Hospital HVAC retrofit uses external facade return air ducts
Limited indoor space wasn't about to thwart Saint Thomas West Hospital's efforts to retrofit its HVAC exhaust air system for more cost-cutting energy recovery, preheating increased outdoor air and enhanced indoor air quality (IAQ).
Instead of space-consuming conventional indoor HVAC ductwork, the recent retrofit created return air distribution through ducts constructed of acoustical panels on the exterior of the Nashville, Tenn.-based hospital, according to a release. What now appears as a seven-story-high architectural accoutrement on three sides of Saint Thomas' 43-year-old, seven-story K Tower exterior, is actually three HVAC exhaust air distribution risers to new rooftop energy recovery ventilators (ERV), according to the release from SEMCO, maker of the acoustical panels that cover the units.
St. Louis-based Ascension Health, the nation's largest not-for-profit hospital network and Saint Thomas' parent organization, considers environmental stewardship as important as its health ministry. The estimated savings for recovering exhaust air energy in the ERVs promises to save the hospital significant annual energy costs with a short payback, said Don King, CEO of Saint Thomas West and Saint Thomas Midtown, both which are part of Saint Thomas Health's nine-location network.
Mechanical contractor Nashville Machine Co., project architecture firm Freeman White and consulting engineer TME used acoustical panels manufactured by SEMCO. The trio innovated the external acoustical panel ductwork plan, along with fabricating a support mount system that's completely hidden from the building facade.
External conventional sheet metal ductwork and visible support brackets would have detracted from the building.
"We didn't want it to look like something bolted onto the side of the building," King said.
The HVAC project was part of a four-year, multi-phase $95 million renovation executed throughout the 2.1 million-square-foot campus of the 550-bed Saint Thomas, which has made the Top 100 Hospital list the last 16 years of both Truven Health Analytics and Modern Healthcare.
Besides environmental stewardship and energy savings, the HVAC retrofit brings the circa 1973 building into current ASHRAE Standard 170 "Ventilation for Health Care Facilities," enhances indoor air quality (IAQ) for patients and employees, and helps assure future high hospital industry ratings.
Previously, return air was simply exhausted outdoors, which was customary in 1970s-style building HVAC design strategies. Now the three 4-inch-thick, 5 x 5-foot-rectangular, R-16 insulated, seven-story-high acoustic panel risers, each deliver 45,000-cfm of return air from the wings to their respective single-wheel desiccant ERV to precondition outdoor air.
While supply air continues to be delivered via an existing chase inside the building, the acoustical panel riser is divided internally into two 2.5-square-foot sections that deliver general exhaust and isolation exhaust separately. Nashville Machine pressure tested both passageways to assure no future contamination occurs between isolation and general exhaust. The panels' airtight tongue-and-groove design eliminated any air leak repairs commonly required after conventional ductwork fabrication and installation. Three isolation fans by Twin City Fan were also specified.
The most critical factor in choosing panels over sheet metal ductwork was aesthetics. The panels' custom factory powder-coated sandalwood color matches the building's exterior and blends into the facade's exterior. The panels also have custom factory-designed 12.6-foot lengths, that when combined with a six-inch wide metal support band, matches up with the building's pre-cast concrete seams that are at 13-foot increments. Thus the panel and building seams match and appear continuous.
While the risers only service floors two through seven as they ascend to their respective rooftop ERV plenum, Nashville Machine fabricated an awning that gives a finished, complementary appearance at the bottom of each riser at eight feet above exit doorways. SEMCO, which also manufactures custom ductwork, chilled beams and ERV, active desiccant and dedicated outdoor air systems, supplied the awning metal with a matching powder coated finish.
Custom-designed suspension system
The riser suspension system was custom-designed by Nashville Machine engineers to remain unseen and allow for seismic and wind loads. It uses hidden wedge anchors drilled into the precast concrete. Connecting to the wedge anchors are unseen 5-inch by 5-inch tubular steel stubs incorporated into the building-side of the panels. The result is a riser with no visible support structure.
After recovering heat and moisture, if needed in drier winter time operation, the ERVs' return air is exhausted into another acoustical panel plenum that travels 77-feet in the opposite direction from of an additional 90-foot-long extended plenum that collects outdoor air on the opposite side of the tower for ERV conditioning. This separation of exhaust and outdoor air prevents cross contamination.
Hughes sees exterior-mounted acoustical panels ductwork as the wave of the future for older hospital buildings with space restraints for returning air to ERVs. "This is a great way to bring buildings into compliance of updated codes," he said.
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