Introduction of Fire Detectors

Overview

The fire detector is a device used in the automatic fire alarm system for fire protection to detect the scene and find the fire. The fire detector is the “sense organ” of the system, and its function is to monitor whether there is a fire in the environment. Once there is a fire, the characteristic physical quantities of the fire, such as temperature, smoke, gas and radiation intensity, are converted into electrical signals, and an alarm signal is sent to the fire alarm controller immediately.

Working principle

Sensitive element: As part of the construction of the fire detector, the sensitive element can convert the characteristic physical quantities of the fire into electrical signals.

Circuit: Amplify the electrical signal converted by the sensitive element and process it into the signal required by the fire alarm controller.

1. conversion circuit

It converts the electrical signal output by the sensitive element into an alarm signal with a certain amplitude and in line with the requirements of the fire alarm controller. It usually includes matching circuits, amplifier circuits and threshold circuits. The specific circuit composition depends on the type of signal used by the alarm system, such as voltage or current step signal, pulse signal, carrier frequency signal and digital signal.

2. Anti-interference circuit

Due to external environmental conditions, such as temperature, wind speed, strong electromagnetic field, artificial light and other factors, the normal operation of different types of detectors will be affected, or false signals may be caused to cause false alarms. Therefore, the detector should be equipped with an anti-jamming circuit to improve its reliability. Commonly used are filters, delay circuits, integrating circuits, compensation circuits, etc.

3. protect the circuit

Used to monitor detectors and transmission line failures. Check whether the test circuit, components and components are in good condition, monitor whether the detector works normally; check whether the transmission line is normal (such as whether the connecting wire between the detector and the fire alarm controller is connected). It consists of a monitoring circuit and an inspection circuit.

4. Indicating circuit

Used to indicate whether the detector is active. After the detector moves, it should give a display signal by itself. This kind of self-action display usually sets the action signal light on the detector, which is called the confirmation light.

5. Interface Circuit

It is used to complete the electrical connection between the fire detector and the fire alarm controller, the input and output of the signal, and to protect the detector from damage due to installation errors.

It is the mechanical structure of the detector. Its function is to organically connect components such as sensing elements, circuit printed boards, connectors, confirmation lights and fasteners into one, to ensure a certain mechanical strength and achieve the specified electrical performance, so as to prevent the environment such as light source, light source, Sunlight, dust, airflow, high-frequency electromagnetic waves and other interference and destruction of mechanical force.

Application

The automatic fire alarm system consists of a fire detector and a fire alarm controller. Once there is a fire, the characteristic physical quantities of the fire, such as temperature, smoke, gas and radiant light intensity, are converted into electrical signals and act immediately to send an alarm signal to the fire alarm controller. For flammable and explosive occasions, the fire detector mainly detects the gas concentration in the surrounding space, and alarms before the concentration reaches the lower limit. In individual cases, fire detectors can also detect pressure and sound waves.

Classification

(1) Thermal fire detector: This is a fire detector that responds to abnormal temperature, temperature rise rate and temperature difference. It can also be divided into fixed temperature fire detectors – fire detectors that respond when the temperature reaches or exceeds a predetermined value; differential temperature fire detectors that respond when the heating rate exceeds a predetermined value: differential fixed temperature fire detectors – A temperature-sensing fire detector with both differential temperature and constant temperature functions. Due to the use of different sensitive components, such as thermistors, thermocouples, bimetals, fusible metals, membrane boxes and semiconductors, various temperature-sensitive fire detectors can be derived.

(2) Smoke detector: This is a fire detector that responds to solid or liquid particles produced by combustion or pyrolysis. Because it can detect the concentration of aerosols or smoke particles generated in the early stage of combustion of substances, some countries call smoke detectors “early detection” detectors. Aerosol or smoke particles can alter light intensity, reduce the ionic current in the ionization chamber and alter certain properties of the electrolytic constant semiconductor of air capacitors. Therefore, smoke detectors can be divided into ion type, photoelectric type, capacitive type and semiconductor type. Among them, photoelectric smoke detectors can be divided into two types: light-reducing type (using the principle of blocking the light path by smoke particles) and astigmatism type (using the principle of light-scattering by smoke particles).

(3) Photosensitive fire detectors: Photosensitive fire detectors are also known as flame detectors. This is a fire detector that responds to the infrared, ultraviolet, and visible light radiated by the flame. There are mainly two types of infrared flame type and ultraviolet flame type.

