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The Official Website of Dr. John H. Klote, PE -  Updated June 22, 2015

Smoke is the major killer in building fires, and smoke control

provides significant protection from the threat of smoke.

Physical Mechanisms of Smoke Control


A smoke control system is an engineered system that uses all methods that can be used either alone or in combination to modify smoke movement to protect people and/or property. The methods that are used by engineers to provide this protection are the physical mechanisms of smoke control: (1) compartmentation, (2) pressurization, (3) airflow, (4) buoyancy, and (5) dilution.


Compartmentation


For centuries, compartmentation has been recognized as a way of controlling the spread of fire and smoke. When a person closes the only door to a burning room, smoke flow from the room decreases considerably. Also, the amount of air available to the fire drops off. This passive smoke protection is recognized in many building and fire codes even without a design analysis. Engineered smoke control systems that use only passive smoke barriers can be tenability systems. A tenability system is one where occupants can be exposed to some combustion products that are so diluted that an environment is maintained that is not life threatening. Methods of analysis of such tenability systems are discussed in Chapter 19 of the Handbook of Smoke Control Engineering.


Pressurization


Many systems rely on the pressure difference across a barrier to control smoke movement. Airflow through construction cracks and gaps around doors prevents smoke infiltration to protected spaces. Full scale fire tests have shown that pressurization can provide smoke protection even with a fully developed fire on the low pressure side of the barrier. In a fully developed fire, everything in the fire room that can burn is burning.


Airflow


Airflow can control smoke flow, and some applications of airflow are for transit tunnels, corridors, open doorways, and between an atrium and other spaces. Fir airflow to control smoke, the velocity needs to be at least the critical airflow, and this critical value is different for different applications. Probably the most extensive use of the airflow method is in transit tunnels where the airflow method is often referred to as ventilation. The airflow method has the disadvantage of requiring large amounts of supply air. The airflow method can supply oxygen to the fire, which can result in catastrophic failure. Even sprinkler protection does not completely eliminate this potential. For any application that uses the airflow approach, this failure mode needs to be addressed in the design analysis.


Buoyancy


Buoyancy of hot smoke is employed in atrium smoke control systems. Because of buoyancy, the smoke rises above the fire to form a smoke layer under the ceiling of the atrium. Below the smoke layer there is a “smoke free” lower layer, and the intent of conventional atrium smoke control systems is that occupants can evacuate atrium through this lower layer. Buoyancy and dilution are the driving forces behind atrium tenability smoke control systems.  


Dilution


The products of combustion can be diluted such that a tenable environment is maintained. As stated above, a tenable environment is one in which smoke and heat are limited such that the impact on occupants is not life threatening. For example in a large and high atrium, there is considerable dilution in the smoke plume rising above the fire. If the there is sufficient dilution in the plume, it may be possible that the atrium smoke control can be done as a tenability system.





Pressurization can control smoke at a barrier

from a fire at a remote location.

Pressurization can control smoke at a barrier

from a fully developed fire next to the door.

Airflow can control smoke provided that the velocity is sufficient.

Conventional atrium smoke control system.