Figure 1. – Air washer flow diagram.
The National Gallery of Art is responsible for housing, protecting, and exhibiting some of the world's most treasured works of human artistic creativity. The Gallery is open 363 days a year to more than four million annual visitors from across the United States and around the world. The Gallery buildings, designed by famed architects John Russell Pope and I.M. Pei, are themselves considered national art treasures.

Acknowledging the Gallery's unique status, architects and engineers were decidedly challenged by the enormous responsibility in approaching needed renovations and alterations to this renowned institution. They had to proceed with great care, creating change that would not encroach on historic character.

Art Conservation Demands HVAC Systems with Stringent Environmental Control

The National Gallery's exacting interior environment requirements demand specialized HVAC systems that can operate reliably 24 hours a day, seven days a week, 365 days a year. This high performance level is a must for the Gallery's mechanical and electrical equipment to fulfill the mission of protecting and preserving priceless collections.

The Gallery needed a plan for future renovations that would safeguard its treasures, public, and staff; extend facility life; incorporate technological advances in environmental conditioning and electrical support; and reduce the risks of future systems failures. The Gallery hired architectural/engineering firm VITETTA to develop a master facilities plan (MFP) with a flexible framework to guide phased renovation efforts over a period of years and to dovetail construction budgets with federal funding streams.

Shortly after the MFP was published, Hayes, Seay, Mattern and Mattern, Inc. (HSMM) joined the team as a subconsultant to VITETTA, reviewing the document and providing engineering studies and life-cycle cost analysis to complete the preliminary assessments with detailed engineering recommendations. HSMM then produced construction documents, including detailed phasing approaches, to implement the MFP.

Concurrently, the Gallery began renovation preparations, with initial activity conducted from the Office of the Administrator, Darrell R. Willson. The Administrator later formed the Office of Capital Projects (AOCP) to support the renovation program with a skilled staff of design and construction management professionals. AOCP manages the architects, engineers, and construction contractors, and coordinates with the affected Gallery groups and functions.

To date, HSMM has designed five MFP projects, with three constructed and complete: work area 1, work area 2, and the electrical upgrade. Two additional projects - the emergency power supply system and chiller plant renovation -have been designed and are awaiting contractor selection.

The mall entrance to West Building of the National Gallery of Art. Copyright © National Gallery of Art, Washington, DC, The content of the image may not be forwarded, copied, published without permission of the National Gallery of Art.

Environmental Control: Air Washers vs. Conventional Air Handling

One major work item involved the heart of the HVAC system, the air washer AHUs. These units operate continuously, maintaining temperature and humidity throughout the Gallery. Their internal configuration is two spray trees with spray nozzles facing each other above a sump (Figure 1).

Each spray tree has a horizontal header at the top, with a number of vertical standpipes connected to the header, and spray nozzles that are located periodically along the standpipe's length. Each air washer has between 170 and 200 spray nozzles. Baffles are upstream from the spray trees, while mist eliminators are downstream. A spray pump and its associated piping are located externally to the air washers.

In the summer, chilled water is introduced to the spray circuit directly to cool and dehumidify the airstream. Excess chilled water flows over an internal weir and returns by gravity to the building chilled water pit. An airside economizer operates in the winter, with spray water recirculated to humidify the airstream, and domestic water used for makeup. When sufficient dry air is introduced to lower the discharge temperature, preheat coils activate to preheat the mixed air.

Compared to conventional air-handling equipment, air washers bring disadvantages as well as advantages to the Gallery application. One disadvantage is that the air washer is a cross-flow heat exchange device, where the leaving chilled water temperature cannot exceed the leaving air temperature as would normally occur in a closed, counter-flow cooling coil. To hold 70°F and 50% rh in the galleries, the air washer needs to operate at 49° off the air washer. With 42º chilled water supply temperature, this effectively limits the chilled water temperature differential to 6° or 7°, resulting in chilled water flows that are double those of conventional chilled water systems with closed coils.

Another air washer disadvantage is the need for significant regular maintenance. The air washer continuously scrubs dirt and debris out of the airstream. This material falls out in the sump and collection pits and clogs spray nozzles and strainers. Furthermore, domestic water is relatively hard, typically forming scale on baffles, eliminators, and spray trees.

The air washers have a proven history at the Gallery, being in place and providing reliable temperature and humidity control since 1941, when the West Building opened. Air washers offer specific advantages over conventional air-handling equipment in a Gallery application. One advantage is that air off an active air washer is always at saturation. The control system can use a simple drybulb sensor at the air washer discharge to control discharge temperature and, at the same time, produce stable humidity in the galleries.

Contrast this with a conventional system, where one control loop controls temperature off the cooling coil, another controls a humidifier at the AHU (subject to a duct high limit of typically 80%), and yet another controls a booster humidifier downstream of the reheat coil serving a particular space in order to get the subject room's relative humidity up to 50%.

Another air washer advantage is its inherent scrubbing action. Air washers at the Gallery typically have excellent upstream filtration already in place with 50% bag pre-filters and 95% bag final filters. However, the units still wash significant amounts of dirt from the airstream. This debris collects in the sump, necessitating a weekly cleaning. Coupon testing of various airstreams was conducted in late 1999 and early 2000, revealing that the galleries' inside air typically had significantly lower corrosion rates than outside air, a fact credited to the air washers' scrubbing action. Contaminants in the outside air included sulfur-based compounds, oxides of nitrogen, and acid gases, urban pollution typically expected in an environment such as Washington, D.C.

Replacing air washers with more conventional HVAC systems was considered, but, given the air washers' proven track record at the Gallery for maintaining the desired indoor environment, reducing pollutants, and their advantages of stable humidity control, the project team decided to rehabilitate existing units by replacing worn components and adding new elements that reduce maintenance and improve unit performance.

Improving Air Washer Performance and Maintenance

The air washers' existing carbon steel supply fans had deteriorated from the 100% humidity environment immediately downstream from the units. These fans were replaced with ones of equivalent capacity, but with all airside surfaces manufactured with Type 316 stainless steel. A typical existing fan had a 54-in. wheel in a single-width, single-inlet configuration. They were replaced with fans specified with a horizontal flanged housing split, allowing disassembly for moving through tight aisles and access openings, and then reassembly in place.

Various enhancements were also considered that would extend the air washers' maintenance interval from once a week to once every two to three weeks. Maintaining existing units meant removing dirt and debris from the sump, cleaning clogged spray nozzles, and removing baffle and eliminator scale.

Since most material collected in the sump's bottom, indicating particles with a specific gravity greater than 1, a centrifugal separator was applied to the spray circuit. This device was equipped with an automatic purge valve that operates eight seconds every hour to clear the separator. The separator now collects material that used to gather in the sump, and the purge periodically cleans the separator.

The previous spray nozzles were brass, with hard-machined orifices that clogged frequently. These were replaced with units with a flexible rubber insert at the discharge opening that allows solid particles to pass through the nozzle without clogging.

Reducing scale was another challenge. Traditional water treatment would introduce chemicals to the airstream, possibly harmful to the art collection or to building personnel. Instead, a trial magnetic water treatment has been installed to evaluate that process's effectiveness for reducing scale, using six pairs of permanent magnets, mounted on the outside of the spray water piping. While there is considerable industry debate over magnetism's effectiveness, the DOE has endorsed the treatment as a viable technology to precipitate minerals from hard water (allowing the centrifugal separator to remove them from the spray water).1

The existing eliminators, a six-bend design spaced on 11/8-in. centers, were difficult to clean internally. The replacement eliminators are similar, except they are hinged on the upstream edge, allowing maintenance personnel to remove downstream spacer strips and open the eliminators to clean between blades, much like turning book pages.

In the Sculpture Gallery, a fountain showcasing “Venus and Cupid.” A water mist fire protection system was installed in several galleries instead of a traditional sprinkler system, due to the potential water damage they may cause. The mist system uses a fraction of water. Copyright © National Gallery of Art, Washington, DC, The content of the image may not be forwarded, copied, published without permission of the National Gallery of Art.

Controlling Airflow Key to Conditioned Air Quality

Airflow measuring stations were installed in the minimum outdoor air sections of the mixing boxes, ensuring that necessary outside air quantities are being introduced through the air washers to meet code requirements and maintain positive building pressurization. One lesson from work area 1 is that electronic airflow measuring stations using thermistor sensors work well in an outside airstream, but not immediately downstream from the air washer. These stations were initially installed in the inlet bell of the supply fan and return fan, with the intent of implementing a return fan tracking to maintain a cfm offset between supply and return flows.

However, the stations in the supply fan inlet bell were subject to condensation and water droplets that passed the eliminators. Since the stations used heat transfer as an indicator of velocity, the liquid water cooled the sensing elements, giving inaccurate readings. Vortex shedding-type meters were considered, but an algebraic fan curve fit algorithm was used to establish return fan speed, which has proven reliable for achieving the required offset.

The AHU controls are a DDC system incorporating a new custom-tailored operational sequence. The controls were networked into the BAS for central monitoring of all HVAC system components.

New Fire Protection Systems

The MFP recommended further study of the Gallery's fire protection and life safety systems. Hughes and Associates, Inc., (HAI) joined the team as fire protection engineers, initially performing a fire risk assessment (FRA) on the West Building, and, more recently, on the East Building and Connecting Link. HAI performed extensive smoke and fire modeling of the Gallery as part of developing the FRAs.

One FRA recommendation that affected air washer design was smoke control. The existing air washers used a smoke removal cycle that basically exhausted areas indexed for smoke removal through the return system.

However, Gallery air distribution typically is supplied through high registers concealed above cornices and returned through low ornamental grilles near the floor. Smoke modeling indicated major improvement if the air were exhausted high. In many fire scenarios, this contains smoke to one Gallery, above door openings and, consequently, above most of the works of art.

To exhaust smoke high in the galleries, two approaches had to be taken. For air washers serving ground floor galleries in the West Building, a smoke exhaust fan was connected to the supply ductwork immediately downstream of the supply fan. Motorized dampers were arranged to isolate the air washer, with the smoke exhaust fan producing a reverse flow in the supply duct, exhausting air out of the supply registers located high in the galleries.

West Building main floor galleries were treated differently because they have laylights separating the galleries from attic space. The laylights' glass panels allow daylight from attic skylights into the galleries. Typically, two laylight panels in each Gallery were modified with a hinged support frame and electric actuator. In case of fire, the panels tilt open to let smoke enter the attic space. The attic heating and ventilating units' exhaust fans activate to draw smoke from the respective galleries through the open laylight panels and remove smoke from the attic.

Sprinkler systems have not been used in many galleries because of potential water damage to artworks. HAI designed a water mist system for the NGA galleries with moderate to high fire fuel loads. This system is now installed in selective areas of work area 1 and work area 2, and will be extended throughout the West Building as renovation proceeds. The mist system needs only a fraction of the water used by normal sprinklers. The water mist fire pump has four motors driving eight pumps that raise water pressure to more than 1,600 psig. The high-pressure water is distributed through a stainless steel system to mist nozzles in the galleries, with water supplied only to zones where fire is detected.

Hot Water Replaces Steam

The West Building had extensive steam piping that fed heating equipment throughout the building. Steam/hot water converters and associated pumps were installed in work area 1 and sized for the entire West Building. As renovation proceeds, heating hot water equipment will replace steam heated equipment, including preheat coils in the air washers, reheat coils serving various zones, and cabinet heaters below windows. This significantly reduces maintenance by eliminating a large number of steam traps.

Heating hot water pumps and converter installed in Work Area 1 and sized for entire West Building. Copyright © National Gallery of Art, Washington, DC, The content of the image may not be forwarded, copied, published without permission of the National Gallery of Art.

Improved Electrical Service Facilitates Digital Monitoring and Control

The electrical upgrade project addressed a number of vital Gallery components that had reached the end of their useful service life. Changes to the duct bank, switchgear, transformer, and switchboard allowed enhancements in addition to providing safer, more efficient and more flexible power, and eliminated the remnants of environmentally hazardous PCB contaminants that were part of the original building transformers.

The revitalized electrical system permitted introduction of a power monitoring and control system for observing and tracking trends in frequency, current distribution, voltage fluctuations, demand, total harmonic distortion at various system points, and other important operating factors. The new system triggers alarms to operators when a component factor goes outside its normal operating range, and it allows manual intervention from a workstation as needed to control breakers and other electrical distribution equipment. The system is expandable and will incorporate other areas of the National Gallery as they are renovated.

More Gallery Renovations to Come

Two additional projects designed by HSMM and slated for construction are the emergency power supply system and the chiller plant renovation. The first will provide an emergency generator for the West Building and improve existing emergency, standby, and emergency lighting systems, while the second will remove existing chillers using CFC refrigerant R-12 and discontinue use of Tidal Basin water as condenser water. The chiller project will connect the Gallery to the chilled water utility that the U.S. General Services Administration (GSA) maintains on the National Mall.

Since GSA's chilled water system is a closed system using chemical treatment, it will be isolated from the Gallery chilled water system with plate-and-frame heat exchangers with a 1°F approach. The pumping scheme uses utility water pumps to push water through the heat exchangers on the utility side.

On the Gallery side of the heat exchangers, primary chilled water pumps will pump from the chilled water pit and through the heat exchangers, while secondary pumps will distribute chilled water throughout the entire facility. The plant will be sized for an ultimate load of 3,000 tons. Pumps and heat exchangers will be provided in an N+1 arrangement for redundancy. VSDs will be provided on all pumps to allow maximum flexibility to match the utility water flow to the actual building load. A standby 2,000-ton chiller will supply chilled water if the utility fails or cannot deliver supply water at a low enough temperature. The chiller will be able to use either utility water or domestic water for condenser heat rejection.

Over the past six years, the MFP has continued to guide Gallery design and renovation efforts. Managing this intricate web of change has required detailed planning and careful phasing to ensure that each aspect progressed in an orderly fashion, never interfering with normal museum operations or leaving art treasures vulnerable to threat. Each renovation phase has been approached with meticulous planning and coordination without losing its focus, despite the complexity, to provide solutions that respected the National Gallery's unique stature and responsibility.

In summary, these projects provided the Gallery, VITETTA, HSMM, HAI, hazardous materials consultant General Physics, Inc., and the construction contractors with significant challenges in a highly sensitive work environment. The end result of the collaborative effort has met the Gallery's needs by fulfilling project goals of providing more reliable and more efficient systems. The careful implementation of the enhancements should provide many years of enhanced service for the National Gallery of Art. For the viewing public, the renovations ensure the protection and preservation of Gallery treasures for generations to come. ES