When control loops are not properly tuned, actuators may fail prematurely, and the system can provide poor temperature or flow control.
Three conditions are usually caused by poor control loop tuning: oscillation, hunting, and dithering. Oscillation is constant changes in position. Hunting is wide, constant swings in control signal. Dithering is small, sometimes imperceptible changes in signal. For floating actuators, the time and frequency of the pulse, rather than signal voltage, is the control output but the result is the same.
Oscillating control loops caused by being over sensitive can result in more than a million actuator movements in just a few years and can cause actuator failures. In sluggish control loops, actuator movement does not occur or is too small to affect flow, resulting in extreme system overshoot and comfort problems.
There is an easy trade-off between tight control and a reasonable number, and size, of actuator movements. Space temperature changes occur slowly in a stable process. When a room fills with people, it takes about 5 minutes for a 1 degree temperature change to occur. It may take another 5 minutes for a wall sensor to register the change. Fast movements of the actuator are not necessary.
Mixed air control is also a very stable process. Rapid changes in outdoor air temperature or average return air temperatures are rare. On the other hand, discharge air temperature control with a hot water or chilled water valve is a fast process because the sensor is close to the coil. A bare averaging thermistor sensor may respond in 30 seconds to a 1-degree change. Encapsulated sensors may take 2 minutes.
During start-up of a system, more actuator movements may be necessary to find the correct operating condition, but the space is not occupied and there is time to stabilize the system without fast actuator responses. In an ideal control loop, each actuator movement is productive.
Proportional constants (and also integral and derivative) should be set so that continuous oscillation of actuators does not occur. Actuator movement should not occur before the effects of previous movements have had time to affect the sensor. This may be 10 minutes for a space sensor or 1 minute for an air sensor. Movement of damper or valve actuator when air temperature is within .5 degrees of set point, or when mixed-air is within 1 degree of set point usually will yield stable control; attempts to achieve more precise control may result in unstable control and excessive actuator movement.
When 3-point floating control is used, the total run-time of the actuator should be entered into the program logic. When the actuator is at either the fully open or fully closed position, continual pulsing of actuator against end-stops (end-stop dithering) should not occur. The actuator should either stop and hold or drive continuously against the end-stop.
Each VAV box and actuator control loop should be individually tuned. Where high turbulence in the sensor flow control loop is causing dithering (rapid oscillation) of the actuator control loop, the sensor loop should be corrected mechanically or with software averaging. Where the control loop is not fully adjustable, it may be necessary to find a method unique to the individual controller.
If an actuator is “chasing” a process which occurs faster than the actuator movements are able to correct, as may occur in the situation described above, the result will be many more actuator movements than are necessary to affect flow. The average flow should be adjusted, but turbulence is normal and should be ignored.
- Hunting: wide oscillations
- Dithering: small, unproductive movements of the actuator
- End-Stop Dithering: the actuator or damper is at the end of its rotation, but the control loop continues to pulse.
- Reduce the proportional or integral gain.
- Use a deadband.
- Add time delay.
- Reset constants.
- Shut down control when at end-stops.
- Do not control process faster than components response.
In the US, Canada, and Latin America fire, smoke, and combination fire and smoke dampers are used in two general categories:
- Containment of fire and/or smoke to maintain building compartmentation. These are installed based on Chapter 7 of the International Building Code (IBC) which is the primary model code. These are sometimes referred to as passive systems although the dampers do close and fire alarms operate when a smoke detector operates.
- Engineered smoke control systems use dampers, fans, and some architectural features in a wide variety of applications. These are based on Chapter 9 of the IBC.
In the Americas smoke dampers are always actuated; fire dampers use mechanical means of sensing heat (fusible links that melt and gravity or spring release for closure). They and can be actuated for ease of periodic inspection and maintenance. Smoke must be sensed using electrical sensing – smoke detectors. Spring return actuators are used to close the dampers and then the actuator motor used to open the damper. Combination fire and smoke dampers are actuated due to the smoke function.
Many smoke control applications require modulating control of dampers. Stairwell pressurization and underfloor air-conditioning are examples where they can be utilized.
