Understand the working principle and characteristics of flame detectors in one article

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A flame detector, also known as a photosensitive fire detector, is a device designed to respond to the optical characteristics of fires; it detects the light intensity generated by burning flames as well as the frequency of flame flickering. There are two types of such detectors: point-type flame detectors and beam-type flame detectors. Point-type flame detectors are more widely used, and Changhui Instruments focuses on introducing these in this article. Point-type flame detectors are photosensitive detectors that respond to infrared and ultraviolet light in the flame radiation spectrum. Based on their working principle, they can be classified into point-type UV flame detectors, point-type IR flame detectors, and point-type combined IR-UV flame detectors. The point-type infrared flame detector is currently the most widely used and technologically advanced type. Changhui Instruments, yunrun.com.cn – Working principle and characteristics of flame detectors. 1. Point-type ultraviolet flame detector: A point-type ultraviolet flame detector is a fire detection device that responds to ultraviolet radiation from an open flame; it is suitable for use in large warehouses, hangars, chemical production and storage facilities, power stations, etc. Its core component is a UV phototube, which is a phototube based on the principle of the photoelectric effect. ①The photoelectric effect is the phenomenon in which, under the influence of light, electrons within a material are ejected from the surface of that material. The electrons that are excited are called photoelectrons. The photoelectric effect is essentially the phenomenon of photoelectron emission that occurs when electrons on the surface of a material absorb the energy of a photon sufficient to overcome their own work function. Whether an electron can be excited depends on the frequency of the incident light; the higher the frequency, the greater the energy of the photons. In this way, a particular element corresponds to a minimum spectral frequency that can excite electrons from it, namely the red limit frequency. ②Ultraviolet phototubes: The ultraviolet phototubes used in most ultraviolet flame detectors, also known as Geiger-Müller tubes, consist of two electrodes enclosed in glass tubes filled with a low-pressure inert gas; tungsten is typically used as the cathode material. When ultraviolet radiation reaches the cathode, it emits photoelectrons, which move toward the anode under the influence of the voltage between the electrodes, thereby generating a current. Due to the effect of the inert gas, photoelectrons collide with gas atoms on their way to the anode, causing the gas to ionize and releasing more charged particles. These charged particles continue to collide with gas atoms, thereby amplifying the current and resulting in an avalanche discharge. Within the atmosphere, thanks to the protection provided by the ozone layer, electromagnetic waves with wavelengths of 280 nm or less in solar radiation are almost completely absorbed. Due to the light-transmitting limitations of the glass envelope surrounding the phototube, the spectral response range of the cathode material in ultraviolet phototubes is from 185 nm to 260 nm. In this way, ultraviolet phototubes that use metal materials such as tungsten as cathodes can achieve a \"day-blind\" effect, effectively preventing false alarms from ultraviolet flame detectors. ③Signal acquisition: After an avalanche discharge occurs in the ultraviolet phototube, its internal resistance decreases, and the voltage on the capacitor is rapidly discharged through the phototube. When the voltage across the capacitor drops to the point where it can no longer drive the emitted photoelectrons toward the anode, the discharge of the phototube stops, and the capacitor continues to charge. When the voltage across the capacitor reaches a level sufficient to move the photoelectrons to the anode, the internal resistance of the phototube decreases again. In this way, each repetition generates a pulse, and the frequency of the pulses depends on the intensity of the ultraviolet light and the electrical parameters of the circuit. When the circuit remains unchanged, the stronger the light, the higher the frequency. When the measured pulse frequency is higher than the alarm set value, the detector emits a fire alarm signal. The point-type UV flame detector is not affected by wind, rain, high humidity, changes in air pressure, etc., and can be used outdoors. However, in some polluted areas where the ozone layer is thin, part of the ultraviolet radiation can penetrate the atmosphere and reach the ground, which has an adverse effect on the operation of point-type ultraviolet flame detectors in outdoor environments and increases the likelihood of false alarms. When using in such areas or in locations where lightning occurs frequently and a large amount of arc light is generated, certain measures must be taken to prevent false fire alarms. Generally, in such locations, it is recommended to use point-type infrared flame detectors or point-type composite infrared-ultraviolet flame detectors. 