When we speak of air conditioners' operation, the majority of people have in mind compressors and evaporators. But there is another valuable working behind the scenes component, which is the expansion valve. Otherwise referred to as a metering device, it is a small but significant feature that controls refrigerant flow and is largely responsible for system efficiency.

Let's examine more closely the function of an expansion valve, the types generally used in air conditioners, and how they operate.

What Is an Expansion Valve in an Air Conditioner?

The expansion valve is one of four major constituents of a cooling system, the others being the compressor, evaporator, and condenser. It is the interface between the system's high-pressure and low-pressure sides—with the compressor.

Located near the evaporator coil in the indoor unit (also known as the air handler), the expansion valve regulates the flow of liquid refrigerant into the evaporator. Without it, the system cannot maintain proper pressure and temperature balance necessary for successful cooling.

Although its function is vital, the expansion valve is easily overlooked because it is small, nested in the evaporator compartment, and not easily accessible without taking it apart.

Types of Air Conditioner Expansion Valves

There are several expansion devices used in AC systems, yet the most common are:

- Thermostatic Expansion Valve (TXV or TEV)

- Capillary Tube

- Automatic Expansion Valve (AEV)

We will be discussing the first two, which are common in residential as well as commercial air conditioning systems.

1. Thermostatic Expansion Valve (TXV)

The thermostatic expansion valve is an active device that controls the flow of refrigerant by temperature and pressure in the evaporator. Its basic function is to control the degree of superheat—the temperature of the refrigerant vapor above its boiling point—inside the evaporator.

A TXV has a sensing bulb which is mounted on the suction line. The bulb holds a temperature-sensitive fluid. It's connected to the valve through a capillary tube, which relays the pressure signal to a diaphragm inside the valve. When the evaporator heats up, the pressure in the bulb rises, opening the valve and allowing more refrigerant to enter. As it falls as it cools, the valve closes the flow.

This auto-compensation allows the system to function efficiently with a wide range of loads and is optimum for systems whose performance requirements fluctuate.

2. Capillary Tube Metering Device

The capillary tube is a much simpler expansion component. It is essentially a long, narrow tube of fixed internal diameter. The length and diameter determine the amount of pressure drop as the refrigerant passes through, thus metering flow into the evaporator.

Although not actively controlled, the capillary tube works reasonably well in steady-load situations like small air conditioners and home refrigerators.

Newer models feature larger internal diameters and longer tube lengths for more reliability. Larger bores reduce the risk of blockage from debris or oil, and the longer tubes provide the required resistance necessary to regulate pressure.

An inlet filter or drier would typically be installed to prevent blockage, which is one of the extremely few maintenance concerns with this equipment.

One special benefit of capillary systems is off-cycle pressure equalization. This makes it possible to use low starting torque compressors, which are lower in noise and cost less energy. Furthermore, capillary systems don't need a liquid receiver, as the refrigerant remains within the evaporator during shutdown.

However, they do incorporate a suction accumulator to preclude liquid refrigerant from entering the compressor during low load situations—saving it from damage.

A TXV can actually surpass a capillary in wildly fluctuating loads, but capillary tubes remain excellent performers, especially in lower or economical applications.

Conclusion

Whether a thermostatic valve reacting to load change or a capillary tube providing consistent flow resistance, the expansion valve is the foundation of AC efficiency. Expansion valves ensure the evaporator coil receives the proper amount of refrigerant at the proper pressure, improving performance, conserving energy, and protecting the compressor.

Should you be looking to produce or design air conditioner systems—especially those with brass components for the valves—then our people at iPower Machinery can help. We provide high-precision production lines and solutions geared toward AC expansion valve production.