A New Approach to Museum HVAC Design PHOTO CREDIT JON MCNEAL, SNØHETTA.JPG 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. BY STEVEN T. TAYLOR, P.E. FELLOW ASHRAE; DAVID HEINZERLING, P.E. MEMBER ASHRAE 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. 34 ASHRAE JOURNAL ashrae.org AUGUST 2018
FIRST PLACE 2018 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 ft 2 (45 000 m 2 ) 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 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. 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 dewpoint 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 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% AUGUST 2018 ashrae.org ASHRAE JOURNAL 35
Relative Humidity (%) 65 60 55 50 45 40 35 Dehumidification Setpoint (Highest Allowable RH Level) Humidification Setpoint (Lowest Allowable RH Level) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec FIGURE 1 Relative humidity seasonal setpoints. 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. 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-saturation 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 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 engineer 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 expensive 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 saturated 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 schematically 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 problems of the existing system and included additional features to further improve energy efficiency and temperature and humidity control: 36 ASHRAE JOURNAL ashrae.org AUGUST 2018
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 position. Centralized humidity control with direct evaporative (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 efficient than return fans in this low-pressure plenum return application. 2,3 The dual-fan/dual-duct system fully implements sequences from ASHRAE Guideline 36-2018, High-Performance Sequences of Operation for HVAC Systems, including snap-acting dual-duct VAV box logic in non-humiditycontrolled zones, which eliminates simultaneous heating and cooling. Gallery zones with humidity control use a dual-duct mixing logic that minimizes simultaneous heating and cooling while allowing for some zone-level humidity control through different dew-point temperatures in the hot deck (higher) and cold deck (lower). All gallery and high occupancy zones have CO 2 sensors, and all other spaces have occupancy sensors, with demand-controlled ventilation sequences to eliminate energy associated with excessive ventilation. Minimum airflow rate setpoints are the lowest allowed to meet ventilation requirements and air is shut off entirely during unoccupied hours for non-gallery spaces. This dramatically improves efficiency compared to constant volume systems recommended by the ASHRAE Handbook, HVAC Applications. Despite low airflow rates, trend data show temperature and humidity setpoints are consistently maintained (see Figure 9, page 42). The new air handlers have multiple plenum fan arrays, a total of 84 fans, each with its own variable frequency drive and near-zero pressure drop backdraft dampers. Fan arrays have few if any disadvantages compared to the large vane-axial fans they replaced in Expansion of the new SFMOMA. Outside Air Relief Air VSD Relief Fan Economizer Cooling Fan Array Filter Bank Cooling Direct Evap VSD Humidifier Coil Filter Bank Heating Coil Steam Humidifier VSD FIGURE 3 Dual fan/dual duct air handler schematic. Return Air Plenum Cooling Supply Air Duct Heating Supply Air Duct Dual Duct VAV Box the existing building, including reduced space requirements, eliminated sound attenuators, improved redundancy, improved low load efficiency, and much easier future motor and fan replacement. Compared to smaller distributed air handing units, large air-handling units are less expensive, require less space, have lower maintenance costs, and, in this application, are more energy efficient. 4 The cooling towers are very high efficiency (>100 gpm/hp at ASHRAE/IES Standard 90.1 conditions). Heat rejected to the closed condenser water loop (which serves water-cooled kitchen and cold room R.A. PHOTO IWAN BAAN, COURTESY SFMOMA 38 ASHRAE JOURNAL ashrae.org AUGUST 2018
refrigeration systems, water-cooled IT air conditioners, and other process loads) is recovered as preheat for the domestic hot water system. Heating hot water is generated by three 3,000 kbh high efficiency condensing boilers with primary-only variable flow with only two-way valves and oversized heating coils, resulting in high ΔTs and thus low return water temperatures ensuring condensing and high boiler efficiency. The chilled water system is primary-only, variable speed serving two existing 365 ton (1284 kw) variable speed centrifugal chillers a new 100 ton (352 kw) scroll chiller. Centralized Humidity Control with Direct Evaporative Humidifiers The primary design innovation for this project is the humidification design. The direct-evaporative humidifiers (DEHs) on the cold deck humidify outdoor air during cool and cold weather and provide evaporative cooling during warm dry weather. The electric steam humidifiers on the hot deck provide the additional humidification required due to infiltration of outdoor air at museum entries. The energy savings of the DEH in cold weather at high airflow * can be seen in Figure 4, which shows the psychrometric process superimposed on San Francisco weather data. The conventional system (Figure 4a) has the economizer shut off to reduce humidification loads, so both chiller and humidification energy (typically electricity) are required. But the DEH system (Figure 4b) uses warm return air to evaporate the humidification moisture needed and uses no chiller or humidifier energy. Figure 5 shows the performance in warm, dry weather. The conventional system (Figure 5a) uses both chiller and humidification energy while the DEH system (Figure 5b) becomes a direct evaporative cooler to reduce chiller energy and eliminate humidification energy. The annual performance of the DEH versus a conventional system is shown in Figure 6; it lowers or eliminates energy costs when the outdoor air dew-point temperature is less than desired supply air dew-point temperature (about 50 F [10 C]) and has the same costs at higher dew-point temperatures. Alexander Calder s Untitled (1963) on view in the Evelyn and Walter Haas, Jr. Atrium at the new SFMOMA. Energy Usage The project is LEED 3.0 Gold certified, with 43% annual site energy