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Fire Alarm - Field Sensor Devices This section is going to be about Smoke Detectors, Heat Detectors, Sprinkling System Detectors, and Hand Pulls.
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SMOKE DETECTORS
Smoke detectors fall into three (3) basic technologies:
Photoelectric – This is the most common technology used in commercial and residential applications. The concept is based on carbon smoke particles reflect light from an LED (light emitting diode) into a receiving LED sensor. A specified increase in the amount of light at the receiving sensor will initiate an alarm.
Ionization – This concept relies on a radiation source (transmitter and receiver) across a sampling area in the smoke detector. Particles or gases released by combustion, absorb some of the gamma radiation particles as they pass through the sampling area. When the radiation, receiver detects a specified decrease in the gamma radiation level, an alarm is initiated.
Laser diode – This is the newest and most sensitive concept. The laser diode smoke detector was introduced in the mid-1990’s. It operates on a light threshold concept similar to a photoelectric smoke detector. It is extremely sensitive and, as a result, can detect combustion at very early stages.
Most commercial (approximately 80%) and all residential smoke detectors utilize the photoelectric concept. The photoelectric smoke detector is inexpensive, durable, and has reasonable sensitivity to combustion (smoke). False alarms are minimized since the photoelectric concept requires visible smoke for activation.
Ionization and Laser diode smoke detectors do not require combustion particles in the visible spectrum. These detectors are very sensitive and can have a high incidence of false alarms as a result. These detectors are normally used only in high volume air flow environments such as air ducts, return air plenums, etc.
A common use of ionization detectors is as duct detectors in many industrial applications. An alarm condition will not only trip the fire alarm system audibles but will also shut down the applicable air handling system.
The laser diode smoke detector has been used mostly in critical environments such as communication and computer processing equipment areas. The laser diode detector in much more expensive than either the photoelectric or ionization detectors.
HEAT DETECTORS
Heat detectors are required for early detection of those fires that have rapid heat rise with little visible smoke.
Heat detectors are of two (2) types:
fixed temperature
rate of rise.
Because heat detectors can be relatively small in size and inexpensive, they are normally included in (built into) a photoelectric smoke detector.
Fixed temperature heat detectors have special application around work areas that have very high ambient temperatures (i.e. foundries, welding, heat processing applications, etc). The normal fixed temperature detector is set at 186°F., but special application detectors are set to trip at temperatures up to 300°F.
SPRINKLING SYSTEM DETECTORS
Fire sprinkling systems are a necessary part of many larger buildings. These systems typically have a fixed temperature element in each sprinkler head. Fire will melt this element in the sprinkler head(s), thus releasing water to extinguish the fire in the immediate area. In most cases, the resulting fire(s) will be extinguished immediately The real problems are:
detecting that the sprinkling system has been activated
getting the water shut off to reduce subsequent water damage to the building and its contents.
Water flow switches are key to detecting that the sprinkling system has been activated. Water flow switches are commonly vane type switches located within the vertical riser pipe. When water starts flowing through the pipe, this vane switch is activated (much like a flapper valve), causing an alarm signal. Water flow switches can also be activated by pressure differential. This concept is used in dry system applications.
A valve position switch is also used to monitor open/close position of all critical shut off valves in the sprinkling system. These switches are most critical in re-establishing the integrity of the sprinkling system after construction or work on the sprinkling system when valves are closed temporarily.
HAND PULLS (MANUAL PULL STATIONS)
Although there are a variety of hand pull devices on the market, they all accomplish the same thing. A signal from a hand pull device is universally accepted as the most reliable type of fire alarm signal possible because of its origin. Hand pull stations are required in most business/industrial applications to meet the applicable fire/safety codes.
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