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The major fire risks and detection difficulties within Clean Rooms arise as a result of the following:
Utility and process tool equipment faults.
Electrical faults in cabling or other electrical and electronic equipment.
The Over Head Transport (OHT) System.
The presence of large amounts of flammable and explosive materials.
Potential rapid fire growth resulting from the delayed control of forced ventilation.
Rapid fire spread as a result of the air circulation used to filter out pollutants.
Air movement dilution of and interference with the normal dispersion of smoke making its detection by conventional technologies extremely difficult.
Why Use VESDA Smoke Detection?
It is essential that fire events in Clean Rooms be detected as early as possible, to avoid asset damage, leading to costly business disruption, and to ensure occupant safety.
The limitations of point (spot) type smoke (even the ones with high sensitivity) and heat detectors must be considered. The comparatively low sensitivity of point (spot) type smoke detectors, for instance, can mean that fire events will not be detected soon enough in many cases. Rather than behaving as they normally would, both smoke and heat from low energy fires will tend to follow the air streams created by the AHUs with the following consequences:
Air movement, filtering and the introduction of clean air during the air conditioning cycle will all cause smoke dilution. This consequence impairs the performance of point (spot) type smoke detectors
The cooling effects of the air conditioning will decrease the temperature of the smoke plume. This consequence impairs the performance of both point (spot) type smoke and heat detectors.
The Very Early Warning Fire Detection (VEWFD) capability of the VESDA system allows it to minimize the risks, and combat the detection difficulties caused by air movement in the following ways:
VESDA detectors can be configured as both early warning and very early warning devices, thus, if both are required only one technology need be installed.
A VESDA system can detect fires very early, at the incipient (smoldering) stage. This provides staff with an opportunity to investigate and take action, before the smoke contamination can irreversibly damage process tools or the products being manufactured.
The very early warning capability of the VESDA system also minimizes the rapid fire growth and spread facilitated by the high air movement.
Since the security precautions necessary to ensure the maintenance of the sterile environment within the Clean Room also complicate evacuation, very early warning will allow more time to execute an evacuation.
A VESDA system has a better chance of detecting smoke that has been diluted than conventional point (spot) type smoke detectors. This is due to the fact that air collected by several sampling holes, at different locations within the protected area, is being analyzed simultaneously by the same detector (aggregated sampling effect).
A VESDA system actively draws air into its sampling holes which increases the chance of smoke being detected. Passive smoke detectors rely on smoke reaching them via diffusion or using the thermal energy of the fire. Thus the detection of smoke by passive detectors would be less likely where air movement is being artificially directed as well as being cooled.
There is a comparatively low incidence of nuisance alarms with a VESDA system; a feature which will minimize the possibility of unnecessary evacuations.
In cases where fire suppression is to be included, as part of the overall fire protection system, the VESDA detectors’ wide sensitivity range of 0.005 to 20%Obs/m (0.0015 to 6%Obs/ft) means that appropriate alarm thresholds can be set for both early detection and, at a later stage in the fire event, the activation of the suppression release mechanisms.