Metering Devices Review and Comparison
Metering devices all perform the same basic refrigeration-cycle job, but they do not all control refrigerant flow in the same way. Some use a fixed restriction, some respond mechanically to pressure and temperature, and others use electronic sensors and controllers.
This final lesson brings the entire Metering Devices subsection together so you can compare the devices, recognize their applications, and explain what happens to the refrigerant as it passes from the high-pressure side of the system into the evaporator.
What You Will Review
By the end of this lesson you should be able to:
Explain the common purpose of all metering devices.
Describe how each device controls refrigerant flow and establishes the pressure drop between the liquid line and evaporator.
Compare the five major device types.
Distinguish capillary tubes, fixed orifices, TXVs, AEVs, and EEVs by how each device controls refrigerant flow.
Explain refrigerant state across the metering device.
Identify high-pressure liquid before the device and a low-pressure liquid/vapor mixture after the pressure drop.
Explain flash gas.
Describe why only part of the liquid refrigerant boils immediately after the pressure drop.
Match metering devices to applications.
Recognize why particular devices are used in different types of air-conditioning and refrigeration equipment.
Apply the correct troubleshooting approach.
Use the control method of the device to determine what measurements and system conditions should be checked.
Every Metering Device Must Do Two Things
No matter how simple or sophisticated the metering device is, it must perform two fundamental refrigeration-cycle functions.
Control Refrigerant Flow
The device limits the amount of refrigerant entering the evaporator so refrigerant feed matches the system design and operating conditions.
Create the Pressure Drop
The device separates the high-pressure liquid side from the low-pressure evaporator side and allows refrigerant pressure to fall to evaporator pressure.
Refrigerant should normally reach the metering device as liquid from the condenser and liquid line.
Part of the liquid flashes into vapor while the remaining liquid enters the evaporator.
A capillary tube, fixed orifice, TXV, AEV, and EEV all create the same basic high-pressure-liquid to low-pressure liquid/vapor-mixture transition.
Only Part of the Liquid Boils During the Pressure Drop
When high-pressure liquid refrigerant passes through the metering device, its pressure suddenly decreases. The lower pressure corresponds to a lower saturation temperature, so some of the liquid immediately boils.
Flash gas is the portion of liquid refrigerant that immediately changes to vapor because of the pressure reduction through the metering device.
The remaining refrigerant is still liquid. That liquid must absorb heat in the evaporator before it can boil.
Remember the term flash gas. The metering device does not convert all of the refrigerant to vapor. The remaining liquid must boil in the evaporator.
Compare the Control Methods
Capillary Tube
A long, very small-diameter tube creates resistance to refrigerant flow.
Fixed Orifice
A calibrated opening provides the required refrigerant restriction.
TXV
A needle and seat move in response to sensing-bulb pressure, evaporator pressure, and spring pressure.
AEV
A needle and seat move primarily in response to evaporator pressure and spring pressure.
EEV
An electronic controller uses sensor information to position an electrically operated valve.
Fixed Restriction Through Length and Diameter
The capillary tube is one of the simplest metering devices. It has no moving mechanical parts and cannot actively reposition itself when system load changes.
Inside Diameter
A smaller inside diameter generally increases resistance to refrigerant flow.
Tube Length
A longer capillary tube generally increases resistance and pressure drop.
The original course specifically identifies diameter and length as the factors that determine capillary-tube refrigerant flow and pressure drop. :contentReference[oaicite:5]{index=5}
Correct replacement dimensions, cleanliness, and system charge are especially important because the capillary tube cannot adjust itself to compensate for incorrect operating conditions.
A Calibrated Opening Controls Refrigerant Flow
The fixed orifice performs the same basic job as the capillary tube, but the restriction is concentrated in a short, precisely sized opening rather than distributed through a long section of small tubing.
Simple Construction
The basic metering action requires no sensing bulb, diaphragm, stepper motor, or controller.
Correct Size Is Critical
An oversized or undersized orifice changes evaporator refrigerant feed and therefore changes system operation.
The physical opening does not change, but refrigerant flow through the device can change as the pressure difference and other system operating conditions change.
The TXV Responds to Evaporator Outlet Conditions
The thermostatic expansion valve is a mechanically modulating metering device. Its valve position results from the balance of three forces.
Sensing-Bulb Pressure
Bulb pressure acts on the diaphragm and tends to open the valve.
Evaporator Pressure
Evaporator pressure acts against the diaphragm and tends to close the valve.
Spring Pressure
Spring pressure adds a closing force and establishes the valve’s superheat setting.
