Modeling and control of a direct expansion (DX) variable-air-volume (VAV) air conditioning (A/C) system.

摘要

A direct expansion (DX) variable-air-volume (VAV) air conditioning (A/C) system, a typical type of air-cooled packaged A/C systems found in commercial buildings, consists of a VAV air-distribution sub-system and a DX refrigeration plant. Currently, the major deterrence to its wider application is to continuously match the output cooling capacity from its DX refrigeration plant with the varying cooling load in its VAV air-distribution sub-system.; This thesis firstly reports on the development of an experimental rig for a DX VAV A/C system. The rig was operated by a computerized logging and control supervisory system.; Secondly, a complete dynamic mathematical model for the experimental DX VAV A/C system has been developed based on the principle of mass and energy conservation, and using the correlations describing the operational performance of various components in the experimental DX VAV A/C system. The model was component-based and of partial-lumped-parameter type. Both the steady-state and dynamic behaviors from both the DX refrigeration plant and the VAV sub-system can be simultaneously simulated by the model.; Thirdly, the experimental validation for the dynamic model developed using the experimental rig is presented. The open-loop responses from both the DX refrigeration plant and the dynamic sub-model representing the DX plant were compared and found in good agreement. Fourthly, the thesis reports on the simulated closed-loop responses from the DX VAV A/C system with all its conventional proportional-integral control loops being enabled, using the validated model. Simulation results demonstrated that the dynamic model developed can behave as expected in a similar manner to a real DX VAV A/C system with all its control loops enabled.; Finally, the development of a novel DDC-based feedforward capacity controller, for matching the output cooling capacity from the DX refrigeration plant with the varying cooling load in the VAV sub-system, is reported. The feedforward controller consisted of both a numerical calculation algorithm, and a dead-band for avoiding unstable compressor operation. Controllability tests for the feedforward controller have been carried out in the experimental rig. Test results suggested that the novel feedforward controller developed was able to provide satisfactory control sensitivity and accuracy.