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Smoke control systems using mechanical equipment, such as fans and dampers, rely on the integrity of this equipment to control the spread of smoke within a building. Fire/life safety and HVAC systems must be carefully integrated to ensure reliability in smoke control systems.
A smoke control system is a system that is used to limit the migration of smoke within a building due to a fire. There are several methods to limit this migration, and some are designed to provide a tenable environment for occupants to egress the building. A smoke control system can include physical barriers that limit smoke from migrating outside the zone, a combination of physical barriers and mechanical systems, or only mechanical systems to control the spread of smoke
Many of the model building and fire codes and recognized fire protection standards outline the requirements for the design and installation of smoke control systems. They provide guidance on the performance criteria for the various systems as well as requirements for equipment related to these. Typical performance requirements for smoke control systems using mechanical systems include a pressure difference between the fire zone and adjacent zones, or exhausting the fire zone so that the smoke layer is maintained a certain distance above the highest occupied floor to allow occupants to evacuate the fire zone.
When mechanical systems are employed, a fire event will cause the equipment to be configured to their smoke control mode. A control system, such as the fire alarm system, receives signals from sensors in the field and provides outputs to equipment in the building to start, stop, open, or close. This includes fans, dampers, doors, shutters, and other equipment related to the system. The equipment is monitored for desired positions and their position is displayed on the panel, either graphically or through other types of annunciation equipment.
Dedicated, nondedicated systems
NFPA 92: Standard for Smoke Control Systems defines two types of smoke control systems: they are either dedicated or nondedicated systems. Dedicated systems use equipment that is installed for the sole purpose of providing smoke control. Nondedicated systems share components with some other systems, such as the building HVAC system. A nondedicated system changes the normal operation of the equipment to smoke control mode when a fire is detected. Dedicated systems typically are found where no other fans or dampers are used in the normal operation of the building, such as pressurization of stairwells or elevator hoistways. Nondedicated systems are typically found where other equipment normally is installed, such as a HVAC system for climate control.
It makes sense to use HVAC systems for smoke control purposes for a variety of reasons. The foremost of these is the reduced cost. If there are fans, dampers, and ductwork already in place, why install another system to do smoke control when the HVAC system may be more than adequate to fulfill this function? However, HVAC equipment may need certain enhancements to fulfill the duty of smoke control. This can include the number of belts, the service factor of the motor, and the temperature rating of the fan, among other things. The fans also will need to be served by standby power systems in order to allow operation when normal power is lost. The designer and installer of the HVAC system needs to understand what is required if HVAC equipment is used for smoke control. The following sections highlight some of the things a designer and installer need to consider.
HVAC system fans can be adapted to be used for smoke control purposes. There are several things to consider when using HVAC fans for smoke control:
The International Building Code (IBC) requires fans used for smoke control purposes to have 1.5 times the number of belts required for design duty, and no fewer than two. The manufacturer for the fan can be consulted to confirm the number of belts being used for design duty, but typically smoke control fans will be provided with a minimum of two belts. This provides redundancy in the drive should one belt break or come off during operation. Direct drive fans do not have the same requirement for drive redundancy because they are not susceptible to broken belts.
The IBC also requires fan motors to operate within their nameplate ratings. This requires fans to operate at or below their rated horsepower and to be selected with a minimum service factor of 1.15. The service factor increase allows the motor to run in a nominal overload condition, thereby mitigating damage to the motor. However, the fan is required to operate at nameplate capacities. The increased service factor is intended to improve the reliability of the motor because it is expected to operate in fire conditions.
Fans used for smoke control are required to be designed to run in a stable portion of the fan curve. All fans have performance curves based on the airflow being provided and the static pressure present. If the fan is running outside of its stable region, then the performance of the fan is not easily predicted. Generally this occurs at lower airflow rates or when the static pressure is higher. When either or both of these conditions occur, there is an increased chance that the fan is running outside of its stable region. This can become an issue when HVAC fans are sized to deliver more airflow in normal conditions and significantly lower airflow rates in smoke control.
Smoke control fans are required to operate at elevated temperatures. This is especially true for exhaust fans because they exhaust air that has been heated due to the fire conditions in the space. Therefore, the code will require the fan to be rated for the probable temperature rise that can be expected. There are several ways to calculate the required temperature rating for the fan. The most common approach is to use the equations in the IBC or NFPA 92. These are based on the heat release rate of the fire and the exhaust rate. In certain conditions, this equation can produce high temperature rates that might not likely occur, especially in buildings with automatic sprinkler protection. In buildings with automatic sprinkler protection and relatively low ceilings, temperatures are not expected to increase beyond 200 F because activation of the sprinklers will cool the surrounding areas. In these conditions, using calculation methods that include diluted air would be advantageous. The IBC recognizes this and allows this approach as an exception.
