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an ejector system. An important function of the
airflow is to purge any flammable vapours from the
engine compartment. By keeping the airflow minimal,
the power plant drag is minimized and, as the
required quantity of fire extinguishant is in proportion
to the zonal airflow, any fire outbreak would be of low
intensity.
9. On some engines a fireproof bulkhead is also
provided to separate the ’cool’ area or zone of the
engine, which contains the fuel, oil, hydraulic and
electrical systems, from the ’hot’ area surrounding
the combustion, turbine and exhaust sections of the
engine. Differential pressures can be created in the
two zones by calibration of the inlet and outlet
apertures to prevent the spread of fire from the hot
zone.
10. Fig. 14-2 shows a more complex cooling and
ventilation system used on a turbo-fan engine. Air is
induced from the intake duct and also delivered from
the fan to provide multi-zone cooling, each zone
having its own calibrated cooling flow.
FIRE DETECTION
11. The rapid detection of a fire is essential to
minimize the fire period before engine shut-down drill
and release of extinguishant is effected. It is also
extremely important that a fire detection system will
not give a false fire warning resulting from short
circuiting caused by chafing or the ingress of
moisture in the case of electrically operated systems
and chafes of the capillary resulting in loss of the
contained gas in the case of the gas filled continuous
element sensing type,
12. A detection system may consist of a number of
strategically located detector units, or be of the
continuous element (gas filled or electrical) sensing
type that can be shaped and attached to pre-formed
tubes. The sensing element can be routed across
outlet orifices, such as a zone extractor ventilation
duct, to give early detection of a fire (fig. 14-3).
13. In the case of electrical systems the presence of
a fire is signalled by a change in the electrical characteristics
of the detector circuit, according to the
type of detector, be it thermistor, thermocouple or
electrical continuous element. In these cases the
change in temperature creates the signal which,
through an amplifier, operates the warning indicator.
14. Both the thermocouple and thermistor detectors
have properties making them ideally suited to this
application. The thermocouple comprises two
dissimilar metals which are joined together to form
two junctions. As the temperature difference between
the two junctions increases an E.M.F. is produced in
the circuit and it is this E.M.F. that triggers the fire
Fire protection
154
Fig. 14-1 A typical cooling and ventilation system.
warning displays. The thermistor consists of a semiconductor
material whose resistance changes as
temperature increases, with a corresponding change
in the current flowing in the circuit. It is this change in
Fire protection
155
Fig. 14-2 Cooling and ventilation - turbo-fan engine.
the current that operates the warning indicators. A
thermistor may be used as a single point detector or
as a continuous element sensor.
15. Another form of continuous element sensor
takes the form of a capacitor consisting of a tube
containing a dielectric material with a conductor
running through the centre. A voltage difference is
applied between the tube and the centre conductor.
As the temperature increases then the properties of
the dielectric change with a corresponding change in
the value of capacitance. This change of capacitance
is displayed as a fire warning.
16. The gas filled detector consists of stainless
steel tubing filled with gas absorbent material and in
the event of a fire or overheat condition the
temperature rise will cause the core of the sensing
loop to expel the absorbed active gas into the sealed
tube causing a rapid increase in pressure. This build
up of pressure is sensed by the detector alarm
switch. Should the sensing loop become damaged
causing a loss of the pressurized gas, an integrity
switch will indicate a detection loop fault on the
appropriate engine. Fire indication is given by a
warning light and bell.
17. At high Mach numbers, the considerably higher
temperature levels may be such as to render the
thermistor or thermocouple fire detection system
unsatisfactory. Thermal detectors that sense either a
temperature rise, or a rate of temperature rise, may
therefore prove most suitable.
18. Alternatives to the above types are surveillance
detectors that respond to light radiation from a fire.
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