Consulting-Specifying Engineer - May 2008 - (Page 17) installation of two smoke detectors in the same area that are run on separate circuits. In this case, when a fire is detected, the panel will require activation of both circuits before the release of a Figure 1: Air sampling type smoke detector provides four alarm statfire-extinguish- utes: Action, Alert, Fire 1, and Fire 2. ing agent. Cross- Source: Xtrailis zoned detection systems minimize possibility of accidental Water-based suppression systems discharge due to false alarm and may proWater-based extinguishing systems fight vide an additional opportunity for on-site fires in two dimensions, both in the air and personnel to eliminate an impending risk on the equipment, but not underneath factor. (Note: In a data center, only smoke equipment cabinets and other computer detectors should be cross-zoned, heat room fixtures. Ideal for structural protecdetectors need not be.) tion, water-based extinguishing systems are available in either wet or pre-action Fire extinguishing systems configurations (see Figure 3). While detection is crucial to alerting Pipes in a wet system are filled with water personnel in the room, fire damage can at all times, discharging it over the fire when be minimized where an effective extinthe sprinkler head fuses due to heat build-up. guishing system is installed and properly Although the accidental discharge rates of maintained. Both water and gas-based fire a wet sprinkler system are minimal, an accisuppression systems can be found in today’s dental discharge due to a damaged sprinkler data centers. activation pipe or fitting leak could cause considerable damage to information technology equipment, possibly resulting in a large or even fatal loss in data processing. The more appropriate water-based extinguishing system for a data center is a pre-action system, available in both single and double interlock options, as its pipes remains dry until a reliable fire condition is detected. The single interlock pre-action system requires the activation of heat or smoke detectors before water enters the piping, discharging it over fire upon fusion of a sprinkler glass bulb or solder link due to heat buildup in the space. Loss of air due to a damaged sprinkler will transmit a supervisory alarm at the control panel. With the double interlock system, water again enters the pipes upon heat detection and loss of air due to fusion of glass bulb or solder link. In both cases, when the sprinkler is merely damaged or there is a Figure 2: Air sampling smoke detector piping network simple pipe leak, the pre-action system will Source: Xtrailis installed in the supply and return ducts of HVAC systems to prevent smoke spread by initiating control of fans, dampers, etc. As they sense particles of combustion, they transmit a supervisory alarm to the fire alarm panel urging it to shut down the air handling unit. The International Building Code requires high-rise buildings with air in the return and exhaust air plenums with a capacity greater than 2,000 cfm to have a duct smoke detector in a serviceable area downstream of the last duct inlet. An aspirating smoke detection system, on the other hand, is an active VEWSD, constantly sampling particles of air for the presence of visible or invisible smoke, with its piping network, an aspirating fan and a highly sensitive detector. Less dependent on the airflow in the room to transport smoke to the detector, the VEWSD carries extremely low accidental discharge rates, with a maximum transport time from the most remote sampling port not to exceed 120 seconds. Air sampling systems (see Figures 1 and 2) are specified where very early detection is crucial, in data centers and other high-tech communications facilities. After evaluating EWSD and VEWSD systems, a critical facility should consider employing its smoke detectors as part of a cross-zoned scheme, which requires the Consulting-Specifying Engineer • MAY 2008 17
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