Source: ASHRAE Handbook, HVAC Applications, Smoke Management, Ventilation of the Industrial Environment, and Industrial Local Exhaust Systems Chapters
Fire protection within facilities is intended to prevent fires from starting. However, when and if a fire starts, a provision must be made to manage the exposure and threat impact. The latter is usually accomplished with sprinklers, firewalls, fire doors, and fire dampers. The exposure is managed using exit systems, smoke control, smoke venting, and smoke barriers.
In building fires, smoke often flows to locations remote from the fire, threatening life and damaging property. Stairwells and elevators frequently become smoke-filled, thereby blocking or inhibiting evacuation. Smoke causes the most deaths in fires.
In the late 1960s, the idea of using pressurization to prevent smoke infiltration of stairwells began to attract attention. This concept was followed by the idea of the "pressure sandwich," i.e., venting or exhausting the fire floor and pressurizing the surrounding floors. Frequently, a building's ventilation system is used for this purpose. Smoke management systems use pressurization produced by mechanical fans to limit smoke movement in fire situations.
This segment discusses fire protection and smoke control systems in buildings as they relate to the HVAC field. For a more complete discussion, refer to Design of Smoke Management Systems. The National Fire Protection Association Guide for Smoke and Heat Venting (NFPA 204M) provides information about venting of large industrial and storage buildings. Other references are the Recommended Practice for Smoke Control Systems (NFPA 92A) and the Guide for Smoke Management Systems in Malls, Atria, and Large Areas (NFPA 92B).
The objective of fire safety is to provide some degree of protection for a building's occupants, the building and the property inside it, and neighboring buildings. Specific life safety objectives differ with occupancy; for example, nursing home requirements are different from those for office buildings.
The basic approaches to fire protection are to prevent fire ignition and to manage fire impact. The building occupants and managers have the primary role in preventing fire ignition. The building design typically includes features to assist the occupants and managers in this effort. Because it is impossible to prevent fire ignition completely, managing fire impact has become significant in fire protection design. Examples of fire impact management include compartmentation, suppression, and control of construction materials, exit systems, and smoke management. The Fire Protection Handbook (NFPA 1991a) contains detailed fire safety information.
The HVAC system is considered a potentially dangerous penetration of natural building membranes (wall, floors, etc.) that can readily transport smoke and fire. For this reason, the systems have traditionally been shut down when fire is discovered. Although shutting down the system prevents fans from forcing smoke flow, it does not prevent smoke movement through ducts due to smoke buoyancy, stack effect, or the wind. To solve the problem of smoke movement, methods of smoke control have been developed; it should be viewed as only one part of the overall building fire protection system.
A smoke control system must be designed so that it is not overpowered by the driving forces that cause smoke movement, which include stack effect, buoyancy, expansion, the wind, and the HVAC system. In a fire, smoke is generally moved by a combination of these forces.
Stack Effect - When it is cold outside, air often moves upward within building stairwells, elevator shafts, dumbwaiter shafts, mechanical shafts, or mail chutes. This normal stack effect occurs because the air in the building is warmer and less dense than the outside air. Normal stack effect is great when outside temperatures are low, especially in tall buildings. However, normal stack effect can exist even in a one-story building. When the outside air is warmer than the building air, a downward airflow, or reverse stack effect, frequently exists in shafts. Smoke movement from a building fire can be dominated by stack effect.
Buoyancy - High-temperature smoke from a fire has a buoyancy force due to its reduced density. The neutral plane is the plane of equal hydrostatic pressure between the fire compartment and its surroundings. For a fire with a fire compartment temperature at 1470°F, the pressure difference 5 ft above the neutral plane is 0.052 in. of water. Much larger pressure differences are possible for tall fire compartments where the distance from the neutral plane can be larger. If the fire compartment temperature is 1290°F, the pressure difference 35 ft above the neutral plane is 0.35 in. of water. This is a large fire, and the pressures it produces are beyond present smoke control methods.
In sprinkler-controlled fires, the temperature in the room on fire remains at that of the surroundings except for a short time before sprinkler activation. Sprinklers are activated by a thin (2 to 4 in.) layer of hot gas under the ceiling called the ceiling jet. For most commercial applications, the ceiling jet is between 180 and 300°F.
Expansion - In addition to buoyancy, the energy released by a fire can move smoke by expansion. In a room on fire with only one opening to the building, building air will flow in, and hot smoke will flow out. For a room on fire with open doors or windows, the pressure difference across these openings due to expansion is negligible. However, for a tightly sealed room on fire, the pressure differences due to expansion may be important.