The original course describes the TXV as responding to load variations for the purpose of maintaining evaporator superheat. :contentReference[oaicite:6]{index=6}
Load Increases
Outlet conditions tend to increase bulb pressure, which opens the valve farther and increases refrigerant feed.
Load Decreases
Bulb pressure decreases relative to the closing forces, so the valve moves toward closed and refrigerant feed decreases.
Before adjusting or replacing the valve, check liquid supply, bulb installation, equalizer operation, evaporator load, distributor condition, and manufacturer specifications.
The AEV Controls Evaporator Pressure
The automatic expansion valve is also called a constant-pressure expansion valve because it modulates refrigerant flow in response to evaporator pressure.
Evaporator Pressure Rises
Higher evaporator pressure creates more closing force, so the AEV tends to move toward closed.
Evaporator Pressure Falls
Lower evaporator pressure allows spring pressure to open the valve farther.
The original course specifically identifies the AEV as a device designed to maintain constant evaporator pressure. :contentReference[oaicite:7]{index=7}
A TXV generally opens farther as evaporator load increases. An AEV tends to move toward closed as evaporator pressure increases. Do not treat the two valves as interchangeable control devices.
Electronic Sensors and a Controller Position the Valve
The electronic expansion valve uses electrical control rather than a purely mechanical force balance.
Measure system conditions
Calculates required response
Moves the valve
Changes evaporator conditions
The original course describes communication among the microprocessor, step motor, control algorithm, and sensor as a feedback loop. :contentReference[oaicite:8]{index=8}
A refrigerant-feeding problem may be caused by the valve, actuator, temperature sensor, pressure sensor, wiring, controller, refrigerant circuit, or operating conditions.
Metering Device Comparison Table
| Device | Restriction Type | Primary Control Method | Actively Modulates? | Typical Strength |
|---|---|---|---|---|
| Capillary Tube | Long small-diameter tubing | Diameter, length, and pressure difference | No | Simple and inexpensive |
| Fixed Orifice | Calibrated opening | Orifice size and pressure difference | No | Simple fixed metering |
| TXV | Variable needle and seat | Bulb pressure, evaporator pressure, and spring pressure | Yes | Mechanical superheat control |
| AEV | Variable needle and seat | Evaporator pressure and spring pressure | Yes | Constant evaporator pressure |
| EEV | Electronically positioned valve | Sensors, controller, and actuator | Yes | Precise control over a wide operating range |
Why Are Different Metering Devices Used?
No single metering-device design is ideal for every refrigeration system. Equipment designers choose a device based on capacity range, load variation, cost, control precision, refrigerant, operating temperatures, and equipment complexity.
Small Refrigeration Appliances
Capillary tubes are well suited to compact factory-designed systems operating within a known range.
Fixed-Capacity Air Conditioning
Fixed orifices have been widely used where a simple fixed restriction can provide acceptable performance over the equipment’s intended operating range.
Commercial Refrigeration and AC
TXVs are widely used where evaporator load varies and mechanical superheat control is useful.
Relatively Steady Evaporator Pressure Applications
AEVs are suited to applications where maintaining evaporator pressure is the primary control objective.
Modern Variable-Capacity Equipment
EEVs are particularly useful in refrigeration, high-efficiency air conditioning, heat pumps, variable-speed systems, chillers, VRF equipment, and ductless mini-splits where refrigerant flow must be controlled across a wide operating range.
What Tells Each Device What to Do?
No Sensor
The fixed tubing restriction and system pressure conditions determine refrigerant flow.
No Sensor
The fixed opening and pressure difference determine refrigerant flow.
Temperature + Pressure
Sensing-bulb pressure, evaporator pressure, and spring pressure determine valve position.
Evaporator Pressure
Evaporator pressure and spring pressure determine valve position.
Electronic Inputs
Sensors provide information to a controller that commands valve position.
How Superheat Relates to the Metering Device
Superheat tells the technician what is happening to refrigerant after all liquid has boiled in the evaporator. It is therefore useful when evaluating evaporator feeding.
Higher Than Expected Superheat
The evaporator may be underfed, but possible causes include low refrigerant charge, restrictions, inadequate liquid supply, incorrect metering-device size, control problems, or excessive evaporator load.
Lower Than Expected Superheat
The evaporator may be overfed or lightly loaded. Possible causes include excessive refrigerant feed, incorrect control response, low airflow, or low evaporator load.
Always consider superheat together with refrigerant charge, subcooling, pressures, airflow or fluid flow, refrigeration load, and manufacturer information.
