DESCRIPTION AND APPLICATIONS

In fire fighting the automatic fire detection is of paramount importance because an early warning of hazard allows a timely intervention and an easier and more successful suppression.


There are three detecting methods:

Smoke detection;
Heat detection;
Flame detection.

Smoke Detection

It is one of the mostly used methods. The European rule EN 54 issued by CEN ,Comitato Europeo di Normalizzazione defines smoke detectors as those that react to the volatile particles produced by combustion.

Fundamentally, there are two types:

Optical smoke detectors;
Optical smoke detectors with ionisation chamber.

These optical detectors can also work in two ways:

Extinction optical smoke detectors;
Diffusion optical smoke detectors.

The extinction optical smoke detectors utilise the physical effect of reduced light intensity when its beam is wrapped by smoke.

The diffusion optical smoke detectors utilise the same principle but employ a screen placed between the emitting and the receiving source. When smoke is not present, light is intercepted by the screen and does not activate the photoelectric cell.

The optical smoke detectors with ionisation chamber react to combustion when conductivity in a chamber is modified. By applying a different calibration to the two electrodes the conductivity can be measured. When smoke interferes, a reduction in the degree of ionisation is immediately detected. This variation activates the alarm system.

Heat Detection

They react to an increase of temperature in a certain environment.

They are of two types:

Thermostatic detectors;
Thermo speedometer detectors.

The thermostatic detectors are equipped with a sensor reacting to temperature increase.
The sensor can be a thermo store, a slow blow fuse, a metallic foil or the thermal expansion of a liquid.
It is completely inactive until temperature stays below the calibration level.

This can be an advantage when a rapid variation of temperature occurs in places such as kitchen or boilers rooms, but it can be a disadvantage when the rapid increase of temperature is due to a situation of fire.

Another defect of the thermostatic detector lies in the heat inertia of the sensor, so that also in case of high values of temperature some time elapses before the warning sign is activated.
Therefore this type of detector finds its best application in outdoors situations where, in case of fire, a rapid increase of temperature is expected.

The thermo speedometer detectors are equipped with a sensor device calibrated at a certain fixed temperature measured in °C/ min.
They operate according to the principles of thermistors or electric resistance of cables or expansion of liquids.
They are activated when the temperature reaches the calibration level and the more rapid is the increase of temperature the more timely is the warning signal.
These detectors do not react when a slow increase of room temperature occurs due to a compensating effect between the external sensor and the calibrated one.
They find an optimal application in the protection of rooms where a flagrant flame hazard exists.

Optical Detection

In this case the electromagnetic radiations produced by the flame are utilised for signalling a hazard situation.
The sensor therefore translates the optical signal into an electrical one.

It is important for the reliability of this method to make a distinction between signals coming from a real flame and optical situations that have nothing to do with fire, such as sun light, reflected lights, lightings and other optical interferences.
For this reason the sensitivity of detectors is limited between prefixed values in the range of ultraviolet and ultra red electromagnetic radiations.

These flame detectors are of two types:

Optical flame detectors with single channel;
Optical flame detectors with two channels.

The first ones find their best application in rooms where materials with carbon compounds burn with a flame.

The second ones are equipped with two pyroelectric sensors reacting to the infra red radiations at two different length waves. The first sensor reacts to the infra red radiations in the range of carbon dioxide emissions produced by materials such as wood, oil products, plastics, alcohols etc. The second sensor instead monitors external sources emitting signals similar to fire such as sun light, artificial light, or emissions from hot materials.
The signals from the two sensors are compared in their width and synchronism by an electronic circuit. This type of control is very scarcely influenced by external situations similar to flames.

Smoke Detector

They can be divided into two types:

Punctiform detectors reacting in an area which can be assimilated to a fixed point of control;
Linear detectors reacting along an area assimilated to an ideal continuous fixed line.

The linear smoke detectors work under the principle of reduced brightness in the presence of smoke.

They find their best application in the protection of loft buildings, barns, garages, premises with strong air streams, cable lines etc.