Several types of smoke detection devices can be used for duct applications. These include smoke detector sensors within a housing mounted outside the duct and utilizing sampling tubes, area smoke detectors listed for in-duct or partial in-duct mounting, a light beam detector consisting of projector and receiver mounted within the duct, and an in-stream air sampling type detector. In all cases, alarm contacts are available either in the detector or in the fire alarm control unit to initiate air movement control or perform other control functions. An externally mounted duct smoke detector enclosure may incorporate either an ionization or photoelectric type smoke sensor. The light beam detector performs best when utilizing long beam paths, i.e. in the large plenums rather than short or small ducts. Air stream sampling is accomplished with sampling tubes that penetrate and traverse either the supply or return air ducts. The sampling tubes are positioned so air may be drawn through the detector, sampled, and then returned to the air stream in the duct. All types of air sampling detection units are extremely sensitive and can detect minute changes in air particle concentration. An air sampling detection tube or head is installed in the air stream of the duct. Air is drawn out and through the detection device, and is typically calibrated to an operating level of normal background particle concentration for the building. An alarm will sound when the concentration exceeds a preset alarm threshold value at the fire alarm control panel. In next month’s bulletin, we will re-visit these devices.
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