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10/08/2016 / Graham Lowe

Large open spaces: Understanding your options

When faced with a large open space, such as a sports facility, shopping centre, museum or warehouse, you might be wondering which technology is most suitable. Vast open spaces can lead to rapid fire spread, therefore early detection is vital in order to reduce damage levels and avoid loss of life.

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Your first option is probably going to be optical beam detectors. The advantage of beam detectors in these applications is their ability to detect when smoke is scattered over wide areas.  This is achieved due to the beam detectors using the obscuration method, which detects fire due to smoke particles obscuring the infra-red light along the sensing path.  When sufficient smoke blocks the infra-red light beam, reducing the received signal strength by a pre-determined setting, a fire condition is generated.  Such detection sensitivity enables the beam to identify a fire before it spreads.

Of course, there are limitations to beam detectors; for example, reflections from nearby objects can cause problems with a beam detectors ability to cover the required distance, and sunlight can also interfere with the beam; but careful positioning can avoid these issues.

BEAMS OBSCURATIONOptical beam detectors can be installed at a height of up to 25 metres, which means that very little unwanted obscuration is likely to occur, however a well-designed beam detector will still be designed to distinguish between unwanted obscuration (such as a bird breaking the beam) and a genuine fire condition. Unfortunately, mist, steam and dust can all obscure a beam in a similar way to smoke which can potentially cause a false alarm, therefore the installer will need to adjust the sensitivity of the beam detector to suit the environment in which it is being installed.

Warehouses and other areas that suffer from extreme changes in temperature, can pose particular problems; for example, a beam detector is susceptible to condensation and so in addition to regular cleaning, an anti-fogging kit may be required. Also, as the building expands and contracts with the differences in temperature, the beam may need re-aligning. This isn’t a problem with many modern beam detectors as they can automatically re-align themselves, however in an area that is regularly used for an activity such as welding, you will need to turn the auto-alignment function off as it will struggle to re-align due to the welding smoke.

Where a large volume is to be covered and high sensitivity is required, an aspirating system could be a viable second option. Air sampling systems consist of lengths of pipe with strategically placed sampling holes along its length. An air impeller is then used to draw air along the pipe from the sampling holes and through the detector measuring chamber. Most high sensitivity aspirating detectors use the obscuration method, and when a pre-defined density of smoke appears in the chamber, the system will go into alarm.

In most instances an aspirating system is set up using absolute scaling, whereby the system assumes that the normal environment has zero amount of smoke pollution. The sensitivity can then be set; for example, 0.05% obscuration per meter would be highly sensitive, which would be ideal for clean rooms and computer rooms etc. However, if you have a slightly more polluted environment, relative scaling might be more suitable. Relative scaling allows you to establish a pollution scale relative to the background level of pollution. ‘Zero’ would be defined as the average background level of pollution. This therefore means that aspirating systems can be used in less traditional environments, such as warehouses.

Again, when using aspirating systems, there are some potential issues to bear in mind; for example, dilution can be a potential problem if the pipe design has not been carefully considered. For example, if the detector only draws smoke in through 1 out of 10 sample holes, the density of smoke may be diluted so much that it fails to go into alarm. Other issues include; dirt building up in the sample pipes, moisture problems and air flow faults.

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Dirt can build up in the pipes due to the continuous air flow, therefore they need to be cleaned regularly. This can be tricky if you have long or intricate pipe work, and often you will require a high pressure cleaning system to ensure it is done properly.

Aspirating systems are also susceptible to moisture, you may therefore need a moisture trap and a heater box (should the environment necessitate it).

A change in air pressure (in an aircraft hangar for example) can lead to an air flow fault. The installer will therefore need to position the exhaust inside the sample area, but make sure that the detector is outside the area prone to pressure change.

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Ok, so let’s consider price point. Aspirating systems are designed to provide maximum performance on the assumption that the customer will be willing to pay more. So, if you require high performance, high reliability, and stability, then it is worthwhile investing in an aspirating system.

Beam detectors are less expensive, and if using a reflective beam detector instead of a transmitter and a receiver, costs are significantly reduced again.

Of course, ultimately, it depends on the environment; we are not saying that beam detectors and aspirating systems are completely interchangeable; they’re not! It’s just important to understand the scope and application of the different technology available!

For more information about Hochiki’s high sensitivity aspirating system, FIRElink, please click here.

For more information about Hochiki’s  FIREbeam please click here.

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