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:
Heat Detectors
· FIXED TEMPERATURE TYPE
· RATE OF TEMPERATURE RISE TYPE
Smoke Detectors
· LIGHT OBSTRUCTION TYPE
· LIGHT SCATTER TYPE
· IONIZATION TYPE
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.
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. ......... SEE HOW IT WORKS.
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.
LIGHT SCATTER TYPE SMOKE DETECTOR
When a beam of light extends 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.
If the atom is subjected to radiation from a radioactive 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.
FLAME DETECTOR (INFRA-RED TYPE)
· In addition to light, fires generate infrared and ultraviolet radiations which are less affected by smoke than normal white light.
· By employing a photoelectric cell with infrared filters or an ultraviolet sensor, the presence of flames can be detected in spite of the smoke generated.
· The signal from the photoelectric cell with infrared filters or an ultraviolet sensor can be arranged to trigger the alarm.
· These devices suffer from the disadvantage that they will react to infrared or ultraviolet radiations from other sources and hence are prone to spurious alarms.
· This disadvantage is overcome in a combined smoke and heat detecting system in which a modulated infrared beam is transmitted across the area to be protected.
· Smoke and heat haze interfere with the beam and this is detected in the receiving unit which initiates the alarm.