Lesson System components · Expansion and accessories
Expansion devices: TXV and capillary
The expansion device drops liquid refrigerant from high pressure to low pressure and meters feed into the evaporator. A TXV tries to hold superheat; a capillary tube (or fixed orifice) feeds based mainly on pressure difference and charge. Wrong device diagnosis wastes hours—learn how each thinks before you turn adjustment stems or add gas.
Common job
Both devices create the pressure drop so liquid can boil in the evaporator. Difference is control strategy.
Thermostatic expansion valve (TXV)
Forces acting on the diaphragm typically include:
- Bulb pressure (opens with rising suction temperature)
- Evaporator pressure (closes as pressure rises)
- Spring (superheat spring setting)
Result: valve opens/closes to chase a superheat target.
Bulb installation rules (field critical)
- Mount on horizontal suction when possible, at OEM clock position.
- Tight metal contact; conductive paste as specified.
- Insulate bulb from ambient.
- External equalizer taps downstream of the bulb on the suction line when used.
Loose bulb → valve thinks evaporator is cold → flood. Wrong equalizer → hunting or starve.
Internally vs externally equalized
Pressure drop across large distributors needs external equalizer so the valve sees true outlet pressure. Using the wrong style undersizes or miscontrols feed.
Capillary tube / fixed orifice
No moving parts. Length, diameter, and charge set performance. Superheat varies with load and ambient. Charging is often by weight, glass, or manufacturer chart—not “TXV superheat targets” alone.
Restriction (kink, oil, debris) acts like a partial close; overcharge floods easily.
Comparing behavior
| TXV | Capillary / orifice | |
|---|---|---|
| Superheat | Controlled | Varies |
| Load change | Adapts | Limited |
| Charge method | Weight + SH/SC | Critical charge / OEM |
| Dirt | Screen / power head | Tube plug |
Hunting
TXV may oscillate (hunt) with oversized valve, wrong bulb charge, unstable load, or low load. Stabilize airflow and verify sizing before “chasing” with charge.
Power element and charges
TXV power heads match temperature ranges (including MOP styles). A head that lost charge starves the coil. Do not swap heads across refrigerants casually.
Screens and inlet dirt
Inlet screens load after burnouts and dirty openings—mimicking a bad valve. Inspect before turning the superheat stem.
Capillary special problems
Oil logging, kinks, and partial plugs create permanent starve. Critical charge means recover and weigh-in after openings—not “top off to a memorized psig.”
Adjustment discipline
Mark the original stem position, change slowly, and allow minutes to stabilize. Hunting often worsens with aggressive stem turns when the bulb or load is the real issue.
Field case
Situation. A/C with TXV: intermittent flood, compressor sweating. Superheat swings 2–25°F. Bulb hanging on insulation with a zip tie.
How to think it.
- Wild SH + poor bulb mount = control signal garbage.
- Remount on clean copper, correct position, insulated.
- SH stabilizes near target; flood stops.
Takeaway: TXV mechanics are only as good as the bulb.
In the field
Symptom
High/low SH, hunt, flood, starve
Where to look
Device type, bulb, equalizer, screen, charge method
Likely causes
- Bulb, restriction, wrong charge, oversized TXV
What to measure
- SH stable vs swinging
- liquid line SC
- ΔT across drier
What not to do
- Adjust TXV before bulb and airflow are correct
Checklist
- I explain pressure drop + metering as the expansion job
- I contrast TXV superheat control vs capillary fixed feed
- I inspect bulb mount and equalizer before adjusting
- I use OEM charge method for capillary systems
- I recognize hunting as a stability problem