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Ultimate climate control

The reconstructed U.S. Botanic Garden Conservatory's integrated mechanical systems provide tailored environments for 4,000 varied plants

January 01, 2003 |

A $33.5 million renovation of the U.S. Botanic Garden Conservatory on the U.S. Capitol grounds in Washington has not only enlarged the 70-year-old facility by 40%, but has made it substantially more able to provide optimum environments for the nearly 4,000 plants it houses. Some plants represent endangered species. Others, such as orchids, are noteworthy because of their value. Also on display are plants not in cultivation that have been given to the conservatory by scientists.

The four-year project included the reconstruction of eight greenhouse areas and the construction of a new, 15,900-sq.-ft. south addition.

"This was renovation on a monumental scale," says project designer Michael Brainerd, of the Washington office of Los Angeles-based A/E DMJMH+N, whose firm worked in association with Rodney Robinson Landscape Architects, Wilmington, Del., on the project.

"Since it's both a greenhouse and a monument, it had to be functional as well as beautiful," says Brainerd. "That alone was a heck of a challenge."

The original facility was naturally ventilated, primarily by the opening and closing of windows in the walls and along the roof peak to maintain a desired environment. Radiators around the perimeter of each gallery provided heat. With the incorporation of new HVAC and fogging systems, operable shades and vents, and paddle fans, sophisticated temperature and humidity control has been achieved.

One of the more visible elements used to control temperature and humidity is the fogging systems that discharge water in a fine mist that evaporates almost immediately. The system, which utilizes 1/4- and 3/8-in. diameter stainless steel pipe, normally operates at a pressure of 1,200 psi. The fogging system in the Jungle, originally called the Palm House, delivers 1.5 gallons of water per minute, the conservatory's highest fogging rate.

Environmental control is accomplished in a sequential manner in the greenhouses that have mechanical ventilation systems, explains Alfred Marks, who headed DMJMH+N's mechanical/- electrical design team. Starting on a typical warm day, the first step is to use natural ventilation, supplemented as needed with the fogging system to provide temperature and humidity control, Marks says. When the temperature exceeds the capability of this scenario, the vents close and the mechanical ventilation system takes control. Temperature is then maintained with variable speed fans, the use of evaporative coolers in the air handlers, and the fogging system. The greenhouses are divided into eight separate mechanical zones.


Visitors who enter through the main entrance are welcomed into the 9,000-sq.-ft. Garden Court. Its limestone walls are the only retained exterior feature of the original building, which was completed in 1933.

In summer, the fogging system can lower the temperature as much as 10 degrees. In winter, when the humidity is normally low, the system can boost it as high as 70% in pavilions that require it. Humidity can be controlled within a range of 10%.

Pure water supply is critical

The high-pressure fogging system, with its small nozzles, requires extremely pure water that is produced by a reverse osmosis (RO) filtering process. The use of untreated water would allow dissolved solids, such as calcium, to form deposits on the nozzles, ultimately impeding the water flow. This also could cause water to drip onto the plants, with resultant spotting and burning of the plants.

The conservatory's RO water treatment equipment operates continuously. Some of the processed water is piped into a 2,500-gallon storage tank in order to be able to keep up with demand during the summer, when the need for evaporative cooling increases.

Before the decision was made to specify a high-pressure fogging system, a low-pressure system was considered. However, it was determined that the low-pressure system would be three times more expensive, largely because it would have required side-by-side pipes, one for water flowing at a standard domestic water pressure of 30-40 psi, the other for compressed air delivered at 60-80 psi.

While the low-pressure system's larger nozzles would be less susceptible to clogging, reliable system operation nevertheless would have dictated the use of RO water, according to Frank Murphy, chief plumbing engineer with DMJMH+N. This precluded any savings that would have resulted from elimination of the RO water treatment equipment. The two-pipe system also would have hindered the designers' objective of keeping piping runs as inconspicuous as possible.

The temperature and humidity of each gallery, ranging from the humid Jungle to the drier Desert houses, vary according to the requirements of each environment.

