Working Principles and Differences Between Internal Equalized and External Equalized Expansion Valves

The Role of Expansion Valves in Refrigeration Systems

Although the expansion valve is only a relatively small component within an entire cold storage refrigeration system, its importance is extremely high. The expansion valve directly affects whether evaporator liquid supply remains stable and whether refrigeration efficiency operates normally.

The primary function of an expansion valve is to control refrigerant flow entering the evaporator and automatically regulate refrigerant supply so that the evaporator always remains in an appropriate operating condition. Simply put, it works like an “automatic water flow regulating faucet.” When cold storage heat load increases, it automatically opens wider. When cold storage temperature decreases, it automatically reduces opening.

If the expansion valve opens excessively, large amounts of liquid refrigerant may not completely evaporate before returning directly to the compressor, causing liquid return. In severe cases, this may even damage the compressor. If the expansion valve opens too little, evaporator liquid supply becomes insufficient, preventing the latter portion of the evaporator from fully participating in heat exchange, causing cooling speed to decrease. Therefore, the operating condition of the expansion valve directly affects the stability of the entire refrigeration system.

In practical applications, thermostatic expansion valves are divided into two types: internal equalized expansion valves and external equalized expansion valves. Although these two valves look similar externally, their internal operating methods are completely different.

Working Principle of Internal Equalized Expansion Valves

The most obvious characteristic of an internal equalized expansion valve is that it only has two main connections. Structurally, it appears relatively simple, which is why many small refrigeration units and standard air conditioning systems use this design.

To understand internal equalized expansion valves, it is first necessary to understand how they “sense temperature changes.”

At the top of the expansion valve, a thin capillary tube connects to the sensing bulb. The sensing bulb is usually attached to the evaporator outlet pipe and contains a small amount of refrigerant inside. When evaporator outlet temperature increases, refrigerant inside the sensing bulb begins evaporating and generates pressure. This pressure is transmitted through the capillary tube to the upper side of the diaphragm inside the expansion valve.

As pressure above the diaphragm increases, the diaphragm is pushed downward and drives the valve needle downward. Once the valve needle moves, the previously closed valve port gradually opens, allowing more refrigerant into the evaporator. As liquid supply increases, evaporator heat absorption capacity increases, and cold storage temperature gradually decreases.

When cold storage temperature decreases, evaporator outlet temperature also decreases. At this time, refrigerant inside the sensing bulb condenses again, reducing pressure. The downward force acting on the diaphragm weakens, and the expansion valve gradually closes.

However, valve closure does not depend solely on spring force. Another force participating in equilibrium is evaporator pressure itself. Inside the internal equalized expansion valve, there is a very small passage. Pressure near the evaporator inlet enters the space beneath the diaphragm through this small passage, creating an upward force on the diaphragm. Meanwhile, the spring inside the expansion valve also pushes upward against the diaphragm.

Therefore, the entire valve actually operates under the balance of three forces:

• Sensing bulb pressure pushes downward

• Evaporator pressure pushes upward

• Spring force also pushes upward

When these three forces reach equilibrium, the valve stabilizes at a certain opening position.

The reason it is called “internal equalization” is because evaporator pressure is obtained directly through an internal passage within the valve body, requiring no additional tubing. Therefore, pressure acquisition occurs internally.

Thermostatic Expansion Valve (TXV) Working Principle

Problems Associated with Internal Equalized Expansion Valves

Internal equalized expansion valves have simple structures, relatively low cost, and easy installation, making them widely used. However, they have one obvious problem: the pressure they measure is not the true evaporator outlet pressure but pressure near the evaporator inlet.

Many beginners in refrigeration systems often overlook pressure losses inside evaporators. In reality, after refrigerant enters the evaporator, it must travel through long coil circuits. During this process, pressure continuously decreases because of frictional resistance and evaporation effects.

This means evaporator inlet pressure and outlet pressure are often different.

