The main function of the thermal expansion valve in the HVAC system is to automatically adjust the refrigerant flow rate according to the load changes of the evaporator, control the superheat of the evaporator outlet, and achieve efficient and energy-saving operation of the refrigeration system. However, due to the complex and changeable working conditions, the thermal expansion valve is very prone to failure, becoming a weak link affecting the performance of the refrigeration system. In this article, let's talk about the thermal expansion valve.
The thermal expansion valve automatically controls refrigerant flow. It adjusts the valve opening based on the refrigerant superheat at the evaporator outlet. When the evaporator load increases, the outlet superheat decreases. This reduces the pressure in the temperature-sensing package. The valve opening increases, allowing more refrigerant to enter. When the load decreases, the outlet superheat rises. The pressure in the temperature-sensing package increases. The valve opening decreases, reducing refrigerant flow. By adjusting the valve opening, the thermal expansion valve maintains the optimal evaporation temperature and superheat. This ensures efficient and energy-saving control of the refrigeration system.
The thermal expansion valve consists of a valve body, valve needle, valve seat, adjustment spring, temperature-sensing package, and balance pipe. The valve body connects the high and low-pressure sides, with the valve needle adjusting the opening under the spring and temperature-sensing package's force. The temperature-sensing package detects evaporator outlet superheat and converts it into a pressure signal. The balance pipe connects the evaporator outlet to the valve diaphragm, balancing evaporation pressure with the temperature-sensing package pressure. These designs determine the valve's control accuracy and service life.
Fault cause: spring deformation, impurity blockage, poor valve stem lubrication.
Fault manifestation: inflexible valve opening adjustment, abnormal refrigerant flow, large fluctuation of evaporation temperature.
Solution: clean the valve body channel, replace the spring and valve stem, and ensure smooth valve lubrication.
Fault cause: leakage of temperature sensor, blockage of capillary tube or improper installation.
Fault manifestation: loss of temperature feedback, resulting in severe overheating or liquid return of the evaporator.
Solution: check the temperature sensor and capillary tube, replace them if necessary, and ensure the correct installation position.
Fault cause: solid particles in the refrigerant, and the valve needle material is not wear-resistant.
Fault manifestation: the valve is not closed tightly, the pressure on the low-pressure side increases, and the evaporation temperature rises.
Solution: replace the valve needle and valve seat, install the filter, and select wear-resistant materials.
Fault cause: liquid impact, water hammer, long-term overload operation.
Fault manifestation: The valve loses its regulating ability and the low-pressure side is working abnormally.
Solution: Replace the diaphragm, optimize the system design, and avoid liquid hammer.
Fault cause: Spring fatigue, seal aging, valve seat oxidation.
Fault manifestation: A large amount of refrigerant leaks and the refrigeration effect decreases.
Solution: Replace the sealing ring, valve stem and valve seat, check the spring strength, and ensure the sealing performance.
Fault cause: Improper filling of the temperature sensor package and the pressure balance is destroyed.
Fault manifestation: Unable to adjust the superheat and the evaporator temperature fluctuates frequently.
Solution: Recalibrate the temperature sensor package and restore the automatic adjustment function.
Fault cause: The evaporator is too cold and the refrigerant contains water.
Fault manifestation: The expansion valve is frosted, the flow capacity decreases, and the temperature rises.
Solution: Clean up the water and impurities, and adjust the evaporation pressure and supercooling.
Fault diagnosis of thermal expansion valve should follow the principle of "first overall then local, first common then rare", and proceed according to the steps of refrigeration system analysis, fault hypothesis, information collection, cause verification, and troubleshooting.
The general diagnostic process is as follows:
(1) Understand the use of the equipment and analyze the operating conditions of heat pump system;
(2) Observe the external manifestations of the fault and put forward a preliminary hypothesis of the possible cause of the fault;
(3) Check the pipeline connection, measure the operating parameters, and collect relevant information about the fault;
(4) Compare with the standard operating conditions, verify the cause of the fault, and narrow the scope of investigation;
(5) Disassemble and inspect suspicious components, eliminate local faults, and restore equipment performance.
Common Methods for Diagnosing Thermal Expansion Valve Faults
(1)Parameter Comparison: Measure system pressure and temperature, then compare with standard values to identify faults.
(2)Experience Judgment: Use maintenance experience to quickly determine the fault cause based on its appearance.
(3)Parts Inspection: Check components like pipelines, sensors, and valve bodies to locate the fault.
(4)Reverse Thinking: Start from the worst fault outcome and trace back to its possible cause.
(5)Scenario Simulation: Simulate specific fault scenarios, observe results, and gain diagnostic experience.
To extend the service life of the thermal expansion valve in heat pump systems and improve fault diagnosis efficiency, follow these use and maintenance guidelines:
(1)Remove system moisture thoroughly during winter shutdown to prevent the valve body from freezing and cracking.
(2)Check the valve body for patency, ensure the temperature sensor is intact, and clean the filter before installation.
(3)Follow installation regulations strictly to avoid collisions or sand holes and ensure the valve stem moves flexibly.
(4)Use the balance pipe reasonably to prevent oil backflow and liquid impact, reducing valve body wear.
(5)Inspect the temperature sensor regularly to confirm it is securely fixed and in tight contact.
(6)Analyze any abnormal parameters promptly to prevent the issue from worsening.
(7)Test the valve action sensitivity and actuator reliability comprehensively before the refrigeration season.