As noted above, one of the common issues associated with HVAC fans in smoke control systems is managing the airflow requirements for both uses. The required airflow for climate control may be more or less than that required for smoke control systems. Two-speed or variable-speed fans can be used to address this; however, the airflow rates must be within the overall range of the fan at stable performance. Understanding the performance criteria of the smoke control system is critical when sizing the fans. For systems using a pressure difference, the airflow rate may need to be adjusted either up or down based on the construction of the building.
The tighter the construction, the less air is needed to maintain the pressure differences. The looser the construction, the more air is required. Smoke zone barrier maximum allowable leakage ratios, based on the category of the barrier (i.e., walls, exit enclosure, shaft, floor, or roof) are provided within the applicable design standards and/or codes enforced by the authority having jurisdiction (AHJ). These ratios are to be used when determining the calculated total leakage area of a given smoke zone, which is also required to include any other gaps or openings, such as gaps around closed doors, elevator doors, windows, or air transfer grills. The final actual total leakage area, once constructed, is generally determined by workmanship, with the compliance of the systems being determined through achieving the required smoke control system performance criteria. This is often not determined until acceptance testing, long after the fan has been selected and installed. Since this may impact the size of the fan and motor, it is in the designer’s interest to assume loose construction without oversizing beyond stable performance.
One of the methods for smoke control, commonly known as the pressurization method or zoned smoke control, is to set up a negative pressure in the zone of origin and exhaust the space providing no make-up air. All of the fans go to full exhaust and supply air is shut down. This creates a negative pressure within the zone relative to adjacent zones and is intended to maintain the smoke inside the zone. The minimum and maximum pressure differentials across a smoke zone barrier are dictated by the applicable design standard and/or building code enforced by the AHJ. Determining the minimum pressure differential is based on whether the associated smoke zone or building is sprinklered or not. If not sprinklered, sufficient exhaust quantities must be provided to ensure the zone will not be overcome by the buoyancy forces of hot gases resulting from a fire.
The maximum pressure differential is determined by maintaining the required door opening or closing forces below code allowed maximums, for doors that are located within the smoke zone barriers. Zones that have a large amount of general utility exhaust fans need to be evaluated to determine if these utility fans need to be turned off during the smoke control sequence or can continue to run without impact to the performance of the smoke control system. If allowed to run, they should be turned off during testing to simulate normal power shutdown of the fans and confirm the pressure difference is met without them. If the zone can reach minimum pressure differences without the fans, and there are no adverse impacts as a result of utility exhaust fans operating and increasing pressure differentials, they do not need to be tied into the smoke control sequencing. If this is not the case, then the fans need to be configured so that minimum pressure differentials can be met in either normal or standby power modes without over-pressurizing the zone. If the zone has too much of a pressure difference, doors will require too much force to open. This would also apply to kitchen areas where kitchen exhaust fans may not be tied into the smoke control sequencing. When dealing with kitchen exhaust fans, care must be taken that the proposed smoke control sequencing does not impact the operational capabilities of any suppression systems related to these kitchen exhaust fans.
Some of the code requirements for high-rise buildings today require smoke removal systems. While not smoke control systems, they anticipate the use of the HVAC system for manual smoke removal after a fire incident. These are not automatic systems. If smoke removal requirements cannot be met through the use of natural ventilation techniques, which is also allowed by code, this will require mechanical systems that generally require four air changes/hour (ACH) as part of the performance criteria of the system. If the HVAC system does not incorporate exhaust/relief fans, is not sized for four air changes, or provides a means where return/exhaust air contaminated with fire combustion by-products cannot be moved directly to the exterior without recirculation to other areas of the building, then modifications may be required to accomplish the smoke removal requirements. Understanding the criteria is essential in designing the HVAC system if dual use is anticipated.
Variable frequency drives
The variable frequency drive (VFD) is increasingly becoming a common component of modern smoke control systems. Whether through nondedicated systems where VFDs may be implemented as a means to reduce energy use in HVAC systems or through dedicated systems where VFDs may be used for system balancing, VFDs are required to be listed and conform to recognized standards. For example, all IBC Section 909 requirements that apply for fans and smoke control equipment also apply to VFDs, though they are not specifically mentioned. It is the designer’s responsibility to ensure that VFDs and other sensitive electronic equipment are located in a protected environment and installed so that the expected ambient temperatures do not exceed the limitations of the device. This may be in a temperature-controlled portion of the building, such as a mechanical equipment room or an exterior enclosure, as long as the enclosure can be maintained within the temperature limitations of the equipment.
