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Flame Detection A flame detector responds either to radiant energy visible to the human eye (approx. 4000 to 7700 A) or outside the range of human vision. Similar to the human eye, flame detectors have a 'cone of vision', or viewing angle, that defines the effective detection capability of the detector. FIGURE With this constraint , the sensitivity increases as the angle of incidence decreases. Such a detector is sensitive to glowing embers, coals, or flames which radiate energy of sufficient intensity and spectral quality to actuate the alarm. Each type of fuel, when burning, produces a flame with specific radiation characteristics. A flame detection system must be chosen for the type of fire that is probable. For example an ultraviolet (UV) detector will respond to a hydrogen fire, but an infrared (IR) detector operating in the 4.4 micron sensitivity range will not. FIGURE It is imperative, therefore, that a qualified fire protection engineer be involved in the design of these systems, along with assistance from the manufacturer's design staff. Due to their fast detection capabilities, flame detectors are generally used only in high-hazard areas, such as fuel-loading platforms, industrial process areas, hyperbaric chambers, high-ceiling areas, and atmospheres in which explosions or very rapid fires may occur. Because flame detectors must be able to 'see' the fire, they must not be blocked by objects placed in front of them. The infrared-type detector, however,has some capability for detecting radiation reflected from walls. UV Flame Detectors ___________________________________________________________________ Ultraviolet (UV) detectors generally use either a solid-state device, such as silicone carbide or aluminum nitride, or a gas-filled tube as the sensing element. UV detectors are essentially sensitive to both, sunlight and artificial light. A UV flame radiates in the 1850 to 2450 angstrom range. Virtually all fire emit radiation in this band, while the sun's radiation at this band is absorbed by the Earth's atmosphere. The result is that the UV detector is solar blind, meaning it will not cause an alarm in response to radiation fromthe sun. The implication of this feature is that it can easily be used both indoors and outdoors. UV detectors are sensitive to most fires, including hydrocarbon, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays used in nondestructive metal testing equipment, and radiaacctive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to 'see' a flame. Likewise, the presence of an oil mist in the air or an oil film on the detector window will have the same effect. UV/IR Flame Detector An ultraviolet/infrared flame detectorconsists of an UV and single-frequency sensor, paired together to form one unit. The two sensors individually operate the same as described in the UV and IR sections, but additional circuitry processes signals from both sensors. A fire alarm is produced only when both sensors detect a fire. The result is that a UV/IR system has better false alarm rejection capabilities than either detector by itself. Since the UV/IR detector pairs two sensor types, it is subject to the limitation of both.
___________________________________________________________________ An infrared (IR) detector basically is composed of a filter and system used to screen out unwanted wavelengths and focus the incoming energy on a photovoltanic or photoresistive cell sensitive to infrared radiation. IR flame detectors can respond to the total IR component of the flame alone or in combination with flame flicker inthe frequency range of 5 to 30 Hz. A major problem in the use of infrared detectors receiving only IR radiation is the possible interference of solar radiatino in the infrared bandwidth. When detectors are located in places shielded from the sun, such as vaults,then filtering and/or shielding the unit from the sun rays is unnnecessary. IR detectors are sensitive to most hydrocarbon fires (liquid, gases and solids). Fires, such as burning metals, ammonia, hydrogen, and sulfur do not emit significant amounts of IP radiation in the 4.4 micron sensitivity range of most IR detectors. Thorough planning is required to aplly IR fire detctors to non-hydrocarbon fueled fire applications. A buildup of ice and water film on the detector viewing window will greatly reduce its sensitivity. IR detectors are less affected by smoke, oil, and certain gases and vapors than UV Detectors.
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