School uses DV in underground athletic facility
For the subgrade athletic center at Sidwell Friends School, a K-12 school on 15 acres in Washington, D.C., displacement ventilation (DV) was ideal for the 23,000-sq-ft, four-court gymnasium, which is equipped with retractable bleacher seating for 600. In this enclosed, high-ceilinged space, airborne contaminants consist mainly of carbon dioxide and odor from athletes and spectators.
For the new subgrade athletic center at Sidwell Friends School, a K-12 school on 15 acres in Washington, D.C., displacement ventilation (DV) was ideal for the 23,000-sq-ft, four-court gymnasium, which is equipped with retractable bleacher seating for 600. In this enclosed, high-ceilinged space, airborne contaminants consist mainly of carbon dioxide and odor from athletes and spectators.
In addition to the gymnasium, the 73,000-sq-ft underground facility houses a fitness facility, a wrestling room, a dance room, coaches’ offices, and locker rooms for students, faculty, and officials. All that is visible from above is a small headhouse of a few hundred square feet. U.S. Green Building Council LEED Silver certification is currently under review for the athletic facility, and DV is playing a major role in achieving this certification. The new athletic facility supplements the gymnasiums already on campus and gives Sidwell options for future programming changes. Shortly following the completion of construction documentation in July 2008, extensive excavation began, with the new building occupied by the owner in fall 2010.
The DV approach is expected to use 60,000 kwh less per year than a conventional mixed-flow system, and it is a major factor in the facility’s eligibility for LEED Silver certification. This savings will be verified after commissioning is complete.
The gymnasium’s DV system dovetailed neatly with the facility’s design. Storage rooms and egress corridors along the perimeter of the lowest level are long and thin, accommodating the ductwork to the large supply air grilles DV requires. In the gym, the air is supplied through register assemblies evenly spaced along the perimeter wall, about 8 ft above the floor. The functional needs of the storage rooms and surrounding corridors did not permit duct drops down to the floor.
The building’s subgrade location offered opportunities that are not conventionally applied to gymnasium facilities. The space’s heat gain characteristic made it a good candidate for DV. The gymnasium’s interior walls are, for the most part, constant temperature and do not affect naturally occurring plumes. Due to the slope of the site, the eastern exposure of the gymnasium is aboveground and has a long strip of glazing. This high-performance glass is washed with moderately spaced linear ceiling diffusers, and the downward flow of the supply air has not disrupted the DV flow.
The gymnasium is supplied with 45,000 cfm of supply air at 65 F to provide the 68 tons of cooling required under peak conditions. This provides about 3.5 air changes per hour. The air change rate is held constant, and the supply air temperature is increased to approach 70 F when occupancy and internal heat gain decrease, both supporting the DV flow and saving cooling energy.
The gymnasium is ringed with 64 custom-fabricated DV supply diffusers. Each diffuser supplies 700 cfm. The large supply ducts discharging from each air-handling unit split into mains that connect to the DV supply diffusers, creating a north zone and a south zone. The ductwork is conventionally sized and fabricated to fit within the available ceiling space. Balancing dampers are installed in the branch ducts serving the diffusers.
Concentration of lighter-than-air pollutants, such as the carbon dioxide exhaled by athletes and spectators, increases from floor to ceiling. This internal vertical stratification improves ventilation effectiveness. Because upward free convection around a person brings air from below to the breathing zone, the inhaled air has a lower concentration of pollutants than it would with a mixed-flow distribution system.
The humidification requirements for the wood gymnasium floor limited the relative humidity (RH) range to 35% to 55%. More important, it limited the rate of change of RH to keep the wood material stable. RH range is maintained through the air handling system with two 200 lb/h electric steam-generating humidifiers for winter operation and ducted chilled water coil bypass for summer operation. In bypass mode, all of the outside air and some of the return air passes through the chilled water coil, where it is cooled and dehumidified. The air discharges from the cooling coil at 55 F and mixes with the remaining return air to provide a supply air temperature of 65 F and above while maintaining RH below 55%. The amount of outside air brought into the gymnasium is minimized with carbon-dioxide-based demand-controlled ventilation.
Keeping underground locker rooms healthy
Sidwell Friends School’s underground athletic facility includes a dozen underground locker rooms with accompanying shower rooms and toilet rooms. These spaces require a healthy ventilation rate of more than 1.2 cfm/sq ft. Because the cool basement walls absorb a great deal of the heat generated in the space by lights and people, the supply air replacing the exhausted air needs to be about room temperature. But it also needs to be dehumidified.
Two 25,000 cfm modular air-handling units serve the gymnasium. Each air handler operates as a single-zone constant-volume component with the flexibility to reduce supply airflow, if needed. Because the majority of bleacher seating is located in the north half of the gym, the north air-handling unit is selected for 45% outside air, and the south air-handling unit is selected for 28% outside air. The air handlers are cooled from a centralized chilled water plant and are expected to provide supply air at 65 F and above. This higher supply air temperature both prevents cold pockets and increases the number of hours annually when free cooling can be used. The air-handling units bypass return air around the chilled-water coil producing this higher mixed supply air temperature. The volume of air passing through the chilled water coil is taken below dew point and then mixed with the bypassed return air to maintain relative humidity at 50% to 60%.
Schultz is an associate vice president at Cannon Design. His expertise includes all facets of mechanical systems, from heating and cooling plants to distribution and terminal units. He most recently served as mechanical engineer for Kaleida Health’s Gates Vascular Institute at the State University of New York at Buffalo.