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  • A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 183 4

    ASHRAE TECHNOLOGY AWARD CASE STUDIES 20

    18

    The San Francisco Museum of Modern Art (SFMOMA) consists of a 10-story addition and a renovated existing five-story building. Pictured is the view from the Yerba Buena Gardens.

    PHOTO CREDIT JON MCNEAL, ©SNØHETTA.JPG

    BY STEVEN T. TAYLOR, P.E. FELLOW ASHRAE; DAVID HEINZERLING, P.E. MEMBER ASHRAE

    A New Approach to Museum HVAC Design

    This article was published in ASHRAE Journal, August 2018. Copyright 2018 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

    This�file�is�licensed�to�Steven�Taylor�(staylor@taylor-engineering.com).�Copyright�ASHRAE�2018.

  • A U G U S T 2 0 18 a s h r a e . o r g A S H R A E J O U R N A L 3 5

    FIRST PLACE | 2018 ASHRAE TECHNOLOGY AWARD CASE STUDIES

    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.

    Building at a Glance San Francisco Museum of Modern Art (SFMOMA) Location: San Francisco

    Owner: San Francisco Museum of Modern Art

    Principal Use: Museum

    Includes: Art galleries, theater, administrative offices, library, café, event space, retail shop, wood shop, art conservation studios, cafeteria, and cold and cool storage rooms.

    Employees/Occupants: 470 staff and 1.2 million visitors in first year

    Gross Square Footage: 486,000

    Conditioned Space Square Footage: 350,000

    Substantial Completion/Occupancy: June 2016

    Occupancy: 100%

    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, cafete- ria, 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 condi-

    tions for artwork and still maintain-

    ing 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 con-

    trol allowed the design team to con-

    sider 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 enve-

    lope, 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 condi-

    tion: saturated air with a dew-

    point temperature just above that

    at the lowest acceptable space

    temperature and lowest accept-

    able relatively humidity, in our

    case 70°F (21.1°C) and 45% relative

    This�file�is�licensed�to�Steven�Taylor�(staylor@taylor-engineering.com).�Copyright�ASHRAE�2018.

  • A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 183 6

    20 18 ASHRAE TECHNOLOGY AWARD CASE STUDIES

    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 pro-

    vided 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 wa-

    ter 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.

    • The blow-through arrangement of the SFDD system results in nearly saturated cold duct supply air when

    mechanical cooling is active. This resulted in over-satu-

    ration and condensation on the supply air ducts leaving

    the cold deck discharge plenum due to the pressure

    drop as air accelerated into the supply air mains. This

    65

    60

    55

    50

    45

    40

    35

    Re lat

    ive H

    um idi

    ty (%

    )

    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

    Dehumidification Setpoint (Highest Allowable RH Level)

    Humidification Setpoint (Lowest Allowable RH Level)

    FIGURE 1 Relative humidity seasonal setpoints.

    FIGURE 2 Psychrometric process of centralized humidity control.

    resulted in microbial growth and all cold duct acoustical

    lining had to be removed.

    • Access to the coils, filters, and fans of the field-built air handlers was very poor, requiring the building en-

    gineer to climb over obstructions with ladders to reach

    this equipment. Replacing the 100 hp (75 kW) supply fan

    motors and 40 hp (30 kW) return fan motors bordered

    on impossible.

    • The variable pitch vane-axial fans required annual tear-down and rebuild, made more difficult and expen-

    sive by the poor access.

    • The humidifiers were located in ceiling plenums that were difficult to access for maintenance. They also

    caused condensation in ductwork due to the nearly satu-

    rated supply air when the chillers and cooling coils were

    active, which was most of the time. Some were relocated

    to the hot decks to avoid this problem. Humidity control

    was accordingly very poor.

    These two air-handling systems were gutted and

    replaced with dual-fan/dual-duct (DFDD) systems with

    relief fans and central humidification shown schemati-

    cally in Figure 3. A third DFDD system was installed in

    the expansion building. Together the systems totaled

    350,000 cooling cfm (165 000 L/s) and 123,000 heating

    cfm (58 000 L/s).

    The revised design resolves all the operational prob-

    lems of the existing system and included additional

    features to further improve energy efficiency and tem-

    perature and humidity control:

    This�file�is�licensed�to�Steven�Taylor�(staylor@taylor-engineering.com).�Copyright�ASHRAE�2018.

  • A S H R A E J O U R N A L a s h r a e . o r g A U G U S T 2 0 183 8

    20 18 ASHRAE TECHNOLOGY AWARD CASE STUDIES

    • The use of a DFDD design instead of SFDD resolved the first two issues listed above. DFDD also has lower

    fan energy because with SFDD systems, duct pressure

    is always higher than it needs to be in one of the two

    supply air ducts. DFDD systems can maintain hot and

    cold duct pressure independently with independent

    pressure setpoint reset based on VAV box damper posi-

    tion.

    • Centralized humidity control with direct evapora- tive (adiabatic) humidifiers reduces energy use, first

    costs, and maintenance costs (see next section for

    details).

    • The use of relief fans instead of return fans allowed the layout of the mechanical rooms to be improved,

    resolving the maintenance access issues. Relief fans are

    more flexible because, unlike return fans, they need

    not be in series with the supply fans and can be located

    anywhere in the common return air path. Relief fans in

    this application are also more eff