How an Expansion Valve Works: A Simple Explanation

Learn how an expansion valve works and fails

Let’s keep this simple; the job of the expansion valve is to regulate the flow of high-pressure liquid refrigerant through a small opening into the evaporator. There are two types of automotive expansion valves, but they all work on the same basic principle; the valve remains open to allow refrigerant flow until the evaporator temperature drops to a certain point. Then the valve closes to prevent superheating and evaporator freeze up. Here’s how an expansion valve works.

How a remote sensing bulb expansion valve works

A remote sensing bulb expansion valve has a power head, capillary tube, and a sensing bulb. The expansion valve is connected to the evaporator inlet/outlet. The remote sensing bulb is attached to the fins on the evaporator to sense the evaporator temperature.

The power head, capillary tube, and bulb are filled with a gas that expands and contracts with changes in evaporator temperature. As the remote bulb senses changes in the surface temperature of the evaporator, and the gas expands and contracts, the gas pushes up or down on the power head which moves the diaphragm, pushing the metering pin into or out of the machined orifice. It’s the pin movement that allows or restricts the flow of refrigerant into the evaporator.

Typical remote bulb or capillary tube expansion valve

High-pressure liquid refrigerant pushes against the seated metering pin in the expansion valve. When the evaporator is warm the gas in the capillary tube expands and pushes down on the power head and diaphragm, which pushes the metering pin down, allowing refrigerant to flow into the evaporator. As the evaporator chills, the sensing bulb cools, causing the gas to contract, allowing the superheat spring to push up on the metering pin and close off the flow of refrigerant.

How an H block expansion valve

An H-block expansion valve performs the same refrigerant metering function as a remote sensing bulb system, but it does it in a slightly different way. Instead of sensing the surface temperature of the evaporator, it senses the temperature of the refrigerant leaving the evaporator.

The low-pressure refrigerant gas passes out of the evaporator and around the hollow temperature-sensing pin located in the outlet port. The hollow pin is filled with a gas that expands and contracts in relation to refrigerant temperature. The gas moves the diaphragm in the power head, which then moves the metering ball valve located in the valve inlet, allowing refrigerant metering based on exiting refrigerant temperature.

An H-block metering valve eliminates the sensing tube kinking issues commonly found in a remote sensing expansion valve.

See this post for tips on how to diagnose an automotive expansion valve

See this post for symptoms of a bad expansion valve

©, 2019 Rick Muscoplat