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By Nadav Malin

Increasing Airflow Makes People Comfortable at Higher Temperatures

One of Yeang's naturally ventilated projects is the 21-story Menara UMNO office building in Penang, Malaysia, completed in 1998. Penang, near the equator, is warm in the summer-an average of 90 degrees Fahrenheit during the day, with humidity levels hovering around 76 percent. Offices have a high internal heat load from lighting, machines, and people. Fortunately, the building is on an open site, which means there is little to interfere with the prevailing breezes.

Yeang positioned the building to "collect" the wind at an oblique angle-generally 30 to 60 degrees away from the direct impact. This angling creates a pressure gradient along the windward walls, increasing wind velocity. Wing walls, small projecting walls placed perpendicular to the major ventilation openings, act as pockets or scoops to collect the wind and further increase airflow in the floor spaces. The building also includes balconies at either end of each floor, with operable doors to allow cross ventilation. Windows opposite the balconies also admit breezes.

There is a limit, however, to how much air movement can be increased in office buildings. "If papers are not flying around, the comfort zone can be extended about two degrees Fahrenheit with extra air motion-using a ceiling or oscillating fan at low speeds, or strategically placed open windows with inlets directed at people," says Subrato Chandra, a researcher at the Florida Solar Energy Center. Windows are good for releasing heat when it is cooler outside than inside, but fans are a better way to create consistent airflow, Chandra says.

A combination of systems

Mixed-mode buildings are more common than naturally ventilated buildings; they are more practical in extreme climates and provide building owners with some backup in case occupants are uncomfortable. It's not easy to find mechanical engineers willing to work with mixed-mode systems, however. Integrating the passive and mechanical air-distribution systems is tricky, and most engineers prefer not to deal with the less-than-predictable variable of occupants who undermine the mechanical system by opening their windows when the equipment is operating.

Many mixed-mode spaces switch from natural to mechanical ventilation on a daily or seasonal basis. Rafael Viñoly's 1.3 million-square-foot David L. Lawrence Convention Center in Pittsburgh, to be completed in 2003, will switch to natural ventilation when outdoor conditions allow it. The large exhibit hall and circulation areas, for example, will be naturally ventilated much of the year, according to Dave Lineman, director of engineering for Burt Hill Kosar Rittleman, who's collaborating with Viñoly on the project. Cool air, conducted via plenums from openings along the river- and the city-end of the building, will be distributed to meeting rooms as well. This type of cooling is especially appropriate when convention displays are installed and dismantled, since little heat is generated during these times and comfort needs are fewer. The natural ventilation is likely to save about 25 percent in energy costs over comparable buildings, Lineman says.

The pros and cons Because it fundamentally affects both building form and mechanical systems, natural ventilation requires extremely close collaboration between architects and engineers. "I can't think of anything, except possibly daylighting, for which design integration is more essential," says Harry Gordon, AIA, with Burt Hill Kosar Rittleman.

Close collaboration with clients, who must recognize that indoor climate conditions will be more variable than in a conventional air-conditioned building, is also needed. In specifying the allowable conditions, it is best not to set absolute limits on acceptable indoor temperatures, McGregor says. "Specify allowable variances instead, in terms of how many hours the space will be above a given temperature," he suggests. "Those terms are more acceptable to clients."