Choosing The Proper Dampers; Placing Air Inlets And Outlets - Simplex 4100U Application Manual

Designing a Smoke Control System,
Selecting the Proper
Fans and Duct Work
Choosing the
Proper Dampers
Placing Air Inlets
and Outlets
Fans must be capable of reaching the required pressure setting within 60 seconds. Each fan must
also have a pressure monitor so that the smoke control system can receive feedback on the status
of the fan to determine whether it is actually working. It is the responsibility of the system
designer to select duct work that meets the temperature and fire ratings for the specific application.
In some climates, the outside air can be so cold that drawing it directly inside the building for
pressurization can damage the building's interior fixtures or equipment (e.g., freeze pipes or
damage temperature-sensitive equipment). In these cases, some sort of pre-heater needs to be
installed on the air inlet. The smoke control system does not need to control this air intake heater
as closely as one on an HVAC system, since maintaining comfort levels is not an issue. It simply
has to make sure the air sent into an area is warm enough to not damage the building's equipment.
The dampers used to isolate the smoke zone must be smoke dampers. Smoke dampers are dampers
that meet the requirements given in UL 555S, Standard for Leakage Rated Dampers for Use in
Smoke Control Systems. Following this standard ensures that the dampers are able to block the
smoke when they are fully closed. These dampers may be different from those you might use in an
HVAC system that does not perform smoke control.
In a smoke control system, the dampers must be able to travel to their desired setting in a maximum
time of 75 seconds (see note below). All dampers must be fitted with end-position switches to
provide feedback to the smoke control system. These switches let the control system know the
position of the dampers, since smoke dampers are usually either fully-closed or fully-open.
Note: Local codes may specify a shorter maximum time.
Dampers sometimes function as both smoke dampers and fire dampers. Fire dampers are dampers
that block a fire from penetrating a fire-rated partition via a duct. These dampers are normally
open, held in place by a fusible link. The fusible link is a heat-sensitive device that releases the
dampers when it is heated to a certain temperature. Once the fusible link releases, the dampers
close by the force of gravity. This is required so that fire dampers operate even if the local electric
service has failed. The specifications for fire dampers appear in UL 555, Standard for Fire
Dampers.
If you want a damper to function as both a smoke damper and a fire damper, it must meet the
requirements for both devices. A damper can be operated by an electric motor or pneumatics.
However, it must have a fusible link or some other means of automatic closure (like a regular fire
damper). Since the control system can override the damper closure if the temperature warrants, the
damper needs the fusible link in case the damper's automatic control is interrupted.
You need to carefully consider the placement of the air inlets and outlets on your building. If you
place an outlet that vents smoke too close to an air inlet, the air intake can draw the smoke back
into the building. Since smoke rises, the exhausts that vent smoke should be placed well above air
inlets. The exhausts should be placed several feet above the roof level to allow space for the
smoke to rise and disperse.
Keeping smoke outlets far away from air inlets does not guarantee that the air brought into the
building is always smoke free. You may want to place smoke detectors in air inlets that operate
during a smoke emergency.
Note: In some cases, smoke detection in the air inlet is required to have the capability of being
overridden by the responding authority after the situation has been investigated.
If the detector finds smoke in the incoming air, it alerts the control system. The control system
must then decide whether or not to shut down the air inlet.
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