Engineered Smoke Control Systems

Engineered Smoke Control Systems

Smoke control systems use any and all methods possible to protect from smoke spread.  Doors, fans, sprinklers, dampers, and alarms are unified into one coordinated system.  Coordination of all the smoke control tactics is typically performed by a fire alarm/smoke control panel.  In most systems, fire fighters have override control from a Fire Fighters’ Smoke Control System (FSCS) Panel located in a lobby or a protected area.

Overrides and status indication of all equipment are present on the face of the FSCS or a computer screen display.  Figure to the left shows a detail of a typical override switch and indicator lights on a FSCS panel.

Smoke control tactics

Where strategy looks at the overall picture, the individual tactics are used to achieve the goals. The main purpose of this booklet is to explain details of how these mechanical and electrical systems operate with respect to dampers. The Fire Marshals, Building Officials, design engineers, and contractors are often called upon to go beneath the overall operation of a subsystem and look at the details.

Where devices and wiring interconnect two disciplines, there is a tendency for those involved to have only a fuzzy concept of the whole, interrelated design.

Some system dampers are applied in other ways to control air flow and smoke. Air Handling Units (AHU) are often shut down if any smoke detector in the area they serve senses smoke. However, in engineered smoke control systems the fans may continue to run while the AHU dampers position so that all return air is dumped outside and only fresh air is brought into the building. For large spaces that exhaust smoke in case of an event, dampers located on outside walls (with ducts where appropriate) open to allow outside air to enter to replace air and smoke pulled out by exhausts.

Figure 1: Relief damper variation of stairwell pressurization.

Smoke exhaust or extraction

In large spaces, there is no way to pressurize the large area to prevent smoke movement into the space. It is best to exhaust high volumes to remove the smoke. Atria and large spaces, particularly malls, have exhaust fans to remove smoke and keep it at least six feet above the occupied levels for 20 minutes to allow escape. Lower level make-up air dampers open to the outside to admit fresh air to replace the smoke.

Smoke vents

In certain warehouse and storage occupancies, smoke vents are prescribed by the codes. These can be automatic or manually operated. The goal is to remove hot, buoyant smoke to provide clear air for occupants and fire fighters. When wind is a potential problem, powered fans are used. These are part of an engineered system with the switches to operate them located outside the building where the fire service has quick access. (Section 910. IBC2009.)

Smoke shafts

In some buildings there are shafts extending the height of the building. Fans are mounted at the top and closed dampers are mounted in the wall of each floor. In case of fire, the fan turns on and the damper on the fire floor opens. Smoke is pulled out of the fire floor. A variation of this is the use of the HVAC ducts to pull smoke out of a building by sucking with the return air fan and opening the exhaust damper and closing the return air damper. The HVAC components are not typically designed for this application and the volume of smoke removed may be insufficient.

Zoned smoke control.

In some buildings entire zones or floors are exhausted or pressurized to prevent smoke migration. The fire zone or floor is placed under a negative pressure, often by the HVAC return duct damper and fan. The adjacent floors are placed under a positive pressure to prevent smoke migration. This is a “sandwich pressurization system.” If all the floors except the fire floor are positive, the system is known as a “building pressurization system.” Zoned smoke control was mandatory in high-rise buildings in the legacy codes, but the present IBC does not require them. They may still be found in some local codes and in underground buildings which are particularly dangerous since escape paths are highly restricted. See Below Figure.

Corridor pressurization

If only the corridors are zone pressurized as above, the system is called a corridor pressurization system. When smoke fills a corridor, it is very hard to see exit signs and people become disoriented. A combination of intake and exhaust fans can clear smoke. Corridor dampers normally provide ventilation air and exhaust stale air. However, they can be converted to smoke control dampers very easily. If a fire starts, the floors above and below the fire floor open their corridor ventilation dampers 100% to pressurize the floors while they close their return air dampers. This is identical in concept to the floor pressurization system discussed above. See Figure below.

Supply and return ducts in corridor protected by fire & smoke dampers.

Stairwell pressurization

The IBC requires that stairwells be designed as smoke proof enclosures. There are variations allowed by the code for when automatic sprinkles are provided and some architectural differences. Stairwell pressurization can be accomplished a number of ways. The IBC (IBC. 2012) requires vestibules in unsprinklered buildings.  This can be supplemented with stairwell pressurization.  In sprinklered buildings pressurization alone is allowed.  One should consult the IBC for details of requirements.

