When fire department personnel respond to a commercial kitchen range hood fire with fire possibly extending into the exhaust duct above in a high-rise building, the two primary considerations that must be determined while the fire is being brought under control are: Where does the duct terminate...
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The fire started in a restaurant directly beneath the trading floor for "the Merc," with fire extending into the exhaust duct. These occupancies were situated between the two towers comprising the complex. The smoke everyone saw was the grease burning in the kitchen exhaust duct and discharging out the roof of the south tower. The chiefs at the command post knew me and I volunteered to find the engineer and chase down the heating, ventilating and air conditioning (HVAC) system drawings to determine exactly where the duct penetrated (horizontally and vertically) the south tower to assist fire crews assigned to checking extension.
The fire was quickly controlled in the restaurant, but the still-heavy volume of smoke coming out the roof meant we still had fire in the ductwork. After gaining access to the necessary print and seeing where the duct traveled in the void spaces, I went with a crew up to the second floor of the south tower and entered a storage room, pointing to where the general area was that the duct should be. The crew brought all the appropriate equipment to do the job, including a thermal imaging camera. They were very experienced firefighters and had a good working knowledge of how to properly use the camera. As ceiling tiles were being carefully removed, they immediately found the duct and determined there was fire in the duct at that location. They set up a stepladder, opened up the duct and easily extinguished the fire with a portable extinguisher. It did not appear that the fire was extending vertically into the office tower's upper floors. With the use of the camera, much damage was avoided and the entire fire was brought under control within 30 minutes. It showed me how effective a tool these cameras can be on these types of fires.
The challenging aspect of fighting these fires is trying to figure out where the duct terminates and where it penetrates various spaces. Some exhausts may discharge directly outside the building at that floor, some may travel up a few floors and discharge outside the tower at a level well above the kitchen, while others travel the entire length of the building up and out the roof (see graphics). Depending how much grease is built up in these ducts and how much of it is burning, these fires can be either rapidly handled very early into the fire or they can turn into multiple-alarm, raging fires if the fire shows in multiple points and/or extends out of the duct through a crack or open crease in the duct, allowing fire to leave the shaft and enter the building it is housed in. It must be noted that with this occurring, even in a "fully sprinklered building" (very loose statement), it will be breaking out in void spaces where there likely will be no fire suppression systems present. Fire crews must locate and fight these fires manually. If these ducts pass through a ceiling void that is also used as an open plenum return, this may present an additional problem if the fire is pulled back toward the core by the HVAC system, while feeding on plenum wiring.
Not all buildings have return duct detectors that will shut down fans should smoke be present in these shafts, plus there is not always a guarantee the system will indeed shut down anyway, as systems do fail through lack of maintenance or testing, even improper installation of key components. A well-constructed duct is well-suited to containing fire, but where the sections connect can be avenues for fire travel outside the duct if they are not properly sealed. Many times, building owners hire a professional cleaning firm to clean the ducts and clear the grease built up over time. However, too often, a poor job is done, resulting in a continual buildup until one day the temperature is reached where the grease ignites and fire erupts. This is especially true when the duct is difficult to clean due to numerous turns and elbows. Maintenance is vital to preventing fires in these shafts.
CURTIS S.D. MASSEY is president of Massey Enterprises Inc., the world's leading disaster-planning firm. Massey Disaster/Pre-Fire Plans protect the vast majority of the tallest and highest-profile buildings in North America. He also teaches an advanced course on High-Rise Fire Department Emergency Operations to major city fire departments throughout the U.S. and Canada. Massey also regularly writes articles regarding "new-age" technology that impacts firefighter safety.