The No. 1 question about fire and smoke dampers that mechanical engineers and contractors ask is, “How do the controls work with the actuators?” This article explains that.
We must clearly distinguish among three types of dampers.
Fire dampers, which rarely are actuated, close when a rise in temperature occurs and stay shut to stop fire from passing through a barrier. About 90 percent of fire dampers are curtain-type dampers that close when a mechanical fusible link melts, releasing a closing mechanism. A damper must be in the plane of a firewall, although some jurisdictions make exceptions if ducts are heavy duty. Out-of-partition dampers are available from most manufacturers.
In a few cases, auxiliary contacts are needed to prove opening and closing in periodic testing. Dampers with blade switches or actuators with internal auxiliary switches provide proof of closure.
Fire dampers normally close when the temperature of the fusible link reaches 165°F, although some local variations exist. For example, if steam coils are present in ducts, use of a 212°F high limit may be used. The fire-damper temperature then may rise a couple thousand degrees, but the damper will hold for the time rating for which it was designed.
Each year, several million fire dampers are installed in barriers designed to slow the spread of fire (Photo A).
Smoke dampers, which are actuated, must open and close when required to provide fresh air or to stop smoke passage. They do not have to have high-limit sensors or fusible links to close automatically. Smoke dampers often provide signaling for indicator lights, as discussed later in this article.
Practices vary by geographical region and by type of smoke control mandated by codes. Most commonly, a smoke detector (or two) inside of ducts will shut down fans and close dampers if smoke is detected. Area smoke detectors sometimes are wired to a central fire-alarm panel, while a panel contact or remote relay initiates closing.
Smoke dampers do not need to be within a wall providing a smoke barrier; they merely need to be within 24 in. of the wall. As a result, the actuators may be axle-shaft mounted. However, most are jackshaft mounted with linkage to the damper blades (Photo B).
Combination fire and smoke dampers
Because combination fire and smoke dampers, which are more common than simple smoke dampers, are fire-rated, their blades must be in a barrier wall. The actuator cannot be in the wall, so a jackshaft and linkage are employed with the actuator connected to the jackshaft extension outside of the damper sleeve. (There are other methods. However, this is standard in the United States.) Almost all current actuators are direct-coupled.
Page 2 of 3
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.
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.
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.
Page 3 of 3
A firefighters' smoke-control system (FSCS) has status-indication lights in addition to manual hand-off-auto switches. The lights allow for verification of damper position and override status. They can be initiated from auxiliary switches on an actuator, damper-blade switches, or proximity switches.
Actuator switches often are preferred, as they are as reliable as blade-switch indication. The linkage between actuator jackshaft and damper is more robust than the typical coat-hanger connection from damper blade to switch package.
Fully engineered smoke-control systems exist in about only 10 percent of applications. Firefighters have control of dampers in these systems. Indicator lights show the systems' status.
Figure 3 shows a simple method of indicator-light wiring. The lights could be in a panel or local junction box, which could be located in the ceiling. An FSCS panel also would have an amber light indicating fault. If the damper were closed, the red light would be on. If the damper were open, the green light would be on. A variety of wiring methods are possible.
In some jurisdictions, the red light means “fire” or “problem,” while the green light means “OK.” In other areas, green means “on,” and red means “off,” particularly for fans. Although rare, some dampers normally are open when not powered, and the red and green lights could be switched. This can confuse a firefighter during the commotion of a fire. Indication lights should be discussed with a local fire department to ensure correct operation.
PROPORTIONAL DAMPER CONTROL
Figure 4 shows two proportional applications. The pressure in a stairwell or duct may need to be controlled at a certain set point. A smoke damper is required if a wall also is part of a smoke barrier. Given typical space constraints, one damper and actuator are technically and economically superior to two (or three, if a wall also is a fire barrier).
There are two ways to provide proportional damper control:
No proportional electric actuator meets Uniform Building Code (UBC) criteria for 15 sec of operation. These actuators cannot be installed in UBC regions (which currently are California and a few outside cities). In these cases, two dampers are required. A two-position fire and smoke damper sits in the rated wall. A standard proportional damper and actuator are installed in series.
In International Building Code regions, a proportional fire and smoke damper and actuator can be installed because the code (based on Underwriters Laboratories 555S, Standard for Smoke Dampers) requires a 75-sec maximum for operation. Actuators that meet the 75-sec requirement (20 sec to spring closed, 75 sec to drive open) are available.
The type of damper used depends on the application. Actuation is provided for smoke-control dampers and combination fire and smoke dampers. Containment systems are the most common, while reopenable dampers provide fire departments with more options.
By following the wiring diagrams in this article and comparing them with the sequence of operation, one can gain an understanding of the operation requirements for interfaces between mechanical and control systems.
Codes and practices differ geographically. One should consult local inspectors, contractors, and representatives with specific questions. The methods shown in this article are typical, although practices may vary.
For past HPAC Engineering feature articles, visit www.hpac.com.
Larry Felker is a product manager for Belimo Americas. He is a member of the International Code Council, the National Fire Protection Association, and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). He serves on ASHRAE Technical Committee (TC) 1.4: Control Theory and Application and is a corresponding member of ASHRAE TC 5.6: Control of Fire and Smoke.