In this article the common control methods for fire and smoke dampers (typically Chapter 7 applications) are described in order to help distinguish among applications. Then modulating control of the same dampers in different applications (typically Chapter 9 applications) is discussed and explained.
Containment fire and smoke damper controls
Figure 1 shows (from left to right) a duct smoke detector, high temperature switch, and actuated damper. Roughly 80% of fire and smoke dampers are installed this way although the smoke sensing may be via area smoke detection and a relay employed to operate the damper. The damper protects the integrity of the wall to maintain compartmentation so that neither smoke nor fire can pass to an adjacent compartment.
Figure 1 Typical installation of a combination fire and smoke containment damper.
Figure 2 shows the wiring. Starting at the far left, hot power is run to the smoke detector. As long as smoke is not present the detector passes power to the temperature switch. Power to the actuator drives the damper open and holds it in the open position.
If smoke is detected power is removed from the actuator and the alarm contact on the detector closes to issue an alarm. If an area smoke detection system is used, the smoke control system has a relay connected in place of the smoke detector contact.
In case smoke is not detected but the temperature at the damper rises to 165°F (74°C), then the temperature responsive switch opens. This cuts power to the actuator and the damper springs closed. The temperature switch is manual reset so the damper remains closed during the event.
In the cases where the damper is only a smoke damper, the temperature switch is not present. The smoke detector or a relay from the smoke control system is the only operating control.
Figure 2 Smoke detector and combination fire and smoke damper wiring.
Engineered Smoke Control System Dampers
Roughly 80% of fire and smoke dampers are installed in containment applications as shown above. About 20% are installed in more customized applications that are designed by the fire protection and mechanical engineers. Typical applications are atria, stairwell pressurization systems, underfloor air conditioning, underground floors, and large spaces like malls, auditoriums, and stages.
Figure 3 shows the basic controls employed in a smoke control system for one damper. The Fire fighters’ Smoke Control System (FSCS) panel allows override control and provides status indication for all components of the system.
Figure 3 FSCS panel and remote smoke damper wiring.
The dampers used for smoke control are typically of the same construction as containment. The primary difference is in the control methods. The damper blade position indication switches may be auxiliary switches on the actuator, damper blade switches, or magnetic contact switches. The smoke control system has a relay that allows the FSCS panel switches to place it in automatic, closed, or open position. Figure 3 also shows the connections to a networked system. The relays or cards are isolated from the line or 24V power used to operate the actuator.
The smoke control system components are UL 864, UUKL listed. The actuator has UL 873 or UL 60730 electrical listing and UL 2043 low smoke generation listings. The damper and actuator as a unit is UL555S listed.
Figure 4 shows a reopenable damper. Wiring for the Auto-Off-On Override switch is shown connected directly to the FSCS panel although typically there are network relays present to perform the functions. This damper serves both in containment and smoke control functions. It is connected to the FSCS panel so that the fire department incident commander can reopen the damper to remove smoke or pressurize a space. Status indication is provided.
Figure 4 Reopenable damper.
Sequence of Operation
In Automatic mode the smoke relay responds to the programming of the control panel to cut power and spring the damper closed when appropriate. Alternately, if a fire is present and the temperature in the duct rises to 165°F (74°C) the primary temperature switch opens and the damper springs closed.
If the panel switch is moved to Override, then the smoke relay and primary sensor are bypassed. The actuator is again powered and the damper opens. However, if the temperature at the damper continues to rise then the secondary sensor opens at 250°F (121°C). (The fire is close enough that there is danger of flames or heat moving through the damper to the other side of the wall.)
In addition, if the fire department moves the switch on the FSCS panel to Off, then power is removed from the actuator and the damper closes.
Modulating Control System Dampers
Some systems require proportional control of the dampers in the fire and smoke applications discussed above. The controls must combine typical temperature and/or pressure control methods as well as fire and smoke functions.
Figure 5 shows the simplest of modulating control methods for a fire and smoke damper. It is used commonly for corridor ventilation. The potentiometer sets a balance position for the damper during normal operation. The relay can close the damper in event of fire avoid smoke spread.
