The premise of smoke control is to protect occupants, allowing them to safely evacuate or relocate by maintaining a tenable environment for a reasonable time period. Smoke control is not required to maintain a tenable environment in the compartment of fire origin nor is it intended to protect the contents of the building.
Smoke control systems can be active or passive. They can consist of simple components, such as open window to ventilate (OWTV) with air system shutdown and isolation to engineered smoke control systems employing integration of the fire alarm system and BAS that properly sequence the smoke control components, along with identification of those components required to be on emergency power.
BASICSDefinitions of smoke control features and components are generally consistent throughout the language of the various codes. This is important in the system design and in discussion with the AHJ as it helps eliminate possible ambiguity.
Minimum smoke control system features include fans for introduction of outside air for purging the fire compartment, fans for smoke exhaust/removal, smoke dampers for isolation of equipment and smoke compartments, devices for detection of combustive products, HVAC control system to direct operation of the equipment, and a fire alarm system to both receive the initial signal and signal the BAS. All system components - fan motors, damper power supplies, detection devices, and control panels - must be connected to the essential power system to provide standby power in the event of normal electrical power failure.
An example of a basic system might include an air-handling system serving a single-story building. Smoke control in this building would consist of shutting down the supply fan upon detection of combustive products by a smoke detector in the return and/or supply duct. This would restrict the smoke migration through the duct system from the area of fire origin to the other building areas, allowing for a safe means of egress for the occupants. NFPA 90A requires isolation of the air handler with smoke damper if it is over 15,000 cfm, unless it is located within the smoke compartment it serves. This isolation further restricts smoke migration through the duct system.
Smoke control system complexity can increase if the single story building is an ambulatory health care (23-hr stay) occupancy with a minimum of two smoke compartments. It further increases if the occupancy is changed to I-2 (hospital) and contains overnight patient sleeping areas and anesthetizing locations, such as operating rooms (ORs). The air system would then require smoke dampers, additional detection devices, a logic-based HVAC control system, and integration with the fire alarm system.
USE AND OCCUPANCIESThe 2003 International Building Code (IBC) has detailed requirements based on use and occupancy. High-rise buildings (Section 403) require an automatic fire sprinkler system, a fire command center, standby power (emergency power systems), and smoke-proof exit enclosures (or pressurized stairways). Section 404 on atriums requires an automatic fire sprinkler system, smoke control system, and standby power. Underground buildings (Section 405) require an automatic fire sprinkler system, fire detection system, compartmentation, smoke control system, and standby power. Occupancy Group I-2 (hospitals) requires an automatic fire sprinkler system, smoke barriers and compartmentation, and automatic fire detection.
There seems to be a pattern here. Each of these conditions is referenced to specific code sections regarding compliance with required construction features. A health care facility can fall under the requirements for each of these occupancy types, as it can be a hospital with two or more levels below ground and contain an atrium as part of the public space and patient healing environment, but generally there is one overriding occupancy classification for the building.
There are further requirements for health care facilities regarding these occupancies and related requirements identified in NFPA 101 (Life Safety Code), Chapter 18 for New Health Care, and Chapter 19 for Existing Health Care. Fortunately, smoke control system basic requirements and those for smoke-proof enclosures are similar to those in the IBC.
NFPA 99 has further requirements for anesthetizing locations, which include administering agents for conscious sedation. Buildings with an I-2 primary occupancy classification can also have business occupancy on some levels as long as they are properly separated by two-hr construction. How complicated can this get if you have a high-rise hospital with a four-level atrium, two levels of office/clinic, bed floors, and a level of operating suites? It's very complicated if we let it be, but taken a piece at a time with proper planning, it's relatively painless.
ANESTHETIZING LOCATIONSAnesthetizing locations are defined by NFPA 99 as any area of a facility that has been designated for the administration of nonflammable inhalation anesthetic agents for relative analgesia. Relative analgesia is defined as a state of sedation and partial block of pain perception provided in a patient by the inhalation of concentrations of nitrous oxide insufficient to produce loss of consciousness (conscious sedation).
