Fault detection and diagnostic system for a refrigeration circuit

1. A fault detection and diagnostics (FDD) system comprising:

a refrigeration circuit comprising an evaporator and a compressor configured to circulate a refrigerant through the evaporator;

a communications interface configured to receive a measurement of a thermodynamic property affected by the refrigeration circuit; and

a processing circuit having a processor and memory, wherein the processing circuit is configured to:

use the measured thermodynamic property to determine an expected suction entropy of the refrigerant at a suction of the compressor;

use the expected suction entropy to determine an expected thermodynamic discharge property of the refrigerant at a discharge of the compressor;

determine an actual thermodynamic discharge property of the refrigerant at the discharge of the compressor; and

detect a fault in the refrigeration circuit by comparing the expected thermodynamic discharge property with the actual thermodynamic discharge property.

2. The FDD system of claim 1 , wherein:

the refrigerant absorbs heat from a secondary fluid in the evaporator; and

the measured thermodynamic property is a measured temperature of the secondary fluid downstream of the evaporator.

3. The FDD system of claim 2 , wherein determining the expected suction entropy comprises:

using the measured temperature of the secondary fluid and an expected approach of the evaporator to determine an expected suction temperature of the refrigerant at the suction of the compressor; and

calculating the expected suction entropy corresponding to a saturated vapor state of the refrigerant at the expected suction temperature.

4. The FDD system of claim 3 , wherein:

the communications interface is configured to receive a measured discharge pressure of the refrigerant at the discharge of the compressor; and

determining the expected thermodynamic discharge property comprises using the measured discharge pressure and the expected suction entropy to calculate an expected isentropic discharge temperature of the refrigerant at the discharge of the compressor.

5. The FDD system of claim 4 , wherein determining the expected thermodynamic discharge property comprises:

calculating an expected suction enthalpy corresponding to a saturated vapor state of the refrigerant at the expected suction temperature;

using the isentropic discharge temperature and the measured discharge pressure to calculate an isentropic discharge enthalpy of the refrigerant at the discharge of the compressor.

6. The FDD system of claim 5 , wherein determining the expected thermodynamic discharge property comprises:

identifying an isentropic efficiency of the compressor; and

using the expected suction enthalpy, the isentropic discharge enthalpy, and the isentropic efficiency to calculate an expected discharge enthalpy of the refrigerant at the discharge of the compressor.

7. The FDD system of claim 6 , wherein determining the expected thermodynamic discharge property comprises:

using the expected discharge enthalpy and the measured discharge pressure to calculate an expected discharge temperature of the refrigerant at the discharge of the compressor.

8. The FDD system of claim 1 , wherein:

the expected thermodynamic discharge property is an expected discharge temperature;

the actual thermodynamic discharge property is a measured discharge temperature; and

detecting the fault in the refrigeration circuit comprises comparing the expected discharge temperature with the measured discharge temperature.

9. The FDD system of claim 1 , wherein:

the expected thermodynamic discharge property is an expected amount of superheat corresponding to a difference between an expected discharge temperature of the refrigerant and a saturation temperature of the refrigerant at a measured discharge pressure;

the actual thermodynamic discharge property is an actual amount of superheat corresponding to a difference between a measured discharge temperature of the refrigerant and the saturation temperature of the refrigerant at the measured discharge pressure; and

detecting the fault in the refrigeration circuit comprises comparing the expected amount of superheat with the actual amount of superheat.

10. The FDD system of claim 1 , wherein detecting the fault in the refrigeration circuit comprises:

calculating an amount by which the actual thermodynamic discharge property exceeds the expected thermodynamic discharge property;

comparing the calculated amount with a threshold value; and

determining that an evaporator fouling fault is detected in response to the calculated amount exceeding the threshold value.

11. The FDD system of claim 1 , wherein:

the measured thermodynamic property is a measured suction temperature or pressure of the refrigerant at the suction of the compressor; and

determining the expected suction entropy comprises calculating an expected entropy corresponding to a saturated vapor state of the refrigerant at the measured suction temperature or pressure.