Power is placed on the actuator terminals 1 and 2. The potentiometer has a varying signal of from 2 to 10VDC that goes to terminal 3, the signal input. The actuator positions from 0 to 100% to open the damper to the balanced position. The common acts as a source of electrons and carries both AC and DC currents. In an event, the Override relay can cut power to the actuator which then springs the damper closed.
Figure 5 Potentiometer control of a smoke damper with override closed.
Figure 6 shows the same smoke damper as in Figure 4 with an added relay to override the damper open. By shorting hot power to terminal 3 of the actuator, it will drive open. While not always necessary, a contact opens to disconnect the signal terminal on the potentiometer. This prevents hot 24VAC from damaging the signal output. On DDC systems this is important.
There are optional wiring configurations that work just as well as that shown. For example, Override relay 2 could be placed in the common 24VAC wire. At times it is important to arrange the relay contacts so that in case of failure of one relay, the failsafe condition is the safest.
Figure 6 Potentiometer control of a smoke damper with override open or closed
In Figure 7 instead of a minimum potentiometer controlling the actuator, a building automation system, direct digital control sends the signal to terminal 3 and the actuator is continuously adjustable. (Default is 2V, closed and 10V, full open. This is reversible when needed for some applications.) The signal path is from Sig + on the controller to 3 through the actuator electronics to 1 and back out to the controller Com. A complete loop is always needed for current flow out and into any device.
Figure 7 Typical analog 2-10VDC actuator control circuit.
Figure 8 adds a high temperature switch. It is shown here in the common wire, but could be placed in the hot wire also. If the temperature at the damper rises to 165°F (74°C) the switch opens to cut power to the actuator and it springs the damper closed.
Figure 8 Control of a fire and smoke damper showing high temperature switch.
Normally, the damper modulates based on the output signal from the BAS controller. Typically, if smoke is detected, the automatic response is to make Override relay 2 and spring the damper closed. If the FSCS panel is set to Open, then Override relay 2 is de-energized and Override relay 1 is energized. The damper is then open 100%. However, if the temperature in the duct going into the damper reaches 165°F (74°C), then the damper again closes.
Figure 9 adds a secondary high temperature switch and a bypass relay in the common wire.
Figure 9 Reopenable combination fire and smoke damper.
The sequence of operation is as follows:
With 24VAC present and all controls in the normal state, the actuator opens damper to the position the Signal indicates. Actuator will modulate to maintain the setpoint.
Cutting 24VAC power or making Override relay 2 closes the damper.
If the temperature at the damper reaches 165°F (74°C), the primary sensor opens and the damper springs closed.
If the FSCS panel switch is set to Open, several actions occur.
a. The primary sensor is bypassed reconnecting the common power to the actuator.
b. Override relay 1 is made and Override relay 2 goes to normal. This causes the actuator to drive full open. (Hot 24VAC is shorted to the actuator terminal. Hot 24VAC is not allowed to reach Signal of DDC controller as that would destroy the output’s electronics.)
However, if the duct temperature reaches 250°F (121°C), then the secondary temperature switch opens and the damper again closes. The FSCS panel cannot override this and manual reset is necessary. It is presumed that the fire is too close to the damper and compartmentation is at risk.
Underfloor Air Conditioning Example
Figure 10 shows an example of an underfloor air conditioning system and how a modulating actuator could function.
The shaft wall is a fire barrier and a smoke partition and therefore requires either separate dampers or a combination fire and smoke damper. The pressure under the floor must be maintained at somewhere between 0.05 and 0.10 in. w.c. (12 to 25 Pa). This would require another damper and modulating actuator. However, by using a modulating fire and smoke damper, only one damper and actuator can do the job of three. This saves material and labor costs and also helps alleviate space constraints.
Figure 10 Underfloor air conditioning example.
It would be up to the fire protection engineer and the local authority having jurisdiction to determine if this damper is considered part of containment (Chapter 7) or part of the engineered smoke control system (Chapter 9). It could be used for both. If it is part of the smoke control also, then status indication and overrides would be required.
