The Importance Of Stopping SMOKE
Section 101.3 (Intent) of the 2000 International Building Code states, "The purpose of this code is to establish minimum requirements to safeguard the public health, safety, and general welfare through structural strength, means of egress facilities, stability, sanitation, adequate light and ventilation, energy conservation, and safety to life and property from fire and other hazards attributed to the built environment." In 2003, this section added a clause that also included "a reasonable level of safety for firefighters and other emergency responders."
According to the National Fire Protection Association (NFPA), fire deaths due to smoke inhalation outnumber deaths due to burns by a 2-to-1 margin if death certificates from 2002 are used, and by 3-to-1 if death certificates prior to 1999 are used. It is also estimated that smoke inhalation is the primary cause of death in 60% to 80% of burn victims each year. Firefighters certainly are not immune to the danger (according to the NFPA, 2,890 firefighters were injured from smoke inhalation in 2003), often thrusting themselves into the hazard.
By definition, inhalation injury is the aspiration of superheated gases, steam, hot liquids, or noxious products of incomplete combustion that cause thermal or chemical injury to the airways and lungs. The combustion of all natural and manmade products results in the production of various chemicals, including hydrogen cyanide, aldehydes, hydrochloric acid, and acrolein, which produce the changes in the airway and lungs that are characteristic of inhalation injury. The injury can occur above or below the vocal cords, or in both locations at once. Injuries above the vocal cords are typically caused by inhaled heat, while those below the cords are usually caused by toxins and particulate matter. Because dry heat does not easily penetrate as far as the lower respiratory tract, true thermal damage of the lungs is rare.
The presence of inhalation injury doubles the predicted mortality rate associated with any size burn in all age groups.
RISK FACTORSOnce a fire reaches flashover, production of carbon monoxide and hydrogen cyanide increases, consumption of oxygen intensifies, and incapacitating conditions are induced within 2 min, possibly causing death of those exposed within 10 min. On November 28, 1942, in one of the most infamous fires in U.S. history, patrons of the Coconut Grove nightclub in Boston were trapped inside a building whose only exit was a revolving door blocked by the bodies of more than 200 people overcome by smoke. Most of the 492 deaths that night were due to smoke inhalation, as were most of the subsequent deaths among hospitalized victims. Almost 40 years later, 84 deaths and 679 injuries resulted from smoke spreading through seismic joints, pipe chases, and ductwork shafts in the MGM fire in Las Vegas - with the majority of deaths and injuries occurring in upper floors far from the source of the fire in the casino.
Although these notable fires are infamous due to their size, the people at the greatest risk for inhalation injury are those who are asleep when a fire starts in a building. Production of carbon monoxide deepens the sleep state, making escape less likely. Conditions that lead to changes in consciousness, such as cerebral vascular accidents, seizures, or alcohol abuse, are also risk factors for inhalation injury.
It is interesting to note that there are more than 8,000 fires in educational occupancies in the U.S. each year, yet fire-related deaths in schools average less than one per year. In fact, from 1989 to 1998, public assembly and educational properties show no deaths in reported fires. This is in spite of the fact that over two-thirds of public assembly and educational facilities do not have sprinklers installed. Why are there so few deaths in school fires? Because educational facilities meet many of the criteria for mitigating smoke inhalation and fire-related deaths. Educational buildings are occupied largely during the day, they are built of non-combustible and fire-resistant materials, egress areas are easy to access and well understood by occupants, and many zones are fire and smoke compartmentalized. We can learn a lot from schools - even after graduation.
BALANCE AND REDUNDANCY IS IMPORTANTRegardless of whether a building is sprinklered or not, both building codes and experts agree that stopping smoke migration is important for both primary and redundancy reasons. Dr. John Klote, a fire and smoke control consultant in McLean, VA,. stresses this point, "From a primary standpoint, even under successful sprinkler suppression, smoke is still generated and can travel through duct openings if not properly isolated," he said.
Emphasizing the importance of redundancy, Klote further cited floods that inundated parts of Iowa in recent history. "A local fire chief (in Iowa) said that because the water system is down, you can't occupy a high-rise building. Some buildings are too vulnerable to fire without sprinkler systems. There's no backup in some designs." Vickie Lovell, a building code consultant in Margate, FL, agrees, "Sprinkler systems must be coupled with compartmentalization for balanced, optimum protection."
