The design takes advantage of the arid climate to provide cost-efficient evaporative cooling with a variable air volume (VAV) design that still manages to deliver good indoor air quality (IAQ) during the winter. This is unusual for VAV designs, which tend to decrease the supply air volume to the building as outdoor air temperatures drop. However, this particular air-handling unit (AHU) configuration includes an air-to-air heat exchanger, which can be used to preheat outdoor air, so that outdoor air minimum flow settings still exceed code.
One hundred percent outdoor air was desirable not only because of the high-density people loading associated with classroom environments, but also because this multifunction facility would provide limited dental care to uninsured members of the community, potentially increasing the number of airborne viruses.
Saving With IDEC
Three Des Champs Oasis IDEC (indirect/direct evaporative cooling) AHUs were selected for the new, three-story, 38,500-square-foot health science building. These units work off of 100 percent outdoor air and a two-stage evaporative cooling process, designed to eliminate the need for chiller operation, except during extreme outdoor temperatures.In the cooling cycle, 100 percent outdoor air enters the unit and flows across a heat pipe that extends across the supply and return sides of the unit. The return air side of the heat pipe is fitted with direct water sprays, designed to provide indirect evaporative cooling. This enhances the cooling effect of the heat pipe.
During this indirect stage, both the dry bulb and wet bulb temperatures of the outdoor air are lowered. Downstream, this indirectly cooled air passes through a wetted medium, designed to further lower the temperature of the supply air. This is the direct stage of the evaporative cooling process.
A Record-Setting Day
The net result is a 38 percent reduction in cooling tonnage on a design day. This was confirmed by monitoring the system on a record temperature day of 101 degrees F. On this extreme temperature day, the direct and indirect stage cooling systems were able to maintain building dry bulb temperatures of 71 degrees to 76 degrees and relative humidity between 50 percent and 60 percent.Another study demonstrated that the direct evaporative cooling stage alone provides nearly 73 percent of the building's required cooling, while the chiller is required for less than 5 percent.
To provide stable IAQ, engineers also incorporated ceiling diffusers with a high air diffusion performance index to ensure good mixing of room and primary supply air. This, along with the 100 percent outdoor air, is designed so minimum ventilation rates of 15 to 20 cfm per person are met at all times. At the same time, 100 percent outdoor air means lower CO2 levels and efficient exhausting of outgases from building materials.
"This project shows that IDEC technology can substantially reduce building energy usage while maintaining high indoor air quality and occupant comfort," said Tony Costa, of Costa Engineers Inc. The Napa, Calif., firm has been honored with both a regional American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) award and an ASHRAE Technology Award for the Santa Rosa project.
Two Buildings, One Chiller
Jay Carpenter, director of facilities operations at the college, said the IDEC system makes the Race building one of the least expensive campus buildings to operate in terms of energy.In fact, because the Race chiller is so underutilized, the school is going to use it for another adjacent building that is currently being remodeled and retrofitted with another IDEC system. The two buildings will share the same chiller.
"We are so satisfied with it that we are not only installing another system like it in the adjacent building, but are also going to install the same system in our new library at our Petaluma campus," said Carpenter.
Publication date: 06/27/2005
