Is There a Future for Underfloor Air?
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.
By Consulting Specifying Engineer Staff
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."