One method uses a constant volume fan capable of pushing air through any stair door that opens.  A barometric damper in the stairwell roof or wall relieves excessive pressure.  See Figure 1. In Figure 2 a combination vestibule with barometric is shown. There are designs by different fire protection engineers that use lobbies under positive pressure and others using negative pressure (IBC method) by exhausting. For the most part these designs do not use automated dampers in the periphery.

Since most buildings are sprinklered, pressurization systems alone are more common. A duct system can be run the height of the stairwell and proportional actuated dampers located every few floors with local pressure sensors.  If a floor door opens, the damper(s) nearest it modulate(s) open as necessary to maintain pressure.  A certain amount of smoke may enter the stairwell when any door is opened if there is a lot of pressure behind it.  Typically, the expansion of heated air does provide pressure.  It takes some time for the sensor, controller, and actuator to respond and open the local dampers further.  See Figure 3. The fan may be controlled by a VFD for better control.

Figure 2: Vestibule variation and supplemental stairwell pressurization

Other variations are possible and research is incomplete with regards to which is best in what geometric arrangement of stairs, stack effect, or height of stairs.  One variation is a second fan that turns on when the egress level door is opened.  Then that door does not relieve all the pressure necessary for the floors.  Some research has shown that sufficient ventilation alone during a fire will keep the stairwell tenable.  This employs a supply fan at the bottom of the stairwell and an exhaust fan at the top.  It can be combined with door pressurization by using variable frequency drive (VFD) fans.

Figure 3: Stairwell pressurization system using proportional damper control

Stairwells are built to be smoke proof compartments.  The occupants can escape into the stairwells and be protected from smoke while they escape the building.  When floor doors are opened, smoke must not enter the stairwell.  Since several architectural and control design methods are used examination of each system is necessary to understand its intent.  Testing using smoke generators helps to ensure the system works as required. Pressure in the stairwell must be below that which would hinder the opening of doors.

Elevator lobby pressurization

The lobbies of elevators can be pressurized to keep smoke from entering. These lobbies are sometimes areas of refuge and must be kept clear of smoke. The codes typically require that the elevator lobbies, where pressurized as a smoke compartment, be kept positive with respect to the occupied spaces. This is achieved by balancing the air systems to provide more air to the lobbies or by injecting air with a separate unit.

Special fire and smoke proportional or three-position actuators can be used to control the corridor dampers. The dampers must be partially closed for balancing, however they must reopen 100% to pressurize the floors adjacent to a smoke floor or to exhaust smoke as quickly as possible. Two speed fan motors or VFD’s prevent noise due to dampers that must be near closed during normal operation to avoid imbalance in design flow. Standard balancing dampers would restrict the full flow when needed. All other floors’ corridor dampers close so that a higher pressure and more air movement are available for the sandwich floors. Smoke causes most of the deaths in fires and smoke exhaust or pressurization methods can constrain it. However, in all of the methods discussed, too much oxygen cannot be injected and thus feed the fire. When fans are used to pressurize or add air for smoke removal, smoke detection on the inlet of the fan is used to avoid injecting smoke if the fire is near the inlet of the fan. Sprinklers are essential for fire protection. However, they are insufficient for fully balanced protection in large buildings. A balanced approach between active and passive measures produces the safest conditions. Compartmentation is the primary protection method for fire and smoke control. Maintaining the integrity of walls prevents fire passage and smoke spread. Containment duct and shaft dampers protect from smoke transport across compartment walls. About 85% of smoke dampers are used to maintain compartment containment. All means of egress must be protected – stairwells, elevator hoistways, lobbies, corridors, and paths to the outside. In addition, dampers are required where ducts penetrate shaft walls. Shaft dampers are the only way to restrict smoke movement. Air handling unit shutdown is insufficient alone. Large spaces like atriums, stages, malls, and stadium seating require smoke exhaust to keep the smoke layer above the level of the occupants’ heads. Engineered smoke control systems use mostly pressurization to prevent smoke migration. About 15% of actuated dampers are installed in them.

In general, any damper that is part of a smoke control system must be a UL555 (fire) and/or a UL555S (smoke) rated damper. In some cases exceptions are allowed since the damper is not meant to stop smoke. Examples are outside make-up air intakes and exhaust dampers on the outside of the building. They are usually open during an event and do not stop spread of fire and smoke.

Dampers are required to maintain compartmentation and restrict fire and smoke from spreading outside of the area of origin. However, full engineered smoke control systems can actively manage smoke and ensure means of egress for occupants. Exhausting large spaces with fans removes smoke. Preventing smoke from entering exit corridors, lobbies, elevators, and stairwells is critical in allowing escape. Other smoke control methods prevent the spread of smoke in buildings and along with architectural planning can protect occupants.