Clearing The Smoke About Smoke Control
For the majority of buildings - restaurants and retail included - the life safety system design is simple and straightforward, with little interface between the system and the building’s HVAC, security, and other systems. In such applications, the interface between the life safety system and other systems may be as straightforward and limited as duct-mounted smoke detectors on AHUs in excess of 2,000 cfm. For buildings with high security, the fire alarm may interface with the security system to unlock magnetic door locks to allow egress. But for buildings with smoke control systems, the interfacing among the different building systems can raise several more complex design and interface issues.
Currently, the International Building Code (IBC) requires smoke control in areas such as high-rise buildings, buildings with atria, underground parking, and areas where smoke-protected seating is used. In structures such as these, a smoke control system requires interfacing of the fire alarm and detection system with the building environmental control system to control and monitor fans and dampers.
Fundamentals of Smoke Control SystemsWhile many believe smoke control systems are designed to remove smoke from a zone of incident, the intent of smoke control systems is to provide a tenable environment to allow a means of egress or to contain the smoke in the zone of origin. There are a variety of options to accomplish smoke control in a fully sprinklered building. Less common methods of controlling smoke include the air change method and the airflow method. The engineer of record and the local AHJ will dictate the method of smoke control utilized for the application. A few of the most common methods used to control smoke are listed below.
Passive method. The passive method utilizes architectural barriers to maintain the smoke in the zone of origin, as opposed to an active method utilizing mechanical equipment (dampers, fans, etc.) to maintain or control the smoke. The passive method of smoke control is typically found in utility rooms or hotel guest rooms. For example, if a fire were to originate in a hotel guest room, the compartmentalization of the room (rated walls and doors, smoke-rated doors, and self-closing doors) is intended to prevent the smoke from migrating outside the hotel room. A door fan assembly is typically used to verify the integrity of the passive zone. When the room is pressurized, the leakage is measured to confirm that the space maintains the correct volume calculated by the engineer of record. Visual verification of fire and smoke-rated doors, as well as smoke seals, is also required. Where door fan assemblies are not utilized, an inspection of all barrier walls should be performed to confirm that all penetrations are fire-stopped with an UL-listed fire and smoke assembly.
Pressurization method. In this method, the primary means of controlling smoke utilizes mechanical equipment, such as AHUs, exhaust fans, fire/smoke dampers, etc., to provide a minimum negative-pressure differential (0.05 in. w.c. in the zone of origin) relative to the adjacent spaces. The intent of the pressurization method is to maintain the smoke in the zone of incident. A tenable environment is not required in the smoke control zone of origin. The floor of a hotel tower is an example of a space where the pressurization method is utilized. When smoke is detected on a hotel floor, the following is a typical sequence employed to maintain the required minimum 0.05 in. w.c.
- The corridor smoke exhaust fan will start.
- The supply/makeup air damper will close on the floor of incident. The exhaust damper on the floor of incident will open.
- The exhaust dampers on all other floors will close.
- The supply/makeup air dampers will remain open.
A digital monometer is used to verify the pressure differential between the floor of incident and the stair enclosure between the floor of incident and the adjacent passive hotel guestrooms (and when required, to the adjacent floors). In order to maintain a safe means of egress, the stair enclosures are provided with a pressurization fan to prevent smoke from migrating into the enclosure. A minimum of 0.05 in. w.c. is required between the stair enclosure and the adjacent floor and/or vestibule without exceeding a 30-lb door opening force.
Exhaust method. The intent of this method is to maintain the smoke layer 10 ft above the walking surface. This is accomplished by calculating the smoke plume using a design fire not less than 5,000 Btu. The exhaust method requires both smoke exhaust fans and makeup air. The makeup air can be from mechanical means or natural ventilation, but it cannot exceed a flow of 200 fpm toward the fire. The exhaust method is typically used in large open spaces such as atria, covered malls, and casinos. To confirm correct operation of the system utilizing this method, the exhaust cfm must be measured to verify the velocities are in accordance with the design parameters established by the engineer of record. The makeup air must be measured to confirm that no more than 200 fpm is recorded.
It should be noted that in all methods listed above, the design and functionality of a smoke control system is predicated on a fully functional automatic fire sprinkler system installed in accordance with the NFPA-13. TestMarcx has been involved in the commissioning and special inspection of some of the largest and most complex smoke control systems in the world. Regardless of the size and complexity of the system, certain design recommendations will help any team ensure the minimum amount of conflicts and facilitate opening the building on schedule. Here are some basic recommendations.
- Use a fire protection engineering firm to assist in the design of the smoke management system. Just as mechanical and electrical engineers are experts in their respective fields, fire protection engineers are experts within their field. A fire protection engineering firm can provide a project with expertise and resources that a typical MEP firm cannot. The fire protection consultant can provide the system basis of design, develop required smoke control rational analysis, and size the required smoke control fans through the use of computational fluid dynamics (CFD) computer fire modeling. CFD modeling alone can save a project thousands of dollars in fan size and other expenses. When a fire protection consultant is engaged on a project, it is extremely important to comply with his recommendations. Too often, TestMarcx has witnessed instances in which the design team did not act in accordance with the recommendations of the fire protection consultant to the detriment of the project.
- Specify and approve UL-listed equipment for smoke control. Fans must be UL-listed or capable of meeting the requirements of smoke control, including the fan being rated for high temperatures, being equipped with 1.5 times the required belts, and having a service factor of 1.5. The fire alarm and all related components must be UULK-listed for smoke control, not just UL-listed for fire. Oftentimes to save money, fire alarm contractors install older, off-the-shelf fire alarm systems. These systems are approved and installed with the involved parties unaware of the UULK requirement. This results in costly modifications when discovered by the special inspector.
- Keep it simple. Design the environmental system around smoke management. Keep environmental systems within smoke zones. Every duct that crosses a smoke boundary requires a fire smoke damper that must be controlled and monitored. The fewer the dampers required within a system, the better.
- Keep the smoke control system separate from the BAS. UULK-listed fire alarm systems have been designed specifically to meet the requirements of control and monitoring smoke control systems. Experience has shown that systems work smoother, faster, and are easier to service when the fire alarm panel contains all smoke control logic, system control, and monitoring components. Anytime the BAS is used to control fans and dampers for smoke control, the system becomes much more complex. In these circumstances, it can be difficult to find hardware - such as programmable logic controls (PLCs) - listed for smoke control. Using fire alarm control modules and monitor modules is a much cleaner and simpler method.
- Hire a special inspector during the design phase. The IBC requires that all smoke control systems be inspected by an independent, third-party special inspector. The special inspector ensures that the system functions as designed. By incorporating the special inspector early, mistakes or design issues can be caught before they become costly changeorders. This also speeds up the formal special inspection process because the special inspector works with the construction team to make sure they are aware of what is expected during the process. Oftentimes, the special inspector is called out to examine the building a week before it is scheduled to open, only to find major issues that will take weeks to correct. Hiring the special inspector early can eliminate many of these potential delays.
While commissioning smoke control systems, TestMarcx has witnessed both good and poor examples of planning and designing for smoke control systems. Due to the complexity of smoke control systems, even the most experienced design teams can encounter unexpected problems. Keeping smoke control a priority and following the guidelines listed above will help minimize the conflicts and problems encountered and help ensure that you get your building open on time and on budget. ES