savings compared to the Standard 90.1-2007 baseline (Figure 7) and 33% energy cost savings. The first operational year of the completed project used 34% less electricity/ft 2 and 44% less gas/ft 2 than the original museum did in its last year of operation (2012-2013) as shown in Figure 8. The overall EUI of the building is 70.9 kbtu/ft 2 yr. (224 kwh/m 2 ) compared with 112.5 kbtu/ft 2 yr. (355 kwh/m 2 ) for the original museum, a 37% reduction. Despite having a more energy intense program than the original museum, with new large cold storage rooms and on-site art-restoration laboratories, the new museum uses significantly less energy/ft 2 than the original museum, due in large part to the energyefficient HVAC system design. PHOTO IWAN BAAN, COURTESY SFMOMA * Museums are very internally load dominated due to high lighting and people loads and small or zero envelope loads since most galleries are internal spaces without windows. Hence it is not uncommon to have high airflow requirements in cold weather. 40 ASHRAE JOURNAL ashrae.org AUGUST 2018
FIRST PLACE 2018 A. A. B. B. FIGURE 4 Cold weather, high airflow. A. Conventional system (top). B. Evaporative humidifier (bottom). FIGURE 5 Warm dry weather. A. Conventional system (top). B. Evaporative humidifier (bottom). Indoor Air Quality and Thermal Comfort The air-handlers have design minimum outdoor airflow setpoints 30% higher than ASHRAE Standard 62.1-2013; however, given the mild climate of San Francisco, for the vast majority of the year higher ventilation rates are provided due to the use of the airside economizer. Air quality was improved by eliminating lined ductwork, a potential source of microbial growth given the continuously saturated supply air, made possible, in part, through the use of quiet and efficient custom fan-array air handlers. The risk of microbial growth on the direct-evaporative humidifiers was mitigated by UV radiation and conductivity control of the water circuit, along with a control sequence where each section of the evaporative media is allowed to fully dry out sequentially once per day. Given the responsibility of the museum to keep the art within these agreed upon environmental conditions at all times, the temperature and humidity of FIGURE 6 Performance summary of direct evaporative humidifier. each zone is tracked closely and trend reports are regularly generated to show consistent compliant conditions, and consequently high thermal comfort. AUGUST 2018 ashrae.org ASHRAE JOURNAL 41
Energy Consumption (Million Btu) 20,000 18,000 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 0 Baseline Design FIGURE 7 Energy modeling results (courtesy of Atelier 10) (green zone is the allowable range, minimum RH for July is 44% and maximum is 56% per Figure 1, temperature is always 70 F to 75 F [21.1 C to 23.9 C]). Similar charts have been made for all zones in all seasons, showing compliance. Cost Effectiveness Compared to a traditional VAVreheat system, dual-duct has more ductwork, but no zone hot water piping and less expensive VAV zones, resulting in first-cost savings, improved efficiency, and, most importantly to the museum, elimination of risk of damage to art due to a piping leak. The central humidification also reduces first costs compared to zonal humidifiers, and also eliminates leak issues and maintenance issues due to poor access to humidifiers located above high ceilings. Zone Relative Humidity (%) Zone Temperature ( F) Special attention was paid to reusing as much existing equipment as possible, reducing first costs. The two existing 365 ton (1284 kw) variable speed centrifugal chillers were retained and only a new 100 ton (352 kw) 43% Proposed Building 60 58 56 54 52 50 48 46 44 42 40 78 76 74 72 70 68 80 70 60 50 40 30 20 10 0 Space Cool Space Heat Heat Rejection Ventilation Fans Pumps & Auxilliary Hot Water Miscellaneous Equipment Ext. Usage Decorative + Art Lighting Area Lights Annual Site Energy Intensity (kbtu/ft 2 yr) Figure 9 shows aggregated five-minute temperature and relative humidity data for every gallery zone served by AH-1, showing conditions within design parameters Electricity Power Density (kwh/ft 2 ) Gas Gas Density (MBtu/ft 2 ) 2.50 2.00 1.50 1.00 0.50 0.00 4.50 3.50 2.50 1.50 0.50 Old SFMOMA 34% Total Savings New SFMOMA Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun 2012/2016 2013/2017 44% Total Savings Old SFMOMA New SFMOMA Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun 2012/2016 2013/2017 FIGURE 8 Electricity (top) and gas usage density (bottom). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 FIGURE 9 Humidity and temperature trend data for all gallery zones on AH-1. Maximum 90 th Percentile 10 th Percentile Minimum scroll chiller was added to the central plant. The fact that 10 stories were added to the existing museum, more than doubling the overall square footage, and only an additional 100 ton (352 kw) chiller was added to supplement the existing 730 tons (2567 kw) is testament to 42 ASHRAE JOURNAL ashrae.org AUGUST 2018
engineering right-sizing resulting in significant first-cost savings. Reusing existing pumps upgraded with variable frequency drives also helped reduce first costs for the project. Conclusions The new SFMOMA building is a beautiful space, housing a worldclass art collection, kept comfortable for both the visitors and the artwork using a very energy-efficient HVAC system that is also cost-effective and easily operated and maintained. Key elements of the innovative design include dual fan/dual duct variable air volume distribution systems with air economizers and centralized humidity control using direct evaporative humidifiers. The system is much more energy efficient and less expensive than traditional constant volume systems with zonal humidity control, yet it provides the same tight humidity and temperature control required in museums. Because of the combination of low cost and high efficiency, the design should be considered for all museums and other applications requiring tight humidity control. Acknowledgments The authors would like to thank Jeff Phairas, SFMOMA s Chief Engineer, for providing the energy data for this article, and for his salient feedback and insights throughout the project. References 1. 2015 ASHRAE Handbook HVAC Applications, Chapt. 23. 2. Taylor, S. 2000. Comparing economizer relief systems. ASHRAE Journal, 42(9). 3. Kettler, J. 2004. Return fans or relief fans. ASHRAE Journal, 46(4). 4. Taylor, S. 2018. Designing mega AHUs. ASHRAE Journal, 60(4). AUGUST 2018 ashrae.org ASHRAE JOURNAL 43