©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 8, August 2012.
By Tim Gasper, P.E., Member ASHRAE
About the Author
Tim Gasper, P.E., is a solutions engineer at Brady Trane in Raleigh, N.C.
Hamstrung for years by budget issues that did not allow the North Carolina Museum of Art (NCMA) to spend the money it needed to maintain the mechanical system, in 2005, the museum experienced variances in humidity of 30% in the winter and up to 60% in the summer. This was hindering the ability to host many national traveling art exhibits. After careful review of the facilities, it was determined that ASHRAE Class AA environmental conditions could be achieved through an HVAC system overhaul. An added benefit of the upgrade would be energy savings totaling 57% or more.
NCMA was constructed in 1982 with a gross square footage of 171,870 divided among four levels. The facility includes offices, exhibit space and restaurant/catering operations. Open to the public Tuesday through Sunday and occupied seven days a week, the museum averages 25,000 visitors per month with a peak daily occupancy at approximately 2,500 visitors and 150 staff.
Existing Systems and Equipment
The existing system consisted of four constant-volume, chilled-water air-handling units (AHUs) with hot water reheat boxes, each including a steam humidifier. A central utility plant that included two R-11 centrifugal chillers manufactured in 1979 and two constant speed blow-through cooling towers supplied chilled water, hot water and steam. Serving the museum humidifiers, steam-to-hot water heat exchangers and domestic hot water system were two 600 hp (5886 kW) fire-tube boilers operating at 110 psig (759 kPa). The museum’s annual energy costs were $6/ft2 ($64.58/m2) at approximately 300 kBtu/ft2 (11 357 kJ/m2).
Getting the Outside Air Right
Controlling the outside air was the first consideration. Raleigh, N.C., has a subtropical climate with winter lows averaging 35°F (2°C), and summer highs averaging around 90°F (32°C). To manage the amount of outside air entering the facility, a dual-mode, dedicated outside air unit was installed to pretreat, measure and control the airflow and dew point of the outside air. The unit is set to maintain the building at a slightly positive pressure. During unoccupied hours, the airflow is reduced to account for the reduction in exhaust air from the building since a number of exhaust fans are cycled off at the end of the day.
Using a single unit for all outside air pretreatment greatly simplified the museum’s climate control system. Using hot water pre-heat coils, chilled water cooling coils and a high-pressure, high-purity, cold-fog humidification system, the pretreated air is then distributed to the main air-handling units at a constant 52°F (11°C) dew point. Since the museum’s temperature requirements are 70°F (21°C) in all zones, this reduced the number of humidifiers from 92 distributed steam humidifiers down to one centralized cold-fog humidifier. This also eliminated the need for boiler system steam for humidification. The addition of a single pretreated outside air system eliminated the need for untreated outside air mixing at each of the air-handling units and virtually eliminated the need for individual zone reheat in the summer months. The air-handling units now mainly condition space-sensible heat only.
Paying for the Improvements
One of the museum’s goals was to host more high profile traveling exhibits. To meet the strict environmental requirements that many traveling exhibitions require, the museum needed to upgrade the mechanical systems. Since the museum’s energy and operating costs were relatively high, the recently enacted North Carolina performance contracting legislation and process was used to fund and deliver the project. Performance contracting allowed government-owned facility operators to finance and pay for renovations and improvements based on guaranteed future utility and operational savings.
Citation: ASHRAE Journal, vol. 54, no. 8, August 2012
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