New handbook address smoke control challenges
Handbook of Smoke Control Engineering discusses myths about stack effect and the challenges of elevator pressurization
By John H. Klote, PE, DSc; and Douglas H. Evans, PE, FSFPE,
Smoke is recognized as the major killer in building fires, and a new smoke control handbook provides the latest information to help the design team develop systems to provide protection from smoke: Handbook of Smoke Control Engineering, published by ASHRAE. Earlier books about smoke control were textbooks; the handbook format was chosen for this new book to make it more useful to design engineers. To still be useful as a textbook, the new handbook has an appendix of derivations of equations.
The handbook has completely new chapters on climatic data, controls, network modeling, CONTAM, fire and smoke control in transport tunnels, and full scale fire testing. This article focuses on sustainability, myths about stack effect, and the challenges of elevator pressurization.
Sustainability has attracted considerable attention in recent years, and the design of green buildings requires ingenuity and understanding of the technology. Sustainable smoke control systems can be used in place of code-mandated systems, but such alternates need to be approved by the authority having jurisdiction (AHJ). Alternate systems need to provide at least equivalent protection to that of the code-mandated approaches.
A note about sustainability in the front of the new handbook states that the handbook can be thought of as a treatment of sustainability to the extent that designers can use it to help develop sustainable smoke control systems. Smoke venting and smoke filling are two alternatives to atrium smoke exhaust systems. With smoke venting, the smoke exhaust is replaced with a vent that opens to allow smoke to flow out of the atrium as shown in Figure 1. Smoke filling is applicable in very large atria such that the time for smoke to fill the atrium to a specified level is sufficient for evacuation of the occupants. These alternatives have been used for many decades in Europe, Australia, and Japan. They eliminate the need for smoke exhaust fans, and reduce or eliminate electrical power needed for weekly self-tests.
Alternates to pressurization smoke control systems can rely on passive protection or dilution of smoke. An example of an alternate is using stairwell ventilation in place of stairwell pressurization. Stairwell ventilation is a new concept that relies on dilution to maintain tenable conditions in stairwells during building fires. The supply air needed for stairwell ventilation is relatively independent of height, but the supply air needed for pressurized stairwells is nearly proportional to stairwell height. This means that stairwell ventilation has the potential to reduce significantly the amount of supply air needed for stairwells in tall buildings.
Myths about stack effect
It is common to have an upward flow of air in building shafts during winter. These shafts include stairwells, elevator shafts, dumbwaiters, and utility shafts. The upward flow is caused by the buoyancy of warm air relative to the cold outdoor air. With stack effect, there is a horizontal plane where the pressure in the shaft is the same as that outdoors, and this plane is called the neutral plane. Air flows into the shaft below the neutral plane, and out of the shaft above the neutral plane. In summer there is a downward flow in shafts that is called “reverse stack effect.”
The well-known stack effect equation is used to calculate the pressure difference caused by stack and reverse stack effect. This pressure difference is nearly proportional to two things: the shaft height and the temperature difference from the shaft to the outdoors.
The shaft temperature is often the same as that of the building, but the shaft can be relatively cold in winter and warm in summer. This is especially so when one or more shaft walls are on the outside of the building. A shaft temperature that is different from that of the building temperature can have a major impact on stack effect.
The term “stack effect” is often used in another way. Sometimes engineers will say that a pressurized stairwell (or elevator) needs to be designed to account for stack effect. This means that the shaft pressurization needs to be designed to account for the shaft-to-outdoor temperature difference that is the cause of stack effect. It may seem that this distinction is not very important, but misunderstandings can have significant consequences.
Myth: The pressure difference due to stack effect is nearly proportional to the temperature difference between the building and the outdoors.
Fact: This pressure difference is nearly proportional to the temperature difference between a shaft and the outdoors.
For shafts pressurized with untreated outdoor air, the shaft-to-outdoor temperature difference is small, and stack effect is also small. Often floor-to-floor variations in floor size and leakage to the outdoors are much more important than stack effect. For example, pressurizing a stairwell that serves a parking garage, a hotel lobby, hotel meeting floors, and guest room floors can be a challenge. Pressurized stairwells are designed to operate between a minimum and a maximum design pressure. The minimum pressure difference is intended to prevent smoke flow into protected spaces. The maximum pressure difference is intended to prevent excessive door opening forces. For buildings with such complex stairwells, analysis with CONTAM is recommended to determine if a particular smoke control system is capable of being balanced such that it will perform as intended.
CONTAM is a network model developed by the National Institute of Standards and Technology that simulates the flows and pressures throughout a building, taking into account wind, any pressurization systems, indoor temperature, shaft temperature, and outdoor temperature. Because CONTAM has a sophisticated graphic interface and superior numerical routines, it has become the de facto standard for analysis of pressurization smoke control systems. CONTAM can be downloaded free.
Challenges of elevator pressurization
The elevator pressurization systems discussed here are intended to prevent smoke from flowing through an elevator shaft and threatening life on floors remote from the fire. Pressurization of elevators has the added benefit of providing elevator smoke protection for the fire service. Smoke control for emergency elevator evacuation is not discussed here, but it is addressed in Chapter 12 of the new smoke control handbook.
