The use of aspirating smoke detection has significantly increased since its introduction and now provides the key to solving a vast range of fire detection obstacles.
Commonly used as an alternative to point or beam detectors, an aspirating smoke detection system is an elemental part of the over all fire alarm system. Therefore the system designer must continue to comply with National standards, particularly for fault monitoring, zoning and battery standby.
The advantages of ASD are plentiful offering the facility to provide very early warning of fire, enhanced smoke sensitivity and an alternative to point or beam detectors. ASD can also provide invaluable detection for restricted or difficult to access areas, areas with high ceilings and hazardous environments to name but a few.
ASD Sampling Types
For the purposes of definition, the Fire Industry Association (FIA) consider there to be five different approaches to sampling types of aspirating smoke detection: primary sampling, secondary sampling, localised sampling, in-cabinet sampling and duct sampling. However, for the purposes of this article I am just going to be talking about the first two; primary and secondary sampling.
According to the FIA code of practice “Primary Sampling is arranged to sample from specific locations in the protected area where smoke is most likely to travel / accumulate.”
Sampling points may be situated anywhere that smoke is likely to travel. However, as smoke tends to travel with the air flow, sampling points are most commonly placed at air intake grills of Air Handling Units (AHUs) or Pressure Relief Vents (PRV). Primary sampling detection is typically viewed as supplementary to other forms of detection with its response capability being dependant on other systems such as the air movement provided by AHUs. However, this type of sampling is still widely considered to provide the earliest possible warning of fire.
It is recommended that when a primary sampling system is installed to provide early warning of fire or to counter the obstacle of air movement in a particular area, that a Class A system such as the EF-LASD Aspirating Smoke Detector is used, although a class B system may be specified for some small or ‘dirty’ applications.
The FIA code of practice describes secondary sampling as “arranged such that the air sampling points are sited and spaced as an alternative to point type smoke detectors.”
Sampling points are situated in place of point detectors in accordance with National or International Standards, ensuring that the performance of each sampling point (or group of sampling points if specified) is equal to or surpasses the minimum requirements of a point detector.
A simple equation can be applied to estimate the sensitivity of each air sampling unit (given that every sampling point has been designed to maintain a balanced system):
Individual Sampling Point = Smoke Sensing Element Sensitivity x Number of Sampling Points
For example, an aspirating smoke detector that can provide a 0.1% obscuration/m sensitivity can provide a sensitivity level equivalent to 2% obscuration/m over 20 perfectly balanced sampling points. This means that where each sampling point is monitoring separate areas each point can be considered to be equivalent to a 2% obscuration/m point detectors.
When smoke can enter more than one sampling hole in any given area the response of the ASD system would be considered somewhat greater. This is a particularly advantageous feature of aspirating smoke detection systems and is known as the cumulative effect.
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