Clearing the air
When a fire develops, a smoke plume comprising hot gases and smoke rises upward into the space directly above the fire. When the smoke plume impacts the ceiling, the hot gases and combustion products spread out horizontally under the ceiling surface, quickly reaching areas of the building that are removed from the immediate location of the fire. The rapid flow of hot gas moving in a shallow layer beneath the ceiling surface is referred to as the ceiling jet. The smoke layer flows under the ceiling jet. As the fire continues to burn, more smoke and hot gases rise to the ceiling.
Ambient air is also entrained into the smoke plume. As a result, the layer of hot gas and smoke at the ceiling thickens, and the temperatures of the ceiling jet and smoke layer rise. The accumulation of smoke inside the building limits visibility, which can make it more difficult for occupants to escape and firefighters to gain access to the fire. The intense heat building at the ceiling will eventually begin to weaken the structure, increasing the potential danger to the people and firemen on the scene and the amount of damage caused by the fire.
Underwriters Laboratory (UL)-listed smoke vents are required to open automatically and be equipped with both an interior and exterior pull release that allows firefighters to open them manually. Automatic smoke vents can be actuated by fusible links or electrically by a heat or smoke detector or sprinkler water flow switch.
The automatic smoke vent or vents located closest to the source of the fire open first, providing the most direct exit for the hot gases and smoke produced in the combustion zone. This reduces the spread of hot gases and smoke throughout the facility, and it slows the thickening rate of the ceiling jet and the smoke layer. With the development of the ceiling jet and smoke layers better controlled, occupants will have time to evacuate safely, and firefighters can enter the building and fight the blaze from the floor.
The automatic smoke vents installed at Dick Cold Storage include a Thermolatch® II positive hold/release mechanism that ensures reliable vent operation when a fire occurs. The vent automatically releases upon the melting of a UL-listed 74 C (165 F) fusible link. The curb-mounted fusible link housing allows the latch to be quickly and easily reset from the roof level.
It is also critical that smoke vents open in snow and wind.
“NFPA 204 requires opening mechanisms to remain free and clear, and require periodic inspection, testing, and maintenance,” Solomon said. “Additionally, NFPA 204 requires automatic vents to be fail-open, meaning that if there is a failure in the mechanism of the vent, it will automatically open.”
Acoustic vents work similarly, but they also have the added feature to guard against noise intrusion. Acoustic vents have a sound transmission class (STC) sound rating, which is a figure that rates how well a vent attenuates airborne sound. The rating roughly reflects the decibel reduction in noise that a vent can provide. Acoustic vents are ideal for concert halls, theaters, and other interior applications that require limited noise from the outside.
In the Hale Centre project, architects used BILCO smoke vents with an STC-46 rating. The theater is situated directly in the flight path of a nearby airport and a busy interstate runs next to it.
“The sound issues were pretty intense,” architect Lyle Beecher said. “We knew it was going to be an issue.”
One of the most important sections in NFPA 204 is a section that requires vents to be designed so that they can be inspected visually. Solomon said Section 5.2.4 of the standard requires the supporting structure and means of actuation have to be designed to be inspected visually.
“This is very important because too often mechanical equipment is installed in a manner which makes regular inspection, testing, and maintenance very difficult or infeasible,” he said. “The vent layout as presented on the contract plans is also critical. The vent size, arrangement, and geometry are often times based around the location of draft curtains and intended storage configurations. Deviations for the vent location may impact their effectiveness. Any changes from the original design need to be carefully evaluated.”
Solomon said smoke and heat vent requirements can be found in local building and fire codes. The decision to provide smoke and heat vents should be made by an engineer, as there are times when smoke and heat vents should not be provided.
“Vents should not be provided if the building is equipped with an early suppression fast response (ESFR) sprinkler system or if an alternative means of smoke ventilation is provided, such as an active smoke control or smoke evacuation system.” Solomon said. “The benefits of smoke and heat vents are also highly dependent on the size and geometry of the building and the expected size of the fire. They are most effective in large, open spaces with high ceilings with a significant fuel load.”
Difference between vents and sprinklers
Smoke vents and sprinklers serve different fire protection and life safety purposes. Sprinklers control or suppress a fire until the fire department can respond to extinguish it. Smoke vents allow for smoke and heat to leave the building, which can help increase the safe egress time for building occupants. They will also allow for increased visibility for both occupants and firefighters, and reduce interior temperatures for firefighting operations.
“Smoke vents are most effective in large, open areas with high ceilings and have limited benefit in a building which is highly subdivided,” Solomon said. “While the two can be used together, NFPA 204 requires an engineering analysis to make sure the design of the smoke vent system will not have a detrimental impact on the sprinkler system performance.”
“Proper operation of an automatic sprinkler system should result in a reduction in the amount of smoke produced,” Koffel added. “While the amount of smoke produced may be less, there are fire scenarios, especially in storage occupancies, in which a considerable amount of smoke is still produced.”
Solomon said there is some concern vents may create a vacuum effect and bring more outside air into the building or pull a fire in the direction of a smoke vent, which will help the fire continue to grow and spread.
While smoke and heat vents do increase entrained air into the building, the benefits from the significant reduction the amount of smoke and superheated gases in the space usually outweigh the possible fire growth due to the entrained air,” he said.
In June 2018, Dick Cold Storage opened a new facility with 15-m (50-ft) ceilings for expanded vertical storage, 169,902 m3 of storage space, 15,000 pallet positions, and seven multi-temperature storage rooms.
The new building also had many of the fire-protection technology and safeguards that were lacking in the original structure, such as horns and strobes, pull stations at doors, linear heat detection in freezers, and 18 automatic smoke vents.
The automatic smoke vents installed in the new facility feature a positive hold/release mechanism to ensure reliable operation. When the temperature at the ceiling of the facility reaches 74 C, the fusible link will melt and release the vent covers, allowing the hot gases and smoke to exit the building. Gas spring operators are designed to open the covers against snow and wind loads and include integral dampers to assure the covers open at a controlled rate of speed.
While devastating, the fire at the General Motors plant ushered in a new era in building design and safety. Automatic smoke vents are a powerful fire-response solution that can deliver significant protection to the people and property inside a burning building and help create an environment that allows firefighters to access and contain a fire as quickly and safely as possible.
All information listed in this section was submitted by The Bilco Company.
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