TR2017-137

Exploiting Refrigerant Distribution for Predictive Control of Multi-Evaporator Vapor Compression Systems



An observed behavior of refrigerant mass distribution in multi-path heat exchangers is exploited for control purposes. In this paper, we describe the following empirical property exploited for control: as the inlet valve position is decreased, refrigerant mass flow rate entering the heat exchanger is reduced, and for some flow rates, refrigerant is shown to preferentially flow in some paths more than others, causing maldistribution. This uneven refrigerant distribution is repeatable, reduces the capacity in a continuous manner and can be exploited with feedback controllers to regulate the perzone cooling.
A controller is designed to provide stability and robustness to per-zone conditions and setpoints for this controller that relate per-path superheat temperature to overall evaporator capacity is created in such a way as to be robust to changes in local zone temperatures and the overall system evaporating temperature. This strategy provides zone decoupling and ultimately creates a virtual control input for a model predictive controller. Experiments demonstrate the effectiveness of this approach on a two-zone air conditioner in laboratory tests.