A fire detector is a sensor designed to detect and respond to the presence of fire or fire, allowing flame detection . Response to a detected flame depends on installation, but may include sounding an alarm, disabling fuel lines (such as propane or natural gas channels), and activating a fire suppression system. When used in applications such as industrial furnaces, its role is to confirm that the furnace is correct; in which case they do not take direct action outside informing the operator or the control system. Fire detectors can often respond faster and more accurately than smoke or heat detectors because of the mechanisms used to detect flame.
Video Flame detector
Detektor api optik
Detektor ultraviolet
Ultraviolet (UV) detector works by detecting UV radiation emitted at the time of ignition. While capable of detecting fires and explosions in 3-4 milliseconds, a delay of 2-3 seconds is often incorporated to minimize false alarms that other UV sources can trigger such as lightning, arc welding, radiation, and sunlight. UV detectors typically operate with wavelengths shorter than 300 nm to minimize the effects of natural background radiation. Band wavelength UV sun blind is also easily blinded by oily contaminants.
Near IR array
The near infrared (IR) array detector (IR) array (0.7-1.0 Âμm), also known as a visual fire detector, uses fire recognition technology to confirm fire by analyzing near IR radiation using a charge-coupled device (CCD). Near infrared sensors (IR) are particularly able to monitor the phenomenon of fire, without too much obstruction from water and moisture. The pyroelectric sensors operating at these wavelengths can be relatively inexpensive. Some channels or pixel array sensor monitors in the near IR band are arguably the most reliable technology available to detect fires. The emission of light from the fire forms a fire image at a certain moment. Digital image processing can be used to recognize flame through video analysis made from near IR images.
Infrared
Infrared (IR) or wideband infrared (1.1Ã,Âμm and higher) flame detector monitors infrared spectral bands for specific patterns released by hot gas. This is felt using a special fire-fighting shooting camera (TIC), a kind of thermography camera. False alarms can be caused by other hot surfaces and background heat radiation in the area. Water on the detector lens will greatly reduce the accuracy of the detector, as it will be exposed to direct sunlight. Specific frequency range is 4.3-4.4 Âμm. This is the resonance frequency of CO 2 . During combustion of hydrocarbons (eg, wood or fossil fuels such as oil and natural gas) a lot of heat and CO 2 are released. The hot CO 2 emits a lot of energy at its resonation frequency of 4.3 Âμm. This causes a peak in total emission of radiation and can be detected well. In addition, the "cold" 2 CO in the air keeps sunlight and other IR radiation filtered off. This makes the sensor in this frequency "sun blind"; However, the sensitivity is reduced by sunlight. By observing the flicker frequency (1 to 20 Hz) the detector is made less sensitive to false alarms caused by heat radiation, for example caused by a heat engine. The multi-infrared detector utilizes the algorithm to suppress the effects of background radiation (black radiation), again the sensitivity is reduced by this radiation.
A severe disadvantage is that almost all radiation can be absorbed by water or water vapor; this is especially true for infrared flame detection in the region of 4.3-4.4 m. From around. 3.5 Ã,Âμm and higher absorption by water or ice almost 100%. This makes infrared sensors for use in external applications very unresponsive to fires. The biggest problem is our ignorance; some infrared detectors have a (self) self-test window detector, but this test only monitors the occurrence of water or ice on the detector window.
Salt films are also dangerous, because salt absorbs water. However, water vapor, mist or light rain also make the sensor almost blind, unbeknownst to the user. The cause is similar to what a fireman does when he approaches a hot fire: he protects himself by using a water vapor screen against very large infrared heat radiation. The presence of water vapor, mist, or light rain will also "protect" the monitor causing it to not see fire. However, visible light will be transmitted through the water vapor screen, as it can be easily seen by the fact that humans can still see fire through the water vapor display.
The usual response time of the IR detector is 3-5 seconds.
Infrared thermal camera
IRIR infrared (IR) cameras can be used to detect heat and with certain algorithms can detect hot spots in a scene and fire to detect and prevent fire and fire risks. This camera can be used in total darkness and operate both inside and outside.
UV/IR
The detector is sensitive to UV and IR wavelengths, and detects flame by comparing threshold signals from both ranges. This helps minimize false alarms.
IR/IR flame detection
The IR Dual IR detector (IR/IR) compares the threshold signal in two infrared ranges. Often one sensor looks at 4.4 micrometers of carbon dioxide (CO2), while other sensors look at the reference frequency. Feel the appropriate CO2 emissions for hydrocarbon fuel; for non-carbon-based fuels, for example, hydrogen, broadband airband is felt.
