Avoiding coil freeze in HVAC systems
An HVAC system includes an evaporator. The evaporator includes a sensor configured to measure a property value (i.e., a saturated suction temperature or a saturated suction pressure) associated with saturated refrigerant flowing through the evaporator. The system includes a variable-speed compressor configured to receive the refrigerant and compress the received refrigerant. The system includes a controller communicatively coupled to the sensor and the variable-speed compressor. The controller monitors the property value measured by the sensor and detects a system fault, based on the monitored property value. In response to detecting the system fault, the controller operates the compressor in a freeze-prevention mode, which is configured to maintain the property value above a setpoint value by adjusting a speed of the variable-speed compressor. This prevents or delays freezing of the evaporator during operation of the system during the detected system fault.
1. A heating, ventilation, and air conditioning (HVAC) system, the HVAC system comprising:
an evaporator configured to receive a refrigerant and transfer heat from a flow of air to the refrigerant, the evaporator comprising a sensor configured to measure a property value associated with saturated refrigerant flowing through the evaporator, wherein the property value is one of a saturated suction temperature and a saturated suction pressure;
a variable-speed compressor configured to receive the refrigerant and compress the received refrigerant;
a controller communicatively coupled to the sensor and the variable-speed compressor, the controller configured to:
monitor the property value measured by the sensor;
detect a system fault, based on the monitored property value; and
in response to detecting the system fault, operate the variable-speed compressor in a freeze-prevention mode, the freeze-prevention mode configured to maintain the monitored property value above a setpoint value by adjusting a speed of the variable-speed compressor, thereby delaying freezing of the evaporator during operation of the HVAC system during the detected system fault.
2. The HVAC system of claim 1 , wherein the controller is configured to detect the system fault by determining that the property value is less than or equal to a predefined threshold value.
3. The HVAC system of claim 1 , wherein the controller is configured to:
monitor the property value over a period of time;
determine, based on a trend of the monitored property value over the period of time, an estimated value of the monitored property value at a future time, the future time following the period of time;
detect an anticipated system fault, based on the estimated value of the monitored property value; and
in response to detecting the anticipated system fault, operate the variable-speed compressor in the freeze-prevention mode.
4. The HVAC system of claim 1 , wherein, during operation in the freeze-prevention mode, a discharge air temperature setpoint of the HVAC system is ignored.
5. The HVAC system of claim 2 , wherein the controller is further configured to, in response to detecting the system fault:
decrease the speed of the variable-speed compressor to a first decreased speed;
following decreasing the speed of the variable-speed compressor to the first decreased speed, determine that the property value is not less than or equal to the predefined threshold value; and
in response to determining that the property value is not less than or equal to the predefined threshold value, decrease the first decreased speed of the variable-speed compressor to a second decreased speed, the second decreased speed less than the first decreased speed.
6. The HVAC system of claim 5 , wherein the controller is further configured to determine the second decreased speed based on a difference between the monitored property value the predefined threshold value following decreasing the speed of the variable-speed compressor to the first decreased speed.
7. The HVAC system of claim 1 , further comprising:
a discharge air temperature sensor configured to measure a temperature of a flow of air discharged by the HVAC system; and
wherein the controller is communicatively coupled to the discharge air temperature sensor and further configured to:
after causing the compressor to operate in the freeze-prevention mode, continue to monitor the property value;
determine that the monitored property value is greater than a threshold value; and
in response to determining that the monitored property value is greater than the threshold value, operate the variable speed compressor in a normal mode, the normal mode configured to maintain the temperature of the flow of air discharged by the HVAC system near a discharge temperature setpoint value by adjusting a speed of the variable speed compressor.
8. A method of operating a heating, ventilation, and air conditioning (HVAC) system, the method comprising:
monitoring a property value measured by a sensor, the sensor configured to measure a property value associated with saturated refrigerant flowing through an evaporator, wherein the property value is one of a saturated suction temperature and a saturated suction pressure;
detecting a system fault, based on the monitored property value; and
in response to detecting the system fault, operate a variable-speed compressor of the HVAC system in a freeze-prevention mode, the freeze-prevention mode configured to maintain the monitored property value above a setpoint value by adjusting a speed of the variable-speed compressor, thereby delaying freezing of the evaporator during operation of the HVAC system during the detected system fault.
9. The method of claim 8 , further comprising detecting the system fault by determining that the property value is less than or equal to a predefined threshold value.
