Where There's Fire There's Smoke
Patients in hospitals, outpatient facilities and nursing homes require special protection from the elements of a fire, of which smoke is one of the most dangerous. In fact, smoke control is especially critical in these facilities. The 2000 Life Safety Code (LSC)-National Fire Protection Association...
J. Patrick Banse, P.E., Smith Seckman Reid, Inc., Houston
Patients in hospitals, outpatient facilities and nursing homes require special protection from the elements of a fire, of which smoke is one of the most dangerous. In fact, smoke control is especially critical in these facilities.
The 2000 Life Safety Code (LSC)-National Fire Protection Association (NFPA) 101-states in chapter 9 that the purpose of smoke-control systems is to confine smoke to the general area of the fire origin and to maintain the use of means of egress. It further states that smoke-control systems shall have approved maintenance and testing programs to ensure operational integrity.
The LSC and other model building codes address construction type, corridor width and related egress requirements. One must identify design elements that are necessary for smoke control: horizontal exits, smoke compartments and the sealing and fire-safing of wall and floor penetrations and corridor walls. State health codes and the American Institute of Architects' Guidelines for Design and Construction of Hospital and Health Care Facilities also limit the flame- and smoke-spread ratings of interior finishes and materials. Addressable fire-detection and alarm systems are essential features of smoke-control systems in health-care facilities.
There are, however, specific ways to mechanically control smoke in certain areas of these facilities-anesthetizing locations, smoke compartments, smoke-proof enclosures and atriums.
NFPA 92A defines a smoke-control system as "an engineered system that uses mechanical fans to produce airflows and pressure differences across smoke barriers to limit and direct smoke movement." Operational ways to control and limit smoke movement include engineered smoke-control systems and stair pressurization, smoke-exhaust systems, fan-system shutdown and smoke-compartment exhaust/pressurization.
Not all health-care facilities require engineered smoke-control systems, but all require some means of smoke control. The size and layout of each building must be considered. State and local codes that identify the construction requirements of buildings include smoke-control features, if required. The local authority having jurisdiction (AHJ) may also have specific design requirements. For example, the AHJ may enforce a building codes' high-rise provision that is normally restricted to business and residential occupancies.
Health-care facilities may be high-rise buildings and will most likely have anesthetizing locations. They may have windowless areas below ground or multilevel atriums and may also be one- or two-level structures with access at grade.
Chapter 5 of the 1999 Edition of NFPA 99 requires that ventilation systems for anesthetizing locations automatically prevent recirculation of smoke originating within the surgical suite. These systems must also prevent the circulation of smoke entering the system intake without interfering with the exhaust function of the system. It also requires that supply and exhaust systems for windowless anesthetizing locations be arranged to automatically vent smoke and other products of combustion. Anesthetizing locations are usually confined to operating rooms, delivery rooms, special procedure rooms and trauma rooms. A typical operating-room (OR) suite has:
Four operating rooms.
A patient holding area.
A recovery area.
A substerile area.
Sterile storage and related areas.
All of these areas are contained within the OR suite-the "red-line" area-except the recovery area. The red-line area is usually served by one air-handling system comprising: mixed-air inlet, 30-percent prefilter, preheat coil, access section, cooling coil, fan section, diffuser plate, 90-percent final filter, humidifier and discharge plenum. Air is supplied through laminar-flow ceiling diffusers at 25 air changes per hour and is removed through two low returns at opposite corners of each OR. Smoke detectors are located within each OR and in the rest of the suite, as well as in the supply-air duct, common return air and in the outside air near the unit (see Figure 1 opposite page.)
To comply with NFPA 99 requirements, the control sequence to ventilate products of combustion are as follows:
Upon detection of products of combustion by a smoke detector within the surgery suite, the relief damper opens, the maximum outside-air damper opens, the return damper closes and the supply fan and return/relief fan continue to operate at full speed. The system operates in this manner until the fire-alarm system is reset. The return damper should be a Class II (minimum) leakage-rated smoke damper to allow as little leakage as possible.
If products of combustion are detected in the outside air or the supply air and the system is in a smoke-evacuation mode, the supply fan shuts down, the outside dampers close and the return fan continues to operate in the relief mode.