The sequence of operation is:
- During normal operation the pressure under the floor is maintained by modulating the damper mounted in the shaft wall.
- If a fire occurs and the temperature at the damper reaches 165°F (74°C), then the damper closes.
- If smoke is present in duct (or space area), then damper closes.
There are a large number of methods to modulate fire and smoke dampers and apply fire and smoke safety controls. In containment applications, the damper is closed when either high temperatures or smoke is observed. In smoke control systems a number of ways exist to either open or close the damper to purge or pressurize spaces to prevent smoke from spreading.
Some, not all, of the methods of control are shown and explained in this article. Consult the referenced Codes and Standards or contact the author for additional information.
Author - Larry Felker
Larry Felker is a mechanical engineer and member of ICC (International Code Council), NFPA (National Fire Protection Association), and a life member of ASHRAE (American Society of Heating, Refrigeration Air Conditioning Engineers). He is a Product Manager for Fire & Smoke Actuators for Belimo Americas who has specialized in fire and smoke dampers and actuators since 2002. Previously he was a temperature control system designer and before that a mechanical and electrical contractor. He is the co-author (with Travis Felker) of Dampers and Airflow Control, ASHRAE Special Publications, 2010.
(IBC) International Building Code, 2012, International Code Council, Inc. (ICC), Country Club Hills, IL 60478-5795
(IFC) International Fire Code 2012, ICC, ibid.
(IMC) International Mechanical Code 2012, ICC, op. cit.
UL 555 Standard for Safety for Fire Dampers, Edition 7, 2006, Updated 2010, Underwriters Laboratories Inc. (UL), 333 Pfingsten Road, Northbrook, IL 60062-2096
UL 555S Standard for Safety for Smoke Dampers, 4th Edition, 1999, Updated 2012, ibid.
UL 864 Standard for Safety Control Units and Accessories for Fire Alarm Systems, 9th Edition, 2010
UL 873 Standard for Temperature-Indicating and -Regulating Equipment (Ed. 12), U, 2007
UL 2043 Fire Test for Heat and Visible Smoke Release for Discrete Products and Their Accessories Installed in Air-Handling Spaces, Standard 2043, Edition 4, 2013
UL 60730 Standard for Automatic Electrical Controls for Household and Similar Use, 2010
 “Primary heat responsive device” in UL 555 terminology.
 “Normal” is defined as the de-energized or low variable condition. For example, low smoke is the normal condition.
Corridors are typically a means of egress during fires or emergency events. During normal operation they require ventilation. In some code jurisdictions or building design requirements, pressurization or smoke extraction is also required.
This article presents several means to provide pressurization or smoke extraction with ventilation and details the operation of the controls, dampers, and actuators.
The operation of corridor smoke exhaust influences and is influenced by the other smoke control tactics in a fire and the fire protection and mechanical engineers model the airflows with respect to one another. Figure 1 shows a larger picture than just the corridors.
Fiqure 1: Some of the strategic elements in fire and smoke control.
The model codes used in the United States and some other countries – International Building Code ((IBC 2012) and the International Fire Code (IFC 2012) along with the International Mechanical Code (IMC 2012) have various requirements for corridors in commercial buildings. The walls must be constructed as smoke partitions and in some cases as fire partitions. Minimum widths are established. Mechanical ventilation, travel distances, and other requirements are also covered. Chapter 4 of the IBC establishes requirements based on occupancy type. Chapter 7 details the requirements with respect to structure. Chapter 9 details the active or engineered system
All smoke control equipment status must be indicated on the fire-fighter’s smoke control system (FSCS) panel. This includes smoke control dampers (IFC 909.16.1). Control of all smoke control equipment must be possible from the FSCS (IFC 909.16.2) with the exception of complex systems where other provisions are allowed.
In corridors there are jurisdictions and individual projects where corridor damper and fan systems are required to clear the corridor of smoke and prevent spread to adjacent floors. Since ventilation is also required, the two functions must be coordinated. This can be achieved with dedicated or common (non-dedicated) equipment.
