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Kaustubh Phalak, University of Miami
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Gang Wang, P.E., University of Miami
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Single zone (SZ) air handling units (AHU) are widely applied in a large conditioned spaces. A SZ AHU typically consists of a chilled water cooling coil, a hot water heating coil and a supply fan. For a constant volume (CV) SZ AHU or variable air volume (VAV) SZ AHU operating at a minimum airflow, the control valve of either the cooling coil or heating coil is modulated to vary the supply air temperature and consequently control space air temperature. Traditionally, a single control loop is applied to modulate the control valve directly based on the space air temperature. The traditional control is simplistic in nature however, suffers significant drawbacks. Due to the thermal capacity of both the water in the coils and the air in the conditioned space, the system often becomes unstable if the controller is not well tuned. On the other hand, cascade control makes the control system more adaptive and robust. A cascade control can be applied to SZ AHU in order to stabilize the system. The primary controller reads the room air temperature and determines the required supply air temperature for secondary controller which then controls the heating/cooling coil valve. The purpose of this paper is to demonstrate the stability of the cascade control method in SZ AHU system, theoretically and experimentally. First a theoretical model of the single zone AHU system with transfer functions is developed and root-locus analysis is performed with MATLAB. Then the experiment is conducted on a SZ AHU to evaluate the system performance using the traditional and cascade control respectively. The simulation and experiment results shows the cascade control stabilizes the system operation.