As previously mentioned, the IBC requires that fans used for smoke control operate under stable conditions and not exceed their nameplate horsepower rating. Likewise, VFDs are not permitted to be used if their setting could destabilize fan performance or exceed the motor horsepower rating. In addition, the minimum service factor required by the IBC of 1.15 applies. The IBC also requires elements of a smoke management system that rely on volatile memory be supplied with an uninterruptable power supply (UPS) of sufficient duration to span a 15-minute primary power interruption. The designer must take into account potential power surges or interruptions and determine whether a VFD contains nonvolatile memory or if a UPS will be used. This should be clearly identified on the construction documents for each VFD specified for smoke control service.
VFDs are typically designed with a touchscreen or keypad that can accommodate VFD programming and controls. When the smoke control system is activated either automatically or manually at the firefighter’s control panel, this keypad needs to be overridden and all control/programming functions disabled. In addition, the keypad may have an “off” feature. Although not required by code, it is recommended that the keypad be removed and stored in an approved location. This will ensure that the “off” feature is not misunderstood as a safety override in an alarm mode. VFD manufacturers may require the keypad to be replaced with a blank cover plate in order to maintain the listing of the VFD and protect any connections. It is the designer’s responsibility to determine if protection of the connections is necessary and address the method of protection.
The IBC requires that fire detection systems be equipped with a control unit complying with UL 864. When the fire alarm control panel signals an alarm to the building management system (BMS), a listing in accordance with UL 864/UUKL is required for the BMS also. UUKL is a listing category under UL 864 for smoke control system equipment. Equipment that receives the UUKL listing has been tested for integrity and long-term reliability.
The UL 864 listing requirement does not necessarily apply to VFDs used for smoke control. VFDs are not required to be UL 864-UUKL listed provided that they are applied as an end device, as they are effectively no different than a motor starter. This implies that the failure of one VFD will only affect the associated fan. However, if the VFD is connected as a controller as part of a control system where its failure will affect smoke control equipment or other operations other than the end device, the VFD may need to be UUKL listed. The designer will need to confirm the application and system architecture when using VFDs as part of a smoke control system.
Dampers for smoke control systems are really no different than dampers installed to protect rated assemblies. They are required to be listed and conform to recognized standards. For systems using the IBC as the code, fire dampers are required to meet UL 555. Smoke dampers are required to meet UL 555S. This is consistent with those requirements found in NFPA 92.
The IBC further regulates the type of smoke damper to be used. They are required to have leakage ratings no less than Class II and elevated temperature ratings of not less than 250 F. This limits the allowable leakage from the damper at the higher temperature expected in the system.
If combination fire/smoke dampers are used, the smoke damper requirements stay the same, but the temperature rating for the fire damper actuating device is required to be increased to account for elevated temperatures in the system. This is intended to prevent the fire damper from closing when needed for smoke control normal operation. Often fire dampers are selected at lower release temperature for HVAC-only systems. These are typically below the anticipated temperatures in smoke mode. The IBC requires the rating to be approximately 50 F above the normal operating temperature in the duct (in this case under smoke control mode). The operating temperature cannot exceed 350 F.
One of the things that can be confusing when applying combination fire/smoke dampers in smoke control systems is the elevated temperature requirements for the smoke damper and the elevated temperature requirements for the fire damper. The elevated temperature rating for the smoke damper is 250 F, whereas the fire damper is a maximum 350 F. These are not always the same operating parameters. The elevated temperature of the smoke damper is the temperature at which the damper will continue to maintain minimum leakage per its listing as well as when the damper actuator will still continue to operate. The 350 F fire damper setting is associated with the actuating device. For most combination fire/smoke dampers, the linkage and actuator provide the same function so the two should be coordinated. If the normal operation plus 50 F is less than 250 F, then the minimum elevated temperature requirements for the smoke damper apply; if it is greater than 250 F, then a higher rating is required.
Smoke dampers found in smoke control systems are generally located at smoke barriers or fire-resistance rated walls. However, as defined they include any damper in the air distribution system, whether it is located in the field or at the unit. HVAC units often use control dampers to mix outside air, return air, and exhaust air. These dampers need to be configured when entering a smoke control mode to allow the system to perform under its expected operation. These dampers also need to be able to prevent the migration of smoke and, per NFPA 92 and IBC, be listed.