Start With the System, Then Move Toward the Device
Identify the refrigerant, equipment type, metering-device type, and operating mode.
Verify evaporator airflow or fluid flow, refrigeration load, and condenser operating conditions.
Allow the equipment to operate long enough to reach reasonably stable conditions.
Measure pressures, temperatures, superheat, subcooling, and relevant line temperatures.
Determine whether the evaporator appears normally fed, underfed, or overfed.
Verify that high-pressure liquid refrigerant is reaching the metering-device inlet.
Inspect the specific control elements used by the device, such as TXV bulbs and equalizers or EEV sensors and actuators.
Compare system operation with manufacturer specifications before adjusting or replacing components.
Vocabulary From the Metering Devices Subsection
Metering Device
A device that controls refrigerant entering the evaporator and creates the pressure drop between the high and low sides.
Flash Gas
The portion of liquid refrigerant that immediately boils because of the pressure reduction through the metering device.
Capillary Tube
A fixed metering device using a long length of very small-diameter tubing.
Fixed Orifice
A fixed metering device using a calibrated opening.
TXV
A thermostatic expansion valve that mechanically modulates refrigerant feed in response to evaporator outlet conditions.
External Equalizer
A pressure-sensing connection that allows a TXV to respond to evaporator outlet pressure.
Distributor
A component that divides refrigerant leaving a TXV among multiple evaporator circuits.
AEV
An automatic expansion valve designed to maintain relatively constant evaporator pressure.
EEV
An electronic expansion valve positioned by an electrical actuator under the command of an electronic controller.
Feedback Loop
A control process in which operating conditions are measured and used to continually correct valve position.
Quick Identification Review
Follow the Refrigerant From Condenser to Compressor
Condenser
Refrigerant rejects heat and becomes high-pressure liquid.
Liquid Line
High-pressure liquid travels toward the metering device.
Metering Device
Refrigerant flow is controlled and pressure drops.
Evaporator
The low-pressure liquid/vapor mixture absorbs heat and the remaining liquid boils.
Compressor
Low-pressure vapor returns to the compressor and the refrigeration cycle continues.
Can You Explain Metering Devices?
You should be able to answer these questions after completing this subsection.
- What are the two basic functions performed by every metering device?
- What refrigerant state should normally enter the metering device?
- What refrigerant state enters the evaporator after the metering device?
- What is flash gas?
- Does the metering device boil all of the liquid refrigerant?
- What determines refrigerant flow through a capillary tube?
- How does a fixed orifice differ physically from a capillary tube?
- What three forces control a TXV?
- What is the primary control goal of a TXV?
- Why is an external equalizer used on some TXV systems?
- What is the purpose of an evaporator distributor?
- What system condition does an AEV attempt to maintain?
- How does an AEV respond when evaporator pressure increases?
- How does an EEV determine valve position?
- What is a feedback loop?
- Why are EEVs especially useful in variable-capacity and ductless equipment?
- What does high superheat often suggest about evaporator feeding?
- What does low superheat often suggest about evaporator feeding or load?
- Why must the condition of the liquid line be checked before condemning a metering device?
- Why should manufacturer service information be used before adjusting or replacing a metering device?
What You Should Have Learned
All metering devices control refrigerant flow into the evaporator and create the pressure drop between the high-pressure and low-pressure sides of the refrigeration system.
High-pressure liquid refrigerant enters the metering device and a low-pressure liquid/vapor mixture leaves it and enters the evaporator.
Flash gas is the portion of liquid refrigerant that immediately changes to vapor because of the pressure drop through the metering device.
Capillary tubes and fixed orifices are fixed metering devices whose physical restriction does not actively change during operation.
TXVs mechanically modulate refrigerant flow in response to sensing-bulb pressure, evaporator pressure, and spring pressure to maintain the intended evaporator superheat.
AEVs modulate refrigerant flow primarily in response to evaporator pressure and are designed to maintain relatively constant evaporator pressure.
EEVs use electronic sensors, a controller, and an actuator to precisely position the refrigerant valve and can operate as part of a closed feedback loop.
Metering-device diagnosis requires evaluating the complete refrigeration system, including refrigerant supply, pressures, temperatures, superheat, subcooling, airflow or load, control components, and manufacturer specifications.
Metering Devices in Air Conditioning and Refrigeration
You have completed the Metering Devices subsection. You should now be able to identify the major metering-device types, explain how each device controls refrigerant flow, describe the refrigerant state before and after the pressure drop, explain flash gas, and use system operating information to begin diagnosing refrigerant-feeding problems.