Either a “sandwich” or “building” pressurization type of approach is usually used. See Figure 2.
In a sandwich pressurization system –
a) The corridors on the fire floor are negative with the fan pulling smoke out of the floor. (Supply closed, return or exhaust open fully.)
b) The floors above and below the fire floor are pressurized more than other floors. (Supply fully open, exhaust off or closed.)
c) The corridors on other floors of the building operate normally. (Typically partially open supplies and exhausts.)
In a building pressurization system approach –
a) The corridors on the fire floor are negative with the fan pulling smoke out of the floor.
b) All other floors operate normally. They are under a positive pressure with ventilation air. Since the fire floor is very negative, the difference in pressure is large enough to prevent smoke spread to the non-fire floors.
Figure 2: “Building” vs. “sandwich” pressurization system.
Figure 2, shows the overall concept of a non-dedicated system – the ducts move ventilation air under normal circumstances and are used for smoke control only in an emergency. However, variations are common in corridor systems as there are a number of ways to achieve the goals. Among the possible methods are:
a) Two rooftop fans (supply and exhaust) and separate ducts to the corridors.
b) One reversible fan that delivers ventilation air in normal operation and exhausts air during an event. In this case there are other provisions for make-up air, reliefs or local exhausts. Various factors influence the approach that is best. All pressures – positive or negative – due to stack effect; lobbies, elevators, or natural ventilation, or attached rooms and spaces are considered.
c) If there is sufficient make-up air elsewhere, an exhaust fan alone may be used to move air out of the corridor. No supply fan.
See Figures 3 and 4 for drawings of the two approaches. Figure 5 shows a vertical representation of a high-rise corridor duct system in a multistory building. The point is that the same duct feeds or draws from all of the dampers. The system must be balanced in order to provide the correct amount of ventilation air on each floor. When a fire event occurs, the air flow requirements change.
Figure 3: Separate supply and exhaust ducts in a corridor.
Figure 4: Single duct serves as supply and smoke exhaust in a corridor.
Figure 5: Fan and ducts in 10 story building.
The pressure at the discharge of the fan is higher than at the bottom of the building. However, the required quantity of air going through each damper is the same during normal operation.
The damper at the top of the building will be open much less than that at the bottom of the building. Even with careful calculations to use different sizes of dampers, balancing will be needed to get the correct flow through each damper. In addition to pressure losses in the duct and across the dampers, local exhausts and some stack effect will cause variations that cannot be precisely calculated. The goal will be to have the furthest damper full open to use the least fan energy while delivering the right amount of ventilation air on every floor.
The fan sequence for each of the cases above is straightforward. With the Figure 3 two duct system both the ventilation supply fan and exhaust fan are on when occupied (or optimizing or under control of an air quality sensor). If a fire alarm activates or smoke is detected during unoccupied periods, the fans are turned on again. This is the same for both sandwich and pressurization systems. At the same time the stairwell pressurization system is activated and alarms are issued. This is beyond the scope of this article.
If using the Figure 4 geometry, then the fan is on and supplying air during normal occupied times. In event of a fire, the fan goes to the reverse air flow direction – exhaust mode.
Any individual damper in either type of system must perform several functions. These dampers can be parallel blade (PB) or opposed blade (OB). In most cases an OB will have more accurate resolution for setting minimum position and full open the flow is the same for either type.
The dampers must be UL 555S (UL 555S) listed as smoke dampers (IFC 909.10.4). In some cases the damper must also be a fire damper that meets UL 555 (UL 555) as well so combination fire and smoke dampers would be required. Most corridor walls must be fire rated. However, if the fire damper function could interfere with the smoke control system operation, then installation of a fire damper is not required (IBC 717.2.1).
The actuated damper operation will be similar for all cases. Here we will discuss the detailed operation for only the Figure 3 supply and return duct case in a sandwich pressurization system.
Normal operation. Both supply and exhaust dampers open to a minimum position. Balancing dampers in series with the smoke control dampers cannot be used. They would block some flow when the damper went to 100%.