All greenhouses have upper and lower sashes for air intake and exhaust, and ceiling fans facilitate the natural stratification of hot and cold air. Ceiling fans in the Jungle, the only area conditioned entirely by natural circulation, represent its sole mechanical intervention for environmental control. Traditional air conditioning is provided only in the new addition, the gift shop, the east and west display halls, and the Cool Desert. The Cool Desert's air-conditioning system is designed to provide a temperature of 80-90 F and a relative humidity of 20-50% during the peak summer months. Chilled water from the Capitol power plant is used for air conditioning.

Mechanical supply air systems place each house under positive pressure. To prevent plant damage, the maximum velocity of air discharged into the greenhouses is 250 fpm.

Plants rule

The requirements of plants take precedence over the comfort of human visitors in a greenhouse environment, notes Marks. Conservatory visitors should experience the environment associated with specific plant types, not just the plants themselves, he says.

Water used for general irrigation is routed through a sand filter and a charcoal filter. The latter primarily removes chlorine, which is especially detrimental to orchids (RO water is supplied to the Orchid House). After being drawn from city mains, the water is heated to room temperature before being routed for RO treatment or for use in general irrigation. In winter, the temperature of supplied water may drop to about 35 degrees, which would shock plants kept in an 80-degree environment. In summer, when the delivered water temperature rises to the 70-degree range, this differential is less of a concern.

Sun control was another important consideration in designing the reconstructed conservatory. For example, the clear glass on the east elevation of the Jungle House receives direct morning sun. This condition was addressed by installing movable shades, whose operation is controlled by photocells. The shades were supplemented with the installation of tinted glass in locations such as triangular areas, where the use of mechanical shading devices would be impractical.


The 93-ft. height of the 6,800-sq.-ft. Jungle provides room for the upward growth of plants, and a second-level walkway enables visitors to obtain a bird’s-eye view of them. The Jungle’s glass dome was precisely replicated.

The former Palm House, the conservatory's central feature and the first U.S. structure to use aluminum, was dismantled in 1992 after being deemed unsafe for occupancy. Its upper structure was removed and replaced with new aluminum framing; the lower structure was of steel construction.

Palm House dome re-created

In 1998, the entire conservatory was closed and stripped to its salvageable essentials. Limestone walls were the only exterior feature retained.

Glass panes, etched by chemicals and time, were removed and discarded from eight glass galleries. The framing of the 18-ft.-high low galleries was sandblasted and repainted.

Renovation of the 93-ft.-high former Palm House was the most dramatic aspect of the renovation. Its glass dome was precisely re-created. Aluminum-coated steel frames of the seven original smaller glass houses were sandblasted bare and fitted with new tempered, laminated glass panes.

The 37x250-ft. addition creates a new south entrance on Independence Avenue. Its lower level houses new mechanical equipment.

Frank Ferrucci, project manager with general contractor Clark Construction Group, Bethesda, Md., says construction coordination was made more difficult by the need to accommodate multiple systems, sometimes in very tight spaces.

Ferrucci compares the equipment-filled basement of the new addition to the mechanical room of a ship. He says the project was essentially completed in layers, beginning with excavation for two large utility ducts, continuing with grade-level work, and concluding with the installation of glass systems and mechanical systems. The need to establish an appropriate climate for various types of plants before they were installed underscored the importance of the timely completion of mechanical and electrical systems.

Exhibits on the east side of the conservatory feature themes relating plants to their natural environments, and include World Deserts, Oasis, Plant Adaptations and the Garden Primeval (a representation of the landscape as it might have appeared 150 million years ago). The relationship of plants to humans is the focus of exhibits on the west side. This area includes Rare and Endangered Species, Plant Exploration, Orchids, and Medicinal Plants.

Christine Flanagan, the conservatory's public programs manager, says the facility is "clearly much busier now." A regular weekday with no special exhibits can draw 1,000 to 2,000 visitors, compared to 200-300 before the renovation. The project also enables the conservatory to grow a greater diversity of plants.

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