Types of fire detectors

INTRODUCTION :

Success in controlling fire lies in its early detection. With die passage of each moment the fire spreads progressively and within a few minutes it turns violent. Fke detection systems are designed to detect fire at the earliest practicable moment so that appropriate actions can be taken for its control.

MAIN OBJECTIVES :

  • Quick detection
  • Reliable transmission of detection signal
  • Translation of signal into clears alarm and indicates location of fire.

GENERAL REQUIREMENTS :

  • The system should be reliable and should not be liable to disturbance by any other system whether associated with it or not.
  • The system should not be rendered in operative partially or totally by the fire.
  • Operation of an installed system should be tested for satisfactory performance periodically.
  •  The system should not create direct or indirect hazards.
  • The power supply to the system shall be from two sources with an automatic change over switch.
  • If the signals are not received or acknowledged within two minutes, an audible alarm shall be automatically sounded.
  • Detectors shall be operated by heat, smoke or other products of combustion like flame or combination of these factors. Flame detectors shall only be used in addition to smoke or heat detectors.

CATEGORIES OF FIRE DETECTION SYSTEM:

These fell into two important categories:

  1. Heat Detectors
    1. FIXED TEMPERATURE TYPE
    2. RATE OF TEMPERATURE RISE TYPE
  2. Smoke Detectors
    1. LIGHT OBSTRUCTION TYPE
    2. LIGHT SCATTER TYPE
    3. IONIZATION TYPE

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Details of different types of fire detection system

 

1.FIXED TEMPERATURE TYPE DETECTOR

Which operate at fixed temperature

A bimetallic strip is used to make or break an electric circuit at a preset temperature. When the temperature near the bimetallic strip rises to a preset temperature the electric circuit is made which makes the bell to ring.

 

2.RATE OF TEMP RISE TYPE DETECTOR

Which operate when the rate of rise in temperature of the surrounding air reaches a set limit

IT SUBJECTED TO MEASURES THE SUDDEN CHANGE IN SURROUNDING TEMPERATURE.  

A sealed chamber when subjected to heat expands and pushes a flexible diaphragm to make an electrical contact complete for the alarm. then it gives the signal to the fire sprinklers which starts working within few seconds after taking signal

 

3. LIGHT OBSTRUCTION TYPE SMOKE DETECTOR

A beam of light is arranged to fall on a photo electric cell and if the atmosphere between the light source and the cell is clear, the balance of the electric circuit is not disturbed.

When smoke passes between the source of light and the photocell, the intensity of light falling on the photocell is reduced. This change in the electric output is used to operate an alarm at a predetermined level.

 

4.LIGHT SCATTER TYPE SMOKE DETECTOR

                    When a beam of light extend across a transparent medium e.g., air, its intensity is reduced by absorption and partially by scattering. Scattering takes place due to suspended particles such as dust, liquid particles or smoke.

                  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 photo electric cell to change the electrical; balance.

               This change in current is used to signal the alarm.

5.IONIZATION TYPE FIRE DETECTOR

                If the atom is subjected to radiation from a radio active source some electrons become detached. As a result the atom becomes positively charged. Such charged atoms are called “Ions” and the process that created them is called “Ionization”.

                 If the atoms of air in a container are subjected to radiation, ionization will take place and the ions will move haphazardly. If we then introduce a positively charged plate and a negatively charged plate to the container, the positive and the negative ions are attached to the oppositely charged plate. This will produce a small current in the external circuit.

                 This current is disturbed when smoke enters the chamber, the reduction in the current is used to make up the circuit for the alarm.