Preventing fires with aspirating detectors

It will be obvious to any installer of automatic fire detection systems that there is a wide range of differing building types in existence. Each individual building will usually have its own layout, style and function. It is therefore no surprise to realise that it is impossible to have a single smoke detection system type that would be ideally suited to them all.

Choosing the most effective smoke detection system for a specific building can be a confusing and troublesome business. One building can differ tremendously from the next, whether by its shape, its occupancy, the construction materials, the type of assembly or the air conditioning system.

All too often fires go undetected or are identified at a late stage because a building is installed with an inappropriate detection system. There are other factors.

What about the location? Is there an adjoining building? How long will it take emergency services to respond? On top of this, what is inside the building must be considered. Is it a manufacturing plant that uses combustible materials? Is it a historical building or museum with irreplaceable contents? Or is it just full of people? These are just a few of the questions that any installer must take into account. They all require different solutions, placing pressure on the installer to select the most effective detection system.

Early detection
The most widely used detection technology is the traditional point-type detector. These can provide an adequate level of protection if they are installed in the appropriate environment. While they are suitable for protecting many standard installations, they will not give a high level of protection for many building types: warehouses; those with high ceilings; areas with high airflows, such as telecomms or computer rooms; dusty/dirty areas; areas with very high levels of radio frequency; very hot or cold areas such as cold stores; or areas that need true incipient (very early warning) fire detection.

Point-type detectors are also considered by some to perform inconsistently in areas where the normal environmental smoke density fluctuates between day and night. This can be due to standard processes that may, for example, produce smoke, such as kitchens, soldering areas, furnaces and standby generators.

Traditional point-type detectors commonly have a sensitivity value of between 3-8% obscuration per metre (obs/m). If this is compared to some types of aspirating smoke detector, these can be capable of signalling changes in smoke density of 0·003% obs/m. This is literally a thousand times more sensitive and thereby provides much earlier warning than other systems.

What may be surprising is that despite a substantial sensitivity advantage, some types of aspirating detector are highly suitable for the protection of the problematic environments described above.

Creating a nuisance
Experienced fire alarm installers are only too aware that many businesses have been subject to nuisance alarms when an inappropriate detector is installed and cannot operate effectively.

Installing a point or beam detector may be thought suitable where the requirements are relatively simple or the area is considered low risk. However, in a warehouse or factory, where there is constant manufacturing and distribution work taking place, inappropriate detectors have been the source of many nuisance alarms.

The Sony Distribution Centre in Thatcham, Berkshire had been using beam detectors for several years and was suffering three to four false alarms per month. Staff were frequently being evacuated and this was having a significant impact on production.

The main reason for these nuisance alarms was that the detectors were often having their beam's line of sight impaired by wind loading on the building, thermal expansion or stock being put in front of them.

To remedy this situation, Sony looked to a sophisticated aspirating smoke detection system developed and manufactured by AirSense Technology of Hitchin, Hertfordshire. The AirSense system is known as Stratos-HSSD. It is capable of detecting even incipient fires and consequently provides more than adequate early warning to any potential incident.

Since installing a Stratos-HSSD system, Sony has managed to detect two potential fires in the form of electrical overloads. There have been no nuisance alarms.

Nuisance-free alarms
Aspirating smoke detectors usually draw a continuous sample of air through a system of sampling pipes or directly from an air conditioning system. They are available from several manufacturers and range in sensitivity from 'ordinary' to 'very sensitive indeed'. Operation at sensitivity levels above ordinary level is unsurprisingly associated with nuisance alarms. This need not be the case.

The Stratos-HSSD system contains within it sophisticated perceptive artificial intelligence algorithms called ClassiFire. This process allows the detector to literally think for itself. It is capable of continually modifying the system's sensitivity so that it maximises usable performance, while not allowing it to use inappropriately high sensitivity. Further, it allows the system to make major and instantaneous sensitivity changes, based upon what it has learned about its operating environment. This can be of use during night-time or holiday shutdown periods.

Further advantages of the system are that it minimises the need for skilled or specialist knowledge on the part of the installer. It also provides compensation for the highly negative effect of contaminated air filtration systems used on some aspirating smoke detection systems, which otherwise are known to negatively effect sensitivity.

Foreseeing stratification
A major consideration when designing a smoke detection system for a large, tall space is smoke stratification.

Smoke rises because it is hotter than the surrounding air. When it cools to a temperature equivalent to the surrounding air, it stops rising and starts to spread horizontally – a process termed stratification. The height at which stratification occurs is dependent primarily upon the temperature gradient between the fire and detection locations. For example, on a hot summer day the temperature at the top of a glazed atrium may be surprisingly high and consequently the stratification level would be relatively low. At night or during cold periods, the stratification level would be higher within the space or probably non-existent.

Ventilation, either fixed or forced, will also have an effect on the stratification level. A system capable of taking a sample of air from multiple heights within the area therefore becomes increasingly advantageous.

If stratification is anticipated, the sampling points are usually positioned at multiple heights within the area, thus ensuring detection from any stratification level.

Maintaining aesthetics
In many situations, smoke detection systems would ideally be invisible. Aspirating systems provide the possibility in many situations to install detection systems that are virtually impossible to see.

Routing main sampling pipes above ceilings or behind facades, with only virtually invisible, small diameter, capillary pipes protruding into the protected area, will usually achieve this. The detectors consist of small cabinets that are capable in some circumstances of protecting up to 2000 m2. These can be conveniently located and hidden in positions that are easily accessible for routine maintenance.

Being prepared for fire
It is clear that in order to select the most efficient and appropriate smoke detection system there are more factors to be considered than might be immediately thought.

Installers should consider all aspects of the building and its environment, and which of those will impact the functionality of the detection system. Aspirating smoke detection is a perfect solution to most troublesome environments.

The benefits of installing such a system can be substantial. Gains in performance will normally be massive and savings may be expected in terms of their inherently low cost of ownership. The detectors are also able to readily interface with existing systems making them generally a sound investment.