Introduction
In refrigeration and air-conditioning systems, expansion valves, four-way valves, and stop valves play important roles. These components affect system efficiency and reliability and are key elements of refrigeration technology. This article provides a technical analysis of their structure, working principle, common faults, and detection and maintenance methods.
1. Expansion Valves
1.1 Electronic Expansion Valve
The electronic expansion valve adjusts the refrigerant supply to the evaporator according to a preset program. Its drive scheme uses a controller that computes parameters acquired by sensors and issues control commands to a driver board. The driver board outputs electrical signals to actuate the electronic expansion valve. The valve can move from fully closed to fully open in only a few seconds, with fast response and no static superheat. Opening and closing characteristics and speed are configurable, making electronic valves suitable for units with highly variable operating conditions.
From a control implementation perspective, an electronic expansion valve comprises three parts: the controller, the actuator, and the sensors. The controller's core hardware is a microcontroller. By drive type, expansion valves are classified as electromagnetic or electric; the most widely used type is the electric electronic expansion valve driven by a four-phase stepper motor.
1.2 Thermal Expansion Valve
The thermal expansion valve uses a sealed sensing assembly at the top, consisting of a sensing bulb, capillary tube, corrugated diaphragm, and a sealed cover. The sensing assembly is filled with refrigerant to form a closed sensing system; the charge can be the same refrigerant as the system or a different one.
The sensing bulb measures the superheat at the evaporator outlet. Changes in superheat cause pressure variations inside the sensing system (composed of the bulb, capillary, diaphragm, and corrugated tube). These pressure changes act on the diaphragm, causing it to move up or down. Through a linkage, the diaphragm movement is transmitted to the valve stem, moving the needle valve to increase or restrict flow. This reduces pressure and throttles the refrigerant flow, automatically regulating the refrigerant supply to maintain a set superheat at the evaporator outlet. That helps ensure full use of the evaporator heat transfer area and reduces the risk of liquid slugging.
Thermal expansion valves generally have a limited modulation range. Systems that must operate in both cooling and heating modes across wide ambient temperature ranges, and systems with multiple compressors where refrigerant flow varies dramatically with compressor count, can exceed the regulation capability of a single thermal expansion valve. As a result, some large heat pump systems adopt single-loop single-compressor designs or separate expansion valve circuits for cooling and heating, which increases system complexity and manufacturing cost.
Structurally, thermal expansion valves are classified as internally balanced or externally balanced. When the pressure drop across the evaporator corresponds to an evaporating temperature drop exceeding about 2 to 3 °C, an externally balanced thermal expansion valve is typically required to reduce operating superheat and improve evaporator heat transfer utilization.
2. Stop Valves
Stop valves are used in refrigeration systems to control the opening and closing of fluid flow and are important for system operation and maintenance. A stop valve controls flow by a vertically movable valve stem. The stem's movement fully opens or fully closes the flow path.
Common Faults and Causes
Stop valve quality issues reported in the market mainly manifest as leakage or inability to operate correctly. Common causes include:
- Valve core leakage: Two situations occur: leakage when the valve core is closed, and leakage when the valve core is open. Market feedback usually reports the latter. Typical causes include lack of cooling protection during brazing and system impurities that damage sealing rings.
- Thread stripping at the connection between the stop valve and the indoor/outdoor connection nut: Mainly caused by improper installation or excessive force during assembly.
- Low-pressure valve core leakage: Typically caused by system impurities, inadequate cooling after brazing, insufficient straight-fit length at the mating section between valve body and valve core leading to sticking on reset, or inherent valve core quality issues.
- Copper tube to valve body brazing leaks and valve core too tight to open: Often caused by overtightening the valve when closed.
- Copper nut cracking leading to leakage.
- Incorrect use of torque on stop valves.
3. Four-Way Valves
Four-way valves in refrigeration and air-conditioning systems are used to change the direction of refrigerant flow, enabling switching between cooling and heating modes. The valve redirects the refrigerant by moving an internal sliding spool, allowing the same system to operate in different modes.
3.1 Common Faults and Diagnostic Methods for Solenoid Four-Way Valves
Gas bypass detection for four-way valves: Start the compressor and initiate valve changeover. Simultaneously touch the three pipes labeled E, S, and C. If all three pipes are warm, the valve has not completed changeover.
Common reasons a four-way valve fails to change direction:
- Damaged solenoid coil; pilot valve does not actuate.
- The internal sliding spool is stuck by debris (scale, foreign material, degraded oil). In some cases, lightly striking the valve body with a wooden or rubber mallet can free the spool.
- Valve body deformation due to external impact (dented body) prevents spool movement; this is visible on inspection.
- Liquid hammer inside the system has broken the spool guide or deformed the end cap, preventing changeover.
- Excessive internal clearance or slight damage from seat welding causes excessive leakage, resulting in pressure balance across the spool so it cannot be driven to change position.
- System pressures cause the main spool to fracture, preventing changeover.
- Pilot valve cavity contamination or blockage prevents the pilot valve from working.
- At startup the main spool may be positioned near the center of the body so that energizing the valve cannot establish the required pressure differential; this may be resolved in some cases by tapping the valve body and recharging refrigerant.
- System has a slow leak and insufficient refrigerant to build the differential needed for changeover.
- Leak detection of valve body, capillary tube, or welds: Valve bodies often have oil residue on the surface. Apply soapy water to the valve body; bubbles indicate refrigerant leaks. If bubbles appear on the valve body, capillary, or capillary welds, replace the four-way valve. If bubbles appear at the flare joints of E, S, C, or D pipes, the leak can usually be repaired by re-brazing or re-welding those joints.
