Detectors that respond to heat Flashcards
At what stage of the fire do heat detectors react
Heat detectors are designed to detect fire in its
more advanced stages when the temperature in the
protected area starts to rise
The effects of heat which provide the basic operating
principles for heat detectors are
Melting (or fusion) in metals or plastics.
n Expansion in solids, gases and liquids.
n The electrical effect.
In discussing heat detectors reference will be made
to ‘fixed temperature’ detectors and ‘rate-of-rise. What are they
A ‘fixed temperature’ detector is one that
responds only when a predetermined temperature
is reached.
A ‘rate-of-rise’ detector is one that responds when
the rate of temperature rise is abnormally rapid.
w a diagram and discuss Heat detectors using the
principle of expansion. Expansion of a single metal strip
A piece of metal will expand when heated; this
expansion is most noticeable in a length of metal
with its ends unrestrained.
If both ends of the metal are secured to a solid base
and the metal is then subjected to heat the effect of
the expansion is to cause the metal strip to bow. If
contacts are added, as shown in Figure 14.1, the
principle can be used in a detector to complete an
electrical circuit when a predetermined temperature
is reached.
Draw a diagram and discuss Heat detectors using the
principle of expansion. Expansion of a bi-metallic strip
The bi-metallic strip is a development of the basic
principle of metal expansion due to heat and makes
use of the fact that, when heated, some metals
expand at a greater rate than others (Figure 14.2).
If these two metals are bonded together to form a
bi-metallic strip and then subjected to heat the strip
will bend (Figure 14.3) to accommodate the differing
rates of expansion. Figure 14.4 shows a simple
example of the use of a bi-metallic strip as a heat
detector.
Bi metallic strips can be used in fixed temp and rate of rise. true or false
Bi-metallic strips are also used as the heat sensitive
elements in some ‘rate-of-rise’ detectors
List the main advantages and disadvantages of The main advantage of detectors operating
on the expansion-of-metal principle
1.suffer no damage from operation
2. generally self-resetting. They
are therefore back on standby automatically
immediately any fire has been dealt with.
3.Where there is likely to be a large but gradual
variation in ambient temperature during
normal processes, the ‘rate-of-rise’ detector
has the advantage of giving a quick response
to any sudden abnormal temperature rise
whilst minimising the number of false
alarms.
4. where a rapid rise in temperature
is a normal result of work processes, the
fixed temperature detector is to be preferred.
In this type of situation it is less prone to
false alarms than the ‘rate-of-rise’ type
5. A
fixed temperature detector will take longer to
respond in a cold area than in a warm one.
6.A ‘rate-ofrise’
type on the other hand will take the
same time to respond in both situations - it
reacts to the relative rise in temperature.
Describe a typical rate of rise detector
The detector head comprises a circular white plastic
moulding to which is attached a metal cap
forming a sealed chamber (Figures 14.6 and 14.7).
The upper surface of the air chamber includes a
diaphragm and a leak element, which communicates
with the upper part of the detector. The rapid
rise of ambient temperature due to a fire causes an
equally rapid expansion of air inside the chamber.
This deflects the centre of the diaphragm (1),
which rises to make an electrical contact (2). This
triggers the circuit to ‘fire’ condition, sounds the
alarm and illuminates the ‘fire’ indicator (3) in the
detector
Label the diagram of a typical rate of rise detector
BI metallic disc Diaphragm Contact point Common base Alarm indicator Leak element
List the general problems with fire detection in tunnels and the advantage of heat detectors
There is a problem when protecting cable tunnels,
conveyors and similar areas with lengthy runs. Point detectors may have to be unacceptably close, or dense, to be effective.
Flame detectors are one method used but, with cables, flame is not necessarily the first manifestation of fire, and beam detectors
have to depend on the configuration of the tunnel,
Heat detectors are able to detect overheat conditions possibly even before a fire occurs and can,for instance in a cable tunnel, follow the risk closely wherever it runs
Name the 3 types of linear heat detectors LHD according to method of operation
(i) Relying on the effect of heat on electrical
resistivity.
(ii) Relying on the effect of heat on the insulation
between two conductors.
(iii) Relying on the melting of a thermo-plastic
tube containing compressed air or an inert
gas.
Discuss the resistivity type of linear heat detector
In this type the conductors are separated by a sensitive dielectric material which, when subjected to heat, decreases in resistance and allows a measurable leakage current to develop .
This type is limited in the length of risk zone that
it can cover - basically, because resistivity is not
only related to temperature but also to length.
An advantage, however, is that, following an overheat condition, the LHD will reset, providing that it has not been damaged or destroyed by the fire.
This type can also be programmed to take account of different ambient temperatures prevailing in any
zone it passes through.
Discuss the Insulation type of Linear heat detector
This type relies on the melting, at a predetermined
temperature, of the insulation separating two conductors.
A fire alarm is, therefore, indicated by a
short circuit, whilst an open circuit gives a fault
indication.
There is no limitation on the length of the insulation
type but that section of the cable which has
detected the fire will have to be replaced
Discuss the compressed gas type of Linear heat dector
Some areas to be protected are very hostile to both
the resistivity type and the insulation type. This
compressed gas type utilises a thermo-plastic
small-bore tube fed by a source of compressed air (or inert gas) at, perhaps, about 5 bar pressure.
This pressure is used to hold back a mechanism
which, when the pressure is released, trips and
transmits the alarm. The tube is designed to melt at
a certain temperature within a 5% tolerance.
It is relatively inexpensive to install and replace
when it is damaged or has detected a fire. It is
basically mechanical and any fall in pressure,
e.g. due to leaks or damage, will raise the
alarm.
What is thermal lag in a heat detector
As the air will heat more quickly
than the detector, the operating element in the
detector will usually be at a slightly lower temperature
than the surrounding air. This difference in
temperature is referred to as ‘thermal lag’ and
could in some circumstances delay a detector’s
response. Its extent will depend on a number of
factors, e.g. the surface area of the detector, the
amount and speed of air passing the device.
