Fabric Duct Promotes Even Underfloor Air Distribution in University Library Addition

March 1, 2012
Use of fabric-ducted zones prevents thermal decay

In September 2010, Seattle University formally dedicated Lemieux Library and McGoldrick Learning Commons. Representing the largest investment in academics and scholarship in the school's 120-year history, the $55 million project involved the gutting of the original 92,700-sq-ft Lemieux Library and the addition of a three-story, 33,000-sq-ft teaching, learning, and research center.

"I'm the service tech who gets around 10 hot/cold calls per day on campus, but rarely is there a call about the addition's temperature," Patrick Baldwin-McCurdy, the university's lead building-control technician, said. "I don't feel a temperature difference anywhere in that new addition, which can be attributed to the design."

The design team chose a raised-access-floor design with underfloor air distribution (UFAD).

"We recommended the raised-floor concept because its efficiency would help with LEED (Leadership in Energy and Environmental Design) credits, plus the building design featured 18-ft floor-to-floor spaces that would be difficult to heat efficiently," Mark Stavig, PE, principal, CDi Engineers, said.

Typically, with UFAD, a pressurized plenum disperses heated or conditioned air evenly up through floor vents into a space.

Plenum pressurization and thermal decay in perimeter and high-heating- and cooling-load areas near windows are minimized with 24 runs of UnderFloorSox (UFSox) fabric ductwork, manufactured by DuctSox Corp., per floor. Non-vented lengths distribute air to vented lengths incorporating a permeability and linear-orifice design factory-engineered for the project.

"We knew thermal decay at the perimeters is a potential problem, but the fabric duct allowed us to put conditioned air anywhere we wanted in the UFAD system," Stavig said.

While a conventional overhead system typically supplies 55°F air and cools from the top of a space to the bottom, a UFAD system supplies 65°F air and uses air displacement to cool the bottom 5 ft of the addition’s 18-ft-high spaces. CDi Engineers determined that a UFAD system would earn nine LEED credits and, compared with a conventional overhead system, result in 32-percent operational-cost savings and hundreds of thousands of dollars in energy-cost savings over its life.

With the various sizes and types of rooms, such as classrooms and high-heat-load media suites, in the addition, the combination of zoned UFSox ductwork supplied by variable-air-volume (VAV) boxes offers the capability of zoned individual temperature control. Each zone has its own temperature sensor, which is mounted 5 ft high and monitored by the university’s Delta Controls building-automation system. A more conventional alternative of using 100 or more fan-powered boxes throughout the UFAD plenums was considered too maintenance-intensive by the university’s facilities people.

Solving Challenges
The architect’s use of stone flooring at a feature entrance presented a challenge, which CDi Engineers met with perimeter placement of slot floor diffusers by Titus that have alternating supplies of UFAD air diffusion for cooling and hot-water coils for heating.

The open floor plan between two of the addition’s floors and the original building presented another challenge, one regarding the new UFAD system coexisting with a conventional overhead system. CDi Engineers used computational-fluid-dynamics modeling software from TASS Americas to help determine the best placement of UFAD and overhead diffusers near the two buildings’ common areas.

The UFAD system is very flexible because of its many zones. Floor reconfigurations involve only the rerouting of underfloor fabric duct and the repositioning of diffusers. Heating can be increased with the addition of an electric heater to any zone’s VAV box.

CDi Engineers also used chilled beams from TROX USA Inc., which radiate cooling down into the personal-computing area. Combined with reduced floor-vent airflow, this creates a comfortable space.

The existing building’s 100,000-cfm HVAC system uses a Multistack chiller for cooling. The chiller system’s tonnage had been increased incrementally from 200 tons to 650 tons the previous few years. Tonnage was added easily through Multistack’s modular concept to accommodate additional cooling loads.

Two 20,000-cfm rooftop air handlers from AAON provide supply air through metal duct chases that connect with each floor’s multiple zones. Other equipment includes 2-MBtuh Lochinvar boilers and Bell & Gossett pumps.

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