If evaporator coils are relatively short, this pressure difference may not be significant, allowing internal equalized expansion valves to function normally. However, if evaporators are long or belong to low-temperature systems, internal pressure drops become much more significant.

At this point, internal equalized expansion valves still regulate based on inlet pressure, causing misjudgment. This situation can be compared to someone trying to determine whether water pressure at the end of a pipe is sufficient but only measuring pressure at the pipe entrance. Since resistance exists throughout the pipe, high inlet pressure does not necessarily mean high pressure at the outlet.

Similarly, because internal equalized expansion valves cannot detect true outlet pressure, they may close prematurely. The result is insufficient liquid supply, causing the latter part of the evaporator to participate less effectively in heat exchange and ultimately reducing refrigeration performance.

Many cold storage systems experience situations where “the front section frosts while the rear section remains insufficiently cooled.” In many cases, this is directly related to uneven refrigerant distribution.

Working Principle of External Equalized Expansion Valves

To solve errors caused by evaporator pressure drop, external equalized expansion valves were developed.

The biggest characteristic of external equalized expansion valves is that they have one additional connection compared with internal equalized valves. This connection usually attaches to a small equalizing tube directly connected to the evaporator outlet pipe. As a result, the pressure sensed beneath the diaphragm is no longer evaporator inlet pressure but the actual evaporator outlet pressure.

Although this only adds one small tube, the operating logic changes significantly. Because external equalized expansion valves directly collect evaporator outlet pressure, they are not affected by pressure losses inside the evaporator. The data obtained becomes more accurate, naturally resulting in more precise valve regulation.

When evaporator load changes, external equalized valves can more accurately determine actual evaporator operating conditions and adjust liquid supply accordingly.

The greatest advantage of this regulation method is more precise superheat control. Superheat can simply be understood as the additional temperature rise of refrigerant after leaving the evaporator. Excessive superheat indicates insufficient liquid supply, while insufficient superheat may indicate liquid return risk.

Because external equalized expansion valves provide more precise control, they allow evaporators to maintain more ideal operating conditions.

Advantages of External Equalized Expansion Valves in Cold Storage Applications

Low-temperature cold storage systems typically feature long evaporators, complex piping, and large internal pressure drops[  Low-temperature cold storage is inherently more difficult to cool, requiring larger heat exchange areas. Larger heat exchange areas inevitably create greater flow resistance.]. In these systems, continuing to use internal equalized expansion valves often results in insufficient liquid supply.

Especially in large air-cooled cold storage systems, evaporator coil lengths may become extremely long. After refrigerant passes through these coils, pressure decreases significantly. Since internal equalized expansion valves cannot detect actual outlet pressure, they often incorrectly assume evaporator pressure remains high and prematurely reduce refrigerant supply.

The result is that the latter portion of the evaporator cannot be fully utilized, wasting available heat exchange area. External equalized expansion valves, however, directly read evaporator outlet pressure, enabling more accurate adjustment of valve needle opening so that the entire evaporator coil participates in heat exchange.

This not only improves refrigeration efficiency but also provides more stable cold storage temperatures. Therefore, in low-temperature cold storage systems, medium-to-large cold storage facilities, long evaporator coils, and systems requiring high temperature control accuracy, external equalized expansion valves have already become the mainstream choice.

The Importance of Understanding Expansion Valve Principles

Many refrigeration system failures are actually related to abnormal expansion valve regulation. Problems such as insufficient liquid supply, uneven evaporator frosting, excessively high suction temperatures, and compressor liquid return may all be related to abnormal expansion valve operation.

Truly professional refrigeration technicians do not simply solve problems by replacing components. Instead, they can quickly identify system abnormalities through pressure changes, temperature changes, and frosting conditions.

The foundation of all such analysis is understanding expansion valve operating principles. Only by truly understanding the differences between internal equalized and external equalized expansion valves can technicians more accurately determine why problems occur in actual refrigeration systems and how adjustments and repairs should be performed.