by thermal buoyancy to the ceiling, where conventional type smoke detectors are installed. This led to the development of ‘primary detection’ using a high sensitivity aspirating detector taking a continuous air sample from the airflow returning to air handling plant. Rather than working against it, rapid air movement works in favour of the detection system, carrying any diluted smoke particles on the airstream.
Primary detection offers an extremely efficient method of protecting an area subject to high air movement. But if the air handling system is switched off, detection will suffer – any smoke will rise to the ceiling in the normal way. Consideration should therefore be given to installing secondary detection either through conventional point detectors or, if high sensitivity is required, through additional aspirating smoke detectors positioned on the ceiling and in any void spaces considered a risk.
Returning to the subject of unwanted alarms, the very nature of aspirating smoke detection systems means that detection is set at a much higher sensitivity than normal – so it is even more important that the technology is designed with this in mind. Some high sensitivity ASD systems employ electronic laser dust discrimination, others use filtration, and at least one uses both technologies, to provide a system virtually immune to alarms caused by most dust particles.
In environments where unwanted alarms can be caused through external pollution being introduced into the protected area, a ‘reference’ detector system can be used to monitor the pollution density of the air. The signal from this detector (or a proportion of it) is then ‘subtracted’ from that from the detectors used to protect the area, so rendering them ‘insensitive’ to externally introduced smoke.
Unlike conventional detectors, aspirating detectors do not actually need to be located inside the area to be protected. So air sampling pipes can be installed in areas subject to extremes of heat, cold and/or humidity, transporting the air to the detector located outside the area. ASD is therefore widely used in cold stores, food preparation areas, textile dying areas and tobacco plants.
A good example of a current application for ASD is demonstrated at a major UK waste recycling facility in Luton, Bedfordshire, which has seen a reduction in dust caused nuisance alarms from up to five a day to zero by replacing a poorly designed point detection system with a modern, high sensitivity ASD system. Other good examples include textile manufacturing areas, paper mills, flour mills, wood recycling, metro tunnels, cable tunnels and coal conveyors.
Manufacturers of aspirating detection systems have long recognised their systems’ relevance to the petrochemical and related industries. The attractiveness of an ‘Ex’ certified ASD system, capable of offering fire ‘prevention’ as opposed to fire ‘detection’ will be obvious. Yet until recently the requirement for hazardous area certification effectively excluded ASD from such areas. Now, however, a fully certified ‘Ex’ version of an ASD detector, suitable for continuous use in an explosive hydrogen atmosphere, is available.
Covert detection
Aesthetics are often a prime consideration in buildings with ornate interiors, such as cathedrals and palaces, while vandalism of detectors can be an issue in prisons. In both instances, ASD can provide the solution with the fire detection equipment effectively made invisible, through the use of capillary tubes to carry the air sample to the pipework. The capillary tubes can be flush terminated, or, because they are so small, easily concealed in ceiling features such as light fittings.
Smoke ‘stratification’ is the term used when smoke has insufficient buoyancy to reach high ceilings. This affects buildings with high ceilings, and can render ‘point’ detectors ineffective – unless fire has reached the conflagration stage. Warehouses, distribution centres, hotel lobbies, atria, stairwells, lift shafts and aircraft hangars all fall into the category of building where the potential of stratification should be considered.
Maintenance at such heights can also prove costly and difficult,and other inaccessible areas, such as ceiling and under-floor voids or escalators, can equally benefit in this and other aspects of ASD.
Codes of practice on the release of automatic gaseous fire extinguishing systems, such as FM200, call for two levels of detection, formally known as ‘co-incidence’ release but commonly known as ‘double-knock’, before the extinguishing gas may be released. This is to reduce the probability of an unwanted release, and to allow people time to evacuate the area. The early warning from some ASD systems allows them to be used to provide the first level of detection with point detection, or a lower sensitivity ASD system. providing the second.
Coming of age
Aspirating smoke detection is no longer a new technology. It has been tried and tested in many varied applications. Although it continues to be widely used in IT and telecommunications environments (the ‘core’ applications for which the technology was originally developed) it is now seen in a whole host of situations – from heritage buildings and retail complexes to clean rooms, flour mills and even ‘high end’ residential premises. The fact that ASD provides such an early warning at the incipient stages of a fire means that it can effectively be viewed as preventing rather than merely detecting a fire. With conservative estimates for fire losses in the UK alone being some GB pound 7.7 billion a year, the need to minimise damage, reduce downtime and prevent harm to people or the environment is abundantly clear.
Peter Fox is MD of Airsense Technology in the UK. www.airsensetechnology.com
