Systems and methods for controlling differential refrigerant pressure
Systems and methods are provided for controlling compressor systems to ensure sufficient pressure differentials to provide cooling. A compressor system includes a compressor, a suction pressure sensor at a suction of the compressor, a discharge pressure sensor, a condenser, an expansion device, a liquid line, a liquid line pressure sensor, an evaporator, a condenser blower and a controller. The method includes determining a pressure target based on an intermediate pressure within the compressor and a threshold cooling differential pressure value, determining a pressure ratio setpoint based on the pressure target and a liquid line pressure measured by the liquid line pressure sensor, controlling the condenser blower to operate based on the determined pressure ratio setpoint, determining a subcooling setpoint based on the pressure target and the liquid line pressure in the compressor system, and controlling the expansion device to operate based on the subcooling setpoint.
1. A method of controlling a compressor system, comprising:
determining a pressure target based on an intermediate pressure within a compressor of the compressor system and a threshold cooling differential pressure value;
determining a pressure ratio setpoint based on the pressure target and a liquid line pressure within a liquid line of the compressor system;
operating a condenser blower at a speed, the speed determined based on the determined pressure ratio setpoint;
determining a subcooling setpoint based on the pressure target and the liquid line pressure in the compressor system; and
operating an expansion device based on the determined subcooling setpoint.
2. The method of claim 1 , wherein the intermediate pressure is determined based on a suction pressure of the compressor and a discharge pressure of the compressor.
3. The method of claim 1 , wherein determining the pressure ratio setpoint comprises:
comparing the pressure target and the liquid line pressure;
when the pressure target exceeds the liquid line pressure, setting the pressure ratio setpoint to a current pressure ratio multiplied by a ratio of the pressure target to the liquid line pressure;
when the liquid line pressure meets or exceeds the pressure target, setting the pressure ratio setpoint to the current pressure ratio.
4. The method of claim 3 , wherein determining the speed comprises referencing a mathematical model correlating pressure ratio and condenser blower speed.
5. The method of claim 3 , further comprising applying a smoothing function to control a rate of change of the pressure ratio setpoint.
6. The method of claim 3 , further comprising comparing the determined pressure ratio setpoint to a pressure ratio limit and setting the pressure ratio setpoint to the pressure ratio limit when the determined pressure ratio setpoint exceeds the pressure ratio limit.
7. The method of claim 1 , wherein determining the subcooling setpoint comprises:
determining a liquid pressure difference based on the pressure target and the liquid line pressure;
determining a subcooling offset based on the liquid pressure difference;
determining the subcooling setpoint based on the subcooling offset and a current subcooling value.
8. The method of claim 7 , wherein determining the subcooling offset comprises referencing a mathematical model correlating the subcooling offset with the liquid pressure difference.
9. The method of claim 7 , further comprising applying a smoothing function to control a rate of change in the subcooling setpoint.
10. The method of claim 7 , wherein operating the expansion device based on the subcooling setpoint comprises setting an aperture size of the expansion device based on the subcooling setpoint.
11. A compressor system, comprising:
a compressor;
a condenser;
an expansion device fluidly connected to the condenser by a liquid line;
a liquid line pressure sensor at a liquid line of the compressor system;
a condenser;
a blower configured to drive an airflow over the condenser; and
a controller, configured to:
determine a pressure target based on an intermediate pressure within the compressor and a threshold cooling differential pressure value;
determine a pressure ratio setpoint based on the pressure target and a liquid line pressure measured by the liquid line pressure sensor;
control the blower to operate at a speed determined based on the determined pressure ratio setpoint;
determine a subcooling setpoint based on the pressure target and the liquid line pressure in the compressor system; and
control the expansion device to operate based on the subcooling setpoint.
12. The compressor system of claim 11 , wherein the expansion device is a controllable electronic expansion valve.
13. The compressor system of claim 12 , wherein the processor is configured to control the expansion device to operate based on the subcooling setpoint by setting an aperture size for the controllable electronic expansion valve.
14. The compressor system of claim 11 , further comprising a suction pressure sensor located at the suction of the compressor and a discharge pressure sensor located at the discharge of the compressor, and wherein the processor is further configured to determine the intermediate pressure within the compressor based on a suction pressure measured by the suction pressure sensor and a discharge pressure measured by the discharge pressure sensor.
15. The compressor system of claim 11 , wherein the controller is configured to determine a pressure ratio setpoint by:
comparing the pressure target and the liquid line pressure;
when the pressure target exceeds the liquid line pressure, setting the pressure ratio setpoint to a current pressure ratio multiplied by a ratio of the pressure target to the liquid line pressure; and
when the liquid line pressure meets or exceeds the pressure target, setting the pressure ratio setpoint to the current pressure ratio.
16. The compressor system of claim 11 , wherein the controller is configured to determine a subcooling setpoint by:
determining a liquid pressure difference based on the pressure target and the liquid line pressure;
determining a subcooling offset based on the liquid pressure difference; and
determining the subcooling setpoint based on the subcooling offset and a current subcooling value.