Going large

04 May 2022

The protection of large premises from fire presents a specific challenge for system designers. The first issue to address is the rapid and reliable detection of smoke. Experience has shown that optical beam smoke detectors offer an excellent solution, says Nigel Ward

LARGE INTERNAL spaces – including airport and rail terminals, sports centres and warehouses – play a hugely important role in modern society. Warehouses and distribution centres in particular have become central to the UK’s economy in recent years due to the rise in popularity of online shopping and home delivery services. The latter, of course, have accelerated even further during the COVID-19 pandemic.

Large-scale buildings of this nature present particular challenges for those designing the fire protection systems that safeguard these vital assets. Detection of a fire is obviously a critical step in the safety strategy. Any fire detection option must address a number of key requirements if it’s to be widely adopted. These are the rapid detection of a fire anywhere within the space, the avoidance of false alarms and a solution that’s straightforward and cost-effective to both install and maintain.

Taken together with the level of ongoing activity within them (which is often quite considerable), the sheer size of large-scale premises merely serves to magnify the challenge in meeting each of these requirements. Smoke from fires will disperse into a large volume of air. It follows that smoke detection must be highly sensitive without causing spurious false alarms. The coverage needed implies a complex network of detectors that could be costly to install.

Finally, the detection systems employed must be accessible for maintenance without unduly affecting day-to-day operations at the premises. Any disruption to transport hubs and warehouses, for example, can often lead to costs for the host organisation rising at a truly frightening rate.

Meeting the challenge

Optical beam smoke detectors have proven their worth in meeting the challenge of fire detection in large premises over many years now. A beam of light (usually in the infrared region of the spectrum) is projected across an open space between a transmitter and a receiver. When smoke passes through this invisible beam, the solid particles and liquid droplets in the smoke reflect and scatter the photons of light. 

The result is that the intensity of the light is reduced on the far side of the cloud of smoke. Known as obscuration, this reduction in intensity can be detected and used to signal a fire.

The detection of smoke by obscuration is the principle behind many types of smoke detector. The most significant difference between the available technologies is how the protected area is monitored. 

The obscuration effect in beam smoke detectors occurs equally at all points along the beam path, no matter where the smoke passes through the beam, such that the detector behaves effectively as though it was made up of an infinite number of point detectors arranged along a line. This renders optical beam smoke detectors ideal for protecting long distances (up to 120 metres for a single beam, in fact), provided that the beam is completely unobstructed and correctly aligned with the detector (more of which anon).

There are two main configurations of optical beam smoke detector: reflective and end-to-end. Both consist of a transmitter ‘T’ (the light source) and receiver ‘R’ (the detector). Each configuration has advantages and disadvantages. Reflective detectors are easier to install, align and maintain as access is only needed to a single device at one end of the protected space. As a result, they’re more widely used than the end-to-end variety, which is reserved for those applications where the available space for the beam is limited. This is because reflected beams diverge further than end-to-end beams so are more likely to be obstructed by the building’s structure or its contents.

Applying Best Practice

Although highly effective, beam smoke detector systems still require careful planning if they are to detect fires rapidly and reliably. In the UK, BS 5839 Part 1: Fire Detection and Fire Alarm Systems for Buildings – Code of Practice for the Design, Installation, Commissioning and Maintenance of Systems in Non-Domestic Premises encompasses automatic fire detection systems. This document recommends that smoke detectors are mounted quite close to ceilings to take advantage of the accumulation of the smoke plume that occurs there during a fire scenario.

However, the plume also disperses as it rises, leading to a reduction in the concentration of the smoke particles. The British Standard therefore stipulates a maximum height for different types of detectors, depending on their sensitivity. The long sampling length for the projected beam allows low concentrations of smoke to be detected. As such, the maximum height permitted for beam detectors is greater than that for point detectors. This can be a distinct advantage in very tall buildings.

Another reason for mounting detectors at height is that the beams are less likely to be accidentally obstructed by operations within the protected volume. In some circumstances, such as glass atriums, the space immediately under the ceiling may become quite warm and prevent smoke from rising to the ceiling surface. In these instances, the beam can be angled to allow the stratified smoke layer to be detected.

