©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 8, August 2018.
By Steven T. Taylor, P.E. Fellow ASHRAE; David Heinzerling, P.E. Member ASHRAE
About the Authors
Steven T. Taylor, P.E. and David Heinzerling, P.E., are principals at Taylor Engineering in Alameda, Calif. Taylor is a member of SSPC 90.1 and GPC 36. Heinzerling is a member of SSPC 55.
The San Francisco Museum of Modern Art (SFMOMA) consists of a 10-story new addition to a fully renovated existing five-story museum. Program elements for the 486,000 ft2 (45 000 m2) project include art galleries, theater, administrative offices, library, café, event space, retail shop, wood shop, art conservation studios, cafeteria, and cold and cool storage rooms. The entire project is served by an innovative HVAC system that could become a new standard for museums and similar applications.
Museum Environmental Criteria
Museums are traditionally large energy users because of the need to provide tight humidity control. The design team worked closely with SFMOMA conservationists to study various published environmental criteria for museums as well as those from major museums across the country. Through this roundtable process, the team concluded that a seasonally adjusted relative humidity setpoint (Figure 1) could be used while still maintaining acceptable conditions for artwork and still maintaining a Class A rating.1 Concurrent temperature control was specified to be 72.5°F ± 2.5°F (22.5°C ± 1.4°C).
This relaxation in humidity control allowed the design team to consider centralized, rather than zonal, humidification systems. Zonal humidity controls can handle wide variations in humidity loads from people and infiltration, but they cost more, have higher maintenance costs, and are less energy efficient. Centralized humidity control, on the other hand, relies on low zone humidity loads from infiltration, but the relaxed humidity setpoints in Figure 1, along with a tight envelope, allows it to provide acceptable control because the infiltration loads tend to vary in the same way as the humidity setpoints.
The concept behind central humidification is to maintain a nearly constant supply air condition: saturated air with a dew-point temperature just above that at the lowest acceptable space temperature and lowest acceptable relatively humidity, in our case 70°F (21.1°C) and 45% relative humidity, where RH is adjusted based on time of year as discussed above. For zones that are unoccupied with low cooling loads, the resulting space condition is the “Unoccupied” point in Figure 2. For spaces that are fully occupied, the room temperature is allowed to rise to 75°F (23.9°C) and, with the moisture added by people, the resulting condition is the "Fully Occupied" point. Thus, with a single supply air condition, all spaces can be maintained in the required humidity range provided humidity loads from infiltration, especially of cold, dry air, are small. Where they are not expected to be small, e.g., at entries, local humidifiers can be added to augment the centralized system.
Existing System Upgrades
The two air handlers serving the existing museum were single-fan/dual duct (SFDD) systems with return fans and steam humidifiers in the cold duct mains on each floor. Operational problems with the systems included:
The economizer on the SFDD significantly increases heating energy use on the hot deck because the hot water coil entering air temperature is the same as the cold deck supply air temperature. The added outdoor air to the hot deck also increases the humidification load. The economizer had to be disabled even at mild outdoor air conditions, causing the chiller plant to run most of the time.
Figures
Figure 1
Figure 2
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