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.
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
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.
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.
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
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.
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.
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.
Table 1. Pressure Differences Criteria for Elevator Pressurization Simulations5
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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. Ground floor of the example building with the ground floor lobby (GFL) system.
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.
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