2.1. Stack Effect in High-Rise Buildings
When the stack effect occurs in a high-rise open-plan office building, the greatest pressure differences across the elevator doors are usually measured at the first floor and the top floor (Figure 1
a). However, when the upper floors are pressurized by the HVAC systems, the air flow rate from the elevator shaft to the office spaces on the upper floors is reduced, and the air flow rate from the lobby space on the first floor to the elevator shaft is also reduced (Figure 1
b). This method can greatly alleviate problems such as malfunctioning elevator doors and noise by reducing the pressure differences across the elevator shaft. Pressurizing every floor uniformly has little effect on the pressure differences across the floors and vertical shafts. If stack pressure across the entrance door is a concern, the pressurization of the first floor is often adopted [17
]. In addition, when the inside of an elevator or stairwell shaft is pressurized, the pressure difference across the elevator doors on the first floor decreases while one on each floor of the neutral pressure level or above increases [18
2.2. Previous Studies
] used field measurements and computer simulations to identify various pressure profiles under which the stack effect occurs according to the outdoor temperature, partitioning schemes, and operation conditions of the ventilation systems. Tamura developed a stack effect evaluation index called the thermal draft coefficient (TDC). The TDC indicates the level of air-tightness of the building envelope relative to the interior divisions. In that study, various architectural and mechanical measures were suggested to reduce the stack effect. While Tamura and Wilson originally applied the TDC concept to a whole building, Hayakaya et al. [6
] applied the TDC to individual floors. Jo et al. [7
] evaluated the characteristics of pressure distributions in high-rise residential buildings according to interior partitioning and elevator zoning schemes, and suggested a separation method by installing so-called “air lock doors” between the elevator doors and entrances to the residential units to reduce the pressure differences across those doors. Lstiburek [8
] suggested the inclusion of air barriers to control infiltration and exfiltration, thereby maintaining the air-tightness of the interior spaces and the building as a whole. Based on the information and the design guidelines suggested by these studies to reduce the stack effect, most high-rise buildings recently built in Korea have been designed to include various architectural measures such as air-tightened building envelopes, revolving doors for the main entrance, air-lock doors for elevator halls, dedicated elevators for underground parking lots, and vertical zoning of elevator shafts.
These architectural measures have practically solved stack effect problems in high-rise residential buildings, in which numerous walls separate the housing units. However, because commercial buildings have many open-plan office spaces, the stack effect in these buildings has not been effectively reduced by architectural measures alone. Therefore, other measures to reduce the stack effect in high-rise commercial buildings have been developed and applied to several buildings. These measures include natural or mechanical cooling of elevator shafts and mechanical pressurization of elevator shafts and office spaces. Yu et al. [9
] suggested that elevator shafts should be located in the perimeter zone to lower the air temperature within elevator shafts. In the study, stack effects were measured and compared between an elevator shaft located in the core zone and another one located in the perimeter zone with a glass wall exposed to the outdoors. The study found that the perimeter shaft had a much weaker stack effect owing to the lower air temperature within the shaft, resulting in a pressure difference across the elevator door in the perimeter shaft that is about half that of the core shaft. Lee et al. [20
] conducted computer simulations and measurements of a building with multiple elevator shafts, located in Seoul, to investigate the effectiveness of mechanically cooling the elevator shafts with cold OA to reduce the pressure difference across elevator doors. In the computer simulations, the pressure difference was reduced by about 27% by cooling the elevator shafts from 22 °C to 12 °C; field measurements showed a reduction of about 25% in air velocity through the elevator door on the first floor and by about 10% on the upper floors when the elevator shafts were cooled. However, this method requires extra building space for fan systems and ductwork, which should be prepared from the design stages, and also incurs extra costs for the mechanical systems. Therefore, this method may not be a practical solution for existing high-rise buildings if there is not enough space to accommodate the added mechanical systems and ductwork.
Some notable studies [3
] mostly related to mechanical measures to reduce stack effect were conducted with various operation schemes using the HVAC systems to pressurize building spaces. Tamura and Wilson [11
] used the ventilation system to reduce the air pressure in the upper zone and increase the pressure in the lower zone to reduce infiltration through the building skins. As a result, the operation could reduce infiltrated air volume by reducing the pressure differences across the building skins, but the pressure differences across the vertical shafts, including elevator doors and stairwell doors, increased.
On the other hand, Tamblyn [12
] suggested increasing air pressure in the upper zone and decreasing pressure in the lower zone in order to reduce the pressure difference across the elevator shaft. Tamblyn emphasized the importance of airtightness of the building envelope to suppress infiltration, which might increase due to the lowered interior air pressure at the lower zone.
In the research project conducted by ASHRAE [13
] for an existing high-rise building, the upper zone and mid zone were pressurized and the lower zone excluding the first floor for the lobby was depressurized. The results showed that the pressure differences across the elevator doors for the upper zone, mid zone, and the lobby floor were reduced. However, it was found that the pressure differences across the elevator doors for the lower zone exceeded 25 Pa, which was known as the maximum pressure difference that does not cause the malfunction of the elevator doors. The research suggested that the mechanical balancing of air pressures for a limited number of floors at the top and bottom would be effective for buildings with loose airtight skins and buildings of more than 180 m in height.
From these previous studies, it was revealed that the depressurization of the lower zone caused high pressure differences across the elevator doors and building envelope for the lower zone. Therefore, pressurization of the upper zone only was adopted and tested in this study. Then, this study focused on finding the optimum vertical floor ranges and volumes of air for pressurization.