There is no real difference between ductwork in the distribution system used for HVAC and smoke control systems. The only changes are that the ductwork is required to be supported by fire-resistance rated structural elements using noncombustible supports. There is no specific requirement as to the type of material to be used, other than what is required for HVAC systems. Ducts can be made from sheet metal or can be made of drywall.
Duct materials and joints are required by the IBC to be capable of withstanding the probable temperatures and pressures to which they will be exposed. They are required to be leak tested to 1.5 times the maximum design pressure. The designer needs to document the design pressure of the duct, which is often different than the fan static pressure. The measured leakage cannot exceed 5% of the design flow. While not specifically outlined in the code, leak testing is typically done only for exhaust systems that traverse other smoke zones to confirm they will not leak significant amounts of smoke when exhausting the fire zone. Ducts that are contained only within the zone or supply air systems do not have the potential to convey smoke outside of the initial zone. Therefore, it is typical to see the designer identify those ducts requiring leak testing and not all ducts. It would be reasonable to expect vertical ducts traversing multiple floors to be leak tested, but not the portion of the duct that is isolated to the zone.
Another issue that has been encountered in the use of HVAC systems for smoke control is the use of ducts made of drywall materials. Leakage rates for HVAC systems may not be impacted for climate control the same as they are for smoke control. Experience has shown that when using drywall ducts for smoke control systems, it is difficult at best to meet the leakage testing requirements. Often metal ductwork has had to be installed to replace the drywall ducts to meet the leak testing. Therefore, metal ductwork should be used wherever possible to prevent impact by leak testing during commissioning of the system.
Additional considerations should be taken when determining the location of exhaust/return fan discharge outlets and their associated fans. Fans used for exhaust in a smoke control or smoke removal system should not discharge at locations where the potential exists for reintroducing smoke into the building or any adjacent buildings.
Smoke control mode, as defined by NFPA 92, is a predefined operational configuration of a system or device for the purpose of smoke control. This is accomplished by a set of predefined sequences programmed into the smoke control system. The smoke control system can be either a subset of the building’s fire alarm system, or a combination of the building’s automation and fire alarm system. When HVAC systems are being used, some form of building automation system (BAS) is commonly part of the overall smoke control system.
A smoke control system requires input from the field to determine when to configure the system. This is done primarily through the fire alarm system’s smoke detectors or monitoring of the building’s automatic sprinkler system. When the fire alarm system detects one of these conditions, a signal is processed and the smoke control system is configured for its predetermined sequence for that zone in the building. Depending upon the control system concept employed, either the fire alarm system drives all sequences overriding the BAS or a handoff of signals occurs allowing the BAS to control all or a portion of the system. In a typical high-rise building, it is usually a combination of the BAS and fire alarm system controlling equipment.
It is common to see both BAS and fire alarm systems control the overall smoke control system. While it is also common to see just the fire alarm system, this can duplicate controls and wiring and drive costs. If the BAS is suited for smoke control functions (in the United States, UUKL listed by UL for smoke control purposes), then it is acceptable to have both systems control fans and dampers appropriately. In high-rise office buildings where HVAC systems are used for both climate and smoke control, the fire alarm system can hand off a signal to the HVAC/BAS to configure dampers and fans already being controlled for climate control purposes. Other fans, such as stairwell pressurization fans, do not have HVAC controls and can be controlled directly by the fire alarm system.
The designers of the smoke control and HVAC systems must collaborate to determine the most cost-effective and reliable source for controlling the system. If the designers are one and the same, consideration should be given to how to achieve the proper control functions using reliable equipment already in place. This not only applies to the field circuits and controls but also to graphic interface panels. It is not practical to have two separate graphic control panels for one overall smoke control system. When using two control systems, the user interface needs to be considered to allow for efficient control and overrides of the system in an emergency condition.
Overall, it is common to see HVAC systems employed for smoke control purposes. It can save money and can provide for reliable operation. When considering the use of dual-purpose equipment, such as HVAC systems, take into account the impact the smoke control operation and code requirements have on HVAC functions and whether the HVAC system can function as intended for emergency conditions.
Allyn J. Vaughn is CTO/president at jba consulting engineers. He has been in Las Vegas for more than 15 years providing fire protection system design and code consulting services, including design and commissioning of smoke control systems. Stephen Haines is project engineer with jba consulting engineers. He has more than 5 years of experience designing HVAC systems for smoke control in various high-rise buildings in Nevada, Arizona, California, and Macau, China.
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