A New Sequencing Control Strategy for Air-Handling Units

Abstract

Air-handling unit (AHU) controllers commonly use sequencing logic to determine the most economic way to use the components of the AHU to maintain the supply air temperature at a setpoint value. The objective of this paper is to introduce the finite state machine (FSM) sequencing control strategy for AHUs and to compare this strategy with a traditional split-range sequencing control strategy. The FSM control strategy represents a paradigm shift in the way sequencing logic is developed, illustrated, and implemented for the control of HVAC systems. Simulation results obtained from representative weather data for each month of the year are presented to demonstrate the performance of the two sequencing strategies with respect to the controller gains (proportional gain and integral time) used for the heating coil, cooling coil, and mixing box dampers. Simulation results demonstrate that in comparison to the split-range control strategy, the FSM control strategy leads to more stable operation of the AHU when the feedback controller(s) used to control the supply air temperature is (are) poorly tuned.