Displacement Ventilation

Displacement ventilation is the name given to a number of techniques for letting natural forces distribute air in spaces by having cooler air displace warmer air in a space, much as filling a bathtub with cold water will eventually displace hot water flowing out the drain. Displacement ventilation has a number of virtues for the open-plan office environment, or cubicle farm. It uses far less energy than fan-forced ventilation (the ubiquitous overhead air diffusers that put a cold draft right on top of you), it moves contaminants out of spaces because air is not recirculated in spaces, and it allows people to adjust their own temperature and airflows inside of cubicles from low-velocity air diffusers located in floor tiles.

Displacement ventilation is one name given to underfloor air distribution (UFAD) systems that are beginning to see widespread use in the US. An update of the older computer-room floor, UFAD systems also accommodate electrical wiring and data cabling, enabling workstations to be easily moved as companies and agencies redistribute people in an office. The difference is that the moveable floor, a concrete tile about two-feet

This nine-panel mock-up shows the components of an access flooring system: the access floor with modular wiring, both a variable-air-volume system (VAV) and a swirl diffuser for underfloor air, and one-to-one-fit modular carpet tile.

square weighing 40 pounds, is typically 14 inches to 16 inches above the base floor, versus the older-style flooring at 6 inches, to allow for easier airflow under the floor.

Displacement ventilation works on the principle that more-dense cooler air falls and less-dense warmer air rises, something we all know but seldom appreciate. Typically, displacement air is introduced into a space at 62°F or 63°F, 7°F or 8°F warmer than the traditional 55°F of overhead diffusers, which saves energy in summer because the outside air doesn't need as much cooling to be usable. In some climates such as the maritime Pacific Northwest, for most of the year, air is cooler than 62°F during the day, so there is no need for mechanical air conditioning to get cool air into a space. Engineers use what's called an "economizer cycle" to bring in cool outside air whenever possible. Since most office buildings require cooling year-round, owing to the density of people and all the electrical equipment used, this is an effective strategy. As the warmer air rises, it exits at return-air grilles typically located high on a wall. In effect, with a 9-foot or 10-foot ceiling, displacement ventilation allows cooling only of the "occupied zone," i.e., up to about 6.5 feet, saving energy on cooling the entire volume of a room, as a traditional overhead ventilation system would do.

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