Civil Engineering Reference
In-Depth Information
A form of fi ltering is normally combined with fl ame
detectors to ensure that static light emitting sources are
not mistaken as a fi re. Due to their principle of operation,
fl ame detectors need a direct line-of-sight with the fi re to
allow them to detect it. Therefore, it is often necessary
to train a number of detectors at specifi c areas in order
to provide for full coverage. Even then, objects, furniture,
etc., introduced after the detectors have been placed,
can reduce their detection function. In some cases it is
possible to utilise building materials to 'refl ect' light from
dead areas onto the detector. However, once a fl ame
is seen by the detector, operation is virtually instantan-
eous. Therefore, fl ame detectors are particularly useful
as part of an explosion suppression system.
Figure 9.77
Beam detector
holes through which the air is drawn. Aspirating smoke
detection systems are commonly used for applications
such as laboratories or computer rooms, they can also
be used to monitor individual computer cabinets for the
fi rst signs of overheating cables or components.
Linear heat detecting cable (LHDC)
Linear heat detectors comprise heat detecting cables
which respond to temperature along their length. There
are two types of cable:
Figure 9.78
Flame detector
➤
Integrating cable, where heat distribution along
the length is summed and averaged such that the
resultant signal given does not necessarily equate to
the highest temperature at any point on the length
9.5.17
Fire panels and zoning
Fire alarm panels comprise control and indicating equip-
ment and perform three principal functions:
➤
Non-integrating cable, where heat sensed at any
point along the length will be detected and signalled
as appropriate.
➤
Automatically monitor and control fi re detection and
fi re alarm devices and their power supply
➤
Indicate fi re signals, system faults and locations
➤
LHDC is used in a wide variety of applications but is
particularly suited where there is a harsh environmental
condition, a physical or hazardous maintenance access
constraint to the protected area, and/or a requirement
to cost-effective install detection in close proximity to
the fi re risk. The main benefi ts of LHDC systems are that
they are effective in detecting a rise in temperature at any
point along their length, and can be used in environments
that may be potentially explosive and therefore require
the use of intrinsically safe equipment (see Chapter 8).
Provide a means of manual control to facilitate test-
ing, disabling of devices, triggering and silencing of
audible warning systems and resetting of the sys-
tem following a fi re signal.
The complexity and type of the control and indicat-
ing required will vary according to the size and type of
premises.
When small and less complex systems are installed
they are housed within a single control panel or box.
In larger multi-building and complex premises it is
likely that the control equipment will be provided with
a main fi re panel with repeater panels located within
each building (many such panels are fi tted at alternative
points of entry for the fi re service).
Flame detectors
Flame detectors operate by recognising the specifi c
bandwidths of light emitted by a fi re. They are used in
locations where immediate detection of fi re is needed
such as petrochemical installations.
