State-space model for transient behavior of water-to-air surface heat exchanger

Abstract

Adequately representing the dynamic performance of a water-to-air heat exchanger under transient conditions would help design optimized control strategies for a high-energy-efficiency HVAC system. In this paper, the water-to-air heat exchanger is considered as a multi-input-and-multi-output system (MIMO), and the corresponding dynamic model is developed with the mass and energy balance equations based on which the state-space model for transient behavior of the heat exchanger is derived.The model has been validated by a series of dynamic response experiments including the transient responses of exit air temperature and humidity as well as exit water temperature subjected to the disturbances of inlet water temperatures (Case I: Start up the chiller; and Case III: Stop the chiller) and a sudden change in the water flow rate (Case II: The water flow rate increases by 0.058kg/s). The time of experimental validation on the transient response simulation is set as 1200s in all the cases. The simulated results obtained by the state-space model have been compared with the experimental data, which shows that the state-space model developed in this study is capable of predicting the transient performance of the water-to-air heat exchangers satisfactorily (the average error is less than 15.0%). Meanwhile, the influence of input disturbances and initial conditions on the transient behaviors of the heat exchanger is discussed through the model simulation. The results show that the input disturbances and the initial conditions will produce significant influence on the proportionality coefficient of the response variables, but little influence on the time constant of the response ones.