AHUs contribute to a case of alchemy at Dallas service center
January 1, 2007
The City of Dallas’s fleet of over 1,200 service vehicles is the largest in the nation to run primarily on compressed natural gas (CNG) as part of an effort to make the city greener. In addition, the new maintenance and operations facility that services the fleet - the Northwest Service Center (NSC) - is designed to meet city-mandated green requirements.
Those requirements challenged the facility’s designers to develop an HVAC system that provides safe and adequate ventilation for a vehicle maintenance and operation facility while also meeting LEED® Silver guidelines established largely for office buildings. The project was so successful that the NSC is anticipating not Silver, but Gold certification for its efforts, utilizing the LEED v2.1 NC (new construction) and Campus Setting requirements.
Industrial-Strength ChallengeIn 2003, the Dallas City Council mandated that all new city facilities achieve LEED Silver certification. One of the biggest challenges for the NSC project was designing an energy-efficient industrial complex using LEED guidelines modeled on commercial office building energy consumption. Located on a 17-acre brownfield redevelopment site, the center provides space for city building services, vehicle maintenance, and the sanitation and streets divisions. While it includes 16,000 sq ft of office space for the streets and sanitation departments, the site is primarily industrial space: 42,000 sq ft for fleet vehicle maintenance, 10,000 sq ft for vehicle fueling and washing, and 25,000 sq ft for covered storage.
LEED-NC certification is structured on gains in energy cost savings compared to a typical office building as defined by ASHRAE 90.1 standards. At present, LEED certification programs don’t consider the unique requirements of industrial applications. For example, while A/C is the largest energy consumer in a typical commercial office building, the NSC has a relatively small cooling requirement: only the 16,000-sq-ft offices are cooled.
In the center, the largest source of energy consumption would come from a requirement for 100% outdoor makeup air exchangers in the vehicle service area to ensure adequate and safe air exchange for occupants. That’s where the engineers would have to gain the efficiencies they needed to meet the LEED requirement.
Outside InTo meet the 100% outdoor air exchange requirement in the vehicle service area, three large McQuay Vision AHUs were installed. These can pull in outside air at a total exchange rate of approximately 40,000 cfm (two units at 15,000 cfm and one unit at 10,000 cfm). Additional safety measures in the building include a gas detection system and emergency exhaust systems.
To achieve the required energy efficiencies in the service area, Sergey Aleksanyan, P.E. with Huitt-Zollars, the mechanical EOR, specified energy recovery wheels for each Vision unit. These capture energy from the heated exhaust airstream (up to 75% of it) and transfer it to the cool outside air being pulled into the space.
“Our solution for meeting the energy requirement was to gain winter efficiencies to offset the lack of a cooling load,” said Aleksanyan. “The exhaust air energy is used for preheating the makeup air. The rotary wheels recover energy from the exhaust air and return it to the supply air.” In addition to achieving LEED points for energy efficiency, this solution will earn the center an additional LEED point for innovation because energy recovery wheels are not considered standard design.
To achieve additional efficiencies, the office area is conditioned by two McQuay Vision AHUs with VAV controls, energy recovery wheels, and hot water heating. The VAV controls provide energy efficiency by matching airflow volume to load requirements; the motor runs at a lower amp draw under part load, thereby using energy only as needed to meet conditions. The energy recovery wheels lower heating and cooling costs by pre-treating the outside air using building exhaust air. A high-efficiency McQuay Model AGZ air-cooled, scroll compressor chiller (130 tons) provides chilled water for cooling. The fact that the chiller operates using an HCFC-free refrigerant (R 407C) gained the project an additional LEED point.