The technology for sensing airborne particles of combustion includes ionization, photoelectric, and incipient stage/early detection devices. In general, flaming fires are more quickly detected by ionization detectors, and smoldering or slow-developing fires are more quickly detected by photoelectric detectors. Ionization detectors have widespread use and are suitable for many applications. Selection of the proper detector type depends on the nature of the hazard; refer to the NFPA Handbook and/or fire protection specialists. Smoke detectors are commonly located in areas such as
• Control rooms
• Computer rooms
• Instrument rooms
• Office areas
• Electrical equipment rooms
In areas where smoke detection is used to actuate fire suppression systems,
cross-zoning or other voting type detection schemes are often used for actuation.
Subfloor and above-ceiling areas containing electrical and instrumentation
cabling or other combustibles may also need smoke detection. Critical
and high value control cabinets or panels which arc not ventilated may require
internal smoke detection since an incipient stage firc could escalate beyond
control inside a cabinet prior to room detection.
Numerous conditions in chemical facilities can have a negative impact on smoke detector performance. These conditions can result in false alarms. Some detectors are sensitive to humidity and dirty atmospheres. lonization detectors are also sensitive to chlorine, hydrogen chloride, hydrogen fluoride, phosgene, trichlorethylene, and other chemicals.
Smoke detectors are commonly used in electrical equipment rooms, control rooms, and other areas containing electrical equipment. Often the detectors are located at the ceiling. Electrical equipment failures often generate “cold smoke,” which will not rise to the ceiling where the detectors are normally located. Additional detectors may be required at the bottom of electrical cabinets or rooms to sense this condition.