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A flame detector is a sensor designed to detect and respond to the presence of a flame or fire. Responses to a detected flame depend on the installation, but can include sounding an alarm, deactivating a fuel line (such as a propane or a natural gas line), and activating a fire suppression system. When used in applications such as industrial furnaces, their role is to provide confirmation that the furnace is properly lit; in these cases they take no direct action beyond notifying the operator or control system. A flame detector can often respond faster and more accurately than a smoke or heat detector due to the mechanisms it uses to detect the flame.
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Ultraviolet Detectors
Ultraviolet (UV) detectors work by detecting the UV radiation emitted at the instant of ignition. While capable of detecting fires and explosions within 3–4 milliseconds, a time delay of 2–3 seconds is often included to minimize false alarms which can be triggered by other UV sources such as lightning, arc welding, radiation, and sunlight. UV detectors typically operate with wavelengths shorter than 300 nm. The solar blind UV wavelength band is also easily blinded by oily contaminants.
Near IR array
Near infrared (IR) array flame detectors, also known as visual flame detectors, employ flame recognition technology to confirm fire by analyzing near IR radiation using a charge-coupled device (CCD).
Infrared
Infrared (IR) flame detectors monitor the infrared spectral band for specific patterns given off by hot gases. These are sensed using a specialized fire-fighting thermal imaging camera (TIC), a type of thermographic camera. False alarms can be caused by other hot surfaces and background thermal radiation in the area. Water on the detector's lens will greatly reduce the accuracy of the detector, as will exposure to direct sunlight. A single-frequency IR flame detector is typically sensitive to wavelengths around 4.4 micrometers, which is a spectral characteristic peak of hot carbon dioxide as is produced in a fire. The usual response time of an IR detector is 3–5 seconds.
Infrared Thermal Cameras
MWIR Infrared (IR) cameras can be used to detect heat and with particular algorithms can detect hot-spots within a scene as well as flames for both detection and prevention of fire and risks of fire. These cameras can be used in complete darkness and operate both inside and outside.
UV/IR
These detectors are sensitive to both UV and IR wavelengths, and detect flame by comparing the threshold signal of both ranges. This helps minimize false alarms.
IR/IR flame detection
Dual IR (IR/IR) flame detectors compare the threshold signal in two infrared ranges. Often one sensor looks at the 4.4 micrometer carbon dioxide (CO2), while the other sensor looks at a reference frequency. Sensing the CO2 emission is appropriate for hydrocarbon fuels; for non-carbon based fuels, e.g., hydrogen, the broadband water bands are sensed.
IR3 flame detection
Triple-IR flame detectors compare three specific wavelength bands within the IR spectral region and their ratio to each other. In this case one sensor looks at the 4.4 micrometer range while the other sensors look at reference wavelengths both above and below 4.4. This allows the detector to distinguish between non-flame IR sources and actual flames which emit hot CO2 in the combustion process. As a result, both detection range and immunity to false alarms can be significantly increased. IR3 detectors can detect a 0.1m2 (1 ft2) gasoline pan fire at up to 65 m (215 ft) in less than 5 seconds. Triple IRs, like other IR detector types, are susceptible to blinding by a layer of water on the detector's window.
Most IR detectors are designed to ignore constant background IR radiation, which is present in all environments. Instead they are designed to detect suddenly changing or increasing sources of the radiation. When exposed to changing patterns of non-flame IR radiation, IR and UV/IR detectors become more prone to false alarms, while IR3 detectors become somewhat less sensitive but are more immune to false alarms.
Visible sensors
In some detectors, a sensor for visible radiation (light) is added to the design in order to better discriminate against false alarms or to improve the detection range.
Video
Closed-circuit television or a web camera can be used for visual detection of (wavelengths between 0.4 and 0.7 µm). Smoke or fog can limit the effective range of these, since they operate solely in the visible spectrum.
Ionization current flame detection
The intense ionization within the body of a flame can be measured by means of a current that flows when a voltage is applied by the phenomena of Flame Rectification. This current can be used to verify flame presence and quality. Such detectors are used in large industrial process gas heaters and are connected to the flame control system. They usually act as both flame quality monitors and for flame failure detection.
These types of sensors are also common in a variety of household gas furnaces.
Thermocouple flame detection
Thermocouples are used extensively for monitoring flame presence in combustion heating systems and gas cookers. A common use in these installations is to cut off the supply of fuel if the flame fails, in order to prevent unburned fuel from accumulating. These sensors measure heat and therefore are commonly used to determine the absence of a flame. This can be used to verify the presence of a Pilot flame.
Applications
UV/IR flame detectors are used in: