Download pdf - A New Approach to Museum HVAC Design - Taylor Engineering · Handbook, HVAC Applications. Despite low airflow rates, trend data show temperature and humidity setpoints are consistently

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 STUDIES20

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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�([email protected]).�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 GlanceSan 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



2018 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 UpgradesThe 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

Relat

ive H

umidi

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:




• 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 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-Per-

formance Sequences of Operation for HVAC Systems, including

snap-acting dual-duct VAV box logic in non-humidity-

controlled zones, which eliminates simultaneous heat-

ing 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 tempera-

tures in the hot deck (higher) and cold deck (lower).

• All gallery and high occupancy zones have CO2 sen-

sors, 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 al-

lowed 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

the existing building, including reduced space require-

ments, eliminated sound attenuators, improved redun-

dancy, 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 main-

tenance 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

EconomizerOutside Air

Cooling Fan Array

Filter Bank Cooling

Coil

Direct Evap Humidifier

VSD

Heating Coil

Steam Humidifier

VSD

VSD

Relief Air

Cooling Supply Air Duct

Heating Supply Air Duct

Dual Duct VAV Box

Return Air Plenum

Relief Fan

R.A.

Filter Bank

FIGURE 3 Dual fan/dual duct air handler schematic.PH

OTO

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MOM

A

Expansion of the new SFMOMA.




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 out-

door 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 infil-

tration 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 psychromet-

ric 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 humidi-

fication 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 per-

formance 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.

PHOT

O ©

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BAA

N, C

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SFM

OMA

* 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.

Energy UsageThe 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 sav-

ings. The first operational year of the completed project

used 34% less electricity/ft2 and 44% less gas/ft2 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/ft2·yr. (224 kWh/m2) compared with 112.5

kBtu/ft2·yr. (355 kWh/m2) 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/ft2 than

the original museum, due in large part to the energy-

efficient HVAC system design.



FIRST PLACE | 2018 ASHRAE TECHNOLOGY AWARD CASE STUDIES

Indoor Air Quality and Thermal ComfortThe 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 pro-

vided 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 satu-

rated 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 humid-

ifiers 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 condi-

tions at all times, the temperature and humidity of

each zone is tracked closely and trend reports are

regularly generated to show consistent compliant

conditions, and consequently high thermal comfort.

FIGURE 6 Performance summary of direct evaporative humidifier.

FIGURE 4 Cold weather, high airflow. A. Conventional system (top). B. Evaporative humidifier (bottom).

A.

B.

FIGURE 5 Warm dry weather. A. Conventional system (top). B. Evaporative humidifier (bottom).

A.

B.




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 supple-

ment the existing 730 tons (2567 kW) is testament to

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 EffectivenessCompared to a traditional VAV-

reheat 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 com-

pared to zonal humidifiers, and

also eliminates leak issues and

maintenance issues due to poor

access to humidifiers located

above high ceilings.

Special attention was paid to reusing as much exist-

ing 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)

FIGURE 8 Electricity (top) and gas usage density (bottom).

Powe

r Den

sity (

kWh/

ft2 )

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

2012/2016 | 2013/2017

2.50

2.00

1.50

1.00

0.50

0.00

Old SFMOMA New SFMOMA

Electricity

34% Total Savings

Gas D

ensit

y (M

Btu/

ft2 )

Old SFMOMA New SFMOMA

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

2012/2016 | 2013/2017

4.50

3.50

2.50

1.50

0.50

Gas

44% Total Savings

6058565452504846444240

Zone

Rela

tive

Hum

idity

(%)

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.

Zone

Tem

pera

ture

(°F)

78

76

74

72

70

68 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

Maximum

90th Percentile

10th Percentile

Minimum

FIGURE 7 Energy modeling results (courtesy of Atelier 10)

20,00018,00016,00014,00012,00010,0008,0006,0004,0002,000

0

Ener

gy C

onsu

mpt

ion (M

illion

Btu

) 43%

Baseline Design Proposed Building

Annu

al Si

te E

nerg

y In

tens

ity (k

Btu/

ft2 ·yr)

80

706050

40

3020

100

Space Cool Space Heat Heat Rejection Ventilation Fans Pumps & Auxilliary Hot Water Miscellaneous Equipment Ext. Usage Decorative + Art Lighting Area Lights

Figure 9 shows aggregated five-minute temperature and

relative humidity data for every gallery zone served by

AH-1, showing conditions within design parameters

(green zone is the allowable range, minimum RH for



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 world-

class 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 evapo-

rative 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 con-

sidered for all museums and other

applications requiring tight humidity

control.

AcknowledgmentsThe 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.

References1. 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).
