Abstract
The air handling unit (AHU) in a variable air volume (VAV) heating, ventilation and air conditioning (HVAC) system consists of outside air, return air and exhaust air dampers. The volume flow of air through a damper can be modified by changing the damper angles. In a typical AHU, outside air is mixed with recycled air and this mixture is then circulated to the various rooms in a building. A portion of the net volume flow out of these rooms is exhausted while the remainder is recycled. The net volume flow is usually determined by a supply fan in the duct, whose speed is dependent on the quiescent thermal load. In the absence of any regulation, the volume flow of outside air depends on the speed of the supply fan. The indoor air quality (IAQ) standards require a constant intake of outside air, independent of the speed of the supply fan. Additionally, motivated by the fact that air entering the system through the exhaust air damper is not as appropriately preconditioned as that entering the system through the outside air damper, it is required that air does not enter the AHU through the exhaust air damper. Using flow conservation equations, we derive a model for the AHU. This model was validated by experimental data collected on a full-scale HVAC test facility. Using this model, we provide a control policy that achieves the objective of maintaining IAQ in a VAV HVAC system where the flow across the exhaust air damper is never reversed. This control policy is validated manually using experimental data.
Published Version
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