Technical / Extra Edition

Under Floor Ventilation Maintains Comfort for Library Addition

February 20, 2012
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Patrick Baldwin-McCurdy fields daily indoor air temperature complaints from employees and students; however, Seattle University’s lead buildings control technician rarely hears HVAC complaints from the college’s new library addition featuring under floor air distribution (UFAD).

The $55 million project included the construction of a new three-story, 33,000-square-foot teaching, learning, and research center addition and the renovation of the university’s 92,700-square-foot, 45-year-old A.A. Lemieux library. The centerpiece of a successful university capital campaign, the complex is now called The Lemieux Library & McGoldrick Learning Commons. It boasts a Leadership in Energy and Environmental Design (LEED®) Gold certification.

While efficient technology, such as chilled beam technology helped rack up nine LEED HVAC credits, the addition achieves further efficiency plus its tight temperature control tolerances with the new concept of fabric ductwork inside the UFAD called UnderFloorSox™, manufactured by DuctSox Corp., Peosta, Iowa. “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,” said Baldwin-McCurdy, who with Steve DeBruhl, Seattle University’s senior project manager, handles HVAC and other mechanical responsibilities. “I don’t feel a temperature difference anywhere in that new addition, which can be attributed to the design.”

The design team included consulting engineering firm, CDi Engineers, Lynnwood, Wash.; construction manager, David Bonewitz, president, Bonewitz, Seattle; mechanical contractor, McKinstry, Seattle; and architect firms Pfeiffer Partners Architects, Los Angeles; and Mithun, Seattle.

The design team chose a raised access floor design with UFAD, because unlike the six-story original building, the addition has a huge collection of state-of-the-art media electronics. Raised floors hide the needed cabling associated with the 80 desktop and 120 laptop computers, plus a high-definition video studio, 11 multimedia editing stations, and video monitors throughout the space. The anticipated reconfigurations and serviceability required with such a media-rich space are easily accomplished with the moveable raised floor’s panels.

In addition, UFAD offers quietness, even temperature control, and the ability to aesthetically hide support utilities and HVAC ventilation. Typically, UFAD acts as a pressurized plenum that disperses heated or air conditioned air evenly up through floor vents into the space. “We recommended the raised floor concept because its efficiency would help with LEED credits, plus the building design featured 18-foot floor-to-floor spaces that would be difficult to heat efficiently,” said Mark Stavig, P.E., principal, CDi Engineers, who oversaw mechanical system design along with Leslie Jonsson, P.E., LEED AP, a CDi mechanical engineer.

UFAD has resurged recently after engineers have found solutions to reoccurring challenges of plenum pressurization and thermal decay in perimeter and high heat or cooling load areas near windows. Pressurization and thermal decay issues were minimized at Seattle University by CDi’s design calling for 24 fabric duct runs per floor of UnderFloorSox, which is designed specifically for UFAD. The under floor fabric duct runs use a combination of non-vented lengths distributing air to vented lengths that incorporate a permeability and linear orifice design factory-engineered specifically for the project. Manufacturers representative, Air Commodities Inc., Seattle, assisted in specifying the fabric duct to fit the application. “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,” said Stavig, who had specified a half-dozen UFAD projects previously.

While a conventional overhead system typically supplies 55°F and cools from top to bottom, the UFAD supplies 65° and uses air displacement to cool the bottom five feet of the addition’s 18-foot-high areas. CDi’s LEED analysis found that the UFAD’s inherent energy-saving concept would generate nine credits and save the university approximately 32 percent in operational energy costs and a total energy savings of hundreds of thousands of dollars over the UFAD lifecycle.

With the prevalence of various room uses and sizes, such as classrooms and high-heat-load media suites, the combination of zoned under floor fabric duct supplied by VAV boxes offers the addition the capability of individual zoned temperature control. Each zone has its own temperature sensor that’s mounted five-foot-high and monitored by the university’s Delta Controls, Surrey, British Columbia, building automation system. A more conventional alternative of using 100 or more fan-powered boxes throughout the UFAD plenums was considered as too maintenance intensive by Seattle University’s facilities people. Plus, the use of fabric duct helps projects generate LEED credits.

Solving Challenges

One UFAD challenge was the architect’s use of stone flooring at a feature entrance, which CDi solved with perimeter placement of slot floor diffusers by Titus, Richardson, Texas, that have alternating supplies of UFAD air diffusion for cooling and hot water coils for heating.

For CDi Engineers, the open floor plan between two of the addition’s floors and the original building created a potential challenge of the new UFAD system coexisting with a conventional overhead system. CDi used computational fluid dynamics (CFD) modeling software by Tass Americas, Livonia, Mich., to help determine the best placement of UFAD and overhead diffusers located near the two buildings’ common areas.

The library UFAD system is also very flexible because of its many zones. Floor reconfigurations only need to reroute the under floor fabric duct and reposition diffusers. Heating also can be increased by adding an electric heater to any zone’s VAV box.

CDi also used chilled beams by TroxUSA, Cummings, Ga., that radiate cooling down into the personal computing area combined with reduced floor vent airflow to create a comfortable space.

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

Two rooftop 20,000-cfm air handlers by Aaon, Tulsa, Okla., provide supply air through metal duct chases that connect with each floor’s multiple zones. Other equipment included in the project include Lochinvar, Lebanon, Tenn., 2-Mbtuh boilers and Bell & Gossett, Morton Grove, Ill., pumps.

Publication date: 02/20/2012

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