Integration: HVAC fans and smoke control

Fire, life safety, and HVAC systems must be carefully integrated to achieve reliability in smoke control systems.

05/20/2013


Learning objectives

  1. Learn the requirements of fire, life safety, and HVAC equipment in relation to smoke control.
  2. Know how to integrate HVAC equipment into mechanical smoke control systems.  

Figure 1: This diagram shows a typical HVAC and dedicated smoke control system design for a high-rise building. Courtesy: jba consulting engineersSmoke 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 fans 

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:

  1. Make sure the motor and number of belts complies with minimum code requirements.
  2. The temperature rating of the fan needs to be adequate for smoke control use.
  3. Determine whether the fan has adequate capacity to deliver the performance criteria of the smoke control system, while operating at stable performance.  

Figure 2: These fan motor belts have the minimum number of belts, two. Courtesy: Trane Co.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.


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