IR3 flame detection
The Triple-IR fire detector compares three specific wavelengths within the IR spectrum region and compares it to each other. In this case one sensor looks at the 4.4 micrometer range while the other sensors see the reference wavelengths above and below 4.4. This allows the detector to distinguish between non-fire IR sources and actual flame emitting hot CO 2 in the combustion process. As a result, the range of detection and immunity against false alarms can increase significantly. IR3 detector can detect 0.1 m 2 (1Ã, ft 2 ) gasoline pan fire at up to 65 m (215Ã, ft) in less than 5 seconds. Triple IRs, like other IR detector types, are susceptible to constriction by the water layer in the detector window.
Most IR detectors are designed to ignore the constant IR background radiation, which exists in all environments. Instead they are designed to detect sudden changes or increase the source of radiation. When exposed to a non-fire IR radiation pattern change, IR and UV/IR detectors become more susceptible to false alarms, while IR3 detectors become somewhat less sensitive but more resistant to false alarms.
Visible sensors
Visible light sensors (eg camera: 0.4 to 0.7 Âμm) capable of displaying images, which can be understood by humans. Furthermore, complex image processing analysis can be performed by computers, which can recognize flame or even smoke. Unfortunately, the camera can be blinded, like a human, by thick smoke and by fog. It is also possible to mix visible light information (monitors) with UV or infrared information, to better discriminate against false alarms or to increase detection ranges. Corona cameras are an example of this equipment. In this equipment the information from the UV camera is mixed with the visible image information. This is used to track defects in high voltage equipment and fire detection at a distance.
In some detectors, sensors for visible radiation (light) are added to the design.
Video
Closed circuit television or web camera can be used for visual detection (wavelengths between 0.4 and 0.7 Âμm). Smoke or fog can limit this effective range, as they operate only in visible spectrum.
Maps Flame detector
Other types
Flame detector ionization detection
The intense ionization in the flame body can be measured by using the current that flows when the voltage is applied by the Flame Rectification phenomenon. This current may be used to verify the presence and quality of the flame. The detector is used in large industrial process gas heaters and is connected to a flame control system. They usually act as both a fire quality monitor and for flame failure detection.
These types of sensors are also common in domestic gas furnaces.
Thermocouple flame detector
Thermocouples are widely used to monitor the presence of flame in combustion heating systems and gas stoves. A common use in this installation is to cut off the fuel supply if the flame fails, to prevent unburned fuel from accumulation. This sensor measures heat and therefore is usually used to determine the flame absence. This can be used to verify the presence of a Pilot flame.
Apps
The UV/IR fire detector is used in:
- Hydrogen station.
- Gas-fired stove
- Industrial heating and drying system
- Domestic heating system
- Industrial gas turbines
Radiation emission
The fire emits radiation, which the human eye experiences as a fire and a visible yellow red fire. In fact, during a fire, the UV energy is relatively rare and visible light energy is emitted, compared to the emission of infrared radiation. Non-hydrocarbon fires, for example, one from hydrogen, do not show CO 2 peaks at 4.3 μm because during burning hydrogen no CO 2 is released. The peak of 4.3Ã,Âμm CO 2 in the image is exaggerated, and in fact less than 2% of the total fire energy. A multi-frequency-detector with sensors for UV, visible light, near IR and/or IR wideband so as to have more "sensor data" to calculate with and therefore be able to detect more types of fires and to detect these types of fires with better. : hydrogen, methanol, ether or sulfur. It looks like a static image, but in reality the energy fluctuates, or flashes. These twinkling are due to the fact that burnt and flammable oxygen is currently burning and permeating new oxygen aspirations and new combustible materials. This small explosion causes flickering flame.
Sunlight
The sun emits large amounts of energy, which would be harmful to humans if not for the vapor and gases in the atmosphere, such as water (clouds), ozone, etc., where sunlight is filtered. In the picture it can be seen clearly that CO "cold" 2 filters out about 4.3 Âμm solar radiation. The infrared detector uses this frequency because it is a blind sun. Not all fire detector manufacturers use sharp filters for 4.3 Âμm radiation and thus still take up quite a bit of sunlight. This cheap fire detector can hardly be used for outdoor applications. Between 0.7 Âμm and around. 3 Âμm there is a relatively large absorption of sunlight. Therefore, this frequency range is used to detect flame by some fire detector manufacturers (in combination with other sensors such as ultraviolet, visible light, or near infrared). The big economic advantage is that window detectors can be made from quartz, not from expensive sapphires. This combination of electro-optical sensors also allows the detection of non-hydrocarbons such as hydrogen fires without the risk of false alarms caused by artificial light or electric welding.
