Energy Recovery Retrofit at Nashville Hospital
Many hospitals throughout the United States face the same indoor air quality (IAQ) challenges that Ascension Health’s Saint Thomas Hospital in Nashville, TN was facing. Prior to IAQ codes in the 1980s, healthcare buildings exhausted vast amounts of indoor air in order to bring in more fresh outdoor air to fight challenges such as hospital acquired infections (HAI). Now a trend is to recover the cooled or heated exhaust air and use it to cool or heat the tremendous amounts of outdoor air hospitals are bringing in for patient health reasons.
However these HVAC retrofits are sometimes impossible because of the disruption of hospital operations or there just isn’t room to install the necessary ductwork. This article contributed by SEMCO, a HVAC products and services company, examines this type of scenario at St. Thomas Hospital — and the solution employed.
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 retrofit created return air distribution through ducts constructed of acoustical panels on the exterior of the Nashville, TN based hospital. What now appears as an 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).
St. Louis, MO-based Ascension Health, the parent organization of St. Thomas, considers environmental stewardship as important as its health ministry. Therefore the estimated savings for recovering exhaust air energy in the ERVs promises to save the hospital significant annual energy costs with a short payback, according to Don King, CEO, Saint Thomas West and Saint Thomas Midtown, both which are part of Saint Thomas Health’s nine location network.
The project wouldn’t have ensued however, if not for the collaborative brainstorming of mechanical contractor Nashville Machine Co., Nashville, TN; project architecture firm, Freeman White, Brentwood, TN; and consulting engineer TME, St. Louis, MO. Using acoustical panels manufactured by SEMCO LLC of Columbia, MO, the trio innovated the external acoustical panel ductwork plan along with fabricating a support mount system that is 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,” said King.
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”.
Previously, return air was simply exhausted outdoors, which was customary in 1970’s style building HVAC design strategies. Now the three 4″ thick, 5 ‘x 5′ 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 of Minneapolis, MN, were also specified.
The outdoor air and heat recovery, along with the existing variable air volume system supplied by three chillers manufactured by the York division of Johnson Controls, Inc. (JCI) of Milwaukee, WI; one chiller by Carrier division of United Technologies of Syracuse, NY; and three boilers by Cleaver-Brooks of Lebanon, PA are all controlled by the facility’s JCI Metasys building management system (BMS).
The most critical factor in choosing panels over sheet metal ductwork was aesthetics.
The panels’ custom, factory powder-coated, sandalwood color matches the hospital exterior and blends into the facade’s exterior. The panels also have custom factory-designed 12.6′ lengths, that when combined with a 6″ wide metal support band, matches up with the building’s pre-cast concrete seams that are at 13′ increments. Thus the panel and building seams match and appear continuous. “This is the result of close collaboration between manufacturer and our installation team,” said Travis Hughes, project manager, Nashville Machine, which has experience using acoustical panels as plenums and ductwork on several past projects.
While the risers only service floors two through seven as they ascend up 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, and Nashville Machine fabricated it in its sheet metal shop.
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″ x 5″ tubular steel stubs that are incorporated into the building side of the panels. The result is a riser with no visible support structure.
Collecting the exhaust air from each wing riser is a 6’w x 12’h x 63′ l plenum also constructed of acoustical panels. The plenum aesthetically appears as a rooftop architectural facade and hides the three packaged ERV units from street-level view.
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′ 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.
The alternative to the Saint Thomas project extended plenum was most likely sheet metal duct with visible welded supports anchored to the exterior of the duct riser and the building wall. The sheet metal would have needed painting and insulation, plus SMACNA code-mandated interior supports, because the interior dimensions of any ductwork beyond 4′ x 4′ require structural reinforcement.
The ductwork would have required a shop-painted exterior coating needing job site touch-up because of the inevitable event of nicking the surface during shipping and installation. Additionally, factory powder-coating is superior in longevity, durability and labor costs compared to site-applied primers and coatings, according to Hughes. “It would have cost the project approximately 20% more if insulated sheet metal duct was used instead of the panels even though we have our own sheet metal shop,” he said.
Furthermore, when ductwork dimensions approach 5′ x 5′ and larger, the economic law of diminishing returns kicks in. Worker handling/safety, structural steel costs, field installation time, and other job costs all increase as sheet metal duct becomes larger. With panels however, all of those concerns remain more constant as their easy modular assembly does not change as drastically in proportion to the size of the duct.
Once the panels arrived at Nashville Machine, they were assembled into 12.6′ long sections, shipped to the job site and hoisted into place using rental cranes ranging from 460-ton for the top floors, down to 120-ton cranes for lower floors.
With the trend toward 100% outdoor air in the healthcare industry, 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,” said Hughes.
Acoustical Panels For Noise Abatement
Acoustical panels live up to their name and do provide noise abatement features, although these functioned as plenums and ductwork in the Saint Thomas Hospital project. Equipment and airflow noise wasn’t a prime consideration for the project, because the mechanical ERV equipment was remotely located on the rooftop, If noise was a concern however, the panels for the ERV plenums, as well as the externally mounted exhaust air ducts could have been constructed with a perforated inner wall that envelopes interior sound absorbing materials. Insertion loss abatements (a reduction of sound and reverberation as it travels down the ductwork) vary by size and length of the ductwork. Co-efficiency of performance levels can be custom built into a panel system, depending on the degree of noise reduction the end-user requires.
Article courtesy of Facility Executive.