Calif. library's HVAC design earns rave reviews for IAQ
Photos courtesy of Gregory Cortez
The new $47 million Mitchell Park Library & Community Center recently opened to rave reviews for its indoor air quality (IAQ) and controllable air comfort designed and installed by the HVAC engineers and contractors.
The 40,000-square-foot, Palo Alto, Calif.-based library and its accompanying 16,000-square-foot community center ranks first in air comfort among the city's nearly 100 facilities, said Debra Jacobs, project engineer for the Palo Alto public works department. "This is by far the best air comfort of any city-owned building we have."
She reports first-hand experiences since relocating her office into the facility during the final six month start-up and commissioning, according to a press release.
The library is one of California's most sustainable, illustrated by the project's Leadership in Energy and Environmental Design (LEED) platinum certification. The center also won last year's Center of Excellence in the "Facility Design--Community Center/Mixed Use" category awarded by the California Parks and Recreation Society (CPRS).
The design includes cutting-edge technologies including solar electric panels providing 25 percent of the library's electricity, solar hot water panels, night sky radiant cooling with cold water storage, water reclamation, green living roofs and raised floors with underfloor air distribution (UFAD) fabric ductwork for uniform air dispersion.
One of the two-story library's most innovative and sustainable HVAC designs is the UFAD. It provides energy savings, because the HVAC supply systems use less energy to maintain set point temperatures. UFAD also provides a more even temperature control in the three to six-feet-high occupied zone versus the impractical and less efficient cooling methodology of conventional overhead ductwork, especially with Mitchell Park's high ceilings, designers said.
UFAD offers advantages of positioning floor diffusers where air is most needed. The raised floor also provides a space to conceal, add or reroute electronic cables, piping and other utilities for any future data equipment upgrades or room reconfigurations.
"Compared to UFAD, recessed or hung ceiling ductwork would be more difficult to maintain," Jacobs said. "Cooling air from above the occupied zone is less efficient and the ductwork would also reduce the aesthetic floor-to-ceiling window views of the adjacent park and sports fields."
UFAD has disadvantages, but the design solved them. Supply air temperature differentials of as much as 4 degrees are common in UFAD designs, especially in buildings with window-dominated walls, such as Mitchell Park. The thermal swings are due to low-set point operating discharge temperatures required to meet the perimeter high solar loads, which in turn can create excessively cool interior area temperatures.
Slab and floor temperatures tend to increase the supply air temperature when it travels more than 50 feet, especially during early morning ramp-up periods, designer Tunde Munz of Guttmann & Blaevoet said.
In Mitchell Park's design, the design solution delivered air strategically through the custom linear vents to facilitate a better distribution via textile duct with an adjustable air releasing end cap, Munz said.
Reducing solar gain helps minimize UFAD perimeter thermal decay. Therefore the design team specified highly-insulated Solarban 70XL glazing by PPG Industries, Pittsburgh. The glass has a U-factor of 0.439 and a shading coefficient of 0.359. The building also employs sun shades custom-designed and manufactured for each exposure. The walls and roof have a U-value of 0.05.
Supplying the UFAD system, constructed of flooring panels, are two 91-ton chillers and two 1,250-MBh boilers that provide tempered water to air handlers and mixed into the plenum using the fabric. Supply airflow is dispersed into the occupied space through floor mounted adjustable diffusers or through floor mounted displacement diffusers (interior) or linear diffusers (perimeter). Fan-powered variable air volume (VAV) reheat boxes draw airflow from the plenum and connect to the perimeter linear grills ensuring quick adjustment for additional perimeter load for cooling or heat.
The library's cutting-edge IAQ monitor and control includes air handlers that supply economizer air when outside conditions are optimal. Additional house exhaust fans are used for cooling with mechanical ventilation. CO2 detectors also sense contaminants from large gatherings and activate natural ventilation.
The building management system (BMS) monitors and controls the indoor environments.
"This project has more than 350 points (measurement and control) compared to the 50 points at our City Hall, which is an older building with similar space volume and the same BMS network," Jacobs said.
Besides IAQ, Jacobs attributes the air conditioning system's low decibel operational sound to the UFAD, fabric duct distribution and its linear dispersion which all contribute to airflow noise suppression that's critical in library environments.
In addition to LEED platinum, the facility is recognized by the Santa Clara Valley Urban Runoff Pollution Prevention Program for "Site Design and Low Impact Development" and by the California Library Association "Public Relations Excellence Award." It was also a film location for a video depicting the environmental award the city won from Acterra.
In previous buildings on the site, occupants walked through zones and experienced noticeable temperature differentials. The Mitchell Park Library & Community Center has even temperatures throughout, which is a credit to the efficient building envelope, pinpoint BMS monitoring/control, the UFAD and the underfloor air dispersion and appropriate temperature zoning.
"As I walk through the building the temperature is amazingly consistent, even when comparing the west and east portions of the building in the late afternoon's setting sun," Jacobs said.
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