Proper expansion valve selection is critical for refrigeration system efficiency and reliability. This guide outlines the systematic approach to sizing and selecting expansion valves based on system requirements and operating conditions. Expansion valves regulate the flow of refrigerant into the evaporator, making their correct selection essential for system performance, energy efficiency, and compressor protection.
Types of Expansion Valves in Refrigeration Systems
The expansion valve is a crucial component in refrigeration systems, responsible for reducing the pressure of the refrigerant, allowing it to expand and become cold. There are two primary types of expansion valves: thermostatic expansion valves (TXVs) and electronic expansion valves (EEVs).
Thermostatic Expansion Valves (TXVs)
Thermostatic expansion valves consist of two primary components working together: the thermostatic element (acting as the system actuator) and the orifice (performing the actual expansion of refrigerant). They are designed to maintain a constant superheat at the evaporator outlet by modulating refrigerant flow according to cooling load demands. The thermostatic element senses the temperature of the refrigerant at the evaporator outlet and adjusts the valve opening to maintain the desired superheat.
Internally Equalized Valves
Internally equalized valves sense evaporator pressure at the valve outlet. These valves receive warm, high-pressure liquid refrigerant and reduce its pressure, allowing it to expand and cool significantly. These valves are suitable for evaporators with minimal pressure drop (below 0.02 MPa or 0.2 kgf/cm²). The advantages of internally equalized valves include:
- Simple design and low cost
- Easy to install and maintain
- Suitable for small to medium-sized refrigeration systems
However, internally equalized valves may not be suitable for systems with large pressure drops or complex evaporator configurations.
Externally Equalized Valves
The operation of externally equalized valves is similar to internally equalized valves, except that evaporator pressure is fed against the bottom of the valve diaphragm from the evaporator outlet tube via an equalizer line. This design balances the expansion valve temperature during phase conversion and is necessary when there is appreciable pressure drop between the valve outlet and evaporator outlet. Externally equalized valves are suitable for:
- Large refrigeration systems with complex evaporator configurations
- Systems with high pressure drops (above 0.02 MPa or 0.2 kgf/cm²)
- Applications where precise control of superheat is required
The advantages of externally equalized valves include:
- Improved superheat control
- Suitable for large and complex refrigeration systems
- Can handle high pressure drops
However, externally equalized valves are more complex and expensive than internally equalized valves.
Electronic Expansion Valves (EEVs)
Electronic expansion valves offer more precise control through pulse-width modulation. Their selection criteria differ from thermostatic valves, typically being sized at 70-80% load capacity for air conditioning and refrigeration applications, providing reserve capacity for events such as pull-down periods. EEVs are suitable for:
- High-precision temperature control applications
- Systems with variable cooling loads
- Applications where energy efficiency is critical
The advantages of EEVs include:
- High precision temperature control
- Energy efficient
- Suitable for variable cooling load applications
However, EEVs are more complex and expensive than thermostatic expansion valves.
Comparison of Expansion Valves
The following table summarizes the key characteristics of thermostatic expansion valves and electronic expansion valves:
Valve Type | Superheat Control | Pressure Drop | Complexity | Cost |
|---|---|---|---|---|
Internally Equalized TXV | Moderate | Low | Low | Low |
Externally Equalized TXV | High | High | Medium | Medium |
Electronic Expansion Valve (EEV) | High | Variable | High | High |
Key Parameters for Expansion Valve Selection
To correctly select an expansion valve for a refrigeration system, several parameters must be considered:
- Refrigerant type: Different refrigerants require specific valve designs
- Evaporator capacity (Qe): The cooling load determines the base capacity requirement
- Evaporating temperature/pressure (Te/pe): Affects valve capacity and operation
- Condensing temperature/pressure (Tc/pc): Determines the high-side conditions
- Liquid refrigerant temperature (Tl): Critical for determining subcooling and correct valve sizing
- Pressure drop in liquid line, distributor, and evaporator (Δp): Affects valve performance and selection of equalization type
Step-by-Step Selection Process
Step 1: Determine Pressure Drop Across the Valve
The pressure drop is calculated using the equation:
Δptot = (Pc – Pe) – Δp
Where:
- Pc = condensing pressure
- Pe = evaporating pressure
- Δp = sum of pressure drops in liquid line, distributor, and evaporator
This calculation is critical as pressure drop directly affects the valve’s capacity.
Step 2: Determine Required Valve Capacity
Use the evaporator capacity (Qe) to select the required valve capacity at a given evaporating temperature. If necessary, correct the evaporator capacity based on subcooling value.