Chapter 6, Environmental Systems, of NFPA 99 in Subsection 6.4.1 requires supply and exhaust systems be arranged to automatically vent smoke and combustive products. Ventilating systems for anesthetizing locations are required to automatically prevent recirculation of smoke originating within the surgical suite and prevent the circulation of smoke entering the system intake, without, in either case, interfering with the exhaust function of the system. Once it is detected, smoke needs to be removed from the location using a fan and duct system that brings in air from the outside and pushes air through the space, while another fan removes or exhausts the air and smoke to the outside with the discharge without any re-entrainment of the smoke.
A typical OR air-handling system and components introduce air to the OR through ceiling diffusers in a laminar flow arrangement and remove air through a return duct system from low wall return registers. This return can occur though supply fan suction or a return relief fan (Figures 1 and 2). Many hospitals' air systems are set up to have economizer systems, allowing the use of outside air for cooling as an energy saving opportunity. Figure 1 shows typical components in such a system. By having these components in place, removal of smoke is relatively easy. The trick is to set up the control sequence to position dampers and fan speed to make this happen. Also, due to temperature and air filtration requirements, many air systems serving OR suites only serve the OR suite.
In large OR suites with six, 10, or more ORs, it is common for multiple air handlers to serve the area, as users want to continue to use the operating suite should one of the air units be out of service. If the climate is not suitable for an economizer, then an exhaust fan (Figure 2) is required to remove the air and prevent any recirculation. The motorized dampers in the return air and outside air ducts must be minimum Class II leakage rated dampers by code, again to ensure no recirculation and no circulation of smoke as noted in NFPA 99, Section 6.4.1.
So how does all of this operate and sequence? To comply with NFPA 99 requirements, the control sequence to ventilate the products of combustion would be as follows:
- Upon detection of combustive products by a smoke detector within the surgery suite, the relief damper and maximum outside air damper opens, the return damper closes, and the supply fan and return/relief fan continue to operate at design speed. The system continues to operate in this manner until the fire alarm system is reset.
- If combustive products are detected in the outside air or the supply air and the system is already in a smoke evacuation mode, the supply fan shuts down, outside air dampers close, and the return fan continues to operate in the relief mode.
- If combustive products are detected in the outside air and the system is in a normal operating mode, the minimum and maximum outside air dampers close and air continues to recirculate with the supply and return fans. If smoke is detected in the suite or by the supply air smoke detector, the supply fan shuts down, return damper closes, and relief fan continues to operate.
- If combustive products are detected by the return air smoke detector while in a normal operating mode, the system would be placed in a smoke evacuation mode as noted above.
The key to effective smoke control is to have the system planned to function regardless of which detector is the first to detect smoke. A control system priority must be created depending upon which detector initiates first, meaning the system cannot reset once it is in smoke evacuation mode without risking introducing smoke into or not relieving it from the space of origin. BAS may be overridden, but supply air smoke detectors must always remain functional to ensure smoke is not introduced to a space.
Some jurisdictions require smoke removal from the ceiling in anesthetizing locations. This requires leakage-rated dampers to be mounted in ductwork with control sequencing that will isolate the low returns and open the ceiling exhaust. Placement of these additional ceiling air devices will be challenging as many OR ceilings contain three or four ceiling-mounted booms, one or two light hubs, and additional lighting.
Twenty-five to 40 ach of air are provided through 16 to 18 2-ft by 4-ft laminar airflow diffusers. While warm smoke will rise to the ceiling, the placement of the exhaust may also provide for short-circuiting of the supply air pattern, which may cause more turbulence than removal. Placement and air patterns should be planned carefully.
SMOKE COMPARTMENTATION / PRESSURIZATIONNFPA 90A defines smoke control as a system that utilizes fans to produce pressure differences to manage smoke movement. The goal in health care settings is to put the compartment of fire origin under negative pressure with the adjacent or surrounding compartments under positive pressure. The recommended minimum pressure difference across the smoke barrier including the common compartment openings, such as corridor egress doors, is 0.05 in. w.g. in fully sprinklered buildings. The IBC also states that the primary means of controlling smoke is through pressure differences across smoke barriers. In health care occupancies where occupant relocation occurs more often than evacuation, this function is extremely critical. Also critical to the maintaining of pressure differentials is tightness of building construction.
