Smoke detection technologies are classified into EWSD (Early warning Smoke Detection) and Very Early Warning Smoke Detection (VEWSD). EWSD detectors are of ionisation or photoelectric type. Ionisation detectors are designed to detect microscopic particles such as the type produced by flammable liquids. Photoelectric sensors detect larger particles such as those produced by non-natural materials like PVC.
An EWSD provides detection of a fire condition before it becomes threatening to the occupants of the building. Generally, this is the time that smoke is visible. Let us use the example of paper catching fire within an office. Seconds after the paper has ignited, smoke will generate and rise to the ceiling. This visible and hot smoke will eventually enter the smoke detection chamber and trigger the alarm to alert the occupants that a fire has commenced.
The EWSD are passive in the sense that the smoke has to find its way to the detector. These detectors wait for smoke and rely on the airflow to transport the smoke to the sensor.
Therefore their performance is affected by airflow. If a computer terminal within a room had a fault in its electronics resulting in a thermal event, it might smoulder for hours before a flame ignites. This smouldering stage is the embryonic stage of a fire. During this embryonic stage, the human eye will not see the particles but may smell them.
EWSD are not sensitive enough to detect smoke at the incipient stage of an electrical type fire. Only a VEWSD will detect a developing fire and thus the term “ VERY EARLY WARNING”.
This stage of a fire could last for hours or even days. Since the rate of smoke generation in a smouldering fire is relatively low, and the airflow velocity in the room is relatively high, the movement of smoke is influenced by the airflow of the mechanical systems.
Furthermore, the smoke generated during the embryonic stage is not hot. Therefore, there is very little thermal lift. This often restricts smoke movement directly to the ceiling, where spot type detectors are located, causing the smoke to dissipate more widely. The aspirated smoke detection or VESDA is “active “, constantly sampling the air from multiple points throughout the environment. Therefore, it is not totally dependant on thermal energy to transport smoke to enter the smoke detector.
Aspirated Detection: Aspirated smoke detection (ASD) systems are quite different from conventional spot type smoke detectors. Aspirated systems typically comprise several small-bore pipes laid out above or below a ceiling in parallel runs, some metres apart. Small holes, also some metres apart, are drilled into each pipe to form a matrix of holes (sampling points), providing an even distribution across the ceiling. Air or smoke is drawn into the pipework through the holes and onward to a very sensitive smoke detector mounted nearby, using an aspirator's negative pressure (air pump).
How much smoke should we detect?
An obscuration is a unit of measurement that has become the standard definition of smoke detector sensitivity used in the industry today. Obscuration is the effect that smoke has on reducing visibility. Higher concentrations of smoke result in higher obscuration levels, lowering visibility. Typical Smoke Detection Ratings for smoke detectors: ¨ Photoelectric : 3.0 % – 12 % obscuration per metre ¨ Beam : 3.0 % – 7.0% obscuration per metre ¨ VESDA : 0.005 % - 20 % obscuration per metre
Note that the obscuration level at which VESDA detects smoke is lesser (0.005 %) compared to the lower limit of 3.0 % obscuration level for Photoelectric or Beam detector.
Design Techniques for Very Early Warning Smoke Detection:
The following have to be considered when designing a VEWSD system.
1. The coverage area per detector or sample point
2. The sensitivity required per sampling point
3. The airflow characteristics and the air change rate within the room.
4. The room size and features – raised floor, tall ceilings etc.
5. The annunciation of emergency response systems.
6. The activation of mechanical control systems such as air extraction and suppression systems.
Coverage Area of ASD
The maximum coverage area of the ASD detector is 2000 sq. meters (this is the maximum area coverage permissible within the codes, BS, AS, NFPA). The sample point of an ASD detector is treated the same as a spot type detector. The area of coverage for a sampling point is effectively around 100-sq. Metre. For ASD applications in high airflow environments, we can decrease coverage for the sample point by adding more holes and making the distance between each pipe less.
Sensitivity of ASD
The smoke that reaches the detector is the cumulative smoke, which has entered all the network sampling points. Take the example of a 200 square metre room with 10 sample points on the ceiling. If the detector sensitivity is set to 0.1 % Obscuration / m this effectively makes each sample points sensitivity as 0.1 X 10 = 1.0 % Obscuration / m. That is, if only one sample point were exposed to smoke, it would require 1.0 % Obscuration / m to trigger an alarm. This is because we take into account the dilution caused by the other holes.
Taking the same example, If smoke enters three holes, then the adequate sensitivity required to trigger an alarm is 0.1 x 10 divided by three = 0.33 % Obscuration / m per sampling point. Cumulative sampling allows much lower levels of smoke and thus enable very early detection. If the same room was designed with EWSD and each detector was rated at 5 Obscuration /m, the alarm would only trigger once the smoke density has reached this point throughout the room or at one sensor. Usage of VEWSD in Computer Room and Aiir Condiitiioniing (CRAC) Systems: In the diagram given below, The sampling pipes on the ceiling and within the floor void are used for detection where the CRAC is out of service. The pipe used to detect smoke across the return air path is used for detection where CRAC is operational. The enabling and disabling of the sampling pipes helps to achieve this. VEWSD techniques are preferred in areas where Air Handling Units (AHU’s) and other DX systems are installed because the air changes which happen in these rooms will dilute the smoke, thus making smoke detection difficult for EWSD.
Usage of VESDA in Explosion Prone Environments
VESDA is installed in hazardous areas like petrochemical plants and also warehouses storing solvents and alcohol. These are areas where explosive mixtures of gases or vapours can accumulate. These gases, if ignited, would cause an explosion. Equipment installed in hazardous areas must be CE marked and have appropriate Ex ratings. Flameproof enclosures (Exd) will hold ignition sources within them so that any ignition of the hazard inside the enclosure will not be transmitted to the atmosphere outside the section.
Glossary
Hazardous area
An area in which flammable substance in the form of gas, vapour or dust, when mixed with air, is present in such proportions that it can explode when in contact with an ignition source.
Explosion Proof Apparatus
This apparatus is enclosed in a case capable of withstanding an explosion of a specified gas or vapour that may occur within it. It also prevents the ignition of a specific gas or vapour that may occur within it. It also prevents ignition of a particular gas or vapour surrounding the enclosure by sparks, flashes, or explosion of the gas or vapour within and operates at an external temperature that will not ignite a surrounding explosive atmosphere.
Flame Proof
The equipment's enclosure will withstand an internal explosion and prevent the passage of flame to the surrounding atmosphere. ExD: Abbreviation for “Eex d” method for flameproof protection. This includes the use of explosion-proof enclosure and flame arresters. This protection level is deemed suitable for electrical equipment in Class I, Zone 1 or Zone 2 environments.
Class I
Locations where flammable gases or vapours are or may be present in the air in quantities sufficient to produce ignitable mixtures.
Class II
Locations, which are hazardous due to the presence of combustible or electrically conductive clouds of dust in sufficient quantities for a fire or explosion hazard to exist.
Class III
Locations, which are hazardous due to the presence of easily ignitable fibres flyings. However, the material is not suspended in the air in quantities sufficient to produce volatile mixtures.
Zone 1
Where ignitable concentrations of flammable gases, vapours or liquids can exist some of the time under normal operating conditions. (IEC adds: Typically between 10 to 100 hours per year)
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