Patent ID: 8205465

Claim:
A control system for an expansion valve and an evaporator of an air conditioning system, comprising: an evaporator coil having an inlet and an outlet, the evaporator being configured to conduct heat to a refrigerant flowing through the evaporator coil, wherein the temperature of the refrigerant exiting the evaporator coil outlet is dependent on the flow rate of the refrigerant to the evaporator coil; a valve having a valve element that is movable relative to a valve port for varying an opening area of the valve port to regulate the flow of refrigerant through the valve and to the evaporator coil; an evaporator coil outlet temperature sensor configured to sense the temperature of refrigerant within the evaporator coil near the outlet of the evaporator coil; at least one other evaporator coil temperature sensor upstream of the outlet temperature sensor, which is configured to sense temperature of refrigerant within the evaporator coil at a point upstream of the outlet temperature sensor; a controller in communication with the evaporator coil outlet temperature sensor, the controller being configured to determine a control set point based on the sensed temperature of refrigerant near the outlet of the evaporator coil, which control set point is used to determine a valve opening area sufficient to regulate the flow rate of refrigerant into the evaporator coil such that the temperature of the refrigerant at the outlet of the evaporator coil is within a temperature band having a lower bound that is within 12 degrees Fahrenheit above the liquid-to-vapor transition temperature of the refrigerant; wherein the controller is configured to determine a temperature difference over time, between the temperature sensed by the at least one other evaporator coil temperature sensor and the evaporator coil outlet temperature sensor, to capture oscillations in the sensed temperature of the refrigerant at the evaporator coil outlet, which define a minimum and maximum sensed temperature within the oscillations and an amplitude “A” of the oscillations in the sensed temperature; and wherein the controller is configured to determine, based on the temperature difference over time between the at least one other evaporator coil temperature sensor and the evaporator coil outlet temperature sensor, an optimum control set point used to determine an optimum opening area for establishing a flow rate of refrigerant that maintains the oscillating temperature of the refrigerant at an optimal range in which the minimum sensed temperature of the refrigerant at the evaporator coil outlet is within a fraction of the oscillation amplitude “A” from the liquid-to-vapor transition temperature, and the oscillating temperature of the refrigerant at the evaporator coil outlet does not fall to the liquid-to-vapor transition temperature for more than a time duration of one-quarter of the period of the oscillations in the temperature of the refrigerant at the evaporator coil outlet.