Explain the working of a fire alarm with a labelled diagram
Dear student,
The figure below shows a fire alarm or thermal fire detector.Bimetal strips form the basis of thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature. When temperature increases, the bimetal curves as the metal with higher coefficient of expansion lying on the outer side of an arc undergoes a greater increase in length. With one end fixed, the movement of the free end of the strip can be arranged to close an electric circuit that operates an alarm.
Thermal fire detectors are least sensitive type of detector. They have a high thermal inertia and the fire has to produce large amount of heat before the temperature at the detector is sufficient to cause it to operate. Consequently, they are normally used in spaces such as laundries, drying rooms, galleys, and pantries, where other detectors are susceptible to false alarm from water vapour or smoke.
In the most recent detector designs the bimetal strips have been replaced by thermistors (solid state devices of, for example, nickel, manganese and cobalt, whose electrical resistance changes significantly with temperature ). The principle of operation is, however, no different as one thermistor is exposed to the air and one is shielded.
Regards
The figure below shows a fire alarm or thermal fire detector.Bimetal strips form the basis of thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature. When temperature increases, the bimetal curves as the metal with higher coefficient of expansion lying on the outer side of an arc undergoes a greater increase in length. With one end fixed, the movement of the free end of the strip can be arranged to close an electric circuit that operates an alarm.
Thermal fire detectors are least sensitive type of detector. They have a high thermal inertia and the fire has to produce large amount of heat before the temperature at the detector is sufficient to cause it to operate. Consequently, they are normally used in spaces such as laundries, drying rooms, galleys, and pantries, where other detectors are susceptible to false alarm from water vapour or smoke.
In the most recent detector designs the bimetal strips have been replaced by thermistors (solid state devices of, for example, nickel, manganese and cobalt, whose electrical resistance changes significantly with temperature ). The principle of operation is, however, no different as one thermistor is exposed to the air and one is shielded.
Regards