Wind - In many instances, wind can have a pronounced effect on smoke movement within a building. In general, wind velocity increases with building height. The effect of wind on air movement within tightly constructed buildings with all exterior doors and windows closed is slight. However, the effects of wind can be important for loosely constructed buildings or for buildings with open doors or windows.
Frequently in fire situations, a window breaks in the fire compartment. If the window is on the leeward side of the building, the negative pressure caused by the wind vents the smoke from the room on fire. This reduces smoke movement throughout the building. However, if the broken window is on the windward side, the wind forces the smoke throughout the fire floor and to other floors, which endangers the lives of building occupants and hampers fire fighting. Pressures induced by the wind in this situation can be large and can dominate air movement throughout the building.
HVAC Systems - Before methods of smoke control were developed, HVAC systems were shut down when fires were discovered because the systems frequently transported smoke during fires. In the early stages of a fire, the HVAC system can aid in fire detection. When a fire starts in an unoccupied portion of a building, the system can transport the smoke to a space where people can smell it and be alerted to the fire. However, as the fire progresses, the system transports smoke to every area it serves, thus endangering life in all those spaces. The system also supplies air to the fire space, which aids combustion. Although shutting the system down prevents it from supplying air to the fire, it does not prevent smoke movement through the supply and return air ducts, airshafts, and other building openings due to stack effect, buoyancy, or wind.
Smoke management includes all methods that can be used singly or in combination to modify smoke movement for occupant safety or firefighters or for the reduction of property damage. The use of barriers, smoke vents, and smoke shafts are traditional methods of smoke management. The effectiveness of barriers is limited by the extent to which they are free of leakage paths. Smoke vents and smoke shafts are limited by the fact that smoke must be sufficiently buoyant to overcome any other driving forces that could be present. Fans have been used with the intent of overcoming the limitations of traditional approaches. The mechanisms of compartmentation, dilution, airflow, pressurization, and buoyancy are used by themselves or in combination to manage smoke conditions in fire situations.
Barriers with sufficient fire endurance to remain effective have a long history of providing protection against fire spread. In such fire compartmentation, the walls, partitions, floors, doors, and other barriers provide some level of smoke protection to spaces remote from the fire. There is both passive compartmentation and the use of compartmentation in conjunction with pressurization. Many codes, such as NFPA 101, provide specific criteria for the construction of smoke barriers (including doors) and smoke dampers in these barriers. The extent to which smoke leaks through such barriers depends on the size and shape of the leakage paths in the barriers and the pressure difference across the paths.
Dilution Remote from Fire - Smoke dilution (smoke purging, smoke removal, smoke exhaust, or smoke extraction) can be used to maintain acceptable gas and particulate concentrations in a room subject to smoke infiltration from an adjacent space. It can be effective if the rate of smoke leakage is small compared to either the total volume of the safeguarded space or the rate of purging air supplied to and removed from the space. Also, dilution can be beneficial to the fire service for removing smoke after a fire has been extinguished. Sometimes, when doors are opened, smoke flows into areas intended to be protected. Ideally, the doors are only open for short periods during evacuation. Supplying outside air to dilute the smoke can purge smoke that has entered spaces remote from the fire.
Caution About Dilution Near Fire - Many people have unrealistic expectations about what dilution can accomplish in the fire space. No evidence indicates that using a building's HVAC system for smoke dilution will significantly improve tenable conditions within the fire space. Because HVAC systems promote a considerable degree of air mixing within the spaces they serve and because building fires can produce very large quantities of smoke, it is generally believed that dilution of smoke by an HVAC system in the fire space will not result in any practical improvement in the tenable conditions in that space. Thus smoke-purging systems should not be used to improve hazard conditions within the fire space or in spaces connected to the fire space by large openings.
Pressurization - Systems that pressurize an area using mechanical fans are referred to as smoke control NFPA (1993b). A pressure difference across a barrier can control smoke movement. Within the barrier is a door. The high-pressure side of the door can be either a refuge area or an egress route. The low-pressure side is exposed to smoke from a fire. Airflow through the gaps around the door and through construction cracks prevents smoke infiltration to the high pressure side.
Some fire code officials say that if a building has sprinklers, it is not necessary to pressurize stairwells. Safety officials disagree - they feel as an exit path pressurized stairwells are an important aspect of getting people safely out of the building.
Buoyancy - The buoyancy of hot combustion gases is employed in both fan-powered and non-fan-powered venting systems. Such fan-powered venting for large spaces is commonly employed for atriums and covered shopping malls, and non-fan-powered venting is commonly used for large industrial and storage buildings. There is a concern that the sprinkler flow will cool the smoke, reducing buoyancy and thus the system effectiveness.
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