Smoke control systems
System design success
The design of an effective exhaust-type smoke removal system starts with a proper analysis of the use of the space and the expected contents of the space. This important step establishes the potential sizes and locations of a fire within the space, which leads to the growth rate and smoke production expectation. Because the development of these parameters sets the foundation for the calculation and the design of the system, it requires a significant amount of experience and expertise. Underestimating the energy content or burning speed of the fuel load will lead to the design of a system that is incapable of extracting the smoke fast enough to maintain a tenable environment; overestimating the energy content or burning speed of the fuel load will lead to the design of a system that is unnecessarily expensive, complex, or energy consuming.
While overdesigning a smoke control system may seem like an innocuous way to ensure that the design was conservative, there are serious cost and functional issues that must be considered. Overdesign generally means that the exhaust fans are either larger or more numerous than necessary. The immediate cost implication of this obvious; more exhaust fans will cost more. But the cost implication goes beyond the initial cost of the fans. The total cost of the excess capacity includes the additional ductwork associated with the fans, the additional roof support, the additional wiring, and the added standby power generation capacity needed to supply the added electrical demand. Then, there is the additional makeup air required for the excess exhaust. Additional makeup air may be available by simply automating the operation of exterior doors or windows, but it may require the addition of supply fans and ductwork to ensure a sufficient volume of air.
Determining the proper amount and locations of exhaust, the location and method of proving makeup air, and the activation sequence of the smoke control system requires coordination between the IBC and NFPA 92. As noted earlier, the IBC dictates where smoke control is needed, and it prescribes certain features and characteristics of the system, but NFPA 92 provides the detailed guidance on the calculation of the fire plume, smoke production rates, and exhaust requirements. These calculation methods are the result of many years of research, observation, and testing. While they work well for simple geometries, they fall short when applied to more challenging geometries and non-uniform vertical spaces. This is where computational fluid dynamics (CFD) modeling becomes invaluable.
Modeling and simulation
CFD modeling allows the engineer to build a computer model of the space to simulate various fire scenarios that can be used to optimize the design. The engineer can run the model with various fire sizes and locations as well as modify the airflow rates and locations of exhaust and makeup air. Although this sounds simple, it is a time-consuming process because these models are very data intensive and it can take from hours to days to run a single scenario. Because time is always a constraint in the design process, the engineer can use the equations from NFPA 92, along with knowledge of airflow and smoke development gained from past experience, to build the initial model that will serve as a reasonable starting point to be tweaked and modified to optimize the system design.
The optimized system will not only maintain the smoke layer at more than 6 ft above the highest walking surface, it will minimize initial and ongoing costs, minimize energy consumption, and consider the ease of maintenance and reliability. It is not enough to simply consider if the system meets the requirements of the applicable codes, it must also consider the needs of the owner and the users of the system.
When smoke control cannot be achieved through the exhaust method, it is necessary to consider alternatives. For example, when the highest walking surface in a prison has only an 8-ft ceiling, it is impractical to assume that an exhaust system will keep the smoke layer to less than 2 ft thick (i.e., more than 6 ft above the highest walking surface). Instead, it is more practical to confine the smoke to the area of fire origin and protect those in the surrounding areas from the migration of smoke. This can be achieved through the pressurization method.
If a fire begins in the common area of a pod of cells in a prison without a smoke control system, it is possible for the smoke to migrate from the area of fire origin and affect occupants throughout the prison. The pressurization method of smoke control minimizes the migration of smoke by creating a lower relative pressure in the area of fire origin so that airflow through unprotected openings is directed into the area of fire origin, thus preventing smoke from escaping. This approach can be effectively tailored down to small groups of cells to prevent smoke from a fire occurring in a cell from impacting the occupants of the neighboring groups of cells.
In one prison in Northern Virginia, the smoke detection, HVAC, and smoke control systems were designed to treat each pod of cells as having five distinct areas: the common area in the center, and four distinct sets of cells (east ground, west ground, east mezzanine, and west mezzanine). The pressurization system was programmed to reduce the relative pressure in the area of fire origin within the pod while still reducing the relative pressure of the entire pod relative to the remainder of the building. For example, a fire beginning in a cell in the east mezzanine causes the HVAC system to damper off supply air and increase exhaust for the east mezzanine while drawing down the pressure in the overall pod. This effectively contains the smoke to the east mezzanine, but testing proved that this method actually contained smoke only to the single cell that was the origin of the fire.
It is not enough to simply draw pressure in the area of fire origin negative with respect to the surrounding areas; one must also consider the effects of the pressure differential on the egress doors. Minimum pressure differentials are provided in NFPA 92, but the maximum pressure must be derived from the maximum opening forces permitted by the IBC or NFPA 101: Life Safety Code. Equation 909.1 of the IBC allows one to calculate the maximum pressure differential across a door based on the dimensions of the door and the maximum force permitted to open the door (as found elsewhere in the IBC).
F=Fdc +K(WA∆P)/2(W- d)
A = Door area, sq ft (m2)
d = Distance from door handle to latch edge of door, feet (m)
F = Total door opening force, pounds (N)
Fdc = Force required to overcome closing device, pounds (N)
K = Coefficient 5.2 (1.0)
W = Door width, feet (m)
∆P = Design pressure difference, inches of water (Pa)
When calculating the airflow rates and pressures for the design of a pressurization system, the engineer must make assumptions about the air leakage rates through walls, shafts, windows, etc. Because of the complexity of the systems, their interaction with systems in adjacent areas, and air leakage, it is not uncommon for systems to require adjustment during commissioning. Keeping the pressure differential across the doors to a space between the minimum and maximum limits can be a delicate balance, but with good planning it is achievable.
While there is no one-size-fits-all solution to the control of the migration of smoke, the exhaust and pressurization methods have proven effective and CFD modeling is improving the design of these systems every day.
Paul J. Pinigis is director of life safety at H&A Architects & Engineers. He leads a team that focuses on building code and life safety consulting, CFD fire modeling, special suppression systems design, automatic sprinkler system design, and fire alarm and mass notification systems design.