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Cross-flow water-to-air heat exchangers are a common element in heating ventilating and air condition (HVAC) systems. In a typical configuration the outlet air temperature is controlled by the flow rate of water through the coil. In this configuration the heat exchanger exhibits non-linear dynamics. In particular the system has variable gains. Variable gain presents a challenge for the linear controllers that are typically used to control the outlet air temperature. To ensure stability over the entire operating range controllers need to be tuned at the highest gain state. This leads to sluggish response in lower gain states.

A new nonlinear control architecture, error modulated state setting control, was used to control an experimental cross-flow water-to-air heat exchanger. The controller outperformed a properly tuned nominal PI controller for both input tracking and disturbance rejection.

Citation: ASHRAE Trans., vol. 118, pt. 2, San Antonio, TX