FIRE DAMPERS VS. COMBINATION FIRE/SMOKE DAMPERSDampers are designed to assist the mechanical designer with compartmentalization needs. There is a significant difference between a "fire damper" and a "combination fire/smoke damper." A fire damper closes once duct temperature reaches a high enough level to melt a fuse link. A combination damper closes based on high duct temperature or upon a smoke detector signal. Additionally, fire dampers are not UL leakage-rated to stop smoke. A smoke damper, or combination fire/smoke damper, is a leakage-rated device.
The 2003 IBC recognizes the importance of stopping smoke by requiring combination fire/smoke dampers in shafts penetrated by ducts or transfer openings for all building classification types. Smoke dampers are required at each point a duct or air transfer opening penetrates a corridor. Additionally, smoke-proof enclosures or pressurized stairways must be provided in buildings with a floor surface located more than 75 ft above the ground, or 30 ft below the level of exit. This is designed to mitigate smoke migration and smoke damage and to permit a safer exit for building occupants.
In Section 716 of the IBC, leakage ratings for smoke dampers (which include combination fire/smoke dampers) shall not be less than Class II (less than 10 cfm/sq ft at 1 in. static pressure). Elevated temperature ratings for this leakage class cannot be less than 250°F.
Fire, smoke, and combination fire/smoke dampers are a vital part of an adequate design. If there is a fire emergency, the fire, smoke, and combination fire smoke dampers will help contain the fire and resulting smoke to the compartment of origin and thus minimize life and property loss while helping the firefighters extinguish the blaze.
FIRE, SMOKE, AND COMBINATION DAMPERS: AN APPLICATION SUMMARYSmoke dampers are operated by either a factory-installed electric or pneumatic actuator. They are controlled by smoke detectors and/or fire alarms. Smoke dampers are qualified under UL Standard 555S, Smoke Dampers, and are designed to resist the passage of air and smoke. Smoke dampers have two general applications:
- Part of a "passive smoke control system" in which they close upon detection of smoke and prevent the circulation of air and smoke through a duct, transfer, or ventilation opening.
- Part of an "engineered smoke control system" designed to control smoke migration using walls and floors as barriers to create pressure differences. Pressurizing the areas surrounding the fire prevents the spread of smoke into other areas.
Smoke dampers have the following installation requirements:
- Location: Smoke dampers are for use in or adjacent to smoke barriers. They must be installed no more than 24 in. from the smoke barrier. Of course, smoke dampers that are used to isolate air handlers are not limited to this distance requirement. NFPA 90A states that smoke dampers are to be used to isolate AHUs over 15,000 cfm.
- Sleeves and attachment: Smoke dampers do not necessarily have to be installed in sleeves. They can be installed directly in the duct. The manufacturer's installation instructions will include the approved method for attachment and spacing of the attachment.
- Sealing: The joints between the damper frame and the duct must be sealed to prevent unwanted air leakage. Smoke damper leakage ratings are based on leakage through the blades and not additional leakage between the damper frame and duct or sleeve.
Fire dampers (Figure 1) are installed in a wall or floor, at the point of duct penetration, to retain the integrity and fire rating of a wall - whether it is a ducted or openplenum return application. They are designed and tested to maintain the integrity of the fire-rated separation. Fire dampers are equipped with a fusible link (rated for 165°, up to 286°), which "holds" the blades open until it melts. When the fusible link melts, the blades close and stop the flame from moving into an adjoining compartment.
There are two types of applications for fire dampers: static and dynamic. Static fire dampers can only be applied in HVAC systems that are designed to shut down in the event of a fire. Dynamic fire dampers have been tested for closure under airflow and carry both an airflow velocity (fpm) and pressure differential rating. The minimum rating for all dynamic fire dampers is 2,000 fpm and 4.0 in. wc. The minimum ratings are based upon closure at a minimum airflow of 2,400 fpm and 4.5 in. wc. Higher ratings than the minimum are established in increments of 1,000 fpm and in increments of 2 in. wc.
Fire dampers are available in two basic designs: curtain-type and multiple-blade-type. Curtain-type dampers are the most common and consist of a "curtain" held up by a fusible link. Multiple-blade dampers are similar to control dampers with "blades" located in the air-stream. Multiple-blade fire dampers generally offer greater restriction to airflow than a curtain-type fire damper for the same size duct. However, multiple-blade fire dampers can be applied in situations when the system air velocities exceed the curtain-type fire damper closure ratings. Multiple-blade fire dampers have been UL tested and are dynamic rated for closure at 4,000 fpm and 8.0 in. wc.