Other recommendations in BS 5839 Part 1 include the maximum spacing between detectors and minimum distances from vertical surfaces (ie outer walls, internal partitions and racking, etc). These recommendations help to ensure that the airflow which carries the smoke to the detector isn’t impeded or unduly delayed in any way, thereby enabling the rapid detection of a fire. 

Even when Best Practice techniques are applied, it’s important to note that all detection technologies have their disadvantages. Beam detectors are no exception to that rule. However, recent innovations in solutions now available on the market have purposefully addressed previous limitations.

Maintaining alignment

Perhaps the biggest constraint with beam detectors when compared to other smoke detection methods concerns the alignment. The beam must be correctly aligned between the transmitter and the receiver at all times if smoke is to be reliably detected. It’s therefore essential to mount the detectors on solid structural surfaces within the building. Ideally, those experiencing minimal movement due to temperature changes and vibration, etc. This allows the beam to be accurately aligned during commissioning by adjusting the beam direction to maximise the signal measured at the receiver.

Some products now offer an automatic alignment feature which greatly assists the installation engineer. For example, the Fireray One reflective beam detector uses motorised mounts to adjust the direction of the beam very precisely. The engineer makes use of an integrated laser to point the detector towards the reflector, after which the system aligns and centralises the beam on the reflector, and then subsequently adjusts the beam power and receiver gain in order to optimise the signal.

This process can – and should – be repeated at regular intervals to check that the beam remains in alignment. Large temperature swings between day and night or during different seasons of the year can cause expansion or contraction of the building’s structure and may well lead to the beam becoming misaligned. New foundations will also experience a degree of settling in their early years for which automatic alignment can duly compensate, at least to some extent.

In days gone by, high levels of ambient light present in the vicinity of the beam have caused some issues with beam detectors. These can arise due to the presence of skylights in glass atriums, for example, or if reflective surfaces are located close to the beam path. Modern buildings often include automatic blinds or louvres to control light levels and regulate heating within. These can change the reflectivity of surfaces close to the beam.

The end result can be that light which isn’t from the beam is measured by the detector. That not only leads to false fire alarms but also, in some instances, to a lack of response to an actual fire due to saturation of the receiver. Both of these outcomes are highly undesirable as they can serve to endanger the occupants and fabric of the building or otherwise become such an ongoing nuisance that the operator loses faith in the fire detection system altogether.

Many products include a filter to remove unwanted portions of the signal, but that isn’t always an effective strategy as this can also remove some of the signal needed for smoke detection. An alternative approach is to subtract the background light signal to extract the infrared beam signal in its entirely. This can then be analysed. 

Such an approach, which has been patented by ourselves and is known as Light Cancellation Technology, has been shown to prevent faults or false alarms in those systems where light levels can be prone to vary quite widely.

Additional coverage

Although reflective beam detectors are able to protect distances of up to around 120 metres, there are some premises where this isn’t far enough and a ‘face-to-face’ configuration is necessary using a central double-sided reflector. Further, the coverage of more complex building geometries might require beams that cross each other.

Historically, such situations have realised some difficulties when it comes to system design because the divergence of the beams can cause a signal to be recorded at a separate device, leading to false alarms or otherwise spurious fault warnings. Previously, the only solution was either extra shielding around reflectors with extensive on-site testing to confirm the design’s efficacy or the use of point detectors instead.

An innovation introduced to tackle this issue has been built into the Fireray One reflective beam detector. Called Dynamic Beam Phasing, the process uses a built-in algorithm to create a virtually unique pattern for the pulses which make up the beam from every detector. This means that the pulses reaching a receiver from a separate beam will have a different pulse pattern to those of its own beam. The detector will ignore any pulses which do not fit the expected pattern, with the overall result that beams can be positioned to give the best possible coverage for the application without causing any faults or false alarm scenarios.

The importance of protecting large premises has never been clearer, while optical beam detectors have been a key part of the solution for such buildings across many years now. Recent innovations have tackled long-standing constraints with the technology and, pleasingly, look set to increase the range of applications that can be diligently served by these invisible beams.

Nigel Ward is Technical Sales and Support Engineer at FFE 

For more information:

Tel: 01462 444740