In a floor with two smoke compartments, each served by a separate air-handling system, the control and function of the supply and relief fans will be relatively easy to sequence. With a fire in compartment A the following sequence for equipment serving the two smoke compartments might look like this:
- Compartment A
- Supply fan goes to 60% design airflow, using 100% outside air
- Return air damper - closed, relief damper open
- Return/relief fan to 100% design speed
- Compartment B
- Supply fan goes to 100% design speed with all terminal units at design airflow
- Return fan goes to 30% of design speed in a recirculation mode
VFDs on each fan motor allow these functions to occur. The operation sequence for each system must be clearly written to indicate performance and desired results. It must also identify which smoke damper in each of the supply and return duct system must remain open or closed during which sequence. The less dampers there are, the easier this task is.
In another example, a multilevel hospital with differing patient care functions on each floor, and with limited space assigned for mechanical equipment placement, the HVAC design indicates that a large air unit will be used to serve two floors, in lieu of one unit per floor or compartment.
Additionally, the floor function and egress requirements call for three smoke compartments of irregular shape that must be continuous from exterior wall to exterior wall. Duct crossing of a smoke compartment (smoke barrier) wall is unavoidable (Figure 3).
The Life Safety Code allows smoke dampers to be omitted from duct penetrations in fully ducted HVAC systems; however, in this example, the building code does not. So, fire smoke dampers are placed in the ducts as they cross the smoke partition. There are two AHUs in a penthouse with ductwork serving approximately one half of each of floors 1 and 2. Return relief fans are designed for economizer use and smoke purge.
Let's say a fire starts in Compartment 2 of Level 1 and the space smoke detector initiates a fire alarm signal. What needs to occur to protect occupants of adjacent compartments? One of the first occurrences is occupant notification through the fire alarm system with audible and visual indicators. As required by hospital policy and other codes, the staff must be fully trained in emergency procedures to protect the occupants. This procedure may involve relocation of patients to adjacent compartments. The fire alarm signal will also be sent to the BAS, indicating the HVAC system should be placed in a smoke evacuation mode.
Chapter 9 of the 2003 LSC requires that where smoke control systems are required, the smoke control system shall be installed, inspected, tested, and maintained in accordance with nationally recognized standards, engineering guides, or recommended practices. It also states the engineer of record shall clearly identify the system intent, design method used, appropriateness of the method used, and required means of inspecting, testing, and maintaining the system. Acceptance testing is conducted by a special inspector in accordance with Section 9.8.
Clearly, maintaining pressure differentials in the example will be difficult. The goal is to create negative pressure in the compartment of fire origin and positive pressure in the adjacent compartments. This requires a detailed operation sequence with appropriate components identified. Detailed duct system planning and design is also required to isolate and close off branch ducts to allow for proper pressurization.
One way to do this is to design separate supply and return duct risers for each smoke compartment. This would eliminate the need to cross smoke barrier walls with horizontal duct runs, making the sequencing of dampers on a compartment-by-compartment basis easier to identify and control (Figure 4). Another way would be to place modulating control dampers in the branch ducts in addition to the smoke damper. Modulating of a smoke damper actuator is not recommended.
With a fire in Compartment 2 of Figure 3, with ducts crossing the smoke partition, a sequence of operation might include the following:
- Fire in Compartment 2, Level 1
- The supply air terminal units in Compartments 1 and 3 go to 100% design flow
- The supply air terminal units in Compartment 2 go to 50% design flow
- The return air control damper serving Compartments 1 and 3 go to 50% design flow position
- The fire smoke dampers in the ducts penetrating the smoke barrier between Compartment 1 and 2 and 2 and 3 remain open
- The supply fans in the air units go to 100% design flow using 100% outside air
- The return air control dampers serving Compartment 2 remain fully open
- If smoke was detected by the supply air duct detector or the outside air duct detector while in a smoke evacuation mode, the supply fan would shut down, the fire smoke damper in the supply duct would close, and the relief fan would continue to operate.
Now, if the system is ducted as shown in Figure 4, the control sequence would be similar, but there would not be any dampers at the smoke partitions to control. Pressurization control would need to be performed by terminal unit damper position and by control dampers in the return ducts. It also may be achieved by closing some fire smoke dampers in the branch duct, provided air movement from the smoke compartment is not impeded.
Care must be taken to avoid closing fire smoke dampers that may have a detrimental impact to the duct system, such as collapsing or exploding the duct. The designer is cautioned to consider the duct construction pressure rating required for return and exhaust ducts.
