Expansion valve control

A method for controlling a refrigeration system having a compressor, heat rejecting heat exchanger, expansion valve and heat absorbing heat exchanger circulating a refrigerant in series flow, the heat absorbing heat exchanger in thermal communication with working fluid, the method includes obtaining an expansion valve position set point; using a feedback control loop to generate a controlled expansion valve position; obtaining a rate of change of an operating parameter of the system; using the rate of change of the operating parameter to generate an adjustment; modifying the controlled expansion valve position using the adjustment; and controlling the expansion valve using the modified controlled expansion valve position.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to controlling an expansion valve, and more particularly to controlling an expansion valve using an anticipatory process to accommodate fast load changes in a refrigeration system.

Expansion valves, such as electronic expansion valves (EXVs) are used for metering refrigerant flow to an evaporator. The valves are typically slow moving and unable to keep up with fast loading (at startup or during rapid load change). Existing control methods may pre-open the expansion valve by a fixed number steps (or few discrete # of steps—e.g 50% and 100%). However, this may cause a low suction pressure fault (if the # of steps are too small compared to loading rate) or may cause compressor flooding (if the # of steps are too large compared to loading rate). Existing control methods do not employ provisions for pre-closing the valve, in case of load reduction, which exposes the chiller to potential compressor flooding.

BRIEF DESCRIPTION OF THE INVENTION

According to an aspect of the invention, a method for controlling a refrigeration system having a compressor, heat rejecting heat exchanger, expansion valve and heat absorbing heat exchanger circulating a refrigerant in series flow, the heat absorbing heat exchanger in thermal communication with working fluid, the method includes obtaining an expansion valve position set point; using a feedback control loop to generate a controlled expansion valve position; obtaining a rate of change of an operating parameter of the system; using the rate of change of the operating parameter to generate an adjustment; modifying the controlled expansion valve position using the adjustment; and controlling the expansion valve using the modified controlled expansion valve position.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises motor speed of the compressor.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises temperature of the working fluid entering the heat absorbing heat exchanger.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises a variable indexing value for the compressor.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises liquid level in the heat rejecting heat exchanger.

According to an aspect of the invention a refrigeration system includes a compressor; a heat rejecting heat exchanger; an expansion valve; a heat absorbing heat exchanger in thermal communication with working fluid; a controller to control the expansion valve, the controller performing operations comprising: obtaining an expansion valve position set point; using a feedback control loop to generate a controlled expansion valve position; obtaining a rate of change of an operating parameter of the system; using the rate of change of the operating parameter to generate an adjustment; modifying the controlled expansion valve position using the adjustment and controlling the expansion valve using the modified controlled expansion valve position.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises motor speed of the compressor.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises temperature of the working fluid entering the heat absorbing heat exchanger.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises a variable indexing value for the compressor.

In addition to one or more of the features described above or below, or as an alternative, further embodiments could include wherein the operating parameter comprises liquid level in condenser.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a schematic view of an embodiment of a heating, ventilation and air conditioning (HVAC) unit, for example, a chiller10. A compressor16receives vapor refrigerant14supplies refrigerant14to a heat rejecting heat exchanger18(e.g., condenser or gas cooler). Heat rejecting heat exchanger18outputs a flow of liquid refrigerant20to an expansion valve22. The expansion valve22outputs a vapor and liquid refrigerant mixture24toward the heat absorbing heat exchanger12(e.g., evaporator). The heat absorbing heat exchanger12places the refrigerant in thermal communication with a working fluid44(e.g., air, brine, water, etc.), causing the refrigerant to assume a vapor state, while cooling the working fluid44.

A controller50is coupled to the expansion valve22and controls the position of the expansion valve22using an adaptive process. Controller50may be implemented using known processor-based devices. Controller50receives sensor signals from one or more sensors52. Sensors52may sense a variety of operational parameters of the system10. Examples of such sensors include thermistors, pressure transducers, RTDs, liquid level sensors, speed sensors, etc. Sensors52can monitor a variety of parameters, directly or indirectly, including but not limited to: discharge pressure, discharge and suction superheat, subcooling, condenser and cooler refrigerant level, compressor speed, etc.

FIG. 2depicts a control process for controlling position of an expansion valve in an exemplary embodiment. The control process ofFIG. 2may be implemented by controller50to control the position of expansion valve22in an anticipatory manner. The controller50obtains a control variable (e.g., expansion valve position) set point100generated based on a first control loop. The expansion valve position set point100provides a desired opening for the expansion valve based on current conditions of system10(e.g., superheat, condenser liquid level, etc.). A feedback controller102receives a difference between expansion valve position set point100and the current controlled expansion valve position from output140and generates a controlled expansion valve position. The controlled expansion valve position may be limited by section104, which may alter the controlled expansion valve position based on factors such as limits on the physical valve and current position of the valve. The controlled expansion valve position is then used by output140to generate the controlled expansion valve position to the expansion valve22.

The control process ofFIG. 2also uses an anticipatory loop to adjust the controlled expansion valve position based on a rate of change of an operational parameter of the system. As shown inFIG. 2, a rate of change of an operational parameter of the system is obtained at150. The operational parameters may relate to load on the system10or capacity of system10. The operational parameter(s) may be one or more factors, such as change in temperature of working fluid44entering the heat absorbing heat exchanger12, motor speed of compressor16, a variable index value for compressor16, liquid level in the heat rejecting heat exchanger18, etc. These values may be provided by sensors52to controller50, which computes the rate of change of the operational parameter. The rate of change of the operational parameter is used by a feed forward controller152to generate an adjustment used to modify the controlled expansion valve position. The adjustment to the controlled expansion valve position can be positive or negative (or zero). The adjustment to the controlled expansion valve position compensates to rapid changes in operating parameters of the system10.

FIG. 3depicts plots of expansion valve position and chiller load versus time in an exemplary embodiment. As shown inFIG. 3, the combination of the feedback control and anticipatory feed forward control allows the expansion valve opening to increase upon anticipating an increased load. The feedback control alone would not anticipate the load change on the compressor and would result in a low suction pressure shutdown. By anticipating the load increase, the feed forward control generates an adjustment that increases the expansion valve opening, and accommodates the increased compressor speed. On the other hand, when the compressor speed falls rapidly in response to a reduction of fluid flow or reduction in load, the feedback controller102will not be able to anticipate the load change. It will cause the EXV to remain open and that will cause liquid carryover and low discharge superheat. Both of these are detrimental to compressor reliability. By anticipating the load decrease, the feed forward control152generates an adjustment that decreases the expansion valve opening, and accommodates the decreased compressor speed.

Embodiments provide a number of benefits including, but not limited to, (1) allowing the chiller to load and unload quickly (2) avoiding nuisance trips during fast loading (3) improved reliability by reducing chance of compressor flooding and loss of liquid seal and (4) improving settling time (time to reach steady state) of the chiller because the pre-open/pre-close value used is proportional to actual load change. In some embodiments, the anticipatory control is active only when it is necessary (during a change of load or other system parameter(s)). The anticipatory control is activated (turned on) when the magnitude of the rate of change of an operating parameter(s) and the load exceeds a certain threshold and it is de-activated when the magnitude of the rate of change of operating parameter(s) and the load falls below a certain threshold. It is understood that the anticipatory control may be active at all times, or activated based on other conditions.