Subcooling is calculated as:
ΔTsub = Tc – Tl
Where:
- Tc = condensing temperature
- Tl = liquid temperature
The required valve capacity is determined by:
Qv = Qe / Fsub
Where:
- Fsub = subcooling correction factor from manufacturer tables
HVAC Valve Selection Calculator
This calculator determines the required valve capacity for an HVAC system based on the pressure drop across the valve and the evaporator capacity.
Step 3: Select Appropriate Orifice Size
Using the calculated pressure drop across the valve, the evaporating temperature, and the corrected evaporator capacity, select the corresponding orifice size from the manufacturer’s capacity tables for the specific refrigerant. The expansion valve capacity must be equal to or slightly greater than the calculated evaporator capacity.
Step 4: Choose the Thermostatic Charge
Select the appropriate thermostatic charge based on the application and temperature range. Different charge types include:
- G-Charge (Gas): Commonly used in air conditioning but loses control if the valve body temperature falls below the sensing bulb temperature
- L-Charge (Liquid): Provides precise control when the valve body is colder than the sensing bulb
- C/CL/CY-Charge (Cross charge): Used in low-temperature applications, maintains control regardless of valve body temperature relative to the sensing bulb
- S/SA/SL-Charge: Provides precise control and maximum operating pressure (MOP) protection
Step 5: Select Internal or External Equalization
Choose between internal or external equalization based on the pressure drop in the evaporator:
- Use internal equalization when pressure drop across the evaporator is negligible (below 0.02 MPa or 0.2 kgf/cm²)
- Use external equalization when there is significant pressure drop between valve outlet and evaporator outlet, or when using a pressure-drop-type refrigerant distributor at the evaporator inlet
Impact of Operating Conditions on Valve Selection
Effect of Liquid Temperature and Subcooling
Liquid temperature significantly affects valve selection. As demonstrated in one case study, a difference in liquid temperature from 43°C to 21°C (due to a sub-cooler in a two-stage compressor) changed the required valve from a TE 12-5 to a much smaller TE 5-4 model.
All standard capacity tables are typically calculated for a subcooling value of 4°C. When actual subcooling differs, the valve capacity must be adjusted using correction factors provided by manufacturers.
Effect of Pressure Drop in the Evaporator
In systems with appreciable pressure drop across the evaporator, externally equalized valves should be used. For R134a, a pressure drop of 0.01 MPa (0.1 kgf/cm²) increases static superheat by approximately 1°C, which restricts refrigerant flow and reduces system capacity.
An internally equalized valve in a system with significant evaporator pressure drop will operate at a higher than desired superheat. For example, in a system with a 6 psi pressure drop across the evaporator, an internally equalized valve would operate at 13°F superheat instead of the desired 9°F.
Superheat Settings and Adjustment
Superheat is critical for proper valve operation and system protection. In air conditioning, most valves are set to 10°F superheat. Superheat adjustment changes the static superheat, which is the superheat at which the valve begins to open from a fully closed condition.
To adjust superheat:
- Measure the temperature at the evaporator exit (T)
- Measure the evaporating pressure as close to the evaporator as possible
- Determine the evaporating temperature (ET) from pressure-temperature charts
- Calculate superheat as: superheat = T – ET
- Adjust the TXV if necessary
Valve Sizing and System Performance
Consequences of Improper Sizing
Proper valve sizing is crucial for system performance. An undersized valve will starve the evaporator of refrigerant, while an oversized valve can flood the compressor with refrigerant. Either situation leads to inefficient operation, reduced system capacity, and increased energy consumption.
For electronic expansion valves, the selection criteria typically recommend:
- 70-80% load capacity for normal refrigeration applications
- 50-60% for low-temperature applications to ensure sufficient capacity during pull-down periods
Energy Efficiency Considerations
The expansion valve directly impacts system energy efficiency. TXVs were reintroduced in the late 1980s as energy consumption became an important issue, replacing fixed metering devices like capillary tubes and metering pistons in many applications.
A properly sized and adjusted expansion valve ensures optimal evaporator performance and compressor protection, leading to energy-efficient operation. Too little superheat can damage the compressor, while too much superheat causes poor evaporator performance and compressor overheating.
Conclusion
The proper selection and sizing of refrigeration expansion valves is a systematic process requiring careful consideration of multiple parameters. Selecting the right expansion valve involves understanding system requirements, calculating correct capacities, and accounting for operating conditions such as subcooling and pressure drop.
A well-chosen expansion valve not only ensures optimal system performance but also enhances energy efficiency and extends equipment life. The selection process should follow manufacturer guidelines and consider specific application requirements to achieve the best results.
By following the step-by-step approach outlined in this guide, engineers and technicians can confidently select expansion valves that provide precise refrigerant control for reliable and efficient refrigeration systems.
