An Alarming Discovery
Duct smoke detectors help prevent the spread of smoke in air ducts
- By Andrew Kuester
- Jul 01, 2006
FIRE and smoke not only endanger lives, they also can cause varying degrees of damage to any type of facility. National and local safety standards and codes address the ability of air duct systems to transfer smoke, toxic gases and flame from area to area in facilities. It is this threat that is confronted with the use of duct smoke detectors because oftentimes in fire scenarios, smoke can be of such quantity that it poses a serious hazard to the safety of people several floors removed from the actual fire.
It is critical that facility managers, engineers and architects understand that the primary purpose of duct smoke detection is to prevent injury, panic and property damage by reducing the spread and re-circulation of smoke.
Preventing Injury and Damage
A duct smoke detector is a device or group of devices used to detect the presence of smoke in the air stream of ductwork sections of the heating, ventilating and air conditioning air handling systems.
Duct smoke detection is extremely useful in preventing injury and property damage in all types of facilities. It can serve to protect an air conditioning system from fire and smoke damage, and also can be used to assist in equipment protection applications, for example, in the ventilation/exhaust duct work leading to the areas that house mainframe computers and tape drives.
Duct smoke detection also can be the first line of defense to shut down the system's blowers and ensure dampers are actuated when there is a fire. For instance, should an HVAC fan motor overheat, the resulting smoke is sensed by the duct smoke detector installed in the main supply duct. The 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 hallways and classroom areas.
If a fire starts on the second floor, for example, and the HVAC system serving the second floor also serves floors one through four, the smoke also will spread to those floors. If area smoke detectors are not provided, the only means of automatic detection are the duct smoke detectors located in the return air ducts on each floor ahead of the main return plenum. The quantity of smoke in the duct eventually reaches proportions sufficient to alarm the second floor duct smoke detector, which transmits a signal to the fire alarm system. Evacuation signaling and HVAC shutdown functions are then provided by the duct detector's auxiliary relay contacts. In these and other situations, duct smoke detection devices are proven effective in helping prevent injury and property damage.
>Installation, Maintenance and Testing
An HVAC system supplies conditioned air to virtually every area of a building. Thus, smoke introduced into this air duct system has the potential to reach the entire building. Because smoke detectors designed for use in air duct systems are used to sense the presence of smoke in the duct, it is critical that they are properly installed, maintained and tested.
NFPA 90A, "Standard for Air Conditioning and Ventilating Systems," requires that smoke detectors listed for duct installations be installed at a suitable location in the main supply duct on the downstream side of the filters to automatically stop the supply fans in systems more than 2,000 cubic feet per minute. For systems more than 15,000 cfm, additional detectors are required in the return system of each floor, at the point of entry into the common return, or a system of smoke detectors is required to provide total area coverage.
Smoke detectors are designed to be as maintenance-free as possible. However, dust, dirt and other foreign matter can accumulate inside a detector and change its sensitivity. This is especially true with duct-type smoke detectors in educational facilities. They can become more sensitive, which may cause unwanted, or less-sensitive alarms, which may reduce the level of protection. Both are undesirable.
According to chapter 10 of NFPA 72, 2002 edition, duct smoke detectors should be tested upon acceptance and re-tested annually. Always check your local code requirements to determine if more frequent testing is required.
Under normal conditions, detectors require routine maintenance at least twice a year or more frequently in dirtier than normal environments. Notify the proper authorities that the smoke detector system is undergoing maintenance and that the system will be temporarily out of service. It also is imperative that the zone or system undergoing maintenance is disabled to prevent unwanted alarms and possible dispatch of the fire department.
Most duct smoke detectors have detector sensors that can be accessed for cleaning. To clean the sensors, remove dust from all openings on and around the sensor, the sampling tube and the exhaust tube. Some detectors can be removed for more thorough cleaning, if necessary.
It also is important to test each detector's sensitivity. If a detector's sensitivity is within specifications, nothing further needs to be done. If the detector's sensitivity is outside its listed specifications, either clean or replace the detector.
Understanding the purpose of duct smoke detectors, as well as the importance of their proper installation and care, is the key to protecting property and saving lives.
Intelligent Laser Smoke Detectors for Critical Applications
Electronic equipment is particularly vulnerable to smoke, much more so than heat. In many cases, a fire is contained to a small physical area and extinguished relatively quickly, but the smoke still negatively affects the entire facility.
For example, smoke in telecommunications facilities can disrupt service for up to several weeks, cause major disruptions in telephone communications and result in costly damage to equipment, even when the fire is promptly controlled. In other critical applications, such as switching stations, computer rooms, clean rooms, hospitals, museums, archives and historic buildings, installing the highest sensitivity spot-type smoke detectors is the best bet for protection against damage to electronic equipment.
Traditionally, the only way to achieve highly sensitive smoke detection was through the use of aspirating smoke detection systems. Aspirating systems operate by drawing air and smoke through a network of pipe or tubing, which is then routed throughout the protected space. By their nature, these types of systems are subject to the effects of dilution.
During an actual fire, smoke is drawn into the aspirating system's pipe through one of its sampling ports. At the same time, the system's other sampling ports may continue to draw clean air into the pipe from areas that the smoke has not yet reached. As a result, the sensitivity level in an aspirating system's smoke sensor must be set much higher to offset the effects of dilution.
In critical applications that require very early warning detection, you can use either an aspirating system or a laser model. If the building already has a fire alarm control panel, or if one will be installed, the laser detector will be more cost effective for several reasons. For one, the laser detector will be installed on the same pair of wires as the other detectors that have been installed. Also, the company that installs the fire alarm control panel will install, service and maintain these detectors too.
Intelligent, microprocessor-controlled smoke detectors that incorporate laser-based technology are able to pinpoint the exact locations of a fire by identifying the address of the detector sensing the smoke. This can greatly reduce response time in real fire situations because smoke at such low levels is not visible to the human eye.
Ideal for critical applications, a laser detector is capable of sensing the presence of smoke and triggering an alarm up to 20 minutes earlier than standard photoelectric smoke detectors. While this type of ultra-sensitive detector works on the same light-scattering principle as standard photoelectric detectors, a laser detector can operate with sensitivity levels 100 times greater.
Laser-based, spot-type detectors are not subject to dilution. In fact, they perform as good or better than aspirating technologies in high-sensitivity environments. In an aspirating system, the smoke is sensed at one central unit, allowing dilution to negatively affect response time.