If products of combustion are detected in the outside air and the system is in a normal operating mode, the minimum and maximum outside-air damper closes and air continues to recirculate with the supply and return fans. If smoke is then detected in the suite or by the supply-air smoke detector, the supply fan shuts down, the return damper closes and the relief fan operates in relief mode.
If products of combustion are detected by the return-air smoke detector, the system is placed in a smoke-evacuation mode as noted in No. 1 above.
Freeze-protection control modifications-such as freeze-start lockout or the opening of the chilled-water coil valve and the starting of a chilled-water pump-may be required to prevent cooling coil freeze-up during introduction of 100-percent outside air. A preheat coil may also be used.
The key to effective smoke control is a system that functions regardless of which detector is the first to detect smoke. A control-system priority is created depending upon which detector initiates first, meaning that the system cannot reset once it is in a smoke-evacuation mode without risking the introduction of smoke into an area or failing to relieve it from the space of origin. Energy-management systems must be overridden, but supply-air smoke detectors must always remain functional to ensure smoke is not introduced into a space.
In addition, to satisfy NPFA 90A a system-shutdown switch must be located within the surgery suite or other "approved location" to shut down the supply and return fans. Some states require them within the suite, while others accept the disconnect at the unit. This really applies to all air-moving systems, not just those serving anesthetizing locations. This kill switch should be properly labeled and covered to avoid accidental shutdown. Mounting the switch at 7 feet, 6 inches above the floor is a good deterrent to unwanted shutdowns.
A variation of this system would have an exhaust fan in lieu of the return/relief fan. The sequence would open the relief damper, start the exhaust fan and close the return-air damper upon surgery suite or return-air smoke-detector initiation.
The air-moving equipment in the OR suite smoke-removal system does not have to be high-temperature-rated, because normal exhaust-fan construction can usually handle these air temperatures. In the area where the patient is, air temperature can be maintained for the short time required to safely bring the patient out of the fire area, allowing other fire-suppression systems to bring the fire under control to minimize smoke.
As health-care facilities are required to have fully automatic fire-sprinkler systems by the LSC, properly designed air-handling equipment may be used in lieu of natural ventilation-operable windows-to vent smoke.
The air-handling systems-including motors, dampers and controls-must be connected to the emergency-power system and arranged to exhaust at least 6 air changes per hour directly to the outside without recirculation. Supply fans and return/relief fans equipped with variable-speed drives make this possible on a compartment-by-compartment basis. All such systems must be connected to a standby power system, usually the equipment branch of the essential power system.
Each floor of a hospital is divided into at least two smoke compartments by a smoke partition, depending upon floor area, occupant load and travel distance. Air is supplied and removed from each space creating a generally equal-if not slightly positive-pressure on the floor. Each smoke compartment has access to at least one stair. Cross-corridor doors isolate each smoke compartment and form a part of the smoke partition as shown above in Figure 2.
NFPA 92A identifies zoned smoke control as a system that includes smoke exhaust for the smoke zone and pressurization for contiguous smoke-control zones. The recommended pressure differential across the smoke-compartment corridor doors is 0.05 inches water gauge (wg) in a fully sprinklered building.
A simple way to provide zoned smoke control is to have an air-handling system for each smoke compartment. This allows independent control of air volumes for each smoke compartment, which is necessary to maintain the pressure difference without relying on modulating smoke dampers in ducts penetrating the smoke partition.
For example, when smoke compartment 1 (see Figure 2 on page 40) goes into a smoke-control mode, the following sequence occurs:
Air-system supply fan in affected compartment goes to 50-percent speed with 100-percent outside air.
Return/relief fan goes to 100-percent speed.
Relief damper opens.
Return air damper at air unit closes.
Supply fan in the adjacent compartment goes to 100-percent speed.
Return fan in the adjacent compartment goes to 30-percent speed.
Pressure differential measurements taken across the corridor doors would show 0.05 inches to 0.10 inches wg with airflow inward toward the fire compartment. These fan speeds were determined from an actual project commissioning; different air systems and building configurations may require different fan speeds or arrangements. Also, systems serving multiple floors may require modulating smoke dampers.
