Understanding technology
Smoke from a moderate sized fire in a large open area can be extremely difficult to detect. Conventional fire detection technology requires that a certain concentration of smoke be detected inside the chamber before it declares an alarm – typically around 2% to 4% obscuration. In large open areas conventional detection systems generally see only enough smoke to declare an alarm once a fire is already well advanced, perhaps into its flaming stage, causing confusion and possible panic among occupants.
Aspirating smoke detection systems that draw sampled air into detectors are similarly challenged in high ceiling structures as smoke still has to reach the sampling position in the roof before the alarm isactivated.
A spot-type detector samples smoke at its one particular “spot”. The smoke that enters the chamber may be diluted to less than the alarm threshold, which is the level of smoke needed for an alarm. In contrast, beam smoke detectors look across the entire smoke field intersecting the beam from a high ceiling standpoint and may be more responsiveto slow or smoldering fires than spot-type detectors.
Beam smoke detectors offer unique capabilities that can overcome many of the challenges associated with the high ceiling airport setting. Projected beam smoke detectors consist of a transmitter that projects an infrared beam across the protected area to a receiver containing a photosensitive cell, which monitors the signal strength of thelight beam. Single ended reflective beam detectors consist of a transmitterand a receiver in one unit, with a reflector used on the other endto return the light. One of the advantages of these units is that wiring across the room (transmitter to receiver) is no longer required.
Accessories to the beam smoke detector may include remote annunciators and remote test stations for the periodic electronic and/or sensitivitytesting of the detector. Intelligent fire alarm systems can give the beam smoke detector a discrete address to provide better annunciation of the fire location. Conventional systems may use relays to annunciate remotely.
Spot-type smoke detectors are considered to have a maximum coverage of 900 square feet or 30 feet by 30 feet. The maximum length between detectors is 41 feet when the width of the area being protected does not exceed 10 feet, as in a hallway.
Beam smoke detectors generally have a maximum range of 330 feet and a maximum distance between detectors of 60 feet. This gives the beam smoke detector theoretical coverage of 19,800 square feet. Manufacturers’ recommendations and other factors, such as room geometry, may impose practical reductions of this maximum coverage.
Even with these reductions, a beam smoke detector can cover an area that would require a dozen or more spot-type detectors, which generally decrease in response as their distance from the fire increases. Another advantage of the beam smoke detector is that fewer devices translate to lower installation and maintenance costs. They can be mounted on walls, which are more accessible than ceilings.
The major limitation of projected beam smoke detectors is that these units are line of sight devices and are, therefore, subject to interference from any object or person entering the beam path. This may make it impractical for normal ceiling height situations. However, modern airports have areas where beam smoke detectors are the detector of choice.
While the potential benefits are clear and desirable, a firm understanding of the technology and limitations of specific smoke detectors is essential to a successful airport fire alarm system.
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Tags: Smoke detection
Posted in Airports, Audible/Visible Notification, Commercial, Guidelines, Spring 2008
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