Shopping in safety: based on the American malls, shopping centres have flourished in the last ten years, with their success based on having a wide range of shops to suit everyones' needs, all under one roof. With furious competition between the shopping centres to maintain their popularity, providing a pleasant and safe environment is of paramount importance - Passive Protection

With a multitude of shops containing substantial value of stock, and with thousands of visitors present at any one time, the provision of adequate protection against fire is vital. Life safety has to be the primary concern, and ensuring customers can be evacuated in the event of a fire safely and quickly is of overriding importance. However, providing adequate protection to such large areas is complicated by a number of issues and it is worth considering how these can be best overcome.

Modern buildings are commonly built around a central atrium to provide a bright airy atmosphere to the occupied spaces. In multi-storey buildings such as shopping malls there is the potential for smoke and combustion products to enter the atrium from many sources such as shops, cafes etc. The two main reasons why it may be difficult to reliably detect smoke are that there is usually a limited number of positions available to mount smoke detection systems without spoiling aesthetic appeal and more significantly, the problem of smoke not reaching the detector height or location for reasons dealt with more fully below.

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 smoke cools to a temperature equivalent to the air, it stops rising and starts to spread horizontally. The exact height at which stratification occurs is dependent primarily upon the temperature gradient between the fire and detection locations.

On a hot summer day, the temperature at the top level in 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. A system capable of taking a sample of air from multiple heights within the area would therefore be advantageous. If stratification is anticipated, the sampling points may usually be positioned at multiple heights within the area--thus ensuring detection from any stratification level.

DETECTION METHODS

Traditionally, infrared beams have been installed to protect atria but they have become less popular recently, as Chubb Fire Projects Manager Terry Basleigh, explains: "The growing realisation is that they are rarely at an ideal height to cope with differing stratification heights and that access for maintenance can be problematic. This latter situation is compounded if the building is open to the general public, since the maintenance may have to be undertaken at premium rate times, adding extra cost to the client. Another problem is that infrared beam detectors rely on stable mounting points. Modern steel frame buildings flex in response to solar heating and wind pressure. This flexing can cause the beams to go out of alignment. Equally, `black heat' radiators are often used to heat these buildings and are installed at a high level. This can cause beam detectors to provide unwanted alarms." So what is the solution?

ASPIRATING DETECTION

A method for providing a degree of protection in atria that a few years ago would have been unthinkable is the modern high sensitivity aspirating smoke detection system. These are small localised cabinets protecting up to 2,000[m.sup.2] of space, located in positions convenient for maintenance, that draw a sample of air through a system of small diameter--monitored sampling pipes that are strategically positioned throughout the protected space.

The major benefit of this type of system is their extremely high sensitivity, which in turn can provide very early warning of incidents. In comparison with a traditional point type detector, the most advanced aspirating detectors can be more than 1,000 times more sensitive. These detectors would typically be networked with other detection devices used in less problematic areas, thereby adding to existing systems, rather than replacing them. Conventional point-type detectors can effectively protect lower-risk areas and are appropriate for protecting smaller areas in shopping centres and shops themselves and should be installed in accordance with applicable codes, standards and regulations.

The secret of being able to apply true high sensitivity aspirating detectors in an environment with a fluctuating level of background smoke (caused by changes in ventilation systems for example), is that the system has to be capable of making continuous adjustments to ensure that the level of sensitivity is never inappropriately high. Equally, sensitivity has to be sufficiently high to give reliable warning of incidents that if left unattended could develop into a fire. Therefore highly sophisticated perceptive artificial intelligence (PAI) algorithms have been created, contained in the detector, which are capable of continually interrogating the output from a sensor to maximise useable performance.

Some detectors accomplish this by continually monitoring the ambient conditions for noise (normal fluctuations in ambient smoke density), which allows the system to become more sensitive if installed in a quiet location. If the location becomes noisy, the sensitivity automatically decreases but will never lower itself outside of its approved range, providing consistent protection. The system is also capable of making immediate sensitivity switches, with no external input, when it learns to recognise changes such as day/night. This technology provides the highest possible sensitivity with the minimum risk of unwanted alarms.

ATRIA APPLICATION

The image shows how an aspirating system can be installed using both horizontal and vertical runs of sampling pipe to protect the space under virtually all circumstances.

It can be seen in this example that the detector has the ability to sense smoke over the entire atrium height to capture any eventuality for smoke stratification. By using an appropriate material or finish for the sampling pipe it can usually be blended with the architectural features. For aesthetic purposes it is important to note that the rules and recommendations governing the position of detection remain unchanged.

This means that the system designer may have to devise a system that fulfils the spirit of the rules and regulations yet remains unobtrusive. Aspirating systems give the designer the ability to install systems that are efficient, practical, cost-effective and easy to maintain.

Peter Fox is Managing Director of AirSense Technology Ltd, based in Hitchin, Hertfordshire, with associated companies worldwide. Peter was the first person in Europe to be involved with high sensitivity smoke detection systems. www.airsense.co.uk

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