10. The method of claim 8 , further comprising:
monitoring the property value over a period of time;
determining, based on a trend of the monitored property value over the period of time, an estimated value of the monitored property value at a future time, the future time following the period of time;
detecting an anticipated system fault, based on the estimated value of the monitored property value; and
in response to detecting the anticipated system fault, operating the variable-speed compressor in the freeze-prevention mode.
11. The method of claim 8 , wherein during operation in the freeze-prevention mode, a discharge air temperature setpoint of the HVAC system is ignored.
12. The method of claim 9 , further comprising, in response to detecting the system fault:
decreasing the speed of the variable-speed compressor to a first decreased speed;
following decreasing the speed of the variable-speed compressor to the first decreased speed, determining that the property value is not less than or equal to the predefined threshold value; and
in response to determining that the property value is not less than or equal to the predefined threshold value, decreasing the first decreased speed of the variable-speed compressor to a second decreased speed, the second decreased speed less than the first decreased speed.
13. The method of claim 12 , further comprising determining the second decreased speed based on a difference between the monitored property value the predefined threshold value following decreasing the speed of the variable-speed compressor to the first decreased speed.
14. The method of claim 8 , further comprising:
after causing the compressor to operate in the freeze-prevention mode, continuing to monitor the property value;
determining that the monitored property value is greater than a threshold value; and
in response to determining that the monitored property value is greater than the threshold value, operating the variable speed compressor in a normal mode, the normal mode configured to maintain a temperature of a flow of air discharged by the HVAC system near a discharge temperature setpoint value by adjusting a speed of the variable speed compressor.
15. A controller for operating a heating, ventilation, and air conditioning (HVAC) system, the controller comprising:
a memory configured to store a setpoint value associated with operating the HVAC system in a freeze-prevention mode;
an input/output interface configured to:
receive measurement data from a sensor, the configured to measure a property value associated with saturated refrigerant flowing through the evaporator, wherein the property value is one of a saturated suction temperature and a saturated suction pressure; and
transmit a control signal to a variable-speed compressor of the HVAC system; and
a processor coupled to the memory and the input/output interface, the processor configured to:
monitor the property value measured by the sensor;
detect a system fault, based on the monitored property value; and
in response to detecting the system fault, cause the input/output interface to transmit the control signal, the control signal instructing the variable-speed compressor to operate in the freeze-prevention mode, the freeze-prevention mode configured to maintain the monitored property value above the setpoint value by adjusting a speed of the variable-speed compressor, thereby delaying freezing of the evaporator during operation of the HVAC system during the detected system fault.
16. The controller of claim 15 , wherein the processor is configured to detect the system fault by determining that the property value is less than or equal to a predefined threshold value.
17. The controller of claim 15 , wherein the processor is further configured to:
monitor the property value over a period of time;
determine, based on a trend of the monitored property value over the period of time, an estimated value of the monitored property value at a future time, the future time following the period of time;
detect an anticipated system fault, based on the estimated value of the monitored property value; and
in response to detecting the anticipated system fault, cause the input/output interface to transmit the control signal, the control signal instructing the variable-speed compressor to operate in the freeze-prevention mode.
18. The controller of claim 15 , wherein during operation in the freeze-prevention mode, a discharge air temperature setpoint of the HVAC system is ignored.
19. The controller of claim 16 , wherein the processor is further configured to, in response to detecting the system fault:
decrease the speed of the variable-speed compressor to a first decreased speed;
following decreasing the speed of the variable-speed compressor to the first decreased speed, determine that the property value is not less than or equal to the predefined threshold value; and
in response to determining that the property value is not less than or equal to the predefined threshold value, decrease the first decreased speed of the variable-speed compressor to a second decreased speed, the second decreased speed less than the first decreased speed, wherein the second decreased speed is determined based on a difference between the monitored property value the predefined threshold value following decreasing the speed of the variable-speed compressor to the first decreased speed.
20. The controller of claim 15 , wherein the processor is further configured to:
after causing the compressor to operate in the freeze-prevention mode, continue to monitor the property value;
determine that the monitored property value is greater than a threshold value; and
in response to determining that the monitored property value is greater than the threshold value, cause the input/output interface to transmit a second control signal, the second control signal instructing the variable speed compressor to operate in a normal mode, the normal mode configured to maintain the temperature of the flow of air discharged by the HVAC system near a discharge temperature setpoint value by adjusting a speed of the variable speed compressor.