firesmokedampercontrol_hpac_engineering01_2008

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The inner workings of controls and

actuators in three types of fire and

smoke dampers are explained

F i r e - a n d S m o k e -

Damper Control

testing. Dampers with blade switches or actuators with internal auxiliary switches provide proof of closure.

Fire dampers normally close when thetemperature of the fusible link reaches 165˚F,

although some local variationsexist. For example, if steam coilsare present in ducts, use of a 212˚Fhigh 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 wasdesigned.

Each year, several million fire dampers areinstalled in barriers designed to slow the spread of fire (Photo A).

Sk ds. Smoke dampers, which areactuated, must open and close when requiredto provide fresh air or to stop smoke passage.They do not have to have high-limit sensorsor fusible links to close automatically. Smokedampers often provide signaling for indicatorlights, as discussed later in this article.

Practices vary by geographical region andby type of smoke control mandated by codes.Most commonly, a smoke detector (or two)inside of ducts will shut down fans and close

The No. 1 question about fire and smokedampers that mechanical engineers andcontractors ask is, “How do the controls

work with the actuators?” This article explainsthat.

Damper typeS

We must clearly distinguishamong three types of dampers.

F ds. Fire dampers, which rarely are actuated, close when a rise intemperature occurs and stay shut to stop fire frompassing through a barrier. About 90 percent of fire dampers are curtain-type dampers that close

when a mechanical fusible link melts, releasinga closing mechanism. A damper must be in theplane of a firewall, although some jurisdictionsmake exceptions if ducts are heavy duty. Out-of-partition dampers are available from mostmanufacturers.

In a few cases, auxiliary contacts are neededto prove opening and closing in periodic

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, Refrigerat-ing and Air-Conditioning Engineers (ASHRAE).He serves on ASHRAE Technical Committee (TC)

1.4: Control Theory and Application and is acorresponding member of ASHRAE TC 5.6:Control of Fire and Smoke.

By Larry FeLKer

Belimo Americas Sparks, Nev.

2 January 2008 • HPAC Engineering

PHOTO A. Curtain fire dampers.

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dampers if smoke is detected. Areasmoke detectors sometimes are wiredto a central fire-alarm panel, while apanel contact or remote relay initiatesclosing.

Smoke dampers do not need tobe within a wall providing a smokebarrier; they merely need to be within24 in. of the wall. As a result, theactuators may be axle-shaft mounted.However, most are jackshaft mounted

with linkage to the damper blades(Photo B).

Cb f d sk ds. Because combination fire and smokedampers, which are more commonthan simple smoke dampers, arefire-rated, their blades must be in abarrier wall. The actuator cannot be in

the wall, so a jackshaft and linkage areemployed with the actuator connectedto the jackshaft extension outside of the damper sleeve. (There are othermethods. However, this is standard inthe United States.) Almost all currentactuators are direct-coupled.

BaSiC ControL

There are several accepted methodsof temperature sensing and actuator

control. A fusible link that restrainsa shaft spring can perform the fire-damper function while an actuator

provides the smoke-control func-tion. The fusible link disconnects thedamper blades from the actuator and

ensures its closure. The actuator canopen and close for smoke managementonly until the fusible link melts.

Alternately, an electric bimetal sensor with a reset button can be used for thesingle primary sensor.

The system in Figure 1 is found inabout 80 percent of the combinationfire and smoke dampers on the markettoday. A smoke detector with a localthermal sensor is used as shown inFigure 1, which is a containment-damper application.

reopenaBLe DamperS

An engineered smoke-controlsystem with reopenable dampers is lesscommon. These have provisions for afirefighter to control the dampers moreprecisely.

Combinat ion f i r e and smokedampers in smoke-control systems

3HPAC Engineering • January 2008

If the temperature at the fire/smoke damperrises to 165˚F, the thermal disk opens andremoves power from the actuator. Whenthis happens, the damper springs closed.

The thermal switch normally is closed below set-point temperature.When the temperature rises above 165˚F, the switch opens. This stopsthe current flow to the actuator, and the damper closes. The switch

must be reset manually before it will close again and open the damper.

Actuator

Thermalswitch

Neutral165˚F

Smoke detectoror other alarmcontacts

Line(voltage)

The actuator is mountedso that the damper canspring closed. The actuatoris powered open for mostof its life.

FIGURE 1. A combination fire and smoke damper open.

PHOTO B. A smoke damper.

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have two sensors—primary andsecondary. The primary can be over-ridden by firefighters’ control; thesecondary is manually reset at thedamper only.

When a dual sensor is used, thefirst sensor always is electric. It opens,removes power, and lets the actuatorspring the damper closed. The secondsensor may be a fusible link or a higher-

temperature electric sensor. Typically,a primary sensor will register 165˚Fbefore closing a damper, while thesecondary sensor will register 250˚F. Inabout 15 percent of cases, the second-ary sensor will register 350˚F.

