Designing Schools with Fire/Life Safety Needs in Mind

Often, a thorough evaluation of effective long-term fire and life safety requirements doesn’t receive the attention it deserves, but design engineers and school facility managers must take a leadership role to identify potential sources of fire in each zone. They should specify fire and smoke detectors based on the expected usage of the space in accordance with local building codes.

What are the Options?

Two types of sensors are the ionization and photoelectric models. Both use different methods to identify particles of combustion. A third type – the thermal sensor – monitors heat registration to identify fire scenarios. When detectors use a combination of any of these sensors, they are deemed multi-criteria detectors.

Ionization sensors almost immediately recognize fast flaming fires that are characterized by combustion particles in the .01 to .3 micron size range, and can reliably detect smoke from most common combustible products. Due to their operating principles, they offer limited capabilities when installed in high altitude locations, areas with high air velocity or near kitchens. They also can be affected by dust or dirt that accumulates on the radioactive element, causing the device to become too sensitive,

Photoelectric sensors very quickly recognize smoldering fires that are characterized by combustion particles in the .3 to 10.0 micron size range, but cannot "see" the full range of smoke at the same intensity as ionization sensors. They instantly identify visible white smoke, while dark smoke produced by fires containing plastics and rubber are not recognized as quickly. Their sensitivity must be amplified to detect this stimuli, but this increases the risk of false alarms. As well photoelectric devices can be "tricked" in intensely lit areas or by steam.

Thermal sensors identify heat energy rather than particles of combustion and will initiate an alarm after temperatures reach a pre-determined level or surpass allowable "rate of rise" temperature increases. Thermal detectors, which are rarely used as single-sensor devices, have carved a niche in harsh environment areas, but should never be installed in spaces that have large temperature swings.

Multi-criteria smoke detectors process inputs from two sensors using software algorithms that equate signals into pre-determined responses, which are based on a "decision tree" programmed to react to defined scenarios. The most redeeming advantages of multi-criteria smoke detectors are quick signal-processing periods and the rejection of false or nuisance alarms.

Specifying a multi-criteria detector entails more than choosing a brand, since the combination of sensor inputs dictates how the unit performs in a variety of fire scenarios. A photoelectric/thermal model is the best combination for a multi-criteria detector, because it has an input that recognizes slow-burning fires and another that identifies fast-flaming characteristics. By having dissimilar inputs, one sensor can "check" with the other to confirm or deny the existence of a fire.

What Should be Specified and Where?

Single-sensor photoelectric detectors provide adequate protection for areas like classrooms, offices and lobbies, while ionization detectors are better suited for areas that could produce fast-flaming fires, such as science labs, libraries or computer labs.

Photoelectric/thermal multi-criteria detectors offer complete protection from both fast-flaming and smoldering fires, providing comprehensive protection in most atmospheres including classrooms, offices, storage areas and lobbies.

However, a multi-criteria detector is not a "catch-all" that should be specified for any and all applications. Since some spaces have many uses, multiple detection parameters are needed to offer complete protection. For example, a cafeteria/multi-purpose room will be vacant during the weekend, but bustling during the school week, resulting in higher airborne particulate counts, increased ventilation activity and additional heat produced from kitchen operations, food service personnel, faculty and students.

Such transitions change a zone’s atmosphere and decrease a sensor’s ability to detect a fire due to changes in temperature, air velocity, stratification and air particulate count. Essentially, the local environment for which the sensor was specified changes into an unfamiliar environment because engineers specify systems for the "uptime" in each zone.

Recent technological advancements have lead to the development of self-adjusting, microprocessor-based multi-criteria detectors that sample and automatically "learn" the local environment. Using advanced software, the detectors continuously monitor and adjust detection parameters and alarm thresholds, without human intervention.

The intent of fire detection has always been to save lives and property. Design engineers and school facility managers are the guardians of students and faculty, as well as their possessions. As a result, they must insist on fire and life-safety systems that offer the greatest return on investment – peace of mind.

John Fitzgerald is vice president, corporate communications, at System Sensor, a division of Honeywell, a manufacturer of fire detection and notification devices. He can be reached at jfitzgerald@systemsensor.com.

Paul Sistare is director, commercial marketing, at System Sensor. He can be reached at paul_sistare@systemsensor.com.