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A Continuous Linear Fire Detector Some Details of the Graviner Firewire Temperature-sensitive System XISTING fire detectors are mainly or the ductor at that point, while remaining for all prac unit type, which is placed at a number of tical purposes an insulator elsewhere. Since the E points in the fire zone. Fire will thus be de temperature/resistance characteristic is continu tected when the temperature of one of the detec ous there is of course an intermediate zone, in tors is raised to its operating value by the flame, which the normal ambient conditions have to be and if the fire happens to arise where there is no taken into account. Increasing the length of the element adds, in effect, more resistance in parallel detector it will not be indicated until it has across the conductors, and the total resistance is spread some distance. The unit detector can be very sensitive, but the operating temperature must thus reduced. The maximum length of the element not be set so low that it may be set off by ambient which can be used without running a risk of trip conditions short of fire, which may no t be readily ping the relay is thus a function of temperature, predictable. The unit detector also requires elec the limitation being as shown in FIG. 1. It might tric wiring, which must be in the fire zone, and an be thought that there is a risk of the element being adequate number of detectors, with the necessary unable to detect the difference between a high ends of the element must be hermetically sealed, temperature applied over a short portion of its wiring, may well be heavy, apart from the diffi while the connexions to the capillary and the cen length, and a lower temperature applied over a culty of providing cables which will stand up to tral conductor have of course to be made. In the fire zone conditions. greater length, since the external indication is, in addition the coupling must withstand the same effect, a function of the mean temperature along temperature conditions as the Firewire itself. The Graviner Firewire has been developed by the whole length of the element. However, the the Graviner Manufacturing Co. Ltd. in collab This has been achieved by means of a Sintox metal very rapid fall in specific resistance with tem oration with R.A.E. Farnborough and Rolls- bonded bush developed in association with perature ensures that the relay will not be tripped Royce Ltd., and overcomes a number of these Lodge Plugs Ltd. The length of detector element by moderately high temperatures over long disadvantages. It replaces the point detecting ele is thus sealed, and a further connexion is pro lengths, subject to the limits shown in FIG. 1, but vided to allow uncoupling for engine servicing. ment with a linear one which may be led abou t in that higher temperatures applied over short This gives a second hermetical seal. Lengths of the fire zone where required. If any portion of the lengths will operate the alarm. The R.A.E. detector can be coupled together, and the coup element is heated by the fire the warning system is standard flame diameter of one inch is fully suffi ling also connects with the bulkhead fittings. operated. The element consists of a stainless steel cient to give a warning. The properties of the tem capillary containing a concentric wire, the two From these wiring runs to the relay box. The cir perature-sensitive material also contribute to the being separated by a material having a negative cuit is as shown in FIG. 2. A double wound trans robustness of the Firewire, since it will deform to temperature coefficient of resistance. An a.c. former provides the voltage required, the supply maintain the separation between the conductors voltage is applied between the capillary and cen-, being either 115 v. or 26 v. 400 c/s, a.c. being re when the element is given rough treatment, such tral wire at each end of a length of detector, and a quired because of the properties of the sensitive as may occur in an engine bay in case of fire. material. This voltage is applied across the capil relay set to operate when an increase in the cur lary and the central conductor at each end of the rent indicates that a portion of the element is being detector. Inserted in series in the circuit, through heated. As the resistance/temperature character Constructional Details a rectifier network, is a relay, which operates a istic of the material is a known continuous curve, The detector element capillary is about 0.1 in. a milliammeter connected in the detector circuit 24 v. d.c. circuit containing the warning light. in diameter, this size being a compromise between will give an indication of the mean temperature There is also, on the relay box, a test switch which the conflicting requirements of robustness and along the path of the detector. This can give valu checks the continuity of the central conductor, in low thermal capacity, the latter being necessary able information about ambient conditions in the serting a series resistance in place of that otherwise to minimize the delay before a warning is given; provided by the detector. Provision is also made fire zone corresponding to various engine condi the statutory requirement is that the warning sys for a remote test switch. tions. tem should operate within four seconds of the The detector itself is extremely robust, and can application of the standard flame. be cut, flattened, bent, and subjected to repeated The temperature-sensitive material has to be Testing and Inspection high temperature without its operation being im kept at a high degree of chemical purity, moisture paired. Continued flattening will eventually short- Since the prime requirement of fire-detecting or natural grease from the hands being enough to circuit the central wire to the capillary, but this equipment is complete reliability and consistency, produce unacceptable changes in the resistance needs a remarkable degree of distortion. As the elaborate test and inspection procedures have characteristics. This involves precautions of sur element is connected to the warning circuit at been instituted in the manufacture of the Fire- gical thoroughness in manufacture, and also the both ends, like a ring main, cutting the element wire. The temperature-sensitive material itself is does not affect its operation, unless it is cut in stable in its properties, but each length is checked two places and the heat applied to the isolated in detail. Samples cut from each length are section. The electrical wiring is all outside the fire stripped of the stainless steel sheath in an acid zone, the element itself being the only part that bath and the remaining material examined micro needs to be there. scopically. The length is checked for insulation resistance, and X-ray photographs of coiled lengths are taken to make sure the central con The Sensitive Material ductor is not displaced. A temperature/current The success of the Firewire is entirely dependent curve is plotted over the range 20-350 deg. C. on the remarkable properties of the filler material The results of all the tests are filed against the used as the temperature-sensitive medium. This reference number. Both the detector elements material—its composition has no t been disclosed and the relay box are tested by immersion under —was suggested by two research chemists at the water and the pressure varied between sea level R.A.E., Mr H. Warburton Hall and Dr R. N . C. and high altitude conditions. Strain. It has the property that its specific resist ance falls very rapidly with increase in tempera Weights ture, being something of the order of one million times as great at room temperature as at 1,100 The sensing elements are made in five-foot and deg. C., the temperature of the R.A.E. standard ten-foot lengths, the respective weights being flame. This means that if a portion of the Firewire 2.5 and 3.5 oz.. including end fittings. The bulk is heated to a temperature such as would exist in a head fittings weigh 2.5 oz. each, and the couplings fire, even over a short length, it becomes a con 0.6 oz. The relay box weighs 1 lb. 10 oz. July 1954
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1954
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