2. Point-type infrared flame detectors: Detectors that respond to infrared radiation with wavelengths greater than 700 nm within the optical radiation emitted by flames are known as point-type infrared flame detectors. Point-type infrared flame detectors can be classified into point-type single-band infrared flame detectors, point-type dual-band infrared flame detectors, and point-type multi-band infrared flame detectors, depending on the number of infrared pyroelectric sensors they contain. ①Infrared pyroelectric sensors: For certain dielectrics, the polarization state that arises under the influence of an applied voltage does not disappear once the voltage is removed; this phenomenon is known as spontaneous polarization. The intensity of spontaneous polarization is related to temperature; it decreases as the temperature rises. When the temperature rises to a certain value, spontaneous polarization disappears suddenly; this temperature is known as the Curie point. When a dielectric material is exposed to infrared radiation, its internal temperature rises, causing the spontaneous polarization intensity to decrease. Consequently, the charges on its surface are released. When the temperature reaches the Curie point, all charges are released. This phenomenon is known as the pyroelectric effect of dielectrics. Infrared pyroelectric sensors are made based on this principle. The pyroelectric element of a sensor is its detection unit. The dielectric acts as an equivalent capacitor; when polarized by an external voltage, it is essentially charged. Under infrared light irradiation, the temperature of the dielectric rises, leading to discharge. In this way, a voltage drop occurs across the equivalent resistance. By measuring the monitoring current, a control signal can be obtained. The control signal is proportional to the change in infrared light intensity. If the infrared light intensity remains constant, the temperature of the dielectric no longer rises, and the surface charge reaches a new state of polarization equilibrium. At this point, it is as though the equivalent capacitor has been recharged to saturation; thus, no discharge occurs, and no signal is output. Only after the surface charges on the dielectric reach an equilibrium state can a further increase in temperature lead to charge release. Therefore, for an infrared pyroelectric sensor to operate, it must be exposed to infrared light with continuously varying intensity in order to generate continuous signal pulses. The measured voltage drop is typically only about 1 mV; therefore, the signal must be amplified using an amplifier. ②Point-type single-band infrared flame detector: Within the infrared spectrum range of radiation emitted by ordinary open-flame flames, the radiation intensity is highest at wavelengths of 4.1–4.7 μm. This is because when hydrocarbons (natural gas, alcohol, gasoline, etc.) burn, they produce a large amount of heated CO2 gas; this heated CO2 has its maximum infrared radiation intensity at around 4.35 μm. On the Earth's surface, due to the absorption by CO2 and water vapor, the infrared light at wavelengths of 2.7 μm and 4.35 μm in the solar radiation spectrum is almost completely absent. Therefore, the detection wavelength chosen for the sensing element of an infrared flame detector can be set around 2.7 μm or 4.35 μm, which allows for maximum absorption of the infrared radiation generated by the flame, improves detection efficiency, and at the same time prevents sunlight from affecting the detector. Currently, most infrared flame detectors choose a response band around 4.35 μm. A narrow-band filter is installed inside the infrared pyroelectric sensor, allowing it to pass only infrared light around 4.35 μm, while preventing solar radiation from passing through. Generally, the transmission range of the selected filters is 4.3–4.5 μm. After infrared radiation reaches the infrared sensor through various optical components, the generated signal is sent to an amplification circuit. If the output signal exceeds the alarm threshold multiple times within a specified time period, the system emits an alarm signal. ③Point-type dual-band infrared flame detectors can detect flames by analyzing the spectral characteristics of hydrocarbon gases. It has been found that the hot CO2 gas in the combustion products exhibits a unique peak radiation band around 4.3 μm. Dual-band infrared detectors are typically designed to respond to this peak radiation; simultaneously, background radiation in the vicinity of this peak band (3.8–4.1 μm) is used as a reference for detection. The signal processing circuit of the detector analyzes and processes the radiation signals received in the above two bands from the following aspects, in order to distinguish flames from other sources of interference: the flicker of the signal, the signal strength received in a single band (threshold analysis), and the ratio of the signal strengths received by the two detectors. Two factors related to CO2 limit the detection range of infrared dual-band flame detectors. Firstly, the CO2 gas present in the air has a strong absorbing effect on the peak radiation emitted by flames at the 4.3 μm wavelength. As a result, the radiation signal undergoes significant attenuation as the detection distance increases. When the detection distance becomes sufficiently large, the received flame radiation signal falls below a certain threshold, causing the flame detector to lose its ability to respond to the flame signal. The attenuation of the signal with detection distance also affects the ratio of the radiation signals received in the two bands. When the signal intensity in the peak radiation band decreases to the same level as that in the background radiation band (i.e., the ratio between the two signals becomes 1:1), the logical analysis circuit of the flame detector also loses its ability to distinguish the flame signal ； Second, the infrared sensors used in currently developed infrared dual-band flame detectors (such as thermopiles, pyroelectric sensors, lead selenide, etc.) all exhibit a certain level of internal noise, and they all suffer from the drawback of a low signal-to-noise ratio. As the detection distance increases, due to the attenuation of the radiation signal, the difference between the intensity of the flame signal received by the sensor and the sensor’s internal noise becomes less distinct; the flame signal may be overwhelmed by the noise from the detector and fail to be detected. In such cases, more complex logical analysis circuits must be employed in order to identify the flame. http://yunrun.com.cn/upload/202304/19/202304192343589698.png Figure 1: Relationship between different wavelength bands and radiation intensity. The point-type multi-band infrared flame detector uses 3 infrared sensors with extremely narrow detection wavelength bands as detection elements; the detection wavelength bands covered by each of these 3 sensors are shown in the figure. As can be seen from the figure, in addition to using the CO2 peak radiation as the main detection target just like ordinary infrared flame detectors, the 3-band infrared flame detector also selects a narrow band on each side of the CO2 peak radiation band as monitoring targets, to distinguish between high-temperature infrared radiation sources and background radiation. Since any infrared radiation source has its own unique spectral characteristics in these three bands, by comparing the mathematical relationships between the radiation intensities in the three bands, it is possible to distinguish a flame from other infrared radiation sources. Furthermore, the infrared three-band flame detector effectively addresses the issue of the detection signal weakening as the detection distance increases. Even though the radiation signals from the three bands are attenuated due to air absorption, the mathematical relationship between their intensities remains unchanged despite this attenuation. By using digital correlation techniques to analyze the received signals, it is possible to detect flame information that would otherwise be obscured by noise due to attenuation, thereby significantly improving the detector’s detection range and sensitivity. Tests have shown that the effective detection range of the infrared three-band flame detector is at least 4 times greater than that of conventional flame detectors. Thanks to the advanced nature of its detection principle, the infrared three-band flame detector is immune to interference from other non-flame infrared radiation sources (such as lighting sources, blackbody and graybody radiators, etc.) except for continuous, modulated, or periodically varying radiation sources; thus, its false alarm rate is extremely low. ⑤Point-type composite infrared-ultraviolet flame detector: A point-type composite infrared-ultraviolet flame detector is one that incorporates both ultraviolet phototubes and infrared pyroelectric sensors in a single detector. By analyzing a large amount of experimental data and using scientific algorithms, an optimal operation mode to minimize false alarms is designed. The detector issues a fire alarm signal only when the infrared radiation level and the ultraviolet radiation level each reach a specific value, thereby preventing false alarms to a greater extent. Related Reading ◆IFD-IR-101_Fire Detector_IFD-UV-101 ◆Application of the IFD-UV-111C fire detector in steel plant boilers