In event of a fire:
a) Supply damper closes so that smoke is not pushed into other areas.
b) Exhaust damper opens 100% to remove smoke.
c) In some cases the dampers are also fire dampers and will close if temperature inside damper frame reaches 165°F. The FSCS panel has override switches to reopen the damper with a secondary sensor to again close damper if the temperature reaches 250°F. This is discussed below.
Floors immediately adjacent to fire floor
a) Supply damper opens 100% to pressurize and restrict smoke entry
b) Exhaust damper closes 100%
All other floors
a) Dampers remain in normal operation
b) Variations do exist
Belimo FSAF24-BAL Solution
Figure 6 shows a corridor damper with the FSAF24-BAL. Several manufacturers produce similar products. Not shown is the front grill. The damper is installed in the corridor wall and the actuator provides the sequence needed for ventilation and smoke control. The actuator is three position – closed, adjustable mid-position, or open 100%. Figure 7 shows operation.
Figure 6: Ruskin FSD60-FA-BAL
With no power the actuator springs closed – unoccupied, fire present at damper, or smoke control stop air flow.
With 24V on wire 2, the actuator opens to the balancing position as set by the potentiometer on the face – this is the normal operation ventilation air position. Each actuator is set at a different potentiometer position as balancer measures flow.
When wire 3 receives 24V the actuator opens 100%. – full pressurization or smoke exhaust mode.
These are the positions needed for the corridor ventilation and smoke control.
Figure 7: Detail of FSAF24-BAL.
Smoke control system program mapped to actuator function
The sequence detailed above under dampers can be mapped against the needed actuator operation and programmed into the smoke control system panel as shown below.
Normal operation: Wire 2 is powered, damper in ventilation position.
a) Supply damper: No power, damper closes.
b) Exhaust damper: Wire 3 powered to open damper 100%.
c) For fire dampers Wire 1, common, is always connected unless the primary sensor opens. See Figure 8 description.
Floor(s) immediately adjacent to fire floor
a) Supply damper: Wire 3 powered to open damper.
b) Exhaust damper: No power, damper closes.
All other floors
Dampers: Wire 2 is powered.
Figure 8, adds more detail to how the damper is controlled. (Note that there are other wiring variations not covered here. For example the secondary sensor could be between the Override relay and wire 3.) A smoke damper would have neither the 165°F high temperature primary sensor nor the 250°F secondary senor.
Figure 8: Control of FSAF24-BAL-S actuator.
In Figure 8 the smoke relay is normally closed and power is delivered to the actuator. The damper drives to the minimum position setting.
Combination fire and smoke dampers have two t”emperature sensors (“heat responsive devices” per UL555) – primary and secondary. If the temperature rises to 165°F (74°C) the primary opens and the damper springs closed. It does not go to the potentiometer position since it does not have power. If the Override relay is made by intervention from the FSCS panel then wire 3 is energized. This bypasses the primary sensor. The damper then opens to the 100% position instead of the ventilation position.
If the temperature at the damper again rises and reaches 250°F (121°C), then the secondary sensor opens and damper springs closed and stays closed until manually reset.
By powering wires 1 and 2 with 24V the actuator drives the damper to the required position for ventilation.
By cutting power the actuator springs the damper closed.
By powering wires 1and 3 the actuator drives the damper 100% open regardless whether wire 2 has power or not.
Figure 9, shows the Fire Fighters’ Smoke Control System panel with indication lights that are given status by the auxiliary switches on the actuator. Each fan and damper has its own status lights and override switch. The signals for light indication at the panel are carried via the network from actuator auxiliary switches, damper blade switches, magnetic contact switches, or programmable actuator signals.
Figure 9: Portion of Fire Fighters’ Smoke Control Panel.
Proportional actuator with Minimum position control
Another way to achieve the same sequence is provided by use of a proportional 2-10V actuator and an SGA24 minimum position switch. This is a different actuator than the BAL shown above. It is a standard 2-10VDC control actuator.
