Smoke control systems
A smoke control system should be chosen by a fire protection engineer with enough knowledge to tailor the system to the characteristics of the building and its occupants.
- Understand which codes/standards govern smoke control systems.
- Learn about active and passive smoke control.
- Understand why more ventilation is not necessarily better.
While we endeavor to prevent, minimize, and control the occurrence and severity of fires by installing sprinkler systems, minimizing the use of combustible materials in construction, and minimizing ignition sources, fires still occur and lives are still lost. By now, we all know that it is the smoke (carbon monoxide and other products of combustion) that is the greatest threat to life in the event of a building fire. While the fire may be localized, the smoke will travel wherever the building airflow and its own buoyancy will take it. Without safeguards, smoke will travel through ductwork, up through building shafts, through openings in walls and floors, and throughout open atria spaces and open malls.
A prime example is the tragic MGM Grand Hotel fire in Las Vegas on Nov. 21, 1980, in which 85 people were killed and about 600 injured. A fire in an unsprinklered area on the ground floor sent smoke throughout the 21-story casino complex. Sixty-one of the fatalities were in the high-rise tower where smoke had traveled through the stairs, floor joints, shafts, and air handling system. Only 18 of the fatalities were on the level where the fire was contained. It was the free movement of smoke that led to the greatest loss of life.
Humans are reactionary by nature, and we react to disasters like the MGM Grand fire by finding ways to prevent them in the future. We do this by developing codes and standards for construction that will apply to all new buildings, or at least those subject to the given code.
The International Building Code (IBC), 2012 edition, addresses the issue of smoke migration through various means, including requirements for sealing penetrations through floors, protection of vertical shafts, smoke barriers and smoke partitions, and smoke control systems. The IBC specifically sets forth requirements for the provision of smoke control systems for malls and atriums that connect more than two stories, underground buildings, and windowless buildings, including prisons.
The IBC identifies only where a smoke control system is required; the type of system must be chosen by an engineer with sufficient knowledge of the individual scenario to tailor the system to the characteristics of the building and its occupants. The engineer can choose to design a mechanical or a passive system. The mechanical system will use either the HVAC system or dedicated smoke control fans to extract smoke from the building, or create a pressure differential to prevent the migration of smoke from the area of fire origin. The passive system relies on the natural buoyancy of the smoke and the stack effect to allow the smoke to exit the building.
Active versus passive smoke control
The stack effect is what allows smoke from a fireplace to travel up the chimney instead of filling a home with the products of combustion. This passive system relies on the temperature differences between the fireplace and the outside air to use the natural buoyancy of the hotter gasses to transport the smoke out of the building. This same approach can be used when designing a smoke control system for a building, but the complexities of variables such as outside temperatures, wind speeds, barometric pressure, entrainment of cooler surrounding air into the fire plume, and pressure differentials induced by the HVAC system limit the appeal and practicality of this approach.
Given the limited application of passive smoke control systems, the focus is generally on active mechanical systems. These systems fall into two categories: exhaust and pressurization. Each of these types of systems has its place, and each can be a very effective means of safeguarding the building occupants from exposure to the products of combustion, but the building characteristics will dictate which type of system is appropriate.
Large open spaces like malls and atria tend to expose all of the occupants to the same environment and the same threats. As we learned from the MGM Grand fire, a fire beginning in the casino of a large atrium hotel has the potential to endanger not only those people in direct proximity to the fire, but those many stories above. The potential for danger is even greater when the casino is actually part of the atrium. In these large open spaces, it is generally not practical to try to contain the smoke to the area of the fire origin because there are no walls, but smoke control can be achieved through mechanical exhaust.
Exhausting large-volume spaces is not as simple as placing a large exhaust fan on the roof and expecting it to keep up with the production of smoke. The design of the smoke exhaust system requires a thorough analysis that considers the geometry of the space, the fuel load (the expected magnitude of a fire in the space), the means of introducing makeup air to replenish the volume gas being removed by the fans, and the effects of the air movement on the adjacent egress components. The IBC sets the objective of an exhaust-type smoke control system to keep the smoke layer at least 6 ft above the highest walking surface in the atrium. While the IBC is used to determine when a smoke control system is required, NFPA 92: Standard for Smoke Control Systems provides the guidance on how to achieve this goal. The Handbook of Smoke Control Engineering provides extensive information about smoke control that can be helpful to designers; this handbook is jointly published by ASHRAE, International Code Council, NFPA, and Society of Fire Protection Engineers.