(4) Gas fire detector: This is a fire detector that responds to gases produced by combustion or pyrolysis. In flammable and explosive occasions, the concentration of gas (dust) is mainly detected, and the alarm is generally adjusted when the concentration is 1/5-1/6 of the lower limit concentration. The sensing elements used for gas fire detectors to detect gas (dust) concentration mainly include platinum wire, diamond palladium (black and white elements) and metal oxide semiconductors (such as metal oxides, perovskite crystals and spinels).

(5) Composite fire detector: This is a fire detector that responds to more than two fire parameters. There are mainly temperature-sensing smoke detectors, photosensitive smoke detectors, photosensitive temperature-sensing fire detectors, etc.

Selection guide

1. In most general places, such as hotel rooms, shopping malls, office buildings, etc., point-type smoke detectors should be used, and photoelectric smoke detectors should be preferred. In occasions with more black smoke, ion smoke detectors should be used.

2. In places where it is not suitable to install or install smoke detectors that may cause false alarms, or where there is less smoke and rapid temperature rise when a fire occurs, fire detectors such as temperature sensors or flames should be used.

3. In tall spaces, such as exhibition halls, waiting halls, tall workshops, etc., infrared beam smoke detectors should generally be used. When conditions permit, it is advisable to combine it with the TV monitoring system, and select image-type fire alarm detectors (dual-band flame detectors, optical cross-section smoke detectors)

4. In special important or high fire hazard places where fire needs to be detected early, such as important communication room, large computer room, electromagnetic compatibility laboratory (microwave darkroom), large three-dimensional warehouse, etc., it is advisable to use high-sensitivity. Air duct style smoke detector.

5. In the places where the accuracy of the alarm is high, or the false alarm will cause losses, the composite detector (smoke temperature composite, smoke light composite, etc.) should be selected.

6. In the places that need to be linked for fire extinguishing control, such as controlling the computer room gas fire extinguishing, controlling the deluge system fire extinguishing, etc., in order to prevent misoperation, two or more detectors and doors should be used to control fire extinguishing, such as point-type smoke detection. And heat detectors, infrared beam smoke and cable temperature detectors, smoke and flame detectors, etc.

7. In large bays where the detection area is not required to be used as an alarm area in detail, such as garages, etc., in order to save investment, non-address code detectors should be selected, and several detectors share one address.

8. According to the “Code for Design of Garages, Repair Garages and Parking Lot” and the current high requirements for automobile exhaust emission standards, in order to achieve early warning, smoke detectors should be used in well-ventilated garages, but it is necessary to install smoke detectors. It is set at a lower sensitivity.

In some places where the space is relatively small and the density of combustibles is high, such as under electrostatic floors, cable trenches, cable wells, etc., temperature sensing cables can be used.

Maintenance

After the detector has been put into operation for 2 years, it should be cleaned every 3 years. Now taking the ion detector as an example, the dust in the air sticks to the surface of the radioactive source and the ionization chamber, which weakens the ion flow in the ionization chamber, which will make the detector prone to false alarms. The radioactive source will be slowly corroded, and if the radioactive source in the ionization chamber is corroded more than the radioactive source in the reference chamber, the detector will be prone to false alarms; on the contrary, the alarm will be delayed or not alarmed. In addition, the parameter drift of the electronic components in the detector cannot be ignored, and the cleaned detector must be electrically calibrated and adjusted. Therefore, after changing the source, cleaning, and adjusting the electrical parameters of the detector, and its index reaches the index of the new detector when it leaves the factory, these cleaned detectors can be replaced. Therefore, in order to ensure that the detector can work normally for a long time, it is very necessary to send the detector to a professional cleaning factory for regular overhaul and cleaning.

Matters needing attention

1. Make a record of the address of the tested smoke detectors, so as to avoid repeated testing of the same point;

2. In the process of adding smoke test, record the delay of the detector alarm, and through the final summary, have a general understanding of the working status of the smoke detectors in the whole station, which is the next step whether to detect the smoke detector. Provide evidence that the device is cleaned;

3. During the test, it should be checked whether the address of the smoke detector is accurate, so as to re-adjust the address of the smoke detector whose address and room do not match the number in time, so as to prevent wrong instructions to the central control during the disaster relief process. room.

Troubleshooting

First, due to environmental pollution (such as dust, oil fume, water vapor), especially after environmental pollution, smoke or temperature detectors are more likely to generate false alarms in humid weather. The treatment method is to remove the smoke or temperature detectors that have falsely alarmed due to environmental pollution, and send them to professional cleaning equipment manufacturers for cleaning and re-installation.

Second, a false alarm is generated due to the circuit failure of the smoke or temperature detector itself. The solution is to replace the new smoke or temperature detector.

The third is that a false alarm occurs due to a short circuit in the line of the smoke or temperature detector. The processing method is to check the line related to the fault point, and find the short circuit point for processing.