Metering Devices in Air Conditioning and Refrigeration
The metering device controls the flow of refrigerant into the evaporator and creates the pressure drop between the high-pressure and low-pressure sides of the refrigeration system.
In this subsection you will study fixed and modulating metering devices, learn how refrigerant changes as it passes through the device, compare mechanical and electronic control methods, and apply those concepts to system troubleshooting.
What You Will Learn
Metering devices are used throughout air-conditioning and refrigeration equipment. Although every metering device performs the same basic refrigeration-cycle function, the method used to control refrigerant flow varies considerably from one device to another.
Refrigerant Flow and Pressure Drop
Understand why high-pressure liquid enters the metering device and a low-pressure liquid/vapor mixture enters the evaporator.
Flash Gas
Explain why only part of the liquid refrigerant immediately boils during the pressure drop and why the remaining liquid must boil in the evaporator.
Fixed Metering Devices
Study capillary tubes and fixed-orifice devices and learn how physical restriction controls refrigerant flow.
Mechanical Expansion Valves
Learn how TXVs and AEVs modulate refrigerant flow using pressure, temperature, and spring forces.
Electronic Expansion Valves
Examine sensors, controllers, actuators, stepper motors, and closed-loop refrigerant control used in modern equipment.
System Diagnosis
Use superheat, subcooling, pressures, temperatures, system load, and device-specific control information to diagnose refrigerant-feeding problems.
Every Metering Device Separates the High Side From the Low Side
Refrigerant arriving from the condenser should normally reach the metering device as liquid.
The device restricts refrigerant flow and reduces refrigerant pressure to the evaporator pressure.
Some liquid has flashed into vapor, while the remaining liquid must absorb heat and boil inside the evaporator.
The metering device does not normally turn all of the liquid refrigerant into vapor. Only a portion immediately boils because of the pressure reduction. That vapor is called flash gas.
Metering Devices Lessons
Complete the lessons in order. Each lesson builds on the refrigeration-cycle and evaporator concepts introduced earlier in the HVAC/R training material.
Introduction to Metering Devices
Learn where the metering device is located, what it does, what happens during the pressure drop, and why flash gas forms.
Flash Gas
Device Types
Capillary Tubes and Fixed-Orifice Devices
Study fixed refrigerant restrictions and learn how capillary-tube length, diameter, and calibrated orifice size affect refrigerant flow.
Fixed Orifices
System Charge
Thermostatic Expansion Valves (TXVs)
Learn how sensing-bulb pressure, evaporator pressure, and spring pressure work together to regulate refrigerant flow and evaporator superheat.
Three Forces
Load Response
TXV Applications and Service
Examine sensing-bulb installation, internal and external equalization, refrigerant distributors, valve adjustment, and specialized TXV designs.
Equalization
Distributors
Automatic Expansion Valves (AEVs)
Learn how the constant-pressure expansion valve responds to evaporator pressure and why its load response differs from a TXV.
Spring Force
Load Response
Electronic Expansion Valves (EEVs)
Explore electronic refrigerant control using sensors, controllers, stepper motors, feedback loops, and modern variable-capacity equipment.
Feedback Loops
Variable Capacity
Metering Device Troubleshooting and System Diagnosis
Use refrigerant conditions and system measurements to distinguish normal feeding, underfeeding, overfeeding, restrictions, load problems, and control faults.
Overfeeding
Diagnosis
Metering Devices Review and Comparison
Compare all five metering-device types and review refrigerant state, flash gas, control methods, applications, and troubleshooting principles.
Applications
Final Review
From Basic Restriction to Electronic Control
Understand the Pressure Drop
Begin with refrigerant state, pressure reduction, and flash gas.
Study Fixed Restrictions
Learn how capillary tubes and fixed orifices meter refrigerant without actively changing their opening.
Study Mechanical Modulation
Learn how TXVs and AEVs change valve position using pressure, temperature, and mechanical forces.
Study Electronic Modulation
Learn how EEV controllers use electronic sensors and actuators to continually adjust refrigerant flow.
Diagnose the Complete System
Combine refrigerant knowledge, superheat, subcooling, load, airflow, and control information to troubleshoot evaporator feeding.
Five Metering Devices — Five Control Methods
| Metering Device | Type | What Controls Refrigerant Flow? | Primary Characteristic |
|---|---|---|---|
| Capillary Tube | Fixed | Tube diameter, length, and pressure difference | Simple fixed restriction |
| Fixed Orifice | Fixed | Calibrated opening and pressure difference | Simple fixed restriction |
| TXV | Mechanical modulation | Sensing-bulb pressure, evaporator pressure, and spring pressure | Controls evaporator feeding based on superheat |
| AEV | Mechanical modulation | Evaporator pressure and spring pressure | Maintains relatively constant evaporator pressure |
| EEV | Electronic modulation | Sensors, electronic controller, and actuator | Precise control over a wide operating range |
These Principles Apply Across HVAC/R Equipment
Metering devices are not limited to residential air-conditioning equipment. The same refrigeration principles are used in comfort cooling, heat pumps, commercial refrigeration, refrigerators, freezers, ice machines, walk-ins, display cases, chillers, variable-capacity systems, VRF equipment, and ductless mini-splits.
Air Conditioning
Fixed orifices, TXVs, and EEVs are encountered across residential, commercial, packaged, and split-system equipment.
Commercial Refrigeration
TXVs and EEVs are widely used where evaporator load, temperature, and system operating conditions change.
Small Refrigeration
Capillary tubes remain common in compact factory-designed refrigeration appliances and equipment.
Variable-Capacity Equipment
EEVs allow refrigerant flow to be coordinated with variable-speed compressors and changing system capacity.
Start With the Refrigeration-Cycle Fundamentals
Begin with Lesson 1 to establish the pressure-drop and refrigerant-state concepts used throughout the remainder of the subsection.