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PDS Fire detection - Types of detector
FIRE DETECTION
IN BUILDINGS
- Detectors
Smoke detector
- Device that detects smoke
- Used in commercial, industrial, and mass residential areas
Electrical heat detector
- Primary heat sensing device is a thermistor
- Triggered when there is a rise in temperature.
Placement of detectors
- Alarm system
Fire sprinkler system
- Closed-head sprinkler held closed by a heat-sensitive glass bulb
Fire alarm system
- Bulb breaks as a result of the thermal expansion of the liquid inside the bulb.
- Monitors inputs and system integrity, controls outputs and relays information.
- Designed to alert people with visual and audio alarms when triggered
- Sprinkler system
Scope
- Fire detection in buildings
Particulate sensing
- aerosol produced from flaming or smoldering combustion.
- spot detector
-detector is located at the sensing location
- aspirated system
- remote with the smoke drawn through piping to the detector location
Ionization detector
Light scattering detector
- suited for detecting a flaming fire with high concentration of small smoke particles
Solid state smoke detector
- a light source(light emitting diode in the visible or IR spectrum)
- produces a collimated beam that extends across the sensing region
these ions flow to the oppositely charged plates generating a small current
-two opposing plates are held at a fixed voltage potential
- a photo-detector is located such that direct light from the source is blocked
-alpha particles radiating from a source interact with air molecules creating charged ions
-suits for detecting a smoldering fire accompanied by relatively large particles which scatter much more light than the small particles produced in flaming combustion
- Semi-conductor absorb carbon particles in smoke
- changes the resistance value of conductor
smoke particles entering the detector flow between the plates and scavenge ions decreasing the electrical current.
- Smoke particles cause the infra-red light to scatter in all directions
Tom Hopton