Indirect/direct evaporative cooling system gets clean bill of health
The design takes advantage of northern California's arid climate to provide cost-efficient evaporative cooling with a VAV design that still manages to deliver excellent 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 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 multi-function facility would provide limited dental care to uninsured members of the community, potentially increasing the number of airborne viruses.
Saving with IEC/DECThree Des Champs Oasis IDEC (indirect/direct evaporative cooling) AHUs were selected for the brand new, three-story, 38,500-sq-ft health science building. These units use 100% outdoor air and a two-stage evaporative cooling process that eliminates the need for chiller operation, except during extreme outdoor temperatures.
In the cooling cycle, 100% 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 that provide indirect evaporative cooling, which in turn enhances the cooling effect of the heat pipe.
During this indirect stage, both the drybulb and wetbulb temperatures of the outdoor air are lowered. Downstream, this indirectly cooled air passes through a wetted medium, which further lowers the temperature of the supply air. This is the direct stage of the evaporative cooling process.
A Record-setting DayThe net result is a 38% reduction in cooling tonnage on a design day. This was confirmed by monitoring the system on a record temperature day of 101°F. On this extreme temperature day, the direct and indirect stage cooling systems were able to maintain building drybulb temperatures of 71° to 76° and rh between 50% and 60%.
Another third-party study demonstrated that the direct evaporative cooling stage alone provides nearly 73% of the Race building's required cooling, while the chiller is required for less than 5%.
To ensure 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% outdoor air, ensures that minimum ventilation rates of 15 to 20 cfm per person are met at all times. At the same time, 100% 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," remarked Tony Costa, of Costa Engineers, Inc. The firm has been honored with both a regional ASHRAE award and an ASHRAE Technology Award for the Santa Rosa project.
Two Buildings, One ChillerJay Carpenter, director of facilities operations at Santa Rosa, said that 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.
"The Race HVAC system has been successfully functioning for four years. We are so satisfied with it that we are not only installing another system like it in the adjacent building, but we are also going to install the same system in our new library at our Petaluma campus," said Carpenter. ES