Page 164 - The Jet Engine

Fire protection

However, if ignition should occur, this high velocity air FIRE DETECTION
stream may have to be shut off, otherwise it would
increase the flame intensity and reduce the effective- 11. The rapid detection of a fire is essential to
ness of the extinguishing system by rapid dispersal minimize the fire period before engine shut-down drill
of the extinguishant. and release of extinguishant is effected. It is also
extremely important that a fire detection system will
External cooling and ventilation not give a false fire warning resulting from short
8. The engine bay or pod is usually cooled and circuiting caused by chafing or the ingress of
ventilated by atmospheric air being passed around moisture in the case of electrically operated systems
the engine and then vented overboard (fig. 14-1). and chafes of the capillary resulting in loss of the
Convection cooling during ground running may be contained gas in the case of the gas filled continuous
provided by using an internal cooling outlet vent as element sensing type,
an ejector system. An important function of the 12. A detection system may consist of a number of
airflow is to purge any flammable vapours from the strategically located detector units, or be of the
engine compartment. By keeping the airflow minimal, continuous element (gas filled or electrical) sensing
the power plant drag is minimized and, as the type that can be shaped and attached to pre-formed
required quantity of fire extinguishant is in proportion tubes. The sensing element can be routed across
to the zonal airflow, any fire outbreak would be of low outlet orifices, such as a zone extractor ventilation
intensity. duct, to give early detection of a fire (fig. 14-3).

9. On some engines a fireproof bulkhead is also 13. In the case of electrical systems the presence of
provided to separate the 'cool' area or zone of the a fire is signalled by a change in the electrical char-
engine, which contains the fuel, oil, hydraulic and acteristics of the detector circuit, according to the
electrical systems, from the 'hot' area surrounding type of detector, be it thermistor, thermocouple or
the combustion, turbine and exhaust sections of the electrical continuous element. In these cases the
engine. Differential pressures can be created in the change in temperature creates the signal which,
two zones by calibration of the inlet and outlet through an amplifier, operates the warning indicator.
apertures to prevent the spread of fire from the hot
zone. 14. Both the thermocouple and thermistor detectors
have properties making them ideally suited to this
10. Fig. 14-2 shows a more complex cooling and application. The thermocouple comprises two
ventilation system used on a turbo-fan engine. Air is dissimilar metals which are joined together to form
induced from the intake duct and also delivered from two junctions. As the temperature difference between
the fan to provide multi-zone cooling, each zone the two junctions increases an E.M.F. is produced in
having its own calibrated cooling flow. the circuit and it is this E.M.F. that triggers the fire

Fig. 14-1 A typical cooling and ventilation system.

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