Heat radiation
The infrared flame detector suffers from infrared heat radiation that is not emitted by a possible fire. One can say that fire can be covered by other heat sources. Any object that has a temperature higher than the absolute minimum temperature (0 kelvin or -273.15 Â ° C) emits energy and at room temperature (300 K) this heat is already a problem for the infrared detector with the highest sensitivity. Sometimes a hand moves enough to trigger an IR flame detector. At 700 K the hot object (black body) begins to emit visible light (light). The double or multi-infrared detector suppresses the effects of heat radiation by using sensors that detect only the peak of CO 2 ; for example at 4.1 Âμm. Here it is necessary that there is a large difference in the output between the applied sensors (eg S1 and S2 sensors in the picture). The disadvantage is that the radiation energy from the possibility of fire should be much larger than the current background heat radiation. In other words, the fire detector becomes less sensitive. Every multi-infrared flame detector is negatively affected by this effect, regardless of how expensive it is.
The vision cone
The fire detector cone vision is determined by the shape and size of windows and housing and the location of the sensor in the housing. For infrared sensors also laminate of the sensor material plays part; it limits the cone vision fire detector. The wide vision cone does not automatically mean that the fire detector is better. For some applications, fire detectors need to be aligned properly to be careful not to detect potential background radiation sources. The vision cone of a fire detector is three-dimensional and not necessarily perfectly round. Horizontal and vertical viewing angles are often different; This is largely due to the shape of the housing and by the mirror parts (intended for self-test). Different fuels can even have different angles in the same fire detector. Very important is the sensitivity at an angle of 45 Â °. Here at least 50% of the maximum sensitivity on the central axis must be achieved. Some fire detectors here reach 70% or more. Even this fire detector has a total horizontal viewing angle of more than 90 Â °, but most manufacturers do not mention this. High sensitivity at the edges of the sight angle provides an advantage for the projection of fire detectors.
Detection range
The range of the fire detector is largely determined by the location of the installation. In fact, when making projections, one should imagine what fire detectors "see". A rule of thumb is, that the height of the fire detector installation is twice as high as the highest object in the field of view. Also the accessibility of the fire detector must be taken into account, due to maintenance and/or repair. A stiff lamp post with a pivot point for this reason can be recommended. A "roof" above fire detector (30 x 30 cm, 1 x 1-foot) prevents rapid contamination in outdoor applications. Also the shadow effect must be considered. The shadow effect can be minimized by installing a second fire detector in the first detector opponent. The second advantage of this approach is that the second fire detector is excessive, if the former is not functioning or blinded. Generally, when installing multiple fire detectors, one must let them "see" each other so they do not look at the wall. Following this procedure blind spots (caused by shadow effects) can be avoided and better redundancy can be achieved than if the fire detector will "see" from the center position to the area to be protected. The range of fire detectors to an industry standard fire 30 x 30 cm, 1 x 1-ft is expressed in data sheets and factory manuals, this range may be affected by the previous de-sensitization effects of sunlight, water, fog, steam and black body radiation.
Quadratic law
If the distance between the flame and the flame detector is large compared to the fire dimension then the square law applies: If the fire detector can detect fire with area A at a certain distance, then a 4 times greater fire area is required if the distance between the fire detector and the flame multiplies. In short:
Duplicate = four times larger fire area (fire).
This law applies equally to all optical fire detectors, including video-based ones. Maximum sensitivity can be estimated by dividing the maximum fire area A by the square of the distance between the fire and the fire detector: c Ã, = / d 2 . With this constant c can, for the same fire detector and the same fire type, the maximum distance or minimum fire area is calculated: a = cd 2 and d = ? A / c
But it must be emphasized that the square root in reality is no longer valid at great distances. At a distance, other parameters play an important part; such as the occurrence of water vapor and cold CO 2 in the air. In the case of a very small flame, on the other hand, the decrease of flickering flame will play an increasing part.
Better contact - occurs when the distance between the flame and the small flame detector - between the radiation density, E , the detector and the distance, D , between the detector and the flame of the radius effective, R , emits energy density, M , given by
E = 2? MR 2 / ( R 2 D 2 )
When R & lt; & lt; D then the relation is reduced to quadratic law (inverse)
E = 2? MR 2 / D 2
See also
- Flame
- Active fire protection
- Flame ionization detector
- Gas detector
- Fire alarm system
References
Source of the article : Wikipedia
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