The elevator pressurization systems discussed in this article are intended to prevent smoke from flowing through an elevator shaft and threatening life on floors remote from a fire. Elevator pressurization is an alternative to enclosed elevator lobbies. The material in this article is based on the treatment of pressurized elevators in SFPE’s smoke control seminars1, 2 and a new smoke control handbook.3 This article does not address smoke control for elevator evacuation, which is discussed in the new handbook.

Many pressurized elevators are in buildings that have pressurized stairwells, and the focus of this article is on both of these pressurization systems operating together. In the rare situation where pressurized elevators are the only pressurization smoke control system in a building, the information in this article should be useful.

The pressures produced by elevator car motion has the potential to adversely impact the performance of a pressurized elevator system, and this elevator piston effect should be taken into account in the design of a pressurized elevator system. For more information about elevator piston effect, see the smoke control handbook.3

DESIGN ANALYSIS

Network analysis models are often used for design analysis of pressurization smoke control systems, and CONTAM4 is so extensively used for such analysis that it has become the de facto standard. CONTAM was used for the simulations discussed in this article. Generally a CONTAM analysis is needed to determine if pressurized elevators and pressurized stairwells of a particular building are capable of being balanced to perform as intended.

Design of pressurized elevators is much more complicated than design of pressurized stairwells, but there are a number of systems that can deal with this complexity. The reasons for this complexity are: (1) often the building envelope is not capable of effectively handling the large airflow resulting from both elevator and stairwell pressurization, (2) open elevator doors on the ground floor tend to increase the flow from the elevator shaft at the ground floor, and (3) open exterior doors on the ground floor can cause excessive pressure differences across the elevator shaft at the ground floor.

In most large cities, the fire service props open exterior doors when they get to a fire to speed up mobilization, and the International Building Code5 considers that elevator pressurization functions with open exterior doors. Occupants also open some exterior doors during evacuation. In this article, it is considered that elevator pressurization needs to operate with a number of exterior doors open. If the system cannot also operate as intended with all exterior doors closed, some of these doors may need to open automatically before the elevators are pressurized. At locations where the fire service does not prop open exterior doors, a different approach to open exterior doors may be appropriate.

The elevator pressurization systems discussed here are: (1) the basic system, (2) the exterior vent (EV) system, (3) the floor exhaust (FE) system, and (4) the ground floor lobby (GFL) system. The following discussion of these systems is for buildings that also have pressurized stairwells.

CONTAM SIMULATIONS

Thirty-six CONTAM simulations were used to study the performance of the systems in the example building of Figure 1.1 The example building was chosen to illustrate the elevator pressurization systems mentioned above, and it was based on an actual building to assure that it had appropriate elevator capacity. For this reason, the example could be thought of as an update to an existing building rather than a building designed to a specific code.

Figure 1. Floor plans of the example building for the CONTAM simulations.

For the simulations, the pressure difference criteria listed in Table 1 were used, and these criteria are consistent with pressure differences requirements in the International Building Code.5 The minimum pressure difference criteria are intended to prevent smoke flow into the elevator shafts and stairwells. The maximum pressure difference criteria for stairwells are intended to prevent excessive door opening forces. The maximum pressure difference criterion for elevators is intended to prevent the elevator doors from jamming.

For the CONTAM simulations of the example building, supply air was injected at the top of the elevator shafts, but about half the supply air was injected at the top of the stairs and the rest at the second floor.

Building leakage has an impact on the performance of pressurized stairwell systems, and various leakage values were used in the CONTAM simulations. The leakage of exterior walls has a major impact on system performance, and the leakage classifications of exterior walls were tight, average, loose, and very loose.

System Minimum in. H2O (pa) Maximum in. H2O (pa)
Pressurized Elevators
0.10 (25)
0.25 (62)
Pressurized Stairwells
0.10 (25)
0.35 (87)
The above criteria are for the elevator simulations discussed in this article, and some projects may have different criteria depending on code requirements and requirements of specifi c applications.

Table 1. Pressure Differences Criteria for Elevator Pressurization Simulations5

In 1999, Persily studied building leakage, and found that many buildings were relatively leaky in spite of the energy conservation concerns of the time.6 The leakages of toilet exhausts and the HVAC system were not explicitly included in CONTAM simulations discussed here, but it was recognized that they are part of the leakage of the building envelope.

For all the systems, the amount of pressurization air needed depends on the leakage of the elevator shaft walls and the elevator doors. For the simulations, the leakage of interior walls was loose, and that of elevator doors was about average. Relatively large floor-to-floor leakage (paths in floor slabs and gaps between the floor slab and curtain wall) tends to even out extremes of pressure differences across stairwells and elevator shafts, and the simulations showed that this leakage was important for the GFL system.

BASIC SYSTEM

In the basic system, each stairwell and elevator shaft has one or more dedicated fans that supply pressurization air. As mentioned above, the building envelope is not capable of effectively handling the large airflow from both the elevators and stairwells, and this is why the basic system does not result in successful pressurization for most buildings. By successful pressurization it is meant that the pressure differences across the elevator shaft (or stairwell) are within the design minimum and maximum values of Table 1.

