FIRE ALARM SYSTEM 2012 2.0 LITERATURE REVIEW
From the moment a fire begins, changes can be felt in the surrounding area. These changes from the ambient conditions are termed “fire signatures” and manifest themselves in the form of smoke, heat, light and gas. Appropriate detection devices are selected to respond to one or more of these fire signatures. Such detection devices are the heat detector and the smoke detector. These detection device systems are often referred to as the passive system, whereby it is a system that does not require any external movement in other word it is a static device. 2.1 HEAT DETECTOR
Figure 2.1 Shows a common heat detector.
Heat detector is a fire alarm device that is specifically design to react to a specific amount of temperature from the thermal output of a fire inside a close area. The heat that is generated spread throughout the area by laminar and turbulent convective heat flow created by heated gases. The sensitivity of a heat detectors to a given fire situation depends on the gas temperature which are affected by a few factors such as the proximity to the walls, room configuration and ceiling obstructions. The heat detectors are classified by their operating principles: 1. Fixed Temperature Detectors. 2. Rate-of-Rise Detectors. 2.1.1 FIXED TEMPERATURE DETECTORS
Considers as the simplest type of heat detectors, this device operates in a way when the sensor of the device reaches a predetermined temperature, the alarm goes off. The system operation principles are the same as the sprinkler head system. The surrounding air temperature that has surpasses the heat detector rating which is commonly 136.4 ᵒF (58ᵒ (58ᵒC) will raise the heat detector element to the operating element. This condition is known as thermal lag. The heat sensitive eutectic alloy inside the detector reaches the eutectic point changing state from a solid to a liquid releasing a spring under tension and initiates an alarm signal. This is known as the t he fusible element type.
FIRE ALARM SYSTEM 2012 Another type is the continuous line type heat detector which consists of parallel wire separated by a heat resistive insulation. As the insulation melts due to a fire, the wires come into contact and an alarm is initiated. These detectors are applicable within tunnels, cable trays and other. The last type is the bimetal type of heat detector, it consist of two joined metals that differs in the coefficient of expansion. As the metal expand when is heated, the bimetal will deflect toward the metal with the lowest coefficient. This deflection is normally designed to close a contact and initiate an alarm. 2.1.2 RATE-OF-RISE HEAT DETECTORS
This heat detector are designed to function when there is a rapid increase in the surrounding air temperature of 120ᵒ to 150ᵒF (67ᵒ to 83ᵒC) increase per minute. These detectors are engineered to accommodate normal changes in the surrounding air temperature, which are anticipated under normal conditions. There is other rate-of-rise detector that operates using pneumatic tube filled with air with a relief vent. A normal heated air will expand the tube with the excess volume exhausted through the vent port before the pressure build up. But a heated air that is caused by a high thermal source, such as fire, will exceed the capacity relief of the vent hence building a pressure an initiate an alarm.
2.2 SMOKE DETECTORS
Figure 2.2 Shows a common smoke detector.
Smoke detector is engineered to detect smoke, which are one of the fire signatures. Upon detecting smoke, it issues a local audible or visual alarm from the detector itself. There are two basic type of smoke detector, ionization and photoelectric. The sensing chambers of these detectors use different principles of operation to sense the visible or invisible particles of combustion given off in developing fires.
FIRE ALARM SYSTEM 2012 2.2.1 IONIZATION TYPE DETECTORS
Figure 2.2.1 Shows the operations of Ionization Type Smoke Detector.
Generally, the ionization chamber consists of two electrically charged plates and a radioactive material (Americium 241) to ionize or electrically charge the air between the plates. The ionization are due to the radioactive source emits particles that collide with the air molecule and dislodge their electron generating positively and negatively charged ion, the ionization of the air permits the air to conduct electricity between the two plates within the chamber. As smoke enters the chamber, the smoke particle become ionized and reduces the conductivity of the air between the plates. The reduction in conductivity between the plates is sensed and will cause the detector to respond. Typically, these detectors are the lowest cost, and respond to smaller smoke particles which are resulted from high-energy fires. But the fault of this device is that it is easily set off from cooking and other non-fire source such as dust particles.
FIRE ALARM SYSTEM 2012 2.2.2 PHOTOELECTRIC TYPE DETECTORS
Figure 2.2.2 Shows the operation of Photoelectric Type Smoke Detector.
Smoke that is produced during a fire can affect the intensity of li ght beam passing through air by blocking or obscuring it. It can also cause the light to scatter in different direction due to reflection of the smoke particles. Photoelectric smoke detectors are engineered to detect smoke by utilizing these effects of smoke on light. Photoelectric detectors operate different type of principles: a. Light scattering principle detector. b. Light obscuration principle detector. a) Light Scattering Principle Detector.
This principle operates by utilizing light source and a photosensitive device. A light-emitting diode (LED) is beamed into a path that is not directly toward the photosensitive device which generally is photodiode. When the smoke particles enter the light path, the particle reflects the light as it hit it onto the photosensitive device causing the detector to respond. b) Light Obscuration Principle Detector.
This principle operates by projecting a light beam toward the photosensitive device, such as photodiode. As smoke particles enter the chamber, the smoke particle partially block the light beam, the light intensity diminishes and alerting the photosensitive device and alter its output. The change in output is sensed by the detector’s circuitry, and when the threshold is crossed, an alarm is initiated. Most light obscuration detectors are the beam type and are used for smoke detection of large open areas.
FIRE ALARM SYSTEM 2012 2.3 EFFECTIVENESS OF THE HEAT AND SMOKE DETECTOR
There are a few factor that need to be considered in order to get the full effectiveness of the fire alarm devices, one of them is location, as such the smoke detector must avoid any places that produce or generated a large amount of particle, due to the smoke detector can not differentiate between smoke and dust particle while the heat detector must avoid any places that have direct contact with high thermal emitting devices. Another is the configuration of the room, if a room is packed with electrical devices that generates high temperature, it is possible that it can trigger the alarm, and by placing the heat detector in a an air-conditioned room that is frequently turned on and off, the changes in temperature could also trigger the alarm.
Reference , http://www.pdhcenter.com/courses/m110a/Module5.pdf , http://www.systemsensor.com/pdf/A05-1003.pdf.
