Page 2 of 3

FIGURE 1. A combination fire and smoke damper open.

BASIC CONTROL

There are several accepted methods of temperature sensing and actuator control. A fusible link that restrains a shaft spring can perform the fire-damper function while an actuator provides the smoke-control function. The fusible link disconnects the damper blades from the actuator and ensures its closure. The actuator can open and close for smoke management only until the fusible link melts. Alternately, an electric bimetal sensor with a reset button can be used for the single primary sensor.

The system in Figure 1 is found in about 80 percent of the combination fire and smoke dampers on the market today. A smoke detector with a local thermal sensor is used as shown in Figure 1, which is a containment-damper application.

REOPENABLE DAMPERS

An engineered smoke-control system with reopenable dampers is less common. These have provisions for a firefighter to control the dampers more precisely.

Combination fire and smoke dampers in smoke-control systems have two sensors — primary and secondary. The primary can be overridden by firefighters' control; the secondary is manually reset at the damper only.

When a dual sensor is used, the first sensor always is electric. It opens, removes power, and lets the actuator spring the damper closed. The second sensor may be a fusible link or a higher-temperature electric sensor. Typically, a primary sensor will register 165°F before closing a damper, while the secondary sensor will register 250°F. In about 15 percent of cases, the secondary sensor will register 350°F.

FIGURE 2. dual-sensor reopenable damper wiring.

The wiring of a dual-sensor combination fire and smoke damper can be seen in Figure 2. If a fire were to break out and duct temperature were to increase to 165°F, the damper would spring closed to keep the fire from spreading. However, firefighters could choose to reopen the damper for smoke-control purposes. If the temperature reached the 250°F (sometimes 350°F) limit, power again would be cut off and the actuator would spring closed. It could not be opened again until a reset button were pressed. Firefighters also could close an open damper to prevent oxygen from feeding a fire or air pressure from pushing smoke into other areas.

Original dampers had release mechanisms that only included external springs. Actuators did not have an internal spring. Later, a fusible link and dual springs were standard. One fire spring was used to close the damper until the fusible link could be replaced, while a separate actuator spring was used to close the damper if the actuator lost power, although the actuator could reopen against the spring. Now, most manufacturers use electronic methods of sensing and control. The actuator spring is the only spring in most recent designs.