Nowadays, more and more refrigeration equipment is using electronic expansion valves to replace traditional thermal expansion valves. While electronic expansion valves and thermal expansion valves serve the same basic purpose and come in various structures, they exhibit significant differences in performance.
01 adjustment range
Currently, the adjustment range of thermal expansion valves is generally narrow. However, heat pump units must handle both cooling and heating, operating in ambient temperatures ranging from -15°C to +43°C, with corresponding refrigerant evaporation temperatures between -25°C and 5°C.
Moreover, when multiple compressors are present in the refrigeration circuit, the number of running compressors changes according to user load, causing drastic fluctuations in refrigerant flow.
As a result, a single thermal expansion valve is far from adequate for the operating conditions of large heat pump units. Currently, many large heat pump products adopt a single-circuit, single-compressor design system, along with separate expansion valve systems for cooling and heating modes. This inevitably increases system complexity and manufacturing costs. In contrast, electronic expansion valves can achieve precise adjustments within a range of 15% to 100%.
Based on current usage results, a single electronic expansion valve can meet the regulation demands of heat pump units under the aforementioned conditions. Additionally, this adjustment range can be customized according to different product specifications, offering greater flexibility.
02 Superheat control
(1) Superheat control point:
Thermal expansion valves can generally only control superheat at the evaporator outlet. Electronic expansion valves, however, demonstrate superiority—in semi-hermetic and fully hermetic compressor systems, the control point can be set not only at the evaporator outlet but also at the compressor suction port, enabling control of compressor suction superheat to ensure compressor efficiency.
(2) Superheat setpoint:
For thermal expansion valves, the superheat setpoint is typically fixed by the manufacturer during production, usually at 5°C, 6°C, or 8°C. In contrast, electronic expansion valves allow manual adjustment of superheat based on product characteristics—for example, evaporator outlet superheat can be set to 6°C, while compressor suction superheat can be set to 15°C, offering high flexibility.
(3) Stability of superheat control under non-standard conditions:
The superheat setpoint of thermal expansion valves is calibrated for standard operating conditions. Due to the properties of the charged refrigerant, when the system deviates from standard conditions, the superheat often shifts from the setpoint as condensation pressure changes. This not only reduces system efficiency but also causes operational instability. Electronic expansion valves, however, have a manually set superheat via the controller, while the actual superheat is calculated from sensor data at the control point, eliminating such issues.
(4) System regulation intelligence:
Thermal expansion valves control superheat based solely on the current state of the control point, determined by the properties of the charged refrigerant, and cannot predict system trends. In contrast, electronic expansion valves employ various intelligent control systems tailored to different product designs and manufacturing characteristics. They not only adjust to the current system state but also analyze system behavior through parameters like superheat change rates, enabling adaptive control strategies for different operational trends.
Consequently, their response speed and precision in adapting to system changes are superior to those of thermal expansion valves.
03 response speed
The operation of thermal expansion valves relies on the thermodynamic properties of the charged refrigerant, resulting in the following characteristics:
(1) Relatively low response sensitivity and slower opening/closing speed.
(2) Generally, the opening and closing speeds are relatively consistent.
(3) During unit startup, static superheat exists. The superheat (SH) of a thermal expansion valve consists of static superheat (SS) and opening superheat (OS). Due to the presence of static superheat, there is a tendency for delayed valve opening during startup.
In contrast, electronic expansion valves are driven by control signals. The controller processes parameters collected by sensors, sends adjustment commands to the driver board, which then outputs electrical signals to actuate the electronic expansion valve.
An electronic expansion valve can transition from fully closed to fully open in just a few seconds, offering fast response and actuation. It exhibits no static superheat phenomenon, and both its opening/closing characteristics and speed can be manually configured, making it particularly suitable for heat pump units operating under highly variable conditions.
04 Diversity of control functions
To prevent compressor overload caused by excessive refrigerant pressure and flow on the evaporator side during initial startup, thermal expansion valves are generally equipped with MOP (Maximum Operating Pressure) functionality—the valve only opens when the evaporation pressure is below the set value. However, compared to electronic expansion valves, this feature remains relatively simplistic.
Structurally, electronic expansion valves can be viewed as an integrated combination of a throttling device and a solenoid valve, regulated via a controller. Therefore, depending on product characteristics, they demonstrate superior diversity in control functions during unit startup, load variations, defrosting, shutdown, and fault protection.
For example, besides regulating the evaporator, electronic expansion valves can also adjust refrigerant flow to control the condenser.
When evaporator conditions permit, if the condensing pressure is too high, the expansion valve can be partially closed to reduce refrigerant flow, lowering condenser load and thereby decreasing condensing pressure. This ensures efficient and reliable operation of the unit.
