Introduction to the principles and classification of fire alarms

    There are many types of fire detectors, which are roughly as follows: ion smoke detectors, photoelectric smoke detectors, temperature detectors (including fixed temperature and differential temperature), gas detectors, infrared detectors, and ultraviolet detectors. Here I would like to introduce to you the most commonly used fire alarm principles.



1. Fire Alarm Principle Smoke Fire Detector

    The fire development process can be roughly divided into the initial stage, the development stage, and the attenuation and extinguishing stage. The function of the smoke fire detector is to automatically send out a fire alarm signal in the initial stage of smoke generation, in order to extinguish the fire before it becomes a disaster. According to different structures, smoke detectors can be divided into ion smoke detectors and photoelectric smoke detectors.

    ① Ion smoke detector The

    ion smoke detector is composed of two series chambers containing Am241 radioactive sources, field effect tubes and switching circuits. The inner ionization chamber is the compensation chamber, which is sealed and smoke is not easy to enter; the outer ionization chamber is the detection chamber, which is open and smoke can enter smoothly. Directly connect the 24V DC power supply to both ends of the two ionization chambers in series. When a fire occurs, smoke enters the detection ionization chamber, and the α rays generated by Am241 are blocked, which reduces its ionization ability, thereby reducing the ionization current and increasing the equivalent impedance of the air in the detection ionization chamber. Since there is no smoke entering the compensation ionization chamber, the impedance of the ionization chamber remains unchanged. Therefore, the voltage divider ratio applied to both ends of the two ionization chambers changes. When the voltage increase at both ends of the detection ionization chamber reaches a certain value, the switch circuit operates and sends out an alarm signal.

    ② Photoelectric smoke detector

    Photoelectric smoke detector consists of a light source, a photoelectric element and an electronic switch. The light scattering principle is used to detect the smoke generated in the early stage of the fire and send out an alarm signal in time. According to the light source, it can be divided into four types: general photoelectric, laser photoelectric, ultraviolet photoelectric and infrared photoelectric.

    a. General photoelectric smoke detectors can be divided into two types according to their structural characteristics: light-shielding type and scattering type.

    The light-shielding type photoelectric smoke detector consists of a light source (bulb or light-emitting diode) and a photoelectric element installed in a small dark room. In the absence of smoke, the light emitted by the light source is focused into a beam through the lens, irradiated onto the photoelectric element, and converted into an electrical signal, so that the entire circuit is maintained in a normal state and no alarm is issued. When a fire occurs and smoke enters the detector, the propagation characteristics of the light are changed, the light intensity is significantly weakened, the normal state of the circuit is destroyed, and an alarm signal is issued.

    The positions of the light-emitting diode and the photoelectric element of the scattered photoelectric smoke detector are not corresponding. The photoelectric element is set in a porous small dark room. When there is no smoke, the light cannot be projected onto the photoelectric element, and the circuit maintains a normal state. When a fire occurs, smoke enters the detector, and the light reaches the photoelectric element through reflection or scattering of smoke particles. The optical signal is converted into an electrical signal, which is amplified by the amplifier circuit and drives the automatic alarm device to issue an alarm signal.

   b. Laser smoke detector. It consists of a laser transmitter (including a pulse power supply and a laser generator) and a laser receiver (including a photoelectric receiver, pulse amplification and alarm).
It uses the characteristics of strong directivity, high brightness, monochromaticity and good coherence of lasers. In the absence of smoke, the pulse laser beam is projected onto the photoelectric receiver and converted into an electrical signal, and the alarm does not issue an alarm. Once the laser beam is blocked by smoke during the emission process and weakened to a certain extent, the photoelectric receiver signal will be significantly weakened, and the detector will send out an alarm signal. Among the various types of laser light sources, semiconductor lasers are valued for their advantages such as low required excitation voltage, high efficiency, large pulse power, small device size, shock resistance, long life and low price.

   c. Ultraviolet and infrared smoke detectors. They have the advantages of high sensitivity, stable performance, reliability, accurate detection direction, etc., so they are widely valued and have become an important equipment and development direction of fire detectors.

    Photoelectric smoke detectors are developing rapidly and their types are increasing. In terms of their functions, they can realize early fire alarm. In addition to being used inside large buildings, they are also particularly suitable for places with high electrical fire risks, such as computer rooms, instrument rooms, cable trenches, tunnels, etc.


2. Principle of Fire Alarm Temperature-sensitive Fire Detector

    ① Bimetallic type uses two metal sheets with different expansion coefficients as sensitive elements. It is generally made into two forms: differential temperature and fixed temperature. The fixed temperature type is that when the ambient temperature rises to the set temperature, the fixed temperature component immediately acts and sends out an alarm signal; the differential temperature type is that when the ambient temperature rises sharply and its temperature rise rate (℃/min) reaches or exceeds the action temperature rise rate specified by the detector, the differential temperature component immediately acts and sends out an alarm signal.

    ② The membrane box type detector consists of a corrugated plate to form an air chamber, and the indoor air can only communicate with the atmosphere through the small hole of the air plug screw. Under normal circumstances (the ambient temperature rise rate is not more than 1℃/min), the air chamber is heated, and the expanded gas in the room can leak into the atmosphere through the small hole of the air plug screw. When a fire occurs, the temperature rise rate increases sharply, the air pressure in the air chamber increases, the corrugated plate bulges upward, pushes the elastic contact piece, connects the electrical contact, and sends out an alarm signal.

    ③The electronic temperature detector consists of two thermistors with the same resistance and temperature characteristics and an electronic switch circuit. One of the two thermistors can directly sense changes in ambient temperature, while the other is enclosed in a small ball with a certain heat capacity. When the external temperature changes slowly, the resistance of the two thermistors is basically close to the temperature change, and the switch circuit does not work. When a fire occurs, the ambient temperature rises sharply, the resistance of the two thermistors changes differently, the original stable state is destroyed, the switch circuit opens, and an alarm signal is issued.