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SCS Series Manual —
P/N 15712:L 7/18/16
Concepts of Smoke Control
Principles of Smoke Control
2.2.6 HVAC Systems
Before the development of smoke control, HVAC systems were shut down when fires were discovered for two main reasons:
•
The HVAC system frequently aided the movement of smoke during a fire condition, transporting smoke to every area that it serves.
•
The HVAC system also supplied air to the fire space, which has a tendency to fuel a fire.
An HVAC system, however, may aid in the detection of a fire in its early stages by transporting smoke from an unoccupied area to a space
where building occupants can be alerted to the fire. Once a fire is detected, the HVAC system should be designed to either shut down the fans
or provide a special smoke control mode. If neither of these steps is taken, the HVAC system will transport smoke to every area that the sys-
tem serves, thus endangering life, damaging property, and inhibiting fire fighting.
However, shutting down the fans does not prevent smoke movement through the supply and return air ducts, air shafts, and other building
openings due to stack effect, buoyancy, and wind. Installation of smoke dampers for when the system is shut down can help inhibit smoke
movement in this case.
Utilizing an HVAC system for smoke control will be discussed in detail in the sections that follow.
2.3 Principles of Smoke Control
The idea of using pressurization to prevent the movement of smoke into
stairwells began to attract attention in the late 1960s. This was followed by
utilizing the ventilation system of a building to vent or exhaust the fire floor
and pressurize surrounding floors. The term "smoke control" was used to
describe these systems that use pressurization, produced by mechanical
fans, to limit smoke movement in fire situations. The three major consider-
ations for smoke control are smoke containment, purging, and door-opening
forces. These principles are described below.
2.3.1 Smoke Containment
The area where the fire is located is referred to as the smoke zone. There are two basic principles for containing smoke within that smoke
zone:
•
Air pressure differences across barriers can act to control smoke movement. This is referred to as pressurization.
•
Airflow by itself can control smoke movement if the average air velocity is of sufficient magnitude.
Pressurization
Pressurization results in airflows in the small gaps between smoke control zones preventing smoke movement through these openings. Tech-
nically, pressurization uses airflow and thus is a special case of the second principle, but considering the two principles separately is advan-
tageous for smoke control design.
Pressurization is employed by creating pressure differences across partitions that separate the smoke zone from other areas. This can be
accomplished by making pressure in the area surrounding the smoke zone higher than pressure in the smoke zone itself (refer to Figure 2.4).
Airflow through construction and door cracks prevents the movement of smoke to the high-pressure side. The pressure difference must be
sufficient to contain the smoke in the smoke zone and at the same time allow doors leading to exit routes to be opened.
Pressurization is the most desired means of controlling smoke.
Air Flow
When the door in the barrier is open, air flows through the opening. When the air velocity is low, smoke can flow from the smoke zone into
unwanted areas. Airflow is employed by controlling the flow of air into a smoke zone and that flow must be sufficient to prevent the migra-
tion of smoke from the zone. This process is usually used to prevent the flow of smoke down corridors or through open doorways, as shown
in Figure 2.5.
Airflow is not the most practical method of limiting the movement of smoke because of the large quantities of air required.
Figure 2.4 Pressurization
Door
High
Pressure
Side
Low
Pressure
Side