Combination fire/smoke dampers meet the requirements of both the UL555 fire damper and UL555S smoke damper standards and application requirements as described above. They are used in HVAC penetrations where a wall, floor, or ceiling is required to have both a fire damper and smoke damper. They close upon the detection of heat (via duct temperature) or smoke (via a smoke detector) and "seal" the opening. Unlike regular fire dampers, however, fire smoke dampers are available with electric heat release devices instead of fusible links. The electric release devices are resettable and allow the damper to close in a controlled manner rather than slamming closed and causing problems with the HVAC system.
System designers should insist upon these electric fuse links when selecting a combination fire/smoke damper. Fire/smoke dampers designed with airfoil blades perform better (less pressure drop) than others. Less pressure drop in a system means energy savings. System designers should select fire/smoke dampers that certify their performance through a third party, like the Air Movement and Control Association (AMCA).
INCREASED APPLICATION FLEXIBILITYThe installation of fire dampers should be accomplished in accordance with the manufacturer's instructions as tested by UL or another code-compliant approval agency. Recently, many damper designs - especially combination fire/smoke dampers - have increased their application and installation flexibility. The number of UL tests has increased dramatically to accommodate the new designs and installation methods. These designs have made it easier to select and install the right damper without creating undue burden on the engineer, contractor, and the AHJ. Mechanical inspectors and plan review teams are being educated on these changes and are becoming increasingly aware of the paradigm shift. Again, it is important that the damper manufacturer's approved installation sheets be available for the AHJ during installation review.
Some of the important design and application changes for fire and combination fire/smoke dampers include:
- One-sided angle installation: Until recently, all fire and combination fire/smoke dampers required mounting angles on both sides of the wall. In addition to added cost, it made it extremely difficult to install dampers in finished shaft walls. Many designs are now UL approved to be installed with a mounting angle on one-side only - thereby cutting damper installation time in half. This applies to both rectangular and round dampers.
- Damper to sleeve connections (Figure 2): The installation instructions are required to show the method of attaching the damper to the sleeve and the spacing of attachments for UL-compliant breakaway connections. Newer damper designs have been tested to accommodate a cadre of various flanged connection combinations to comply with UL's breakaway requirements. This permits the contractor to use any flange type and not worry about a special connection to the damper sleeve.
- Out-of-the-wall (Figure 3): For years, UL damper installations were approved only if the damper blades were in the wall. Occasionally, other "hidden" items (e.g., hydronic piping, ductwork, cables, etc.) interfered with the clearance of the damper actuator. Newer designs now accommodate installation "out-of-the-wall" by an additional 8 in. The damper sleeve is wrapped with a heat-resistant material that effectively extends the wall rating to the damper.
- Vertical-bladed installation (Figure 4): Another method to assist in eliminating interference is with a damper designed with vertical blades. This permits access of the actuator from below or above the damper. This again can solve interference problems - especially with side-by-side ducts. This configuration also permits a slightly lower pressure drop on multistory supply shafts.
- Corridor fire/smoke dampers (Figure 5): Since codes require that many corridors be protected with fire/smoke dampers, many designs have been developed to permit installation from the corridor. In these designs, dampers can be mounted and fastened directly to the wall or floor - without traditional mounting angles (and without traditional expansion gaps). Aesthetically pleasing flush-mounted grilles provide both airflow and a means in which to access the actuator without mounting an access door in the corridor.
- Underfloor dampers: There are installation methods now approved that eliminate the 1-½ in. mounting angle around the perimeter of the damper. This frees 3 in. of space, which is often critical for underloor applications. The damper height can now be maximized to minimize pressure drop.
- Maximum damper size: Maximum UL-approved damper sizes have increased in recent years. This is especially accommodating on large open-air returns. These maximum size limitations are based solely on dampers that were tested to meet UL555 requirements. Maximum sizes for vertical installations may be different from maximum sizes for horizontal installations. UL requires manufacturers to list the damper maximum sizes on the installation instructions. By maximizing these UL-approved sizes, the extra cost of subdividing ductwork can be eliminated.