Author(s): Doug Horton. Published on July 1, 2015.

WHILE NFPA 96 PROVIDES comprehensive guidance on solid-fuel cooking systems—including the critical areas of cleaning and maintenance—there may be opportunities for further research, especially regarding detection and suppression.

Investigations are finding that, in some cases, conventional systems are not reliably detecting or extinguishing fires in hood plenums and ducts over solid-fuel cooking. Some creosote fires are igniting in upper portions of ducts, above conventional fusible link detectors, while other solid-fuel fires are starting below the conventional detectors and moving into ducts faster than the detectors can activate suppression systems. Detector fouling by grease accumulation can result in fusible links failing to separate, especially if the grease is aged and solidified.

Modern electronic fire suppression systems may represent a technological advancement over fusible links, and this could be a valuable research question to address. Electronic systems include additional temperature sensors high in ducts, beneath each exhaust fan inlet, in addition to usual detector locations over appliances and in duct exits from hoods. The added sensors can detect fires that start in ducts above the location where conventional fire suppression system detectors are installed, and they can activate fire systems by detecting either high temperatures or high rates of temperature rise.

Even if a fire is detected, conventional fire suppression systems are limited to the amount of liquid fire suppressant in one or more prefilled canisters. Such systems spray the appliances, hood plenum, and lower duct for a limited time, on the order of one minute. Conventional fire suppression systems also include interlocked gas valves or electric circuit breakers to shut off the respective cooking energy sources when fire systems are activated. With solid fuel appliances, however, there is no means of turning off the fuel, a condition that is exacerbated by persistent use and high operating temperatures, which in some cases can approach 1,000 degrees F (538 degrees C).

Fire suppression system requirements for solid-fuel cooking are provided in Section 14.7 of NFPA 96 and call for water-based systems. Suppression technology exists that discharges an unlimited amount of water, or water and surfactant—a compound designed to coat the fuel and prevent its contact with oxygen—on solid-fuel fires, a potential improvement over the finite amount of suppression material contained in prefilled canisters. Newer technology also includes systems that generate a shielding mist of water droplets, enhanced by surfactant, to coat grease and creosote and provide a thermal blanket that limits surface temperatures to 212 degrees F (100 degrees C). The water mist droplets absorb heat from the fire and vaporize, expanding more than 1,000 times to displace oxygen and further suppress combustion—an especially important feature, considering that solid-fuel cooking systems like rotiserries or pizza ovens do not include automatic shut-offs. Newer systems can also be electronically monitored and can notify operators if the systems require maintenance. Newer systems are tested and listed to UL 300 and meet the requirements of NFPA 96 and NFPA 17A, Wet Chemical Extinguishing Systems.

In UL 300 tests of modern fire suppression systems with electronic detection, surfactant, and unlimited water, these systems were consistently more responsive and effective than conventional fire suppression systems with fusible links and a limited amount of suppression agent. Significantly, the electronic detection systems on the modern systems actuated more often from the rate of temperature rise than from reaching a set point of 360 degrees F (182 degrees C)—a detection feature not available with fusible links.

Taken together, these topics may provide a possible roadmap for further research and, ultimately, help provide improved confidence in solid-fuel fire prevention. 

DOUG HORTON, MS, CFSP, is principal consultant at D.J. Horton and Associates, Inc. in Batavia, Illinois.