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Types of Fire Detectors

Types of Fire Detectors

Overview: 


Fire detection systems are designed to detect the presence of the following changes/conditions: heat, smoke, flame, infrared and or ultraviolet light radiation. 


The main objective of a fire detection system is to alert the presence of the above-mentioned conditions & relay information to fire response systems or personnel. 


Basic fire detection technology typically consists of the following parts 

  • Input: Which would be the smoke detectors, heat detectors or ‘emergency break glass’ units etc.  
  • Control panel: Transmits the information to the output devices & may alert the fire units as well. 
  • Output: These devices are the ones that will perform the task of alerting in case of a fire. The most common output devices are LED flashers, alarms (hooters), water sprinklers and door releases. 

Types of fire detectors:  


There are three main categorizations of fire detectors: 

  • Smoke detector
  • Heat detector
  • Flame detector


All three types of detectors have subcategories


Smoke detectors:

A smoke detector is a device that senses visible & invisible smoke, typically as an indicator of fire. These smoke alarms sense the presence of smoke differently.  There are two types of smoke detectors in general use: photoelectric (optical) and ionization.

  • The photoelectric detector or optical detector uses light as a detection mechanism. It contains a source of infrared, visible, or ultraviolet light, a lens, and a photoelectric receiver (typically a photodiode). Photoelectric smoke detectors can further be categorized under
  1.  Light obscuring(Blocking) A beam of light is arranged to fall on a photoelectric cell and if the atmosphere between the light source and the cell is clear, the balance of the electric circuit is not disturbed. 
  2.  Light scattering (Sensing) As long as there are no suspended particles in the air, the electrical balance of the detector remains undisturbed. But as soon as smoke enters the detector the light rays are scattered around the light barrier and reach the photoelectric cell to change the electrical balance.
  • Ionization smoke detector: This detector contains a radioactive source, usually an alpha particle, which ionizes the air while passing through the chamber, where a small current flows between two electrodes. When the smoke enters this chamber the current flow decreases. This drop in current flow is used to initiate an alarm. This detector automatically resets when the atmosphere clears.
Heat detectors:

A heat detector is a device that responds to very high temperatures that are only present during fires. Standalone heat detectors are often used in rooms where smoke detectors may cause false alarms. They usually take longer to respond to fire than smoke detectors. There are two types of heat detectors: Fixed temperature detector & Rate of Rise detector (ROR)

  • Fixed temperature detectors contain a bi-metallic switch element that shuts at a specified temperature limit. The switch is composed of two metals, each having different coefficient temperatures of expansion. As this bimetallic element heats the metal with a higher coefficient of expansion, it causes the switch to bend or curve, closing the switch; thus indicating an alarm condition. The most common fixed temperature point for electrically connected heat detectors is 58°C (136.4°F).

Rate-of-Rise (ROR) heat detectors trigger the fire alarm when the rate of temperature increase in the surroundings rises above a certain rate. The air in the air chamber will expand and close the contact which is used to generate the alarm. There are two types of ROR:

  1. Pneumatic Type
  2. Electric Type
Flame detector

As the name suggests is a type of sensor that can detect and respond to the presence of a flame. These detectors have the ability to identify smokeless liquid and smoke that can create open fire. They can also optically sense radiation given off by flames or glowing embers and often respond faster and more accurately than a smoke or heat detector. Types of flame detectors include: Infrared (IR) & Ultra Violet (UV)

  • The infrared flame detector senses light at the extreme, high end of the light spectrum. Flicker-flame detectors are infrared detectors capable of sensing the typical flicker of a flame. It can't be installed in direct sunlight and near an electric heater.
  • The ultraviolet flame detector's sensing element usually consists of either a solid-state device, such as silicon carbide or aluminium nitride, or a gas-filled tube in which the gas is ionized by ultraviolet radiation. This causes the materials to become conductive, resulting in the initiating of an alarm. The Sun's rays and artificial rays have no effect on UV detectors. But electric arc has an effect, so where there is electric welding going on, it cannot be installed. UV detectors are sensitive to most fires, including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulphur, hydrogen, hydrazine and ammonia.

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