12. The FDD system of claim 11 , wherein:

the expected thermodynamic discharge property is an isentropic discharge property resulting from an ideal isentropic compression of the refrigerant from a saturated vapor at the suction of the compressor to superheated vapor at the discharge of the compressor;

the actual discharge property is based on a measured discharge temperature and a measured discharge pressure of the refrigerant at the discharge of the compressor; and

detecting the fault in the refrigeration circuit comprises comparing the isentropic discharge property with the actual discharge property.

13. The FDD system of claim 12 , wherein detecting the fault in the refrigeration circuit comprises determining that a liquid carryover fault is detected in response to the isentropic discharge property exceeding the actual discharge property.

14. A fault detection and diagnostics (FDD) system comprising:

a refrigeration circuit comprising an evaporator and a compressor configured to circulate a refrigerant through the evaporator;

one or more sensors positioned to measure a thermodynamic property of the refrigerant at a suction of the compressor and a thermodynamic property of the refrigerant at a discharge of the compressor; and

a processing circuit having a processor and memory, wherein the processing circuit is configured to:

use the measured thermodynamic properties to calculate enthalpy values comprising an actual suction enthalpy of the refrigerant at the suction of the compressor, an actual discharge enthalpy of the refrigerant at the discharge of the compressor, and an isentropic discharge enthalpy of the refrigerant at the discharge of the compressor;

use the calculated enthalpy values to calculate an isentropic efficiency of the compressor;

identify a threshold isentropic efficiency of the compressor; and

detect a fault in the refrigeration circuit by comparing the calculated isentropic efficiency with the threshold isentropic efficiency.

15. The FDD system of claim 14 , wherein the thermodynamic properties measured by the one or more sensors comprise:

a measured suction temperature or pressure of the refrigerant at the suction of the compressor;

a measured discharge pressure of the refrigerant at the discharge of the compressor; and

a measured discharge temperature of the refrigerant at the discharge of the compressor.

16. The FDD system of claim 15 , wherein calculating the isentropic efficiency of the compressor comprises:

calculating a suction enthalpy and a suction entropy corresponding to a saturated vapor state of the refrigerant at the measured suction temperature or pressure;

using the suction entropy and the measured discharge pressure to calculate an isentropic discharge enthalpy at the discharge of the compressor; and

using the measured discharge pressure and the measured discharge temperature to calculate an actual discharge enthalpy at the discharge of the compressor.

17. The FDD system of claim 16 , wherein calculating the isentropic efficiency of the compressor comprises:

determining a first amount by which the isentropic discharge enthalpy exceeds the suction enthalpy;

determining a second amount by which the actual discharge enthalpy exceeds the suction enthalpy; and

dividing the first amount by the second amount.

18. A method for detecting and diagnosing faults in a refrigeration circuit, the method comprising:

operating a compressor of the refrigeration circuit to circulate a refrigerant through an evaporator of the refrigeration circuit

receiving, at a processing circuit, a measurement of a thermodynamic property affected by the refrigeration circuit;

using the measured thermodynamic property to determine, by the processing circuit, an expected suction entropy of the refrigerant at a suction of the compressor;

using the expected suction entropy of the refrigerant at the suction of the compressor to determine, by the processing circuit, an expected thermodynamic discharge property of the refrigerant at a discharge of the compressor;

determining, by the processing circuit, an actual thermodynamic discharge property of the refrigerant at the discharge of the compressor; and

detecting, by the processing circuit, a fault in the refrigeration circuit by comparing the expected thermodynamic discharge property with the actual thermodynamic discharge property.

19. The method of claim 18 , wherein detecting the fault in the refrigeration circuit comprises:

calculating an amount by which the actual thermodynamic discharge property exceeds the expected thermodynamic discharge property;

comparing the calculated amount with a threshold value; and

determining that an evaporator fouling fault is detected in response to the calculated amount exceeding the threshold value.

20. The method of claim 18 , wherein:

determining the expected thermodynamic discharge property comprises calculating an isentropic discharge property resulting from an ideal isentropic compression of the refrigerant from a saturated vapor at the suction of the compressor to a superheated vapor at the discharge of the compressor;

determining the actual discharge property comprises using a measured discharge temperature of the refrigerant at the discharge of the compressor;

detecting the fault in the refrigeration circuit comprises determining that a liquid carryover fault is detected in response to the isentropic discharge property exceeding the actual discharge property.