In the fourth of our Fire Doctor series, David Brown responds to frequently asked questions on aspirating smoke detection systems.
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| David Brown is product manager at Hochiki Europe |
I’ve been asked to install an aspirating smoke detection system. How do these systems work?
The first thing to point out is that aspirating fire detection systems are not as complex as some installers think they are. However, you need to understand how they work and have knowledge about the correct installation of the pipework, and the siting and spacing of sampling points.
Aspirating fire detection equipment is generally non-addressable, though they can be installed on addressable systems via suitable I/O interface modules or specially designed Addressable Protocol Interface Cards. The aspirating unit consists of an enclosure, housing the electronics that are powered from a supply, and a fan inside that draws air in via pipes which are connected to the unit. The air that is drawn in then goes into an aspirating chamber after passing through a filter which gets rid of dirt and dust but allows smoke particles to get through.
The air then passes across a laser or LED light source which is projected into the air itself. Should there be sufficient smoke particles detected, an alarm condition will be activated.
What types of environments are most suitable for this type of fire detection system?
Reasons for using an ASD system instead of point or beam detection include high ceilings, environmental considerations, access and maintenance or aesthetics.
Far from being a niche technology, most types of buildings can benefit from having an aspirating fire detection system in place and an increasing number of installers are finding it to be a highly versatile solution.
However, aspirating systems tend to be particularly useful where a very early warning of smoke or enhanced smoke sensitivity is required. Potential applications include warehouses, buildings with high ceilings, data centres and museums.
While BS-5839-1 specifies that an ASD system can be mounted up to 10.5m for normal sensitivity, recent testing carried out by the Fire Industry Association suggests that ASD systems can be as effective as optical beam detectors at heights of up to 25m. However, locating an ASD system above 10.5m would have to be treated as a variation to BS-5839-1.
What regulations and standards govern their installation and use?
BS5839 provides a brief guide to the mounting height restrictions for the aspirators, however, I would thoroughly recommend that you read the Fire Industry Association’s (FIA) Code of Practice for Design, Installation, Commissioning and Maintenance of Aspirating Smoke Detectors. This is a comprehensive document that provides in depth information on this subject and is a useful starting point for anyone wishing to install aspirating fire detection systems. [Download a free copy from the FIA website].
Are there different types of aspirating smoke detectors and, if so, what are they?
There are different types but they are all predominantly optical based and use either LEDs or a laser to detect forward or backscatter light in the presence of smoke. Some older systems use a Wilson Cloud Chamber – a unit which takes an air sample that is diverted in a humidifier at approximately 100 per cent relative humidity.
There are two main forms of analysis used in aspirating detectors to determine the presence of smoke. Some use fixed scaling, which puts the fire threshold level at a fixed point, whilst others use relative scaling to track the background smoke level and adjust the fire threshold accordingly. This means that aspirating systems can be installed in clean or dirty environments as the system sensitivity can be adjusted in a way that reduces the likelihood of unwanted alarms.
Systems are now available which bring the benefits of aspirating technology to smaller scale applications that may not have previously used this type of detection. These allow the system to be configured internally, thus eliminating the need for software, though they will still need suitably spaced sampling points in the pipework. As a cost effective way to specify highly sensitive fire detection equipment, these systems are gaining in popularity.
Do I need to undergo specific training to install aspirating systems?
Most definitely. I would advise anyone who is looking to specify this type of system to take some manufacturer based training to learn how to design, install, maintain and commission it correctly. While the majority of installers will possess the basic installation skills needed, some manufacturers use Windows based software to programme the aspirating unit. This is usually unique to their products and requires specific training.
The knowledge gained by a relevant training course will pay dividends in the long-term and ensure a successful installation which, in turn, will reduce the likelihood of unwanted alarms.
David Brown is product manager at Hochiki Europe. If you have any questions on these topics, email: [email protected]
