[ Page 5 of 6 ]

By Barbara Knecht

“Because air is delivered at a very low velocity, about 40 feet per minute, and at a very low humidity, about 52 degrees dew point, you cannot feel it flowing out of these diffusers, even when you are sitting on the bench with bare legs,” explains Nall. “There is enough diffuser area, however, to provide ventilation air for 600 people and to dehumidify the space.” More diffusers supply dehumidified air at the door opening, to prevent incoming humid air from causing condensation on the cool floor.

Radiant heating and cooling will not work in carpeted spaces, nor does it give the occupants any chance to make individual adjustments to account for differing preferences. In Europe, these problems have been mitigated by raised-floor air distribution, which, like radiant systems, delivers heating and cooling right to individuals instead of over their heads. In Pittsburgh, the Alcoa headquarters uses such a system. Diffusers are installed in relocatable floor tiles. They deliver air in a swirling pattern that mixes quickly with the surrounding air rather than blowing directly on the occupant. The hot air created by office equipment, the sunlit desk, and the occupant rises into the unoccupied strata to the return-air duct. The occupant has individual temperature controls to precisely modulate the local environment.

Cool tools for insuring comfort

Sophisticated computational modeling tools (see sidebar, page 170) can now describe the heterogeneous temperature distribution in a space to design these systems for maximum energy efficiency and maximum human comfort. Flack + Kurtz’s Nall explains what can be studied with a computational fluid dynamics [CFD] analysis, comparing the airflows for underfloor air distribution with conventional overhead-mixing air distribution. “Overhead distribution shows relatively uniform temperature across the height of the space. Underfloor air distribution shows the temperature variation from low to high in the space, commonly known as stratification. It shows how the heat rises off the computers and off the people, in thermal plumes. In this way, part of the heat is carried out of the occupied zone before it has an opportunity to affect comfort.”

 

Photography: © Steve Hall/hedrich-blessing (top); computer renderings by flack + kurtz

Conventional air-handling systems in offices with glass curtain walls are forced to deal with competing thermal needs simultaneously in the winter months. Near the curtain wall is that zone of radiating cold air, and in the center of the building is a zone radiating hot air from occupants and equipment. Losing that cold zone is good for comfort and good for energy consumption. Another concept, well-tested in Europe, is under construction in South Boston for the giant Canadian financial-services company Manulife Financial. Manufactured by the Italian company Permasteelisa, the double-skin glass curtain wall has an 8-inch insulating cavity. The exterior wall is just one of a number of measures Manulife and its architect, the Chicago office of Skidmore, Owings & Merrill (SOM), are taking to achieve a LEED rating from the U.S. Green Building Council. Designed specifically for energy conservation, the wall provides a marked increase in comfort for occupants near the exterior wall as well as increased acoustic insulation, a particularly desirable quality for its location near Logan Airport.

 

Computer renderings by Flack + Kurtz