The w i r ing o f a dua l - s enso rcombination fire and smoke dampercan be seen in Figure 2. If a fire wereto break out and duct temperature

were to increase to 165˚F, the damper would spring closed to keep the firefrom spreading. However, firefighterscould choose to reopen the damperfor smoke-control purposes. If the

temperature reached the 250˚F (some-times 350˚F) limit, power again

would be cut off and the actuator would spring closed. It could not beopened again until a reset button werepressed. Firefighters also could close anopen damper to prevent oxygen fromfeeding a fire or air pressure frompushing smoke into other areas.

Orig inal dampers had releasemechanisms that only inc ludedexternal springs. Actuators did not havean internal spring. Later, a fusible link and dual springs were standard. Onefire spring was used to close the damperuntil the fusible link could be replaced,

while a separate actuator spring wasused to close the damper if the actuatorlost power, although the actuator couldreopen against the spring. Now, mostmanufacturers use electronic methodsof sensing and control. The actuatorspring is the only spring in most recent

designs.

inDiCator LightS

A f iref ighters ’ smoke-controlsystem (FSCS) has status-indicationlights in addition to manual hand-off-auto switches. The lights allow for verification of damper positionand override status. They can beinitiated from auxiliary switches on anactuator, damper-blade switches, or

proximity switches. Actuator switches of ten are preferred,as they are as reliable as blade-switch

4 January 2008 • HPAC Engineering

D a m p e r C o n t r o L

Common24 v or neutral

Green on = openRed on = closed

Normallyopen contact

Changes over when the damperis open greater than 85 degrees

Changes over when the damper is opengreater than 10 degrees

Normallyclosed contact

Contacts can be actuator auxiliarycontacts or damper-blade switches

Hot24 v or Line 1

(Phase 1)

FIGURE 3. Indication-light wiring for a single damper.

Actuator

Neutral

165˚F 250˚F

Smoke detectoror other alarmcontacts

Hot

Hand-off-auto(HOA)

Auto

Off

HandSequence of operation:

opens, power to the actuator is cut and the damper springs closed.

“hand,” then the contact and thermal switch are bypassed, the

damper closed. Manual reset is required.closed.

FIGURE 2. Dual-sensor reopenable damper wiring.



indication. The linkage betweenactuator jackshaft and damper is morerobust than the typical coat-hangerconnection from damper blade toswitch package.

Fully engineered smoke-controlsystems exist in about only 10 per-cent of applications. Firefighters havecontrol of dampers in these systems.Indicator lights show the systems’status.

Figure 3 shows a simple methodo f ind i c a to r - l i ght w i r ing . Thelights 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 indicatingfault. If the damper were closed, thered light would be on. If the damper

were open, the green light would beon. A variety of wiring methods arepossible.

In some jurisdictions, the red lightmeans “fire” or “problem,” while thegreen light means “OK.” In otherareas, green means “on,” and red means“off,” particularly for fans. Althoughrare, some dampers normally are open

when not powered, and the red andgreen lights could be switched. This canconfuse a firefighter during thecommotion of a fire. Indication lightsshould be discussed with a local firedepartment to ensure correct opera-tion.

proportionaL Damper ControL

Figure 4 shows two proportional

applications. The pressure in a stairwellor duct may need to be controlled ata certain set point. A smoke damperis required if a wall also is part of asmoke barrier. Given typical spaceconstraints, one damper and actuatorare technically and economically superior to two (or three, if a wall also isa fire barrier).

There are two ways to provideproportional damper control:

• No proportional electric actuatormeets Uniform Building Code (UBC)criteria for 15 sec of operation. These

actuators cannot be installed in UBCregions (which currently are Califor-nia and a few outside cities). In thesecases, two dampers are required. A two-position fire and smoke damper sits inthe rated wall. A standard proportionaldamper and actuator are installed inseries.

• In International Building Coderegions, a proportional fire and smokedamper and actuator can be installedbecause the code (based on Under-

writers Laboratories 555S, Standard for Smoke Dampers ) requires a 75-secmaximum for operation. Actuators thatmeet the 75-sec requirement (20 sec tospring closed, 75 sec to drive open) areavailable.

ConCLuSionThe type of damper used dependson the application. Actuation is

provided for smoke-control dampersand combination fire and smokedampers. Containment systems arethe most common, while reopenabledampers provide fire departments withmore options.

By following the wiring diagrams inthis article and comparing them with

the sequence of operation, one cangain an understanding of the operationrequirements for interfaces betweenmechanical and control systems.

C o d e s a n d p r a c t i c e s d i f f e rgeographically. One should consultlocal inspectors, contractors, andrepresentatives with specific questions.The methods shown in this articleare typical, although practices may vary.

For past HPAC Engineering feature articles, visit www.hpac.com.

5HPAC Engineering • January 2008

D a m p e r C o n t r o L

Power

165˚F

321

Differential-pressuresensorController

Proportionalactuator

Stairwellpressurization

Above ceiling or tounderfloor distribution

Wall

Shaft

Two position actuatorProportional

Proportionalactuator

FIGURE 4. Proportional applications.

firesmokedampercontrol_hpac_engineering01_2008

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