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Study On Early Fire Detection And Suppression By High Pressure Water Mist In Special Confined Space

Posted on:2008-03-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Q WangFull Text:PDF
GTID:1101360212498620Subject:Safety Technology and Engineering
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The conventional fire suppression agents used in electronic equipment areas such as center control room in industries, telecommunications and utilities industries, et al. were Halon 1211 and 1301 acted as total gas-flooding system via a fixed pipe system from pressurized storage bottles. The advantage of these agents when used as means of fire protection method in sensitive electronic equipment is that they are non-conductive and able to permeate into obscured fires. However, halon agents have been identified as a major ozone-depleting substances. This kind of agent have been phased-out in developed countries and will be phased-out rapidly in developing countries according several relative international Protocols, so there is an urgent need to find suitable replacement fire suppression system. Water mist produced by high pressure nozzle is one of the major possible candidate Halon alternatives for the fire protection in sensitive electronic equipment areas since its advantages of non-toxic, high efficiency, low cost and little damage to the protected objects. But it was found very quickly that it was impossible to extinguish a fire inside an electronic cabinet by application of water mist to the exterior of the cabinet and the in-cabinet water mist fire suppression is necessary, and this brings a challenge to fire detection system also to pin point the location of the fire cabinet in the room. So the combined in-cabinet early fire detection and fire suppression by high pressure water mist system was studied in more details with experimental method in this thesis.On the base of statistics and hazard assessment of fire in electronic cabinets, the printed circuit board(PCB) is not the major fire source, but there are often large masses of cables leading into and out of the electronic cabinets, and if the fire spread into these vertically cables arrays in cabinets, particularly if older-generation non-fire retardant cables are used, the fire will spread in the cabinets and not self-terminate, and pyrolysis reaction will be occurred in the PCBs under this condition. So seven full scale fire experiments of electronic cabinets were carried out in which the cables were mounted vertically in the back of the cabinet, the fire spread rate, temperature rise, Oxygen , CO and CO2 concentration were measured during the experiments, and the critical condition for pyrolysis of PCB was found. The results show that at the critical condition, the typical ventilation rate had low influence on the fire spread rate of the cables, but the temperature in the center of the cabinet and the concentration of toxic gas emission raised rapidly when the ventilation rate were increased.The distributed fiber optical temperature fire detection system based on thetemperature sensitivity of spontaneous Raman backscattering was selected as firedetection method compared with other intelligent fire detection system. On the baseof the analyze of Optical time-domain reflectometry(OTDR) and thetemperature-response performance of Raman scattering in data fiber theoretically,the measures to improve the reliability and distance, temperature and time resolutionof the system was studied systematically. Quantum well diode laser was used aspulse pump source. A specially designed wavelength division multiplexer(WDM)with much low insertion loss and high insulation was used to extract the Stokes andanti-Stokes light from Rayliegh light which was helpful to increase the signal tonoise ratio of the system. Measure to keep the APD works at constant temperature bythermoelectric module was applied to obtain a constant gain since the APD hastemperature-dependent characteristics. A high speed data acquisition card for weaksignal was used to increase the SNR. The reliability and resolution of the firedetection system was improved significantly by the application of those measuresand can be used to locate the fire cabinet to discharge water mist only where it isneeded.The LDV/APV system was applied to measure the distribution of drop velocity and diameter for four kinds of flow rate high pressure water mist nozzles in different distance from the outlet of the nozzles. The results show that the high pressure nozzles can produce fine drop size and high momentum at specified working pressure, and it is suitable for suppression of the fire in electronic equipment.Finally, seven fire suppression tests in electronic cabinet were carried out. The results show that changing water mist nozzle from conical cones to linear spray distribution can enhance the extinguishing capability over the full width and height of the circuit board, control over the direction of water mist to allow it to penetrate the narrow inter-board space has a greater performance on the efficiency of fire suppression, and the fire was suppressed more quickly by higher flux density water mist system than lower one even the total water consumption was almost the same, and the water damage potential to electronic parts was neglectfully in the test conditions.In summary, studies in this thesis show that the combined in-cabinet early fire detection and fire suppression by high pressure water mist system is one of the Halon alternatives for the fire protection in sensitive electronic equipment areas. The study in this thesis was confined to one type of electronic cabinets and more works is required on different types of electronic equipment before a universal protection system is available.
Keywords/Search Tags:special confined space, fire in electronic cabinet, distributed fiber optical temperature fire detection system, high pressure water mist, fire suppression
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