Controls, Feedback Systems, Multi-Evaporator Cycle Control
Multi-evaporator vapor compression systems (ME-VCS) simultaneously provide cooling to multiple zones. The thermodynamic conditions in these zones are independent: the heat loads often differ, and the occupants of these spaces often have different desired room temperatures. Therefore, in order to regulate each zone to its desired setpoint temperature, the amount of thermal energy removed by each evaporator must be modulated independently. However, the common evaporating pressure within all evaporators introduces coupling that makes this objective difficult---the valve and piping arrangement imposes the constraint that all evaporators operate at the same temperature. (Systems considered here do not have valves at the outlet of each evaporator and therefore the individual evaporator pressures cannot be independently controlled.) In order to reduce the per-zone cooling, existing control strategies duty cycle the evaporator (alternate between a fully-open and fully-closed valve). However, duty cycling causes periodic disturbances to not only the local zone, but also to many machine temperatures and pressures, and these disturbances are often not transient but instead persist indefinitely. Fluctuations induced by the periodic disturbances can degrade the ability of the machine to regulate zone temperatures with zero steady state error, cause excessively high or low temperatures during peaks of the period, and couple into most machine signals of interest in ways that are difficult to describe with low order dynamical models. As an alternative to duty cycling, an observed behavior of refrigerant mass distribution in multi-path heat exchangers is exploited for control purposes. Multi-path heat exchangers are characterized by an inlet header pipe that splits refrigerant flow to two or more parallel paths through the heat exchanger and collects those paths into a common outlet header pipe. In the paper, we describe the following empirical phenomenon exploited for control: as the inlet valve is decreased, refrigerant mass flow rate entering the heat exchanger is reduced, and at some critical flow rate, refrigerant is shown to preferentially flow in some paths more than others, causing maldistribution. This uneven refrigerant distribution is repeatable and reduces the capacity in a continuous manner. The refrigerant distribution can be detected by temperature sensors along different paths of a multi-path heat exchanger. As some paths are starved for refrigerant they become superheated, and this uneven superheating process is unstable. A feedback controller is designed to provide stability and robustness to per-zone conditions. Finally, 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, which provides zone decoupling and ultimately creates a virtual control input for a supervisory controller such as a model predictive controller.