The wiring schematic is shown in Figure 10. Figure 11 shows the minimum selector which can be used to set the mid-point balancing position of the actuator.
Figure 10: Proportional actuator controlled by minimum potentiometer.
Figure 11: Proportional minimum potentiometer
The sequence of operation of the wiring diagram in Figure 10 is as follows:
With no power on both Com and Hot, damper springs closed. This would be the typical unoccupied position. The “closed” auxiliary switch indicates the damper is closed and the network card transfers the position indication to the FSCS panel.
With power going to the SGA24 and actuator, the signal out of the SGA on 3 goes to the actuator input 3. (4 is not used in this sequence. It would allow modulating control of the damper in addition.) The 2-10 VDC signal positions the actuator and damper from zero to 90 degrees to be set by the balancing contractor.
If Override relay 1 makes 24V power is delivered to 3 of the actuator which causes it to drive full open. At the same time the 165°F sensor is bypassed. (The 250°F remains in the circuit as a final safety should fire be present too close to the wall.) This would open the damper fully if it is an exhaust on the fire floor or a supply on an adjacent floor.
If Override relay 2 makes, power is cut to 2 of the actuator and it springs closed. This would achieve needed closure of a supply on the fire floor or an exhaust on an adjacent floor in a sandwich pressurization system.
Thus the damper can be placed in closed, open, or partially open as needed for corridor smoke and ventilation control.
Figure 10 shows the primary and secondary sensors for a combination fire and smoke damper along with the override contact on Override relay 1. These are not present if the wall is not a fire barrier or partition requiring a damper. In that case the drawing in Figure 12 would accomplish the smoke damper functions for ventilation or open or closed as required. This is one example where the presence of sprinklers (that might derate the wall) works synergistically with the engineered smoke control system.
In Figure 12, when both relays are normal, the damper goes to its balancing position.
If the damper must open 100% for purge or pressurization, then Override relay 1 is made. Shorting hot to 3 of the actuator drives it full open.
If the damper must close then Override relay 2 makes. This cuts power and the actuator springs closed.
As in Figure 10, actuator auxiliary switches signal damper blade position to the FSCS panel.
Figure 12: Proportional control of a smoke damper by a minimum potentiometer.
Reversible Fan Ventilation and Smoke Removal
Where there is sufficient relief by local exhausts or return airs in adjacent areas, a reversible fan and only one duct run to all floors is possible. In some cases, a make-up air damper can provide for needed relief. A gravity relief damper is another possibility. This removes the need for a second duct and second damper on each floor. Figure 13 shows the concept.
Figure 13 Reversible fan for ventilation or smoke extraction as needed
A sandwich or building pressurization system approach can be used for corridor smoke control to facilitate egress during a fire or other event. The same duct(s) and damper(s) can be used for ventilation during normal occupancy periods.
There are a number of duct and damper choices available for corridor ventilation and smoke extraction. The Belimo FSAF24-BAL or the FSAFB24-SR with an SGA potentiometer can provide the different sequences of operation needed.
International Building Code, 2012, International Code Council, Inc. (ICC), Country Club Hills, IL 60478-5795
International Fire Code 2012, ICC, ibid.
International Mechanical Code 2012, ICC, op. cit
UL 555 Standard for Safety for Fire Dampers, Edition 7, 2006, Updated 2010, Underwriters Laboratories Inc. (UL), 333 Pfingsten Road, Northbrook, IL 60062-2096
UL 555S Standard for Safety for Smoke Dampers, 4th Edition, 1999, Updated 2012, ibid.
Written by: Larry Felker, Mechanical Engineer and member of ICC (International Code Council), NFPA (National Fire Protection Association), and a life member of ASHRAE (American Society of Heating, Refrigeration Air Conditioning Engineers). He is a Product Manager for Fire & Smoke Actuators for Belimo Americas who has specialized in fire and smoke dampers and actuators since 2002. Previously he was a temperature control system designer and before that a mechanical and electrical contractor. He is the co-author (with Travis Felker) of Dampers and Airflow Control, ASHRAE Special Publications, 2010.