Detectors that respond to flame

Question 1

A flame detector is an automatic fire detector,
which responds to the radiation emitted by the
flames from a fire. Name the types of radiation they detect

Answer 1

Ifra-red

Ultra-violet

Question 2

Draw a Schematic
diagram of components
of an infra-red detector

Answer 2

quartz lens/filter
photo electric cell
amplifier/filter
Intergrator/timer
alarm

Question 3

Describe how an Infra red flame detector works

Answer 3

quartz is commonly used as the protective shield in these detectors.
The lens and filter will allow only infra-red radiation to fall onto the photoelectric cell.
On detecting the radiation, the cell will transmit a signal to the filter/amplifier.
Flame, however, may not be the only producer of infra-red radiation in the protected
area; there may be a limited number of other
producers, e.g. sunlight or heaters. To distinguish
flame from other likely sources of radiation, the
detector also recognises the distinctive flicker
given off by flame, normally in the frequency
range of 4Hz-15Hz.
The function of the filter/amplifier, therefore, is
not only to amplify but also to filter out signals not
in this range.
If the signal is in this range (4Hz-15Hz) it is then fed to the integrator/timer
which will activate the alarm circuit only if the signal
persists for a pre-set period (normally 2-15
second

Question 4

Name two types of Infra-red detector

Answer 4

Fixed type

Scanning

Question 5

describe basically, the principle of the scanning Infra-red flame detector

Answer 5

The detector continually scans the protected area
(approximately every 20 seconds). This enables
the detector to monitor 360 degrees in the horizontal plane and a wide angle on the vertical plane.
Immediately the photoelectric cell is struck by
deflected infra-red radiation and the characteristic
‘flicker’ is identified by the filter/amplifier, the
integrator stops the motor in order that the deflector
can ‘view’ the flame source directly and allow
radiation to fall continuously on the photo electric cell

Question 6

Draw a diagram of a scanning type infra red flame detectof

Answer 6

Quartz filter
DEFLECTOR AND MOTOR AND TIMER
photoelectric cell
Amplifier/filter
Intergrator
Alarm

Question 7

What is the range of a scanning infra-red detector

Answer 7

Theoretically, there is no limit to the range of the
infra-red scan detector but, for quick detection in
the early stages of a fire, the radius of detection
should be limited to about 90m.

Question 8

Advantages of scanning infra red flame detectors

Answer 8

Much larger area covered

can be mounted at low level

Question 9

Can infra red flame detectors be used outdoors

Answer 9

yes
Unlike smoke or heat detectors, which can only be
used indoors, the infra-red detector can be equally
efficient inside or out. This is because it simply
needs to ‘see’ the flame, whereas smoke or heat
detectors have to rely on ceiling or walls to direct
combustion products to the sensing device.

Question 10

Infra - red flame detectors, especially scanning type, are useful for what areas

Answer 10

open
storage areas, aircraft maintenance areas (both
inside and out),

Question 11

List some problems infra red flame detectors have identifying fires

Answer 11

Sunlight. especially if flickering thru tree canopies
Rippling pools of water
welding

Question 12

draw an ultra violet flame detector

Answer 12

lens
gas filled tube
photo electric cell
amplifier

Question 13

describe the ultra violet flame detector

Answer 13

When UV radiation strikes the gas-filled tube it
ionises the gas in the tube.
A small current is set up between the two electrodes and the tube becomes
a conductor of electricity.
When the current flow is greater than the set point of the amplifier the alarm relay closes immediately and causes the alarm to sound.
The circuit can also have an integrator
incorporated in it, which will effectively delay the
alarm for 10-15 seconds

Question 14

Is the ultra violet flame defector affected by sunlight

if not what is it sensitive too

Answer 14

The detector is not affected by sunlight or artificial
light but is sensitive to electrical arcs and would
not therefore be recommended for areas in which
welding was being done.

Question 15

the UV detector is most commonly

used for

Answer 15

specialised applications such as
monitoring of aircraft engine nacelles, 
 to protect fuel storage tanks, 
oil drilling rigs, 
warehouses, 
paint spray booths

Question 16

what advantage do flame detectors have in where they are placed

Answer 16

do not have to be ceiling mounted and can be mounted at a low level

Question 17

one disadvantage of UV flame detectors

Answer 17

can be masked by smoke

Ir cannot