A capillary tube is a fixed restriction with no moving parts, a thermostatic expansion valve (TXV) adjusts flow mechanically to hold superheat, and an electronic expansion valve (EEV) adjusts flow using a controller and sensors. TXVs are the common choice for walk-ins, and EEVs are used where tighter control is worth the cost.
All three do the same job: drop the pressure of liquid refrigerant and feed the evaporator with the right amount. They differ in how precisely they respond to changing load.
Side by side
| Type | How it works | Strengths | Limits |
|---|---|---|---|
| Capillary tube | Long thin tube with fixed restriction | Simple, cheap, no parts to fail | Fixed flow, poor at changing loads, sensitive to charge |
| TXV | Sensing bulb moves a diaphragm and needle | Adjusts to load, protects compressor, widely available | Can hunt, clog or lose the bulb charge |
| EEV | Stepper motor and controller with sensors | Precise, flexible, supports advanced control | Costs more, needs controller, harder to diagnose |
Where each is used
- Capillary tube: small, simple, factory-sealed systems. Rare in field-installed walk-ins.
- TXV: the standard in many walk-in coolers and freezers, usually one per evaporator.
- EEV: larger or high-efficiency systems, or where floating head pressure and precise superheat matter.
Common problems
- A TXV that hunts, opening and closing too much, because of a loose bulb or wrong size.
- A blocked screen or orifice from debris or moisture.
- An EEV that cannot move because of a wiring or controller fault.
- A capillary tube kinked or plugged, starving the coil.
Why the difference matters to you
When a technician recommends replacing a metering device, ask why. A hunting valve may need only a bulb adjustment. A clogged one may signal moisture that also requires a new drier. Swapping one type for another is a design change that has to match the coil, refrigerant and controller. It is not a casual fix. If you are choosing new equipment, ask which device it uses and how it is serviced locally.
Charge and metering devices
A capillary system depends heavily on exact charge, so adding or removing refrigerant changes performance. TXV and EEV systems tolerate variation better, but they still need the right charge and superheat. Guessing is never safe.
See the TXV page, the filter drier page and the systems guide for more.
Questions to ask when buying or replacing
- Which metering device does this evaporator use, and is it standard or special order?
- How long does a replacement take to arrive locally?
- Does it need a controller or a special sensor?
- Who in my area can service it?
- What refrigerant is it rated for?
Answers to these matter as much as the technical difference. A slightly less clever device that anyone can service is often the better business choice for a small operator.
Questions about this topic
Which is best for a walk-in?
TXVs suit most walk-ins. EEVs suit systems that need very precise control. The best choice depends on the design.
Can I swap a TXV for a capillary tube?
No. That changes how the system works and can cause damage.
Is an EEV more reliable?
It has fewer wearing parts in the flow path but relies on electronics. Reliability depends on the installation.