Metering Devices
Fixed orifice, thermostatic expansion valve and electronic expansion valve, and how each controls the evaporator.
Safety
Compressor: Low pressure vapor is compressed into hot, high pressure discharge gas.
What it is
The metering device creates the pressure drop between the high and low side and controls how much liquid refrigerant enters the evaporator. It is the dividing line of the refrigeration circuit.
| Device | Control method | Charging method |
|---|---|---|
| Fixed orifice or piston | No control, flow follows pressure difference | Superheat with a manufacturer charging chart |
| Capillary tube | No control, length and bore set the flow | Superheat or exact weighed charge |
| Thermostatic expansion valve | Mechanical, maintains a target superheat | Subcooling to the manufacturer target |
| Electronic expansion valve | Stepper motor driven by a controller | Subcooling, with controller data for verification |
How it works
A thermostatic expansion valve balances three forces: bulb pressure pushing the valve open, evaporator pressure and spring pressure pushing it closed. If the suction line warms, bulb pressure rises and the valve feeds more refrigerant, holding superheat roughly constant across a range of loads.
A fixed orifice has no feedback. As load changes, superheat changes with it, which is why charging charts for these systems require both indoor wet bulb and outdoor dry bulb.
An electronic valve uses a suction pressure transducer and a temperature sensor, and a controller drives a stepper motor. It holds superheat tightly and can close fully on shutdown.
What can go wrong
| Condition | Superheat | Subcooling | Suction pressure |
|---|---|---|---|
| Valve underfeeding or restriction | High | High | Low |
| Valve overfeeding or stuck open | Low | Normal to low | High |
| Lost bulb charge | Very high, valve closed | High | Very low |
| Bulb poorly mounted or insulated | Erratic | Normal | Hunting |
| Plugged filter drier ahead of the valve | High | High | Low, sometimes with a temperature drop at the drier |
How to diagnose it
Verify airflow before touching the refrigerant circuit
Low airflow imitates almost every metering device fault
Measure superheat and subcooling together
The combination distinguishes charge problems from feed problems
Feel for a temperature drop across the filter drier
A noticeable drop indicates a restriction
Confirm the bulb is clamped tight, at the correct clock position and insulated
A loose or uninsulated bulb causes hunting and erratic superheat
Check the external equalizer line for kinks or blockage
A blocked equalizer causes a valve to underfeed
Tools involved
- Digital manifold with temperature clamps
- Manufacturer charging chart
- Infrared or contact thermometer
- Nitrogen and brazing equipment for replacement
Symptoms
Practice & Mastery
- Questions1
- Metering Devices Troubleshooting Scenarios2
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