Expansion Valves in HVAC Systems - Yesncer-fan coil unit,rooftop chiller unit,heat pump-The HVAC Manufacturers of PRC (2024)

Expansion Valves

The expansion valves plays a crucial role in refrigeration systems,such as heat pump, chiller unit. Typically installed between the liquid receiver and the evaporator, it transforms medium-temperature, high-pressure liquid refrigerant into low-temperature, low-pressure wet steam through throttling. The refrigerant then absorbs heat in the evaporator, achieving the cooling effect. By controlling the valve flow based on the superheat at the end of the evaporator, the expansion valve prevents insufficient evaporator area utilization and cylinder knocking.

Types of Expansion Valves

Electronic Expansion Valves: These valves can adjust the flow of refrigerant entering the cooling device according to preset programs. Traditional throttling elements, such as capillary tubes and thermal expansion valves, cannot meet the requirements for comfort and energy efficiency in scenarios with significant load changes or wide operating conditions. Therefore, combining compressor variable capacity technology with electronic expansion valves has become increasingly common. Electronic expansion valves utilize electrical signals generated by the regulated parameters to control the voltage or current applied to the expansion valve, adjusting the liquid supply as needed. The infinitely variable capacity refrigeration system requires a quick response for wide-range liquid supply adjustment, which traditional throttling devices cannot provide, making electronic expansion valves essential.

Thermal Expansion Valves: These valves control the valve opening through the superheat of the gaseous refrigerant at the evaporator outlet, making them widely used in non-flooded evaporators. Based on the balancing method, thermal expansion valves can be categorized into internally balanced and externally balanced types. Different substances and methods can be used for charging in the thermal expansion valve’s temperature sensing system, including liquid-charged, gas-charged, cross liquid-charged, mixed-charged, and adsorption-charged types.

Electronic Expansion Valves vs. Thermal Expansion Valve

• Adjustment Range:
  • Currently, thermal expansion valve have a generally narrow adjustment range. Heat pump units need to provide both cooling and heating, with applicable environmental temperature ranges from -15 ℃ to +43 ℃, corresponding to refrigerant evaporation temperatures ranging from -25 ℃ to 5 ℃. In systems with multiple compressors, the number of running compressors changes with user load, causing significant changes in refrigerant flow. Thus, a single thermal expansion valve cannot handle the operating conditions of large heat pump units. Most large heat pump products use a single circuit with a single compressor design and separate expansion valve systems for cooling and heating modes, which increases system complexity and manufacturing costs. Electronic expansion valves can precisely adjust within a range of 15% to 100%, meeting the regulation requirements of heat pump units under various conditions and providing flexibility.
• Superheat Control:
  • Control Point: Thermal expansion valve generally control the superheat at the evaporator outlet. In contrast, electronic expansion valve can control points at both the evaporator outlet and the compressor suction port in semi-hermetic and hermetic compressor systems, ensuring compressor efficiency.
  • Set Value: Thermal expansion valve manufacturers usually set the superheat value during production, typically at 5 ℃, 6 ℃, or 8 ℃. Electronic expansion valves allow manual superheat setting according to product characteristics, offering significant flexibility.
  • Stability under Non-Standard Conditions: Thermal expansion valves, set for standard conditions, deviate from the superheat value due to changes in condensing pressure and other factors when the system operates under non-standard conditions. This deviation reduces system efficiency and causes instability. Electronic expansion valve, with controller-set superheat and sensor-collected parameters, do not have this issue.
  • System Regulation Intelligence: Thermal expansion valves control superheat based on the current control point’s state. Electronic expansion valve, however, utilize various intelligent control systems based on product design and manufacturing characteristics, not only adjusting the system’s current state but also determining system characteristics based on superheat change rates and adopting corresponding control methods for different system trend changes. This makes electronic expansion valve superior in response speed and specificity compared to thermal expansion valve.
• Response Speed:
  • Thermal expansion valves rely on the thermal characteristics of the charged working substance for driving, exhibiting slow response and opening/closing speeds. In contrast, electronic expansion valve, driven by controllers that calculate parameters collected by sensors, transition from fully closed to fully open in just a few seconds, with quick response and action speed. They do not exhibit static superheat phenomena and allow manual setting of opening/closing characteristics and speed.
• Control Function Diversity:
  • To prevent excessive refrigerant pressure and flow on the evaporator side during initial startup, thermal expansion valve generally have a Maximum Operating Pressure (MOP) function that opens the valve only when the evaporator pressure is below the set value. Electronic expansion valve, regulated by controllers, exhibit diverse control functions during startup, load changes, defrosting, shutdown, and fault protection, making them superior in control function diversity. For example, besides controlling the evaporator, electronic expansion valve can regulate the condenser by reducing refrigerant flow in the system, decreasing condenser load, lowering condensing pressure, and achieving efficient and reliable unit operation.