Efficient and effective wide-area fire detection
Large, wide-area indoor spaces present a challenge to traditional fire safety systems: in order to effectively detect smoke over such a space, complex networks of multiple overlapping sensors will be required. Optical beam smoke detectors, on the other hand, are designed exactly for such situations – one single unit installed on a wall can detect smoke over an area of up to 1500m2 (BS5839) or 19,800 sq ft (NFPA 72). More coverage per detector means fewer detectors, with associated reductions to the time and cost of installation and wiring, as well as a lesser aesthetic intrusion. Mounting on the wall as opposed to the ceiling enables convenient access for maintenance, and a low-level controller further speeds and eases the process. A space which might need as many as 15 point detectors could therefore be maintained from one single low-level controller, as opposed to organising height access to 15 different spots.
There is already a lively debate about the relative merits and drawbacks of different detection systems. A common theme is that beam detection may not be as reliable or trouble-free as other methods, however this is almost always due to incorrect installation. Beams, in fact, can be much more suitable for some situations than other detection systems, and this article will explain how to get the best from beams.
How it all works
Let there be light
An optical beam smoke detector works on the principle of smoke particles interfering with the transmission and reception of a beam of infra-red (IR) light. A transmitter sends out a beam of IR light, and a receiver a set distance away measures the amount of IR light received. When smoke enters the beam’s path, the intensity of IR received is reduced; when this reduction reaches a pre-defined limit the alarm signal is triggered and sent to a fire control panel.
Most beam detector systems consist of a transmitter, receiver and control unit. The transmitter projects the beam, the receiver at the ‘end’ of the beam measures its intensity with a photosensitive sensor, and the control unit analyses and interprets the signal before communicating the detector’s status to a fire control panel. These three elements can either be entirely separate or completely integrated, depending on the system chosen. When the transmitter and receiver are in the same unit, a prismatic panel is fitted to the opposite wall where the receiver would normally go, reflecting the beam back to the source – and further reducing wiring requirements.
FFE multihead transceivers
A good visual analogy is a torch beam of visible light: the beam expands outwards in a cone, its intensity dropping with distance from the central axis. Beam detectors essentially detect how much ‘darker’ the end of the beam has become due to smoke interference. In a torch light, as with IR, beams can cross without scattering, which is what allows reflective beam systems to function. IR light is used as it is significantly affected by both smoke particles and the heat haze of a fire, and is invisible to the human eye – somewhat less intrusive than an actual torch beam.
Combating common problems
A minor, gradual increase in obscuration is not typical of smoke interference, but might well be due to dust and dirt build-up on the active surfaces. Software in more advanced beam detectors can detect this slow change, and increase the gain (a form of signal amplification) to automatically compensate for this. By contrast, sudden and very high beam obscuration is almost certainly a solid object in the beam’s path, and will trigger a ‘Fault’ status so that the path can be cleared. In this way, ‘intelligent’ beam detector systems are able to perform accurately and effectively over a long period of time and with minimal manual maintenance.
Types of Beam detectors and their specific advantages
End-to-End vs. Reflective
As their names suggest, and has been touched upon already, there are two fundamental types of beam detectors. End-to-end systems have the transmitter and receiver on opposite sides of the area to be protected. They can be up to 100m apart, and the receiver can be connected to a control unit installed at ground level for easy maintenance. Reflective systems have the beam transmitter and receiver in the same housing (a transceiver), with a reflective plate on the opposite wall. This can still be up to 100m away, and the plate is prismatic so that it will reflect the beam straight back even if it is not mounted perpendicularly to the transmission path.
End-to-end systems are relatively unaffected by stray reflections from surrounding surfaces and obstructions near the beam path. A reflective system, although potentially susceptible to objects near its line of sight, is easier to install and requires less wiring as power is only needed by the single transceiver unit. Essentially, end-to-end beam detectors can operate effectively through narrower ‘gaps’, and will often be more suitable in more confined areas or those with many obstructions (‘busy’ roof spaces for example). For spaces where this is not an issue, reflective systems will usually be more convenient.
Very recently, technology was also developed that allows the use of multiple transceiver heads running on one single controller. This enables cost-effective protection for larger areas, and improved coverage options for unorthodox indoor spaces.
Motorised vs. Manual Adjustment
New developments in beam detection technology have led to a choice between inexpensive simplicity and intelligent automation. Traditionally, adjusting the beam’s power and direction would have to be performed manually at the time of installation, and then maintained over time to compensate for dust build-up and ‘building shift’. This is where building elements can gradually move in very slight increments, affecting the beam’s aim and effectiveness. Recently, the option has become available to choose automated, motorised beam adjustment. This technology uses data from the unit over time to automatically adjust its direction and sensitivity to keep the beam accurately aligned and the signal at an optimum level. This is fast, reliable, and eases installation as well as reducing both the need and time taken for continued maintenance.
Beams vs. Other Detectors
The right tool for the right job
As already mentioned, by their nature beam detectors cover a huge area, and thus require less units and wiring than other detector types, but there are other things to consider as well. Beams are less affected than other types of detector by high ceilings, harsh environments and airflow blowing smoke away. As a smoke plume rises it becomes less dense, which leads to a maximum operating height for point detectors since the particle density can fall below the alarm threshold. Since a beam operates over a linear path, the density of the plume has no effect – only the total number of smoke particles in the beam path. As the plume widens, it involves more of the beam, making beam detectors more effective as height increases compared to other detectors.
