A NOTE ON FIRE DETECTORS

 

A NOTE ON FIRE DETECTORS

 Fire detectors are meant to detect any one of the below characteristics of a fire

·        Heat

·        Smoke

·        Radiation (given by flame)

 There are different detectors available to choose from, but we cannot conclude that a particular detector is suitable for all applications.  Every detector whether it designed to detect either heat, flame or smoke or sometimes a combination, each one of them has their advantages and disadvantages, and no one type of detector is most suitable for all applications, the choice of a detector depends primarily on

      THE SPEED OF RESPONSE: The faster a detector detects the particular characteristic of fire (smoke, heat or flame) the faster would be the response to extinguish the fire.

       FALSE ALARMS: The false alarms can be an outcome of malicious, mistaken or accidental intention, too much detector sensitivity can also increase false alarms, smoke detectors are faster than heat detectors but may give more false alarms than heat detectors.

3.    NATURE OF FIRE HAZARD: It is important to understand the nature of the fire hazard that exists before selecting the required fire detector.  Some fires give more smoke and some intense heat.  The selection should be based on the requirement

4.    COST:  Using several types of detectors and reducing the space between detectors may help in a better system, but that would increase the cost which may not be feasible for some industries.

5.  SUITABILITY OF THE ENVIRONMENT: Dusty environments will provide false alarms if smoke detectors are used, near heat generation, equipment certain type of heat detectors may give false alarms, the suitability of the environment also plays a vital role

TYPES OF FIRE DETECTORS

Fire detectors can be classified into

·        HEAT DETECTORS

·        SMOKE DETECTORS

·        FLAME DETECTORS

·        SPARK/EMBER DETECTORS

HEAT DETECTORS: These detectors work on the principle of heat detection in the fire. These heat-sensitive fire detectors sense heat due to the temperature rise with the help of probes/sensors which detect heat during the early stages of fire.

Typical examples are bimetallic type, fusible alloy type, pneumatic type, etc.Heat sensitive detectors (both resettable and non-resettable) are of the following types

1.      FIXED TEMPERATURE TYPE HEAT DETECTOR

2.      RATE OF RISING – CUM FIXED TEMPERATURE HEAT DETECTOR

3.      PROBE TYPE HIGH-TEMPERATURE BI-METAL HEAT DETECTOR

4.      LINEAR HEAT SENSING CABLE TYPE HEAT DETECTOR

1)FIXED TEMPERATURE DETECTOR:  These detectors designed to operate at a fixed predetermined value, when the temperature of the detector exceeds the set predetermined value then immediately the probe senses it and gets activated.

2)RATE OF RISING – CUM FIXED TEMPERATURE HEAT DETECTOR: These detectors are designed to operate based on the rate of rising in temperature based on time.

3)PROBE TYPE HIGH-TEMPERATURE BI-METAL HEAT DETECTOR: In places like oven and furnace areas where electronic sensors cannot be used and where the temperature is above 80⁰C these bimetallic heat detectors are highly suitable

4)LINEAR HEAT SENSING CABLE TYPE HEAT DETECTOR: In the case of a Digital sensor, the heat-sensitive insulator separates the two core cables consist of two core cable in which the conductors are separated by a heat-sensitive insulator. When a specified temperature is reached, the cable insulation breaks down and an alarm is indicated

SMOKE DETECTORS: These detectors work on the principle of Invisible aerosols and smoke that gets released in certain cases during the early stage of fire. There are three types of smoke detectors like

1.      IONIZATION SMOKE DETECTOR

2.      OPTICAL (PHOTOELECTRIC) SMOKE DETECTOR

3.      AIR SAMPLING TYPE SMOKE DETECTOR

1)IONIZATION TYPE SMOKE DETECTOR:  These detectors are designed to sense aerosols given off by fires with the help of Ionizing chambers fixed within the smoke detectors.

2)OPTICAL (PHOTOELECTRIC) SMOKE DETECTOR: These detectors are designed to operate when the light gets attenuated or scattered by the presence of smoke particles. This attenuation or scattering of light alters the standard condition and alarm is raised 

3)AIR SAMPLING TYPE SMOKE DETECTOR:  These types of detectors are based on a system of piping or tubing distribution network that runs from the detector to the areas to be protected. An aspiration fan in the detector housing draws air from the protected area back to the detector through air sampling ports, piping or tubing. At the detector, the air is analyzed for fire products.

FLAME DETECTORS: These detectors work on the principle of sensing the emitted radiant energy released from the fire combustion process at specific wavelength bands determined by the combustion chemistry of the fuel. There are classified into

1.      UV FLAME DETECTOR

2.      IR FLAME DETECTOR

3.      IR/UV FLAME DETECTOR

1)UV FLAME DETECTOR:  These detectors use ultraviolet-sensitive photocathode for detecting flame. It has a narrow spectral sensitivity of 185 to 260 nm which is insensitive to visible light.

2)IR FLAME DETECTOR: These detectors are designed to operate in the INFRARED spectrum of light. They sense wavelength in the infrared spectrum

3) IR/UV FLAME DETECTOR: These detectors are designed to operate at both UV AND INFRARED spectrum

EMBER /SPARK DETECTORS: Spark/ember detectors are installed primarily to detect sparks and embers that could if allowed to continue to bum, precipitate a much larger fire or explosion These detectors sense radiant energy from sparks or embers, or both. These devices are normally intended to operate in dark environments and the infra-red part of the spectrum.

These SPARK/EMBER detectors use phototransistors or photodiodes to sense the radiant energy from sparks or embers. These detectors are extremely sensitive (microwatts) and their response times can be made very short (microseconds). Spark/ember detectors are installed primarily to detect sparks and embers that could if allowed to continue to bum, precipitate a much larger fire or explosion.

CONCLUSION:  Each type of detector is designed to detect a particular type of fire. Hence in any building, careful planning for a combination of various detectors is necessary depending upon the type of fire hazard and nature of fire that can exist at that point. The fire behavior of the contents of each part of the building and the design of the building should be considered during planning for installing the detectors. The susceptibility of the contents to heat, smoke, and water damage shall also be considered. Hence any type of detectors / or combination of detectors chosen shall be based on the conditions that provide the earliest reliable warning.

                                                                                                                                                             FIRE Notes by PJ Mohan

                                                                                                                                                                            Sr. Faculty, NIFS