Now that the hot weather is upon us, seasonal humidity can play havoc with fire alarm systems and their connected devices. One such device is the duct smoke detector, which is installed within the air handling systems typically found in large buildings. This device or group of devices can sense the presence of smoke in the air streams of HVAC (heating, ventilating, and air conditioning) ductwork. These detectors are not a substitute for area smoke detectors, an early warning system, or a replacement for a building’s regular fire detection system. The purpose of duct smoke detection is to prevent injury, panic, and property damage by reducing the spread (re-circulation) of smoke. Duct smoke detectors can also protect the air conditioning system itself from fire and smoke damage. They can assist in the protection of specific equipment, such as where installed in the ventilation/exhaust ductwork of mainframe computers and tape drives. Even when a single HVAC fan motor overheats, the resulting smoke can be sensed by a duct smoke detector located in the main supply duct. Each duct smoke detector is equipped with an auxiliary relay that immediately cuts power to the fan motor before significant amounts of smoke can be distributed to occupied areas. In most applications, duct smoke detectors are installed onto both the supply and exhaust ducts. Subject to engineering specifications, they are commonly placed near the input/output filters, but downstream of fans, filters, chillers, heaters, and humidifiers. The duct smoke detector is the final component required to isolate each area of a building, known as a “smoke compartment”, that would otherwise be filled with combustion particles. Duct smoke detectors in the return air streams are normally located at every return air opening (or air exit) within the smoke compartment, or within the duct before air enters the return air system common to more than one compartment.
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