Similarly, airflows that might blow smoke away from point detectors’ tiny sensor chambers are going to have less effect on the long, wide detection pattern of a beam system. Dust and dirt build-up is taken care of by automatic beam signal strength compensation, and extreme temperatures have relatively little effect on the technology – there are even beam detectors suitable for use in explosive atmospheres.
A related, but separate problem can occur when a rising smoke plume draws in surrounding air and cools rapidly as it rises, sometimes actually becoming colder than the air above it. In this situation, most commonly seen in high-ceilinged spaces, the smoke spreads out below the layer of warm air, as though trapped under an ‘invisible ceiling’ of its own. This is known as stratification, and it can render ceiling-mounted detectors ineffective due to the lack of smoke particles reaching them. A typical solution to this problem involves installing supplementary detection at lower levels to detect the stratified layer or even the plume itself. Beam detectors are wall-mounted, typically up to 600mm below ceiling level, thereby giving them a significant advantage in detecting stratification layers.
High Sensitivity Smoke Detection (HSSD) or aspirating systems are another option for large indoor spaces, however they suffer from their complexity and installation requirements. A network of end-caps, sampling pipes, brackets, elbows and labels must be designed, fitted and maintained, which can be costly and inconvenient. The aspirating pipe itself can also be quite obtrusive, and hiding it requires yet further cost and complexity from installing capillary tubes and drilling into the ceiling.
Getting the best out of Beams
Golden rules for a successful installation
As with almost all technology, an optical beam detector will work much better if it is properly installed and maintained. Most reported and ‘common knowledge’ problems with beam detection actually stem from improper installation and usage, but can be easily avoided by following some basic rules coupled with common sense.
A Stable Base
Beam detector elements must be mounted on rigid, stable surfaces to limit the risk of misalignment: as with a torch, a tiny change in the transmitter’s angle will cause a large movement at the other end of the beam. Common problems come from mounting beams on potentially flexible building surfaces such as cladded walls or on free-hanging assemblies. Even building purlins can move, particularly subject to ambient temperature changes causing contractions and expansions, so are not recommended as stable fixing points. So, if direct mounting onto brick or block walls is not possible, it is recommended that beam components be installed onto secure, rigid metal-frame assemblies suspended from RSJs (rigid steel joists).
Bean Detectors - a stable base
Reflective optical beam detectors can be affected by objects or surfaces close to the line of sight between the beam and reflector. Obstructions will not only interfere with the received signal, cutting the IR intensity, but could leave areas hidden by their ‘shadow’. If an obstructive surface were mistakenly used for alignment during initial installation, it would leave the area behind it completely unprotected. Confirming correct alignment is therefore vital, with cover-up tests of the reflector a sound method for ensuring that the whole area is properly protected.
Obstructions can impair reflective beam systems
Beam receivers should always be positioned to avoid other sources of IR light. In the first instance, where multiple beam detectors are in effect, each receiver should only have its associated transmitter’s beam falling on it. If it is within the beam of another detector system, ‘crosstalk’ can occur producing false ‘Fire’ and ‘Fault’ conditions. If two systems must be daisy-chained to cover a long distance, the transmitters should be mounted back-to-back rather than the reflectors or receivers, so as to minimise interference. Other strong IR light sources, such as direct sunlight, can cause IR saturation whereby – much as with the human eye – it will be too ‘bright’ to function properly. Normal fluorescent lights emit very little IR light, though incandescent bulbs, sodium lamps and camera flashes emit more; beams should be positioned to avoid such stray light falling directly onto the receiver.
Correct ‘back-to-back’ transmitter placement
Standards such as EN54-12 and UL268 dictate the design and construction of optical beam smoke detectors. It is important to note, however, that beam installation is governed by the relevant National Code of Practice. Codes can vary by territory in their definition of the accepted width of coverage of a beam, and its allowable height from the ceiling. The operating range (linear distance) for a beam is dictated by the manufacturer’s design and the approval gained for each beam detector product.
Things that go ‘bump’ in the night’
One last, occasional concern is that various ‘creatures of the night’ – bats and owls, usually – might set off false alarms by flying along the apex of a gabled or pitched roof. Although this could conceivably be a problem, some beam detection systems can have a delay timing set. This would then only send a fault or fire signal after that condition had been registered for a certain time – long enough for any flying trespassers to flit away again.
Light at the end of the tunnel (and warehouse, hangar, auditorium…)
This article has explained the mystery of optical beam smoke detection, its viability and benefits, and how to get the best out of it. In short, beam detectors are an excellent option for wide-area smoke detection, covering much larger areas than point-type smoke detectors and with minimal wiring requirements compared to smoke aspirating systems. Different beam systems are available to suit different projects, depending on issues of cost, wiring and space. Possibly the most important point though is that even the best technology in the world is worth nothing if it is not used correctly, so following the golden rules for installation is vital for safety and success. Bearing this information in mind, optical beam smoke detection can – and should – be considered a leading light in fire protection systems for large indoor areas.
Jon Ben is Technical Director at Fire Fighting Enterprises Ltd. His role includes responsibility for all product development, technical support and product training in the field of optical beam smoke detection. Jon has over 25 years experience in highly regulated manufacturing industries and has brought many world-class products to market in the defence, industrial, medical and fire sectors.
For more information please contact:
Fire Fighting Enterprises
201 Hyde Park, Saki Vihar Road
Powai, Mumbai, Maharashtra, 400 072
Tel: 022 6708 0400, Fax: 022 6708 0415
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