Is There a Future for Underfloor Air?

-- Consulting-Specifying Engineer, 1/1/2001

While high-technology facilities have long pioneered the use of raised-floor systems, there is scant agreement among engineers that underfloor air plenums are the next logical step.

The rising heat-load densities of computer and telecommunications equipment-and the need for increased processing speed in a smaller box-is driving the use of underfloor air technologies, says Marc Anderson, P.E., of Syska & Hennessy. "We expect the telecom industry, which traditionally uses free-discharge or overhead ducted systems, to begin using raised floors, [because] the limits of traditional air-delivery methods will soon be reached," he observes.

On the other hand, underfloor systems help deliver what high-tech firms desire: increased airflow, colder discharge temperatures and alternative cooling strategies-such as an increased room-temperature gradient from floor to ceiling and a greater delta T across the coil. Also, in computer rooms where heat dissipation densities are high and relatively constant-or where computer equipment takes in cooling air from below-air from the access floor may work best, adds Calvin Witt, P.E., senior mechanical engineer for Roanoke, Va.-based Hayes, Seay, Mattern & Mattern.

Still, the barriers are significant, say engineers, and include occupant comfort-and cost.

"The prime barrier for most of our clients is the first cost of the raised-floor system and the associated appurtenances: structural equipment stands, grounding, underfloor smoke detectors and plenum-rated cable," says Anderson.

"The industry has introduced special fan-powered diffusers and even systems furniture with individual fans, ducting, integral air outlets and under-desk radiant panel heaters," adds Witt. "However, we believe that projects are rare where the cost of this additional flexibility is justified."

"Designs that employ [those] special features built into systems furniture require extensive design coordination, which is often complicated when the furniture is procured under a separate contract," adds CUH2A's Steven R. Rafferzeder, P.E.

When used, however, the engineers warn of recurring design issues:

• Plenum height. Minimum access-floor depths-usually two feet or more-are critical for ensuring good airflow through the perforated tiles while accommodating additional cabling and piping, says Witt.

• Airflow restrictions. Designers must take care not to choke off underfloor airflow, says Anderson. Underfloor restrictions-piping, conduit and cable-should be placed away from the discharge of precision AC units and preferably routed along the perimeter.

• Temperature and humidity. "Design issues that require special attention include thermal decay at increased distances from the air-handling unit, perimeter heating, and condensation prevention with higher supply-air temperature," adds Witt.

• Good seals. "Other criteria include sealing the plenum airtight," says Anderson. "This is especially true for underfloor gaseous fire-suppression systems, which depend on the ability of pressurizing the underfloor space."

Flexibility is the greatest benefit of these systems, say designers. When done well, underfloor-air-plenum, raised-floor systems pay for themselves quickly as compared to ducted systems, adds Anderson. "It only takes one major change in the equipment layout on the floor to realize this."