In smoke control systems involving AHUs serving multiple floors and multiple smoke compartments, it takes creativity in the duct design to minimize the number of fire smoke dampers as well as have proper pressurization control. Fundamental understanding and concepts must be developed early and discussed with the AHJ to avoid potential startup and commissioning problems.
EDITOR'S NOTE: Part 2 of this article will address stair pressurization and atriums with component, fire alarm, and BAS integration. The question of which system controls, and which system signals will be discussed as well. Stay tuned.
Sidebar: A Case StudyA 14-level (2 below and 12 above grade), 675,000-sq-ft hospital with four-level atrium located in congested medical center. Three smoke compartments per floor required smoke compartment evacuation/pressurization and stair tower pressurization. The building is classified as a high-rise building under Uniform Building Code with special local amendments.
- HVAC systems designed with one air handler per smoke compartment with return/relief fans with VSDs on each fan.
- Ductwork designed to not require cross-rated smoke partitions between compartments.
- Atrium designed for air mechanically introduced at a first-floor level using 60,000 cfm of outside air and removed at roof level through 4 to15,000 cfm smoke removal fans.
- Stair tower pressurization, using two fans for each stairwell with alternate floor air injection (average of 15,000 to 18,000 cfm each). Inlet vanes controlled airflow based on stair pressure sensor feedback. Air quantity designed to maintain 300 fpm velocity across open doorways with four to five doors open including the fire floor, the floor above and below, and the door(s) at the point of exit discharge.
- The exit discharge of one stair contained additional exiting from the ground and first floors as well as those above grade. The exit requirements increased the required exit width from two to four doors. Discussions with the building officials during design confirmed that the building would not be evacuated because it was a hospital and that a fire on an upper level in a sprinklered building would not be cause for the lower levels to be evacuated. This allowed the stair pressurization system to be designed for just the airflow needed with five open doors in lieu of seven.
- Systems commissioned and tested with the design engineer, mechanical contractor, controls and fire alarm vendors, electrical contractors, TAB contractor, and local and state AHJ. Testing procedures per contract documents, local code, NFPA 90A, NFPA 99, and NFPA 92A were followed.
- Planning essentials: Plan early and well with testing programs set up with those needing to witness tests. Discuss during design with AHJ and contractors during construction. Observe critical equipment installations and duct sealing. Remember who your systems will protect.
Sidebar: Definitions Associated With Smoke Control In Health Care Settings(See NFPA and Building Codes for exact definitions as used within the Code of Standard.)
- Anesthetizing location: Any area of a facility designated for the administration of nonflammable inhalation anesthetic agents in the course of examination or treatment, including the use of such agents for relative analgesia.
- Atrium: A large volume space consisting of a floor opening or a series of floor openings connecting two or more stories covered at the top.
- Health care facilities: Buildings or portions of buildings in which medical, dental, psychiatric, nursing, obstetrical, or surgical care are provided. Health care facilities include, but are not limited to hospitals, nursing homes, limited care facilities, clinics, medical and dental offices, and ambulatory care centers, whether permanent or movable.
- High-rise building: A building greater than 75 ft from the lowest level of fire department vehicle access to the highest occupiable floor level. (Some jurisdictions have lowered this to 55 ft.)
- Horizontal exit: A way of passage through or around a fire barrier to an area of refuge on approximately the same level in the same building that provides safety from fire and smoke.
- Means of egress: A continuous and unobstructed way of travel from any point in a building to a public way consisting of the exit access, the exit, and the exit discharge.
- Smoke: Airborne solid and liquid particulates and gases produced when a material undergoes combustion including entrained air.
- Smoke barrier: A continuous vertical or horizontal membrane that is designed and constructed to restrict the passage of smoke. It may carry a fire rating or not.
- Smoke compartment: A space within a building enclosed by smoke barriers on all sides including the top and bottom.
- Smoke-control system: An engineered system that uses mechanical fans to produce airflows and pressure differences across smoke barriers to limit and direct smoke movement.
- Smoke-exhaust system: A mechanical or gravity system intended to move smoke from the smoke zone to the exterior of the building, including fans, duct systems, controls, and related functions.
- Smoke partition: Continuous membrane that is designed and constructed to form a barrier to limit the passage of smoke.
- Smoke-proof enclosure: A stair enclosure designed and constructed to limit the movement of smoke produced by a fire.
- Smoke zone: The smoke-control zone is where the fire is located.