System Components: Metering Devices and Evaporators

What a Metering Device Does

The metering device sits between the high-pressure liquid line and the low-pressure evaporator. Its job is to drop refrigerant pressure sharply, causing it to enter the evaporator as a low-pressure, low-temperature mixture ready to absorb heat. Every metering device does this pressure drop; they differ in how they control the rate of refrigerant flow.

Thermostatic Expansion Valve (TXV)

The TXV is the most capable metering device used in commercial and residential systems. It continuously adjusts refrigerant flow to maintain a set amount of superheat at the evaporator outlet — typically between 8 °F and 12 °F above the evaporator saturation temperature, though the manufacturer sets the exact target.

Construction. Inside the TXV body is a needle and seat that opens or closes to throttle flow. A sensing bulb charged with refrigerant or a special fluid is clamped to the suction line near the evaporator outlet. The bulb connects to a diaphragm chamber through a capillary tube. Three pressures act on the diaphragm:

1. Bulb pressure (P1) — pushes the valve open. Rises when suction-line temperature rises.

2. Evaporator pressure (P2) — pushes the valve closed. Communicated through the external equalizer port on systems with a pressure drop across the evaporator (almost all commercial coils).

3. Superheat spring pressure (P3) — pushes the valve closed. Factory-set; adjustable in the field.

When load increases, the suction line gets warmer, bulb pressure rises, the valve opens wider, more refrigerant flows, and superheat returns to setpoint. When load drops, the reverse happens. This feedback loop protects the compressor from liquid slugging while keeping the evaporator fully active.

External equalizer — without it, a long evaporator with significant internal pressure drop would cause the valve to sense the inlet pressure instead of the outlet pressure, leading to underfeeding and high superheat. Always verify that the external equalizer line is connected and unobstructed.

Automatic Expansion Valve (AXV)

The AXV holds evaporator pressure constant rather than controlling superheat. A spring and atmospheric reference keep evaporator pressure near a fixed setpoint. When compressor suction pulls pressure down, the valve opens; when pressure rises, it closes. AXVs work well on constant, steady loads — small reach-in coolers and some window units — but they hunt and become unstable when load swings widely. They are simpler and less expensive than TXVs but less common in modern systems.

Fixed Orifice Devices: Cap Tube and Piston

Capillary tube (cap tube). A long, small-diameter copper tube — usually 0.026 in to 0.090 in inside diameter — that provides a fixed restriction. Flow rate depends on the pressure difference across the tube and refrigerant density. Cap tubes are used in factory-sealed systems (refrigerators, room air conditioners, small heat pumps) because they are inexpensive, have no moving parts, and require a critically charged system to work correctly. They cannot adjust to load changes.

Fixed piston (orifice). A drilled brass or polymer disc inserted in a fitting. Like the cap tube, it offers a fixed restriction, but it is easier to swap for a different size. Used in many residential split systems. Requires accurate refrigerant charge and proper airflow for correct operation.

Both fixed devices allow refrigerant to flow backward (equalize) when the system shuts off, which lets the compressor start against low pressure — one reason they appear in many residential systems.

Direct-Expansion (DX) Evaporator Coils

The evaporator absorbs heat from the conditioned space. In a direct-expansion coil, liquid refrigerant expands directly inside the coil tubes, boiling as it picks up heat. A typical DX coil consists of copper or aluminum tubes arranged in multiple circuits, with aluminum fins bonded to the tubes to increase surface area. Refrigerant enters as a liquid-vapor mixture and exits as superheated vapor.

Airflow across the coil is critical. As the coil surface temperature falls below the dew point of the air, moisture condenses on the fins — normal and expected. If airflow is severely reduced (dirty filter, failed blower, blocked return), the coil surface temperature drops further, condensate freezes, frost accumulates, airflow drops more, and suction pressure falls. A frosted coil is a symptom of a problem, not a normal operating state outside of defrost cycles.

Safety Points for This Lesson

Refrigerant handling. Any time a refrigerant circuit may be opened, follow your program's procedures for recovering refrigerant before disassembly. EPA Section 608 prohibits venting refrigerants. Wear safety glasses; liquid refrigerant causes freeze burns on skin and eyes.

Pressurized components. Sensing bulbs and TXV diaphragm chambers are under pressure. Inspect threads before reassembly. Do not heat a sensing bulb.

Electrical isolation. On live equipment labs, apply lockout/tagout (LOTO) before any coil or valve work. Verify power is off with a meter before touching terminals.

Frost simulation. Gloves and eye protection when handling ice or frozen surfaces. Do not place hands in the airstream of a running coil without confirming power is locked out.