The elevator pressurization systems discussed here are:
- The basic system
- The exterior vent (EV) system
- The floor exhaust (FE) system
- The ground floor lobby (GFL) system.
The brief discussion that follows is based on CONTAM simulations of these systems in a 14-story example building (see Figure 2).
Elevator pressurization systems need much more supply air than is needed for pressurized stairwells, and this means that the leakage of the building is especially important with elevator pressurization. The pressurization air flows from the elevator shaft through the building to the outdoors, and the concern is that the building envelope may not be able to handle the large amount of pressurization air needed for the elevators. This concern is greater when both elevators and stairwells are pressurized. For this reason, the simulations discussed here include pressurization of both elevators and stairwells.
The classifications of exterior wall leakage are tight, average, loose, and extra loose, and the new smoke control handbook has information about the leakage of these walls and other building components. As is the convention with such simulations, the leakage through the toilet exhaust systems and the HVAC system are not included. It is recognized that there is leakage in the building envelope that is not part of the exterior wall leakage of the simulations. For example, a simulation with loose exterior walls may be representative of an actual building with average exterior walls plus the leakages not included in the simulations. Also, some building envelopes are not as tight as might be expected.
In the basic system, each stairwell and elevator shaft has one or more dedicated fans that supply pressurization air. For most buildings, the building envelope is not capable of effectively handling the large airflow from both the elevators and stairwells, and the basic system does not result in successful pressurization for most buildings.
For the basic system in the example building with average and leaky exterior walls, Figure 3 shows the pressure differences across the elevator shaft at the ground floor greatly exceed the maximum criterion. However, it also demonstrates that with very leaky exterior walls, the basic system is successfully pressurized. For the example building, the air needed for successful pressurization is 27,700 cfm for each of the two elevator shafts and 6,560 cfm for each of the two stairwells.
It is expected that for some relatively leaky buildings, there may be enough wall leakage to accommodate the large amount of pressurization air needed for elevators, and successful pressurization may be possible with the basic system. This should be evaluated with a CONTAM analysis.
Exterior vent system
The purpose of this system is to effectively increase the leakage of the building such that successful pressurization can be achieved. Because the example building is an open plan office building, this can be done by using vents in the exterior walls on all floors. For the example building with the EV system, the vents were sized so that successful pressurization was achieved with the same amount of pressurization air as was needed with the basic system.
For a building that is not open plan, the flow resistance of corridor walls and other walls can have a negative impact on system performance. This negative impact can be overcome by the use of ducts that act as paths for airflow from the elevators and stairwells to the outdoors. The ducts can be located under the ceiling or above a suspended ceiling. The ducted EV system can be used for other occupancies such as hotels and condominiums. Duct penetrations of a fire-rated wall may have fire-resistance requirements depending on code requirements.
The vents should be located in a manner to minimize adverse wind effects, and the supply intakes need to be located away from the vents to minimize the potential for smoke feedback into the supply air. These vents may need fire dampers depending on code requirements.
Floor exhaust system
The FE system deals with the building envelope issue by reducing the amount of supply air used. In the FE system, a relatively small amount of air is supplied to the elevator shafts and the stairwells, and the fire floor is exhausted such that acceptable pressurization is maintained on the fire floor where it is needed. It is common to also exhaust one or two floors above and below the fire floor. Because the FE system only maintains pressurization at some floors, it needs to be approved by the AHJ.
Myth: Stack effect is the major factor impacting stairwell and elevator pressurization.
Fact: The impact of stack effect is a minor factor for most pressurized stairwells and elevators. The pressurization air for many stairwells and elevators is outdoor air that is not heated nor cooled. The temperature of these shafts is often nearly the same as the outdoor temperature, and the impact of stack effect is significantly reduced as compared to shafts pressurized with air treated to the building temperature.
To achieve successful pressurization with the FE system in the example building, each elevator shaft needed 15,100 cfm, and each stairwell needed 3,800 cfm. The floor exhaust ranged from 4800 to 5400 cfm depending on the particular floor being exhausted. For a building with interior partitions, the exhaust needs to be from a space that is open to both the elevator lobby and the stairwell doors.
Ground floor lobby system
This system has an enclosed elevator lobby on the ground floor, but the other floors do not have any enclosed elevator lobbies. This system is the opposite of the normal practice of having enclosed elevator lobbies on all floors except the ground floor, but it has the potential for successful elevator pressurization.
As shown in Figure 3, elevator pressurization systems have a tendency to produce very high pressure differences across the elevator doors at the ground floor, and an enclosed elevator lobby can reduce this pressure difference. The GFL system often has a vent between the enclosed lobby and the building with the intent of preventing excessive pressure differences across the lobby doors. The lobby doors are the doors between the enclosed lobby and the building. For the example building, the air supplied to the shafts was nearly the same as that needed for the basic system and the EV system. The floor-to-floor leakage had an impact on the performance of a GFL system.
John H. Klote, PE, DSc, is a private consultant based in Leesburg, Va., and is the lead author of the Handbook of Smoke Control Engineering. Douglas H. Evans is fire protection engineer with the Clark County Building Department, Nev. He is also a member of the Consulting-Specifying Engineer editorial advisory board.