fire- and smoke-damper control
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smoke damperTRANSCRIPT
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Jan 1, 2008
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Fire and SmokeDamper ControlThe inner workings of controls and actuators in three types of fire and smokedampers are explained
By LARRY FELKER, Belimo Americas, Sparks, Nev.
The No. 1 question about fire and smoke dampers that mechanical engineers andcontractors ask is, “How do the controls work with the actuators?” This article explainsthat.
DAMPER TYPES
We must clearly distinguish among three types of dampers.
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Fire dampers
Fire dampers, which rarely are actuated, close when a rise in temperature occurs and stayshut to stop fire from passing through a barrier. About 90 percent of fire dampers arecurtaintype dampers that close when a mechanical fusible link melts, releasing a closingmechanism. A damper must be in the plane of a firewall, although some jurisdictionsmake exceptions if ducts are heavy duty. Outofpartition dampers are available frommost manufacturers.
PHOTO A. Curtain fire dampers.
In a few cases, auxiliary contacts are needed to prove opening and closing in periodictesting. Dampers with blade switches or actuators with internal auxiliary switches provideproof 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, useof a 212°F high limit may be used. The firedamper temperature then may rise a couplethousand 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 thespread of fire (Photo A).
Smoke dampers
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Smoke dampers, which are actuated, must open and close when required to provide freshair or to stop smoke passage. They do not have to have highlimit sensors or fusible linksto close automatically. Smoke dampers often provide signaling for indicator lights, asdiscussed later in this article.
PHOTO B. A smoke damper.
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 closedampers if smoke is detected. Area smoke detectors sometimes are wired to a central firealarm 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 merelyneed to be within 24 in. of the wall. As a result, the actuators may be axleshaft 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 simplesmoke dampers, are firerated, their blades must be in a barrier wall. The actuator cannotbe in the wall, so a jackshaft and linkage are employed with the actuator connected to thejackshaft extension outside of the damper sleeve. (There are other methods. However, this
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is standard in the United States.) Almost all current actuators are directcoupled.
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. Afusible link that restrains a shaft spring can perform the firedamper function while anactuator provides the smokecontrol function. The fusible link disconnects the damperblades from the actuator and ensures its closure. The actuator can open and close forsmoke management only until the fusible link melts. Alternately, an electric bimetal sensorwith a reset button can be used for the single primary sensor.
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The system in Figure 1 is found in about 80 percent of the combination fire and smokedampers on the market today. A smoke detector with a local thermal sensor is used asshown in Figure 1, which is a containmentdamper application.
REOPENABLE DAMPERS
An engineered smokecontrol system with reopenable dampers is less common. Thesehave provisions for a firefighter to control the dampers more precisely.
Combination fire and smoke dampers in smokecontrol systems have two sensors —primary and secondary. The primary can be overridden by firefighters' control; thesecondary 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 ora highertemperature electric sensor. Typically, a primary sensor will register 165°F beforeclosing a damper, while the secondary sensor will register 250°F. In about 15 percent ofcases, the secondary sensor will register 350°F.
FIGURE 2. dualsensor reopenable damper wiring. TwitterFacebookLinkedInRSS
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The wiring of a dualsensor 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 damperwould spring closed to keep the fire from spreading. However, firefighters could choose toreopen the damper for smokecontrol purposes. If the temperature reached the 250°F(sometimes 350°F) limit, power again would be cut off and the actuator would springclosed. It could not be opened again until a reset button were pressed. Firefighters alsocould close an open damper to prevent oxygen from feeding a fire or air pressure frompushing smoke into other areas.
Original dampers had release mechanisms that only included external springs. Actuatorsdid not have an internal spring. Later, a fusible link and dual springs were standard. Onefire spring was used to close the damper until the fusible link could be replaced, while aseparate actuator spring was used to close the damper if the actuator lost power,although the actuator could reopen against the spring. Now, most manufacturers useelectronic methods of sensing and control. The actuator spring is the only spring in mostrecent designs.
Page 3 of 3
INDICATOR LIGHTS
A firefighters' smokecontrol system (FSCS) has statusindication lights in addition tomanual handoffauto switches. The lights allow for verification of damper position andoverride status. They can be initiated from auxiliary switches on an actuator, damperblade switches, or proximity switches.
Actuator switches often are preferred, as they are as reliable as bladeswitch indication.The linkage between actuator jackshaft and damper is more robust than the typical coathanger connection from damper blade to switch package.
Fully engineered smokecontrol systems exist in about only 10 percent of applications.Firefighters have control of dampers in these systems. Indicator lights show the systems'
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status.
Figure 3 shows a simple method of indicatorlight wiring. The lights could be in a panelor local junction box, which could be located in the ceiling. An FSCS panel also wouldhave an amber light indicating fault. If the damper were closed, the red light would beon. If the damper were open, the green light would be on. A variety of wiring methods arepossible.
FIGURE 3. Indicationlight wiring for a single damper.
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 andgreen lights could be switched. This can confuse a firefighter during the commotion of afire. Indication lights should be discussed with a local fire department to ensure correctoperation.
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PROPORTIONAL DAMPER CONTROL
Figure 4 shows two proportional applications. The pressure in a stairwell or duct mayneed to be controlled at a certain set point. A smoke damper is required if a wall also ispart of a smoke barrier. Given typical space constraints, one damper and actuator aretechnically and economically superior to two (or three, if a wall also is a fire barrier).
FIGURE 4. Proportional applications.
There are two ways to provide proportional damper control:
No proportional electric actuator meets Uniform Building Code (UBC) criteria for15 sec of operation. These actuators cannot be installed in UBC regions (whichcurrently are California and a few outside cities). In these cases, two dampers are
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CONCLUSION
The type of damper used depends on the application. Actuation is provided for smokecontrol dampers and combination fire and smoke dampers. Containment systems are themost common, while reopenable dampers provide fire departments with more options.
By following the wiring diagrams in this article and comparing them with the sequence ofoperation, one can gain an understanding of the operation requirements for interfacesbetween 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 articleare typical, although practices may vary.
Larry Felker is a product manager for Belimo Americas. He is a member of theInternational Code Council, the National Fire Protection Association, and the AmericanSociety of Heating, Refrigerating and AirConditioning Engineers (ASHRAE). He serveson ASHRAE Technical Committee (TC) 1.4: Control Theory and Application and is acorresponding member of ASHRAE TC 5.6: Control of Fire and Smoke.
required. A twoposition fire and smoke damper sits in the rated wall. A standardproportional damper and actuator are installed in series.
In International Building Code regions, a proportional fire and smoke damper andactuator can be installed because the code (based on Underwriters Laboratories555S, Standard for Smoke Dampers) requires a 75sec maximum for operation.Actuators that meet the 75sec requirement (20 sec to spring closed, 75 sec to driveopen) are available.
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very helpful. thank you so much!
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