The expansion valve is one of the four main components that make up a refrigeration cycle.
This article explains the thermostatic expansion valve.

2.1 Main Function of a Thermostatic Expansion Valve

The expansion valve is a component that controls the refrigerant flow rate based on the degree of superheat at the evaporator outlet.
It measures the degree of superheat at the evaporator outlet and increases or decreases the amount of refrigerant flowing into the evaporator accordingly, thereby maintaining a constant degree of superheat.


This ensures that the refrigerant fully evaporates within the evaporator and prevents liquid refrigerant from returning to the compressor, a condition known as liquid refrigerant floodback, as well as the resulting liquid compression.
Because liquid entering the compressor can cause serious damage, the expansion valve is an important component for preventing this problem.

2.2 Main Components

There are many types of thermostatic expansion valves, but all operate according to the same basic principle.
They primarily consist of a valve body, diaphragm, needle, and sensing bulb.

＜Valve Body＞
The valve body houses the internal components and contains an orifice that restricts the flow of refrigerant.
The valve reduces the pressure, allowing the refrigerant to boil at a lower temperature.
An orifice assembly is located inside the valve inlet.
A small orifice (opening), restricted by a spring-loaded stopper, regulates the pressure of the refrigerant flowing into the evaporator (evaporating pressure).

＜Diaphragm＞
The diaphragm is made of a strong, thin, and flexible material, usually metal.

＜Needle＞
The needle is a component that opens and closes the valve attached to the end of a vertically moving pin, changing the size of the opening within the orifice and thereby controlling the flow of refrigerant.

＜Spring＞
The spring counteracts the movement of the needle.

2.3 Operating Principle

When the cooling load increases, the degree of superheat at the evaporator outlet rises.
When the sensing bulb installed at the outlet detects this increase, the charge inside the bulb expands (or boils), causing the pressure to rise.

This pressure pushes the diaphragm downward, causing the connected needle to open the orifice.
As a result, the valve opens against the force of the spring, and a greater amount of refrigerant is supplied to the evaporator (Figure 2-1: Operation when superheat increases).

Figure 2-1 Operating Principle of a Thermostatic Expansion Valve (TEV) When Superheat Increases

Conversely, when the refrigerant supply increases and the degree of superheat decreases, the pressure inside the sensing bulb falls.
The force of the spring then pushes the diaphragm back, restricting the valve opening and reducing the refrigerant flow rate (Figure 2-2: Operation when superheat decreases).

By repeating this process, the valve regulates the flow of refrigerant to maintain an appropriate supply.

Figure 2-2 Operating Principle of a Thermostatic Expansion Valve (TEV) When Superheat Decreases

About This Article

Reference
・The Engineering Mindset
(Last accessed: March 15, 2026)

※This article was prepared with reference to the source listed above and organized based on the author’s understanding.