For the basic system in the example building with average and leaky exterior walls, it can be seen from Figure 2 that the pressure differences across the elevator shaft at the ground floor greatly exceed the maximum criterion. However it also can be seen that with very leaky exteriors walls, the basic system is successfully pressurized. The air needed for successful pressurization is 27,700 cfm (13 m3/s) for each elevator shaft and 6,560 cfm (3.1 m3/s) for each stairwell.

It is expected that for 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. For a specific building, analysis with CONTAM can evaluate if the basic system is feasible. If not, the systems discussed below should be considered.

Figure 2. Elevator pressure differences for basic system in the example building.

EXTERIOR VENT (EV) SYSTEM

The idea of this system is to 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 the use of vents in the exterior walls. For the example building (Figure 3a), the CONTAM simulations showed that the vents can be sized to meet the design criteria. In the example building, the EV system needed 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 as shown in Figure 3b. The ducts are a path for airflow from the elevator to the outdoors thus eliminating the impact of the corridor walls and other walls.

Figure 3. Typical floor plans of buildings with the exterior vent (EV) system.

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. 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.

With open, exterior doors, it is not necessary to have exterior vents on the ground floor. Because the EV system may not be able to achieve acceptable pressurization with some or all the exterior doors closed, it may be necessary to have some of the exterior doors open automatically upon system activation. The number of exterior doors that need to be opened automatically can be evaluated by the CONTAM analysis.

FLOOR EXHAUST (FE) 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 must be approved by the AHJ.

For the example building, the FE system is shown in Figure 4a. The simulations of this building showed that each elevator shaft needed 15,100 cfm (7.1 m3/s), and each stairwell needed 3,800 cfm (1.8 m3/s). The floor exhaust ranged from 4,800 (2.3 m3/s) to 5,400 cfm (2.5 m3/s), depending on the particular floor being exhausted. For a building with many interior partitions, the exhaust can be from the corridor that the elevators and stairwells open onto as shown in Figure 4b.

As with the EV system, some of the exterior doors on the ground floor may need to open automatically upon activation of the FE system, and the number of such doors can be evaluated by the CONTAM analysis.

Figure 4. Typical floor plans of buildings with the floor exhaust (FE) system.

GROUND FLOOR LOBBY (GFL) 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 complete 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 can be seen from Figure 2, 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 doors between the enclosed lobby and the building.

The criteria of Table 1 apply to the GFL system with some modifications. There is no established criterion for the maximum pressure difference across the lobby doors, but the pressure should not be so high as to prevent the doors from remaining closed. This value depends on the specific doors and hardware. For the CONTAM simulations, a maximum pressure difference for the lobby doors was chosen as 0.35 in H2O (87 Pa), but this value might be different for some applications. Because of the enclosed ground floor lobby, the minimum pressure difference criterion does not apply to the elevator doors on the ground floor.

Figure 5 shows the ground floor of the example building with the GFL system. The pressure difference across the lobby door and the elevator door depends on the area of the vent, and this vent needs to be adjustable to allow for balancing during commissioning. CONTAM simulations of the GFL system in the example building showed that the criteria can be met with loose exterior walls, but not with tighter walls. The air supplied to the shafts was nearly the same as that needed for the basic system and the EV system. For the example building, the floor-to-floor leakage can have a significant impact on the performance of a GFL system. This leakage consists of the leakage of the floor and that of the curtain wall gap.

Figure 5. Ground floor of the example building with the ground floor lobby (GFL) system.

John H. Klote, Ph.D., P.E., FSFPE, is with John H. Klote, Inc., Michael J. Ferreira, P.E., is with Hughes Associates, Inc., James A. Milke, Ph.D., P.E., FSFPE, is with the University of Maryland.

References:

  1. Klote, J.H., and Ferreira, M.J. Seminar: Smoke Control Session I— Fundamentals and Pressurization Systems, Society of Fire Protection Engineers, Bethesda, MD, 2011.
  2. Klote, J.H. and Turnbull P.G. Seminar: Smoke Control Session I—Fundamentals and Pressurization Systems, Society of Fire Protection Engineers, October 27, Bethesda, MD, 2010.
  3. Klote, J. H., Milke, J. A., Turnbull, P. G., Kashef, A., Ferreira, M. J. Handbook of Smoke Control Engineering, ASHRAE, Atlanta, GA, 2012. 4 Walton, G. N., Dols, W. S. CONTAM 2.
  4. User Guide and Program Documentation, NISTIR 7251, National Institute of Standards and Technology, Gaithersburg, MD, 2010.
  5. ICC, International Building Code, International Code Council, Country Club Hills, IL, 2012.
  6. Persily, A. K. Myths about Building Envelopes, ASHRAE Journal, Vol. 41, No. 3. 1999.