Fixed vs Adaptive Expansion Valves: What’s the Difference?

An expansion valve is an essential component of any refrigeration system. Whether you're a veteran or new technician in the field, you've probably interacted with this device. But do you the know the difference between fixed vs adaptive expansion valves? For the newbies, don’t feel left out. This ship hasn’t sailed just yet. Let’s get you on board, shall we?

Before we dive into the differences between fixed and adaptive varieties, let's get into the basics of expansion valves.

What is an Expansion Valve?

An expansion valve is a device that controls refrigerant flow in a conditioning or refrigeration system by removing pressure from the liquid refrigeran, allowing it to change from liquid to vapor in the evaporator (expansion). This component serves two major purposes. The valve not only controls the amount of refrigerant making its way into the evaporator, but it also maintains the pressure difference between the evaporator (low-pressure) and the condenser (high-pressure).

In layman’s terms, you could say that an expansion valve injects the refrigerant into the evaporator, letting it absorb heat. This device is sized so that the refrigerant they inject into the evaporator matches the calculated load in the evaporator at any given set of conditions. There are different types of expansion valves on the market. They range from the simple varieties like the manual hand valves to the highly adaptive ones like the electronic options.

With all that said, all these different expansion valves fall under two categories: Fixed and adaptive. Let’s dissect the two classes.

What are Fixed Expansion Valves?

To understand how fixed expansion valves work, you need to note that the evaporator heat load changes depending on a different set of conditions. Got it? Good. A fixed option can't adjust the refrigerant flow into the evaporator to match up with evaporator heat load changes. Well, this doesn’t mean that the amount of refrigerant they inject into the evaporator is constant. No. It might change, but it won’t change purposefully to meet the heat load change.

The most common fixed expansion valves include piston and capillary tubes. Piston expansion valves are simply holes drilled in a metal piece held in a distributor and attached to an evaporator inlet. The amount of refrigerant that flows through it (condenser and evaporator pressure differences) is determined by the pressure difference across it and the size of the hole.

Capillary tubes, on the other hand, are long and narrow and can adapt to condenser pressure changes to some extent. These are ideal in conditions where low and high side pressures are steady. The size of the cap tubes depends on the load at the potential conditions. In other words, you need to determine the load at the desired conditions before determining the ideal diameter and length of the cap tube. If your calculations aren't correct, you’ll end up with too little or too much flow into the evaporator.

What are Adaptive Expansion Valves?

Adaptive expansion valves simply means that the valve will respond to heat load changes in the evaporator by altering the amount of refrigerant entering the evaporator. Thus, adaptive options will inject the right amount of refrigerant that matches the heat load in the evaporator.

Thermal expansion valves (TXVs) are the most popular adaptive valves. These act as a meter that use bulb sensors and pressure in the evaporator to act as signals. The pressure helps open and close the valve as well as act in opposition of each other, controlling the amount of refrigerant released into the system's evaporator by dropping pressure between liquid in the line and evaporator.

Electronic expansion valves (EEVs) are other popular adaptive valves on the market. The EEVs feature a solenoid and body valve with a programmable controller. This allows you to program the valves for specific superheat values and different refrigerant types. The valves use signals for temperature and pressure, determining the appropriate amount of superheat in the evaporator and evaporator saturation temperature to close or open based on what's been programmed.