How an Expansion Valve Works in Auto AC Systems

Learn how an expansion valve works in your car’s AC and also how it fails

The expansion valve is a critical component in auto AC systems. It regulates the flow of refrigerant into the evaporator and reduces the pressure of the refrigerant liquid. That allows the refrigerant to cool and partially evaporate before entering the evaporator. This pressure reduction is essential for the refrigerant to absorb heat effectively inside the evaporator. A faulty expansion valve can stop the flow of refrigerant or get stuck fully open. Both of those conditions will cause a loss of cooling. In order to diagnose an expansion valve problem, it’s important to know how an expansion valve works.

What an expansion valve does

It reduces the high pressure of the refrigerant— The expansion valve receives high-pressure liquid refrigerant from the condenser or receiver-drier. Inside the valve, the refrigerant passes through a small orifice, causing a significant pressure drop. This pressure drop reduces the refrigerant’s temperature, making it ready to absorb heat once it reaches the evaporator.

The expansion valve meters refrigerant flow— The valve is designed to control the amount of refrigerant entering the evaporator based on the cooling demand and the evaporator’s temperature. It does this by adjusting the size of the orifice through which the refrigerant passes. The valve opens or closes in response to the temperature of the evaporator.

It senses evaporator or refrigerant temperature— Many expansion valves have a temperature-sensing bulb attached to the evaporator outlet. This bulb contains a small amount of refrigerant, which responds to changes in temperature. As the temperature of the evaporator outlet changes, the pressure inside the sensing bulb changes as well. This pressure change causes the valve to adjust the orifice size, regulating the flow of refrigerant accordingly.

It maintains optimal cooling— By continuously adjusting the refrigerant flow, the expansion valve ensures that the evaporator maintains an optimal temperature for efficient heat absorption. This regulation prevents the evaporator from becoming too cold (which could lead to icing) or too warm (which would reduce cooling efficiency).

Carmakers use two types of expansion valves: a remote-sensing capillary tube style and an H-block style, although carmakers have been gradually shifting over to the H-block style in recent years.

How a capillary tube expansion valve works

Capillary expansion valve

A thermostatic expansion valve (TXV) with remote sensing utilizes a valve, a diaphragm, and a slender tube with a sensing bulb at one end. The tube’s other end connects to the diaphragm. This assembly is charged a special refrigerant.

The sensing bulb is securely attached to the evaporator coil’s outlet. As the evaporator temperature fluctuates, the bulb’s temperature follows suit. These temperature changes cause the refrigerant within the sensing system to expand or contract.

When the evaporator warms up, the refrigerant in the bulb expands. This expansion pushes on the diaphragm, opening the valve wider to increase refrigerant flow into the evaporator. Conversely, as the evaporator cools, the refrigerant in the bulb contracts. This contraction allows the diaphragm to move in the opposite direction, closing the valve and reducing refrigerant flow into the evaporator.

This mechanism allows the TXV to adjust refrigerant flow dynamically based on the cooling load, maintaining efficient system operation.

How an H-block expansion valve works

Like a capillary tube valve, an H-block expansion valve uses a diaphragm filled with a special refrigerant. The expanding and contracting refrigerant in the diaphragm causes the valve to open and close, regulating refrigerant flow into the evaporator. However, unlike the remote-sensing capillary tube design that senses the temperature of the evaporator, the H-block senses the temperature of the refrigerant leaving the evaporator.

H-block expansion valve

What causes an expansion valve to fail?

Here are the most common causes of air conditioning expansion valve failure.

• Clogging and Blockages— Over time, debris, dirt, rubber hose degradation, and contaminants can accumulate in the AC system. If these particles enter the expansion valve, they can clog the orifice or metering pin, restricting the flow of refrigerant. This blockage can cause the system to lose efficiency or stop functioning altogether.

• Moisture Contamination— Moisture in the air conditioning system can lead to the formation of ice crystals, especially at the expansion valve, where the temperature drops significantly. Ice can block the valve, preventing it from functioning correctly. Moisture contamination occurs when an AC system leaks and moisture-laden air enters the system. That’s why fixing a leaking AC system is important rather than simply adding more refrigerant.

Wear and Tear— Like any mechanical component, expansion valves are subject to wear and tear over time. The moving parts within the valve can wear down, leading to improper regulation of refrigerant flow. Regular inspections and timely replacement of worn-out valves can help maintain system performance.

Thermostatic Element Failure— The thermostatic element (capillary tube, bulb, and diaphragm) can fail. If the sensing bulb loses its charge or the diaphragm becomes damaged, the valve will not respond correctly to temperature changes in the evaporator, leading to inefficient cooling.

If the expansion valve fails, it can cause various issues in the A/C system, such as poor cooling performance, excessive pressure buildup, or compressor damage. Replacing the faulty expansion valve is typically necessary to restore the air conditioning system’s proper operation.

©, 2020 Rick Muscoplat