Smoke detectors are employed where the
type of fire anticipated will generate invisible and visible products
of combustion before temperature changes are sufficient to actuate
heat detectors.
Photoelectric detectors
Photoelectric detectors are of the spot
type or light-scattering type. In each, visible products of
combustion partially obscure or reflect a beam between a light source
and a photoelectric receiving element. The disruption of the light
source is detected by the receiving unit and an alarm actuated.
Combustion products detectors
Ionization detectors and condensation
nuclei detectors are spot detectors that alarm at the presence of
invisible combustion products. Ionization detectors have proven to be
reliable and are the most common type of early warning detectors.
They will alarm in the presence of both
visible and invisible combustion products. Condensation nuclei
detectors operate on the cloud-chamber principle, which allows
invisible particles to be detected by optical techniques.
Flame detectors
Flame detectors are spot detectors that
are usually used in flammable liquids operations and have limited use
in substations. These detectors alarm at the presence of light from
flames, usually in the ultraviolet or infra-red range.
Detectors are set to detect the typical
flicker of a flame. Detectors may be provided with a time delay to
eliminate false alarms from transient flickering light sources.
Air sampling detectors
Air sampling detector systems
continuously draw air samples through sampling heads or ports to
detect submicron particles generated during the incipient stages of a
fire. To detect the presence of these particles, the systems use
either the light-scattering or cloud chamber method.
Both systems are capable of several
levels of preprogrammed alarm thresholds. It is feasible to provide a
staged, early-warning regime that responds to increasing levels of
concern.
Linear beam smoke detectors
Linear beam smoke detectors consist of
a light transmitter and a light receiver that electronically
evaluates the received light. If smoke passes through the beam and
the received signal falls below a preset value, an alarm is
activated.
Slow changes to the received signal
caused by dust accumulation or other environmental influences are
offset by means of a compensating circuit. If the limits of
compensation are reached, if the beam is obstructed, or if the
housing cover is removed, the receiver initiates a fault signal.
These detectors are capable of
monitoring over long distances and are ideal for large or narrow
rooms, e.g., corridors, store-rooms, and machine halls.
No comments:
Post a Comment