It is important to design flexibility into the air system and its components to match the building construction and operation-and to adjust system design parameters if the first system test does not produce the desired results.
Stair-tower pressurization is equally important in high-rise construction. In fully sprinklered buildings, the stair tower can be pressurized to achieve a minimum positive pressure of 0.05 inches to a maximum of 0.35 inches wg by adjusting the airflow of the supply fans. These pressures allow doors to be opened while maintaining airflow outward from the stairs. Fans are positioned to avoid introducing smoke into the stair, and are usually arranged with multiple points of air injection. Inlet guide vanes or variable-speed drives on supply-fan motors, controlled from pressure sensors in the stairwell, work to maintain required pressure. End switches on dampers with 15-second maximum opening time-60 and 75 seconds per NFPA 92A-or time-delay relays in fan starters are required to prevent duct damage during startup (see Figure 3, page 40.)
Smoke detectors at the fan inlet are necessary to shut down the fan and close the inlet damper to prevent recirculation of smoke. Local codes must be reviewed for special requirements such as minimum velocities across door openings and equipment location, and may also require vestibules with 60 air changes per hour of supply air and exhaust air at 150 percent of the supply air, or mechanical air removal at the top of the stair tower.
The supply-fan system must be able to respond quickly to stair door openings and to minimize the entrance of smoke. Maximum air volume must achieve the required relief and minimum pressure differences or air velocities across open doorways-usually on the fire floor, the floor above and below and the point of exit discharge. Care must be taken to count the number of doors at the point of exit discharge. The stair may discharge into an exit passageway which also serves as the exit for the at-grade floors.
Typically, hospitals are not evacuated. Areas of refuge or horizontal exits are designed into the building plan and the stair becomes a tenable environment for firefighters' staging area to fight the fire within the affected compartment.
Other components such as fire command centers may be required for complete control of all heating, ventilation and air-conditioning (HVAC) and smoke-removal systems. Local codes and AHJs must be consulted.
In smaller facilities without requirements for smoke compartment evacuation or pressurization, fan-system shutdown is an effective means of smoke control. Air-handling systems may serve multiple smoke compartments. Smoke detection and sprinkler-flow alarms would be present and serve as the initiation devices for system shutdown. Typically, fire pull stations sound general alarms but do not de-energize fan systems. Smoke dampers-or combination fire/smoke dampers-located at the unit and in duct systems at each smoke-rated wall penetration would isolate the smoke compartment. Smoke detectors in the supply- and return-air ducts shut down the supply fans and close all smoke dampers in the duct system. The fire-alarm system also initiates the closing of cross-corridor doors. Once the fire-alarm system is reset, all systems go back to normal operation. Smoke dampers should have end switches or a time-delay relay built into the supply-fan starter to allow the damper to fully open prior to the fan starting to prevent ductwork damage. This system operation isolates the fire compartment and seals the smoke inside.
Atriums are a design problem unto themselves. The 1997 Standard Building Code outlines requirements for atriums of less than and more than 55 feet in height and with volumes of less than and more than 600,000 cubic feet. Based on the building design, either a minimum of 40,000 cubic feet per minute or 4 air changes per hour should be used as an exhaust requirement. Supply air may be introduced by gravity or by mechanical means. Other codes, such as the 1997 Uniform Building Code, have been modified to require calculations that model a fire in various locations within the atrium to determine the worst case, providing a design basis for air movement and exhaust.
Atriums-within or attached to a health-care facility-must meet design parameters for institutional occupancies, but they must also meet more restrictive smoke-control design requirements. Additional requirements for ductwork construction and testing, fan construction and duty and motor-service factor must be included in the final design. Some codes and AHJs require air to be supplied literally at the floor of the exit level, or within six feet of the floor, to ensure the path of egress stays as smoke-free as possible, while directing the smoke upward for removal by smoke-exhaust fans with capture points or inlets at the roof or highest point.
The design team must protect the occupants of a health-care facility with proper building materials and properly working mechanical and electrical systems.